Guidance for Conducting 

Technical Analyses for

10 CFR Part 61

Draft Report for Comment

Office of Nuclear Material Safety and Safeguards

NUREG-2175
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Guidance for Conducting 

Technical Analyses for

10 CFR Part 61

Draft Report for Comment

Manuscript Completed: January 2015

Date Published: March 2015

Prepared by:

D. Esh, C. Grossman, H. Arlt, C. Barr, P. Yadav

Office of Nuclear Material Safety and Safeguards

NUREG-2175

 
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iii

ABSTRACT

This document provides guidance on conducting technical analyses (i.e., performance 

assessment, inadvertent intruder assessment, assessment of the stability of a low-level waste 

disposal site, defense-in-depth analyses, protective assurance period analyses, and 

performance period analyses) to demonstrate compliance with the performance objectives in 

Title 10 of the Code of Federal Regulations (10 CFR) Part 61, “Licensing Requirements for Land

Disposal of Radioactive Waste.” This document provides implementing guidance for 

amendments to 10 CFR Part 61 that are detailed in the proposed rule, “Low-Level Radioactive 

Waste Disposal,” published in the Federal Register in 2015. As a result, this document is 

written as if the amendments to 10 CFR Part 61 in the cited proposed rule have been enacted; 

this document will be revised, if necessary, if the proposed rule is finalized. The guidance in this 

document is intended to supplement existing low-level radioactive waste guidance on issues 

pertinent to conducting technical analyses to demonstrate compliance with the performance 

objectives. This document provides detailed guidance in new areas that are less covered in 

existing guidance, such as the inadvertent intruder analysis, defense-in depth analyses, and 

analyses for the three phases of the analysis timeframe (compliance period, protective 

assurance period, and performance period). This guidance discusses the use of a graded level 

of effort needed to risk-inform the analyses for the compliance period (1,000 years), the 

protective assurance period (from 1,000 years to 10,000 years after disposal site closure), and 

also covers the performance period analyses that should be performed for analysis of long-lived 

waste beyond 10,000 years.

This guidance should facilitate licensees’ implementation of the proposed amendments as well 

as assist regulatory authorities in reviewing the technical analyses. This guidance applies to all 

waste streams disposed of at a 10 CFR Part 61 low-level waste disposal facility, including large 

quantities of depleted uranium and blended waste. Additional topics covered in this document 

include (1) demonstration that radiation doses are minimized to the extent reasonably 

achievable; (2) the identification and screening of the features, events, and processes to 

develop scenarios for technical analyses; (3) the use of the waste classification tables or the 

results of the technical analyses to develop site-specific waste acceptance criteria; and (4) the 

use of performance confirmation to evaluate and verify the accuracy of information used to 

demonstrate compliance prior to site closure. 
 
v

PREFACE

The U.S. Nuclear Regulatory Commission published the rulemaking, “Low-Level Radioactive 

Waste Disposal,” in 2015 (NRC, 2015a). This rulemaking includes detailed requirements for 

10 CFR Part 61 licensees for performing technical analyses to demonstrate compliance with the 

performance objectives of Subpart C (i.e., 10 CFR 61.41 through 10 CFR 61.44). The purpose 

of this guidance document is to provide licensees with the tools to develop high-quality technical 

analyses, specifically the performance assessment, inadvertent intrusion assessment, site 

stability analyses, defense-in-depth analyses, protective assurance period analyses, and 

performance period analyses. This document should also be used by NRC or Agreement State 

regulators to identify risk-significant aspects of these technical analyses that should drive their 

review and decision-making process for the disposal site. This document refers to the reviewing 

authority as “NRC” or “NRC staff”, but these terms should be interpreted to mean Agreement 

State regulators as well, if applicable.

In addition to the technical analyses, this document covers the process for demonstrating 

compliance with 10 CFR 61.58, for waste acceptance. Licensees and reviewers may use this 

document to assist in developing the waste acceptance criteria for their disposal sites, as well 

as for developing acceptable processes for waste characterization and certification. 

This document is written as if the amendments in the proposed rule have been enacted. 

However, for ease of reference, Appendix A highlights changes to 10 CFR Part 61 that were 

proposed in 2015 that identify new requirements, particularly relating to technical analyses and 

waste acceptance criteria.

This document is intended to provide guidance in a non-prescriptive manner; providing 

reference material for licensees, yet allowing flexibility for adapting this guidance to the specifics 

for each low-level waste disposal site. Additional details and examples are provided in 

appendices to this document, such as (1) hazard maps in Appendix B that may assist licensees

and reviewers in determining site suitability, and (2) lists of example features, events, and 

processes in Appendix C that may assist licensees in defining the scope of the technical 

analyses. A glossary is provided in Section 13.0 that defines many of the terms used in this 

document.

To develop a succinct document, the NRC staff has written this guidance with the assumption 

that the reader has some level of proficiency in conducting technical analyses. For example, 

the NRC staff describes the application of model support and model abstraction for the 

performance assessment and intruder assessment and references existing documents for 

background information on these terms. Therefore, the NRC staff recommends that the reader 

become familiar with documents such as NUREG-1573, “A Performance Assessment 

Methodology for Low-Level Radioactive Waste Disposal Facilities,” issued in October 2000

(NRC, 2000a), and other guidance documents referenced in Section 1.2, as background for this 

document. 

vii

CONTENTS

ABSTRACT.....................................................................................................................................iii

PREFACE........................................................................................................................................v

FIGURES ........................................................................................................................................xi

TABLES......................................................................................................................................... xiii

ACKNOWLEDGMENTS ................................................................................................................xv

ACRONYMS AND ABBREVIATIONS ......................................................................................... xvii

1.0 INTRODUCTION...............................................................................................................1-1

1.1 Background ...................................................................................................................1-1

1.1.1 Performance Objectives ........................................................................................1-1

1.1.2 Safety Case............................................................................................................1-3

1.1.3 Defense-in-Depth Analyses...................................................................................1-3

1.1.4 Technical Analyses................................................................................................1-4

1.1.5 Waste Acceptance Requirements.........................................................................1-7

1.1.6 Agreement State Interactions ................................................................................1-9

1.2 Purpose of This Guidance Document.........................................................................1-10

1.2.1 Relationship to Other NRC Guidance .................................................................1-10

1.2.2 What is New in this Document?...........................................................................1-12

1.3 Document Organization ..............................................................................................1-12

1.4 Risk-Informed Approach .............................................................................................1-14

2.0 GENERAL TECHNICAL ANALYSES CONSIDERATIONS.............................................2-1

2.1 Assessment Process.....................................................................................................2-2

2.1.1 Terms and Definitions............................................................................................2-2

2.1.2 Level of Effort.........................................................................................................2-4

2.2 General Review Considerations ...................................................................................2-5

2.2.1 Data Adequacy.......................................................................................................2-5

2.2.2 Uncertainty .............................................................................................................2-6

2.2.3 Model Support......................................................................................................2-10

2.2.4 Dose Assessment................................................................................................2-14

2.3 Assessment Context...................................................................................................2-19

2.3.1 Context of the Performance Assessment............................................................2-19

2.3.2 Approach to Different Timeframes.......................................................................2-20

2.4 System Description .....................................................................................................2-25
viii

2.5 Scenario Development................................................................................................2-26

2.5.1 Scope of Analysis ................................................................................................2-28

2.5.2 Role of Qualified Specialists................................................................................2-30

2.5.3 Approaches ..........................................................................................................2-30

2.5.4 Constructing Scenarios........................................................................................2-49

2.6 Conceptual Model Development.................................................................................2-51

2.6.1 Alternative Conceptual Models............................................................................2-53

2.7 Numerical Model Development and Assessment.......................................................2-53

2.7.1 Numerical Models ................................................................................................2-53

2.7.2 Model Abstraction and Model Simplification .......................................................2-56

2.7.3 Model Integration .................................................................................................2-60

2.7.4 Analysis and Evaluation of Results .....................................................................2-60

3.0 PERFORMANCE ASSESSMENT....................................................................................3-1

3.1 Performance Assessment Approach............................................................................3-1

3.1.1 Example of a Performance Assessment Approach ..............................................3-2

3.1.2 Role of the Performance Assessment...................................................................3-3

3.2 Source Term..................................................................................................................3-5

3.2.1 Inventory.................................................................................................................3-7

3.2.2 Chemical Environment...........................................................................................3-8

3.2.3 Waste Container ....................................................................................................3-9

3.2.4 Wasteform and Waste Type................................................................................3-10

3.2.5 Aqueous Release.................................................................................................3-12

3.2.6 Gaseous Release ................................................................................................3-13

3.2.7 Direct Release......................................................................................................3-16

3.2.8 Biota Enhanced Release .....................................................................................3-17

3.3 Radionuclide Transport...............................................................................................3-19

3.3.1 Groundwater Transport........................................................................................3-20

3.3.2 Surface Water Transport .....................................................................................3-26

3.3.3 Atmospheric Transport.........................................................................................3-27

3.3.4 Biotic Transport....................................................................................................3-29

3.4 Biosphere ....................................................................................................................3-30

4.0 INADVERTENT INTRUSION............................................................................................4-1

4.1 Waste Acceptance Requirements ................................................................................4-2

4.2 Inadvertent Intrusion Barriers........................................................................................4-2

4.2.1 Risk-Informed Approach to Evaluating Intruder Barriers ......................................4-4
ix

4.2.2 Technical Basis for Intruder Barrier Capabilities ...................................................4-4

4.2.3 Use of Engineered Intruder Barriers......................................................................4-5

4.3 Inadvertent Intrusion Assessment ................................................................................4-7

4.3.1 Receptor Scenario Analysis.................................................................................4-10

4.3.2 Model Abstraction ................................................................................................4-26

4.4 Institutional Controls....................................................................................................4-36

5.0 SITE STABILITY ANALYSES...........................................................................................5-1

5.1 Disruptive Processes and Events.................................................................................5-7

5.1.1 Natural Processes..................................................................................................5-8

5.1.2 Anthropogenic Processes....................................................................................5-12

5.1.3 Subsidence and Differential Settlement ..............................................................5-12

5.2 Technical Assessment................................................................................................5-13

5.2.1 Available Tools and Codes ..................................................................................5-13

5.2.2 Approaches for Assessment................................................................................5-14

5.2.3 Uncertainty ...........................................................................................................5-21

5.3 Engineered Barriers for Site Stability..........................................................................5-21

5.3.1 Existing Guidance................................................................................................5-22

5.3.2 Long-Term Considerations ..................................................................................5-26

5.3.3 Monitoring of Engineered Barriers.......................................................................5-33

6.0 PROTECTIVE ASSURANCE PERIOD ANALYSES........................................................6-1

6.1 Scope of the Protective Assurance Period Analyses ...................................................6-2

6.2 Framework for the Protective Assurance Period Analyses..........................................6-2

6.2.1 Types of Analyses..................................................................................................6-5

7.0 PERFORMANCE PERIOD ANALYSES...........................................................................7-1

7.1 Estimation of Disposal Site-Averaged Isotopic Concentrations...................................7-5

7.2 Disposal Site Characteristics that Enhance Long-Term Isolation................................7-7

7.3 Scope of the Performance Period Analyses.................................................................7-8

7.3.1 Features, Events, and Processes........................................................................7-11

7.3.2 Screening of Features, Events, and Processes Based on the 

Requirements in 10 CFR 61.50...........................................................................7-13

7.3.3 Future Human Behavior.......................................................................................7-14

7.4 Analyses for Long-Lived Waste..................................................................................7-14

7.4.1 Types of Analyses................................................................................................7-16

8.0 DEFENSE-IN-DEPTH ANALYSES ..................................................................................8-1

8.1 Background on Defense-in-Depth ................................................................................8-1
x

8.2 Defense-in-Depth Concepts for a Land Disposal Facility.............................................8-3

8.2.1 Multiple Layers .......................................................................................................8-3

8.2.2 Independent Layers ...............................................................................................8-6

8.2.3 Redundant Layers..................................................................................................8-7

8.2.4 Safety Margin.........................................................................................................8-7

8.2.5 Risk-Informed Approach........................................................................................8-8

8.3 Defense-in-Depth Analyses ..........................................................................................8-8

8.3.1 Identification of Defense-in-Depth Protections......................................................8-9

8.3.2 Description of Safety Functions...........................................................................8-11

8.3.3 Demonstrating Safety Margin ..............................................................................8-13

9.0 WASTE ACCEPTANCE ...................................................................................................9-1

9.1 Waste Acceptance Criteria ...........................................................................................9-2

9.1.1 Allowable Activities and Concentrations................................................................9-2

9.1.2 Acceptable Wasteform Characteristics and Container Specifications................9-12

9.1.3 Restrictions or Prohibitions..................................................................................9-14

9.2 Waste Characterization Methods................................................................................9-15

9.2.1 Acceptable Waste Characterization Methods .....................................................9-16

9.2.2 Data Quality Objectives Process.........................................................................9-21

9.2.3 Documentation.....................................................................................................9-23

9.3 Waste Certification Program.......................................................................................9-24

9.3.1 Certification Program ...........................................................................................9-24

9.3.2 Certification Maintenance ....................................................................................9-28

9.4 Periodic Review...........................................................................................................9-29

9.5 Mitigation .....................................................................................................................9-30

10.0 PERFORMANCE CONFIRMATION...............................................................................10-1

11.0 USE OF OTHER NRC GUIDANCE DOCUMENTS .......................................................11-1

12.0 REFERENCES................................................................................................................12-1

13.0 GLOSSARY ....................................................................................................................13-1

APPENDIX A: CHANGES TO 10 CFR PART 61 MADE IN 2015 RULEMAKING....................A-1

APPENDIX B: HAZARD MAPS..................................................................................................B-1

APPENDIX C: GENERIC FEPs LIST FOR NEAR-SURFACE DISPOSAL OF 

LOW-LEVEL RADIOACTIVE WASTE.............................................................. C-1

APPENDIX D: ADDITIONAL APPROACHES TO CONSTRUCT SCENARIOS AND 

CONCEPTUAL MODELS.................................................................................. D-1

APPENDIX E: SITE STABILITY ASSESSMENT EXAMPLES ..................................................E-1
xi

FIGURES

Figure 1-1 Technical Analyses Relationship to Analyses Timeframes and 

Performance Objectives ........................................................................................1-8

Figure 2-1 Initial Steps of the Performance Assessment Methodology ................................2-3

Figure 2-2 Overall Approach to Uncertainty Analysis for LLW Performance 

Assessment (NRC, 1993a)....................................................................................2-7

Figure 2-3 A Central Scenario with Several Plausible Receptor Scenarios, 

and an Alternative Scenario with a Different Plausible Receptor 

Scenario...............................................................................................................2-15

Figure 2-4 A Central Scenario with Several Plausible Receptor Scenarios, 

and an Alternative Scenario with the Same Plausible Receptor 

Scenarios .............................................................................................................2-15

Figure 2-5 Overview of Abstraction as Applied to a Performance Assessment 

Model....................................................................................................................2-58

Figure 2-6 Abstraction of a Hydrologic Model for a Performance Assessment 

Model....................................................................................................................2-59

Figure 3-1 Example of a Performance Assessment Process.................................................3-4

Figure 4-1 Technical Analyses Required to Demonstrate Compliance with the 

10 CFR Part 61 Performance Objective for the Protection of Individuals 

from Inadvertent Intrusion......................................................................................4-1

Figure 4-2 Example of an Inadvertent Intruder Assessment Process Required Per 

10 CFR 61.42.........................................................................................................4-8

Figure 5-1 Main Elements of Site Stability Analysis ...............................................................5-3

Figure 5-2 Spatial and Temporal Scales and Staff Perspective on the Relative 

Importance of Various Geomorphological Processes Relevant to LLW 

Disposal..................................................................................................................5-6

Figure 5-3 Macroscale Relationship between Climatic Variables, Environments, 

and Rock Weathering Agents: Occurrence of Weathering as a Function 

of Climate (NUREG/CR-2642).............................................................................5-25

Figure 6-1 Analyses Framework for the Minimization Process for the Protective 

Assurance Period Analyses Applied to 10 CFR 61.41(b) .....................................6-3
xii

Figure 6-2 Level Scaling Factors as a Function of the Base Case Dose in 

the Protective Assurance Period (For 10 CFR 61.41 and 

10 CFR 61.42)......................................................................................................6-15

Figure 6-3 Hypothetical Performance Assessment Model Output with the 

Minimization Parameters .....................................................................................6-16

Figure 7-1 Recommended Approach to Conducting Performance Period 

Analysis................................................................................................................7-15

Figure 7-2 Scope of the Technical Analyses for the Compliance Period, Protective 

Assurance Period, and Performance Period for a Hypothetical Site..................7-19

Figure 8-1 Multiple Layers of Defense ....................................................................................8-4

Figure 8-2 Land Disposal Facility Lifecycle and Timeframes for 

Defense-in-Depth Layers.......................................................................................8-5

Figure 9-1 Waste Acceptance Components ...........................................................................9-1

Figure 9-2 Components Of 10 CFR Part 61 Waste Acceptance Criteria...............................9-2

Figure 9-3 Influence of Key Variables on the Required Relative Safety Factors...................9-5

Figure 9-4 Waste Classification and Segregation for Waste Classes....................................9-8

Figure 9-5 Sources of Uncertainty in Various Methods of Determining Radionuclide 

Activities or Concentrations in Waste Packaged for Disposal............................9-19

Figure 9-6 Data Life Cycle Framework for Waste Characterization.....................................9-22
xiii

TABLES

Table 1-1 Crosswalk between Technical Requirements and Document 

Sections...............................................................................................................1-13

Table 2-1 Analyses Required for Included FEPs Based on 10 CFR 61.50 ........................2-35

Table 2-2 Example of Screening Criteria Based on Qualitative Probabilities, 

Consequences, and Uncertainty Associated with a FEP....................................2-45

Table 3-1 List of Parameters and Processes of Concern Associated with the 

Groundwater Transport Pathway.........................................................................3-22

Table 4-1 Comparison and Description of Intruder Receptor Scenario Terms 

Used in this Guidance..........................................................................................4-12

Table 4-2 Exposure Pathways of Generic Intruder Receptor Scenarios ............................4-14

Table 5-1 Summary of Applicability and Limitations of Erosion and Mass 

Wasting Codes for Long-Term Performance Assessment Evaluated 

in Walter and Dubreuilh, 2007 .............................................................................5-15

Table 5-2 Weathering Resistance and Susceptibility Related to Weathering Type 

and Rock Properties ............................................................................................5-27

Table 5-3 Comparison of Chemical Durability (Soxhlet Test) of Waste Glass and 

Common Minerals................................................................................................5-30

Table 7-1 Long-lived Isotopes Potentially Present in LLW Performance Assessment 

Inventories..............................................................................................................7-2

Table 7-2 Expected Level of Review Associated With Performance Period 

Analyses.................................................................................................................7-3

Table 7-3 Disposal Site-Averaged Isotopic Concentrations that Require 

Performance Period Analyses ...............................................................................7-7

Table 7-4 Disposal Site Characteristics that Could Enhance Long-Term Isolation ............7-10

Table 10-1 Regulatory Requirements Supportive of Performance Confirmation.................10-2

Table 11-1 Guidance Crosswalk for Performance Objective 10 CFR 61.41, 

Performance Assessment....................................................................................11-1

Table 11-2 Guidance Crosswalk for Performance Objective 10 CFR 61.42, 

Inadvertent Intruder Assessment.........................................................................11-2
xiv

Table 11-3 Guidance Crosswalk for Performance Objective 10 CFR 61.44, 

Site Stability Analysis...........................................................................................11-4

Table 11-4 Guidance Crosswalk by Topic .............................................................................11-5
xv

ACKNOWLEDGMENTS

The NRC staff would like to thank R.L. Johnson and T. Johnson, who were major contributors to 

the Site Stability Analyses section of this document. Additionally, Adam Schwartzman and 

Christianne Ridge of the Performance Assessment Branch in NRC were significant contributors 

to the Performance Assessment Modeling Issues and Inadvertent Intrusion Assessment 

sections. The NRC staff also appreciates the hazard map figures developed by Allen Gross 

(Appendix B) and the technical review performed by Karen Pinkston, Tim McCartin, and Michael 

Lee.

xvii

ACRONYMS AND ABBREVIATIONS

ACAP Alternative Cover Assessment Project

ADAMS Agencywide Documents Access and Management System

ALARA as low as reasonably achievable 

ASTM American Society for Testing and Materials

BTP branch technical position

BTP CA BTP on concentration averaging and encapsulation

CFR Code of Federal Regulations

CSDMS Community Surface Dynamics Modeling System

DOE U.S. Department of Energy

DQO data quality objective

DU depleted uranium

EPA U.S. Environmental Protection Agency

ET evapotranspiration

FEPs features, events, and processes

FGR Federal Guidance Report

FR Federal Register

GIS Geographic Information Systems

HLW high-level waste

ICRP International Commission on Radiological Protection

IMPEP Integrated Materials Performance Evaluation Program

Kd distribution coefficient

LLW low-level radioactive waste

m meter

mm millimeter

mrem millirem

mSv millisievert

nCi/g nanocuries per gram

NCRP National Council on Radiation Protection and Measurements

NPV net present value

NRC U.S. Nuclear Regulatory Commission

OMB Office of Management and Budget

PA performance assessment

PAP protective assurance period

PMF probable maximum flood

PMP probable maximum precipitation

PRA probabilistic risk assessment

SOF sum of fractions

QA quality assurance

QA/QC quality assurance/quality control

USGS U.S. Geological Survey

yr year

1-1

1 1.0 INTRODUCTION

2

3 The U.S. Nuclear Regulatory Commission (NRC) establishes licensing requirements, 

4 performance objectives, and technical criteria for the disposal of commercial low-level 

5 radioactive waste (LLW) in near-surface disposal facilities. These requirements can be found in 

6 Title 10 of the Code of Federal Regulations (10 CFR) Part 61, “Licensing Requirements for Land 

7 Disposal of Radioactive Waste” (NRC, 1982a; 47 FR 57446). This guidance document is 

8 intended to support the implementation of the requirements for technical analyses and waste 

9 acceptance to demonstrate compliance with the 10 CFR Part 61 performance objectives. This 

10 guidance supplements, rather than replaces, previous guidance. This document applies to all 

11 waste streams disposed at a 10 CFR Part 61 disposal facility, while providing specific 

12 considerations for long-lived and blended wastes (see Section 1.2.2), where appropriate.

13 1.1 Background

14

15 The regulations in 10 CFR Part 61 applies to any near-surface LLW disposal facility licensed 

16 after January 27, 1983. An integrated systems approach is emphasized in 10 CFR Part 61 for 

17 the disposal of commercial LLW, including site selection, disposal facility design and operation, 

18 minimum wasteform requirements, and disposal facility closure. To lessen the burden on 

19 society over the long periods of time contemplated for the control of the radioactive material and 

20 thus lessen reliance on institutional controls, 10 CFR Part 61 emphasizes passive, rather than 

21 active, systems to limit and retard radioactive releases to the environment.

22

23 To grant a license, the NRC (or Agreement State regulator) must conclude that there is 

24 reasonable assurance that the performance objectives of Subpart C will be met. To 

25 demonstrate that they will meet the performance objectives, 10 CFR Part 61 license applicants 

26 need to prepare several technical analyses. The technical analyses required for licensees to 

27 demonstrate that the performance objectives will be met are specified in 10 CFR 61.13. 

28

29 Licensees must also meet specific technical requirements to ensure safe disposal of LLW. 

30 These requirements are specified in Subpart D and include, among others, requirements for 

31 waste acceptance that are specified in 10 CFR 61.58. The waste acceptance requirements are

32 intended to ensure that the waste that licensees accept for disposal together with the disposal 

33 site and facility design provides reasonable assurance that the performance objectives of 

34 Subpart C will be met.

35

36 The regulatory requirements in 10 CFR Part 61 ensure public health and safety are protected 

37 during the operation of any commercial LLW disposal facility. 10 CFR Part 61 is performance38 based and the technical criteria are written in relatively general terms, which allow applicants to 

39 demonstrate how their proposals meet the respective performance objectives for the specific 

40 near-surface disposal method selected. 10 CFR 61.7 identifies the overall philosophy and 

41 concepts that underlie the regulatory requirements of 10 CFR Part 61.

42 1.1.1 Performance Objectives

43

44 The performance objectives for a LLW disposal facility are contained in 10 CFR Part 61, 

45 Subpart C. The general requirement in 10 CFR 61.40 notes that land disposal facilities must be 

In this document, the first 

use of a technical term that 

is defined in the glossary is 

shown in italics.
1-2

1 sited, designed, operated, closed, and controlled after closure so that reasonable assurance 

2 exists that exposures to humans are within the limits established in the performance objectives 

3 in 10 CFR 61.41 through 10 CFR 61.44. During and after facility operations, the performance 

4 objective at 10 CFR 61.41 requires the protection of the general population from releases of 

5 radioactivity. 

6 • The performance objective 10 CFR 61.41(a) provides an annual dose limit of 0.25 

7 milliSievert (mSv) [25 millirem (mrem)] for the compliance period and a requirement to limit 

8 releases to as low as reasonable achievable (ALARA). 

9 • The performance objective in 10 CFR 61.41(b) requires concentrations of radioactive 

10 material that may be released to the general environment in groundwater, surface water, 

11 air, soil, plants, or animals shall be minimized during the protective assurance period, 

12 and that the annual dose shall be below 5 mSv (500 mrem) or a level that is supported

13 as reasonably achievable based on technological and economic considerations. 

14 • The performance objective in 10 CFR 61.41(c) requires releases of radioactivity from a 

15 disposal facility to the general environment at any time during the performance period to 

16 be minimized to the extent reasonably achievable. 

17

18 The performance objective in 10 CFR 61.42 requires that the disposal facility must protect the 

19 inadvertent intruder at all times during the compliance period after active institutional controls 

20 are removed (i.e., 100 years after facility closure). 

21 • The performance objective 10 CFR 61.42(a) provides an annual dose limit of 5 mSv 

22 (500 mrem) for the compliance period. 

23 • The performance objective in 10 CFR 61.42(b) requires that design, operation, and 

24 closure of the land disposal facility shall minimize exposures to any inadvertent intruder 

25 into the disposal site at any time during the protective assurance period and that the 

26 annual dose shall be below 5 mSv (500 mrem) or a level that is supported as reasonably 

27 achievable based on technological and economic considerations. 

28 • The performance objective in 10 CFR 61.42(c) requires exposures to the inadvertent 

29 intruder at any time during the performance period to be minimized to the extent 

30 reasonably achievable.

31

32 Sections 10 CFR 61.41 and 10 CFR 61.42 require a demonstration of protection beyond closure 

33 of the disposal facility. The length of time that these requirements specify is defined in 

34 10 CFR 61.2, “Definitions”. Specifically:

35 Compliance period is the time out to 1,000 years after closure of the disposal facility.

36

37 Protective assurance period is the period from the end of the compliance period through 

38 10,000 years following closure of the site.

39

40 Performance period is the timeframe established for considering waste and site 

41 characteristics to evaluate the performance of the site after the protective assurance 

42 period.

43

44 The performance objective in 10 CFR 61.43 requires protection of individuals during operations. 

45 Compliance with 10 CFR 61.43 is determined largely through compliance with the standards for 

46 radiation protection set forth in 10 CFR Part 20, and therefore, is not discussed further in this 

47 document.
1-3

1 The performance objective set forth in 10 CFR 61.44 requires that the licensee demonstrate that 

2 the disposal facility will be sited, designed, used, operated, and closed to achieve long-term 

3 stability of the disposal site for the compliance and protective assurance periods. Long-term 

4 stability of the disposal site eliminates, to the extent practicable, the need for ongoing active 

5 maintenance of the disposal site following closure so that only surveillance, monitoring, or minor 

6 custodial care is required. 

7 1.1.2 Safety Case

8

9 Section 10 CFR 61.2 defines a safety case as a collection of information that demonstrates the 

10 assessment of the safety of a waste disposal facility. This includes the technical analyses

11 discussed in Section 1.1.4, as well as information on defense-in-depth and supporting evidence 

12 and reasoning on the strength and reliability of the technical analyses and the assumptions 

13 made therein. The safety case also includes description of the safety relevant aspects of the 

14 site, the design of the facility, and the managerial control measures and regulatory controls. 

15

16 A safety case for a land disposal facility covers the suitability of the site and the design, 

17 construction and operation of the facility, the assessment of radiation risks and assurance of the 

18 adequacy and quality of all of the safety related work associated with the disposal facility. The 

19 purpose of a safety case is to provide a sufficient level of detail regarding the description of all 

20 safety relevant aspects of the site, the design of the facility, and the managerial control 

21 measures and regulatory controls to inform the decision whether to grant a license for the 

22 disposal of LLW and provide the public assurance that the facility will be designed, constructed, 

23 operated, and closed safely (IAEA, 2012). 

24

25 Licensing decisions are based on whether there is reasonable assurance that the performance 

26 objectives can be met. Defense-in-depth protections, such as siting, wasteforms, radiological 

27 source-term, engineered features, and natural system features of the disposal site, combined 

28 with technical analyses and scientific judgment form the safety case for licensing a LLW

29 disposal facility. The insights derived from technical analyses include supporting evidence and 

30 reasoning on the strength and reliability of the layers of defense relied upon in the safety case. 

31 These insights provide input for making regulatory decisions. The safety case must conclude

32 that public health and safety will be adequately protected from the disposal of LLW (including 

33 long-lived LLW). A clear case for the safety of a disposal facility also serves to enhance the 

34 communication among stakeholders. 

35

36 Finally, the NRC staff envisions that the safety case for a land disposal facility would evolve 

37 over time as new information is gained during the various phases of the facility’s development 

38 and operation. Therefore, the NRC staff expects that the safety case will be updated as new 

39 information that could significantly impact safety of the facility is learned. Section 

40 10 CFR 61.28(a) requires that the application for closure of a licensed land disposal facility must 

41 include a final revision to the safety case.

42 1.1.3 Defense-in-Depth Analyses 

43

44 Section 10 CFR 61.2 defines defense-in-depth as the use of multiple independent and 

45 redundant layers of defense such that no single layer, no matter how robust, is exclusively relied 

46 upon. Defense-in-depth for a land disposal facility includes, but is not limited to, the use of 

47 siting, wasteforms and radionuclide content, engineered features, and natural geologic features 
1-4

1 of the disposal site. Section 8.0 describes the information that a licensee should provide and a 

2 reviewer should evaluate with respect to the technical analyses for demonstrating that a land 

3 disposal facility incorporates defense-in-depth protections during the operational and post4 closure phases of the land disposal facility lifecycle. Section 10 CFR 61.13(f) requires licensees 

5 to complete analyses that demonstrate the proposed disposal facility includes defense-in-depth 

6 protections.

7 1.1.4 Technical Analyses

8

9 The technical analyses needed to demonstrate that the performance objectives of Subpart C 

10 are met are provided in 10 CFR 61.13(a) through (f). Technical analyses assess the impact of

11 site-specific factors on the performance of the disposal facility and the site environment both (1) 

12 during the operational period, as in the analysis for protection of individuals during operations, 

13 and (2) for disposal of radioactive waste over the longer term, as in the analyses for protection 

14 of the general population from releases of radioactivity, protection of inadvertent intruders, 

15 stability of the disposal site after closure, and assessment of long-term impacts from LLW 

16 disposal over the protective assurance and performance periods. 

17

18 In this document, the term “technical analyses” comprises the performance assessment, 

19 intruder assessment, site stability analysis, defense-in-depth analyses, protective assurance 

20 period analyses, and performance period analyses. A description of each of the analyses is 

21 presented in the following sections.

22 1.1.4.1 Performance Assessment

23

24 A performance assessment (PA) is a type of risk analysis that addresses (1) what can happen,

25 (2) how likely it is to happen, and (3) what are the resulting impacts (Eisenberg et al, 1999). 

26 These impacts can then be compared to the performance objective in 10 CFR 61.41 

27 (radiological protection of the general public). The requirements for a performance assessment 

28 are set forth in 10 CFR 61.13(a). A performance assessment shall:

29 (1) Consider features, events, and processes (FEPs) that might affect demonstration of

30 compliance with 10 CFR 61.41. The FEPs considered must represent a range of 

31 phenomena with both beneficial and adverse effects on performance, and must consider 

32 the specific technical information required in 10 CFR 61.12(a) through 10 CFR 61.12(i). 

33 A technical basis for inclusion or exclusion of specific FEPs must be provided. 

34 (2) Evaluate specific FEPs if their omission would significantly affect meeting the 

35 performance objective specified in 10 CFR 61.41.

36 (3) Consider the likelihood of disruptive or other unlikely FEPs for comparison with the limits 

37 set forth in 10 CFR 61.41.

38 (4) Reflect new FEPs different from the compliance period that address significant 

39 uncertainties inherent in the long timeframes associated with demonstrating compliance 

40 with 10 CFR 61.41(b) only if scientific information compelling such changes is available.

41 (5) Provide a technical basis for either inclusion or exclusion of degradation, deterioration, 

42 or alteration processes (e.g., of the engineered barriers, wasteform, site characteristics) 

43 and interactions between the disposal facility and site characteristics that might affect 

44 meeting the performance objective in 10 CFR 61.41.
1-5

1 (6) Provide a technical basis for models used in the performance assessment such as 

2 comparisons made with outputs of detailed process-level models or empirical 

3 observations (e.g., laboratory testing, field investigations, and natural analogs).

4 (7) Evaluate pathways including air, soil, groundwater, surface water, plant uptake, and 

5 exhumation by burrowing animals.

6 (8) Account for uncertainties and variability in the projected behavior of the disposal system 

7 (e.g., disposal facility, natural system, and environment).

8 (9) Consider alternative conceptual models of features and processes that are consistent 

9 with available data and current scientific understanding, and evaluate the effects that 

10 alternative conceptual models have on the understanding of the performance of the 

11 disposal facility.

12 (10) Identify and differentiate between the roles performed by the natural disposal site 

13 characteristics and design features of the disposal facility in limiting releases of 

14 radioactivity to the general population.

15 1.1.4.2 Inadvertent Intrusion Assessment

16

17 The 10 CFR Part 61 regulations envision a period of active institutional controls for 100 years 

18 following closure of the LLW disposal facility. During that time period, the disposal site and its 

19 contents are protected from disturbance by potential intruders through a series of measures 

20 (e.g., site access controls). At the end of the 100 years of active institutional controls, 

21 10 CFR Part 61 requires licensees to assume there will be no active caretaking of the disposal 

22 site and it is possible for inadvertent intruders to gain access. An inadvertent intruder is defined 

23 in 10 CFR 61.2, “Definitions,” as “a person who might occupy the disposal site after closure and 

24 engage in normal activities, such as agriculture, dwelling construction, resource exploration or 

25 exploitation (e.g., well drilling) or other reasonably foreseeable pursuits that might unknowingly 

26 expose the person to radiation from the waste included in or generated from a disposal facility.” 

27 Licensees should demonstrate that potential inadvertent intruders, who might occupy the site at 

28 any time after institutional controls over the disposal site are removed, will be protected. 

29

30 An inadvertent intrusion assessment (also referred to as an “intruder assessment”) is an 

31 iterative process involving site-specific, prospective modeling evaluations of potential 

32 radiological consequences as a result of reasonably foreseeable human activities that might 

33 unknowingly occur should an individual occupy a near-surface facility for disposal of LLW after 

34 the loss of institutional controls. The intruder assessment is used to evaluate how these 

35 impacts compare to the performance objective in 10 CFR 61.42. 

36

37 Because there is no scientific basis for quantitatively predicting the probability of a future 

38 disruptive human activity over long timeframes, an inadvertent intruder assessment does not 

39 consider the probability of inadvertent intrusion occurring. Rather, the assessment assumes 

40 that reasonably bounding receptor scenarios occur and evaluates the radiological 

41 consequences that could be experienced by inadvertent intruders should institutional controls or 

42 societal memory be lost (NCRP, 2005).

43 As stated in 10 CFR 61.13(b), analyses of the protection of inadvertent intruders must 

44 demonstrate there is reasonable assurance the waste acceptance criteria developed in 

45 accordance with 10 CFR 61.58 will be met, adequate barriers to inadvertent intrusion will be 

46 provided, and any inadvertent intruder will not be exposed to doses that exceed the limits set 
1-6

1 forth in 10 CFR 61.42 as demonstrated in an intruder assessment. An intruder assessment 

2 shall:

3 (1) Assume that an inadvertent intruder occupies the disposal site at any time during the

4 compliance period (i.e., 1,000 years) after the period of institutional controls ends, and 

5 engages in normal activities including agriculture, dwelling construction, resource 

6 exploration or exploitation (e.g., well drilling), or other reasonably foreseeable pursuits 

7 that are consistent with activities in and around the site at the time of closure and that

8 unknowingly expose the intruder to radiation from the waste.

9 (2) Identify adequate barriers to inadvertent intrusion that inhibit contact with the waste or 

10 limit exposure to radiation from the waste, and provide a basis for the time period over 

11 which barriers are effective.

12 (3) Account for uncertainties and variability.

13 1.1.4.3 Site Stability Analysis

14

15 The site stability analysis evaluates the long-term stability of the disposal site and can be used 

16 by a licensee to determine compliance with 10 CFR 61.44 (see Section 5.0). The specific 

17 requirements for the analyses are set forth in 10 CFR 61.13(d):

18 Analyses of the long-term stability of the disposal site and the need for ongoing active 

19 maintenance after closure must be based upon analyses of active natural processes, 

20 such as erosion, mass wasting, slope failure, settlement of wastes and backfill, 

21 infiltration through covers over disposal areas and adjacent soils, and surface drainage 

22 of the disposal site. The analyses must provide reasonable assurance that long-term 

23 stability of the disposal site can be ensured and that there will not be a need for ongoing 

24 active maintenance of the disposal site following closure.

25 1.1.4.4 Protective Assurance Period Analyses

26

27 The primary purpose of the protective assurance period analyses is to provide information that 

28 demonstrates that releases of radioactivity from a LLW disposal facility are minimized during the 

29 protective assurance period, which is the period from the end of the compliance period through 

30 10,000 years following closure of the site. Minimization is the reduction of doses to as low as 

31 reasonably practical with technical and economic factors taken into consideration. The 

32 performance objectives in 10 CFR 61.41(b) and 10 CFR 61.42(b) require that the annual dose 

33 shall be below 5 mSv (500 mrem) or a level that is supported as reasonably achievable based 

34 on technological and economic considerations for (1) concentrations of radioactive material that 

35 may be released to the general environment, and for (2) exposures to any inadvertent intruder, 

36 respectively. Section 6.0 provides guidance on developing the technical analyses for the 

37 protective assurance period. 

38 1.1.4.5 Performance Period Analyses

39

40 The primary purpose of the performance period analyses is to provide information to decision41 makers with respect to the potential performance of the disposal site for the disposal of 

42 long-lived waste. Licensees should conduct performance period analyses for the period after 

43 the 10,000 year (yr) protective assurance period. Performance period analyses are not required 

44 if the disposal facility is not accepting sufficient quantities of long-lived waste. Section 7.0 
1-7

1 provides guidance on how to determine if performance period analyses are necessary. The 

2 performance period analyses are only required for disposal sites with waste that contains 

3 radionuclides with average concentrations exceeding the values specified in Table A of 

4 10 CFR 61.13(e), or if necessitated by site-specific conditions. The performance period 

5 analyses required at 10 CFR 61.13(e) shall:

6 (1) Assess how the disposal site limits the potential long-term radiological impacts, 

7 consistent with available data and current scientific understanding.

8 (2) Identify and describe the features of the design and site characteristics that will 

9 demonstrate that the performance objectives set forth in 10 CFR 61.41(c) and 

10 10 CFR 61.42(c) will be met.

11 The analyses must indicate the long-term performance of the land disposal facility and effort 

12 shall be made to minimize releases of radioactivity from a disposal facility to the general 

13 environment to the extent reasonably achievable at any time during the performance period. 

14 The analyses for the performance period may be similar to the analyses performed for the 

15 compliance period and/or the protective assurance period, but they are not required to be the 

16 same.

17 Figure 1-1 provides the relationship of the major components of the technical analyses with 

18 respect to analysis timeframes and each other. For example, the Assessment Context and 

19 Scenario Development component applies to the performance assessment, intruder 

20 assessment, and site stability analyses (see Sections 2.3 and 2.5).

21

22 Both the performance assessment and intruder assessment are evaluated over all three 

23 timeframes, however, the stability analyses are only applied to the compliance period and 

24 protective assurance period. Defense-in-depth is applicable over all three time periods and all 

25 three analysis types. All of the information contributes to the demonstration that the Subpart C 

26 performance objectives are met.

27 1.1.5 Waste Acceptance Requirements

28

29 Requirements for waste acceptance are specified in 10 CFR 61.58, “Waste Acceptance”. The 

30 regulations require licensees to identify (1) criteria for the acceptance of waste for disposal (i.e., 

31 waste acceptance criteria), (2) acceptable methods for characterizing the waste, and (3) a 

32 program to certify that waste meets the acceptance criteria prior to transfer to the disposal 

33 facility. The waste acceptance requirements are intended to provide reasonable assurance that 

34 the performance objectives of Subpart C will be met.

35

36 The waste acceptance criteria, as specified in 10 CFR 61.58(a) must identify allowable activities 

37 and concentrations of specific radionuclides, acceptable wasteform characteristics and 

38 container specifications, and restrictions or prohibitions on waste, materials, or containers that 

39 might affect meeting the performance objectives. The criteria for allowable activities and 

40 concentrations of specific radionuclides must be developed from either the technical analyses 

41 required by 10 CFR 61.13 for any land disposal facility or the waste classification requirements 

42 set forth in 10 CFR 61.55 for a near-surface disposal facility.
1-8

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40 Figure 1-1 Technical Analyses Relationship to Analyses Timeframes and Performance 

41 Objectives 

Assessment Context and

Scenario Development

Defense-in-Depth

Performance 

Assessment

Intruder

Assessment

Stability 

Analysis

Demonstrate Subpart C 

Performance Objectives are Met

Compliance 

Period

Protective 

Assurance 

Period

Performance

Period
1-9

1 Licensees must also identify acceptable methods for characterizing the waste for acceptance. 

2 The regulations in 10 CFR 61.58(b) specify the minimum information that the acceptable 

3 methods must include to adequately characterize waste for acceptance. The intent of these 

4 requirements is to ensure that knowledge of the waste’s characteristics is commensurate with 

5 the assumptions and approaches employed in the technical analyses used to develop the waste 

6 acceptance criteria and is, thus, sufficient to demonstrate that the waste acceptance criteria are 

7 met.

8

9 10 CFR 61.58(c) requires a program to certify that waste meets the acceptance criteria prior to 

10 shipment to the disposal facility. Certification of waste provides assurance that a disposal 

11 facility operates within the limits established to demonstrate compliance with the performance 

12 objectives of Subpart C. The certification program must:

13 (1) Designate authority to certify and receive waste for disposal.

14 (2) Provide procedures for certifying that waste meets the waste acceptance criteria.

15 (3) Specify documentation required for waste characterization, shipment, and certification.

16 (4) Identify records, reports, tests, and inspections that are necessary.

17 (5) Provide approaches for managing waste to maintain its certification status.

18 Finally, 10 CFR 61.58(f) requires that each licensee shall annually review the content and 

19 implementation of the waste acceptance criteria, waste characterization methods, and 

20 certification program. 

21 1.1.6 Agreement State Interactions

22

23 Agreement State regulators may request technical assistance from the NRC staff to conduct 

24 reviews of the technical analyses listed above and also to assist in interpreting the guidance in 

25 this document. NRC provides three types of technical assistance to Agreement States: routine, 

26 special, and programmatic (NRC, 2013). 

27

28 Routine technical assistance is provided as part of NRC’s daily interaction with Agreement 

29 States. This assistance may include, but is not limited to, the discussion of technical issues 

30 regarding licensing, compliance, and security. Examples of routine technical assistance include 

31 requests for and the sharing of information on licensing, inspection, security, and enforcement 

32 activities. The NRC staff may perform confirmatory reviews of portions of completed Agreement 

33 State technical assessments, on a case-by-case basis, when resources are available.

34

35 Special technical assistance may require specific assignment of the NRC staff or consultants for 

36 a specified period and for a specific job. An Agreement State may not have the special 

37 technical expertise that is required to address a particular need, or an Agreement State may 

38 experience a temporary constraint on resources. Consequently, an Agreement State may 

39 request direct special technical assistance from NRC that would involve NRC licensing and 

40 inspection staff conducting independent licensing. Direct technical assistance to an Agreement 

41 State in these circumstances will be conducted on a case-by-case basis when NRC believes 

42 that such assistance is necessary. The provision of such assistance will be based on the 

43 availability of staff resources and any assistance will be cost-reimbursable. 

44
1-10

1 Programmatic technical assistance is addressed as part of the Integrated Materials 

2 Performance Evaluation Program (IMPEP) process. See Management Directive 5.7 “Technical 

3 Assistance to Agreement States” for additional details on requesting technical assistance from 

4 the NRC staff (NRC, 2013).

5 1.2 Purpose of This Guidance Document

6

7 This guidance document is intended to support the implementation of the requirements for 

8 technical analyses and waste acceptance to demonstrate compliance with the 10 CFR Part 61 

9 performance objectives. 

10 1.2.1 Relationship to Other NRC Guidance

11

12 The NRC staff has issued several guidance documents to assist in the implementation of the 

13 requirements of 10 CFR Part 61. Additionally, in certain cases, guidance for other NRC 

14 regulatory programs (e.g., NUREG-1757) may be adapted to 10 CFR Part 61. Early guidance 

15 for the implementation of 10 CFR Part 61 (e.g., NUREG-1199 and NUREG-1200) was generally 

16 prescriptive. More recently, review of site-specific performance assessments has become more 

17 performance-based, driven in part by the Commission’s 1995 probabilistic risk assessment 

18 (PRA) policy statement (NRC, 1995a). More recent guidance (e.g., NUREG-1573 and 

19 NUREG-1854) has been developed that might be helpful for completing or reviewing a 

20 performance assessment. The following documents, among others provided in Section 11.0 of 

21 this document, are available to 10 CFR Part 61 licensees and reviewers for guidance:

22

23 (1) NUREG-1200, Revision 3, “Standard Review Plan for the Review of a License 

24 Application for a Low-Level Radioactive Waste Disposal Facility,” issued April 1994, 

25 provides regulators with procedures and guidance for reviewing license applications for 

26 new disposal facilities (NRC, 1994). NUREG-1200 identifies areas of review and review 

27 procedures for evaluating technical analyses, including what is referred to today as a 

28 performance assessment (NRC, 1994). NUREG-1200 was developed using a 

29 regulatory approach that was generally prescriptive. 

30

31 (2) NUREG-1573, “A Performance Assessment Methodology for Low-Level Radioactive 

32 Waste Disposal Facilities” was issued in October 2000 to assist licensees in performing 

33 performance assessments to comply with 10 CFR 61.41 (NRC, 2000a) and is 

34 referenced throughout this document. NUREG-1573 developed a bibliography of 

35 technical references applicable to LLW disposal (as of 2000) in its Appendices B and C. 

36 NUREG-1573 provides guidance on an acceptable approach for systematically 

37 integrating site characterization, facility design, and performance modeling into a single 

38 performance assessment process for purposes of demonstrating compliance with 

39 10 CFR 61.41. The guidance in NUREG-1573 might help ensure the consistency of 

40 different reviews.

41

42 In some cases, NUREG-1573 provides information on topics (e.g., analyses timeframes, 

43 climate) that is also covered in this guidance document. Information provided in this 

44 guidance document supersedes guidance in older documents that is inconsistent with 

45 the guidance in this document. The superseded guidance includes: consideration of site 

46 characteristics, timeframe for the analyses, current land use, analysis of engineered 

47 barrier performance, climate change, and consideration of disruptive events. 10 CFR 
1-11

1 Part 61 has changed since NUREG-1573 was developed in 2000 (see Appendix A). In 

2 addition, different types of wastes (e.g., larger quantities of long-lived waste) are being 

3 considered for disposal. Therefore, this guidance also supplements the guidance in 

4 NUREG-1573.

5

6 (3) NUREG-1854, “NRC Staff Guidance for Activities Related to U.S. Department of Energy 

7 Waste Determinations,” issued August 2007, provides guidance specific to the NRC 

8 staff’s review of technical analyses for DOE waste determinations (NRC, 2007a). DOE 

9 uses technical analyses that are documented in a “waste determination” to evaluate 

10 whether the waste at four sites in the States of South Carolina, Idaho, Washington, and 

11 New York, is high-level waste (HLW) or waste incidental to reprocessing (incidental 

12 waste). A waste determination is DOE’s analysis of whether the waste will meet the 

13 applicable criteria to be classified as incidental waste. The four DOE sites are operating 

14 under different requirements for waste evaluation and management; however, they all 

15 include criteria that specifies the waste will be disposed of in compliance with, or with 

16 comparable safety requirements to, the performance objectives in 10 CFR Part 61, 

17 Subpart C. NUREG-1854 contains information that can also be used for conducting the 

18 technical analyses referred to in this document. 

19

20 (4) “Final Branch Technical Position on Concentration Averaging and Encapsulation, 

21 Revision in Part to Waste Classification Technical Position,” dated January 17, 1995 

22 (NRC, 1995b), defines a subset of concentration averaging and encapsulation practices 

23 that the NRC staff finds acceptable in determining the concentrations of the 

24 radionuclides tabulated in 10 CFR 61.55, “Waste Classification.” Although this branch

25 technical position (BTP) is more intended to aid LLW generators in determining 

26 wasteform concentrations, it may prove useful to LLW disposal facility operators in 

27 determining site-specific waste acceptance criteria (see Section 9.0). The NRC staff 

28 updated this BTP and issued “Draft Branch Technical Position on Concentration 

29 Averaging and Encapsulation, Revision 1” in May 2012 that revised the NRC staff 

30 positions in the 1995 version (NRC, 2012a). The NRC staff expects to complete the final 

31 version of this BTP by the end of 2014.

32

33 (5) NUREG-1757, “Consolidated NMSS Decommissioning Guidance: Characterization, 

34 Survey, and Determination of Radiological Criteria,” dated September 30, 2006, provides 

35 guidance on the evaluation of engineered barriers used in site decommissioning (NRC, 

36 2006). If similar engineered barriers are used in land disposal of LLW, then this 

37 guidance might be useful to a licensee. 

38

39 The purpose of this guidance document is to complement the aforementioned documents and 

40 provide guidance in areas not previously covered. The NRC staff has attempted to provide 

41 references to other guidance documents that may be useful for specific topics. Section 11.0 of 

42 this document contains a road map directing the reader to individual sections of the documents 

43 above, as well as to other guidance documents that might be useful to 10 CFR Part 61 

44 licensees and reviewers.

45

46
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1 1.2.2 What is New in this Document?

2

3 A primary purpose of this document is to update guidance on conducting technical analyses to 

4 demonstrate compliance with the performance objectives of Subpart C. Several new areas

5 discussed in this document are: 

6 (1) Acceptable approaches to identify and screen FEPs to develop scenarios 

7 (2) Detailed guidance on the analysis for the protection of the inadvertent intruder 

8 (3) Detailed guidance and examples for conducting site stability analyses that evaluate the 

9 long-term stability of the disposal site and determines compliance with 10 CFR 61.44 

10 (4) Development of waste acceptance criteria, waste characterization methods, and waste 

11 certification 

12 (5) Risk-informing the analyses for the three-tiered analysis timeframe: the compliance 

13 period (1,000 years), the protective assurance period (from 1,000 to 10,000 years), and 

14 the performance period analyses (for long-lived waste beyond 10,000 years), using a 

15 graded level of effort 

16 (6) Defense-in-depth analyses that demonstrate the proposed disposal facility includes 

17 defense-in-depth protections

18 (7) Discussion of a safety case as a collection of information that demonstrates the 

19 assessment of the safety of a waste disposal facility

20 (8) Performance confirmation and the conduct of periodic reviews for technical analyses and 

21 waste acceptance criteria

22 A disposal facility licensed under 10 CFR Part 61 must meet the performance objectives for all 

23 waste disposed of at the site. The technical analyses described in this document should be 

24 performed for the total inventory of waste at the site. As such, this document does not provide 

25 guidance specific to any particular waste. Rather, it provides guidance for the total waste 

26 inventory at each site. However, the NRC staff has attempted to provide examples in this 

27 guidance of the use of graded levels of effort required for the analyses of long-lived waste 

28 versus the analyses of conventional short-lived waste (e.g., long-lived waste often requires 

29 more complex technical analyses). 

30 1.3 Document Organization 

31

32 This document provides guidance on conducting technical analyses to demonstrate compliance 

33 with the performance objectives of 10 CFR Part 61. This guidance document discusses the 

34 parameters and assumptions that can be used in conducting these technical analyses in a 

35 broad sense, rather than in a prescriptive manner, to allow flexibility to licensees (see Example 

36 1.1). The NRC staff considers this flexibility necessary because the site-specific nature of LLW

37 disposal can make specification of particular models or parameter values impractical. 

38 Table 1-1 presents the technical analyses, the relevant requirements in 10 CFR Part 61, and the 

39 individual sections of this document where licensees and regulators can find guidance related to 

40 the analyses. 

41
1-13

1 Table 1-1 Crosswalk between Technical Requirements and Document Sections

Section Number

Technical Requirement Rule

Section 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Performance Assessment 61.13(a)

61.41(a) X X X

Inadvertent Intrusion 

Assessment

61.13(b)

61.42(a) X X X

Site Stability Analysis 61.13(d)

61.44 X X X

Protective Assurance Period

Analysis

61.41(b)

61.42(b) X X X

Performance Period Analysis

61.13(e)

61.41(c)

61.42(c)

X X X

Defense-in-Depth Analysis 61.13(f) X X

Waste Acceptance 61.58 X X

2

3 Section 2.0 summarizes general considerations for conducting technical analyses, such as 

4 model abstraction, model uncertainty, and model support. Sections 2.0 and 3.0 provide

Example 1.1: How much detail is provided on parameters and assumptions that can be used 

in conducting technical analyses?

The NRC staff identifies methodologies for performance assessment and intruder 

assessment that licensees can use to demonstrate compliance with the 10 CFR Part 61 

performance objectives. The guidance discusses important FEPs that should be evaluated 

as part of the scenario analysis process and considerations for abstracting the FEPs into 

computational models in the performance and intruder assessments. The NRC staff presents 

key FEPs in a broad sense to indicate the types of FEPs that should be considered based on 

experience with radioactive waste disposal facilities. For example, the guidance indicates 

that sorption should be considered in evaluating the migration of radionuclides in the 

environment rather than specifying a specific sorption model or parameter values that should 

be used. Licensees are encouraged to consult this guidance document to identify potential 

FEPs for a specific site; however, licensees should develop models and parameters to 

adequately represent their disposal site’s performance consistent with the expected temporal 

behavior of the disposal system. This development might allow for consideration of a subset 

of the FEPs identified in this guidance document or might require the consideration of 

additional FEPs beyond those identified in this guidance.
1-14

1 guidance for specific topics related to performance assessment modeling concerned with 

2 radiological protection of the general public, as required in 10 CFR 61.41. Section 4.0 provides 

3 guidance specific to the analysis required for radiological protection of the inadvertent intruder, 

4 as required in 10 CFR 61.42. Section 5.0 provides guidance on stability of the disposal site 

5 after closure, as required in 10 CFR 61.44. Section 6.0 discusses the protective assurance 

6 period analysis and Section 7.0 provides guidance on the performance period analyses for 

7 10 CFR 61.13(e) associated with the disposal of long-lived waste. Section 8.0 discusses the 

8 defense-in-depth analysis required under 10 CFR 61.13(f). Section 9.0 discusses the process 

9 for waste acceptance, such as developing waste acceptance criteria and performing waste 

10 characterization and waste certification. Section 10.0 discusses conducting performance 

11 confirmation to evaluate and verify the accuracy of information used to demonstrate compliance 

12 prior to site closure. Section 11.0 provides references for the use of other NRC guidance 

13 documents. Section 12.0 provides the references cited in this document and Section 13.0 

14 contains a glossary of technical terms used in this guidance.

15 1.4 Risk-Informed Approach

16

17 This document should be implemented in a risk-informed manner. In NUREG-1614 (NRC, 

18 2012b), risk-informed is defined as “an approach to decision-making in which risk insights are 

19 considered along with other factors such as engineering judgment, safety limits, and redundant 

20 and/or diverse safety systems. Such an approach is used to establish requirements that better 

21 focus licensee and regulatory attention on design and operational issues commensurate with 

22 their importance to public health and safety.” (e.g., the risk to human health associated with 

23 exposure to ionizing radiation). A reviewer should place relatively more emphasis on technical 

information associated with systems that prevent a risk1 24 from being realized or that significantly 

25 reduce the magnitude of a risk. Licensees should perform sufficient evaluation and develop 

26 adequate bases to identify and emphasize the key areas of the performance assessment, 

27 intruder assessment, and site stability evaluation that are expected to have the biggest impact 

28 on public health and safety. The type, quantity, and concentration of waste that a facility 

29 receives will drive the risk which in turn will drive the level of detail of the assessments.

30

31 Various sections of this document provide risk-informed guidance. Although conducting 

32 analyses for the compliance period and the protective assurance period requires projecting 

33 doses for up to 10,000 years, the scope, technical bases, and level of detail should be tailored 

34 to the waste characteristics. For example, the level of effort required for model support for the 

35 performance assessment to demonstrate compliance with 10 CFR 61.41 will most likely be 

36 higher than the level of effort that should be expended for the performance period analyses. 

37 More robust model support may be needed for the performance period analyses if large 

38 quantities and/or high concentrations of long-lived waste are present at the disposal site. 

39

40 The approach to the intruder dose assessment described in Section 4.0 provides methods for 

41 risk-informing scenario selection by considering site environmental conditions and land use 

42 information. The analysis recommended for the site stability evaluation in Section 5.0 will 

43 ensure that the complexity of the evaluation is commensurate with the hazard of the material 

44 that will be disposed. 

45

 1 Risk is a product of likelihood and consequence. Risk-informed review generally focuses on the 

probability-weighted consequences (i.e., the likelihood, the consequences, or both).
2-1

1 2.0 GENERAL TECHNICAL ANALYSES CONSIDERATIONS

2

3 The term “technical analyses” refers to the performance assessment, intruder assessment, site 

4 stability evaluation, and performance period analyses needed to demonstrate compliance with 

5 the 10 CFR Part 61 Subpart C performance objectives. Defense-in-depth analyses are 

6 discussed in Section 8.0. This section is intended to provide general guidance for preparing 

7 these four types of analyses, with emphasis on the performance assessment. The purpose of 

8 this document is to supplement existing NRC 10 CFR Part 61 guidance, such as the guidance 

9 found in NUREG-1199 (NRC, 1991a), and to present new guidance in areas not previously 

10 covered. 

11

12 This section describes the information that a licensee should provide and a reviewer should 

13 evaluate with respect to the basic elements of technical analyses that typically comprise a 

14 performance assessment. Many of the basic elements of the technical analyses that apply to 

15 the performance assessment also apply to the intruder assessment, site stability assessment, 

16 and performance period analyses. For example, data adequacy, uncertainty, and model 

17 support are important with respect to the all of the technical analyses. 

18

19 For efficiency purposes, the general information contained in Section 2.2 that is relevant to the 

20 other technical analyses that are discussed in later sections of this document (e.g., Section 4.0, 

21 5.0, 6.0) is not replicated in those sections. Section 2.2 was written for preparation and review 

22 of performance assessments. Intruder assessments are generally more constrained 

23 assessments using stylized scenarios that involve calculation but do not typically involve 

24 development of integrated conceptual models (see Section 4.0). The guidance in Section 2.2 

25 could be applicable to the site stability assessment if a model-based approach is used, but 

26 would be of limited applicability if a design-based approach is used (see Section 5.0). The 

27 performance period analyses discussed in Section 7.0 may be not required for disposal of 

28 certain short-lived wastes, therefore, the guidance in Section 2.2 has low applicability if only a 

29 screening analysis is performed, but has applicability if a quantitative probabilistic assessment is 

30 developed. 

31

32 Other portions of this section of the guidance may also be applicable to the intruder 

33 assessment, stability assessment, or performance period analyses. To the extent possible, the 

34 NRC staff has noted in individual portions of the text where the material presented for 

35 preparation and review of a performance assessment is also applicable to the other technical 

36 analyses. 

37

38 A technical analysis such as a performance assessment can be a collection of other models 

39 (e.g., submodels or process models) of varying levels of complexity, or it can be an integrated 

40 model. A submodel is a representation of a specific process as part of the technical analysis, 

41 such as a model estimating the rate of infiltration of water to the waste in a performance 

42 assessment. Technical elements that form the basic components of performance assessment 

43 modeling include system description, data adequacy, future uncertainty, model uncertainty, 

44 parameter uncertainty, model support, and integration. These technical elements, though 

45 integral to performance assessment, may also be applicable to the other technical analyses 

46 required by 10 CFR Part 61.
2-2

1 2.1 Assessment Process

2

3 Figure 2-1 provides the steps of the performance assessment process that may be used by 

4 licensees. Development of the assessment context is the first step in the performance 

5 assessment methodology followed by the description of the system (see Figure 2-1). After 

6 development of the system description, a licensee would complete scenario development. 

7 Scenario development includes the identification and categorization of FEPs, the screening of 

8 FEPs, and the representation of the screened FEPs in scenarios. These steps will vary slightly 

9 if the licensee uses a top-down approach to scenario development compared to a bottom-up 

10 approach, as described in Section 2.5. Based on the scenarios that result from scenario 

11 development, a licensee can develop conceptual models that are implemented as numerical 

12 models. As discussed in the following sections, licensees should account for uncertainty 

13 throughout the process. Future uncertainty is accounted for by developing and analyzing 

14 scenarios, or alternative future system states, and model uncertainty is accounted for by 

15 developing and analyzing conceptual models.

16 2.1.1 Terms and Definitions

17

18 Features, events, processes, scenarios, and other relevant terms used in this section are 

19 defined below. Section 13.0 presents a glossary of terms for the whole document. 

20

21 Feature is an object, structure, or characteristic that has a potential to affect the performance of 

22 the disposal system. Examples include rocks within an erosion layer of an engineered cover or

23 a drainage layer of an engineered cover. 

24

25 Event is a qualitative or quantitative phenomenon or change that has the potential to affect the 

26 performance of the disposal system and that occurs during an interval that is short compared to 

27 the analyses timeframe. Examples of events that cause relative rapid change are earthquakes, 

28 floods, storms, well drilling, and excavation. 

29

30 Process is a qualitative or quantitative phenomenon or change that has the potential to affect 

31 the performance of the disposal system and that occurs during all or a significant part of the 

32 analyses timeframe. Examples of processes that cause relative gradual change are 

33 radionuclide transport, differential settlement, leaching, and erosion. 

34

35 FEP categorization is the process of organizing individual FEPs into categories of similar 

36 properties to facilitate FEP screening. For example, FEPs related to natural, human, or waste 

37 phenomena may be grouped into separate categories.

38

39 FEP screening is the process of using regulatory, probability, and consequence criteria to 

40 eliminate FEPs from further consideration that will not significantly impact the performance of 

41 the disposal system or are otherwise excluded by regulation.

42

43 Scenario is a subset of important FEPs that are used to identify a probable future evolution of 

44 the disposal site.

45

46 Central scenario is the scenario that the licensee can best support as to the probable future 

47 dynamic evolution of the disposal site. As a result of the site selection process for LLW 

48 disposal, the central scenario generally will not include disruptive events.
2-3

Assessment Context

Additional Steps of the 

PA Process 

(steps 5 – 12 of Figure 3-1) 

including 

Iterative Steps

System Description

Scenario Development

Conceptual Model Development

Numerical Model Development and 

Analysis

1

2

3 Figure 2-1 Initial Steps of the Performance Assessment Methodology 

4

5 .
2-4

1

2 Alternative scenario is a less likely but still plausible future evolution of the disposal site. 

3 Alternative scenarios may include disruptive events if those FEPs are relevant at a particular 

4 site.

5

6 Conceptual model is a well-defined, connected sequence of phenomena describing the 

7 behavior of the system of concern.

8

9 Alternative conceptual model is an additional and different model on how the system might work 

10 that is consistent with available supporting information. For example, a scenario may have a 

11 matrix flow conceptual model and an alternative fracture flow conceptual model; the model 

12 outputs from each may yield significantly different results.

13

14 Receptor scenario is a type of scenario that describes the FEPs associated with people 

15 becoming exposed to radiation.

16

17 Safety function is defined qualitatively as a function through which a component of the disposal 

18 system contributes to safety and achieves its safety objective throughout the analyses 

19 timeframe.

20

21 Model abstraction is the process of abstracting a conceptual model representing a dynamic site 

22 in the physical world into a mathematical model governed by equations that is implemented 

23 within a numerical model.

24

25 Model simplification is the process of simplifying a complex numerical model into a reduced 

26 numerical model while still maintaining the validity of the simulation results.

27

28 Code is a set of software commands used to solve mathematical equations representing 

29 phenomena of the conceptual model. 

30 2.1.2 Level of Effort

31

32 LLW commonly contains radionuclides that are both short- and long-lived. From a specific 

33 activity standpoint, the short-lived isotopes comprise the dominant fraction of the total activity. 

34 However, when a licensee demonstrates compliance with the 10 CFR 61.41(a) performance 

35 objective, usually the radiological risk is dominated by the long-lived isotopes that remain in the 

36 inventory after the short-lived activity has decayed. LLW may contain large quantities of 

37 depleted uranium (DU) or other long-lived waste. LLW may not decrease in risk after a few 

38 hundred years, although it will decrease in hazard. LLW that contains large quantities of DU 

39 may pose a long-term risk to the public due to the in-growth of progeny. 

40

41 A licensee should use a level of effort for technical analyses commensurate with the risk to the 

42 public from disposal of the waste. Specifically, the level of effort (i.e., the level of detail, 

43 comprehensiveness, completeness, and degree of iteration), is commensurate to the longevity, 

44 concentrations of radionuclides, and quantity of the waste. For example, the level of effort for 

45 the development of the technical analyses for the disposal of LLW will generally be less than 

46 that for the disposal of HLW or spent nuclear fuel. Complex modeling is generally not

47 performed for disposal sites that contain only short-lived radionuclides and have low projected

48 overall risk.
2-5

1 As an example, the completeness and level of detail of a FEPs analysis associated with 

2 disposal of LLW containing predominately short-lived radionuclides will be considerably less 

3 than a proposed LLW disposal site that will contain a large quantity of DU. Historically, as

4 documented in earlier LLW disposals, features and processes not taken into account during the 

5 design and planning phase of a LLW disposal site can significantly reduce the isolation of the 

6 waste in question. Inadequate conceptual models and the absence of alternative conceptual 

7 models have created situations in the past in which the containment of the waste was 

8 compromised and remediation and closure of the disposal facility was necessary (NRC, 2007b). 

9 The NRC staff recommends FEPs analysis and scenario and conceptual model development for 

10 disposal sites dominated by short-lived radionuclides; however, the identification and 

11 categorization of FEPs completed by a licensee for the disposal of short-lived waste is expected 

12 to require less effort since the time period of the analysis is shorter. For longer analyses, there 

13 will be greater uncertainty. Considerably more FEPs may need to be identified, screened, and 

14 integrated into one or more conceptual models. Section 2.5 discusses the importance of 

15 identifying FEPs relevant to the waste characteristics and disposal site, as well as methods to 

16 improve the completeness of FEPs lists and scenarios. 

17 2.2 General Review Considerations

18 2.2.1 Data Adequacy

19

20 It is important for a licensee to develop and use adequate data. The data should be 

21 appropriately representative and complete, of adequate quality, and unbiased. The objective of 

22 the data adequacy review is twofold. First, the reviewer should determine whether sufficient 

23 data have been provided by a licensee to support the performance assessment models. 

24 Second, the reviewer should determine whether those data have been used appropriately. 

25

26 It is generally beneficial to have more data rather than less; however, some amount of 

27 incompleteness in the data may be overcome by appropriately accounting for parameter 

28 uncertainty, as described in Section 2.2.2.1.2. The types of data to be considered by licensees 

29 may include, but are not limited to site-specific data (e.g., laboratory measurements and field30 scale measurements or experiments), data from analogous sites, data from generic sources,

31 output from detailed process-level models, and expert judgment. More objective sources of 

32 data are preferred over more subjective sources of data. A licensee should review published 

33 literature even if they have made site-specific measurements. Review of published literature 

34 may help the licensee identify measurement errors or determine if the data is not representative. 

35 Expert judgment should be used by licensees as a last resort, assigned appropriate uncertainty, 

36 and completed with a formal expert elicitation process when the data are important to the 

37 performance assessment (NRC, 1996). Because performance assessments are completed with 

38 an iterative process, early data for scoping calculations may be from more subjective sources 

39 (e.g., informal expert judgment). If a licensee or regulator determines during independent 

40 analysis that certain data are risks significant; the data should be developed by more objective 

41 sources, when possible. 

42

43 Because performance assessment models can simulate processes and events over a wide 

44 range of temporal and spatial scales, licensees should ensure that the data are representative 

45 of the temporal and spatial scales evaluated in the model. Upscaling of data may be necessary 

46 to achieve representativeness. Upscaling is the modification of data for use at a different scale, 

47 most commonly to take data from fine-scale observations for use at a much coarser scale. If a 
2-6

1 licensee uses upscaling to modify data for use at different scales, they should ensure that the 

2 key features or structure of the data important to the performance assessment are preserved. 

3 For example, precipitation may occur as short duration storm events (minutes to hours),

4 whereas the timeframe for a performance assessment is thousands of years. If a performance 

5 impact, such as erosion of a soil cover, is a non-linear function of runoff that itself is a function of 

6 precipitation intensity and duration, upscaling of the temporal precipitation data in order to 

7 develop the performance assessment calculations may result in the loss of important detail. 

8 Licensees should use caution when estimating whether measurements are outliers, particularly 

9 when the number of measurements is sparse. Licensees should not use subjective 

10 assessments of potential outliers. 

11 2.2.2 Uncertainty

12

13 Sources of uncertainty inherent to waste disposal in the near surface include, but are not limited 

14 to, incomplete knowledge of the natural system, its evolution, and interactions. Regulators 

15 expect that uncertainties that cannot be shown by a licensee to have a minimal effect on safety

16 are avoided or reduced as far as possible (e.g., by means of site selection, site characterization, 

17 disposal design, and, if necessary, research). To some extent, uncertainties in the assessment 

18 results can be counterbalanced by using multiple lines of evidence, or model support. 

19 Licensees should characterize, eliminate (with justification), or bound uncertainties in their

20 technical analyses, as well as document the impact of the uncertainties on performance. 

21

22 The uncertainties in performance assessment have been classified as scenario uncertainty, 

23 model uncertainty (which spans conceptual model uncertainty and mathematical model 

24 uncertainty), and parameter uncertainty (i.e., uncertainty in values used in the numerical model) 

25 (NRC, 1990a). Scenario uncertainty, defined as the consideration of uncertainty in the future 

26 evolution of the site, may result in several different conceptual models for the system, 

27 distinguished by the effects of phenomena on the system. Model uncertainty may be present in 

28 each of the different submodels that comprise the overall system model. Licensees can 

29 evaluate these inherent uncertainties using uncertainty analysis, which is a way of formally 

30 assessing, reducing or managing, and documenting the inherent uncertainty of a system 

31 (Finkel, 1990). For example, an uncertainty analysis could provide information about where a 

32 licensee should focus model support activities, which in turn could reduce uncertainty. 

33 Parameter uncertainty is uncertainty in the parameters used in the technical analyses.

34 2.2.2.1 General Structure of Uncertainty

35

36 Some radioactive waste will present a radiological risk to humans and the environment for a 

37 long time. To help assess this risk, licensees should use predictive models that can describe 

38 the future behavior or performance of disposal systems. Building a computational model that

39 combines and represents all important FEPs at an appropriate level of detail is a complex 

40 process. Uncertainties are greater for long-lived waste (lasting thousands of years or longer) 

41 than they are for short-lived wastes (lasting tens or hundreds of years). Uncertainties must be 

42 handled by licensees within the performance assessment process. Scenario development is a

43 commonly used technique to account for uncertainties about the future. 

44

45 Figure 2-2 shows a general structure of uncertainty analysis that involves separate treatments 

46 of scenario uncertainty (future uncertainty), model uncertainty, and parameter uncertainty (NRC, 

47 1993a). Within each scenario of the future, it is possible to postulate alternative conceptual 
2-7

1 models of the behavior of the disposal system, each of which leads to a particular mathematical 

2 model to describe that behavior. For each conceptual model, it is possible to postulate 

3 alternative sets of input parameter values. In the context of performance assessment for LLW

4 disposal, the primary purpose of an uncertainty analysis is to support a decision about 

5 compliance of the disposal system with the regulatory requirements. 

6 2.2.2.1.1 Scenario Uncertainty

7

8 Uncertainty about the future of the site is the result of inherent lack of knowledge about how the 

9 site will evolve over time. The future climatic, geologic, and population conditions that will 

10 prevail at a site are not known, but the performance assessment process requires that a

11 licensee consider possible future conditions. Scenario uncertainties are illustrated by 

12 considering the events or processes that may significantly influence projected doses to the 

13 receptor. For example, climatic variation may significantly change groundwater flow pathways 

14 over time, necessitating changes to the groundwater flow model or the introduction of new 

15 parameters. If analyses cannot exclude the possibility of either scenario 1) where the 

16 groundwater flow pathways remain unchanged (no major climate variation), or 2) where the 

17 groundwater flow pathways change (climate variation), then the site has two potential routes of 

18 evolutionary development (i.e., two future scenarios that need to be analyzed in this example). 

19 The longer the analysis timeframe, the greater the likelihood of significant changes to the flow 

20 path and properties. Scenario uncertainty is linked to, but not identical with, future parameter 

21 uncertainty, which is discussed in the next section.

22

23

24 Figure 2-2 Overall Approach to Uncertainty Analysis for LLW Performance 

25 Assessment (NRC, 1993a)
2-8

1 2.2.2.1.2 Model Uncertainty

2

3 Model uncertainty encompasses the uncertainty in the conceptualization of the system, the 

4 uncertainty in its mathematical representation, and the uncertainty in the solution of the 

5 mathematical representation (Bonano and Cranwell, 1988). All models will encompass some 

6 simplification of reality (model abstraction) and licensees will have to make a variety of choices 

7 about the level of detail to provide. 

8

9 Conceptual model uncertainty is generally the dominant type of uncertainty in a performance 

10 assessment due to limitations in the available supporting data. The conceptual model should be 

11 as complete and as appropriate to the scenario as possible. The conceptual model should be 

12 based on the information and data available and on previous experience with similar types of 

13 problems. If not all significant processes, features, or significant barriers have been considered,

14 the conceptual model will have deficiencies and the ability of the licensee to bound or otherwise 

15 account for model uncertainty will be reduced (BIOMOVS II, 1996). 

16

17 Licensees should adequately describe and document conceptual model uncertainties. The

18 performance assessment documentation should provide the assumptions, limitations, and 

19 uncertainties of the models. Multiple representations of a model or different models may be 

20 consistent with the available data. In general, licensees should select the models that best 

21 represent available data. However, when data are sparse, multiple models may represent the 

22 available data. In this case, licensees should select the model that provides the most 

23 conservative result, or collect additional data to determine which alternative conceptual model 

24 provides the best representation. Licensees do not need to evaluate all models, but they should 

25 consider all models that are reasonably consistent with available information. Regulators 

26 should perform an independent evaluation of model uncertainty and consider whether more 

27 than one conceptual model should be evaluated, especially for complex sites.

28

29 Other types of model uncertainty include assumptions, decisions or judgments made during the 

30 development of a model, the mathematical form of the conceptual models, and the inexact 

31 implementation of mathematical models in numerical form in computer codes. Related 

32 computer model uncertainties can arise from errors in the computer code used to develop the 

33 model, input data errors, misapplication of the code (e.g., through application of the code to 

34 problems beyond the range for which the code was developed), and approximations in the 

35 solution of the mathematical model (e.g., due to inappropriate grid discretization of a domain or 

36 setting time steps that are too large). 

37

38 The objective of the review of model uncertainty is for the reviewer to determine if the licensee 

39 has considered and appropriately evaluated the impact of model uncertainty. Some 

40 uncertainties are inherent in the application of predictive models to (1) long periods of time for 

41 which direct validation is not possible, and (2) complex systems for which measurement and 

42 characterization may be limited. These uncertainties can be evaluated in the performance 

43 assessment by (1) considering reasonable ranges in conditions and processes to test the 

44 robustness of the model, (2) by using distributions of parameters to represent the likely ranges 

45 in conditions or processes, or (3) by bounding the effects of model uncertainty by using 

46 conservative assumptions. Ideally, a licensee can minimize the impact of model uncertainty by 

47 developing as much model support as practical (see Section 2.2.3).
2-9

1 2.2.2.1.3 Parameter Uncertainty

2

3 Parameter uncertainty can be reducible or irreducible. Technical analyses will need to account 

4 for parameter uncertainty. Although evaluation of parameter uncertainty does not specifically 

5 address scenario uncertainty, parameter values are the building blocks of any model and 

6 therefore, spatial or temporal variations in parameter values are often needed to reflect the 

7 evolution of a site, as well as to evaluate the impact of different representations of what might 

8 happen in the future. For example, the range of flow rates through a hydrogeological unit may 

9 increase over time within one plausible scenario. In an alternative, drier scenario, the same 

10 hydrogeological unit will start out with the same flow rate, but, in contrast with the previous 

11 scenario, may decrease over time. 

12

13 The objective of the review of parameter uncertainty is to determine if the assessment by the 

14 licensee includes parameter uncertainty, which techniques were used to account for parameter 

15 uncertainty, and whether those techniques adequately accounted for parameter uncertainty. 

16 Selection of conservative values is one method a licensee may use to account for parameter 

17 uncertainty. This method works well when the number of uncertain parameters is limited and 

18 conservative values can be clearly established. This method may not work well when there are 

19 many uncertain parameters, especially for complex models. This should not be interpreted by 

20 licensees or regulators to mean that conservative values should not be used for complex sites, 

21 but rather that licensees should demonstrate sufficient knowledge of their performance 

22 assessment so that conservative selections can be reliably made. Variations in the model 

23 responses, such as local minima and maxima data values, can make the determination of 

24 conservative parameter values very challenging. 

25

26 Another method to incorporate parameter uncertainty is the use of some form of probabilistic 

27 analysis. Parameter uncertainty can be propagated through the performance assessment by 

28 distributions of variables (e.g., hydraulic conductivity, porosity, distribution coefficient). When 

29 using probabilistic analysis, licensees should plot measured data on figures showing the 

30 probability distributions assigned to represent the measured data in the modeling to 

31 communicate (1) the amount of data available to construct the distribution, and (2) how the 

32 selected distribution reflects the data. Licensees should preserve the correlation of data for 

33 related parameters (e.g., precipitation and irrigation rate) in probabilistic analysis, as necessary. 

34 Performance assessments that rely on a large amount of generic data (e.g., non-site-specific) 

35 will have comparatively more uncertainty, as discussed in Section 2.2.3. In a deterministic 

36 analysis, licensees can examine the impact of parameter uncertainty with sensitivity analyses. 

37 They can then bound impacts by the selection of conservative values. Section 2.7.4 provides a 

38 discussion of some significant challenges with this approach, as well as the metrics to use for 

39 determining compliance (i.e., the peak of the mean).

40 2.2.2.1.4 Uncertainty Example—Transfer Factors

41

42 One acceptable quantitative approach for modeling the transport of radionuclides through the 

43 biosphere employs steady-state transfer factors (e.g., soil to plant, water to biota) and 

44 bioaccumulation factors. The literature contains many sources for these transfer factors 

45 (Staven et al., 2003; IAEA, 1994; NRC, 1992; Wang et al., 1993; Baes et al., 1984; NRC, 2007c; 

46 NRC, 2003a). Regulatory Guide 1.109, Revision 1, “Calculation of Annual Doses to Man from 

47 Routine Releases of Reactor Effluents for the Purpose of Evaluating Compliance with 

48 10 CFR Part 50, Appendix I,” issued October 1977, provides conservative values for a variety of 
2-10

1 these factors (NRC, 1977). If available, licensees should use site-specific parameter values or 

2 ranges of values, and should document their usage. Site-specific parameter values are values 

3 measured at the site under consideration; true site-specific values are relatively rare. If site4 specific transfer factors are not available, a licensee should document how their site conditions 

5 are comparable to the conditions under which the transfer factors that were used were 

6 collected. Licensees can infer transfer factors for a specific site from compilations in the 

7 literature; however, inferred values carry high uncertainty, which can be difficult to properly 

8 account for in a deterministic assessment.

9

10 Transfer factors can have very large variances and be non-uniformly distributed. These large 

11 variances may require careful treatment in the intruder assessment or performance 

12 assessment. If a transfer factor has few observations and the actual observations span many 

13 orders of magnitude, the appropriate statistic of the inferred distribution to use in the technical 

14 analysis may be difficult to determine. This is because the observations represent both inter15 site and intra-site variability. The technical analysis should not benefit from inter-site variability. 

16 Use of the geometric mean, for example, may result in a high likelihood that data that are not 

17 representative of the specific site have been “credited” in the analysis. Even if intra-site values 

18 are used to extend the “conservative” end of the distribution (e.g., high values of transfer 

19 coefficients), the overall effect may not be conservative because overly broad uncertainty 

20 ranges can lead to risk dilution (Section 2.7.4). If no observations at a site are available and 

21 correlations between the specific site conditions and the conditions for which the observations 

22 were made are not available, then licensees should use conservative statistics for transfer 

23 factors in deterministic analyses. In probabilistic analyses, licensees can use the full non24 truncated distribution (assuming the results are still physically reasonable), and use uncertainty 

25 analyses to determine if the uncertainty in the transfer factors is important. If the uncertainty is 

26 important, then licensees must provide a technical basis to explain why the distribution that 

27 contains inter-site variability used for the technical analysis is protective of public health and 

28 safety, or the licensee may elect to collect additional site characterization data to better define 

29 the range and variability. 

30 2.2.3 Model Support

31

32 Model support is one of the most essential technical elements of a licensee’s analyses to 

33 demonstrate compliance with the Subpart C performance objectives. Performance 

34 assessments and site stability assessments are projections many years into the future, and 

35 therefore, cannot be validated in the traditional sense. Intruder assessments are based on 

36 plausible but hypothetical scenarios. However, support for the calculations is essential for 

37 effective decisionmaking. Model support can help reduce uncertainty and provide a mechanism 

38 to determine when sufficient iteration in the development process has been performed. 

39 Regulatory perspectives on model validation for HLW disposal are provided in NUREG-1636 

40 (NRC, 1999a). Many of the concepts found in NUREG-1636 are generic and are applicable to 

41 LLW disposal as well.

42

43 Model support can be divided into verification-type activities (i.e., determining that the equations 

44 were solved correctly) and validation-type activities (i.e., determining that the correct equations 

45 were solved). Methods for verification of computational models are reasonably well-established. 

46

47 The objective of the review of model support is to determine if the technical analyses have

48 adequate support to justify the estimated system performance. In addition to reviewing a 
2-11

1 licensee’s QA procedures for computational modeling, the reviewer may perform independent 

2 analyses. Independent analyses may be performed with the licensee’s models, with 

3 independent models, or with simplified calculations. The objectives of independent analyses 

4 are to test, confirm, or refute the licensee’s assumptions and analyses.

5

6 Because the primary output of a performance assessment is dose to the public and those doses 

7 are in most cases not expected to occur until the distant future, the performance assessment 

8 cannot be supported by comparing the modeled dose to observed doses. However, a 

9 performance assessment typically includes many submodels that estimate the impact of 

10 processes such as infiltration, leaching of radionuclides from waste, and transport through 

11 groundwater. Licensees can and should support the output from those submodels using 

12 indirect methods if the output from the submodels is not observable in the real world. Model 

13 support that involves multiple sources and types of information is generally more robust. Types 

14 of model support may include laboratory or field tests, comparison to analogous systems, 

15 natural analogs, independent process modeling, formal independent peer review, and 

16 comparison to monitoring data. 

17

18 Upon development, and through early iterations, the performance assessment model (or 

19 submodels) is likely to go through a formal or informal calibration process. Calibration is a 

20 comparison of calculated outputs (e.g., intermediate outputs such as hydraulic head in an 

21 aquifer) with measured or observed data resulting in changes to the numerical calculation to 

22 better represent the observed data. While calibration is important, it does not necessarily result 

23 in confidence in prediction. Confidence in prediction is derived through comparison of the 

24 model with independent data not used in the original development or calibration of the model. 

25 Absolute proof cannot be achieved in performance assessment modeling; however, adequate 

26 confidence in the model and modeling results is essential.

27

28 The quantity and quality of model support should be commensurate with the significance of the 

29 system/subsystem to achieving protection. For example, if the capabilities needed for an 

30 engineered barrier are consistent with past experience in similar conditions, and the barriers 

31 have similar design and quality assurance (QA), then the model support could be considerably 

32 less than for a barrier with projected capabilities that significantly exceed experience with similar 

33 barriers. When considering prior experience, licensees should ensure that the environmental 

34 conditions for the relevant degradation mechanisms are reasonably similar since many 

35 degradation mechanisms can be very sensitive to the environmental exposure conditions.

36

37 Performance assessment modeling may include the projection of performance of engineered 

38 barriers, such as engineered covers or intruder barriers, for long periods of time. Licensees 

39 should consider natural analogs for barriers desired to have very long-term capabilities (e.g., 

40 thousands of years). The greatest uncertainties in predicting future performance stem from 

41 extrapolating the results of short-term tests and observations to long-term performance. 

42 Standard approaches to development of natural analogs frequently assume implicitly that the 

43 initial conditions persist; however, the actual application of a barrier could more appropriately be 

44 viewed as an evolving component of a larger dynamic system (Waugh and Richardson, 1997). 

45 For some types of barriers, natural analogs might provide information about the possible long46 term changes and can be thought of as a long-term, uncontrolled experiment. 

47

48 Licensees should consider the capability of a barrier when developing model support based on 

49 analogs. For example, because of their longevity, Native American earthen mounds may 
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1 provide a reasonable analog for the erosional stability of an engineered cap, but may not be a 

2 reasonable analog for other capabilities (e.g., the hydraulic performance of a cap) (Shetrone, 

3 2004). When evaluating analogs, it is important to note that the structures that have persisted 

4 are most likely the durable structures. That is why licensees should consider analogs that have 

5 persisted, as well as those that may have experienced damage or failure. An additional 

6 complicating factor is that the initial conditions and past exposure environment for the analogs 

7 are not known and may only be estimated. However, developing an understanding of analogs 

8 increases the likelihood that a barrier may be implemented with sustainable long-term 

9 capabilities. Natural analogs are only one element of the technical basis for the capabilities of 

10 engineered barriers. Analog information should not be envisioned as providing absolute proof 

11 of future barrier performance; rather, it provides confidence that the barrier is likely to perform 

12 as intended. Analogs can be also applied to other aspects of the performance assessment 

13 models (e.g., radionuclide transport, geochemistry).

14 2.2.3.1 Peer Review, Expert Judgment, and Expert Elicitation

15

16 Different types of assessments are used in the development, review, and approval of LLW 

17 technical analyses. Scientists and engineers routinely use professional judgment in solving 

18 problems. NUREG-1563, “Branch Technical Position on the Use of Expert Elicitation in the 

19 High-Level Radioactive Waste Program,” issued November 1996, defines “peer review,” “expert 

20 judgment,” and “expert elicitation” as applied to HLW disposal (NRC, 1996). The NRC staff 

21 believes that those definitions and descriptions also can be applied to LLW disposal. The 

22 purpose of this section is to provide a concise summary of information pertaining to the use of 

23 experts and to remind licensees of existing sources of guidance. Methods of reviewing or 

24 supplying information to technical analyses may have different degrees of formality and 

25 independence. Information from experts is necessary when other means of obtaining requisite 

26 data or information are not practical. The types of information that experts can supply include 

27 data (e.g., hydraulic conductivity of basalt), as well as input analogous to model support 

28 (e.g., independent peer review of a hydrogeology model).

29

30 Peer review is distinguished from expert judgment in that peer review seeks judgments from 

31 experts regarding the soundness and quality of an existing or proposed scientific stance or 

32 solution to a problem. Expert judgment is information provided by experts that gives rise to or 

33 contributes to the generation of a scientific stance or solution to a given problem. Expert 

34 elicitation is a formal, highly structured, and well-documented process whereby expert 

35 judgments are obtained (NRC, 1996). The regulatory review of LLW technical analyses is 

36 essentially an independent peer review.

37

38 For all types of information obtained from experts, licensees should describe and fully document 

39 the information developed as well as the process used to obtain the information. Full 

40 documentation entails a description in sufficient detail to allow independent interpretation and 

41 understanding of the information without recourse to the author. Licensees should establish 

42 and describe the independence of the experts and their qualifications.

43

44 If a licensee uses peer review as a form of model support, the experts should have 

45 independence from the activity for their input to be of value. Otherwise, the input is essentially 

46 an extension of the original analysis. The licensee should document how the experts were 

47 selected and their qualifications. Licensees should describe the information that the experts 

48 reviewed and document when the experts’ review and input was received. In other words, 
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1 scope and schedule are important in establishing the thoroughness of the review. The experts’ 

2 reviews should have sufficient detail for independent evaluation by the regulator. Independent 

3 peer review is more subjective than some other forms of model support (e.g., field experiments) 

4 but may be necessary to support the technical analyses for LLW disposal.

5

6 Expert elicitation and expert judgment are means to supply information such as uncertain values 

7 or uncertainty distributions when data are unavailable or inadequate and cannot be obtained 

8 with direct methods such as experiments. Expert elicitation and judgment characterize the state 

9 of knowledge about an uncertain FEP. Expert elicitation and judgment should not be used 

10 instead of “hard” data, but rather in conjunction with those data. NUREG-1563 provides general 

11 guidelines on those circumstances that may warrant the use of a formal process for obtaining 

12 judgments of experts, as well as acceptable procedures for conducting expert elicitation (NRC, 

13 1996).

14 2.2.3.2 Model Support Example –Engineered Cover Performance

15

16 The NRC staff is providing the following example to illustrate a reasonable effort to develop 

17 model support for the long-term performance of an engineered system. The DOE, Office of 

18 Legacy Management, Environmental Sciences Laboratory, developed a multi-faceted strategy 

19 combining monitoring, modeling, and natural analog studies to provide support for the estimated 

20 long-term performance of engineered covers (Waugh, 2004; Waugh 2006). The strategy 

21 utilized various independent sources of information to develop an understanding of the 

22 performance of engineered covers. Laboratory experiments (e.g., the hydraulic conductivity of 

23 clay), field measurements (e.g., the moisture content of the cover materials and tailings), in-situ 

24 field tests (e.g., lysimeter studies), natural analogs, and traditional monitoring data (e.g., radon 

25 fluxes, groundwater concentrations of radionuclides) provided information to compare to 

26 estimates of cover performance. There is uncertainty in any type of observational data. 

27 However, utilization of multiple sources of confirmatory information allows for the development 

28 of a more complete understanding of the uncertainties, and therefore, a lessened chance of 

29 false positive or false negative confirmation of system performance.

30

31 As part of the model support strategy, DOE obtained field measurements using lysimeters to 

32 test the performance of cover designs under the Alternative Cover Assessment Project (ACAP) 

33 (Albright et al., 2004). DOE performed comprehensive lysimeter tests of prototype covers at 

34 landfill sites across the country in climates ranging from arid to humid and from hot to cold. 

35 Some of the cover sites were at locations of full-scale covers operated by DOE. DOE monitored 

36 both conventional, low-permeability and alternative evapotranspiration cover designs in side-by37 side comparisons. The ACAP prototype tests were conducted using 10- by 20-meter drainage 

38 lysimeters instrumented for direct measurement of runoff, soil water storage, lateral drainage, 

39 and percolation flux for a full-depth cover profile. DOE used the lysimeter monitoring data to 

40 develop insights into cover performance.

41

42 Because some processes that may influence the performance of engineered covers are difficult 

43 to address with short-term field tests or existing numerical models, DOE also used natural 

44 analogs to help identify and evaluate likely changes in environmental processes that may 

45 influence the performance of engineered covers (Waugh, 2006). DOE used the natural analog 

46 information to (1) engineer cover systems that mimic favorable natural systems, (2) bound 

47 possible future conditions for input to predictive models and field tests, and (3) provide insights 

48 about the possible evolution of engineered covers as a basis for monitoring leading indicators of 
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1 change. DOE considered a variety of natural analogs, including analogs of future climate states 

2 at the Monticello, Utah site that were developed using paleoclimate data. A preliminary analysis 

3 of paleoclimate data for Monticello yielded average annual temperature and precipitation ranges 

4 of 2 to 10 degrees C and 80 to 60 centimeters, respectively, corresponding to late glacial and 

5 mid-Holocene periods. Additional natural analogs were developed for future soil development 

6 and ecological change. 

7 2.2.4 Dose Assessment

8 2.2.4.1 Human Activity - Scenarios

9

10 Stylized scenarios are commonly defined to represent the human component of the analyses. 

11 Little scientific basis exists for predicting the nature or probability of future human actions over 

12 long timeframes. The use of stylized scenarios is commonly advocated by regulators in order to 

13 avoid excessive speculation. A special category of FEPs are those related to future human 

14 activities that may disrupt the disposal system. Section 4.0 provides guidance on developing 

15 scenarios for the inadvertent intruder assessment. 10 CFR Part 61 requires the consideration 

16 of inadvertent intrusion, and also, to some extent, constrains the types of receptor scenarios 

17 that need to be considered. 

18

19 Licensees should integrate receptor scenarios with the scenarios describing the future evolution 

20 of the disposal site. For example, the landscape and hydrological regime at and around a 

21 hypothetical disposal site may change in response to climate change, and with these changes,

22 receptors and their habits may change. The central scenario for this hypothetical disposal site 

23 may include exposure pathways that result in multiple receptor scenarios. For example, the 

24 most plausible and/or conservative receptor scenarios for an undisturbed central scenario may 

25 be the resident farmer and suburban resident receptor scenarios. However, if a licensee is 

26 evaluating an additional alternative scenario of the future involving climate change for the same 

27 site, the most plausible receptor scenario for the alternative scenario may no longer favor 

28 residential dwellings at the site, but may be, for example, occasional recreational use of the site 

29 by one or more human receptors (Figure 2-3). On the other hand, it is possible that the receptor 

30 scenarios may remain the same (i.e., the resident farmer and suburban resident receptor 

31 scenarios would be evaluated within the undisturbed future scenario and within the alternative 

32 scenario involving climate change) but the dominant pathways of exposure may change within 

33 the same scenario (i.e., irrigation rates increase resulting in additional exposure through food). 

34 (Figure 2-4). 

35 2.2.4.2 Receptor Scenario Development

36

37 After releases of radioactivity to the environment, a receptor may be exposed to contaminated 

38 water, soil, air, or other media. A receptor is a member of the public who may be exposed to 

39 radiation from the disposal facility. Receptors include members of the public who may be on 

40 site after the institutional control period (e.g., an inadvertent intruder), as well as offsite 

41 members of the public. Section 3.4 provides guidance with respect to receptors for the 

42 performance assessment. Section 4.3 provides guidance with respect to receptors for the 

43 intruder assessment. Some of the approaches used in Section 4.3 can be applied in the

44 performance assessment and used to demonstrate compliance with 10 CFR 61.41.

45
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Residential Farmer 

Receptor Scenario

Suburban Resident 

Receptor Scenario

Central 

Scenario

Recreational User 

Receptor Scenario

Alternative 

Scenario

1

2

3 Figure 2-3 A Central Scenario with Several Plausible Receptor Scenarios, and an 

4 Alternative Scenario with a Different Plausible Receptor Scenario

Residential Farmer 

Receptor Scenario

Suburban Resident 

Receptor Scenario

Central 

Scenario

Residential Farmer 

Receptor Scenario

Suburban Resident 

Receptor Scenario

Altrernative 

Scenario

5

6

7 Figure 2-4 A Central Scenario with Several Plausible Receptor Scenarios, and an 

8 Alternative Scenario with the Same Plausible Receptor Scenarios

9

10 Licensees should estimate potential exposure to the average member of the critical group. The 

11 critical group is a group of individuals reasonably expected to receive the greatest exposure to 

12 releases over time, given the circumstances under which the analysis would be carried out. 

13 The average member of the critical group is that individual who is assumed to represent the 

14 most likely exposure situation, based on cautious but reasonable exposure assumptions and 

15 parameter values.

16

17 Scenarios of receptor exposure, also known as exposure scenarios or receptor scenarios, 

18 should not be confused with the general scenarios describing a future evolution of the disposal 

19 system. Receptor scenarios are a subset of the overall scenarios defined for the performance 

20 assessment (e.g., base or central scenario, disturbed or alternative performance scenarios) and 

21 intruder assessment (Figure 2-4). Receptor characteristics and receptor scenarios may vary 

22 from site to site; however, certain pathways commonly contribute to exposure to or intake of 

23 radionuclides. For example, drinking water and agricultural food production (crops, livestock) 
2-16

1 commonly contribute to radionuclide intake by many types of receptors. External exposure to 

2 contaminated soils and inhalation of resuspended contaminants are also common exposure 

3 pathways. Recreational use of surface water (e.g., fishing and swimming, which may lead to 

4 exposure to contaminated sediments) may be an exposure pathway at some sites. 

5

6 The characteristics of receptor scenarios can have a large impact on the projected risks from 

7 the disposal facility. The definition of receptor scenarios can be generic or site-specific. 

8 Regardless of the specific method used to develop receptor scenarios, the licensee should 

9 provide sufficient justification for the approach used. Licensees may use generic receptor 

10 scenarios as described in NUREG-0782 (NRC, 1981a) with site-specific waste streams, or they 

11 may develop site-specific receptor scenarios to evaluate site-specific waste streams. Licensees 

12 may develop site-specific receptor scenarios by modifying the exposure pathways included in 

13 the generic receptor scenarios or build scenarios from scratch based on waste characteristics, 

14 disposal practices, site characteristics, and, when appropriate, projected land use. As the 

15 assessment time increases from the present day, the relevance of current land use to projected 

16 future land use becomes much more uncertain. For long-term assessments, dose calculations 

17 should use reasonably conservative receptor scenarios such as the generic receptor scenarios 

18 (i.e., resident-farmer or resident-gardener). The NRC staff continues to view the generic 

19 receptor scenarios as reasonably conservative to estimate potential radiological exposures to a 

20 member of the public while limiting excessive speculation about future human activities. 

21 Licensees should be cautious about adopting the generic receptor scenarios and exposure 

22 pathways by ensuring that the exposure pathways are appropriate for the technical analysis for 

23 their disposal site.

24

25 Licensees should provide a basis to justify receptor scenario selections. Use of conservative 

26 generic receptor scenarios would require a limited basis, whereas use of novel site-specific 

27 receptor scenarios would require more information. There may be a need to thoroughly 

28 investigate and justify the appropriateness of the selected site-specific receptor scenario(s), 

29 which may include evaluation of alternate receptor scenarios. If a licensee creates a receptor 

30 scenario based on site-specific conditions, it should provide transparent and traceable 

31 documentation of the justification for each assumption used in developing the receptor scenario 

32 (e.g., justify the inclusion (or exclusion) of a particular exposure pathway).

33

34 When assessing the dose to the receptor beyond the site boundary, licensees should consider 

35 potential receptor locations. The site boundary begins at the end of the buffer zone, which 

36 should extend 100 meters from the boundary of the disposal units. Generally, receptor 

37 locations at the site boundary would be expected to receive the greatest radiological exposures 

38 since these locations tend to minimize the opportunity for dilution that may occur at greater 

39 distances from the site. However, this may not always be the case, particularly for sites with 

40 preferential transport pathways (e.g., fracture zones) or physical constraints that might limit 

41 exposure of the receptor to the media (e.g., non-potable groundwater). Licensees should 

42 demonstrate that the selection of an offsite receptor location does not bias the outcome of the 

43 performance assessment such that radiological exposures are significantly underestimated. 

44 Receptor locations may be different for transport in various environmental media; the highest 

45 impact from an air pathway may be at a different location than the highest impact from a water 

46 pathway. However, it is not expected that the peak exposures at different receptor locations 

47 would be additive. Rather, licensees need to demonstrate that the performance objectives will 

48 be met for the location in which the receptor would be expected to receive the largest annual 

49 dose from all significant exposure pathways (e.g., peak annual all-pathway dose). 
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1 Determination of the receptor location should also consider the evolution of the site environment 

2 during the compliance period. For instance, climate change over the compliance period may 

3 alter whether the groundwater is potable or of sufficient yield and thus, potentially, the location 

4 of the offsite receptor well.

5

6 A site-specific assessment would typically consider climatic and environmental conditions. As 

7 discussed in NUREG-1573, performance assessments should consider variability in natural 

8 conditions, processes, and events (NRC, 2000a). The selection of receptor scenarios and 

9 pathways in the performance assessment and intruder assessment should consider the 

10 variability in natural conditions. For a typical commercial LLW disposal facility, where the 

11 hazard from the inventory remaining at 500-1,000 years is expected to be low and allowable

12 limits on long-lived radionuclides can be set, licensees should avoid unnecessary speculation 

13 about major changes to future climate (such as glacier formation). This is because the human 

14 population would be dramatically affected by the natural process, and the radiological impacts 

15 would likely be secondary (in part because the inventory of long-lived waste is limited). 

16 However, licensees should consider more gradual changes in performance assessment 

17 modeling for significant quantities of long-lived radionuclides, especially for performance period 

18 analyses (Section 7.0). For example, natural cycling of climates may induce variation in the 

19 nature, timing, and magnitude of meteorological processes and events. For long-lived waste 

20 streams, licensees should consider these gradual changes when evaluating impacts to 

21 members of the public. If a licensee uses current site conditions to eliminate what would 

22 otherwise be considered credible land use (receptor) scenarios and the waste is long-lived, the 

23 performance assessment and intruder dose assessment should consider expected changes to 

24 climate and environmental conditions as a result of natural cycling of the climate. Changes to 

25 the climate may make the eliminated land use (receptor) scenarios more or less likely to occur 

26 in the future. 

27

28 A licensee’s assumptions about land use should focus on current practice in the region of 

29 concern, which can be as large as an 80-kilometer (50-mile) radius. To narrow the focus of 

30 current land practices, the licensee can use information on how land use has been changing in 

31 the region and should give more weight to land use practices either close to the site or in similar 

32 physical settings. Licensees should also evaluate land uses that occur in locations outside the 

33 region of concern that share characteristics (temperature, precipitation, topography) expected 

34 for the region of concern over the duration evaluated in a site-specific receptor scenario. 

35 Consideration of environmental-analog regions may help identify whether present-day land uses 

36 have been driven by past socio-economic development. Land uses primarily resulting from 

37 socio-economic development are generally more uncertain over longer time periods than land 

38 uses primarily resulting from physical conditions (e.g., climate). 

39 2.2.4.3 Dosimetry

40

41 As described in 10 CFR 61.7(c)(5), the dose methodology used to demonstrate compliance with 

42 the performance objectives of 10 CFR Part 61 shall be consistent with the dose methodology 

43 specified in the standards for radiation protection set forth in 10 CFR Part 20, “Standards for 

44 Protection against Radiation.” The dose methodology is how individual dose factors (for 

45 example: external, ingestion, or inhalation) are calculated for each radionuclide. Licensees may 

46 use updated dose factors, which have been issued by consensus scientific organizations and 

47 incorporated by the U.S. Environmental Protection Agency (EPA) into Federal radiation 

48 guidance. Additionally, licensees may use the most current scientific models and 
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1 methodologies (e.g., those accepted by the International Commission on Radiological 

2 Protection [ICRP]) appropriate for site-specific circumstances to calculate the dose. 

3

4 In the performance assessment and intruder assessment, licensees may use the dose 

5 conversion factors for inhalation and ingestion developed by the EPA as published in EPA6 520/1-88-020, Federal Guidance Report (FGR) No. 11, “Limiting Values of Radionuclide Intake 

7 and Air Concentration and Dose Conversion Factors for Inhalation, Submersion, and Ingestion,” 

8 issued September 1988 (EPA, 1988). Similarly, it is appropriate for a licensee to use EPA’s 

9 external dose factors as published in EPA-402-R-93-081, Federal Guidance Report No. 12, 

10 “External Exposure to Radionuclides in Air, Water, and Soil,” issued September 1993 (EPA, 

11 1993). These dose factors are selected to ensure consistency of the dosimetry models used in 

12 deriving these factors with 10 CFR Part 20, “Standards for Protection against Radiation.”

13

14 Dose conversion factors are further defined by the chemical form of each element, by either its 

15 gastrointestinal tract uptake fraction (known as the f1 factor for ingestion dose factors), or its 

16 solubility class (solubility in lung fluid for inhalation dose factors). Licensees should provide 

17 justification in the performance assessment and intruder assessment for the chemical forms 

18 assumed, particularly if a radionuclide has more than one value (e.g., strontium or uranium). 

19 Licensees should use reasonably conservative solubility classes for modeling radionuclides in a 

20 performance assessment, considering that the element may change in chemical form as it 

21 moves through the environment before reaching receptors. For acute intruder scenarios, 

22 different sets of chemical forms may be expected as the chemical forms of radionuclides in the 

23 disposal unit environment may differ from those in the external natural environment. 

24

25 An example of a conservative approach to selecting dose factors is used in NUREG/CR-5512, 

26 “Residual Radioactive Contamination from Decommissioning, Technical Basis for Translating 

27 Contamination Levels for Annual Total Effective Dose Equivalent,” issued October 1992 (NRC, 

28 1992). Appendix E, Section E.1, of Volume 1 of this NUREG describes the values 

29 recommended for use in the screening models developed for application to decommissioning. 

30 Specifically, Table E.6 gives inhalation (i.e., solubility) class and the f1 factor for each 

31 radionuclide. Because the values are recommended for screening analyses, in most cases, the 

32 solubility class selection will maximize the potential inhalation dose. For plutonium, the solubility 

33 class represents the most common chemical form that will likely be encountered in 

34 environmental situations. For the other radionuclides, the solubility classes and gastrointestinal 

35 uptake fractions are defined for the combination resulting in the highest dose. 

36

37 The licensee should use a consistent dose methodology for all the applicable performance 

38 objectives (i.e., for the performance assessment (10 CFR 61.41), intruder assessment (10 CFR 

39 61.42), and assessing doses during operations (10 CFR 61.43)). If the licensee wishes to use a 

40 methodology not consistent with the definitions in 10 CFR Part 20, they may request an 

41 exemption from the definition of “weighting factor” as defined in 10 CFR 20.1003. For example, 

42 the licensee could request to use (1) the latest dose conversion factors (e.g., ICRP 

43 Publication 72, “Age-Dependent Doses to the Members of the Public from Intake of 

44 Radionuclides, Part 5, Compilation of Ingestion and Inhalation Coefficients,” issued 

45 September 1996 (ICRP, 1996), or (2) EPA-402-R-99-001, Federal Guidance Report No. 13, 

46 “Cancer Risk Coefficients for Environmental Exposure to Radionuclides,” issued 
2-19

September 1999 (EPA, 1999)).1 1 In SRM-SECY-01-0148, the Commission directed that the 

2 NRC staff should continue to consider and grant, as appropriate, licensee requests to use 

3 revised internal dosimetry models on a case-by-case basis (NRC, 2002a).

4

5 Licensees must select organ dose weighting factors and corresponding dose factors (i.e., 

6 extrapolation of the dose to a specific organ to the effective dose to the whole body) that were 

7 developed using the same dose methodology. For example, if a licensee uses the organ dose 

8 weighting factors specified in 10 CFR 20.1003 that were developed from the ICRP 26 dose 

9 methodology (ICRP, 1977), they should use the corresponding dose factors that are tabulated in 

10 FGR Report 11 and ICRP Report 30 (ICRP, 1979; ICRP, 1980; ICRP, 1982). If a licensee uses 

11 the organ dose weighting factors that were developed from the ICRP 60 dose methodology 

12 (ICRP, 1991), they should use the corresponding dose factors that are tabulated in FGR Report 

13 13 and ICRP Report 72. A licensee should not select organ dose weighting factors from 

14 10 CFR 20.1003 and dose factors from ICRP Report 72, because they were not calculated 

15 using consistent dose methodologies.

16

17 Licensees should provide justifications for age-based considerations of scenarios, critical group 

18 assumptions, and the chemical forms, consistent with the dose methodology system being 

19 used. Age-based considerations should evaluate the sensitivity to the total dose, rather than 

20 specific pathways.

21

22 If a licensee chooses to modify their existing performance assessment and intruder analyses 

23 based on the availability of new dosimetry information, the licensee will need to submit the 

24 updated performance assessment and intruder analysis for review and approval, similar to any 

25 other update (see Section 8.0). As stated previously, the licensee may need to request an 

26 exemption to 10 CFR Part 20 to use the latest dose conversion factors to perform dose 

27 analyses for individuals during operations to meet the 10 CFR 61.43 performance objective. 

28 2.3 Assessment Context

29 2.3.1 Context of the Performance Assessment

30

31 The first step in the performance assessment process is the assessment context. In order to 

32 develop the context of a performance assessment a licensee should answer the following 

33 questions: 

34

35 What is being assessed? 

36 Why is it being assessed? 

37 What is the scope of the assessment? 

38

39 Components of an assessment context can comprise the assessment purpose, regulatory 

40 framework, assessment end points, assessment philosophy, waste characteristics, disposal 

41 system characteristics, and assessment timeframes. For LLW, the regulatory framework is 

 1 The regulations at 10 CFR 20.1003 define the weighting factors, based on ICRP 26 (ICRP, 1977) that 

apportion the risk of stochastic effects resulting from irradiation of an organ or tissue to the total risk of 

stochastic effects when the whole body is uniformly irradiated. Dose methodologies that differ from ICRP 

26 may deviate from those weighting factors, thus licensees must request an exemption from the 

definition of weighting factor in 10 CFR 20.1003 to use the alternative dose methodology.
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1 found in 10 CFR Part 61, including the performance objectives found in Subpart C. However, 

2 the purpose of conducting a performance assessment may vary. The licensee should consider 

3 the audience for the performance assessment results. Different strategies for the performance 

4 assessment may be used (e.g., conservative vs. realistic, simple vs. complex, deterministic vs. 

5 probabilistic). A well-defined assessment context can be used to determine the level of model 

6 abstraction, as well as data and computational needs. For example, if the assessment context 

7 calls for a simple modeling approach, a relatively simple mathematical model abstracted from 

8 the conceptual model may be sufficient. In addition, the assessment context may provide a

9 comparison point to other performance assessments. For example, if a similar site has been 

10 modeled by another organization, a comparison, and any possible discrepancies, of the model 

11 outputs will be easier to understand. For a description of an assessment context as it pertains 

12 to biosphere models for geologic disposal, the BIOMASS program, as documented in IAEA 

13 (2003), provides information and guidance.

14 2.3.2 Approach to Different Timeframes

15

16 This section of the guidance document describes the information that a licensee should provide 

17 and a reviewer should evaluate with respect to the timeframe for the analyses for the intruder 

18 assessment, performance assessment, and stability assessment for the compliance period, 

19 protective assurance period, and the performance period. 

20

21 When completing technical analyses, the licensee should select the period of time over which 

22 the potential future behavior of the disposal system will be evaluated against the performance 

23 objectives; this period of time is termed the “analysis timeframe.” 

24

25 Licensees conduct performance assessments to understand how a waste disposal system may 

26 perform with respect to limiting releases to offsite members of the public. Performance 

27 assessments are also used by stakeholders to understand the potential impacts of 

28 uncertainties. Numerous sources of uncertainty are associated with projecting the future 

29 radiological risks from waste disposal for thousands of years, including, but not limited to,

30 natural, engineering, and societal sources. Section 2.2.2 discusses the types of uncertainties 

31 that are commonly explicitly considered in technical analyses by licensees or reviewers.

32

33 One of the outputs of technical analyses for LLW disposal is the projected dose to a member of 

34 the public from radioactivity released to the environment. The projected doses are compared to 

35 regulatory requirements and the regulator determines whether there is an acceptable degree of 

36 confidence that the dose is consistent with the regulatory limit. In the NRC’s terminology, that 

37 degree of confidence is described as “reasonable assurance.” The results of a compliance 

38 analysis should not be interpreted as unequivocal proof of the expected behavior of a waste 

39 disposal facility, because of the uncertainties associated with the projected radiological risk over 

40 long time periods. Over extended periods of time, uncertainties associated with the 

41 performance of natural and engineered systems may increase, and uncertainties associated 

42 with human behavior definitively increase. In some cases, unmanageable uncertainty may be a 

43 suitable reason not to dispose of waste if the consequences have unacceptable outcomes 

44 (represented in that uncertainty). The technical analyses supply information to inform decision45 makers and the public.

46

47 A three-tiered approach to the analyses timeframe is specified in 10 CFR Part 61. When 

48 implemented properly, the three-tiered approach should accomplish the main goal of licensees 
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1 communicating the near- and long-term risks in an appropriate uncertainty context to regulators 

2 and other stakeholders. For discussion purposes in this guidance document, these phases are 

3 defined as follows:

4

5 Analysis timeframe — The timeframe over which a licensee should assess the projected 

6 performance of the disposal facility while factoring in the characteristics of the waste, 

7 engineered barriers, disposal site, and associated uncertainties. The analysis timeframe is 

8 divided into three phases: a compliance period, a protective assurance period, and a 

9 performance period. However, the performance period is only applicable when certain 

10 conditions are met, and therefore, it may not apply to some sites.

11

12 Compliance period — The period of time over which a licensee must demonstrate with 

13 reasonable assurance that the disposal facility will meet the performance objectives found in 

14 10 CFR 61.41(a), 10 CFR 61.42(a), and 10 CFR 61.44. A quantitative assessment should be 

15 performed. The compliance period is defined to be 1,000 years.

16

17 Protective assurance period — The period from the end of the compliance period through 

18 10,000 years following closure of the site. A licensee must demonstrate that releases to the 

19 environment and exposures to an inadvertent intruder have been minimized below 5 mSv (500 

20 mrem) or to a level that is supported as reasonably achievable based on technological and 

21 economic considerations. Licensees should perform quantitative analyses for the protective 

22 assurance period (see Section 6.0).

23

24 Performance period — The period of time over which a licensee evaluates the ability of the 

25 disposal system to contain long-lived waste and demonstrates that releases are minimized to 

26 the extent reasonably achievable (discussed in Section 7.0). The performance period begins at 

27 the end of the protective assurance period and extends as long as necessary to demonstrate 

28 that the metric of the performance period can be met. Licensees should perform a quantitative 

29 assessment, though uncertainties may be large, which may decrease the confidence that a 

30 licensee should place in the results of the analyses. A qualitative interpretation of the 

31 quantitative results by the licensee is appropriate in most cases.

32 2.3.2.1 Compliance Period

33

34 10 CFR 61.2 specifies a period of 1,000 years as the timeframe over which a licensee must 

35 demonstrate compliance with the quantitative limits and stability requirements of Subpart C 

36 (10 CFR 61.41(a), 10 CFR 61.42(b), and 10 CFR 61.44). Licensees should conduct a

37 performance assessment, intruder assessment, and site stability evaluation for the compliance 

38 period. A quantitative assessment of disposal facility performance should be developed

39 covering the timeframe of the first 1,000 years following closure of the disposal facility. The 

40 quantitative assessment may also be risk-informed. The waste characteristics (e.g., the amount 

41 of short- and long-lived wastes), complexity of the disposal facility design, and complexity of the 

42 disposal site and surrounding environment will influence the level of detail that should be 

43 provided to support the technical analyses (see Example 2.1).

44
2-22

1

2 2.3.2.2 Protective Assurance Period

3

4 The protective assurance period is the period from the end of the compliance period through 

5 10,000 years following closure of the site. The analysis for the protective assurance period is 

6 similar to the compliance period – a licensee will complete a performance assessment, intruder 

7 assessment, and site stability analyses. The primary difference between the compliance period 

8 and protective assurance period is that the objective of the protective assurance period is to 

9 minimize impacts to members of the public and the environment from the longer term releases 

10 from the LLW disposal facility. Sections 10 CFR 61.41(b) and 10 CFR 61.42(b) provide dose 

11 goals, rather than dose limits, for the protective assurance period, primarily because the 

12 process licensees may use to minimize impacts based on what is technologically and 

13 economically practical may be different for different types of wastes. A licensee must 

14 demonstrate that releases to the environment and exposures to an inadvertent intruder have 

15 been minimized below 5 mSv (500 mrem) or to a level that is supported as reasonably 

16 achievable based on technological and economic considerations. Section 6.0 of this document 

17 provides detailed guidance on analyses for the protective assurance period.

18

19 The performance assessment for the 10,000 year protective assurance period is intended to be 

20 implemented in a risk-informed, performance-based manner. The type of waste proposed for 

21 disposal by a licensee will affect the complexity and scope of the analysis. A disposal facility 

22 may contain limited quantities or low concentrations of long-lived waste. The mere presence of 

23 long-lived waste should not be interpreted as imposing a burden of quantitative technical 

24 analyses of FEPs extending to 10,000 years. It may be acceptable for a licensee to 

25 demonstrate by use of conservative exposure scenarios that concentrations and quantities of 

26 long-lived waste are sufficiently limited such that longer-term processes are unlikely to result in 

27 unacceptable doses.

28

29 A licensee could demonstrate that a facility accepting only short-lived waste could meet the 

30 regulatory requirements without a quantitative assessment of the impacts extending many 

31 thousands of years into the future. The performance assessment for a facility that accepts large

32 quantities of long-lived waste would need to consider FEPs that could significantly affect 

33 whether the facility could meet the performance objectives for the protective assurance period, 

34 and this set of FEPs may be more extensive than that associated with the disposal of short-lived 

Example 2.1: A facility is expected to receive typical LLW generated by commercial 

entities (e.g., limited concentrations of long-lived waste). The waste has average 

concentrations of long-lived radionuclides at or below one-tenth of the values listed in 

Table 1 of 10 CFR 61.55. Additionally, the facility is expected to receive waste with 

long-lived radionuclides that are not found in Table 1 of 10 CFR 61.55 and are at 

concentrations less than the natural soils surrounding the facility.

Conclusion: A performance assessment, intruder assessment, and stability assessment 

should be completed to demonstrate compliance with 10 CFR 61.41(a), 10 CFR 61.42(a), 

and 10 CFR 61.44. Because the waste is dominated by short-lived activity and long-lived 

concentrations are limited, specialized models and associated model support for long-term 

processes (e.g., cycling of climate) are not necessary.
2-23

1 waste. Disposal of large quantities of long-lived waste will likely require a performance 

2 demonstration by a licensee that is much more complex than the evaluation of the disposal of 

3 short-lived waste. The demonstration of performance for the disposal of long-lived waste will 

4 likely require significantly more resources to complete.

5

6 Licensees and regulators should consider if radiological risk from long-lived waste is being 

7 limited by the waste characteristics or by the facility design and site characteristics. If the risk is 

8 limited by the waste characteristics (e.g., limited inventory) a long-term quantitative analysis 

9 may not be necessary. If the risk from long-lived waste is limited by the facility design and site 

10 characteristics, then a long-term quantitative assessment should be developed. It is appropriate

11 for a licensee to use simplified calculations to determine the detail needed for the longer term 

12 assessment, as long as the simplified calculations are based on reasonably conservative 

13 scenarios and models (see Example 2.2). In other words, the NRC staff expects the process of 

14 determining the level of detail that may be needed for a longer term assessment to be iterative. 

15 In addition, a licensee should consider waste characteristics for the full protective assurance

16 period to account for the potential ingrowth of long-lived progeny when estimating the 

17 concentrations of long-lived radionuclides in the disposed waste.

18

19

20 2.3.2.3 Performance Period

21

22 The performance period is the time after the protective assurance period when longer-term 

23 doses could result from the disposal of certain types of long-lived waste. The objective of 

24 analyses provided for the performance period is for a licensee to demonstrate that releases 

25 from long-lived waste disposed of in the facility have been minimized to the extent reasonably 

26 achievable and that the facility has been designed with consideration of the potential long-term 

27 radiological impacts, consistent with available data and current scientific understanding. The 

28 analyses should identify the features of the design and site characteristics that will reduce long29 term impacts and describe the capabilities of these features. Analyses for long-lived waste 

30 should provide the range of peak annual doses that are projected to occur after 10,000 years 

Example 2.2: A proposed facility plans to receive long-lived waste, but the licensee 

or applicant is unsure how much detail should be provided in the protective assurance 

analyses. The analyses for the protective assurance period reflect longer-term 

processes (those that occur after a thousand years). The licensee completes an 

assessment of the potential long-term risk by evaluating a resident-intruder (with 

conservative biosphere parameters), takes no credit for wasteforms or engineered 

barriers, and completes a groundwater assessment using conservative flow rates and 

distribution coefficients. The licensee finds that the potential doses have been 

minimized consistent with 10 CFR 61.41(b) and 10 CFR 61.42(b).

Conclusion: A complex evaluation of long-term processes such as landform 

evolution and climate change is not necessary to demonstrate compliance with 

10 CFR 61.41(b) and 10 CFR 61.42(b). Disruptive processes would be expected to 

increase dispersion of the waste in the environment. Because the licensee evaluated 

the waste in an undispersed state using conservative scenarios, the licensee has 

demonstrated that the characteristics of the waste will ensure that long-term doses 

are minimized. The licensee is still required to consider the impact of disruptive 

processes on the demonstration of compliance with 10 CFR 61.44.
2-24

1 following site closure, or other metrics such as concentrations of radioactivity in the environment 

2 and flux rates to the environment. The long-term performance period analyses are designed to 

3 complement the technical analyses performed for the compliance demonstration and protective 

4 assurance analyses (10 CFR 61.13(a), (b), (d), and (f)).

5

6 As described below, and in more detail in Section 7.0, a number of approaches are acceptable 

7 for providing the necessary information for performance period analyses. A licensee should 

8 provide sufficient information and analyses for the long-term performance period that

9 demonstrate that the 10 CFR 61.41(c) and 10 CFR 61.42(c) performance objectives will be met. 

10 The licensee should use available data and current scientific understanding to assess the 

11 long-term performance of the waste disposal facility, including uncertainties. 

12

13 The NRC staff recommends the performance period analyses should cover the period of 

14 geologic stability at the disposal site, limited to a maximum of one million years or the peak 

dose2 15 considering uncertainty, whichever comes first. It would not be appropriate to constrain 

16 the analyses to the period of near-surface geologic stability, as one of the reasons for 

17 undertaking the performance period analyses is for a licensee to communicate to decision18 makers the potential range of consequences from the disposal action. Near-surface geologic 

19 instability may result from a process such as fluvial erosion (e.g. driven by lake formation), 

20 which could have severe impacts at an unstable site. Near-surface geologic instability may 

21 indicate that the site is unsuitable for disposing of significant quantities of long-lived radioactive 

22 waste. A licensee should not use near-surface geologic instability as a basis for limiting the 

23 analysis. If the analysis for LLW disposal was limited to the period of near-surface geologic 

24 stability, the analysis could be truncated prematurely and the long-term risks and uncertainties 

25 may not be understood. In addition, instability could be used as a basis to select a site, which is 

26 not acceptable. Section 7.0 provides more detail on specific technical issues that may be 

27 relevant to technical analyses for the performance period.

28 2.3.2.4 Site Characteristics

29

30 Site characteristics are identified in 10 CFR 61.50 that either must be avoided or must be 

31 present at a proposed disposal site. As described in the concepts section (10 CFR 61.7), a 

32 licensee needs to consider site characteristics in terms of the indefinite future, taking into 

33 account the radiological characteristics of the waste. They should evaluate site characteristics

34 for at least a 500-year timeframe and understand that the interpretation of the indefinite future is 

35 different for different types of waste. Flexibility is provided to licensees to ensure that they can 

36 consider site characteristics in a risk-informed manner. The site suitability requirements are 

37 designed to be applied to ensure that the long-term performance objectives of Subpart C are 

38 met. A cornerstone of radioactive waste disposal is the stability of the disposal system. Sites 

39 for which the site characteristics requirements cannot be satisfied are unlikely to meet this 

40 primary regulatory objective.

41

42 For the disposal of any type of waste, the hydrological site characteristics identified in 

10 CFR 61.50 are required3 43 for the next 500 years. For example, disposal should not be 

 2 As discussed in Section 7.0, the performance period analyses may include metrics other than dose 

(e.g., concentration, fluxes). See Section 7.0 for more detail.

3 10 CFR 61.50 lists the hydrological characteristics a site is required to have as well as other 

hydrological characteristics a site must not have.
2-25

1 permitted at a site where the facility is expected to be in the 100-year floodplain over the next 

2 500 years. 10 CFR 61.50(a)(4) specifies other characteristics of the site that should not be 

3 present at any timeframe because they significantly affect the ability of the disposal site to meet

4 the performance objectives of Subpart C (e.g., population growth, tectonic processes). 

5 Appendix B presents currently understood hazard maps related to the features and phenomena 

6 of the 10 CFR 61.50 criteria.

7

8 Whether a licensee’s consideration of site characteristics needs to be extended to the end of the 

9 protective assurance period (i.e., 10,000 years) or into the performance period depends on the 

10 type of waste that will be disposed. For a disposal facility that only accepts short-lived waste 

11 and also contains minimal quantities of long-lived waste, a licensee’s consideration of the site 

12 characteristics over the next 500 years would ensure that a proper site has been selected and 

13 that it would be capable of being characterized, modeled, analyzed, and monitored. A licensee 

14 is not prohibited from considering site characteristics for more than 500 years, and it may 

15 enhance the robustness of their technical evaluation. However, a consideration of site 

16 characteristics for more than 500 years for the disposal of waste with limited long-lived 

17 radioactivity would be unnecessary. To determine if the amount of waste a licensee wishes to 

18 dispose of is a minimal quantity, a licensee may simply calculate the product of the projected 

19 facility volume (or mass) and the concentrations provided in the table in 10 CFR 61.13(e), then 

20 apply the sum of the fractions rule for mixtures of radionuclides described in 

21 10 CFR 61.55(a)(7). In this context, a minimum quantity is defined as the product of volume 

22 and concentration of waste that corresponds to a sum of fractions of one tenth.

23

24 The timeframes that reviewers should use to evaluate the site characteristics requirements are:

25

A) If C < 1/10th 26 X evaluate 500 years

B) If 1/10th 27 X < C < X evaluate 10,000 years

28 C) If C > X evaluate performance period

29

30 where C is the disposal site waste concentration sum of fractions for long-lived waste, and X is 

31 the 10 CFR 61.13(e) Table A sum of fractions. By providing a value for long-lived alpha32 emitting radionuclides in Table A, most isotopes of concern (including uranium isotopes) will be 

33 considered using this approach. However, in some circumstances a disposal facility’s inventory 

34 may contain other long-lived isotopes. If there are long-lived isotopes that are observed to be 

35 key contributors to projected risk that are not included in Table A (e.g. Cl-36), a licensee should 

36 evaluate the site characteristics over a timeframe as long as necessary to support the relevant 

37 performance demonstration. In most cases the licensee would have performed this iterative 

38 evaluation (from characterization to performance analyses and back to characterization) prior to 

39 submittal of the analyses to the regulator.

40 2.4 System Description

41

42 The second step of the performance assessment process is for the licensee to describe the 

43 LLW disposal system and the natural environment of the site. The objective of the system 

44 description review is to ensure that the information that was used to develop the performance 

45 assessment models and the information describing the overall disposal system have been 

46 adequately described. The description should be adequate to allow an independent reviewer to 
2-26

1 understand the LLW disposal system. The system description should provide, at a minimum, 

2 information describing:

3

4 • The site; 

5 • The natural setting; 

6 • The disposal facility; 

7 • The interaction of the site and disposal facility; 

8 • The waste to be disposed of including its radiological, chemical, and physical 

9 characteristics; 

10 • Potential disruptive processes; and 

11 • The characteristics of members of the public potentially affected by the facility. 

12

13 The specific technical information that must be provided is listed in 10 CFR 61.12. The system 

14 description should provide estimates of the temporal changes to the aforementioned 

15 information, especially for disposal of long-lived waste. A practical metric to determine if the 

16 system description is adequate is if the system can be understood without seeking clarification 

17 from the document authors.

18 2.5 Scenario Development

19

20 The third step in the performance assessment process, scenario development, is the process of 

21 developing the scope of the analysis that will be implemented in the conceptual and numerical 

22 models. Development of a model that represents the current and future features, events, 

23 processes, and their interactions, is a complex process. Formal approaches to scenario 

24 development are usually either bottom-up or top-down. The bottom-up approach involves the 

25 identification, categorization, and systematic screening of FEPs. The top-down approach uses 

26 the safety assessment and safety functions to develop scenarios.

27

28 Typically a process or event acts upon a feature, and as time progresses, processes and events 

29 (both can be referred to as a phenomena) act to modify the system. A comprehensive set of 

30 FEPs or safety functions should capture all of the features and phenomena that are potentially 

31 relevant to the near- and long-term performance of a disposal system. 

32

33 For the bottom-up approach, the FEPs analysis developed by a licensee should produce a 

34 FEPs list at a level of detail that is broad enough to produce a systematically categorized but

35 manageable number of FEPs, yet specific enough to provide the complexity required for 

36 screening and/or modeling. From this set of potentially relevant FEPs, a licensee can define a 

37 subset of FEPs that are used to identify a probable future evolution of the disposal site (i.e., a 

38 scenario). The licensee can develop a connected sequence of FEPs describing the behavior of 

39 the system of concern (i.e., conceptual model). Remaining FEPs not incorporated into the 

40 original or central scenario may include disruptive events. Relevant FEPs not incorporated into 

41 the central scenario form the basis for alternative scenarios. Usually, the central scenario does 

42 not include disruptive events (e.g., earthquakes, volcanoes) while alternative scenarios of the 

43 same site may or may not include disruptive events, depending on the results of the scenario 

44 development.
2-27

1 The description of how the disposal system will function, given the FEPs comprising the 

2 scenario, is the conceptual model. A qualitative description of the conceptual model would 

3 include how the FEPs and significant barriers interact with one another and how the site 

4 functions (e.g., porous or fracture flow, precipitation, dissolution, degradation, erosion) for each 

5 scenario. Plausible conceptual models of a system are estimates of how the system may 

6 function. The distinction between a scenario and a conceptual model is not very sharp and can 

7 be somewhat blurred during the performance assessment process. It is important that the 

8 complete set of scenarios developed by a licensee represents the full range of possible future 

9 states of the disposal system, and that the complete set of associated conceptual models 

10 incorporate all of the retained FEPs. 

11

12 Scenarios are often assembled and classified based on their likelihood of occurrence. For 

13 example, the terms “central,” “base case,” or “nominal” scenarios are often used to describe the 

14 expected future state of the system. “Altered evolution” or “alternative” scenarios are typically 

15 considered less probable but still plausible, while implausible “what if” scenarios may be used to 

16 explore the robustness of the system. “Stylized scenarios” may be used to represent future 

17 human actions. Receptor scenarios are subsets of scenarios and describe the end process by 

18 which people may become exposed to radiation. A residential farmer receptor scenario, for 

19 example, is a general description of the pathways leading to possible exposure and of the 

20 behavior and lifestyle of the hypothetical receptor. The relationship between scenarios and 

21 receptor scenarios is discussed further in Section 2.2.4.1.

22

23 A safety function is defined qualitatively as a function through which a component of the 

24 disposal system contributes to safety and achieves its safety objective throughout the analysis 

25 timeframe. Safety functions are used in the top-down approach to scenario development.

26

27 Often, performance assessments will exclude scenarios from the base case based on the 

28 scenario development process but will evaluate scenarios called “what if” scenarios. These 

29 “what if” scenarios include scenarios of varying likelihood – some may be plausible whereas 

30 others are extremely unlikely. Many “what if” scenarios originate from specific concerns 

31 expressed by stakeholders. They can include very conservative scenarios that would normally 

32 have been rejected in a scenario development process due to very low probability of occurring 

33 or low impact on the results. If stakeholder interest is very high, scenarios that are usually 

34 excluded during the scenario development process could still be included in the performance 

35 assessment analyses; however, these scenarios would usually be labeled as very unlikely to 

36 occur and would not be considered as a valid alternative scenario by the analysts. With “what 

37 if” scenarios, distinctions between likely and very unlikely scenarios are not made since they 

38 were constructed outside of the scenario development process. As a consequence, the 

39 interpretation of the results may be difficult since it is not clear which results are realistic and 

40 which ones are very unlikely. Commonly in these cases, all the results are discussed; however, 

41 they are not included in the decision-making process since the results are considered unrealistic 

42 (even though some of the scenarios may have been valid alternative scenarios of future 

43 conditions and events). Therefore, it is generally recommended that licensees avoid “what if” 

44 type scenarios and complete the full scenario development process. If “what-if” scenarios are 

45 used, a qualitative or quantitative likelihood of the scenario should be developed to provide 

46 context for the results.

47
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1 2.5.1 Scope of Analysis

2

3 A performance assessment does not need to incorporate all FEPs for a disposal site. The 

4 performance assessment should include those FEPs that can either individually or in 

5 combination impact the disposal facility’s ability to meet the performance objectives. Because 

6 the significance of FEPs to performance may be difficult to determine a priori, FEP screening a 

7 performance assessment model development are usually iterative processes.

8

9 Some events and processes may be interrelated. For example, a large rainfall event may cause 

10 erosion, as well as damage to a protective layer that then allows more erosion in the future. A 

11 process such as erosion may be driven by events of variable frequencies and duration. Strict 

12 classification of phenomena into events or processes is not as important as ensuring that

13 licensees include the combinations of events and processes that may significantly impact the 

14 performance assessment.

15

16 Different approaches may be used to define and screen the FEPs relevant to a particular 

17 disposal facility. A licensee using internal (i.e., in-house) subject matter experts to define the 

18 scope is an example of informal definition of the scope of the assessment and is appropriate for 

19 simple sites and short-lived waste inventories. A formal process, as discussed in detail in the 

20 following sections, may use internationally defined lists of FEPs (NEA, 2002) with independent 

21 technical review, which is appropriate for complex sites and long-lived waste inventories. The 

22 purpose of defining and screening FEPs is to ensure completeness of the evaluation. Whether 

23 a formal or informal process is used to define and screen FEPs, the assessment should provide 

24 the following information:

25

26 • A clear description of the FEPs included in the assessment;

27 • A description of the FEPs that have been excluded from the assessment and the bases for 

28 their exclusion; and

29 • A description of the process for determining the significance of FEPs and a consideration of 

30 combinations of phenomena for FEPs excluded on the basis of significance.

31 Licensees may exclude FEPs from the assessment based on regulations or due to lack of 

32 relevance (probability) or limited impact (consequence) during the compliance, protective 

33 assurance, or performance period, taking into account the proposed inventory for the disposal 

34 site. The scope of a performance assessment model can be established through either a top35 down or bottom-up approach, as discussed in Section 2.5.3. The bottom-up approach involves 

36 the development of an FEP list followed by a screening process to determine which FEPs may 

37 apply. The bottom-up approach is commonly used for complex sites. The top-down approach 

38 involves the addition of content to a performance assessment model based on the input from 

39 subject matter experts. Both approaches may be iterative.

40

41 A licensee may use an iterative process for FEP identification and selection. As the 

42 performance assessment model is developed, new information needs may be identified which 

43 will allow a licensee further refine the scope of the performance assessment. It may be possible

44 for a licensee to demonstrate that the performance assessment is complete without a formal 

45 FEP identification analysis. This is more likely for simple sites and short-lived waste inventories. 

46 For complex sites and long-lived waste, the likelihood decreases that the performance 

47 assessment can be demonstrated to be complete without an iterative FEP identification and 
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1 screening process. Example 2.3 provides some guidance on determining whether a site is 

2 simple or complex.

3

4 Lists for FEPs (FEPs lists are discussed in Section 2.5.3.1.1; a generic FEP list for LLW 

5 disposal is found in Appendix C) can be quite extensive. Some amount of aggregation may be 

6 necessary to make the screening, assessment, and implementation process manageable. On 

7 the other hand, lists that are too general will not be useful, as key FEPs may not be included at 

8 the implementation stage or may not be included with appropriate responses and functional 

9 behavior because of the coarseness of their definition. The licensee should consider whether 

10 inclusion of the FEP at the next level of detail would improve the assessment of system 

11 performance. In either case, a licensee should demonstrate that the FEPs included in the 

12 analysis are sufficiently comprehensive. Because LLW disposal has been performed at facilities 

13 throughout the world, the licensing and operational experiences of other disposal facilities can 

14 provide a good starting point for licensees at a new facility. 

15

16

17 The analyses required to develop scenarios are closely linked with conceptual model 

18 development and model abstraction. FEP screening and scenario development are increasingly 

19 used as a means to build confidence in the scope of a performance assessment. The more 

20 complete the licensee’s analysis, the more likely they will be able to gain confidence from the 

21 reviewers and stakeholders. When a licensee develops scenarios, the scenarios can be used 

22 to focus stakeholder attention on the key technical issues. Scenarios provide an important area 

23 for communication among various stakeholders and an opportunity to discuss and reach a 

24 consensus on areas of specific importance. Licensees can discuss their use of scenarios and

25 provide a rich and accessible means for public involvement.

26

27 One of the main purposes of FEP screening and scenario development for a radioactive waste 

28 disposal system is to use scientifically-informed expert judgment to guide the development of 

29 descriptions of the disposal system and its future behavior. Scenario uncertainty is handled 

30 directly by describing alternative future states of the system and by allowing for a mixture of 

31 quantitative analysis and qualitative judgments; however, it is not an attempt to predict the 

32 future. The aim is to investigate the importance of particular sources of uncertainty and provide 

Example 2.3: Is my site simple or complex?

Simple sites are generally characterized by few disruptive processes, limited fast transport 

pathways, relatively homogeneous geology, high stability, and stable climatic conditions. 

Complex sites have higher uncertainty, driven by more disruptive processes (individually and 

with cumulative effects); complex geology including fast pathways such as fractures; 

decreased stability; and more highly variable climatic conditions. When there are more 

processes that can lead to significant releases, there will likely be greater complexity in the 

performance assessment of the site. The interpretation of site complexity will also be 

influenced by the type of waste disposed. If the inventory of waste to be disposed of is limited, 

relatively simple conservative analyses may be appropriate even for a complex site. Disposal 

of significant quantities of long-lived waste decreases the confidence that stability can be 

assured and increases the variability in climatic conditions that could be significant within the 

assessment period because of the consideration of longer timeframes. In addition, the longer 

timeframes mean that unlikely disruptive events will be more likely to occur within the period of 

assessment.
2-30

1 meaningful illustrations of future conditions to assist in the decision-making process (NEA, 

2 2001). 

3 2.5.2 Role of Qualified Specialists

4

5 The technical basis for FEPs screening and the approach to scenario formation depends to a 

6 significant extent upon the judgment of the individuals performing the study. The completeness 

7 of a FEPs analysis can be enhanced by including a broad range of people and diverse sources

8 of information. It is usually better to identify and categorize a range of broadly-defined FEPs to 

9 ensure comprehensiveness of the FEP process. If appropriate expertise is not available 

10 internally, a licensee may need to seek the input from experts external to their organization (i.e., 

11 professionals in the field of concern such as hydrogeologists, geomorphologists, seismologists, 

12 chemists, etc.). Licensees should obtain information from different sources, a variety of 

13 methods, and from all relevant disciplines. Licensees should document decisions based on

14 expert judgment. The qualifications of the analysts performing the FEPs screening are also 

15 very important. 

16 2.5.3 Approaches

17

18 There are several methods that can be used by licensees to identify FEPs, screen FEPs, and 

19 construct scenarios. A licensee should ensure that the process is systematic, comprehensive, 

20 logical, traceable, and transparent. Different approaches to scenario formation include bottom21 up, top-down, and a mixture of the two. 

22

23 In bottom-up scenario formation, the screened FEPs are combined to form a limited number of 

24 scenarios for consequence analysis. Sandia National Laboratory developed a structured 

25 approach to scenario selection for the NRC (NRC, 1993a). Initially, this approach was applied 

26 for disposal of HLW, but was later expanded to disposal sites for other radiological source 

27 terms. When using a bottom-up approach, the licensee should develop a comprehensive list of 

28 FEPs as a starting point. Development of a comprehensive list typically involves the use of 

29 generic FEPs lists and the identification of other site-specific FEPs. This is followed by a 

30 screening process to exclude certain FEPs from further consideration. The retained FEPs are 

31 combined into scenarios for evaluation. FEPs screening criteria may include prohibition by

32 regulation, low probability, or limited consequence. The scenario in which disruptive events do 

33 not occur is usually identified as the central scenario and represents a continuation of the 

34 estimated present day undisturbed conditions. However, in some cases the central scenario 

35 could include disruptive processes if they are expected in the normal future evolution of the site. 

36 The projection of present day conditions into the future may include dynamic effects. In other 

37 words, a licensee should not interpret ‘undisturbed’ as ‘static’. For example, degradation of 

38 engineered barriers may be part of a central scenario.

39

40 In the top-down approach, licensees develop scenarios based on analyses of how the safety 

41 functions of the disposal system may be affected by possible events and processes. First, the 

42 licensee should identify the safety functions of the waste disposal system and then consider the

43 combination of conditions that could affect one or more of the safety functions. The top-down 

44 scenario development approach used in the assessment of HLW disposal consisted of iterative 

45 steps and is described in detail in NEA (1992). While the NEA (1992) document may serve as 

46 an example of the process, a lower level of effort should be appropriate for LLW disposal. 

47
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1 Scenarios derived from a top-down approach typically include uncertainties potentially affecting 

2 the safety functions (e.g., barrier performance). However, in order to ensure completeness of 

3 the processes and events used to establish scenarios, a licensee may need to take advantage 

4 of systematic and comprehensive databases of the underlying FEPs. In other words, 

5 information typically used in a bottom-up approach may be useful for a licensee to consider in a 

6 top-down approach.

7

8 Regardless of the method used for developing the scenarios, phenomena and barrier 

9 components that could significantly influence the performance of the disposal system should be 

10 addressed in the assessment. Hence, a licensee should show that potentially significant 

11 transport pathways have been considered and that possible evolutions of the disposal system 

12 have been taken into account. Licensees should give specific consideration to events that could 

13 occur repetitively during the assessment timeframe (e.g., droughts, floods, and earthquakes). 

14 They should consider the performance of the disposal system under both present and future 

15 conditions.

16

17 All of the methodologies share the same basic approach, namely that a central scenario is 

18 considered a starting point. A licensee should demonstrate that relevant FEPs have been taken 

19 into account when developing alternative scenarios. Alternative scenarios are generally less 

20 likely than the central scenario. Alternative scenarios can be developed on the basis of 

21 disturbances to the normal evolution of the disposal system or to represent different amounts of

22 degradation of the safety functions. If uncertainty is high, it may be difficult to classify different 

23 scenarios as central or alternative.

24

25 A licensee should explain and justify which scenarios are regarded as representing the 

26 expected evolution of the system, and which scenarios address FEPs having an unlikely

27 probability of occurrence. The range of future physical environmental conditions at the site and 

28 the range of potentially exposed groups should be identified. Licensees and regulators usually 

29 assume that humans will be present and that they will make use of local resources. It is 

30 appropriate for licensees to assume that humans in the future will have similar habits to present 

31 humans, except where this is clearly inconsistent with the assumed variations in climatic 

32 conditions at a site. Section 2.2.4.2 provides additional guidance to licensees on the 

33 development of receptor scenarios. 

34

35 If disruptive events are represented in a probabilistic performance assessment model, a 

36 licensee should conservatively assign the probabilities of an event occurring if the frequency of 

37 the event is not known. Formal expert elicitation may be necessary to define event frequencies. 

38 Alternatively, the licensee may represent a range of scenarios to capture the potential effects of 

39 disruptive events, without weighting the potential scenarios with probabilities. Some 

40 understanding of the likelihood and frequency of disruptive events is still likely to be necessary 

41 for decisionmakers, unless the dose projections resulting from the evaluation of disruptive 

42 events are below regulatory limits. Section 2.2.3.1 provides additional information of expert 

43 elicitation. 
2-32

1 2.5.3.1 Bottom-Up Approach

2 2.5.3.1.1 Identification and Categorization of FEPs

3

4 A large number of FEP lists, catalogs, and databases have been developed in different 

5 countries and encompass a range of radioactive waste types, disposal system designs, and 

6 geological environments. The size of these FEP lists varies, as do the content and level of 

7 detail of entries. A listing of international published FEP lists, catalogs, and databases have 

8 been compiled by NEA (2002). Licensees should perform a literature search when developing 

9 their scenario analyses because FEP lists continually change as scientific information is 

10 developed. Licensees may want to consider the generic FEP list for LLW disposal that the NRC 

11 staff developed in Appendix C of this document.

12 2.5.3.1.1.1 Identification and Categorization of Present FEPs

13

14 In order for a licensee to identify FEPs important to LLW disposal at their site, they should 

15 review existing FEP lists. The first step in the identification and categorization process consists 

16 of reviewing the information provided by the assessment context, as it relates to the 

17 performance assessment of the site. For example, legislation or regulatory guidance may 

18 predetermine some individual or categories of FEPs that a licensee should consider (e.g., future 

19 human activity, the biosphere system, or the future climate states). A licensee could use a

20 previously compiled generic FEPs list as a starting point to save time and effort. They will need 

21 access to information and documents pertaining to the characterization and description of the 

22 site being considered. 

23

24 The identification and categorization process may be iterative. Initially, the licensee may identify 

25 and categorize the FEPs assuming the site has static conditions. Often the central scenario can 

26 be constructed from present FEPs if the proposed disposal site is in an area of geological and 

27 geomorphological stability. Next, dynamic conditions such as natural degradation processes 

28 are assumed to affect the natural and engineered barriers. This approach is described in the 

29 following steps: 

30

31 (1) Use a generic list to identify features and processes that are currently present. For this 

32 step, features or processes likely to be there in the future or that existed only in the past 

33 should be temporarily set aside until the next phase, as discussed in Section 2.5.3.1.1.2. 

34 (2) Eliminate features and processes in Step 1 that have no potential to affect performance. 

35 (3) Add features and processes that have occurred relatively recently (i.e. within 100 years); 

36 not events that have occurred in the long distant past (e.g., glaciers) or that may occur in 

37 the long-term future (these will be identified later as discussed in Section 2.5.3.1.1.2). 

38

39 The identification step is followed by, and closely linked to, the categorization step whereby 

40 FEPs with certain similar properties are grouped together. Categorization provides a framework 

41 for organizing the scenario development process and the subsequent assessment. In addition, 

42 it provides information on interactions and interrelationships between FEPs. The primary 

43 objective is to uncover missing factors, therefore, categories that examine the system from 

44 different viewpoints should be used (NEA, 1992). 

45

46
2-33

1 Examples of categorization include: 

2

3 • Dividing FEPs related to either natural, human, or waste into separate categories; 

4 • Providing categories according to the time scale during which different events and 

5 processes occur; 

6 • Providing categories of FEPs related to either near-field, far-field, or biosphere; 

7 • Categorizing FEPs according to combinations of different magnitudes of the probability 

8 and the consequence; and 

9 • Classifying according to scientific discipline.

10 2.5.3.1.1.2 Identification and Categorization of Future FEPs

11

12 In the previous section, the NRC staff suggested that a licensee obtain the first FEP list by 

13 assuming the proposed disposal site is not intrinsically dynamic. However, the NRC staff 

14 recognizes that most proposed disposal areas are dynamic (e.g., shifting wind deposition and 

15 erosion). Each disposal site will be in its own state of equilibrium or disequilibrium. Most sites 

16 are chosen because the area is close to equilibrium (i.e., in an area of geological and 

17 geomorphological stability). Events that do occur may be limited in scope, and therefore, are 

18 unlikely to have a significant impact on performance of the proposed site. 

19

20 For some sites, a few hundred years may be sufficient to result in significant changes to the 

21 disposal site, which would make use of a present-day FEP list incomplete. Changes can occur 

22 over different temporal and spatial scales. Some, such as the clearance of woodland and its 

23 replacement by farmland, might occur over relatively short timescales. Others, such as the 

24 closing of open water bodies by the deposition of sediments and changes to the topographical 

25 landscape, may occur over longer timescales and can be considered to be gradual processes. 

26

27 The NRC staff suggests that a licensee continue the identification process after the completion 

28 of the steps listed in the previous section. Features and processes should be identified that 

29 may occur during the analysis timeframe based on documentation and studies of the past 

30 natural history of the area. Processes identified should include processes that are expected to 

31 occur over the entire time period of interest (e.g., leaching, dissolution, fracturing). The analyst 

32 or qualified specialist should always be aware that this step is for the purposes of identification 

33 and categorization of potential FEPs; systematic screening of FEPs occurs in the subsequent 

34 step. Some processes may have slow to moderate rates but may be episodic in nature. It 

35 would be appropriate for a licensee to consider episodic processes if the effect of the process 

36 on the disposal system could be significant. 

37

38 A licensee can identify future FEPs through a qualitative exercise that should not necessarily 

39 involve original research or an effort that would take an extended time to complete; however, it 

40 will require the time of qualified specialists and may involve literature searches. Examination of 

41 past local natural history can often result in the identification of processes that may have been

42 relevant at a site in the past and could be an indicator of potential future behavior. This past 

43 history can be used as a basis to identify and categorize a FEP for further consideration. For 

44 example, a present day desert environment may have had higher average annual precipitation 

45 rates in the past with a higher density of rivers and lakes (e.g., Lake Manly covering Death 

46 Valley in the past) and different distributions of fauna and flora. If the past environment is
2-34

1 considered plausible in the future for the analysis timeframe considered, the associated FEPs 

2 from this past environment would be used as a basis to identify and categorize a FEP for further 

3 consideration. Some of the FEPs from the present will persist in the future, however, some 

4 processes may stop (e.g., groundwater recharge due to permafrost) and other features may 

5 appear (e.g., a forest which was previously scrubland). 

6

7 As discussed for the performance period analyses in Section 7.3, more low-frequency events 

8 may be included in the FEPs list for the performance period than may be included in the FEP list 

for the protective assurance period (e.g., an event with a 10-6 9 per year frequency would be 

10 unlikely in a 10,000 year period but likely in a 1,000,000 year performance period). In addition, 

11 if the same set of FEPs is appropriate for both analyses, they may need to be represented 

12 differently in each analysis due to the longer timeframe and potentially altered conditions. 

13 2.5.3.1.2 Systematic Screening of FEPs

14

15 A licensee should perform systematic screening of identified FEPs and should determine the 

16 subset of FEPs important to disposal system performance, documenting the basis for excluding 

17 or including a FEP for further consideration. A licensee should use clear and justifiable criteria. 

18 Screening approaches can vary from using some type of 'importance' ranking scale (e.g., 0 

19 through 10), or simply an 'include' or 'exclude' system. Exhaustive analysis of importance is not 

20 necessary for FEP screening for LLW disposal. Simple calculations or bounding estimates can 

21 assist with the selection. Licensees should document the assumptions, data, or empirical 

22 information used to make these determinations. They should perform the screening process on 

23 a site-specific basis, and evaluate FEPs or FEP categories one at a time against a screening 

24 criterion. During the screening process, interactions between FEPs should be considered. If 

25 there are uncertainties as to whether a FEP can be screened, then it should be retained. The 

26 FEP can be revaluated at a later stage in the screening and evaluation process. 

27

28 The following subsections discuss various aspects of screening of FEPs. The regulatory aspect 

29 is considered first since these screening criteria are legal in nature and take precedent over 

30 other screening considerations.

31 2.5.3.1.2.1 Regulatory

32

33 FEPs can be screened out based on inconsistency with applicable regulations. The NRC 

34 approach to analyzing timeframes is based on a compliance period of 1,000 years, a protective 

35 assurance period through 10,000 years following closure of the disposal site, and a 

36 performance period of undefined duration during which a licensee must demonstrate that effort 

37 has been made to minimize releases to the extent reasonably achievable. The performance 

38 assessment should reflect changes in FEPs of the natural environment such as climatology, 

39 geology, and geomorphology. The scope of the FEPs considered does not need to be 

40 expanded unless information is available to do so. 

41

42 Section 10 CFR 61.50 is an important regulation that affects the FEPs screening process, as it 

43 provides the disposal site suitability requirements for the land disposal of LLW. The process to 

44 determine if some of these criteria will be met is complementary to the FEPs process. The 

45 criteria from 10 CFR Part 61.50 associated with the process of FEPs analysis are 

46 10 CFR 61.50(a)(2)(i-iv), 10 CFR 61.50(a)(3) and 10 CFR 61.50(a)(4)(ii-iv). 

47
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1 Appendix B includes hazard maps related to the features and phenomena of these criteria. The 

2 hazard maps provide a coarse estimate of impacted areas. The hazard maps may be used to 

3 inform reviews of FEP screening associated with 10 CFR Part 61.50 requirements.

4

5 Site suitability requirements are treated differently in 10 CFR Part 61 depending on the type and 

6 timeframe. Historically, hydrological FEPs were the key drivers of poor performance of early 

7 (pre-10 CFR Part 61) LLW disposal facilities. Therefore, hydrological characteristics of the site 

8 are treated differently from other site characteristics. Regardless of the type of waste disposed, 

9 the hydrological site characteristics are either required to be present for disposal for the 

10 500-year timeframe (e.g., the site must be generally well-drained) or should not be present for 

11 the 500-year timeframe because they will adversely affect the performance of the disposal site 

12 (e.g., waste may not be disposed of in the zone of water table fluctuation). 

13

14 After the 500-year timeframe, the evaluation of hydrological site characteristics can consider the 

15 impact of the characteristics on a licensee’s ability to demonstrate that the Subpart C 

16 performance objectives would be met. Disposal systems with water challenges in the present 

17 day and foreseeable future are not amenable to stability and defensible modeling and 

18 assessment. Table 2-1 provides the analyses that should be completed for the disposal site 

19 suitability requirements in 10 CFR 61.50 that are not excluded based on FEP screening.

20

21 Table 2-1 Analyses Required for Included FEPs Based on 10 CFR 61.50

Characteristic Type Timeframe (years) Analyses Hydrologic

Less than 500 None – site not suitable for 

disposal of low-level waste

Between 500 and up to 10,000 Stability analyses

Performance assessment1

Intruder assessment1

Greater than 10,000 Performance period analyses

Non-hydrologic

Less than 10,000 Stability analyses2

Performance assessment1

Intruder assessment1

Greater than 10,000 Performance period analyses

1 22 If the stability analyses show that the site is stable with the included FEPs (e.g., design-based approach) 

23 then these FEPs would not need to be included in the performance assessment and intruder assessment.

2 24 The stability analyses for included non-hydrologic FEPs may be based on demonstrating that the 

25 10 CFR 61.41 and 10 CFR 61.42 performance objectives will be met.

26

27
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1 Screening of FEPs Based on the Requirements in 10 CFR 61.50

2 10 CFR Part 61.50(a)(4)(ii): Areas must be avoided having known natural resources 

3 which, if exploited, would result in failure to meet the performance objectives of 

4 Subpart C of this part. 

5 Categories of FEPs should be reviewed for known natural resources (i.e., for natural material 

6 currently considered to be a resource and whose range and scope is currently known). 

7 Calculations or estimates on the adjustment in value of a natural resource beyond a decade are

8 not required. If review of the FEPs determines that natural resources of the type described in 

9 10 CFR Part 61.50(a)(4) exist near a proposed disposal site and are likely to be exploited, 

10 resulting in failure to meet the performance objectives of Subpart C, then the site is not qualified 

11 for LLW disposal.

12

13 The NRC staff recommends that a future licensee survey and evaluate an area for potentially 

14 exploitable natural resources within a radius of five kilometers from the future boundary of a 

15 land disposal facility. For potential disposal sites located within valleys or in a riparian setting, a 

16 licensee should consider surveying and evaluating further than five kilometers in the upstream 

17 direction since gravitational forces on air, water, and materials may allow disruptive influences 

18 from the exploitation of natural resources to travel greater distances downstream.

19

20 10 CFR Part 61.50(a)(2)(i): Waste disposal shall not take place in a poorly drained site 

21 or a site subject to flooding or frequent ponding, or in a 100-year flood plain, coastal 

22 high-hazard area or wetland, as defined in Executive Order 11988, “Floodplain 

23 Management Guidelines”.

24

25 A licensee must demonstrate that the disposal site is not located in a projected 100-year 

26 floodplain or a permanent or periodic wetland for a period lasting 500 years after 

27 closure. Section 2.4.1, Appendix A in NUREG-1200 provides additional guidance. For the 

28 ensuing period beginning 500 years after closure of the disposal site the licensee may, in 

29 accordance with 10 CFR 61.50(a)(3), demonstrate that the condition will not exist or that the 10 

30 CFR 61.41 and 10 CFR 61.42 performance objectives can be met irrespective of the 

31 condition. There are two main geomorphic zones to consider: (i) coastal areas, and (ii) flood 

32 plains and wetlands.

33

34 Coastal Areas

35 Coastal high-hazard areas are described as having a high hazard status due to various factors 

36 including proximity to the coastline and elevation of the site. Both the present and future should 

37 be considered and estimated for this criterion. Two aspects are important for the present and 

38 the future: Vulnerability to erosion and vulnerability to flooding. 

39

40 Vulnerability to Erosion: Possible FEPs that could cause detrimental erosion rates include 

41 increased rainfall and/or an increased gradient (and therefore, an increase in the erosional 

42 power of any runoff) due to changes in future climate states. As for the increased precipitation, 

43 the magnitude of the probable maximum precipitation/probable maximum flood (PMP/PMF) is 

44 not likely to change significantly in a wetter climate, based on the conservatism associated with 

45 the estimation of the PMP and the proper computation of the PMF. If the FEP, future erosion,

46 cannot be excluded, it should be included in the analyses described in Table 2-1. 

47
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1 A drop in sea level could change the erosion rates of certain coastal areas due to an increase in 

2 topographic gradients. Sea level variations have been documented by various organizations 

3 and there is a general consensus that sea levels have dropped to 110 to 120 m below current 

4 sea level during the glacial periods of the current Quaternary ice age (NOAA, 2008; Gornitz, 

5 2007; IPCC, 2001). If a proposed disposal facility is associated with a coastal area, the licensee 

6 should perform an erosion analysis assuming a sea level drop of 120 meters (m). Other drop 

7 magnitudes may be evaluated if a licensee provides adequate technical basis for the magnitude 

8 of the sea level drop. The licensee needs to analyze subsequent changes in erosional force 

9 due to sea level drop and evaluate the estimated effects on a future disposal cell. 

10

11 Vulnerability to Flooding: FEPs that could cause detrimental flooding and may exist in future 

12 climates include increased high tides due to higher sea levels and potentially higher storm 

13 surges. Partial deglaciation of the Greenland ice sheet and the West Antarctic ice sheet cannot 

14 be excluded within a million year timeframe and would contribute a 4 to 6 m or more sea level 

rise during the performance period. For performance period analyses on the order of 106 15 years, 

16 high-tide levels near a proposed disposal site could be estimated assuming a sea level increase 

17 of 5 m. A licensee may provide a technical basis for the variation in sea level used in the 

18 analysis for performance period timeframes longer than 10,000 years but shorter than one 

19 million years. If the FEPs cannot be excluded, they should be included in the associated 

20 analyses, as described in Table 2-1.

21

22 Even on a qualitative level, the probability of future tsunami hazards for a particular location 

23 would be too difficult to estimate over the performance period. The level of effort required to 

24 evaluate future tsunami hazards is likely to be excessive and technical analyses is not 

25 warranted considering the short duration that a potential disposal site would be flooded by a 

26 tsunami. 

27

28 Flood Plains and Wetlands

29 Flooding and additional water may impact the performance of a disposal system. Important 

30 aspects include: (1) the location of the current 100-year flood plains and wetlands in relation to 

31 the potential disposal site, and (2) how the current boundaries of the flood plains could change 

32 over the compliance, protective assurance, and performance period, if performance period 

33 analyses are required. 

34

35 Flooding can directly influence the performance of the disposal system or trigger another 

36 process, such as erosion, that can impact the performance of the disposal system. Higher 

37 topographic areas generally have fewer floodplains and wetlands; the effects of erosion usually 

38 become problematic before flooding. On the other hand, depositional areas and relatively flat 

39 areas would have a potential of becoming flood plains and wetlands. 

40

41 Knowledge of the past surface hydrology in the potential disposal area can be used to 

42 strengthen the qualitative FEP assessment. For example, the surface hydrology of southern 

43 Louisiana is quite diverse. The Mississippi River and other rivers have changed river channel 

44 beds numerous times and 100-year floodplains would change in concert with river course 

45 evolution. Disposal sites for significant quantities of long-lived waste should not be located in 

46 the vicinity of these channels. 

47

48 Features and processes identified in studies of the past natural history of the area often provide 

49 an indication of what processes will likely be present in the future. If, for example, an area has
2-38

1 been frequently flooded during and immediately after past glacial periods to create intermittent 

2 glacial lakes (e.g., Lake Missoula, Lake Lewis, and Lake Bonneville), the probability is high that 

3 similar processes and events will occur during the performance period. 

4

5 Geomorphologic evidence may provide useful data on large paleofloods. The evidence can 

6 include slack-water deposits, scour lines, high-water marks, and undisturbed areas (Stedinger 

7 and Cohen, 1986). The advantage of paleoflood data is that it can improve a probabilistic flood 

8 hazard assessment and extend short or non-existent flood records to include the last 1,000 to 

9 10,000 years (Klinger and England, 2013). Paleoflood data can improve the estimates of the 

10 magnitude and frequency of large floods. Licensees should develop an assessment of the 

11 potential for future flooding and for local floodplain and wetland formation in the future.

12

13 In addition, paleoflood data may provide evidence of large, dynamic floods, similar to the 

14 “outburst” floods that occur when the water of dammed glacial lakes are suddenly unobstructed. 

15 Such massive, violent floods could destroy a disposal site located close to the surface and 

16 thereby fail to meet the site stability performance objective (i.e., 10 CFR 61.44). Potential sites 

17 located in areas affected by previous glacial processes and/or subjected to previous massive 

18 flooding would require additional analysis and careful evaluation. Although the site and 

19 immediate area would not be habitable for humans and a potential receptor, the contaminated 

20 material may remain in a relatively concentrated form while being deposited downstream near 

21 more habitable locations where receptors could exist. Potential disposal sites located in areas 

22 subject to previous massive flooding may require deeper disposal and additional man-made 

23 barriers (e.g., engineered surface covers) to mitigate the destructive force of large floods. 

24

25 10 CFR Part 61.50(a)(2)(iii): The disposal site must provide sufficient depth to the water 

26 table that groundwater intrusion, perennial or otherwise, into the waste will not 

27 occur. The Commission will consider an exception to this requirement to allow disposal 

28 below the water table if it can be conclusively shown that disposal site characteristics will 

29 result in molecular diffusion being the predominant means of radionuclide movement 

30 and the rate of movement will result in the performance objectives of Subpart C of this 

31 part being met. In no case will waste disposal be permitted in the zone of fluctuation of 

32 the water table.

33

34 A potential site located in an area with a relatively large number of wells and data on 

35 groundwater fluctuations should be sufficient for a licensee to perform FEP screening 

36 associated with depth to water table. An area without sufficient data on the local water table 

37 and associated groundwater fluctuations may need additional site characterization. Features of 

38 interest for a licensee are current water table elevation and soil types, including sediment layers,

39 with the potential for creating a perched water zone. A licensee should identify the 

40 hydrogeologic units of the layers below the disposal site. For example, a caliche layer or dense 

41 clay may cause temporary perched zones that may only exist on a seasonal basis. It should be 

42 sufficient for a licensee to identify such features in combination with meteorological data, 

43 topography of the disposal system, and processes of the local hydrogeology in their analysis of 

44 current water table behavior. 

45

46 The requirement at 10 CFR 61.50(a)(2)(iii) is closely related to the requirement at 

47 10 CFR 61.50(a)(2)(i) (i.e., poor drainage and flooding). Future water table elevation is partially 

48 a function of the current landscape and potential changes to the landscape in the future. As

49 with the requirement in 10 CFR 61.50(a)(2)(i), topography, hydrogeology, erosion rate, and 
2-39

1 precipitation rates are determining factors for water table elevation. With updated studies on

2 long-term trends, past natural history, and PMP/PMF, it should be possible for a licensee to 

3 develop an assessment to bound potential water table changes. Water tables in topographic 

4 highs or steep areas may have a greater degree of seasonal variability, but generally would be 

5 expected to have a smaller potential to significantly change during the long term than would a 

6 relatively flat, depositional area that is susceptible to drought and flooding. Depending on the 

7 hydrogeology of the region, a low-lying or depositional area today can experience a rising water 

8 table or even become a flood plain in a future wetter climate.

9

10 A number of tools and codes are available to licensees to support analyses and assessments of 

11 groundwater discharge that can be used to assist in making a determination regarding long-term 

12 water table fluctuations. Knowledge of the paleoclimatolgy, paleopedology, and past hydrology 

13 of the potential disposal area can support the assessment of this requirement. For example, 

14 arid regions with wetter conditions in the past may have had higher water tables, but never 

15 close enough to the surface to have posed a threat to a potential future disposal site. However, 

16 evidence of the past has revealed that features such as wetlands and lakes had previously 

17 existed in currently arid areas. Death Valley is a good example: generally dry and waterless, 

18 this valley had been flooded in the past (Lake Manly) and was persistent enough to support a 

19 native population. Death Valley is currently dry because of the extremely low rate of 

20 precipitation at and near the area. However, a geomorphologist, or a qualified specialist, would 

21 recognize that its topography is ideal for lake formation, if more water were to become available 

22 (as evidence from its geological past has shown). Other areas may receive a high rate of 

23 rainfall, but the topography and geology will not allow for lake formation in the future. 

24

25 In general, the climatic history during the Quaternary period has been of a cyclic nature 

26 consisting of glacial and interglacial stages. Features and other evidence of the past natural 

27 history can assist in reconstructing the full cyclic climatic history including precipitation rate and 

28 temperature ranges. Potential disposal sites further to the south may never have experienced 

29 glacial coverage in recent geologic history. Knowledge and evaluation of the full cyclic 

30 extremes, and updated studies of topographic trends (e.g., surface uplift, tectonic subsidence, 

31 erosion vulnerability, etc.), can help a licensee in developing an assessment that could 

32 potentially bound long-term water table fluctuations, even if these fluctuations may have been 

33 extreme. Potential disposal sites further to the north may have been previously covered by 

34 glaciers that cause disruptive surface geologic processes so that the requirement 

35 10 CFR Part 61.50(a)(4)(iv) becomes the primary focus. 

36

37 10 CFR Part 61.50(a)(2)(iv): The hydrogeologic unit used for disposal shall not 

38 discharge groundwater to the surface within the disposal site.

39

40 The criterion in 10 CFR 61.50(a)(2)(iv) is closely related to the criterion in 10 CFR 61.50(a)(2)(i) 

41 and 10 CFR 61.50(a)(2)(iii). Groundwater discharge areas are partially a function of the current

42 landscape and any potential changes to the landscape in the future. As with the requirement in 

43 10 CFR 61.50(a)(2)(i), topography, hydrogeology, erosion rate, and precipitation rates are 

44 determining factors if water will discharge in a certain area. Any time a water table rises higher 

45 than the ground surface, groundwater will discharge to the surface. The assessment done for 

46 10 CFR 61.50(a)(2)(iii) should assist in determining if the hydrogeologic units used for disposal 

47 are able to discharge groundwater to the surface within the disposal site under current or 

48 projected future conditions. A licensee should compare the approximate rise and fall of the 

49 water table assessed in 10 CFR 61.50(a)(2)(iii) to the changes in topography. 
2-40

1

2 Different combinations of trends should be considered by licensees: net mass change 

3 (deposition/erosion) and net change on water table (rising/falling). For example, it is possible 

4 that areas with relatively stable long-term water table levels may also experience relatively high 

5 erosion rates; the net effect is a water table advancing to the ground surface. It is unlikely, 

6 however, that such sites would fulfill the requirement of 10 CFR 61.50(a)(4)(iv) which states that 

7 areas with significant surface geologic processes must be avoided. Site characterization should 

8 provide sufficient data to a licensee on geology, hydrogeology, topography, paleoclimates, and 

9 features and processes to qualitatively assess the plausibility of groundwater discharge 

10 occurring sometime in the future. Evidence supporting the existence of past groundwater 

11 discharge areas, such as calcite deposits or diatomite, vastly increases the chances that the 

12 disposal area will have groundwater discharge occurring during the analyses timeframes. 

13

14 10 CFR Part 61.50(a)(4)(iii): Areas must be avoided where tectonic processes such as 

15 faulting, folding, seismic activity, or volcanism may occur with such frequency and extent 

16 to significantly affect the ability of the disposal site to meet the performance objectives of 

17 Subpart C of this part, or may preclude defensible modeling and prediction of long-term 

18 impacts.

19

20 Tectonic processes, such as faulting, folding, seismic activity, and volcanism, are processes 

21 that might lead to short disruptive events, but unlike the processes associated with other 

22 disposal site suitability requirements, these processes are linked with plate tectonics that

23 proceed at very slow rates. The assessment and evaluation carried out for the compliance and 

24 protective assurance periods to approximate the frequency and extent of faulting, folding, 

25 seismic activity, and volcanism in a particular area, as discussed in NUREG-1200, should not 

26 lead to significantly different results when applied to the performance period. These are long27 term processes and unlike the processes related to the previously discussed site suitability 

28 requirements, water is not directly involved. Tectonic processes are large-scale processes and 

29 the frequency and extent of seismic and volcanic activity will not vary much when extrapolated 

30 to the performance period. The licensee should conduct a thorough assessment of 

31 10 CFR 61.50(a)(4)(iii) based on the guidance in NUREG-1200 and NUREG-1199.

32

33 10 CFR Part 61.50(a)(4)(iv): Areas must be avoided where surface geologic processes 

34 such as mass wasting, erosion, slumping, landsliding, or weathering occur with such 

35 frequency and extent to significantly affect the ability of the disposal site to meet the 

36 performance objectives of Subpart C of this part, or may preclude defensible modeling 

37 and prediction of long-term impacts. 

38

39 FEPs that could cause changes in the frequency and extent of the surface geologic processes 

40 are climate, topography, geology, soil type, type of flora, water chemistry, and the matrix in 

41 which the water flows. Short-term, large-scale topographic change normally does not occur 

42 unless associated with a disruptive event. The frequency and extent of water (i.e., precipitation 

43 rate) has the greatest potential for driving rates of mass wasting, erosion, slumping, landsliding, 

44 or weathering. NUREG-1623 provides guidance on surface geologic processes. FEPs that 

45 could cause unsuitable rates of erosion include increased rainfall and/or an increased gradient 

46 (and therefore, an increase in the erosional power of any runoff). As for the increased 

47 precipitation, the magnitude of the PMP/PMF is not likely to change significantly in a wetter 

48 climate, based on the conservatism associated with the estimation of the PMP.

49
2-41

1 A licensee can use technical assessments to assist in estimating the rates of surface geologic 

2 processes and determine if the FEP should be included in the analyses. The field of 

3 geomorphology has evolved significantly over the last half century and numerous technologies 

4 are available today to facilitate studying surface geologic processes including various programs 

5 such as hydrologic codes, erosion codes, and landscape evolution codes (e.g., CHILD and 

6 SIBERIA). Section 5.0 provides more detail on the technical assessment of erosion. 

7 Landscape evolution is especially important with respect to potential changes to nearby streams 

8 and river channels in unconsolidated material. Given sufficient precipitation, large rain events, 

9 and time, the watershed of an area can change considerably. Specific parts of a facility may 

10 end up close to stream channels and with an increased gradient. Drainage patterns may 

11 change if variations in climate circulation patterns are great enough. Erosion/deposition rates 

12 may vary spatially and temporally across the site. Long-term erosion often concentrates in 

13 gullies which do not uniformly erode over their entire length. Peak erosion depth may translate 

14 into a total breach of a portion of the disposal facility. 

15

16 The massive ice covers of the glacial periods were a source of extensive, large-scale surface 

17 geologic processes. Figure B-9 shows the approximate area covered by glaciers during the last 

18 three glacial periods of the current Quaternary ice age (i.e., the Wisconsin, Illinoisan, and Pre19 Illinoisan glacial periods). Glaciers can cause very disruptive surface geologic processes, and 

20 potential sites located in areas affected by previous glacial processes could require additional 

21 analysis and careful evaluation. 

22

23 Evidence of the past natural history of the area, or from an appropriate natural analog, should 

24 provide support to licensees for excluding, or retaining, the FEPs associated with the surface 

25 geologic processes. Pedogenic processes, biotic activities, and bioturbation are all processes 

26 that may impede or accelerate surface geologic processes. Thick root systems from certain 

27 plants are known to greatly reduce the erosional force; however, it is difficult to rely on the 

28 continuous presence of a specific plant that might be needed for longer timespans. Any number 

29 of factors could influence flora such as drought, fire, disease, fungi, and insects. 

30 2.5.3.1.2.2 Probability

31

32 Licensees use screening criteria, based on the probability of occurrence and/or consequences 

33 to the performance of the disposal system, to screen out FEPs that are unlikely to occur or that 

34 have relatively minor consequences. Three methods that a licensee may consider that handle 

35 probability depending on the extent of quantification of the FEPs concerned include (EC, 

36 2009a): 

37

38 • Quantitative methods, where all FEPs are represented numerically and event probability 

39 is an explicit part of the performance assessment calculation, such as those methods 

40 employed in the probabilistic models used for the Yucca Mountain and Waste Isolation 

41 Pilot Plant projects. For example, a performance assessment required by 

42 10 CFR Part 63 should not include consideration of very unlikely FEPs (i.e., FEPs that 

43 are estimated to have < 1 chance in 10,000 within 10,000 years of occurring). As a 

result, FEPs with probabilities lower than 10-8 44 /yr should be screened out during the 

45 scenario development process. The probability classification in its entirety would include 

46 the following and could be used in conjunction with the qualitative approach as points of 

47 reference: 
2-42

o Implausible: Very Unlikely (<10-8 1 /yr)

o Plausible: Unlikely or Less Likely (<10-5

/yr and >10-8 2 /yr)

o Plausible: Reasonably Foreseeable (>10-5 3 /yr or >1 chance in 100,000 per year 

4 or about a 1 in 10 chance of occurring over a 10,000 year period) 

5 • Qualitative methods, where the probability or likelihood of occurrence of FEPs is 

6 described qualitatively or semi-quantitatively, and probability values are not an explicit 

7 part of the numerical modeling. However, a qualitative description of probability could 

8 still be used (e.g., unlikely vs. very unlikely). Qualified specialists would need to 

9 determine the level of probability, as well as the terminology to be used to describe that 

10 probability. For example, experts on paleofloods may determine that a site’s physical 

11 environment and topography preclude major flooding. The low probability from this 

12 qualitative determination could then be labeled as either “very unlikely” or “implausible”

13 or some other term depending on the terminology agreed upon. The level of effort for 

14 this method is appropriate for LLW disposal sites and is recommended for the analysis 

15 of FEPs. 

16 • Non-consideration of probability, especially where few or no relevant data are available 

17 and there are large uncertainties associated with describing the scenario. With this 

18 method, FEPs are included as a result of lack of information. 

19 One technique applicable to FEPs screening based on probability is the frequentist technique 

20 (EC, 2009b), where probabilities are based on observations on how often a phenomena has 

21 occurred at the proposed site or at a natural analog to the site. Constraints to using this

22 technique include limited data or non-representativeness of the data that is available. 

23

24 Uncertainty Associated with Probability

25 The main consideration in the assignment of probabilities to scenario-forming FEPs is credibility. 

26 This area of the FEPs analysis relies on the skills and experiences of the analysts and on the 

27 qualifications of the independent reviewers. Most probability estimates developed by licensees 

28 will include a substantial amount of judgment. Because the FEP screening process can result in 

29 FEPs not being further considered in the analyses, licensees should make conservative 

30 decisions when screening based on probability. Probability screening will involve the use of 

31 existing data in areas like paleoclimatology, plate tectonics, hydrology, geology, and natural 

32 resources coupled with expert judgment. It is important that the estimates are documented. 

33

34 When there is sufficient and reliable information available, an analyst can have confidence in 

35 probability estimates and the understanding of uncertainty in the probability estimates. If the 

36 sampled population on which the probability is based is small or the quality of the data is poor, 

37 or if the estimates are based on assumptions, then the uncertainty associated with probability 

38 can be high. As previously discussed, long-term analyses or estimates may be more difficult to 

39 quantify due to an increasing scarcity of reliable data. The probability of occurrence for an 

40 earthquake above a certain magnitude over a 100-yr. period would be uncertain however the 

41 confidence in the probability estimate may be relatively high when compared to the uncertainty 

42 in the probability of tundra-like conditions occurring at a particular location many thousands of 

43 years in the future. When using probability to screen FEPs, the uncertainty in the probability

44 must be taken into account. 
2-43

1 2.5.3.1.2.3 Consequence

2

3 Many of the same characteristics and difficulties associated with quantifying probabilities also 

4 apply to attempts to quantify consequences or impacts on disposal site performance. A 

5 conservative semi-quantitative approach is generally better suited than a quantitative approach 

6 when using consequence as a screening criterion. For example, a bounding consequence

7 calculation could be, under the appropriate conditions, a helpful conservative calculation to

8 indicate that the associated impact will be insignificant. One problem with consequence 

9 screening is that as the technical analyses becomes more complex, the output from the 

10 analyses may become more complex and less intuitive, making it difficult to screen based on 

11 consequence without performing a quantitative analysis. Shelf- and cliff-type responses, as well 

12 as local minima and maxima, can confound interpretation of bounding consequence or other 

13 types of significance determinations. Example 2.4 provides an example of a cliff-type response 

14 and the way it may affect FEP screening. 

15

16 A performance assessment model will commonly have inter- and intra-dependent components 

17 or submodels. For this reason, the initial development process should err on the side of 

18 including FEPs of unclear significance. Once the model is developed and the connection and 

19 communication of submodels have been established, then the significance determination 

20 process can more reliably eliminate FEPs.

21

22 The inclusion or exclusion of a FEP in a performance assessment model depends on whether it

23 has a measureable, observable, or significant effect on disposal system performance. Since 

24 FEPs are not measurable or observable in the far future, expert judgment will constitute a key 

25 element of the screening process. Experts or specialists would need to determine the 

26 magnitude of the consequence, as well as the terminology to be used to qualitatively describe 

27 that impact (e.g., significant, major, substantial). For example, if it is plausible that the 

28 consequence of a FEP can be expected to change dose results that are relatively close to the 

29 performance objective, then the impact of the feature or phenomenon in question is significant. 

30 In addition, if the output results change by orders of magnitude depending on the absence or 

31 presence of a feature, or the occurrence or absence of a phenomenon, then the impact of the 

32 feature or phenomenon is significant. Previous performance assessments may provide insights 

33 to consequence, or additional modeling or sensitivity analyses could assist in determing the 

34 impact. Depending on the complexity of the system, this process may need to be iterative.

35

36 Since subsystem-level effects on system-level performance may be masked by certain designs 

37 and/or combinations of input parameter values, the quality of the FEP analysis relies on the 

38 qualifications and judgment of the analysts. In addition, the licensee using consequence-based 

39 FEP screening should consider the interrelationships of the FEPs with one another, and the 

Example 2.4: A good example of a cliff-type response is the transport of a short-lived, 

sorbing radionuclide. At high values of the distribution coefficient, the radionuclide 

decays in place. However, at low values of the distribution coefficient, the radionuclide 

may be transported to a potential receptor location. A measure of the central tendency of 

the distribution coefficient distribution may inappropriately show that the radionuclide 

poses little risk even though a small change to the sorption coefficient would allow the 

radionuclide to arrive at the receptor location during the compliance period or protective 

assurance period and potentially cause a significant consequence. 
2-44

1 effect different combination of FEPs may have on the consequence (e.g., if an engineered 

2 surface barrier cover is performing as planned, the performance of other engineered features 

3 may be masked for a specific period of time). 

4

5 Uncertainty Associated with Consequence 

6 When there is sufficient and reliable information available, an analyst can have confidence in 

7 consequence estimates and the uncertainty may be low. If there is limited information, the 

8 quality of the data is poor, or if the estimates are based on assumptions, then the uncertainty 

9 associated with consequence may be high. In addition, if qualified specialists have difficulty

10 determining masking effects, the uncertainty associated with consequence estimates can be 

11 high. As previously discussed, the interrelationships of the FEPs with one another, and the 

12 effect different combinations of FEPs may have on the consequence should be considered to 

13 the extent possible. For example, variable temperature and chemical composition of 

14 groundwater may affect the rate of radionuclide transport. Distribution coefficient (Kd) values for 

15 radionuclides may also change due to the changing environments. However, these different 

16 FEPs (inherent groundwater variability vs. temporal variability in the environment) may affect the 

17 Kd values for individual radionuclides in opposite ways. In other words, it may be challenging to 

18 determine the consequence of a single FEP when many FEPs are uncertain as to their 

19 influence on the results. High uncertainty in the consequence of a FEP should result in the FEP 

20 being included, unless even with the uncertainty the FEP can be shown to not likely to be 

21 significant or the timing of occurrence will be delayed outside of the regulatory analysis 

22 timeframes.

23 2.5.3.1.2.4 Screening Techniques

24

25 Although there may be different ways to organize, evaluate, and present the FEPs, most 

26 methods have certain similarities and the lists of retained FEPs obtained by different methods 

27 should be similar. Techniques are not mutually exclusive and licensees may use several tools

28 in combination. The following section outlines one systematic screening technique, but there 

29 may be other valid techniques that a licensee may use.

30

31 Systematic FEPs screening by licensees will involve the use of professional judgment. 

32 Screening criteria matrices categorize phenomena into various elements according to the 

33 magnitude of the probability and the consequence (Hommel, 2012; NEA, 1992). For example, 

34 Table 2-2 illustrates the approach of using a combination of probability and consequence of a 

35 FEP, in conjunction with the uncertainty associated with that FEP, to retain or screen out the 

36 FEP (IAEA, 2004). Screening criteria matrices require specialists and experts to select the 

37 FEPs they estimate to be important and include in the assessment. Screening criteria matrices 

38 are one method that can provide transparency. Table 2-2 presents an example matrix that 

39 licensees can use to perform FEP screening for a LLW disposal facility performance 

40 assessment. Factors used for initial screening purposes include (1) probability of the FEP, (2) 

41 consequence of the FEP, and (3) the uncertainties associated with the probability and 

42 consequence.

43

44 Depending on the probability, a phenomenon or feature can be placed either below or above a

45 screening value as seen in column 2 of Table 2-2. FEPs can be screened in or out if a 

46 qualitative screening limit is selected and applied. For example, analysts may decide that FEPs 

47 with “very unlikely” probabilities should be screened out while FEPs with an “unlikely” probability 

48 should be included in the scenario development. The third column divides the uncertainty 
2-45

1 associated with the probability into high and low uncertainty. Consequence is handled in a 

2 similar manner as probability, which can be seen in columns 4 and 5 in Table 2-2. Analysts can 

3 assign a qualitative consequence screening limit in addition to an uncertainty estimate 

4 associated with the consequence. After being evaluated, a FEP will have assigned 

5 designations for the probability, consequence, and their uncertainties. Based on these 

6 designations, the qualified specialists will screen FEPs in or out, as shown in the far right-hand 

7 column.

8

9 Table 2-2 Example of Screening Criteria Based on Qualitative Probabilities, 

10 Consequences, and Uncertainty Associated with a FEP

Case Probability 

Below a Qualitative 

Screening Limit -

e.g., “very unlikely” 

Uncertainty 

Associated with 

Probability

Consequence

Below a Qualitative 

Screening Limit –

e.g., “not 

significant” 

Uncertainty 

Associated with 

Consequence

Screening 

Outcome

1 yes low yes low Out

2 yes high yes low Out

3 yes low yes high Out

4 yes high yes high In 

[Out*]

5 yes low no low Out 

6 yes high no low In

7 yes low no high Out

8 yes high no high In

9 no low yes low Out

10 no high yes low Out

11 no low yes high In

12 no high yes high In

13 no low no low In

14 no high no low In

15 no low no high In

16 no high no high In

* Screening outcome for the protective assurance period if not part of the compliance period

11 Whether a FEP is screened in or out is apparent in most of the 16 cases in Table 2-2. For 

12 example, in case 1, a FEP is clearly screened out since both probability and consequence are 

13 below a screening limit and their associated uncertainties are small. Conversely, FEPs in cases 

14 13-16 are retained for scenario development despite the various degrees of associated 

15 uncertainty since probability and consequence are both above their screening limits. Some of 

16 the cases are not as straightforward and are worth discussing in more detail below. 

17

18 For cases 2 through 4, cases 2 and 3 have probability and consequence below predetermined,

19 qualitative screening limits. For case 2, the uncertainty associated with consequence is low and 

20 for case 3, the uncertainty associated with probability is low, giving the analyst sufficient 

21 confidence to exclude the FEP from further consideration. For case 4, there is less confidence 

22 since both probability and consequence have a high uncertainty associated with them and 

23 although probability and consequence are below screening limits, the uncertainties associated 
2-46

1 with both probability and consequence would lead the analyst to keep the FEP for scenario 

2 development during the compliance period. 

3

4 Cases 5 through 8 all have consequences above the screening limits indicating potential 

5 inclusion of FEPs for further consideration; however, the uncertainty associated with the 

6 probability is important to determining their final inclusion or retention. If there is low uncertainty 

7 associated with a probability lower than a screening limit, there is confidence that despite the 

8 higher impact or consequence, the probability of its occurrence is sufficiently unlikely so that the 

9 FEP could be screened out (cases 5 and 7). Whereas, if there is high uncertainty associated 

10 with the small probability, as in Cases 6 and 8, the FEP should be screened into the analysis. 

11

12 Cases 9 through 12 are the converse of cases 5 through 8; all cases have probabilities above 

13 the screening limits indicating potential inclusion of FEPs for further consideration. However, if 

14 there is low uncertainty associated with a consequence lower than a screening limit, there is 

15 confidence that despite the higher probability of the FEP occurring or being present, the impact 

16 of the FEP is sufficiently insignificant so that the FEP would be screened out (cases 9 and 10). 

17 Whereas, if there is high uncertainty associated with the small consequence, as in Cases 11 

18 and 12, the FEP should be screened into the analysis. 

19

20 Increased uncertainties associated with the phenomena and features of the protective 

21 assurance period compared to the compliance period would be reflected in the potentially 

22 increasing number of FEPs and the uncertainties associated with probabilities and 

23 consequences of the FEPs. As a result, a larger number of FEPs may be retained based on 

24 this increasing uncertainty. However, FEPs from the protective assurance period with 

25 probabilities and consequences that are both below their respective screening limits should be 

26 screened out and not included in any scenarios for the protective assurance period if they have

27 not already been included in the compliance period analysis. For case 4, both probability and 

28 consequence are associated with high uncertainty, although both are below screening limits. 

29 For the compliance period, the uncertainties associated with both probability and consequence

30 would lead the analyst to keep the FEP for scenario development. If the FEP continues to be 

31 present or occur during the protective assurance period, this FEP would be part of the scenario 

32 development for this period. However, if a FEP is new to the protective assurance period and 

33 not a feature or phenomenon of the compliance period, the FEP would not be part of the 

34 scenario development for assessment for that period (see far right-hand column in case 4).

35

36 A licensee may extend compliance period calculations into the future without modification,

37 provided that the calculations are complete with respect to including key FEPs relevant to the 

38 protective assurance period or excluding FEPs only relevant to the compliance period. 

39 However, since the protective assurance period is considerably longer than the compliance 

40 period, FEPs that have been excluded from further consideration in the compliance period may 

41 not be able to be screened out from further consideration for the protective assurance period 

42 analyses. Potentially significant FEPs may need to be considered during scenario development 

43 if disruptive processes are expected to start occurring during the protective assurance 

44 timeframe or if the cumulative impact from repetitive events over the longer timeframes is not 

45 included and the repetition of those processes and events could lead to significant impacts. 

46

47
2-47

1 2.5.3.2 Top-Down Approach

2

3 The concept of safety functions has been used with increased frequency due to recent work on 

4 scenario development methodologies (EC, 2009b). An advantage of the top-down approach 

5 includes focusing on the capabilities of the significant barriers and considering behavior of 

6 individual features in the context of overall system performance relative to the decision to be 

7 made. Some consider the top-down approach a simpler way to develop scenarios since safety 

8 functions are more quickly identified than significant interrelationships between FEPs and since 

9 the probabilities of the resultant scenarios may be easier to estimate. Since the discussion

10 below only provides a brief description of this approach, licensees may refer to the sources 

11 given in the reference section (Section 12.0). As previously stated, both the top-down approach 

12 and bottom-up approach are often used simultaneously in a complementary way.

13 2.5.3.2.1 Safety Assessment and Safety Functions

14

15 Safety assessments are a systematic analysis of the ability of the site and design to provide the 

16 safety functions and meet technical requirements. A safety function is defined qualitatively as a 

17 role through which a component of the disposal system contributes to safety. Safety functions

18 are the diverse capabilities and components of the barriers found within a disposal system that 

19 are used to reduce the potential for the release of radioactive material and to ensure that any 

20 releases are within acceptable limits. The safety functions may differ as the time period 

21 changes. For example, the capabilities of a surface cover component may be relied upon to 

22 achieve short-term safety objectives for a LLW disposal site while the long-term capabilities of 

23 the wasteform itself may be relied upon for the long-term safety functions. Analogous to the use 

24 of a fault tree analysis for nuclear reactor safety (NEA, 1992), knowing when a safety function is 

25 expected to be available and when it can be relied upon will affect scenario development. 

26

27 A safety assessment and the findings of the safety assessment are essential components of the 

28 collection of arguments and evidence in support of disposal system safety. Other components

29 supporting the safety of a radioactive waste disposal facility should include:

30

31 • A description of the waste and the rationale for the chosen waste management strategy; 

32 • Descriptions of the disposal concept, the disposal facility, the disposal site and it’s safety 

33 functions; 

34 • Description of the management system applying to the different phases of facility 

35 development; and 

36 • Any other information that support continued development, operation, and closure of the 

37 facility. 

38

39 Relevant examples of the top-down approach to developing scenarios include HLW disposal 

40 facilities. Due to the relatively recent development and application of the approach for LLW 

41 disposal (EC, 2009b), less examples exist for LLW disposal since performance is more focused 

42 on the relatively active near-surface geomorphology than on the more passive deep geology.

43 The top-down approach is similar for both LLW disposal and HLW disposal, although less effort 

44 is expected to apply the approach to LLW since the level of effort (i.e., the level of detail, 

45 comprehensiveness, completeness, and degree of iteration), is commensurate to the longevity, 

46 concentrations of radionuclides, and quantity of the waste. 
2-48

1 2.5.3.2.2 Safety Functions and Scenario Development

2

3 A number of organizations that have developed performance assessments for waste disposal 

4 develop scenarios using a top-down approach to FEPs. Some national programs link FEP 

5 records with statements about safety functions (e.g., by specific tools such as FEP charts (SKB, 

6 2006)). Uncertainties in the performance of systems may give rise to scenarios. A licensee can 

7 identify plausible, alternative scenarios when safety functions are no longer expected to perform 

8 as intended. The aim of the scenario development process is to identify deviations from an 

9 expected evolution scenario, based on the failure of one or more safety functions or the extent 

10 or form of degradation of one or more safety functions. The main safety functions are 

11 associated with the engineered barrier system and the barriers of the natural system. In the 

12 scenario development process, a licensee can develop altered evolution scenarios by 

13 considering the timing of FEPs, their consequences in terms of safety function effectiveness, 

14 and the status of other safety functions. There is generally no safety function assigned to the 

15 biosphere. 

16

17 The proposed methodology for scenario identification consists of six steps (EC, 2009c):

18

19 1) Define a set of safety functions associated with the engineered and natural barriers for 

20 the considered disposal system.

21 2) Develop a safety concept based on the functioning of the disposal system in the case of 

22 the central scenario. This is strongly directed by the question "when is a safety function 

23 expected to be available or when can it be relied upon.”

24 3) Build a structured set of safety statements. These statements are derived from the 

25 requirements on the disposal system, on the sub-systems and on individual 

26 components.

27 4) Make a systematic analysis of the uncertainty affecting the safety statements.

28 5) Identify a list of possible altered evolution scenarios by considering all identified 

29 uncertainties and by testing if they have the potential to propagate to higher level 

30 statements, and eventually to affect the safety functions.

31 6) Derive a final set of altered evolution scenarios. This is done by constructing functional 

32 diagrams illustrating the impact of the considered uncertainty in a safety statement on 

33 the functioning of the disposal system and by grouping, as far as possible, scenarios 

34 with identical or strongly similar functional diagrams.

35 Structuring and identifying safety-relevant phenomena, information, and uncertainties is a 

36 prerequisite for scenario formulation using a top-down approach. The starting point for the 

37 identification of safety-relevant phenomena and uncertainties is the development of a detailed 

38 description of the initial state of the system and its subsequent evolution. Several tools have 

39 been developed and applied, including system-specific FEP databases, interaction matrices, 

40 influence diagrams, assessment model flowcharts, phenomenological analysis of the disposal 

41 system, storyboards, timeline with subdivision of timeframes, and process description reports. 

42 Further information can be found in NEA (2012). 

43

44
2-49

1 2.5.4 Constructing Scenarios

2

3 A licensee may evaluate multiple scenarios to evaluate scenario uncertainty. Although a 

4 licensee can never eliminate the uncertainty altogether, the technical assessment is an attempt 

5 to constrain the uncertainty associated with future events and processes. The following section 

6 outlines possible methods a licensee may use. There may be other valid techniques a licensee 

7 may consider. Appendix D provides additional information on techniques that may be useful.

8

9 The output of scenario construction is a set of scenarios encompassing most of the plausible 

10 future system states and their potential impact. Scenario development should not be done in 

11 isolation from the rest of the technical analysis process because it is influenced by and uses 

12 information from previous modeling and consequence calculations. The method a licensee 

13 uses for developing and selecting scenarios should be a traceable, structured, and transparent. 

14 A licensee should document and describe the method they have used to identify scenarios and 

15 the technical bases for choosing which scenarios are considered plausible. The licensee should

16 justify that relevant processes and events have been identified and that future evolutions of the 

17 disposal system have been considered in the development of the scenarios. 

18

19 Scenarios are often assembled and classified based on their likelihood of occurrence and on the 

20 probability of the FEPs comprising the scenarios. As introduced in Section 2.5.1.1, scenarios 

21 may be categorized based on their perceived likelihood of occurring. Common terms are 

22 defined below: 

23

24 • The central scenario represents the evolution of the disposal system within the expected 

25 range of uncertainty and in the absence of unlikely disturbances. For some sites, this 

26 may be the only scenario developed. The central scenario has also been referred to as 

27 a main, nominal, normal evolution, reference, design, or base case scenario. 

28 • Altered evolution scenarios, or alternative scenarios, represent less likely, but still 

29 plausible, representations of disposal system evolution, and also describe how 

30 disturbances affect the evolution of the system. 

31 • “What if” or residual scenarios are considered implausible scenarios. They explore the 

32 robustness of the system, such as complete failure of a barrier, without identifying a 

33 particular degradation mechanism. 

34 • Stylized scenarios are typically associated with future human actions (e.g., intrusion) 

35 where few or no relevant data are available and where there are very large uncertainties 

36 associated with describing the scenarios. 

37

38 A licensee should provide the terms used to describe the different types of scenarios in an 

39 assessment and clearly explain their purpose. Once the scenarios have been developed, a 

40 licensee should develop a conceptual model of the disposal system that can estimate the

41 associated release, transport and exposure mechanisms (discussed in Section 2.6). 

42 2.5.4.1 Central Scenarios and Alternative Scenarios

43

44 A licensee should use scenarios to describe the scenario uncertainty associated with the

45 system. The central scenario is the expected evolution of the system; the alternative scenarios 

46 are less likely but cannot be eliminated by the licensee. Scenarios allow for the licensee to use 
2-50

1 a mixture of quantitative analyses and qualitative judgments. The selected scenarios should 

2 together provide an appropriately comprehensive technical description of the estimated 

3 performance of the disposal system. 

4

5 Central scenarios are usually based on extrapolation of existing conditions into the future and 

6 incorporation of changes expected to occur in the future. The central scenario is considered to 

7 be the scenario best supported by available information and is usually considered to be a 

8 benchmark scenario against which the impact of alternative scenarios can be compared. 

9 The central scenario represents how the licensee expects the system to evolve assuming the 

10 proper functioning of the design with anticipated degradation. All of the significant features and 

11 processes that exist at a site should be captured by the central scenario. The central scenario

12 is generally devoid of consideration of major events that change the future evolution of the site 

13 and the performance of the disposal system since most licensees will be selecting a potential 

14 disposal site where such events are not expected. It is acceptable for a licensee to treat 

15 anticipated future evolutions of the disposal system in one numerical model by varying 

16 parameter ranges. However, because the disposal system may evolve differently under 

17 alternative scenarios, it may be difficult to represent the different plausible FEPs all in one 

18 simulation model and additional models may have to be constructed.

19

20 Licensees should also develop plausible, alternative scenarios to investigate the impact of 

21 scenarios that are not expected but cannot be excluded. A licensee is not required to evaluate 

22 implausible alternative scenarios. However, there may be some utility for a licensee to evaluate

23 sequences of events and conditions independent of probabilities, in order to illustrate the 

24 significance of individual barriers and barrier functions. In other words, the robustness of the 

25 disposal system can be examined. These alternative scenarios may represent less likely, but 

26 still plausible, modes of disposal site evolution (e.g., processes that impede the effectiveness of

27 a feature important to waste isolation) as well as scenarios representing extreme natural events 

28 (e.g., earthquakes, volcanic activity, or glaciers) but that are still within the range of realistic 

29 possibilities within the analyses timeframe. Generally, a limited number of external FEPs will be 

30 of concern. 

31

32 Various graphical and tabular techniques have been used to assist in scenario development 

33 (NEA, 1992; IAEA, 2004; NRC, 1995c; SKB, 2008). These techniques may be useful for a 

34 licensee to consider. These techniques include: 

35

36 • Event trees, logic diagrams, and related approaches that analyze alternative 

37 combinations of events and/or of resulting system states 

38 • Fault and/or dependency diagrams that set out in a hierarchical fashion the conditions 

39 and/or processes leading to, or contributing to, an end point of interest

40 • Influence diagrams that map the dependencies or interactions between various 

41 processes, often indicating the importance of the interaction

42 • Interaction matrices that force a comprehensive questioning of the dependencies 

43 between selected key features or processes

44 • Audit tables that force a consideration of the representation of each FEP within the 

45 available models and system representation, and evaluation of bias due to omission or 

46 simplified representation
2-51

1 • Judgmental approaches that rely on specialists in their field and expert judgment

2 The techniques listed above are not mutually exclusive and several tools may be used in 

3 combination. For example, influence diagrams and interaction matrices may be useful to 

4 explore and illustrate the connection between scientific understanding and the numerical 

5 models, whereas event trees and logic diagrams provide a logical structure for selection or 

6 generation of calculation cases. Whatever techniques are used, the judgment of analysts is 

7 critical to ensure that the scientific understanding is appropriately incorporated in the models. A 

8 key value of the graphical and tabular techniques is that they aid communication within projects 

9 enabling experts to see the significance of their knowledge within the system context. The 

10 techniques can also provide logical structure for the comprehensive documentation of the 

11 relevant processes and their representation in models. 

12 2.6 Conceptual Model Development

13

14 A licensee should review the information provided by the assessment context, system 

15 description, and scenario development steps of the assessment approach and use it to develop 

16 a conceptual model of the site. The conceptual model of the site should qualitatively describe 

17 how the FEPs and significant barriers interact with one another and how the site functions.

18 Licensees should describe any simplifying assumptions. Simplifying assumptions may be 

19 necessary when a licensee develops the site conceptual models. Regulators should review all 

20 simplifying assumptions and determine if adequate technical basis has been provided by the 

21 licensee. Simplifying assumptions typically involve the geometry and dimensionality of the 

22 system, initial and boundary conditions, time dependence, and the nature of the relevant 

23 physical and chemical processes. 

24

25 In order to identify areas that require more detailed consideration and reduce model uncertainty, 

26 an initial simple and conservative conceptual model could be developed based on limited data 

27 and design information. Further development of conceptual models by licensees should reflect 

28 an increased focus on significant radionuclides and processes. At all stages of the process, 

29 simplifying assumptions should be clearly identified by licensees. In any single conceptual 

30 model, the simplifying assumptions should be internally consistent and should also be 

31 consistent with existing information. Licensees should justify simplifying assumptions based on 

32 the current level of understanding of the system (NCRP, 2005). 

33

34 For the scenarios that are to be quantitatively assessed, the conceptual model should be

35 amenable to mathematical representation. The conceptual model should have enough detail to 

36 allow mathematical models to be developed to describe the behavior of the system and its 

37 components. Conceptual models developed by the licensee provide the framework for the

38 computational models. It is important that the conceptual model is transparent and supported 

39 with adequate technical basis. More than one conceptual model may be consistent with 

40 available information. Appendix D provides additional information on techniques that have been 

41 used to develop conceptual models. If the set of alternatives does not represent the full range 

42 of possibilities, conceptual model uncertainty will be underestimated. 

43

44 NRC (2003b) discusses conceptual model uncertainty and some of the most important activities 

45 on developing alternative conceptual models to reduce model uncertainty, which include: 

46
2-52

1 • Maximizing the number of experts involved in the generation of alternative 

2 conceptualizations

3 • Minimizing inconsistencies, anomalies, and ambiguities

4 • Articulating uncertainties associated with each alternative conceptualization

5 • Obtaining key data to support each conceptual model alternative

6 • Considering alternative representations of space-time scales and of each feature and 

7 process

8 A variety of approaches have been used to facilitate the development of conceptual models in a 

9 traceable manner. Three examples taken from IAEA (2004) are given below. 

10

11 The “safety assessment comparison approach” relies on the expert judgment and experience of 

12 the analyst carrying out the assessment. The first step is to identify the key release, transport, 

13 and exposure media by reviewing the relevant FEPs associated with each scenario. The 

14 mechanisms by which the associated release, transport, and exposure may occur are 

15 considered for each scenario. 

16

17 Two strategies can be used based on information derived from each scenario:

18

19 • The deductive strategy reviews how release events might occur and considers the 

20 possible transport and exposure mechanisms and the associated impacts; 

21 • The inductive strategy analyzes the impacts, considers the exposure and transport 

22 mechanisms that might have caused the impacts, and the associated release 

23 mechanisms. 

24 The “interaction matrix approach” for developing a conceptual model allows the graphical 

25 representation of system interactions through the use of formalized procedures but does rely on 

26 expert judgment and is data intensive. The approach starts with a top-down approach to 

27 dividing the system into constituent parts. The resulting matrix and the FEP list contents can 

28 later be audited against each other. Using the interaction matrix approach to facilitate 

29 conceptual model development has the advantage of allowing disposal system components to 

30 be included explicitly in the interaction matrix and analyzed in greater detail by creating one or 

31 more sub-matrices. The interaction matrix approach allows FEP interactions and pathways to 

32 be mapped, which is an important step in developing and defining a conceptual model and in 

33 the logical progression to a mathematical model. Moreover, the systematic process of 

34 examining how the system components relate to one another may help to identify new, 

35 previously unrecognized relevant characteristics of the system. When using the interaction 

36 matrix approach for developing a scenario, the convention is to allocate off-diagonal elements in 

37 the direction of contaminant migration. In this way, contaminant migration pathways and the 

38 associated exposure pathways and exposure groups can be traced and translated into the 

39 conceptual model.

40

41 The “influence diagram approach” for developing a conceptual model also allows the interaction 

42 between FEPs to be identified in a logical and systematic way. Advantages and disadvantages 

43 are similar to the interaction matrix approach, although the influence diagram generally contains 

44 more detail than the interaction matrix. FEPs are represented by boxes and interactions 

45 between FEPs are illustrated by arrows showing the influence direction. The number of arrows 
2-53

1 between two FEPs will be equal to the number of influences between them. Only direct 

2 influences should be represented in an influence diagram. 

3 2.6.1 Alternative Conceptual Models

4

5 Licensees should adequately describe and document conceptual model uncertainties. The 

6 performance assessment documentation should provide the assumptions, limitations, and 

7 uncertainties of the models. Multiple representations of the system may be consistent with the 

8 available data. In general, licensees should select the conceptual models that best represent 

9 available data and associated uncertainty. However, when data are sparse, multiple conceptual 

10 models may represent the available data. In this case, licensees should select the conceptual 

11 model that provides the most conservative result, or additional data could be collected to reduce 

12 uncertainty and determine which alternative conceptual model provides the most realistic

13 representation. All conceptual models do not need to be abstracted and evaluated, but all 

14 models that are reasonably consistent with available information should be considered. 

15 Reviewers should perform an independent evaluation of model uncertainty and consider 

16 whether more than one conceptual model should be evaluated, especially for complex sites.

17 2.7 Numerical Model Development and Assessment

18

19 The numerical model development and implementation process typically consists of

20 representing the conceptual models and their associated processes in mathematical models, 

21 and using modeling software to develop numerical simulations of the mathematical models.

22

23 This section provides guidance on the development of numerical models including (1) specific 

24 information on model abstraction of the conceptual model in order for it to be represented 

25 mathematically; (2) model integration; and (3) interpretation of model results.

2.7.1 Numerical Models4 26

27

28 In general, licensees will rely on numerical or computational models to estimate the future 

29 performance of a disposal site. However, the implementation of mathematical models in 

30 computer codes may not be necessary in all cases depending upon the complexity of the 

31 analysis. Additionally, the analyses are intended to be iterative and the level of detail and effort 

32 involved in a particular iteration may vary depending on the phase of facility development and 

33 the level of knowledge about the disposal site. For instance, during siting of the facility, simple 

34 models may initially be used to screen candidate sites, whereas, more sophisticated models 

35 may be needed during licensing of sites with unique characteristics such as complex site 

36 engineering or natural features. Although specific computer codes may be discussed or 

37 referenced in this guidance, the NRC does not endorse the use of any particular code or 

38 modeling software package for analyzing the performance of a LLW disposal site. 

39

40 This section discusses issues licensees should consider when developing numerical models. 

41 First, licensees should select a numerical modeling approach that can appropriately represent 

42 the site’s conceptual model and implement the mathematical model. Second, licensees should 

43 ensure that the selected numerical model is developed with adequate QA/QC.

 4 For this document, the terms computational model and numerical model are used interchangeably. In 

addition, the terms computer code, program, and software are used interchangeably.
2-54

1 2.7.1.1 Selection and Implementation

2

3 Licensees should select a numerical modeling approach that can represent the significant 

4 components of the conceptual model(s) for the disposal site, including the site-specific FEPs. 

5 The numerical model(s) will express the conceptual model as one or more mathematical 

6 expressions (e.g., algebraic and/or differential equations) with a set of boundary and initial 

7 conditions that are then solved. In many cases, more than one mathematical formulation could 

8 appropriately represent a conceptual model. Further, the expressions may be physically-based 

9 or empirically-based depending upon the level of scientific understanding of the physical 

10 processes, information available to parameterize the equations, and the spatial or temporal 

11 scales that the expressions are intended to model. 

12

13 Three approaches are often employed to solve the equations in numerical models: analytical, 

14 semi-analytical, and numerical. These methods are summarized here from IAEA (2004). 

15 Analytical methods provide exact solutions and can be computationally efficient; however, the 

16 methods are typically only available for simple equations involving homogeneous or uniform 

17 spatial domains (e.g., one-dimensional steady-state flow and transport with simple boundary 

18 conditions). Semi-analytical methods are more flexible than analytical methods for solving more 

19 complex problems (e.g., flow and transport involving multiple sources and sinks). Numerical 

20 methods often discretize the spatial and temporal domains into a finite number of compartments 

21 or increments and solve the equations by iteration, matrix methods, or some combination of the 

22 two. Numerical methods can include finite-difference, finite element, method of characteristics, 

23 random walk, and analytic elements. Numerical methods allow for consideration of spatial and 

24 temporal variability, complex geometry and boundary conditions, and dimensionality. However, 

25 numerical methods can be computationally intensive, require highly trained personnel and can 

26 introduce numerical errors. 

27

28 In the documentation of the numerical models, licensees should include information regarding 

29 the following:

30

31 • the mathematical formulation 

32 • model assumptions 

33 • sensitivity to ranges of input data and coefficients 

34 • consistency of the pathways in the numerical model with the pathways of the conceptual 

35 model(s)

36 • accuracy of the software to reflect the model’s mathematical formulation (discussed in 

37 Section 2.7.1.2) 

38 • the correct representation of the process or system for which it is intended 

39 • stability characteristics of the numerical methods employed in the model (discussed in 

40 Section 2.7.1.2) 

41

42 Because long-term behavior of the disposal system is of particular interest for numerical models 

43 that estimate the performance of LLW disposal facilities, licensees should provide comparisons 

44 between theory and experimental results, field observations, and other supporting information 

45 that may involve interpretation and extrapolation. In cases where more than one mathematical 
2-55

1 formulation may be appropriate, licensees should document why a particular formulation is 

2 preferred to the discarded approaches and, importantly, the limitations of the selected approach. 

3 Licensees may also wish to consult Appendix I of Safety Guide No. GS-G-3.4 for information 

4 about controlling the numerical models for waste disposal facilities (IAEA, 2008).

5

6 Reviewers should evaluate the conceptual model(s) and the numerical models to ensure 

7 compatibility with the site conceptual model including the source-term, transport pathways, and 

8 receptor scenario. For example, numerical models designed for the onsite receptor scenario 

9 may not be appropriate for assessment of an offsite receptor scenario. Reviewers should also 

10 ensure that the assumptions in the numerical modeling are consistent with the site conceptual 

11 model and that any deviations will not significantly affect the ability of the numerical model to 

12 represent the conceptual model. Reviewers should investigate the sensitivity of numerical 

13 model calculations to variations in input ranges to ensure that the model is numerically stable 

14 over the range of input parameters for expected site conditions. The range of input parameters 

15 may be different for alternative scenarios compared to the central scenario.

16

17 Licensees may select available modeling software, commercially-available or otherwise, or 

18 develop codes or modeling platforms specifically for site-specific purposes. The modeling 

19 software may be proprietary codes, modified codes, and/or codes specifically developed for 

20 implementation of the chosen mathematical models. Modified codes and codes specifically 

21 developed for a site-specific application are referred to generically in this guidance as “user22 developed” codes. The use of proprietary codes usually has the advantage that the codes have 

23 been previously developed and checked. Proprietary codes may have a history of application to 

24 a range of different analogous problems. In contrast, user-developed codes need to be 

25 developed and checked by licensees. However, they do have the advantage of being tailored to 

26 the needs of the specific problem to be addressed. In all cases, it is necessary for a licensee to 

27 consider the process of software design based on a given mathematical specification (IAEA, 

28 2004). Licensees should ensure that the software used for the numerical models is in 

29 conformance with the active standards of the IEEE Standards for software development, quality 

30 assurance, testing, verification, and validation (e.g., IEEE Standard 730-2002, IEEE Standard 

31 for Software Quality Assurance Plans). Reviewers should be familiar with NRC’s Software 

32 QA/QC guidance (i.e., NUREG/BR-0167, NUREG-0865) to ensure the licensee’s QA/QC 

33 procedures for selection and development of numerical models are comparable. It is 

34 recommended that proprietary codes are peer reviewed by the appropriate technical community 

35 prior to use in a licensing decision; however peer review is not required. The pertinent regulator 

36 may need to perform this function. Further guidance on software QA/QC is provided in the 

37 following section.

38 2.7.1.2 Quality Assurance 

39

40 Licensees may develop computer codes or may use available codes (e.g., previously developed 

41 or commercially available) to simulate the performance of the disposal site using the site 

42 conceptual model. An essential element of modeling software development and/or usage is 

43 quality assurance (QA). Quality assurance associated with the field of code development is 

44 reasonably mature; this section of the document does not attempt to provide detailed guidance 
2-56

1 on software development. However, the main elements of software QA are discussed here. As 

part of QA for software development5 2 , a licensee should develop documentation of:

3

4 (1) Software requirements and intended use

5 (2) Software design and development; the software development process (if used), should 

6 be planned, controlled, and documented. 

7 (3) Software design verification; software verification should be planned, documented, and 

8 performed.

9 (4) Software installation and testing

10 (5) Configuration control

11 (6) Software problems and their resolution

12 (7) Software validation; planning should include validation methods and validation criteria.

13

14 More detailed documentation should be provided if the code or modeling software is developed 

15 by the licensee compared to if a well-established, commercially-available product is used. For 

16 additional information the reader can consider Sections 4.4 and 8.0 of NUREG-1854 as well as 

17 Appendix I of NUREG-1757, Volume 2, and Revision 1.

18

19 The following bullets provide the information a licensee should provide and a reviewer should 

20 evaluate with respect to numerical model development.

21

22 • Valid data should be used in the numerical model. The data used in the analysis should 

be traceable6 23 to their sources through all calculations and data reductions.

24 • The data should have been obtained or qualified under an acceptable QA program (e.g., 

25 an NRC-approved QA program developed to meet the requirements of 10 CFR Part 50, 

26 Appendix B).

27 • The numerical model should be demonstrated to adequately represent the processes or 

28 systems for which it is intended.

29 • The software should perform intended functions, provide correct solutions, and not 

30 cause adverse unintended results.

31 2.7.2 Model Abstraction and Model Simplification

32

33 Model Abstraction

34 A licensee will need to abstract the conceptual models they develop in order to translate the 

35 concepts into mathematical terms. Model abstraction is the process of abstracting a conceptual 

36 model representing a dynamic site in the physical world into a mathematical model governed by 

37 equations which is implemented within a numerical model. It is the process of determining the 

38 level of detail that should be preserved in the overall performance assessment model (or 

39 intruder assessment model). Figure 2-5 provides the types of abstraction that may occur in the 

 5 Steps 4 through 6 apply to application of commercially available or existing codes as well as to 

development of user-developed codes. 6 Traceability is the ability to trace the history, application, or location of an item or like items or activities 

through recorded identification.
2-57

1 model development process. The example provided is for development of a performance 

2 assessment model. Figure 2-6 provides a representation of the development of a hydrologic 

3 model (process model) for use in a performance assessment. Various abstractions may occur 

4 at many different steps in the model development process. Model abstraction builds on insights 

5 gained from FEP development and scenario analysis. In a LLW performance assessment, 

6 several model abstractions typically support the overall assessment of a facility’s ability to 

7 demonstrate compliance with the performance objectives. These abstractions usually include 

8 models of projected climate and infiltration, degradation of barriers, source term release, 

9 transport through environmental media, and potential exposures to a receptor in the biosphere. 

10 Section 3.3 of NUREG-1573 (NRC, 2000a) and Section 3.0 of this guidance discuss issues 

11 related to these model abstractions in the context of a performance assessment. 

12

13 Several factors can affect the complexity of the models that a licensee may use. Some level of 

14 simplification is generally required in order to translate the concepts of a conceptual model into 

15 mathematical terms. This simplification can take several forms, such as:

16

17 • The simplification of the geometry or structure—for example, considering a transport

18 medium to be homogeneous and isotropic

19 • The omission of processes and interactions—for example, neglecting kinetic terms in 

20 chemical reactions

21 • A reduction in spatial or temporal resolution

22

23 Model Simplification

24 Model simplification is similar to model abstraction but more closely tied to modifying numerical 

25 models and consists of simplifying a complex numerical model into a reduced numerical model 

26 that has fewer components and is quicker to run while still maintaining the validity of the 

27 simulation results. For example, it may be appropriate for a licensee to reduce the detail of a 

28 submodel (e.g., for waste release) for incorporation in a performance assessment model. The 

29 model simplification process should demonstrate that the detail that has been lost is not 

30 essential to the estimated performance, which can be difficult to do in complex models and 

31 when supporting information is sparse. In addition, the simplified model should be compared to 

32 the detailed model to show that the simplified representation is appropriate. This comparison 

33 should be performed for base case models as well as for alternative scenario models. A 

34 simplified “model” can be something as simple as a data value or lookup table. For example, 

35 use of distribution coefficients (Kd) for radionuclide transport is actually a simplification of much 

36 more complex phenomena. The process of producing simplified models can introduce 

37 uncertainties and biases.

38

39 It is important that a licensee should clearly document both model abstractions and model 

40 simplifications. A licensee may need to perform abstraction of the real world system into a 

41 mathematical model using an iterative process. It is useful for a licensee to factor in limitations 

42 of the computational platform during conceptual model development. However, it is not 

43 appropriate to limit the assessment to the capabilities of existing computational platforms. In 

44 some cases, a licensee may need to develop a new computational model in order to 

45 appropriately assess the problem.
2-58

1 Figure 2-5 Overview of Abstraction as Applied to a Performance Assessment Model

Site 

characterization 

data and other 

information

Performance 

assessment conceptual 

model development

Hydrologic conceptual 

model development 

(see next figure)

Estimated system

performance

Abstracted hydrologic model
2-59

1 Figure 2-6 Abstraction of a Hydrologic Model for a Performance Assessment Model

Hydrologic 

conceptual model

Boundary conditions

Spatial and temporal

discretization

Numerical model

development
2-60

1 A natural tendency is to preserve all detail that may be developed in models supporting the 

2 performance assessment. However, this approach has disadvantages. The additional detail 

3 can make documentation, review, and understanding more difficult. In addition, many 

4 uncertainty analysis methods work better with a reduction in the number of inputs that do not 

5 affect the outputs. Model simplification should achieve a balance by preserving essential detail 

6 and eliminating unimportant detail.

7 2.7.3 Model Integration

8

9 The objective of the review of integration is to determine if the site and design description, data, 

10 parameter uncertainty, models, and model uncertainty have been appropriately integrated in the 

11 assessment. The reviewer should ensure that representations of the disposal system design 

12 and natural system features have been adequately integrated into the technical analyses. 

13 Assumptions, data, and models should be consistent throughout the technical analyses. 

14 Inconsistencies should be explained in the documentation or be corrected. Boundary and initial 

15 conditions should be consistent between different submodels in the performance assessment. 

16 Information that is passed between submodels should be verified to be of the appropriate 

17 temporal and spatial scale. If different models are used, information passed between models 

18 should be assessed to determine if unit conversions were made. Graphical representations can 

19 be useful to communicate integration in the technical analyses. Influence diagrams can be 

20 useful to document concisely the connections among submodels in the performance 

21 assessment. 

22 2.7.4 Analysis and Evaluation of Results

23

24 Probabilistic approaches to performance assessment are preferred in most cases because they 

25 readily permit the propagation and assessment of the impact of uncertainty on the model 

26 results. However, use of a deterministic model to demonstrate compliance with performance 

27 objectives may be acceptable. In general, if deterministic modeling is used, it should be 

28 reasonably conservative, such that a subject matter expert, with minimal interaction with those 

29 who performed the assessment, could conclude that the analysis is conservative. Independent 

30 reviewers should evaluate the modeling in sufficient detail to be confident that the analysis is 

31 conservative.

32

33 For probabilistic analysis, the appropriate metric to use for comparison with the annual dose 

34 limits is the peak of the mean result. The peak of the mean is calculated by estimating the 

35 mean dose result over all probabilistic realizations, then taking the peak value of the mean 

36 curve. In some cases the mean curve may represent a high percentile of the projected output. 

37 Licensees may propose more conservative metrics; however, the NRC staff believes that the 

38 peak of the mean is protective.

39

40 If a probabilistic approach is used for the performance assessment, the reviewer will need to 

41 determine whether there is significant “risk dilution” affecting the calculation results. Risk 

42 dilution results when overly broad parameter distributions are selected primarily for processes 

43 that affect the timing of impacts. Although large uncertainty ranges may seem to be 

44 “conservative,” overly broad uncertainty ranges can artificially depress the peak of the mean 

45 dose result. For example, selection of an overly broad range for the Kd for neptunium-237 in the 

46 saturated zone may result in the estimated time of arrival of the contaminant at a receptor 
2-61

1 location being artificially spread over the period of performance, thereby "diluting" the risk at any 

2 one point in time. 

3

4 For modeled processes and input parameters that are highly uncertain and cannot clearly be 

5 established as conservative, sensitivity analyses are necessary to establish the relative 

6 importance of these processes and parameters to the performance assessment dose 

7 calculations. NUREG-1757 (NRC, 2006; Volume 2, Appendix I, Section 1.7) and NUREG-1573 

8 (NRC, 2000a; Section 3.3.2) summarize different methods for sensitivity and uncertainty 

9 analyses. Sensitivity analyses may identify the need for additional site characterization to 

10 adequately support the technical analyses.

11

12 Key parameters for further evaluation in a sensitivity analysis may be selected with a variety of 

13 different approaches, and the appropriateness of the approaches depends on the problem. It is 

14 anticipated that the sensitivity analysis and the performance assessment overall may be an 

15 iterative process. The initial approach evaluated may not be the final approach selected. 

16 Regardless of the process, the reviewer should keep in mind that the purpose of the sensitivity 

17 analysis is to evaluate uncertainty and variability in the assessment. One of the simplest 

18 methods uses a top-down approach in which the risk reduction of each component (e.g., 

19 infiltration, unsaturated zone, wasteform, engineered barriers, saturated zone, biosphere 

20 characteristics) of the performance assessment model is identified by starting with a hazard and 

21 calculating how each component reduces the risk from the hazard. Subsequently, for the most 

22 important components, the licensee performs a quantitative or qualitative evaluation of the 

23 parameters to identify those that are most likely to influence the output from the component. 

24 Complications arise because an individual component’s importance in the system can be 

25 relative to the performance of other components. For example, the hydraulic conductivity of a 

26 drainage layer may not appear to be risk-significant as long as a cover is assumed to 

27 significantly limit water flow, but the drainage layer may be very important to performance if the 

28 cover is degraded. Section 7.4.1.2 discusses methods to avoid such problems. Developing an 

29 understanding of the importance of parameters and models in a performance assessment is a 

30 time-consuming process that is best accomplished by exploring a variety of approaches.

31

32 If the licensee has performed a deterministic performance assessment, then the reviewer 

33 should examine the sensitivity analyses provided. The reviewer should evaluate the licensee’s 

34 basis for selecting the parameters and combinations of parameters used in the sensitivity 

35 analysis. The ranges in the parameters selected should be consistent with the variability and 

36 uncertainty in the parameters, and the selected ranges should provide the reviewer with 

37 confidence that the effects of the uncertainty on performance are bounded. The reviewer 

38 should examine the technical basis used to support the variability and uncertainty. Appropriate 

39 combinations of parameters should be used to capture the interdependence of key parameters 

40 and the consequences associated with changes in combinations of key parameters. The 

41 reviewer should consider combinations of parameter values that are likely to occur as a result of 

42 common causes. For example, an aggressive chemical environment could increase both the 

43 corrosion rate of waste containers and the rate of leaching of radionuclides from a wasteform. If 

44 a licensee were to perform analysis of the increased corrosion rate independent from analysis of 

45 increased leaching rate of radionuclides from the wasteform, the true risk significance of an 

46 aggressive chemical environment may not be identified.

47

48 Different approaches to performance assessment calculations (e.g., deterministic, probabilistic) 

49 have advantages and disadvantages with regard to uncertainty and sensitivity analysis. The 
2-62

1 type of analyses that may be suitable for a particular problem will be tied the amount of model 

2 support available. While deterministic analysis can be a suitable approach for performance 

3 assessment, it can also present a challenge for a dynamic system that responds nonlinearly to 

4 the independent variables. When there are numerous inputs (e.g., data or models) that are 

5 uncertain, evaluating the impacts of the uncertainties on the decision can be difficult. Typical 

6 one-off type of sensitivity analysis where a single parameter is increased or decreased will 

7 identify only local sensitivity within the parameter space such that it may not clearly identify the 

8 risk implications. When a licensee must address multiple uncertainties, isolation of the impact 

9 of the uncertainty with one-off sensitivity analyses should be avoided because it can lead to 

10 misleading results (see Section 7.4.1.2). In addition, uncertainties should not be relegated to 

11 representation in one-off evaluations; uncertainties that are expected to apply to the system 

12 should be represented in the compliance case results. A deterministic approach can be useful

13 to bound uncertainty when the analysis can be demonstrated to be conservative. A probabilistic 

14 approach can have distinct advantages when there are a number of uncertainties that may 

15 significantly influence the results of a performance assessment. For example, the uncertainty 

16 introduced by the changing effectiveness of a chemical barrier over time may be represented by 

17 selecting appropriate ranges for the radionuclide transport parameters for the materials of the 

18 barrier. 
3-1

1 3.0 PERFORMANCE ASSESSMENT

2

3 A performance assessment is a type of risk analysis that addresses (1) what can happen,

4 (2) how likely it is to happen, and (3) what are the resulting impacts (Eisenberg et al, 1999). 

5 These impacts can then be compared to the performance objective in 10 CFR 61.41 

6 (radiological protection of the general public). The requirements for a performance assessment 

7 are set forth in 10 CFR 61.13(a), as discussed in Section 1.1.2.1. 

8

9 This section describes acceptable approaches for conducting a performance assessment to 

10 demonstrate that the performance objectives specified at 10 CFR 61.41(a) and (b) would be 

11 met. This guidance supplements, rather than replaces, other NRC guidance on acceptable 

12 approaches for complying with the requirements specified in 10 CFR Part 61 (see Section 

13 1.2.1). Specifically, this guidance supplements the approach discussed in NUREG-1573 (NRC, 

14 2000a). The approaches described in NUREG-1854, for performance assessment in incidental 

15 waste determinations, may also be applicable to the development of a performance assessment 

16 for a land disposal facility for low-level waste (NRC, 2007a).

17 3.1 Performance Assessment Approach

18

19 The essential elements of a performance assessment for an LLW disposal site are:

20

21 (1) A description of the site and engineered system,

22 (2) An understanding of FEPs that might affect the disposal system, 

23 (3) A description of processes controlling the movement of radionuclides from the LLW 

24 disposal units to the environment,

25 (4) A computation of doses to members of the public, and 

26 (9) An evaluation of uncertainties. 

27

28 The methods of performance assessment are matched to the complexity of the problem. 

29 Deterministic, bounding analyses can be used for simple evaluations; however, probabilistic 

30 analyses may be more appropriate for evaluating the disposal of long-lived waste at LLW 

31 disposal sites, to take into account uncertainties over long timeframes. 

32

33 Many FEPs can influence the ability of a waste disposal facility to limit releases of radioactivity 

34 to the environment. While considering the associated uncertainties, a licensee should complete 

35 a performance assessment that identifies the FEPs that might affect the disposal system, 

36 examines the effects of these FEPs on the performance of the disposal system, and estimates 

37 the annual dose to any member of the public caused by relevant FEPs. Section 2.5 provides 

38 guidance on the FEPs identification and screening process, including the development of 

39 scenarios.

40

41 Disposal system behavior is characterized by the disposal facility design, the characteristics of 

42 the waste, geologic and environmental characteristics of the disposal site, and processes and 

43 events that influence the aforementioned features. The performance assessment identifies the 

44 specific characteristics of the disposal site (e.g., hydrology, meteorology, geochemistry, biology, 
3-2

1 geomorphology); degradation, deterioration, or alteration processes of the engineered barriers 

2 (including the wasteform and container); and interactions between the site characteristics and 

3 engineered barriers that might affect the performance of the disposal facility. The performance 

4 assessment examines the effects of these processes and interactions on the ability of the 

5 disposal facility to limit waste releases to the environment that could cause annual dose to a 

6 member of the public. 

7

8 The performance assessment should be performed iteratively and is meant to be a tool for both 

9 the licensee and the regulator to use in assessing whether the disposal facility meets the 

10 10 CFR Part 61.41 performance objective. During the design and licensing of a disposal site, 

11 assumptions may be made, based on expected waste volumes and streams, of the possible 

12 final inventory of a site or a specific disposal unit within a site. As operations occur, these 

13 assumptions should be updated periodically with actual waste volumes and any revised 

14 information on future waste to be received (see Section 10.0). The results of the performance 

15 assessment can then be used to evaluate whether reasonable assurance still remains that the 

16 disposal unit or site will continue to meet the performance objectives. If the performance 

17 assessment shows that meeting the performance objective is uncertain or unlikely, then the 

18 licensee should consider taking the following actions: additional data collection and modeling 

19 could be performed, the facility could be modified, or future waste volumes or specific 

20 radionuclide quantities or concentrations could be reduced (i.e., through setting “allowable”

21 limits, see Section 9.0). The decisions on what actions to take should involve the site operator, 

22 the appropriate regulator(s), and other stakeholders.

23 3.1.1 Example of a Performance Assessment Approach

24

25 An example of an acceptable approach for conducting a performance assessment and 

26 demonstrating that the requirements in 10 CFR 61.41 would be met is outlined below. The 

27 approach is divided into 12 steps, as shown on Figure 3-1:

28

29 • Step 1: Conduct initial data evaluation of information needed to describe the LLW 

30 disposal system environment — Formulate the context of the assessment. Describe the 

31 disposal system and the environment of the site.

32 • Step 2: Describe plausible evolutions of the disposal site — Identify and consider 

33 credible factors or processes that could contribute to affecting a radionuclide release, 

34 including changes to the disposal site over time from natural processes and events, and 

35 construct reasonably foreseeable scenarios to evaluate in Step 5 for compliance with the 

36 performance objective and less likely, but plausible scenarios for evaluating defense-in37 depth in Step 7. 

38 • Step 3: Describe initial conceptual models and parameter values or distributions —

39 Develop site-specific conceptual models based on important disposal site features and 

40 processes. 

41 • Step 4: Formulate mathematical models and select codes — Formulate mathematical 

42 representations of the conceptual models, using appropriate documentation and quality 

43 assurance/quality control (QA/QC). Numerical model development incorporates the 

44 mathematical representations and produces computational models with which to run 

45 simulations. 
3-3

1 • Step 5: Conduct consequence modeling — Estimate performance (e.g., potential dose 

2 to members of the public). See Sections 3.4 and 4.3.1 for further discussion. Much of 

3 the receptor scenario approach used in Section 4.3.1 can be applied in the performance 

4 assessment.

5 • Step 6: Perform sensitivity and uncertainty analysis — Evaluate which models, 

6 assumptions, and combinations of parameters were most significant in producing the 

7 resulting doses. Evaluate the scenario, model, and parameter uncertainties. 

8 • Step 7: Demonstrate defense-in-depth — Evaluate whether defense-in-depth 

9 protections include multiple, independent and redundant layers of defense using the 

10 results of the performance assessment (e.g., uncertainty analysis). See Section 8.0 for 

11 further discussion.

12 • Step 8: Evaluate disposal site adequacy — Compare the results to the performance 

13 objective in 10 CFR 61.41 (radiological protection of the general public). If the 

14 performance objective has not been met, proceed to Step 8. 

15 • Step 9: Reevaluate data and assumptions — Determine what information and/or data 

16 are needed to reduce uncertainty and demonstrate regulations are met. 

17 • Step 10: Collect New Information and/or Change Design — Gather needed information 

18 such as site characterization data and modeling studies, or change the facility design. 

19 • Step 11: Update assumptions — Recalculate site performance using updated data and 

20 assumptions. Steps 9 through 11 can be performed for as many iterations as needed to 

21 demonstrate regulations are met.

22 • Step 12: Final determination — Make final determination that regulations are met. If the 

23 licensee cannot demonstrate that 10 CFR 61.41 would be met at the selected site, they 

24 may choose to reject the site as a potential disposal option or place limitations on the 

25 inventory that can be accepted for disposal.

26

27 Additional guidance on an example performance assessment approach can be found in 

28 NUREG-1573, Section 3.1 (NRC, 2000a). Sections 2.0 of this document provide additional 

29 guidance on Steps 1 through 4 above.

30 3.1.2 Role of the Performance Assessment

31

32 To obtain a license to receive, possess, and dispose of LLW, disposal facility operators should 

33 use the performance assessment to demonstrate, with reasonable assurance, that the 

34 10 CFR 61.41 performance objective for protection of the general public will be met. Further, 

35 the results of a performance assessment can be used to support defense-in-depth analyses 

36 required in 10 CFR 61.13(f). Licensees may use the results of the performance assessment to 

37 quantify barrier capabilities and their associated uncertainties to understand their contribution to 

38 safety and defense-in-depth. Section 8.0 of this document provides additional guidance on 

39 using the results of the performance assessment to demonstrate defense-in-depth protections 

40 are included.

41
3-4

1

2 Figure 3-1 Example of a Performance Assessment Process 

3

4

Demonstrate Defense-in-Depth

Step No. 7

Conduct Consequence Modeling

Step No. 5

Perform Sensitivity and/or

Uncertainty Analysis

Step No. 6

Numerical Model Development

Formulate Mathematical Model(s) and

Select Code(s)

Step No. 4

Conceptual Model Development

Describe Initial Conceptual Models and

Parameter Distributions

Step No. 3

System Description

Conduct Initial Data Evaluation of Information 

Needed to Describe the LLW Disposal Site

Step No. 1

Step No. 2

Scenario Development

Describe Plausible Evolutions of the 

Disposal Site

Collect New Information and/or

Change Design

Step No. 10

Final Determination:

Demonstrate Requirements Met,

Reject Site, or Limit Inventory

Step No. 12

Update Assumptions

Step No. 11

Evaluate 

Disposal Site 

Adequacy

10 CFR 61.41

Performance Objective

is Not Met

10 CFR 61.41

Performance Objective is

Met – Submit to NRC for Review

Steps 1 through 4 

discussed further 

in Section 2.0. 

Step 7 discussed 

further in

Section 8.0.

Staff Review as Part of a 10 CFR Part 61 

License Application (per NUREG-1200)

Questions from the Staff

Reevaluate Data and Assumptions

Step No. 8

Yes

Decision

to

Continue

Proceed

to a

New Site

No

Step No. 9
3-5

1 During construction, operation, and post-closure periods of the LLW disposal facility, the 

2 performance assessment can continue to have an important role in demonstrating that the

3 performance objectives continue to be met. The performance assessment is a projection into 

4 the future to provide the bases for the decision to proceed.

5

6 The other steps in the operation and closure process can be used to confirm (or refute) the 

7 basis for the initial decision. For example, 10 CFR 61.53 requires a licensee to perform 

8 environmental monitoring during the construction, operation, and post-operational periods. 

9 Similarly, 10 CFR 61.28 requires that a final revision to site closure plans should contain any 

10 additional geologic, hydrologic, or other disposal data obtained during the operational period 

11 pertinent to the long-term containment of waste, and the results of tests, experiments, or 

12 analyses pertaining to the long-term containment of waste, including revised analyses for 

13 10 CFR 61.13 using the details of the final closure plan and waste inventory. Site closure may 

14 be authorized only if the final site closure plan provides reasonable assurance of the long-term 

15 safety of the facility.

16

17 The performance assessment can be used to address these requirements by updating the 

18 performance assessment model developed for the initial license application with the new 

19 information from these monitoring programs. These new site data might validate or refute the 

20 key parameters or model assumptions used in the initial performance assessment. Section 9.5 

21 provides information on mitigation, which might be identified as being necessary to continue 

22 meeting the 10 CFR Part 61 performance objectives as a result of conducting an updated 

23 performance assessment upon site closure. Section 10.0 of this document discusses the role of 

24 performance assessment in performance confirmation. Additional discussion of general 

25 technical elements as they relate to performance assessment appears in Sections 2.0 of this 

26 document.

27

28 The following sections are a continuation of the discussion presented in Section 3.3 of 

29 NUREG-1573 (NRC, 2000a), but with an emphasis on analyses for waste containing long-lived 

30 radionuclides. Licensees can use the guidance contained in this section to support 

31 demonstration that the the 10 CFR 61.41(a) and (b) performance objectives are met and in 

32 some cases to support demonstration that the 10 CFR 61.41(c) performance objective is met. 

33 Section 3.2 discusses radionuclide source term modeling and release of radionuclides from the 

34 disposal units. Section 3.3 discusses radionuclide transport through the environment of the 

35 disposal site. Section 3.4 discusses modeling of the biosphere.

36 3.2 Source Term 

37

38 The objective of source term modeling is to calculate radionuclide releases from the disposal 

39 units over time and space. Licensees can use the calculated release rates as input for transport 

40 models (see Section 3.3) that estimate offsite releases for the facility. The source term includes 

41 the inventory, physical and chemical characteristics, and other properties of the waste used to 

42 estimate release rates. The inventory of waste is the physical amount of material and quantity 

43 of radioactivity contained in the waste. Releases generally occur by advective or diffusive 

44 mechanisms, although direct release mechanisms may be possible (e.g., biointrusion, erosion). 

45 Release rates are also a function of the conditions of the environment immediately surrounding 

46 the waste (i.e., the near-field environment). The near-field environment may have hydrological 

47 and chemical conditions that differ significantly from the natural system in which the waste 
3-6

1 disposal facility is located. Additional releases to the aqueous phase and, for certain 

2 radionuclides (e.g., carbon-14, hydrogen-3, radon-222), to the gaseous phase, can also occur.

3 Licensees may simulate many intermediate processes to estimate release rates. Release rates 

4 can be affected by the performance of engineered barriers (e.g., waste containers) and the 

5 wasteforms, the chemical properties of the disposal system, and the interaction of the disposal 

6 system with the natural environment in which it is located. Licensees should identify these 

7 processes using the guidance in Section 2.5 regarding the analysis of FEPs. Some disposal 

8 facilities will require detailed consideration of these processes and conditions, whereas 

9 simplified analyses may be justified for other sites and disposal facilities. Licensees should 

10 carefully consider the source term models for those disposal sites where significant credit is 

11 taken for some aspect of the source-term modeling (e.g., low solubility limits), attributes are 

12 relied upon for defense-in-depth protections, or higher long-term hazard exists, and for which 

13 there is limited model support. Facilities for which a simpler, less complex, analysis may be 

14 acceptable include those where a licensee can show that a simple analysis is clearly 

15 conservative or where a licensee provides a simple model that is well-supported by multiple 

16 lines of evidence, including field tests demonstrating that the model estimates are accurately 

17 representative or bounded by field data. 

18

19 A reviewer may want to use the following simple factors when risk-informing their review effort: 

20 hazard, credit, and support. For example, a disposal site may have a hazard that is large and 

21 persistent, the licensee may have taken significant credit in their technical analyses for release 

22 rate modeling, and they have limited information available to support their release rate 

23 modeling. In this case, the reviewer should apply additional resources to their review effort 

24 compared to an instance where a disposal site presents a small hazard and the licensee 

25 develops well-supported technical analyses.

26

27 This section of the guidance focuses on the assumptions, data, and models (conceptual and 

28 mathematical) that a licensee may use to develop the source term abstraction for the 

29 performance assessment. Most disposal sites and proposed disposal practices vary 

30 significantly, therefore, source term abstractions are usually developed on a site-specific basis. 

31 A licensee should develop the source term abstraction to:

32

33 • include the effects of degradation processes on the performance of the wasteform and 

34 the engineered barriers

35 • consider the physiochemical processes associated with partitioning of radionuclides 

36 between the waste and the physical phases in the disposal unit, as appropriate

37 • include temporal and spatial variation in the inventory, degradation processes, and 

38 physicochemical processes that are significant to performance

39 • adequately represent the features and processes significant to disposal system 

40 performance

41 • simulate the behavior of the system to the extent that disposal system performance is 

42 adequately represented

43

44 The source term model should be integrated with other models, such as climate, infiltration, and 

45 radionuclide transport, over both space and time. Guidance on modeling climate and infiltration 

46 appears in Section 3.3.3 of NUREG-1573 (NRC, 2000a) and Section 4.3.1 of NUREG-1854 
3-7

1 (NRC, 2007a). Section 3.3.7 of NUREG-1573 and Section 4.3.5 of NUREG-1854 present 

2 guidance on radionuclide transport, biosphere characteristics, and dose modeling.

3

4 Section 2.7.2 of this guidance document describes general elements of technical analyses that 

5 are applicable to the review of a source term abstraction for performance assessment. Since 

6 this guidance supplements existing guidance, licensees and reviewers may also consult 

7 NUREG-1573, Section 3.3.5, for further guidance on source term modeling. Also, NUREG8 1854, Section 4.3.3, may assist reviewers in considerations for acceptable source term 

9 modeling that may apply to review of a performance assessment for a LLW disposal facility.

10

11 The following sections of this document provide specific guidance on aspects of source term 

12 modeling. Section 3.2.1 describes approaches for developing the inventory representation. 

13 Section 3.2.2 discusses guidance related to representing the chemical environment in the 

14 performance assessment. Sections 3.2.3 and 3.2.4 discuss guidance on representing the 

15 waste containers and wasteforms, respectively, and the effect of degradation processes on their 

16 longevity. Sections 3.2.5, 3.2.6, and 3.2.7 describe approaches for modeling aqueous, 

17 gaseous, and direct releases.

18 3.2.1 Inventory

19

20 The inventory representation provides the radionuclide inventory for which release rates are 

21 estimated with source term calculations. Sections 3.3.5.1, 3.3.5.2, and 3.3.5.7 of NUREG-1573 

22 (NRC, 2000a) and Sections 3.1 and 4.3.3.1.1 of NUREG-1854 (NRC, 2007a), as applicable to 

23 LLW disposal facilities, provide guidance on recommended approaches for developing and 

24 reviewing waste inventory to support performance assessment modeling.

25

26 The radiological inventory to be disposed of is a key input parameter to the performance 

27 assessment. In addition, the physical and chemical characteristics of the waste can determine 

28 release rates. Physical characteristics can affect the stability of the waste and the stability of 

29 the disposal facility (see Section 5.0). Chemical characteristics can affect the retention of 

30 radionuclides within the disposal facility. Chelating or complexing agents can significantly 

31 increase the mobility and effective solubility of the radionuclides in the disposal environment. 

32

33 A licensee should develop an inventory description that provides the total activity (by 

34 radionuclide), concentrations, and physical and chemical forms of the waste to be disposed of, 

35 including spatial configurations. The inventory description should include the radiological 

36 inventory at the time of disposal, as well as the projection of inventory over the time period of 

37 interest to account for decay and ingrowth. 

38

39 Although Tables 1 and 2 of 10 CFR 61.55 list isotopes for the purposes of waste classification, 

40 the inventory that a licensee evaluates in the performance assessment may be more extensive. 

41 The performance assessment inventory should not be limited to the radionuclides in Tables 1 

42 and 2 unless adequately justified. For example, chlorine-36 and neptunium-237 are not listed in 

43 Tables 1 and 2 but are present in some LLW streams and are long-lived and mobile in the 

44 environment. Licensees should provide estimates of radionuclides that are not listed in these 

45 tables, as well as a basis for their derivation. Licensees may use indirect methods of estimating 

46 the inventory of some radionuclides when direct methods are not available, are not technically 

47 feasible, or are cost prohibitive. Indirect methods of estimating inventory have additional 

48 uncertainty that should be reflected in inventory estimates. When using indirect methods to 
3-8

1 estimate the inventory, the licensee should describe where the waste has been generated and 

2 what isotopes are used or created in the associated processes.

3

4 Different waste streams may have significantly different characteristics. However, it may be 

5 useful for a licensee to compare their projected inventory to the inventory of other LLW disposal 

6 facilities. The similarity or dissimilarity of the radiological inventories can help a reviewer to 

7 identify isotopes that may dictate the need for a more detailed review of the inventory estimates. 

8 In some cases, specific isotopes may be reported as less than a particular value because the 

9 isotope was below the lower detection limit of the characterization technique. It is acceptable to 

10 assign inventory based on the lower detection limit for initial screening. However, if the initial 

11 screening identifies that specific isotope inventory as significant, then additional characterization 

12 (either direct or indirect) may be necessary to determine the actual inventory with greater 

13 confidence. It is generally not acceptable to assume a value of zero for the inventory of 

14 radionuclides that are less than the lower detection limit unless there is adequate justification 

15 that the radionuclide is not present in the waste.

16 3.2.2 Chemical Environment

17

18 Estimation of the chemistry of the environment within the disposal units may be needed for use 

19 in source term models. The chemical environment may have impacts on and be affected by the 

20 lifetime of engineered barriers and waste containers, releases of radionuclides from the 

21 wasteforms, and mobility within the disposal units. The chemical environment in the disposal 

22 site is likely to be dynamic unless the site has been engineered to provide a more stable 

23 chemical environment. The chemistry of the disposal units would likely have an impact on 

24 solubility limits or retardation of radionuclides that are common parameters in modeling release 

25 rates from the source term and through the environment. Geochemical considerations may 

26 include solid composition, pH, buffering capacity, reduction-oxidation potential, partitioning 

27 processes, and the presence of colloidal particles and ligands.

28

29 In general, licensees are encouraged to collect site-specific information to justify parameters 

30 that would be expected to significantly enhance or retard the release and migration of 

31 radionuclides or that are relied upon for defense-in-depth protections. Licensees may use 

32 sensitivity analyses to identify the importance of geochemical parameters on the resulting dose 

33 either to identify whether site-specific data should be obtained for significant parameters, or to 

34 determine whether the range of values used is sufficiently conservative for parameters that do 

35 not have a significant effect. Section 2.2.2 presents guidance on the treatment of uncertainty. It 

36 may also be appropriate to use other means to justify parameters, such as values from available 

37 literature, appropriate geochemical process modeling (e.g., MINTEQA2 (EPA, 1991); EQ3/6 

38 (Daveler and Wolery, 1992; Wolery, 1992a and 1992b; Wolery and Daveler, 1992); PHREEQC 

(USGS, 1999); Geochemists Workbench® (Bethke and Yeakel, 2009a–d)),1 39 or expert judgment 

40 and expert elicitation (see Section 2.2.3.1). Licensees should judiciously select the proper 

41 literature values for significant parameters and transparently describe the similarities and 

42 discrepancies between the site conditions to which they are proposed to represent and the 

43 conditions from which the literature values were derived. A reviewer should determine if the 

 1 NRC does not endorse or recommend any specific code or model for geochemical modeling. Licensees 

are free to select and justify their particular code or model for their application. Regardless of the code 

selected, it is important that the code meet all applicable quality assurance requirements.
3-9

1 literature values were developed under appropriately representative conditions. If the conditions 

2 under which the literature values were derived are not known, then the data should not be used.

3

4 The abstraction for the chemical environment should consider the uncertainty and variability in 

5 developing geochemical parameters. Variability in the chemical environment of a site can occur 

6 spatially and temporally. Spatial variability may arise from heterogeneities in wasteforms and 

7 other materials introduced to the disposal units (e.g., backfill materials, cementitious materials). 

8 Temporal variability may arise from the degradation of engineered barriers, waste containers, 

9 and wasteforms as well as the evolution of the disposal site environment to future conditions. 

10 Licensees should consider the impacts of the evolution of the natural environment on the 

11 chemical environment as a result of degradation of the engineered barriers and natural 

12 processes such as climate change. A licensee should document their treatment of spatial and 

13 temporal variability in the chemical environment model. The documentation should provide the 

14 technical basis for the representation. For example, if a constant environment was used in the 

15 performance assessment modeling, the licensee should demonstrate why the constant 

16 environment was representative or conservative with respect to the projected future chemical 

17 environment.

18

19 Section 3.3.5.6 of NUREG-1573 presents guidance on considerations and issues associated 

20 with developing models for the chemical environment of the disposal units (NRC, 2000a). 

21 Section 4.3.3.1.4 of NUREG-1854 provides guidance on reviewing a chemical environment 

22 representation (NRC, 2007a). Additional considerations, as they apply to a specific facility, may 

23 also be found in Section 4.2.1.3.3 of NUREG-1804, Revision 2, “Yucca Mountain Review Plan,” 

24 issued July 2003 (NRC, 2003c). 

25 3.2.3 Waste Container

26

27 Waste container modeling describes the waste containers and estimates their longevity for use 

28 in source term models. In the past, licensees have generally not taken much credit for the 

29 performance of waste containers in their technical analyses; however, they are not prohibited 

30 from doing so. The waste container modeling should consider degradation processes 

31 (e.g., corrosion, mechanical degradation) for the various waste containers and should be 

32 consistent with the evolution of the chemical environment (see Section 3.2.2). The performance 

33 of the waste containers may also affect the degradation of wasteforms and is important for 

34 estimating radionuclide release in the source term models. Section 3.3.5.4 of NUREG-1573

35 includes guidance on evaluating the performance of waste containers (NRC, 2000a). In 

36 addition, waste containers are a component of an engineered barrier system. Therefore, 

37 reviewers may want to consult applicable guidance on reviewing engineered barriers in 

38 Section 3.3.4 of NUREG-1573 and Section 4.3.2 of NUREG-1854 (NRC, 2007a). Sections 

39 4.2.1.3.1 and 4.2.1.3.2 of NUREG-1804 present additional considerations for the performance 

40 of engineered barriers with respect to the waste containers (NRC, 2003c).

41

42 Metallic containers will undergo corrosion over time. Most metallic containers used in LLW

43 disposal are carbon steel; however, other types of steel may be more commonly used in the 

44 future. If a licensee takes credit for waste containers in the performance assessment, they

45 should provide a degradation analysis to justify the performance of the containers. The 

46 degradation analysis should look at the degradation processes (e.g., general corrosion, 

47 stress-corrosion cracking, localized corrosion, galvanic corrosion, radiolytic effects) and the 

48 disposal environment to estimate future performance. The degradation analysis should 
3-10

1 consider the initial condition of the containers (e.g., corrosion prior to disposal). In humid 

2 disposal environments, failure of carbon steel containers due to general corrosion is expected to 

3 be relatively quick, such that little credit for performance (e.g., delay in releases) is commonly 

4 taken in the analyses. In arid environments, licensees may take more credit for container 

5 performance, if supported by the degradation analysis which will examine the materials, 

6 environmental conditions, and degradation mechanisms. Licensees should support engineering 

7 analysis of container performance with measurements in relevant environmental conditions. In 

8 addition, licensees should provide real-world engineering analogs. The engineering analogs 

9 should be for similar exposure conditions and should not include maintenance, since the 

10 disposal containers will not be maintained after disposal. Section 2.2.3 provides additional 

11 information with respect to developing model support, including the use of analogs. 

12

13 Analysis of container failure will need to consider variability, as leakage from containers may

14 occur as soon as the first perforation occurs. The state of corrosion that defines “failure” will be 

15 different for different release mechanisms. A perforated waste container may still provide a 

16 significant barrier to mass transfer processes for some time. However, in general, when the 

17 corroded area exceeds approximately 10 percent, the container can be considered to be failed 

18 from an advective release standpoint due to shedding of water from intact areas to failed areas.

19

20 Containers made of other (i.e., non-metallic) materials will experience degradation unique to the 

21 material type. Reviewers should evaluate proposed container lifetimes for different materials on 

22 a case-by-case basis. In general, the licensee’s associated degradation analysis will need to 

23 provide the following:

24

25 • an assessment of the relevant degradation mechanisms

26 • a description of the environmental conditions including variability and uncertainty

27 • analysis of the expected performance

28 • an assessment of potential unexpected performance and the likelihood of the 

29 unexpected performance

30 • data from laboratory, field, and/or analog observations that support the expected 

31 performance

32 3.2.4 Wasteform and Waste Type

33

34 The wasteform modeling representation is used to describe wasteform performance, including 

35 variability. Wasteform performance is used in source term models. The modeling should 

36 consider degradation processes for the various wasteforms to estimate their performance 

37 consistent with the evolution of the chemical environment in the disposal units over the time 

38 period of interest (see Section 3.2.2). The performance of the wasteform is important for 

39 estimating radionuclide release in the source term models. 

40

41 The branch technical position on wasteform provides recommendations and guidance on 

42 acceptable methods to demonstrate that waste stability requirements have been met (NRC,

43 1991b). Appendix A to the technical position on wasteform provides methods to ensure the 

44 suitability of cement-stabilized wasteforms. Geochemical Aspects of Radioactive Waste 

45 Disposal describes some wasteforms and their properties (Brookins, 1984). Every wasteform is 
3-11

1 different and can have different characteristics. However, the main elements in Appendix A of

2 the wasteform BTP apply to wasteforms other than cement-based wasteforms. 

3

4 These elements include:

5 • qualification testing, 

6 • quality of sample preparation, 

7 • consideration of variability, 

8 • waste characterization, and 

9 • short- and long-term specimens including surveillance specimens. 

10

11 It is important for licensees and reviewers to note that waste stability from a structural 

12 standpoint does not necessarily translate into acceptable performance for waste leaching. 

13 Depending on the concentration of radioisotopes in the waste, the fractional release rate of an 

14 individual isotope necessary to demonstrate that the performance objectives would be met may 

15 be lower than that necessary to demonstrate waste stability.

16

17 Radionuclide release from wasteforms can be very complex, with a number of processes 

18 controlling the mobility and concentrations of radionuclides. Processes such as complexation 

19 reactions, acid-base reactions, oxidation-reduction reactions, dissolution and precipitation 

20 reactions, sorption and ion-exchange reactions, biodegradation of organic matter, and 

21 radioactive decay and ingrowth can impact waste release rates. A variety of test methods and 

22 modeling approaches have been developed to characterize the release of contaminants 

23 (NRC, 2010). The following guidance is developed on the basis of information in

24 NUREG/CR-7025. In this report, Ebert identified a number of key lessons with respect to waste 

25 release that included:

26

27 • Test results can be misinterpreted if testing artifacts are not taken into account 

28 (e.g., interval for solution replacement, flow rates, failure to reach steady state, modeling 

29 of inappropriate processes).

30 • A single test method may not identify mechanisms controlling release.

31 • It is necessary to identify which processes control release under the conditions of 

32 interest (i.e., the test conditions should be representative of expected conditions, 

33 including variability).

34 • Laboratory test results need to be translated to long-term material behavior 

35 (i.e., appropriate scaling, which may include multiple variables, is necessary).

36

37 The objective of laboratory testing is for a licensee to identify the process (or processes) that will 

38 control the release of radionuclides over long periods of time, collect data to parameterize 

39 models that quantify release, and then integrate the waste release model into a performance 

40 assessment. 

41

42 Test methods can be categorized as static, semi-static (where part or all of the solution is 

43 exchanged during the test), or dynamic (where solution flows continuously). Different test 

44 methods can yield different results; licensees should carefully interpret the test results to avoid 

45 misinterpretation or mischaracterization of the release mechanism. For example, the American 

46 Nuclear Society 16.1 (ANS, 2008) leaching method is a solution-exchange test to characterize 

47 diffusion-controlled release. The test results are sensitive to the sampling intervals that are 
3-12

1 used to measure the extent of component releases. While the test is useful for screening, it is 

2 not recommended for modeling long-term waste degradation. Methods such as American 

3 Society for Testing and Materials (ASTM) C1308-08 (ASTM, 2009), which uses constant 

4 exchange intervals, may be more useful in determining whether waste release is diffusion 

5 controlled or affinity controlled. Column tests are useful in simulating the infiltration of water 

6 through the subsurface to waste and the dissolution of contacted materials. The measured 

7 dissolution rate in column tests is a function of the flow rate of the solution and the reactive 

8 surface area that is contacted. Different column tests may need to be performed for different 

9 materials and to account for the variable emplacement of those materials. A primary challenge 

10 is the scaling of the laboratory results to the field which may have much different flow rates and 

11 reactive surface areas than used in the laboratory. Additionally, the flow rates and reactive 

12 surface areas in the wasteform can vary over time. When some materials leach, the reactive 

13 surface area may increase or decrease from the leaching process. Release rates from some 

14 materials may be much higher under conditions of wet-dry cycling as compared to non-dynamic 

15 conditions. The value of column tests (or tests such as field-scale lysimeters) is to confirm that 

16 the appropriate performance models are used to quantify the processes that control 

17 radionuclide release, especially when calibrated to the system of interest. 

18

19 Because of the complexity of waste release modeling, the NRC staff recommends that 

20 licensees complete a blind validation of the numerical model results to test the predictive 

21 capabilities of the numerical model. This is done by establishing a condition for which waste 

22 release measurements have not been completed and performing numerical simulations of the 

23 waste release that would be expected to occur under those conditions. This is followed by 

24 experimental or field measurements of the actual waste release that is compared to the 

25 numerical modeling. Licensees should establish criteria before the blind comparison to define 

26 acceptable agreement between the numerical modeling results and the experimental 

27 measurements.

28

29 Additional information can be found Section 3.3.5.4 of NUREG-1573, which includes guidance 

30 on information to characterize wasteforms and types (NRC, 2000a). Section 4.3.3.1.2 of 

31 NUREG-1854 presents guidance on wasteforms and their time-dependent degradation 

32 (NRC, 2007a).

33 3.2.5 Aqueous Release

34

35 The models for aqueous releases estimate the rate of radionuclides released from the disposal 

36 units in water contacting the wasteforms. The four general types of aqueous radionuclide 

37 releases are: (1) rinse release, (2) diffusional release, (3) dissolutional release, and 

38 (4) partitioning release. Rinse release refers to washing of radionuclides from the surface of a 

39 wasteform by infiltrating groundwater. Diffusional releases occur when radionuclide movement 

40 through a porous wasteform (e.g., a cement-stabilized wasteform) is limited by diffusion. 

41 Diffusion can occur in any media in the presence of a concentration gradient. Radionuclide 

42 releases resulting from corrosion of an activated metal or dissolution of glass wasteforms are 

43 examples of dissolutional release. Partitioning release results when radionuclide release is 

44 described by a characteristic partitioning parameter (e.g., sorption coefficient) that distributes 

45 the activity between phases in the system (e.g., between the solid wasteform and liquid water 

46 phases). 

47
3-13

1 An aqueous release model will be a function of the wasteforms, radionuclides, geochemical 

2 environment, and amount of water contacting the wasteforms. Licensees should integrate the

3 aqueous release model with these related models. Methods to represent aqueous release have 

4 been incorporated into numerical models such as Disposal Unit Source Term — Multiple 

5 Species — Distributed Failure (DUSTMS-D) (Sullivan, 2006) and Breach, Leach, Transport, and 

6 Equilibrium Chemistry (BLT-EC) (NRC, 1995e). Though it is difficult to provide model support 

7 for the overall performance assessment model, licensees can provide support for individual 

8 process models or abstractions (as discussed in Section 2.2.3). For example, licensees should 

9 provide support for numerical modeling results of aqueous release. 

10

11 Licensees should consider comparison of laboratory test results, such as leaching experiments,

12 with numerical modeling results. If field data are available, they provide information that can be 

13 used for model support. For example, observed concentrations of radionuclides in leachate

14 collection systems can be compared to modeled release rates. Section 4.3.3.2 of NUREG-1854 

15 (NRC, 2007a) and Section 4.3.1.2.4 of NUREG-1804 (NRC, 2003c) provide additional guidance 

16 on potential considerations for development or review of aqueous release models.

17 3.2.6 Gaseous Release

18

19 Models for gaseous release estimate the rate of radionuclides released in the gas phase from 

20 the disposal units to the atmosphere above the disposal facility. A gaseous release model may 

21 also need to consider the generation of nonradioactive gases and their impact on the 

22 capabilities of the disposal facility to contain and isolate the radioactive waste. Examples of

23 important radionuclides that should be considered and evaluated by licensees for gaseous 

24 release include C-14, Kr-85, Rn-222, H-3, and I-129. Gaseous releases are dependent on both 

25 generation of the gas and transport through fluids within the disposal units to the atmosphere. 

26 Discrete features such as fractures or a gap between materials can have a significant impact on 

27 gaseous release rates.

28

29 In Section 3.3.5.7.2 of NUREG-1573, the NRC staff recommends a screening approach to 

30 determine if gaseous releases from the disposal facility might contribute significantly to dose

31 (NRC, 2000a). Licensees may apply this approach to determine whether more realistic release 

32 rates would be needed. More realistic release rates may be developed by licensees using 

33 approaches described in NUREG-1573 or approaches listed in the discussions below. Realistic 

34 release rate modeling should consider the specific processes affecting generation and transport 

35 of gaseous radionuclides or empirical models of gaseous release rates. In general, licensees 

36 are encouraged to collect site-specific information to justify parameters that would be expected 

37 to significantly enhance or retard the release and migration of gaseous radionuclides should 

38 they significantly affect doses to a member of the public.

39 3.2.6.1 Gas Generation

40

41 Gases generated in a LLW disposal facility may result directly from disposal of certain waste 

42 streams (e.g., Kr-85) or from various processes external or internal to the disposal site. 

43 Processes generating gases, many of which are discussed in Section 3.3.5.7.1 of 

44 NUREG-1573, may include: (1) corrosion of metals in waste and its packaging, (2) microbial 

45 degradation of organic waste components, (3) radiolysis of waste, its packaging, or the 

46 surrounding backfill material, (4) volatilization of certain wastes (e.g., iodine-129), and (5) decay 

47 of radionuclides with primordial origin (e.g., uranium-238 or thorium-232) to gaseous progeny 
3-14

1 (e.g., radon-222). Rodwell et al. (2003) summarizes considerations for many of these 

2 processes as they relate to deep geological radioactive waste repositories. This information 

3 may be relevant for licensees to consider in a performance assessment for LLW disposal 

4 facilities. The evaluation of gas generation by a licensee should account for the nature of the 

5 waste, the radionuclides it contains, the waste packaging materials, and the chemical 

6 characteristics of the environment within the disposal units (e.g., saturation, groundwater 

7 composition, pH, Eh, and backfill materials) (NEA/OECD, 2001). 

8 3.2.6.2 Radon Emanation

9

10 Conservation of linear momentum in the alpha-decay process of high-specific-activity 

11 radionuclides can result in transport of radionuclides (i.e., alpha recoil). Typically this process is 

12 not significant for the release and transport of radionuclides from a LLW disposal facility. 

13 However, alpha recoil can be significant for certain gaseous radionuclides (e.g., radon) 

14 associated with some long-lived waste streams. Alpha recoil can result in newly created radon 

15 atoms moving away from their original location. Some end up in the particles (media) where 

16 they were created, some end up in adjacent particles, and some end up in the pore space. The 

17 radon emanation coefficient is a dimensionless parameter that expresses the fraction of radon 

18 released to the pores (compared to the total volume). Guidance on the radon emanation 

19 coefficient is provided here due to the potential significance of this unique process and radon’s 

20 presence in the decay chain for long-lived waste streams associated with uranium.

21

22 The radon emanation coefficient is strongly influenced by the liquid saturation of the medium 

23 (Nazaroff et al., 1988). As liquid saturations vary temporally and spatially, it is expected that 

24 radon emanation coefficients will also exhibit temporal and spatial variability. The values for 

25 radon emanation coefficients depend on the liquid saturation (or moisture content), the media, 

26 and the radon isotope. 

27

28 Yu et al. (1993) provides a limited compilation of radon emanation coefficients. The compilation

29 shows the large variability that can be observed; soil values ranged from 0.02 to 0.70. Nielsen 

30 and Rogers (1988) provides a probability plot of emanation coefficients for 56 soils. The values 

31 range from approximately 0.08 to 0.44 (with a mean of 0.22), consistent with the observed 

32 variability in the data of Yu et al. Regulatory Guide 3.64, “Calculation of Radon Flux Attenuation 

33 by Earthen Uranium Mill Tailings Covers” (NRC, 1989a) for calculation of radon fluxes from mill 

34 tailings covers recommends a default value of 0.35 for design, which would correspond to a 

35 98th percentile value from the Nielson and Rogers data for soil. The challenge is that the data 

36 represent both intra- and inter-site variability. If site-specific measurements are not available, 

37 licensees should select conservative values from the compilations (as was done in Regulatory 

38 Guide 3.64) because the inter-site variability should not be credited in a site-specific analysis. 

39 For long-term performance assessment calculations, it is important to recognize that the values 

40 selected for the analysis are representative of values anticipated for the evolution of site 

41 conditions. Average parameter values (spatial and temporal) can be used if it can be 

42 demonstrated that the variability is not important to estimating radon fluxes. However, because 

43 of the nonlinear processes governing radon release and transport, the variability in emanation 

44 coefficients and other parameters may drive the long-term fluxes. For example, at a moderately 

45 humid site, radon fluxes may be dominated by the periodic dry conditions.

46
3-15

1 3.2.6.3 Gas Transport

2

3 Once gases are released from the wasteforms and containers, they may migrate from the 

4 location where they are generated to the atmosphere above the disposal facility. If gaseous 

5 release is a significant release pathway, licensees may need to perform gas transport modeling. 

6 Mechanisms of soil gas transport in unsaturated systems, as are commonly encountered in the 

7 near surface, and mathematical models to simulate these mechanisms are summarized in more 

8 detail by Scanlon et al. (2000). Similar to aqueous migration in the environment, the transport of 

9 gas can also be described by advection and dispersion. 

10

11 Advection of gaseous radionuclides from a disposal unit may result from pressure gradients 

12 (e.g., barometric pumping) leading to gas flows from areas of higher pressure to areas of lower 

13 pressure. Pressure gradients, particularly those driven by atmospheric conditions, can be 

14 variable, and releases may be dominated by episodic changes. Licensees should assess the 

15 impact of this variability, if significant, on releases of gaseous radionuclides. Advection is 

16 affected by the pressure gradient, gas characteristics (e.g., viscosity), and properties of the solid 

17 media (e.g., permeability) and generally dominates transport under a pressure gradient when 

18 the mean free path of gas molecules is much less than the pore radius and the particle radius of 

19 the solid media through which the gas is moving (Cunningham and Williams, 1980). Darcy’s 

20 Law is typically used to mathematically model advective transport in gases. Depending on the 

21 dimensions of the pore space and pressure, a coupling of advective flow to diffusion occurs in 

22 gas transport as a result of the increasing importance of molecules-to-wall interactions 

23 (Bodvarsson et al., 2000). 

24

25 Diffusive processes may include Fickian molecular diffusion as well as non-Fickian processes 

26 such as Knudsen and non-equimolar diffusion depending on the pressure gradients, dimensions 

27 of the pore space, and mean free paths of the molecular motion. Several mathematical models 

28 (e.g., Fick’s Law, Stefan-Maxwell equations, Dusty Gas Model) exist for diffusive transport 

29 depending on pressure gradient, permeability, and concentration conditions (Scanlon et al., 

30 2000). Because of the challenges in mechanistically modeling diffusion through heterogeneous 

31 media, an empirical effective diffusivity of the gas in the geologic material through which the gas 

32 migrates is often used to account for the various characteristics. The effective diffusivity is 

33 sensitive to the availability of interconnected air space and thus to the total porosity, air-filled 

34 porosity, moisture content, tortuosity of pores and fractures, and soil structure. Various 

35 relationships have been hypothesized to estimate effective diffusion coefficients in soils (e.g., 

36 Buckingham, 1904; Penman, 1940; Millington, 1959; Millington and Quirk, 1960; Troeh et al., 

37 1982; Nielson et al, 1984; Kristensen et al., 2010), but their predictive capabilities may be 

38 sensitive to actual site conditions (Moldrup et al., 2004; Allaire et al., 2008). Also, literature 

39 values for effective diffusion coefficients often employ varied definitions. For example, Culot et 

40 al. (1976) note that various definitions of the effective diffusion coefficient for gas diffusion of 

41 radon in concrete are contained in the literature. Regulatory Guide 3.64 (Calculation of Radon 

42 Flux Attenuation by Earthen Uranium Mill Tailings Covers) provides guidance on an acceptable 

43 method to assess long-term radon diffusion through earthen cover materials for uranium mill 

44 tailings sites and may also be applicable to LLW disposal (NRC, 1989a). Licensees should 

45 demonstrate that parameters describing diffusion of gaseous radionuclides are adequately 

46 justified and account for actual site conditions including uncertainty and variability and the effect 

47 of the evolution of site conditions over time on the parameters.

48
3-16

1 Licensees that undertake more detailed modeling of gas phase transport from disposal units 

2 may need to consider some or all of the aforementioned processes and models in the models

3 for gaseous release, depending on the site conditions. For instance, molecular diffusion may be 

4 the only mechanism to consider for equimolar gases under isobaric, isothermal conditions that 

5 diffuse in pores much larger than the mean free path of the gas molecules, whereas a Dusty 

6 Gas Model approach might be necessary for gas migration through low-permeability materials 

7 under a pressure gradient to account for coupled advective-diffusive processes. Important 

8 parameters of these models (e.g., permeability, saturation, pore space dimensions) should be 

9 well-supported and, to the extent practicable, based on conditions that are representative of the 

10 site environment. In other words, site observations supporting parameter values should be 

11 made under expected conditions for the site and not biased toward conditions that are rare, 

12 extreme, or unlikely at the site, particularly if the unlikely conditions lead to overly optimistic 

13 behavior of the disposal system. Uncertainty in the parameters may also be significant. For

14 instance, permeability may span a broad range of values depending on the soil types (Nazaroff, 

15 1992). Therefore, the analysis of uncertainty in significant parameters should consider the 

16 range of conditions expected for the disposal site. Partitioning of gaseous radionuclides 

17 between phases may also affect transport of radionuclides that are generated in the gas phase. 

18 In addition, biological activity and isotopic dilution (i.e., dilution of a radioisotope with stable 

19 isotopes of the same element) may also have a significant impact on the transport of certain 

20 gaseous radionuclides (e.g., carbon-14) (Bracke and Müller, 2008).

21

22 Changes in cover properties that are responsible for increasing the hydraulic conductivity may

23 also affect the migration of radon-222 through the cover. Licensees can design cover materials 

24 to inhibit radon migration, so that radon flux at the surface is reduced by radioactive decay 

25 during transport to the surface (NRC, 1984). However, preferential flow paths can develop in 

26 clay barriers in conventional covers (Albright et al., 2006a; Albright et al., 2006b). Preferential 

27 flow paths may lead to advective transport of radon. Typically radon migration is controlled by 

28 diffusion which is dependent on the inter-connected porosity and moisture content of the soil or 

29 clay radon barrier (NRC, 1989a). In addition to enhanced diffusive transport, advection may 

30 also contribute to significant migration of radon via these preferential pathways (NRC, 1984). 

31 NUREG/CR-3395, “Influence of Cover Defects on the Attenuation of Radon with Earthen Cover” 

32 states that models for radon migration in cracks are typically designed for simple geometries 

33 that do not account for all dynamic processes resulting in a possible underestimation of radon 

34 flux (NRC, 1983a). The recommendation from NUREG/CR-3395 is to apply cover design and 

35 development methods so as to avoid the formation of defects (i.e., avoid cracks) in light of this 

36 uncertainty.

37 3.2.7 Direct Release

38

39 Licensees may need to represent the direct release of radionuclides into the environment. 

40 Models for direct release estimate the rate of radionuclides released directly to the surface as 

41 the result of natural FEPs. Anthropogenic direct releases are considered in the inadvertent 

42 intrusion assessment, which is discussed in Section 4.0. Direct releases may be caused by 

43 processes such as plant uptake, bioturbation (e.g., burrowing animals), natural disruptive events 

44 (e.g., faulting), and geomorphological processes (e.g., erosion). For guidance on consideration 

45 of natural disruptive events and geomorphological processes, licensees and reviewers should 

46 consult Section 5.0 of this document regarding the demonstration that the performance

47 objective for site stability would be met. Direct release can impact the demonstration that

48 10 CFR 61.41 and 10 CFR 61.42 would be met. Licensees should integrate direct release 
3-17

1 calculations with other models as necessary, such as with climate and radionuclide transport. 

2 Additionally, direct release calculations should consider the long-term evolution of the site 

3 environment and its impact on the evolution of the site’s geomorphology and ecology. For 

4 instance, future climate states may be wetter, leading to increased erosion rates or changes in 

5 vegetation or communities of burrowing animals. Erosion or burrowing animals are processes 

6 that can lead to direct releases. Licensees may use analog sites with climates similar to the 

7 expected future climate for the disposal facility to justify the progression of ecological 

8 communities at the disposal site.

9 3.2.8 Biota Enhanced Release

10

11 “Biota enhanced release” is defined in this section as the direct and indirect processes that 

12 facilitate release of radioactivity from a disposal facility. Damage of an engineered cover by 

13 burrowing animals is an example of an indirect effect of biota on performance of the LLW 

14 disposal facility. Biotic transport, as defined in Section 3.3.4 of this document, is defined as the 

15 transport of radionuclides in the environment via biota after radioactivity is released.

16

17 Biota enhanced release is not usually considered to be a significant pathway in performance 

18 assessments. However, the importance of biotic release and subsequent human exposures 

19 depends on a variety of factors, including site location, design of the disposal facility, and the 

20 wasteforms disposed of at the site. For example, plant uptake and the impact of burrowing 

21 animals is much more likely for shallow disposal than for deeper disposal. Therefore, a 

22 reviewer evaluating radionuclide release from a site should include consideration of the impacts 

23 associated with biotic enhanced release. Current LLW disposal facilities attempt to minimize 

24 the impacts of plants and animals by incorporating design features such as engineered cover 

25 systems and the use of concrete and steel in the construction of the vaults and waste 

26 containers.

27

28 Section 5.9.1, “Background,” of National Council on Radiation Protection and Measurements 

29 (NCRP) 152, “Performance Assessment of Near-Surface Facilities for Disposal of Low-Level 

30 Radioactive Waste,” lists numerous references of studies of biotic processes conducted at LLW 

31 disposal facilities (NCRP, 2005). The long-term contributions of biotic enhanced release 

32 processes, including their impacts on engineered wasteforms and barriers, associated with 

33 recent LLW disposal facilities are uncertain. Biotic enhanced release has been observed for a 

34 wide variety of contaminant release problems; however, biotic enhanced release is typically not 

35 a primary release mechanism.

36

37 A study conducted by McKenzie et al. (NRC, 1982d) identifies and discusses the mechanisms 

38 of biotic release and transport and human exposure associated with arid and moist sites. Based 

39 on this qualitative analysis, the authors of that study concluded that penetration of buried waste 

40 and enhanced release by burrowing animals and plant roots were potentially important 

41 mechanisms in the case of waste disposed in trenches. These studies demonstrate that there 

42 are three general primary effects of biota on an LLW disposal facility: biotic release, biotic 

43 transport (Section 3.3.4), and secondary transport. This section focuses on intrusion and 

44 indirect effects which can enhance release through other mechanisms. Depending on the 

45 characteristics of the disposal facility and its environmental settings, biota could influence the 

46 performance of a disposal system and lead to significant pathways of human exposure.

47
3-18

1 Intrusion and active transport occur when biota penetrate the waste zone causing the 

2 redistribution of waste material or contaminated soil and resultant release from the disposal 

3 facility. Including land management, such as limiting vegetation and minimizing the intrusion by 

4 animals, within the scope of management of LLW disposal facilities can reduce the impact of 

5 biointrusion during the period of institutional control. Licensees may want to consider 

6 observations of biotic intrusion made during the post-closure observation and maintenance 

7 period when revising biotic intrusion assessment in future iterations of the performance 

8 assessment analysis. A licensee should develop an assessment of projected biotic intrusion

9 after the post-closure observation and maintenance period based on the estimated evolution of 

10 the disposal site. The projection should not include land management practices that will be

11 used during the institutional control period to limit the effects of biotic intrusion, unless those 

12 practices are passively effective after the institutional control period. Many engineered cover 

13 designs employ biointrusion barriers. If the licensee provides a technical basis for the barrier’s

14 passive performance, they can take credit for biointrusion barriers in the performance 

15 assessment projections. Analog information may be useful in supporting predictions of the 

16 performance of biointrusion barriers.

17

18 A qualitative assessment of the biotic enhanced release and its contributions to the dose may 

19 be sufficient depending on the site, its characteristics, and the characteristics of the waste 

20 disposed of at the site. In some cases, biological processes can strongly influence the release 

21 of radioactivity from LLW disposal facilities because the near-surface environment is biologically 

22 active. 

23

24 In addition to direct release (e.g., uptake of radionuclides by deeply rooted vegetation), biota 

25 can indirectly enhance release when plants and animals modify the buried waste or the design 

26 of the LLW disposal facility in such a way that there is an increased potential for radionuclide 

27 release and transport. The following are examples of these processes:

28

29 • Burrowing animals or root systems develop tunnels or conduits that can enhance the 

30 transport of groundwater or increase gaseous release (e.g., bioturbation of a clay layer).

31 • Plant roots provide ligands that provide a source for radionuclides to bind, resulting in 

32 the formation of soluble radionuclide-organic complexes.

33 • Microbial degradation of the wasteforms leads to the enhanced generation of gases or 

34 creates waste materials that are more soluble and therefore more easily transported 

35 away from the site.

36 • Microbial enhanced degradation of waste containers or concrete vaults in a disposal 

37 system occurs.

38 • A drainage layer is plugged in an engineered cover system.

39 • Pedogenic processes can modify the soil properties of an evapotranspiration cover 

40 system.

41

42 These examples are not meant to be an exhaustive list; other biological processes may affect

43 the performance of the waste disposal facility. The licensee should provide a technical basis for 

44 its consideration of biological processes on the performance of the disposal facility, including the 

45 basis for excluding biological processes from the evaluation.
3-19

1 3.3 Radionuclide Transport

2

3 This section of guidance focuses on assumptions, data, and models (conceptual and 

4 mathematical) that a licensee may use to develop a radionuclide transport model for the 

5 performance assessment. A licensee may use radionuclide transport modeling to estimate the 

6 transport of radionuclides in environmental media (e.g., waste, soil, air) to receptor locations 

7 (i.e., human access locations) over time. Radionuclides released from LLW disposal facilities 

8 (see Section 3.2) can be transported through the environment by groundwater, surface water 

9 (including suspended sediments), air, and biota (e.g., rodents, insects). Radionuclide doses 

10 may also be linked directly to the media themselves, such as the consumption of contaminated 

11 well water, or indirectly between pathways, such as the transfer of radionuclides from the 

12 groundwater to the surface water and ultimately to the pathways comprising the food chain.

13

14 The significant mechanisms of radionuclide transport from the LLW disposal facility to the 

15 environment accessible to the receptor should be identified and assessed by a licensee. 

16 Section 2.5.3 contains guidance on identifying and screening FEPs. Depending on the 

17 relevance of the FEPs, some disposal facilities will require detailed consideration of these 

18 features and processes in the performance assessment by licensees, whereas simplified 

19 analyses may be justified for other sites and disposal practices. Radionuclide transport models 

20 may need to be considered carefully for those facilities for which licensees take credit for 

21 significant delay in radionuclide migration from the disposal facility to the receptor and those for 

22 which there is little model support. A simplified analysis may be acceptable for facilities for 

23 which the simplified analysis can be shown to be clearly conservative or for which the simple 

24 model is well-supported by multiple lines of evidence, including field tests that demonstrate that 

25 the model and its parameters appropriately represent or bound site conditions.

26

27 The complexity of most sites and proposed disposal practices usually results in radionuclide 

28 transport models being developed on a site-specific basis. The radionuclide transport modeling

29 process should appropriately account for temporal and spatial variability in the environmental 

30 transport pathways resulting from natural heterogeneity in environmental media and the 

31 evolution of site conditions over the analysis timeframe. Variability in natural systems is often 

32 scale-dependent. For instance, wind speeds may vary over a wide range of time scales (e.g., 

33 hourly, daily, annually). The level of variability considered should be consistent with the 

34 assessment context. In this case, the context is annual doses to an offsite receptor. It would be 

35 appropriate to consider how the scale-dependent variability may affect the outcome of the 

36 assessment through sensitivity analyses. If scale-dependent variability is found to be 

37 significant, a licensee may need to include short-term variability in the radionuclide transport 

38 modeling. For example, short-term variability may be considered when developing a distribution 

39 of wind speeds that could occur in any given year (in the above example), while longer-term 

40 variability may be treated as variability in the mean annual wind speed. The approach a 

41 licensee selects to represent the uncertainty and variability resulting from natural 

42 heterogeneities should not bias the outcome of the assessment such that radiological doses to

43 an offsite receptor would be significantly underestimated. Licensees should document analyses 

44 supporting their conclusion that the treatment of uncertainty and variability in radionuclide 

45 transport abstractions does not result in biases that significantly underestimate radiological 

46 doses to an offsite receptor.

47
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1 Licensees should ensure that the modeling of radionuclide transport is appropriately integrated

2 in space and time with the source term releases (see Section 3.2). The radionuclide transport 

3 modeling should also be consistent with modeling of climate and infiltration, as well as the 

4 characteristics of the biosphere and the receptor (e.g., human access locations). Guidance on 

5 modeling of climate and infiltration is provided in Section 3.3.3 of NUREG-1573 (NRC, 2000a) 

6 and Section 4.3.1 of NUREG-1854 (NRC, 2007a). Guidance on biosphere characteristics and 

7 dose modeling appears in Section 3.4 of this document and in Section 3.3.7 of NUREG-1573 

8 and Section 4.3.5 of NUREG-1854.

9

10 Section 2.2 of this document describes general technical considerations applicable to the

11 development and review of a radionuclide transport model for a LLW disposal facility 

12 performance assessment. As mentioned previously, licensees and reviewers may also consult 

13 NUREG-1573, Section 3.3.6, for further guidance on transport modeling. Section 4.3.4 of 

14 NUREG-1854 may assist reviewers by providing technical considerations for acceptable 

15 radionuclide transport modeling. The following sections of this document provide specific 

16 guidance on aspects of radionuclide transport modeling in various environmental media. 

17 3.3.1 Groundwater Transport

18

19 A licensee may perform groundwater transport modeling to assess the concentrations of 

20 radionuclides transported from LLW disposal facilities through the unsaturated and saturated 

21 zones. Groundwater transport is among the most likely processes for radionuclides to be 

22 transported from LLW disposal facilities. In addition to being a direct source of exposure to 

23 radionuclides, groundwater can transfer radionuclides to other exposure pathways that may 

24 ultimately result in doses to members of the public. These transfer processes include 

25 discharges to surface water through seeps and springs and vapor releases to the atmosphere.

26

27 A licensee may need to model the migration of radionuclides in groundwater and determine the 

28 contribution of the groundwater pathway to the total dose to the average member of the critical 

29 group. The complexity of hydrogeologic and geochemical conditions of the site may need to be 

30 reduced to form simplified representations of groundwater flow and transport. A licensee should 

31 provide a technical basis that the reduction in complexity has not resulted in the loss of 

32 information necessary to demonstrate that the performance objectives of Subpart C will be met.

33

34 Development of a groundwater transport model should consider relevant FEPs associated with 

35 the site (Section 2.5 presents guidance on FEPs). The characteristics of the wasteforms 

36 designed to limit releases of radionuclides as well as the physical characteristics of the site 

37 (e.g., hydrogeology, faults, and groundwater flow boundaries) should be considered by 

38 licensees. Physical and geochemical processes (e.g., advection, dispersion, diffusion, sorption, 

39 precipitation) are the primary phenomena included in radionuclide transport models. 

40

41 The transport of radionuclides in groundwater can be affected by many different physical 

42 phenomena associated with the hydrogeology of the system, as well as by the physical 

43 components (e.g., engineered barriers) associated with the facility. Information on the 

44 hydrogeologic characteristics of the site including the stratigraphy, the existence of preferential 

45 pathways (e.g., fractures), spatial variations in properties, and anthropogenic features that may 

46 affect groundwater flow should be considered by licensees. Processes such as diffusion, 

47 mechanical dispersion, and colloid-facilitated transport should be considered. In the case of the 

48 unsaturated zone, variations in properties and features such as thickness of unsaturated strata 
3-21

1 and hydraulic properties can impact water flow. The spatial variation of hydrologic properties of 

2 aquifers and aquitards, as well as geologic features (e.g., fractures) and engineered structures 

3 (e.g., slurry walls), can influence groundwater flow velocities, gradients, volumes, and estimates 

4 of recharge to and discharge from the aquifers associated with the saturated zone. In addition, 

5 licensees should consider differences associated with the unsaturated zone and saturated zone. 

6 Flow of contaminants in the unsaturated zone may be strongly influenced by discrete features, 

7 such as fractures or other heterogeneities. A licensee should provide their site-specific 

8 characterization data of the spatial variation in hydrologic properties and geologic features. If 

9 site-specific information is not available, the licensee should demonstrate how variability in 

10 properties and features was incorporated and evaluated in the radionuclide transport modeling.

11

12 A licensee should evaluate the chemical characteristics of the groundwater and host rocks 

13 including sediments. Chemical processes such as sorption, precipitation, ion exchange, and 

14 redox reactions can affect radionuclide transport in groundwater. NUREG-1573 (NRC, 2000a) 

15 and NUREG-1854 (NRC, 2007a) provide guidance for assessing groundwater transport. 

16 Additionally, NCRP has published a summary of processes and parameters applicable to the 

17 development of groundwater transport models in the performance assessment for a LLW 

18 disposal facility (NCRP, 2005). 

19

20 Table 3-1 summarizes site characteristics, physical processes, and geochemical processes that 

21 should be considered by licensees when developing groundwater transport models. Reviewers 

22 should use Table 3-1 to evaluate the scope of a licensee’s radionuclide transport modeling.

23

24 Modeling groundwater transport can be challenging for a variety of reasons. The characteristics 

25 and features that introduce challenges include the following:

26

27 • the complex interactions of the hydrologic, geologic, physical, and chemical processes 

28 associated with the system

29 • the site-specific flow and transport characteristics for modeling the initial release from 

30 the disposal facility (i.e., source term) through the unsaturated zone to the groundwater

31 • the limited availability of site-specific data to describe the FEPs for a specific 

32 groundwater system

33 • the natural heterogeneity of site characteristics associated with groundwater flow and 

34 transport and changes in characteristics over long times

35 • numerical analysis techniques may introduce modeling artifacts, such as numerical 

36 dispersion, which can bias results

37 A licensee should provide a description in sufficient detail of their radionuclide transport model 

38 to allow a reviewer to independently determine that the aforementioned challenges were 

39 adequately addressed. Some disposal facilities may require detailed consideration of these 

40 challenges in the groundwater transport model, whereas simplified analyses may be justified for 

41 other sites and disposal practices. The groundwater transport models may need to consider 

42 these site-specific complexities for (1) facilities for which licensees take credit for significant 

43 delay in radionuclide migration via groundwater from the disposal facility to the receptor, (2) 

44 those that take significant credit for reductions in concentration during transport, (3) those relied 

45 upon for defense-in-depth, and (4) those for which there is little model support.

46 
3-22

1 Table 3-1 List of Parameters and Processes of Concern Associated with the 

2 Groundwater Transport Pathway

Hydrogeologic Characteristics

Stratigraphy

Geologic structures (e.g., faults)

Ground-water flow boundaries

Zones of groundwater recharge and discharge

Soil and rock characteristics (e.g., porosity, texture, mineralogy)

Fractures and fast pathways (i.e. discrete features)

Quantity and quality of groundwater

Physical Processes

Advection

Dispersion

Diffusion

Deposition

Radioactive decay and daughter ingrowth

Geochemical Processes or Characteristics

Composition of groundwater

Geochemical environment (e.g. pH, Eh, organic content)

Sorption / Desorption

Precipitation

Complexation

Resuspension

Redox reactions

Colloids

Design Features

Well depth

Screen length

Physical components (i.e., engineered barriers)

3

4 A simplified analysis may be acceptable for facilities for which the simplified analysis can be 

5 shown to be clearly conservative or for which the simple model is supported by multiple lines of 

6 evidence, including field tests that demonstrate that the model and its parameters appropriately 

7 represent or bound site conditions. A simplified groundwater modeling approach may be 

8 sufficient to gain an adequate understanding of the FEPs associated with a specific system. 

9 More detailed groundwater flow and transport modeling, however, may be needed to identify 

10 important processes and assess the impacts of uncertainty and variability to support the 

11 simplification of the conceptual groundwater flow system. 

12

13 An important consideration in modeling groundwater flow and transport is the inherent spatial 

14 variability of structural, physical, and chemical properties of the environmental media and the 

15 uncertainty in characterizing the spatial domain. Heterogeneity in flow and transport processes 

16 can vary over many orders of magnitude from the molecule scale to the basin scale (Ledoux 

17 and deMarsily, 1997). Some properties exhibit scale-dependence (e.g., dispersivity) 

18 (Neuman, 1990). When warranted by the significance of the transport pathway, a licensee may 

19 need to include a more explicit representation of the inherent variability. However, it is 

20 challenging to describe the full spectrum of the heterogeneity due to the amount of 

21 characterization that would be needed. Rubin (2003) summarized some approaches (e.g., 
3-23

1 estimation, simulation) that licensees may consider for use in representing the inherent 

2 variability and assessing its impact on groundwater flow and transport.

3

4 Licensees should demonstrate that the method used to assess groundwater transport does not

5 bias the outcome such that radiological doses to an offsite receptor are significantly 

6 underestimated. A possible method to demonstrate this is through comparisons with viable 

7 alternative conceptual models of the groundwater flow system that are consistent with the 

8 current understanding of the site.

9

10 For the performance assessment of a LLW disposal facility, the parameterization of processes 

11 should be made at the scale (spatial and temporal) over which groundwater transport is 

12 evaluated. However, measurements of flow and transport parameters on these large scales are 

13 often not available or practical. Additionally, smaller scale measurements, such as from 

14 laboratory or field tests, can provide useful information to models, but the models can quickly 

15 become computationally complex. For instance, performance assessments often represent 

16 sorption of radionuclides on soils through the use of a distribution coefficient (Kd). In a 

17 groundwater transport abstraction for a disposal facility, this coefficient may be representing the 

18 inherent heterogeneities in the soil properties and groundwater chemistry over large spatial 

19 scales. In practice, distribution coefficients are often measured in the laboratory or field on a 

20 much smaller scale than what is modeled in the performance assessment and may not 

21 represent the actual heterogeneities in the groundwater pathway that the radionuclides might 

22 encounter. Therefore, licensees should consider approaches to demonstrate the 

23 appropriateness of model parameters for the scale of concern for the performance assessment 

24 (e.g., upscaling, inverse modeling). The following two sections provide examples of specific 

25 issues to consider when evaluating the unsaturated zone and the saturated zone.

26 3.3.1.1 Modeling the Unsaturated Zone

27

28 This section highlights issues specific to the flow and transport of groundwater through the 

29 unsaturated zone and the flow and transport modeling that affect the ability of a licensee to 

30 develop a defensible performance assessment. Section 4.3.4.2.3 of NUREG-1854 (NRC, 

31 2007a) and NCRP-152 (NCRP, 2005) provide a general discussion of the issues that should be 

32 covered. Additional details regarding flow and transport in the unsaturated zone appear in other 

33 documents, including Bear (1979), Bear and Verruijt (1987), Domenico and Schwartz (1998), 

34 Evans et al. (2001), Faybishenko et al. (2000), Freeze and Cherry (1979), Hillel (1980), Looney 

35 and Falta (2000), NAS/NRC (2001), Nielsen et al. (1986), and Todd (1980). Documents such 

36 as NAS/NRC (1990) and NCRP (1984) discuss unsaturated flow and transport models. NRC37 sponsored research related to the characterization and performance confirmation monitoring of 

38 the unsaturated zone is included in various references (Meyer et al., 1999; Rockhold, 1999;

39 NRC, 1993b; NRC, 1999b; NRC, 1999c).

40

41 Modeling of flow and transport of radionuclides in the unsaturated zone provides a link between 

42 releases from an LLW disposal facility and the saturated zone. Key inputs for a licensee to 

43 consider when developing unsaturated zone flow and transport models include the infiltration 

44 rate and the release rate of radionuclides from the source; the resulting outputs, flow rate and 

45 radionuclide migration, are used as inputs when modeling saturated zone flow and transport.

46

47 Variations in soil characteristics and moisture content in the unsaturated zone as well as the 

48 complex relationships between hydraulic head, moisture content, and hydraulic conductivity can 
3-24

1 make modeling these systems difficult. Site-specific conditions can further influence modeling 

2 of the unsaturated zone. For example, at moist sites the disposal facility generally lies within a 

3 few meters or tens of meters of an underlying aquifer. The flow and transport of groundwater

4 and radionuclides over this short distance may not be significant when assessing the overall 

5 performance of the site. Modeling the unsaturated zone for these types of sites may require

6 only a simple approach such as not taking any credit for delay or dilution associated with the 

7 transport between the LLW disposal facility and the aquifer. At arid sites, however, the aquifer 

8 may lay several tens or even hundreds of meters below the disposal facility footprint. In this 

9 case the time required for radionuclides to reach the aquifer may allow for more physical and 

10 chemical interactions with the environment and significantly reduce the concentrations or the 

11 timing of when radionuclides reach the aquifer. As discussed in Section 2.2.3, model support is 

12 essential for providing a basis for a licensee’s simulation results. Model support can be used to 

13 justify that the complexity (or lack thereof) in a licensee’s model is appropriate. A licensee 

14 should demonstrate that 1) the complexity is necessary, 2) site-specific information is available 

15 to adequately characterize the complexity, and 3) model support is available to support the 

16 analysis results. 

17

18 Arid sites may experience high evapotranspiration rates creating a situation where there is no 

19 communication or very limited communication between the disposal facility and the water table. 

20 This is especially true for the disposal of inventories containing primarily short-lived 

21 radionuclides. Modeling this situation is typically more complex than a traditional flow and 

22 transport analysis because spatial and temporal variability can more significantly impact the 

23 results. For either thin or thick unsaturated zones, discrete features such as fractures or 

24 abandoned wells can limit the effectiveness of the unsaturated zone as a barrier. Completion of 

25 field tests and analysis of field observations are recommended to reduce the uncertainty 

26 associated with discrete features in the unsaturated zone. Tracer studies over a large area can 

27 be effective.

28

29 Many of the processes that govern transport of radionuclides in the unsaturated (vadose) zone 

30 (i.e., advection, dispersion, and sorption (retardation)) are essentially the same as those that 

31 govern transport in the saturated zone. However, since the pore spaces of the vadose zone are 

32 only partially filled, the effective hydraulic properties have a nonlinear dependence on soil 

33 moisture content. Flow and moisture dependencies are commonly represented with moisture 

34 characteristics curves. Moisture characteristics curves can be difficult to define and to use to 

35 characterize the spatial variability over the area of a disposal site. 

36

37 Because of the nonlinear relationships, flow in the unsaturated zone can be strongly influenced 

38 by extreme conditions (e.g., a zone of discrete features such as fractures, wetter years, or 

39 seasonal variations). Therefore, it is necessary for a licensee to understand how hydraulic 

40 conductivity changes as a function of the amount of moisture and the pressure in the soil. For 

41 example, when the fluid pressure is less than the atmospheric pressure, the pressure head is 

42 negative allowing suction to occur (Freeze and Cherry, 1979). Additionally, the infiltration rate 

43 may vary with time as a result of the intermittent nature of precipitation. It may also vary with 

44 the depth of the unsaturated zone, resulting in greater water content closer to the surface and 

45 dampening with depth. 

46

47 Given these complexities, a graded approach that starts with the simplest and most 

48 conservative approach is recommended for licensees. This technique starts with a relatively 

49 simple representation of the system - assume homogeneous conditions and no time delay or 
3-25

1 dilution. Problems may arise with this simple approach for situations in which the unsaturated 

2 media are known to be highly structured or fractured, or if it is determined that there is 

3 significant time or distance to reach the saturated zone. For these cases, the modeling 

4 approach would need to be more complex. In general, a licensee should only include 

5 complexity if that complexity can be supported. However, in the iterative modeling process a 

6 licensee may need to add complexity to determine if the complexity is significant. If it is, support 

7 may need to be developed or a conservative approach may be necessary.

8 3.3.1.2 Modeling the Saturated Zone

9

10 In a performance assessment for a LLW disposal facility, a licensee should consider the flow of 

11 water through underlying aquifers and the phenomena that influence transport of radionuclides 

12 in groundwater. A licensee should have an adequate understanding of the input flux of 

13 radionuclides from the unsaturated zone. For example, although an increase in flow velocity 

14 into the saturated zone would appear to yield higher concentrations of radionuclides being 

15 released to offsite locations or other exposure pathways due to decreased travel time, it also 

16 results in a greater volume of water available to dilute radionuclide concentrations slowly 

17 percolating into the aquifer from the unsaturated zone. Thus, it is possible that the higher flow 

18 rate can actually result in a decrease in radionuclide concentrations. For a licensee to complete 

19 a sufficient evaluation of the groundwater transport in the saturated zone requires an 

20 understanding of the groundwater velocity field, radionuclide-specific release rates to the 

21 saturated zone, and the phenomena that influence the transport of radionuclides in groundwater

22 (e.g., sorption, advection, diffusion, dispersion, and radioactive decay). Ultimately saturated23 zone radionuclide concentrations are typically converted to two primary types of doses for 

24 receptor scenarios: the groundwater dose resulting from direct ingestion and the groundwater

25 component of the dose resulting from ingestion of foodstuffs grown using contaminated water. 

26 Other pathways are possible and may be significant at specific sites though they are less 

27 common.

28

29 The velocity field provides a representation of the groundwater flow in the saturated zone. 

30 A velocity field, which is a function of location and time, is often derived using indirect, largely 

31 unmeasured, spatially variable, or unknown physical properties and boundary conditions that 

32 can be interpreted only subjectively. Licensees may develop the velocity field by using a site33 specific model of the flow system that has been calibrated using site-specific data on hydraulic 

34 head. The hydraulic head can be obtained using monitoring wells and field experiments (e.g., 

35 slug tests). However, the derived velocity field is likely not going to be a unique representation 

36 for a specific site, as multiple velocity fields can often be used to describe a site equally well. As 

37 a result, licensees should provide justification and support for their assumptions. It is often 

38 useful for a licensee to include the uncertainty associated with the calibration and inverse 

39 modeling to derive groundwater flow fields in the performance assessment groundwater flow 

40 model. Therefore, the significance of the uncertainty can be directly assessed.

41

42 Licensees may describe groundwater flow in the saturated zone using a simplified flow 

43 equation, which combines Darcy’s Law with the principle of conservation of mass. This 

44 approach may be further simplified by assuming a steady-state flow rate and uniform recharge 

45 to the aquifer over the spatial domain being considered. However, uncertainties such as the 

46 presence of heterogeneities in the system, variable or unknown boundary conditions, and 

47 scaling of data obtained from core samples to field conditions need to be evaluated by licensees 
3-26

1 when using this approach to groundwater flow modeling. A licensee must provide model 

2 support for the simplified flow model results.

3

4 Various other phenomena may influence the development of models to represent the transport 

5 of radionuclides in groundwater. Radioactive decay is well understood, presuming that there is 

6 sufficient understanding of the source term; however, approximations may be required when 

7 considering radioactive decay and ingrowth associated with disposal sites containing many 

8 radionuclides. For radionuclides with chain decay that results in multiple progeny, the behavior 

9 of the progeny in the environment may be substantially different than the behavior of the parent 

10 radionuclide (e.g., the Kd for Am-241 may be substantially different from the Kd for Np-237). In 

11 cases where a long-lived radionuclide has decay products of radiological significance, selecting 

12 the lowest plausible Kd value may not result in the highest estimated dose. A licensee should 

13 provide sufficient justification for the use of specific Kd values to evaluate sorption and 

14 desorption. Ideally, site-specific Kd values should be used. However, in cases where site15 specific values are not available generic values may be assessed. Numerous publications have 

16 documented generic values; some even group distribution coefficients according to soil type 

17 (Sheppard and Thibault, 1990; Yu et al., 1993). Licensees should use these values with caution 

18 as they often include variation between sites but do not include potential measurement errors 

19 (e.g., exceeding solubility limits in measurements of sorption parameters). In addition, the 

20 measurements in the compilation may have been taken using significantly different techniques.

21

22 Ultimately, a licensee can best treat uncertainties associated with radionuclide transport in

23 groundwater by using multiple conceptual models. This enables a licensee to examine the 

24 effects of different credible assumptions and provides a better understanding of which 

25 processes are most sensitive and may need to be considered in greater detail.

26 3.3.2 Surface Water Transport

27

28 Surface water transport modeling is used to assess the radionuclide concentrations in surface 

29 water (e.g., rivers, lakes) at human access locations at or beyond the site boundary. By 

30 demonstrating that the requirements for site suitability in 10 CFR 61.50, “Disposal Site 

31 Suitability Requirements for Land Disposal,” and site design in 10 CFR 61.51, “Disposal Site 

32 Design for Land Disposal” are met, it is unlikely that a licensee will estimate that significant 

33 amounts of surface water would directly intersect a waste disposal facility. Therefore, 

34 radionuclides will likely be transported from a LLW disposal facility via other pathways rather 

35 than a surface water pathway. However, radioactivity released to an aquifer may discharge to 

36 surface water bodies before contact with or use by the public. Mechanisms by which 

37 radionuclides may enter the surface water include groundwater discharge and deposition 

38 associated with atmospheric transport, and overland flow (e.g., associated with erosion).

39

40 Licensees should evaluate hydrological and chemical conditions at the site to form simplified 

41 representations of surface water flow and transport if surface water transport is a viable 

42 exposure pathway for public receptors. Simplified analyses to assess surface water transport 

43 may be justified at most disposal facilities sited and constructed according to 10 CFR Part 61 

44 requirements. However, some facilities, depending on site-specific conditions, may require 

45 detailed consideration of surface water transport. Licensees should demonstrate that the 

46 method used to assess surface water transport does not bias the outcome such that radiological 

47 doses to an offsite receptor are significantly underestimated. A licensee should consider 
3-27

1 contributions to surface waters from other environmental media (e.g., groundwater seepage, 

2 atmospheric deposition, overland runoff, and erosion).

3

4 Radionuclides that are released to surface waters may be transported by a variety of processes 

5 including water flow, sediment transport, and bioturbation (Onishi, 2008). Radionuclides 

6 entering surface water systems may remain in solution, be suspended in the water column 

7 attached to particulates, or settle to the bottom and become associated with the sediments. 

8 Advection and dispersion are typically dominant processes, especially for soluble radionuclides 

9 in flowing water. Radionuclides that readily partition to suspended particles or sediments may 

10 also be significantly affected by sediment transport processes (e.g., deposition, erosion,

11 bioturbation). Other hydrologic processes, such as turbulence and thermal and density 

12 stratification, can also affect the distribution of radionuclides in surface waters. The chemistry of

13 the surface water, rocks, and sediments may affect the speciation or partitioning of 

14 radionuclides (e.g., due to sorption, precipitation, ion exchange, volatilization). Licensees 

15 should consider these phenomena when developing surface water transport models.

16 Section 3.3.6.2 of NUREG-1573 provides guidance for the assessment of surface water 

17 transport (NRC, 2000a). Section 4.3.4.1.2 of NUREG-1854 provides guidance for the review of 

18 surface water transport abstractions (NRC, 2007a). Also, NCRP (1996a, 1996b) describes 

19 screening models that may be appropriate for many sites to determine whether more detailed 

20 modeling may be required. Additionally, NCRP (2005) and Onishi (2008) discuss approaches 

21 for modeling the transport of radionuclides in a variety of surface water bodies that may be 

22 appropriate depending on site-specific characteristics.

23 3.3.3 Atmospheric Transport

24

25 Atmospheric transport models estimate concentrations of radionuclides released to the 

26 atmosphere at offsite receptor locations. Section 3.2.6 discusses guidance for assessing the 

27 gaseous release of radionuclides (e.g., C-14, I-129, Kr-85, Rn-222, H-3) from the disposal 

28 facility to the atmosphere. Licensees may also need to evaluate the transport of particulate 

29 releases caused by direct release (e.g., wind erosion), if significant (see Section 3.2.7). Once 

30 released, radionuclides can be transported in the atmosphere to locations downwind from the 

31 disposal facility where they could contribute a nontrivial fraction of the dose to the average 

32 member of the critical group. To evaluate the impacts of gaseous radionuclides downwind from 

33 release points, licensees should consider the following FEPs (Crawford, et al., 2008):

34

35 • source characteristics (e.g., configuration of the release such as the release height from 

36 the surface, puff or continuous releases, gaseous or particulate)

37 • atmospheric transport processes (e.g., wind and turbulence)

38 • radionuclide removal mechanisms (e.g., rainfall, wet and dry deposition)

39 • general topography of the land near the disposal facility

40

41 Source characteristics listed above include information about the material released as well as 

42 the configuration of release. The characteristics of the released material include physical form 

43 (e.g., gaseous, particulate), chemical stability in the atmosphere, and radioactive decay. For 

44 instance, particles with diameters greater than 10 micrometers may experience significant 

45 gravitational settling, and thereby, possibly limited atmospheric transport. The configuration of 

46 the release may also be an important consideration. Information on the configuration includes 
3-28

1 timing and spatial extent. For instance, radioactivity may be released continuously or nearly 

2 instantaneously, as well as from a single point or over an area. Atmospheric transport 

3 processes listed above include a characterization of the physical processes affecting transport 

4 of radionuclides (e.g., advection, dispersion). These processes include movement by wind, 

5 represented by wind direction and speed, as well as mechanical and thermal turbulence which 

6 can result in mixing caused by eddies. Removal mechanisms listed above include deposition,

7 as gases and particles are deposited on surfaces and possibly removed from the atmosphere. 

8 Deposition may result from a variety of processes, both dry and wet, including impingement, 

9 electrostatic interactions, chemical reactions, and rainfall. Site topography as well as nearby 

10 engineered structures could affect atmospheric transport; licensees should exercise caution 

11 when using analog atmospheric transport data for a site-specific evaluation. 

12

13 An important consideration in atmospheric transport modeling is the scale of motion in 

14 atmospheric processes. Atmospheric processes such as wind direction and speed may vary 

15 over a wide range on spatial scales from a kilometer or less to thousands of kilometers, as well 

16 as over the course of hours to years. Therefore, the consideration of atmospheric transport 

17 processes and their parameterization is a function of the spatial and temporal transport scale of 

18 interest. Atmospheric transport from LLW disposal facilities to an offsite human receptor is 

19 typically on the order of less than a kilometer to tens of kilometers. Temporal scales of interest 

20 for LLW disposal range up to 10,000 years to assess the annual dose to an offsite receptor from 

21 releases. Licensees should consider processes consistent with the context of the transport 

22 scale, both spatially and temporally. Additionally, input data should be consistent with the scale 

23 of transport, both spatially and temporally. For instance, winds may vary in speed and direction 

24 with time and height. Therefore, meteorological data to support model parameterization should 

25 be consistent with and account for uncertainty and variability over the spatial and temporal 

26 scales of interest for the site. Licensees should demonstrate the representativeness of model 

27 parameters for the scales of concern for the performance assessment.

28

29 The assessment of the performance of some disposal facilities may require a licensee to

30 evaluate the atmospheric transport of radionuclides associated with particulates. For instance, 

31 progeny of gaseous radon are charged and are attracted to atmospheric particles of opposite 

32 charge. Key factors for a licensee to consider when evaluating the transport of particulates 

33 include: mass loading, resuspension rate, deposition rate, and wind speed. These factors are 

34 dependent on site-specific conditions such as soil type, wind distribution, and other 

35 meteorological conditions, as well as mechanical disturbances.

36

37 The assessment of the performance of some disposal facilities may require detailed 

38 consideration of these processes in the atmospheric transport abstraction, whereas simplified 

39 analyses may be justified for other sites and disposal practices. The atmospheric transport 

40 models may need to consider these site-specific complexities for those facilities for which 

41 licensees take credit for significant delay in radionuclide migration via air transport from the 

42 disposal facility to the receptor, those that take credit for significant dilution, those that are relied 

43 upon for defense-in-depth protections, as well as those for which there is little model support. A

44 simplified analysis may be acceptable for facilities for which the model can be shown to be 

45 clearly conservative or for which the simple model is well supported by multiple lines of 

46 evidence, including field tests demonstrating that the model and its parameters appropriately 

47 represent or bound site conditions. A simplified atmospheric transport modeling approach may 

48 be sufficient to gain an adequate understanding of the FEPs associated with a specific system. 

49 More detailed atmospheric transport modeling, however, may be necessary to identify important 
3-29

1 processes and assess the impacts of uncertainty and variability needed to support the 

2 simplification of atmospheric transport. Licensees should demonstrate that the methods used to 

3 assess atmospheric transport do not bias the outcome such that radiological doses to an offsite 

4 receptor are significantly underestimated.

5

6 NUREG-1573 provides a screening approach to assess whether more detailed consideration of 

7 atmospheric transport modeling is required for gaseous radionuclides (NRC, 2000a). Discussed 

8 in Section 3.3.6.3.2.1 of NUREG-1573, the approach uses the total gaseous radionuclide 

9 release over 1 year and conservative meteorological conditions for wind speed, atmospheric 

10 stability, and atmospheric diffusion. However, the licensee should still provide justification for 

11 the conservatism of the meteorological assumptions and parameters used. NCRP-152 

12 discusses possible approaches for estimating quantities of particulates suspended in the air 

13 (NCRP, 2005). An NRC staff-recommended screening approach to evaluate the transport of 

14 particulate matter, which is suspended in the air and transported downwind, assumes that the 

15 radionuclide concentrations in the atmosphere are equal to the concentrations in the surface soil 

16 or other source from which they originated. Section 3.3.6.3.2.2 of NUREG-1573 provides 

17 guidance for sites where more detailed analyses may be required. In addition, Crawford et al. 

18 (2008), discusses models that have been developed to assess atmospheric transport of 

19 radionuclides. Licensees should be aware that the output of the models is dependent on their 

20 assumptions and the data to support the parameters. Model selection and parameterization 

21 should be consistent with site conditions and supported by field observations, laboratory 

22 experiments, and other relevant information.

23 3.3.4 Biotic Transport

24

25 This section defines biotic transport as the transport of radionuclides from a disposal facility via 

26 biota (NRC, 1982d). Other sections of this document cover the indirect effects of biota on the 

27 disposal system, such as damage of an engineered cover by burrowing animals. This biotic 

28 transport section does not cover plant uptake or radionuclide movement within the biosphere 

29 after radionuclides have been released from the disposal facility via other mechanisms, such as 

30 groundwater release. Instead, this section focuses on offsite transport of radionuclides that 

31 have been released from the disposal facility by biota. One example of intrusion and active 

32 transport involves the Russian thistle (tumbleweed or Salsola kali). The long root system (on 

33 the order of meters) allows the plant to take up radionuclides from the soil (i.e., biota enhanced 

34 release). In autumn, the mature plant dries and detaches from the surface; it is then blown by 

35 the wind until it encounters an obstruction (biotic transport). Thus, the plant can cause both 

36 vertical and horizontal movement of radioactive material from a disposal site.

37

38 A qualitative assessment of the biotic transport pathway and its contributions to the dose may 

39 be sufficient depending on the site, its characteristics, and the characteristics of the waste 

40 disposed of at the site. Biotic transport may be the primary transport mechanism at sites 

41 without viable water pathways.

42

43 Secondary transport occurs after transfer of radionuclides to biotic sources (i.e., plants and 

44 animals) such as the movement of radionuclides within and among the plants and animals 

45 associated with the food chain. Examples of processes that transfer radionuclides to biotic 

46 sources include overland flow, atmospheric deposition, and use of contaminated groundwater

47 for irrigation. Overland flow can result in the deposition of radionuclides into a surface water 

48 body. Once deposited, radionuclides can remain in the water where they may be taken up by 
3-30

1 animals drinking the water or by plants extracting water through their root systems; 

2 radionuclides settling to the bottom sediments may be extracted by plant root systems. 

3 Additionally, radionuclides released to the atmosphere may accumulate on plant surfaces or in 

4 the soil by wet or dry deposition and eventually become incorporated in plants and animals. 

5 The groundwater transport pathway is often the most common means for transporting 

6 radionuclides from LLW disposal facilities to the surrounding biota since the waste is buried 

7 below ground. Secondary transport allows for additional displacement of radionuclides 

8 available to the biota. Food chain activities, such as the consumption of fruits containing 

9 radionuclides by animals, are an example of secondary transport. Upon entering the food 

10 chain, processes such as ingestion and excretion provide a means for the movement of 

11 radionuclides throughout the system. Secondary transport is evaluated using modeling of the 

12 biosphere.

13 3.4 Biosphere

14

15 For the purposes of a performance assessment, the biosphere is the physical environment 

16 accessed by a receptor. The objective of biosphere modeling is to calculate estimates of 

17 radiological exposures to humans, in terms of the average member of the critical group, from 

18 radionuclide releases from the disposal site over time and space. Licensees can then use the 

19 resultant exposures for comparison with the 10 CFR 61.41 performance objective. The 

20 biosphere includes the transfer of radionuclides through the human food chain and human 

21 dosimetry, including characteristics and lifestyles of the human receptors. There are two 

22 specific areas to consider in the assessment of doses to humans. First, the mechanism of 

23 radionuclide transfer through the biosphere to humans needs to be identified and modeled. 

24 This is often termed pathway analysis. Second, the dosimetry of the exposed individual must 

25 be modeled. This is termed the dose assessment. Section 3.3.7 of NUREG-1573 discusses 

26 pathway analysis and dose assessment in detail and provides acceptable approaches for 

27 performing these analyses (NRC, 2000a). 

28

29 The pathways analysis and dose assessment require the development of receptor scenarios 

30 that describe the activities in which an average member of the critical group would be engaged. 

31 Receptor scenarios typically involve input parameters to describe the transport and exposure to 

32 radionuclides that can be generally classified as behavioral, metabolic, or physical. Behavioral 

33 parameters collectively describe the behavior hypothesized for the average member of the 

34 critical group. The behavior is normally consistent with local practices (e.g., time spent 

35 gardening, vegetable consumption rates). Metabolic parameters also describe the exposed 

36 individual, but generally address involuntary physiological characteristics of the individual (e.g., 

37 breathing rates, factors converting intake of unit activity to dose by radionuclide). Physical 

38 parameters collectively describe the physical characteristics of the site (e.g., geological, 

39 hydrologic, geochemical, ecological, and meteorlogic inputs). Section 4.3.5 of NUREG-1854 

40 provides guidance on behavioral, metabolic, and physical input parameters used in the 

41 biosphere modeling (NRC, 2007a). 

42

43 For estimating the performance of a land disposal facility far into the future, licensees may need 

44 to consider changes to the physical environment over time and the impact that may have on 

45 receptor behaviors or physical parameters of the site. For instance, at a site that is currently 

46 inhospitable to gardening because of a harsh environment, a licensee may need to consider the 

47 evolution of the environment over time. If future climates are expected that may significantly 

48 change human behaviors, licensees may need to consider behaviors conducted at other sites 
3-31

1 that are currently analogous to the expected climate. Because metabolic parameters are 

2 involuntary and future evolution of humans is difficult to estimate, licensees should rely on 

3 current information regarding metabolic parameters and do not need to forecast changes to 

4 metabolic parameters over long timeframes.

5

6

7

4-1

1 4.0 INADVERTENT INTRUSION

2

3 Section 61.23(c) specifies the standard that must be met to protect individuals who could

4 occupy the disposal site after closure and unknowingly be exposed to radiation from the waste 

5 (i.e., inadvertent intruders as defined in 10 CFR 61.2). The requirement does not apply to 

6 protection of individuals who may knowingly or deliberately recognize that a radiation hazard 

7 exists and choose to ignore the hazard. The standard states that applicants should provide 

8 reasonable assurance that individual inadvertent intruders would be protected in accordance 

9 with the performance objective in 10 CFR 61.42. Section 61.13 specifies the technical analyses 

10 to demonstrate compliance with the performance objectives in Subpart C. 

11

12 Figure 4-1 illustrates the three analyses necessary to demonstrate compliance with 

13 10 CFR 61.13(b). To protect inadvertent intruders, the analyses require demonstration that (1)

14 waste acceptance criteria developed in accordance with 10 CFR 61.58 will be met, (2) adequate 

15 barriers to inadvertent intrusion will be provided, and (3) exposures to any inadvertent intruder 

16 will not exceed the limits specified in 10 CFR 61.42 as part of an intruder assessment. 

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45 Figure 4-1 Technical Analyses Required to Demonstrate Compliance with the 10 CFR

46 Part 61 Performance Objective for the Protection of Individuals from 

47 Inadvertent Intrusion

Demonstration 

of Compliance 

with Inadvertent 

Intrusion 

Performance 

Objective

Demonstrate 

Compliance with 

Waste 

Classification 

and Segregation 

Requirements

Conduct 

Inadvertent 

Intruder 

Assessment

Demonstrate 

Barriers to 

Intrusion Are 

Adequate
4-2

1 This section describes the information that a licensee should provide and a reviewer should 

2 evaluate with respect to the technical analyses for inadvertent intrusion. Section 4.1 describes 

3 information that should be provided to support a demonstration that the waste acceptance 

4 criteria will be met. Section 4.2 describes information needed to demonstrate that barriers to 

5 inadvertent intrusion will be adequate. Section 4.3 describes information that should be 

6 provided as part of an intruder assessment to demonstrate that exposures to any inadvertent 

7 intruder not exceed the limits specified in 10 CFR 61.42.

8 4.1 Waste AcceptanceRequirements

9

10 Requirements for waste acceptance for land disposal appear at 10 CFR 61.58. The regulations 

11 require applicants to identify (i) criteria for the acceptance of waste for disposal, (ii) acceptable 

12 methods for characterizing the waste, and (iii) a program to certify that waste meets the 

13 acceptance criteria prior to receipt at the disposal facility. Section 9.0 of this guidance 

14 document describes acceptable approaches for demonstrating compliance with the waste 

15 acceptance requirements specified at 10 CFR 61.58. Licensees who limit disposal to waste 

16 under approved waste acceptance plans satisfy this requirement.

17

18 Reviewers should coordinate the review of whether the waste acceptance criteria will be met 

19 with reviews of the waste acceptance requirements. See Section 9.0 for guidance on reviewing 

20 compliance with the waste acceptance requirements. This review should include (1) an 

21 assessment of whether the waste acceptance criteria are adequate, and (2) confirmation that 

22 the licensee has identified acceptable methods for waste characterization and has an adequate 

23 waste certification program to demonstrate that the acceptance criteria are met.

24 4.2 Inadvertent Intrusion Barriers

25

26 Intruder barriers are designed to inhibit contact with the waste (e.g., Class C waste) that is 

27 expected to present a hazard to an inadvertent intruder should institutional controls fail. The 

28 barriers ensure that radiation exposures to an inadvertent intruder will be within the limits of the 

29 performance objective specified at 10 CFR 61.42. Intruder barriers are distinct from institutional 

30 controls, yet complement the protection provided by institutional controls as a measure of 

31 defense-in-depth. An intruder barrier is defined as (1) a sufficient depth of cover over the waste 

32 that inhibits contact with waste and helps to ensure that radiation exposures to an inadvertent 

33 intruder will meet the performance objective, or (2) engineered structures that provide 

34 equivalent protection to the inadvertent intruder. Section 61.52(a)(2) requires that wastes 

35 designated as Class C must be disposed of with sufficient depth (i.e., minimum of 5 meters 

36 below the surface of the cover) or intruder barriers designed to protect for at least 500 years. 

37 Licensees who elect to develop waste acceptance criteria from the results of the technical

38 analyses (i.e., 10 CFR 61.13) may also need to identify certain waste streams that require 

39 intruder barriers to ensure protection of an inadvertent intruder. The inadvertent intruder 

40 assessment should be used to demonstrate that the intruder barrier can limit exposures for at 

41 least 500 years when used for Class C waste, or as long as necessary to limit exposures to an 

42 inadvertent intruder to the limits prescribed in 10 CFR 61.42. The assessment should look at 

43 site-specific conditions such as the length of time over which the disposed waste presents a 

44 significant hazard to an inadvertent intruder. Further, the inadvertent intruder assessment may 

45 identify the need for adequate barriers to demonstrate protection of inadvertent intruders from 

46 disposal of other waste (e.g., Class A and B waste) based on site-specific conditions.

47
4-3

1 While both intruder barriers and institutional controls can limit contact with the waste, they use 

2 different mechanisms. Institutional controls are used to limit intruder access to, or use of, the 

3 site for a period of time following transfer of the disposal site to the owner and cannot be relied 

4 on for more than 100 years under 10 CFR 61.59, “Institutional Requirements.” An institutional 

5 control program includes legal mechanisms (e.g., land use restrictions), environmental 

6 monitoring, periodic surveillance, minor custodial care, or other requirements as determined by 

7 the Commission as well as the administration of funds to cover the costs of these activities. 

8

9 Intruder barriers are passive features of the disposal facility and site that are intended to 

10 enhance a disposal facility’s ability to protect an inadvertent intruder who may engage in normal 

11 activities or other reasonably foreseeable pursuits that are consistent with activities in and 

12 around the site at the time of closure. Intruder barriers may include sufficient depth of cover 

13 over the waste or engineered structures (e.g., reinforced concrete vaults) that can provide 

14 protection to the inadvertent intruder. For more mobile radionuclides that could affect the 

15 intruder through ground water exposure pathways, intruder barriers may also include features 

16 that limit the potential release and transport of radionuclides (e.g., the wasteform).

17

18 Section 61.12(b) requires that the specific technical information describing the land disposal 

19 facility include design features related to inadvertent intrusion. As part of this description, 

20 licensees should identify intruder barriers and describe their capabilities in terms of inhibiting 

21 contact with disposed waste and ensuring that radiation exposures will meet the performance 

22 objective at 10 CFR 61.42. Because of the wide range of radioactive materials at disposal 

23 facilities, the regulations and this guidance are not prescriptive as to the acceptability of site24 specific intruder barriers. Because of this flexibility and because intruder barriers are site25 specific, it is important for licensees to clearly and completely document the description of the 

26 barriers’ capabilities and the supporting technical basis. 

27

28 The information supporting the barriers’ capabilities should be provided for the time period over 

29 which each barrier performs its intended function, including any potential changes to the barrier 

30 during the compliance period. These capabilities, including uncertainties, should be consistent 

31 with the performance attributed to the barriers in the analyses performed in the inadvertent 

32 intruder assessment. The NRC staff will use information gained from the inadvertent intruder 

33 assessment, independent calculations, and other appropriate quantitative analyses to confirm 

34 each barrier’s capabilities. The NRC staff will confirm that the technical bases for barrier 

35 capabilities are consistent with the technical bases for the inadvertent intruder assessment. In 

36 some cases, the barrier capabilities may differ from the information used to support the intruder 

37 assessment; for example, the intruder assessment may have conservatively ignored the 

38 benefits of the intruder barrier.

39

40 The functionality and robustness of the barriers will be determined using the risk-informed 

41 graded approach described in Section 4.2.1 and will be evaluated on a site-specific basis for 

42 each license application. However, the general framework that a licensee should consider will

43 not vary from licensee to licensee; only the depth and breadth of information supplied to 

44 demonstrate the capabilities of the intruder barriers may vary. The guidance that follows 

45 provides the general framework a licensee should consider for intruder barriers.

46

47
4-4

1 4.2.1 Risk-Informed Approach to Evaluating Intruder Barriers

2

3 Licensees are encouraged to use a risk-informed approach to select, design, and provide a 

4 technical basis for the intruder barrier(s) at a specific site. The approach is defined by the 

5 hazard level and likelihood of hazard occurrence. More robust barriers and additional bases for 

6 barrier capability should be provided for higher risk disposal facilities compared to lower risk 

7 facilities. The use of the term “risk” with respect to intruder barriers means the potential for risk; 

8 the analyses and resultant actions should ensure that high risks are not realized by a member of 

9 the public. The NRC would consider disposal facilities where the potential intruder hazards may 

10 be large as high risk. For instance, the NRC would consider a disposal facility with unmitigated 

11 doses following the institutional control period greater than 50 mSv/yr (5,000 mrem/yr) to be 

12 high risk. 

13

14 Licensees should assess the significance of each intruder barrier to mitigate exposures to 

15 inadvertent intruders. Any intruder barrier that reduces unmitigated doses less than or equal to 

16 5 mSv/yr (500 mrem/yr) to smaller doses is of low significance. A barrier that reduces 

17 unmitigated doses ranging between 5 to 50 mSv/yr (500 to 5,000 mrem/yr) to less than 

18 5 mSv/yr (500 mrem/yr), and results in a relative reduction of 50 percent or more, is of moderate 

19 significance. A barrier that reduces unmitigated doses greater than 50 mSv/yr (5,000 mrem/yr) 

20 to less than 5 mSv/yr (500 mrem/yr) is of high significance. Licensees should provide a 

21 technical basis for the capabilities of the barriers commensurate with their significance in 

22 mitigating inadvertent intruder exposures. The technical basis should evaluate the time period 

23 over which each barrier performs its intended function including any changes during the various 

24 periods of performance (i.e., compliance, protective assurance, and performance periods).

25

26 Reviewers will use insights from a licensee’s understanding of each barrier’s capabilities as well 

27 as independent analyses to focus reviews on the technical basis supporting the capabilities of 

28 significant barriers to limit contact with the waste and their integration in the inadvertent intruder 

29 assessment. Section 4.3 discusses guidance on the inadvertent intruder assessment.

30 4.2.2 Technical Basis for Intruder Barrier Capabilities

31

32 There is significant uncertainty concerning the estimation of service life and long-term 

33 degradation of intruder barriers. This section provides guidance on the main elements of 

34 information that should be provided to support the technical basis for the evaluation of intruder 

35 barrier capabilities, including the following:

36

37 • a full description of the design, features, and functionality of the intruder barriers

38 • a full description of the site and environmental conditions an intruder barrier would be 

39 exposed to

40 • a description of potential degradation mechanisms including consideration of combined 

41 and synergistic effects resulting from the service environment expected for the barriers

42 • a description of the suitability of selected numerical models, if used, for the estimation of 

43 intruder barrier capabilities

44 • an estimation of uncertainty in parameters and models used in the assessment of barrier 

45 capabilities and the design of intruder barriers
4-5

1 • parametric or component sensitivity analysis to identify how much degradation of the 

2 intruder barrier is needed for noncompliance to occur

3 • model support for the intruder barrier performance (e.g., analogs, experiments, simple 

4 engineering calculations to demonstrate the reasonableness of the results)

5 • QA and QC for the design, analysis, and implementation of intruder barriers

6

7 The capabilities of some intruder barriers may not be amenable to validation in the traditional 

8 scientific sense because of the long time periods involved. Therefore, licensees should provide 

9 multiple lines of evidence particularly, though not exclusively, to support barrier capabilities 

10 beyond 500 years. As discussed in Section 2.2.3, model support can come in many different 

11 forms, including but not limited to analogs, laboratory experiments, field experiments, formal and 

12 informal expert judgment, and engineering calculations to demonstrate the reasonableness of 

13 the results (e.g., hand calculations when numerical models are used). The level of model 

14 support should be commensurate with the relative significance of the barriers in protecting the 

15 inadvertent intruder. Section 2.2.3 presents more detail on model support.

16

17 During the operational, postclosure observation and maintenance, or institutional control periods 

18 monitoring may be needed to verify the capabilities of intruder barriers. This monitoring involves 

19 both monitoring aspects of the environmental system surrounding the disposal facility and 

20 monitoring the performance of the facility itself. Nondestructive monitoring technologies that 

21 include designed and emplaced sensors are preferred to conventional post-failure monitoring. 

22 To the extent practicable, intruder barriers should be designed to support and simplify 

23 monitoring and maintenance.

24

25 Reviewers should use information from a licensee’s description of intruder barrier capabilities to 

26 focus their review of the adequacy of the technical bases. Reviewers should confirm that the 

27 technical bases are commensurate with the significance of each barrier’s capability and the 

28 associated uncertainties. Based on their reviews of the inadvertent intruder assessment, 

29 reviewers should confirm: (1) the consistency of the technical basis with the intruder 

30 assessment and (2) the quality and completeness of the technical basis for the barrier 

31 capabilities.

32 4.2.3 Use of Engineered IntruderBarriers

33

34 If an engineered intruder barrier is used at a disposal facility, only the barrier’s passive 

35 performance to inhibit contact with the waste and limit radiological exposures (i.e., performance 

36 of the barrier without monitoring, inspection, and maintenance) should be credited in the intruder 

37 assessment to demonstrate compliance with the performance objective at 10 CFR 61.42. The 

38 assessment of the capabilities of the engineered intruder barrier should consider the 

39 reasonableness of a breach and the potential degradation of the barrier over time because 

40 monitoring and maintenance cannot be relied on beyond the period of institutional control.

41 Licensees can find additional information about intruder barriers in:

42

43 • NUREG-1757, Revision 1, Volume 2, Section 3.5.4 and Appendix P (NRC, 2006). 

44 These sections discuss degradation mechanisms and capabilities of common 

45 engineered barriers in greater detail.
4-6

1 • NUREG-1757, Revision 1, Volume 2, Section 3.5.5. This section summarizes existing 

2 guidance and references that may have some relevance to the application of engineered 

3 intruder barriers at disposal facilities.

4 • NUREG/CP-0195 (NRC, 2011a). This document includes reference material and 

5 electronic information sources in the appendices.

6

7 Other reasonably foreseeable disruptive events caused by humans or natural events and 

8 processes should be evaluated, and uncertainty in projecting the passive performance of the 

9 barriers into the future should be considered (see Example 4.1). For example, waste and 

10 disposal facilities might also be subject to instability because of waste characteristics (e.g., 

11 differential settling caused by voids in the waste) or facility design (e.g., long-term physical 

12 instability of covers). Subsidence could have the following effects on an engineered surface

13 cover: small depressions forming on the cover, differential settlement causing an uneven cover 

14 surface, stress cracks forming on the cover surface, and open voids in the cover, each of which 

15 could reduce the depth of cover to an insufficient thickness. 

16

17

18

19

20 Frequently, engineered surface covers rely on vegetation to resist erosion. The probability that 

21 the vegetation cover will deteriorate, due to future drought or disease, is dependent on the 

22 hardiness and diversity of plants established on the cover. Fires, severe storms, and ecological 

23 succession, due to changing temperature and precipitation, could influence the ability of the 

24 vegetation to resist wind and water erosion and maintain a sufficient depth of cover over the 

25 waste. Over long periods of time, a minimum erosion rate can amount to a substantial 

26 thickness of cover loss (e.g., a 0.5 millimeter (mm)/yr rate will reduce a cover’s thickness by half 

27 a meter in 1,000 years).

28

29

Example 4.1

A licensee proposes to use a reinforced concrete vault to limit access to the waste by an 

inadvertent intruder drilling into the waste. The concrete vault would function as a barrier for

as long as it maintains its integrity.

Conclusion: The licensee should consider the effect of reasonably foreseeable processes 

on the degradation of the vault’s mechanical properties that would be relied on to limit access 

to the waste. These processes, which are site-specific, could include seismic activity, 

cementitious material degradation such as sulfate attack, carbonation, leaching of the 

cement, and corrosion of the reinforcing steel. The licensee should also consider local 

drilling practices and estimate when currently used technology would likely penetrate the 

vault given its estimated degradation of mechanical properties. For instance, the likelihood of 

breaching the concrete vault would likely occur earlier in regions of the country where hardrock drilling is common than in regions where hard-rock drilling is not common. Local drilling 

practices vary across geographic regions. Over long periods of time, the assumption of only 

locally-used technology in the geographic region of interest may not be appropriate. At a 

minimum, licensees should communicate the results if drilling were to occur, even if drilling is

not anticipated based on locally-used technology.
4-7

1 4.3 Inadvertent Intrusion Assessment

2

3 The regulations at 10 CFR 61.13 require an inadvertent intruder assessment to demonstrate 

4 that exposures to an inadvertent intruder will not exceed the objectives specified in 

5 10 CFR 61.42. The primary objective of an inadvertent intruder assessment is to quantitatively 

6 analyze the potential radiological exposures to any individual who is assumed to occupy the site 

7 at some time after the loss of institutional controls. The intruder then engages in normal 

8 activities on site, such as agriculture, dwelling construction, resource exploration (e.g., drilling),

9 or other reasonably foreseeable pursuits that are consistent with activities in and around the site 

10 at the time of closure, in which the person might be unknowingly exposed to radiation from the 

11 waste.

12

13 The process for conducting an inadvertent intruder assessment, as shown in Figure 4-2, is 

14 similar to the process for conducting a performance assessment in that it is designed to 

15 evaluate the following questions, often referred to as the risk triplet: 

16

17 • What could occur?

18 • How likely is it to occur? 

19 • What are the consequences?

20

Inadvertent intrusion assessment is an iterative process involving site-specific, 

prospective modeling evaluations of potential radiological consequences as a result of 

reasonably foreseeable human activities that might unknowingly occur should an individual 

occupy a near-surface disposal facility for LLW after the loss of institutional controls. An 

intruder assessment has two primary objectives:

• To determine whether reasonable assurance of compliance with the performance 

objective for protection of inadvertent intruders can be demonstrated 

• To identify insights that support additional site-specific design and control measures 

to preclude the intrusion or to limit radiological impacts to acceptable levels from 

disposed waste should an inadvertent intrusion occur
4-8

1

2

3 Figure 4-2 Example of an Inadvertent Intruder Assessment Process Required Per 

4 10 CFR 61.42

10 CFR 61.42 

Performance 

Objective is 

Not Met

Form Scenario(s)

Conceptualize and 

Abstract System

Conduct 

Consequence 

Modeling

Collect Additional 

Information and/or 

Change Design

Evaluate 

Disposal 

Site 

Update Assumptions 

and/or Parameters

Develop Options:

Refine Analysis

Change Design

Evaluate 

Options

Continue

Perform 

Mitigation for 

Existing Site or 

Select New Site

Complies with 

10 CFR 61.42
4-9

1 Inadvertent intrusion is expected only if required institutional controls or societal memory are 

2 lost. Because of these protections, inadvertent intrusion is considered unlikely. However, as 

3 time passes after closure of the disposal facility, institutional controls or societal memory cannot 

4 be assured and inadvertent intrusion may be possible. Thus, Section 61.59 requires that the 

5 time period over which institutional controls can be relied on may not be more than 100 years 

6 following transfer of control to the disposal site owner. 

7

8 Further, there is no scientific basis for quantitatively predicting the nature or probability of a 

9 future disruptive human activity over long timeframes. This is in contrast to a natural process for 

10 which a scientific basis may be developed to support a probability of occurrence. Therefore, an 

11 inadvertent intruder assessment does not consider the probability of inadvertent intrusion 

12 occurring. Rather, the assessment assumes that reasonably bounding receptor scenarios occur 

13 and evaluates the radiological consequences that could be experienced by inadvertent intruders

14 should institutional controls or societal memory be lost (NCRP, 2005).

15

16 The approach to evaluating the first two questions of the risk triplet is an important difference 

17 between an inadvertent intruder assessment and a performance assessment and a key reason 

18 why the regulations in 10 CFR Part 61 treat these two types of assessments separately. 

19 Intruder assessments qualitatively consider the likelihood of a disturbance of the disposal site 

20 induced by human intrusion after a loss of institutional control for the reasons mentioned in the 

21 preceding paragraph. In contrast, offsite exposures to the general population are driven by 

22 natural processes, such as those evaluated in a performance assessment, and could occur at 

23 any time after disposal. The performance assessment does not assume that institutional 

24 controls or societal memory would be lost (NCRP, 2005). Performance assessments can, 

25 therefore, quantitatively evaluate the likelihood of natural events or processes and their effect on 

26 the performance of a disposal facility to limit radiological consequences to members of the 

27 public beyond the site boundary. The intruder dose assessment is more strongly tied to specific 

28 future human behaviors.

29

30 Given the qualitative assumption that inadvertent intrusion is unlikely, albeit possible, because 

31 of the presence of required institutional controls, a secondary objective of the assessment is to 

32 provide insights that support additional site-specific design and control measures to preclude 

33 the intrusion or to limit radiological impacts to acceptable levels should an inadvertent intrusion 

34 occur. The additional measures could include site-specific inventory limits or other mitigation 

35 techniques such as additional intruder barriers or enhanced disposal practices. See Section 7.0 

36 of this document for additional guidance on defining inventory limits based on the results of the 

37 technical analyses, including the inadvertent intruder assessment.

38

39 As with the performance assessment, scenario analysis and model abstraction are also key 

40 attributes of an inadvertent intruder assessment. Scenario analysis identifies, screens, and 

41 constructs scenarios from relevant FEPs for the disposal facility. For an intruder assessment, 

42 the formation of scenarios is focused on identifying reasonably foreseeable activities that an 

43 inadvertent intruder might engage in on the site (i.e., receptor scenarios). In the near term, 

44 licensees can assess reasonably foreseeable human activities based on site-specific conditions. 

45 However, future human activities are uncertain at longer times. Therefore, 10 CFR Part 61 

46 requires that licensees assess normal activities, such as agriculture or dwelling construction, or 

47 pursuits that are consistent with activities in and around the site at the time of closure to limit 

48 excessive speculation about future human activities far into the future regardless of whether the 
4-10

1 intruder activities and the potential pathways of radiological exposure are likely to be affected by 

2 features and processes relating to or affecting the facility design and site characteristics over 

3 the various periods of performance (e.g., the presence of intruder barriers and their degradation, 

4 future climate conditions). Section 2.5.3 discusses methods to identify and screen FEPs for the 

5 compliance and protective assurance periods. The approaches are also generally appropriate 

6 for the inadvertent intrusion assessment.

7

8 Reviewers should use insights drawn from the review of the intruder barrier analysis (see 

9 Section 4.2 ) to focus their review on topics within the intruder assessment that are important to 

10 inhibiting contact with the waste or ensuring that potential radiation exposures to an inadvertent 

11 intruder will meet the 10 CFR 61.42 performance objective. The following sections provide

12 guidance related to the process for conducting site-specific inadvertent intrusion assessments. 

13 Section 4.3.1 provides guidance on forming receptor scenarios for use in the assessments. 

14 Section 4.3.2 provides guidance related to specific aspects of modeling radiological 

15 consequences for assumed inadvertent human intrusion. When reviewing a licensee’s 

16 inadvertent intruder assessment, reviewers should also consult Section 2.0 for guidance on 

17 general considerations regarding scenario analysis, model abstraction, and performance 

18 assessment that were highlighted in the preceding paragraphs. Section 9.0 also discusses 

19 other considerations related to developing waste acceptance criteria from the technical analyses 

20 (e.g., using the information from intrusion assessments to establish site-specific measures to 

21 limit radiological impacts if inadvertent intrusion occurs).

22 4.3.1 Receptor Scenario Analysis

23

24 In developing 10 CFR Part 61, the NRC staff recognized that it is unlikely, though possible, that 

25 an individual will occupy a site after closure because of the presence of institutional controls. 

26 10 CFR Part 61 does not specify that a particular intrusion scenario be used in the assessment 

27 to demonstrate compliance with the performance objective for protection of inadvertent 

28 intruders. Rather, the criteria are performance-based. Thus, various methods, both generic 

29 and site-specific, are available to licensees, to demonstrate compliance with the criteria. 

30

31 Scenario analysis addresses the following questions pertinent to inadvertent intrusion:

32

33 • What human activities are reasonably foreseeable in the vicinity of the land disposal 

34 facility?

35 • How can an intruder unknowingly come in direct contact with the disposed waste or be 

36 exposed to its radiation?

37 • How does the radioactive material move through the environment on the site?

38 • What are the inadvertent intruder’s habits that will determine exposure?

39

40 Intruder receptor scenarios are defined as reasonable sets of activities related to the future use 

41 of the site. Therefore, receptor scenarios describe potential future land uses, human activities, 

42 and behavior of the engineering design and natural setting. In most situations, possible 

43 scenarios for intruders to interact with or be exposed to radiation from the disposed waste are 

44 numerous. The criteria in 10 CFR Part 61 do not require an investigation of all possible receptor 

45 scenarios; their focus is on reasonably foreseeable activities of the inadvertent intruder. 

46 Licensees do not need to assess inadvertent intruder activities that are considered implausible 
4-11

1 because of persistent physical constraints at the site. The inadvertent intruder is defined in 

2 10 CFR 61.2 as “...a person who might occupy the disposal site after closure and engage in 

3 normal activities, such as agriculture, dwelling construction, or other pursuits in which the 

4 person might unknowingly expose the person to radiation from the waste included in or 

5 generated from a disposal facility.” Assessment of receptor scenarios for an inadvertent 

6 intruder should focus on those activities from which the inadvertent intruder would reasonably 

7 be expected to receive the greatest exposure to radiation from the waste. Because of the 

8 uncertainty in human activities far into the future, the NRC staff recommends a suite of generic 

9 receptor scenarios that are associated with normal activities, described in Section 4.3.1.1, to 

10 represent reasonably foreseeable inadvertent intruder activities. Licensees are also permitted 

11 to use site-specific scenarios based on other reasonably foreseeable pursuits that are 

12 consistent with activities in and around the site at the time of closure to limit speculation about 

13 future human activities. Guidance on developing site specific scenarios is discussed in Section 

14 4.3.1.2. These sections use a number of different terms describing receptor scenarios. Table 

15 4-1 includes a description and comparison of these receptor scenario terms.

16

17 The definition of receptor scenarios and the intruder assessment can be generic or site-specific. 

18 Regardless of the specific method used to develop receptor scenarios, the licensee should 

19 provide sufficient justification for its approach. Licensees may use generic receptor scenarios 

20 similar to those described in NUREG-0782 (NRC, 1981a) with site-specific waste streams, or 

21 they may develop site-specific receptor scenarios to evaluate site-specific waste streams. Site22 specific receptor scenarios may be developed by modifying the exposure pathways included in 

23 the generic receptor scenarios or may be constructed based on waste characteristics, disposal 

24 practices, site characteristics, and, when appropriate, projected land use.

25

26 The NRC staff continues to view the generic receptor scenarios as reasonably conservative for 

27 estimating potential radiological exposures to an inadvertent intruder while limiting excessive 

28 speculation about future human activities. These receptor scenarios are normal activities that 

29 humans typically engage in, in a variety of environments, and they contain a nearly 

30 comprehensive set of exposure pathways reflecting the generic nature of the original analysis 

31 for the development of 10 CFR Part 61 (NRC, 1981a). Licensees may use the generic receptor 

32 scenarios described in Section 4.3.1.1 in an intruder assessment to demonstrate compliance 

33 with 10 CFR 61.42 for site-specific waste streams. However, in some cases generic receptor 

34 scenarios may need to be modified based on site-specific conditions (e.g., waste streams, 

35 facility designs, or environmental conditions) to demonstrate compliance with the inadvertent 

36 intruder performance objective.

37

38 Depending on the method used, licensees should provide justification for their selections. For 

39 some licensees, this may require minimal site-specific data to support the assumptions inherent

40 in the generic receptor scenarios or for removal of specific exposure pathways. Other licensees 

41 may need to thoroughly investigate and justify the appropriateness of the selected receptor 

42 scenarios, which may include an evaluation of alternate receptor scenarios. If a licensee 

43 creates a receptor scenario based on site-specific conditions, they should provide transparent 

44 and traceable documentation of the justification for each assumption used in developing the 

45 receptor scenario (e.g., justify the inclusion (or exclusion) of a particular exposure pathway).
4-12

1

2 Table 4-1 Comparison and Description of Intruder Receptor Scenario Terms Used in 

3 this Guidance

Types of Scenarios

Evaluation 

Purpose Description

Plausible Generic All can be used 

to demonstrate 

compliance 

with the 

inadvertent 

intruder 

performance 

objective.

The scenarios used to inform the waste 

classification criteria at 10 CFR 61.55 that are 

consistent with normal activities including 

agriculture, dwelling construction, resource 

exploration or exploitation.

Site-Specific A scenario developed, using site information, either 

from scratch or by modifying a generic scenario that 

is consistent with activities in and around the 

disposal site at the time of closure.

Reasonably 

Foreseeable

Reasonably foreseeable scenarios are based on 

normal activities or other pursuits that are consistent 

with activities in and around the disposal site at the 

time of closure. Normal activities include 

agriculture, dwelling construction, resource 

exploration or exploitation (e.g., well drilling). The 

NRC staff continues to believe the generic receptor 

scenarios associated with normal activities are 

typically plausible assuming the loss of institutional 

controls and the loss or significant degradation of 

the capabilities of intruder barriers. The NRC staff 

also continues to view the generic receptor 

scenarios as reasonably bounding over long 

timeframes, given the uncertainty in estimating 

future human activities over long time periods. 

However, licensees can also rely on site-specific 

scenarios that are consistent with activities in and 

around the site at the time of closure to limit 

speculation about future human activity.

Less Likely 

but Plausible

Not analyzed 

for compliance, 

but may be 

used to riskinform the 

decision.

Intruder activities that are plausible, assuming the 

loss of institutional controls, based on the 

capabilities of intruder barriers, site characteristics, 

and historical uses, but are not reasonably 

foreseeable considering normal activities or other 

pursuits that are different than activities in and 

around the site at the time of closure. These 

scenarios are usually site-specific.

Implausible No analysis 

required.

Assuming the loss of institutional controls, intruder 

activities that could not occur because of persistent 

physical limitations of the site.

4

5
4-13

1 4.3.1.1 Generic Intruder Receptor Scenarios

2

3 The NRC used a limited number of generic scenarios to inform the development of the 

4 10 CFR Part 61 waste classification criteria (NRC, 1981a, 1982b, 1986a). The receptor 

5 scenarios involve both direct and indirect contact with disposed waste through consumption of 

6 contaminated food as well as receptor scenarios involving a single, acute exposure and 

7 scenarios involving long-term, chronic exposure. The NRC used the direct contact receptor

8 scenarios to develop the 10 CFR Part 61 waste classification and segregation criteria (NRC, 

9 1981a, 1982b) and later, to update the analysis (NRC, 1986a). The receptor scenarios selected 

10 were hypothetical constructs intended to provide reasonable bounds on the exposure of 

11 inadvertent intruders to radiation from the LLW; these receptor scenarios helped establish waste 

12 classification criteria to be applied at all licensed disposal sites. Loss of institutional control is 

13 not expected, but the long-term integrity of the controls cannot be ensured, as the control is 

14 primarily derived from records, markers, and government processes and actions, all of which 

15 may not be durable over many generations.

16

17 Three direct contact receptor scenarios involve acute exposures, one direct-contact receptor 

18 scenario involves chronic exposures, and one groundwater receptor scenario involves chronic 

19 exposures as discussed below: 

20

21 (1) intruder-construction, in which the intruder receives acute exposures while excavating 

22 into disposed waste during construction of a dwelling or building 

23 (2) intruder-discovery, a variant of the intruder-construction scenario, in which the intruder 

24 recognizes the presence of waste during excavation and ceases activity

25 (3) intruder-drilling, in which the intruder receives an acute exposure while drilling through 

26 the waste to install a well

27 (4) intruder-agriculture, in which an intruder receives chronic exposures following 

28 construction of a dwelling built in the intruder-construction scenario

29 (5) intruder-well, in which an intruder is chronically exposed to contaminated groundwater

30 while living on the disposal facility site

31

32 Table 4-2 summarizes these scenarios and the pathways by which the intruder received 

33 exposures. The following subsections discuss the details of the receptor scenarios themselves.

34 4.3.1.1.1 Intruder-Construction and Intruder-Discovery Receptor Scenarios

35

36 The intruder-construction receptor scenario involves the construction of a dwelling directly 

37 above the disposed waste. During construction activities, workers are assumed to come in 

38 contact with some of the waste (e.g., during excavation of a basement). Some of the waste is 

39 also assumed to be dispersed into the air by the excavation and emplaced onto the immediate 

40 area around the dwelling’s foundation. Exposures are estimated for pathways listed in

41 Table 4-2, including inhalation of contaminated dust, exposure to direct gamma radiation from 

42 standing on contaminated soil and being immersed in a contaminated dust cloud, and ingestion 

43 of contaminated dust or food on which contaminated dust has deposited.

44
4-14

1

2 Table 4-2 Exposure Pathways of Generic Intruder Receptor Scenarios

3

Receptor Scenario Exposure Pathway

Inhalation# Ingestion† Direct/External‡

Air Soil Food 

(Air)

Food 

(Soil)

Food 

(Water) Air Soil Surface

Soil 

Volume

Acute Exposures

Intruder-Construction ● ● ● ●

Intruder-Discovery ● ● ● ●

Intruder-Drilling ● ● ● ●

Chronic Exposures

Intruder-Agriculture ● ● ● ● ●

Intruder-Well ● ● ● ●

# 4 Inhalation includes pathways originating via breathing contaminated air due to suspension of soil 

5 particles caused by human activity (air) and caused by natural suspension and volatilization of surface 

6 soil (soil).

† 7 Ingestion includes pathways for plant-to-human, plant-to-animal-to-human, and plant-to-animal-to8 product-to-human uptake. Food (air) considers food pathways originating via atmospheric deposition on 

9 plant surfaces and surrounding soil leading to soil-to-root transfer. Food (soil) considers food pathways 

10 originating via soil-to-root transfer from contaminated soil. Food (water) considers food pathways 

11 originating via irrigation deposition on plant surfaces and the surrounding soil as well as uptake of 

12 radionuclides originating from ingestion of contaminated water (i.e., water-to-human; water-to-animal-to13 human; and water-to-animal-to-product-to-human).

‡ 14 Direct/External includes exposure to gamma rays from standing in homogeneously contaminated air 

15 (air), standing on a homogeneously contaminated surface area (surface), and standing on 

16 homogeneously contaminated ground (volume). 
4-15

1 Since this receptor scenario is limited to construction activities, release and subsequent 

2 exposure occur for a limited period of time sufficient to complete construction activities for a 

3 typical dwelling (i.e., less than a year). The length of time that the intruder is exposed to 

4 radioactivity is a function of the stability of the waste encountered. If the waste is assumed to 

5 be degraded into an unrecognizable form, then it is possible that such construction activities 

6 could proceed following intrusion into the waste. However, if the waste is stabilized to the point 

7 that the waste is clearly distinguishable as something different than natural materials (e.g., soil), 

8 then it is likely that the inadvertent intruder would stop and investigate. In this case, a subset of 

9 the intruder-construction scenario is envisioned and is termed the intruder-discovery scenario. 

10 The exposures in the intruder-discovery scenario are expected to occur over a very limited 

11 period of time (i.e., less than a day) since it is considered unlikely that construction would 

12 resume following the discovery.

13 4.3.1.1.2 Intruder-Drilling Receptor Scenario

14

15 The intruder-drilling receptor scenario is a variant of the intruder-construction scenario that was 

16 developed in an update to the initial impacts analysis (NRC, 1986a). The intruder-drilling 

17 scenario assumes that, in order to build the dwelling, as in the intruder-construction scenario, 

18 the intruder must first install a well to secure an adequate water supply for living needs. During 

19 drilling activities, the drilling crew is assumed to inadvertently drill through the waste, bringing it 

20 up to the surface in the drill cuttings. If resistance is encountered (e.g., from resistant intruder 

21 barriers) during drilling, the crew is assumed to simply move a few yards horizontally to a new 

22 location and drill a second borehole. Exposures are estimated for pathways listed in Table 4-2

23 and include exposure to direct gamma radiation from standing in the vicinity of the borehole, 

24 where drill cuttings collect, or a mud pit if drilling fluids are used. Because this receptor scenario 

25 is limited to drilling activities, release and subsequent exposure occur for a very limited period of 

26 time sufficient to complete drilling activities (i.e., typically less than a day). Though a mud pit 

27 was evaluated in the initial analysis, current practices vary (e.g., drill cuttings are sometimes 

28 spread on the surface). The licensee should justify the assumed cuttings management 

29 practices.

30

31 Exposures are estimated for pathways similar to those evaluated in the intruder-construction 

32 scenario and are listed in Table 4-2. The exposure pathways include inhalation of contaminated 

33 dust from drilling, exposure to direct gamma radiation from standing in the vicinity of the 

34 contaminated drill cuttings, being immersed in a contaminated dust cloud, and ingestion of 

35 contaminated dust or food on which contaminated dust is deposited. The primary difference 

36 between this receptor scenario and the intruder-construction scenario, other than exposure 

37 time, is the volume of waste exhumed. The receptor scenario assumes that drilling is performed 

38 to supply water for living needs for a single dwelling. The volume of material exhumed is limited 

39 to the dimensions of the borehole rather than the dimensions of the dwelling footprint.

40 4.3.1.1.3 Intruder-Agriculture and Intruder-Well Receptor Scenarios

41

42 The intruder-agriculture receptor scenario involves an individual or individuals living in the 

43 dwelling constructed in the intruder-construction scenario. Exposure pathways for the intruder44 agriculture scenario, given in Table 4-2, include those considered for the intruder-construction 

45 scenario in addition to consumption of (1) food grown in the contaminated soil, (2) animals that 

46 consumed contaminated fodder, and (3) contaminated animal products (e.g., milk and eggs).

47
4-16

1 The intruder-agriculture scenario is assumed to be possible only if the waste has been 

2 degraded to a form that is indistinguishable from soil. The length of time that the individuals 

3 would spend in the contaminated area would be greater for this receptor scenario than for the 

4 former intruder-construction scenario because the former scenario is only concerned with 

5 exposures to the intruder during construction activities.

6

7 The intruder-agriculture scenario used in the waste classification tables did not include 

8 consumption of water from an onsite well (as in the intruder-well scenario) because the 

9 exposures from migration of radionuclides in groundwater are much more a function of 

10 site-specific environmental and geohydrological conditions and total activity rather than directly 

11 related to waste concentration. NUREG-0782 (NRC, 1981a) recommends that radionuclides 

12 that are important from a migration standpoint have inventory limits established on a site13 specific basis, based upon groundwater migration considerations. Licensees evaluating the 

14 generic intruder-agriculture scenario should also consider the use of contaminated groundwater

15 from an onsite well in the intruder assessment to demonstrate compliance with Section 61.42. 

16 Incorporating the intruder-well scenario into the intruder-agriculture scenario, ensures that 

17 licensees consider the consumption of food grown in contaminated soil as well as consumption 

18 of contaminated well water and exposure to ground and plant surfaces that are irrigated from 

19 the intruder well.

20 4.3.1.1.4 Criteria for Selecting Generic Receptor Scenarios

21

22 Licensees may adopt the generic receptor scenarios described in Section 4.3.1.1 to 

23 demonstrate compliance if the facility’s design, operation, and site are suitable for their use. 

24 The scenario used to demonstrate compliance with the performance objective should consider 

25 the greatest reasonably foreseeable dose to the inadvertent intruder. Because of the 

26 reasonably conservative nature of the generic-receptor-scenarios approach, the estimated 

27 radiological exposures are anticipated to be greater than estimates using site-specific receptor 

28 scenarios because the generic receptor scenarios usually contain a nearly comprehensive 

29 number of exposure pathways. Use of the generic receptor scenarios may save licensees time 

30 and effort by reducing the amount of site characterization, modeling analysis, and reviews 

31 needed compared to using a site-specific receptor scenario.

32

33 Licensees should be aware that use of the generic receptor scenarios may not be appropriate to 

34 demonstrate compliance for certain sites because of the following factors that may limit or alter 

35 the activities and exposure pathways for an inadvertent intruder:

36

37 • characteristics of the disposal site, such as the presence of adequate water

38 • facility design, particularly the expected long-term capabilities of engineered intruder 

39 barriers

40 • disposal practices, such as waste emplacement as a deterrent to intrusion

41 • waste characteristics, including migration behaviors of radionuclides and progeny

42

43 Licensees should demonstrate that the use of a generic receptor scenario is reasonable at a 

44 particular site and for the facility design and disposal practices. Section 4.3.1.2.1 contains 

45 guidance on using site-specific physical information to justify the scenario(s) used to 

46 demonstrate that the inadvertent intruder performance objective is met. Examples 4.2 through 
4-17

1 4.5 provide examples that should be considered by licensees in selecting a generic receptor 

2 scenario to demonstrate the inadvertent intruder performance objective is met. Depending on 

3 the characteristics of the disposal facility and its environment, licensees may need to or elect to 

4 consider other site-specific receptor scenarios (e.g., industrial, urban, or recreational) or 

5 consider additional exposure pathways (e.g., radon gas migration) to modify the generic 

6 receptor scenarios.

7

8 Reviewers should assess a licensee’s analysis of whether the generic receptor scenarios are 

9 suitable representations of actual site conditions and disposal practices (e.g., the spatial 

10 distribution of radionuclides). The reviewer should consult information provided to demonstrate

11 compliance with 10 CFR 61.12 to support the evaluation. NUREG-1199, Revision 2, “Standard 

12 Format and Content of a License Application for a Low-Level Radioactive Waste Disposal 

13 Facility,” issued January 1991 (NRC, 1991a), provides guidance on information licensees 

14 should submit to demonstrate compliance with 10 CFR 61.12. 

15

16 NUREG-1200, Revision 3 (NRC, 1994), provides guidance for reviewers to evaluate the

17 sufficiency of the information submitted to demonstrate compliance with 10 CFR 61.12. In 

18 general, the generic receptor scenarios are acceptable for demonstrating compliance if 

19 licensees can demonstrate that the scenarios are suitable for the disposal facility and would

20 reasonably be expected to result in greater exposure to radiation from the waste than other 

21 reasonably foreseeable receptor scenarios. The reviewer’s evaluation should also assess the

22 licensee’s justification for the evolution of site conditions over the time periods of interest (i.e., 

23 compliance period, protective assurance period, or performance period) and over what time the

24 conditions that may have supported the licensee’s determination persist. For instance, if a 

25 license applicant, in proposing a new facility, were to rely on a receptor scenario that is 

26 consistent with activities that are anticipated around the site at closure, which could be many

Example 4.2

A licensee proposes to dispose of all waste 10 meters below the ground surface. The 

licensee performs an intruder assessment to demonstrate compliance with 10 CFR 61.42 

using the generic intruder-construction (acute) and intruder-agriculture (chronic) receptor 

scenarios.

Conclusion: The generic intruder receptor scenarios identified above may not be suitable 

to demonstrate compliance because they reasonably assume residential construction that 

typically does not occur deeper than approximately 3 meters. Therefore, these scenarios 

do not directly contact the disposed waste. To justify the suitability of the default scenarios, 

the licensee may evaluate other reasonably foreseeable generic (e.g., intruder-driller, 

intruder-well) or site-specific scenarios in which the intruder contacts the waste to 

demonstrate that the selected scenarios would reasonably be expected to result in the 

greatest exposure to radiation from the waste rather than reasonable alternatives. 

Alternatively, the licensee may justify why other plausible scenarios are not reasonably 

foreseeable. The licensee should consult Section 4.3.1.2 for guidance on screening 

scenarios based on site-specific information.
4-18

1 decades away, the applicant should provide a basis to support future site conditions at the time

2 of closure. Likewise, a licensee proposing to eliminate certain pathways from a receptor 

3 scenario due to the persistence of an engineered barrier should provide a technical basis that 

4 supports the persistence of the engineered barrier to preclude the eliminated pathway(s).

5 . 

Example 4.3

A disposal facility is located in a geographic region that currently lacks of an adequate 

ground water source for purposes of drinking and irrigation. The licensee demonstrates that 

the inadvertent intruder performance objective is met using the intruder-agriculture receptor 

scenario but excludes the exposure pathways resulting from the intruder-well receptor 

scenario.

Conclusion: Excluding exposure pathways from the intruder-well receptor scenario may be 

appropriate for this disposal facility given the site conditions. The licensee should provide a 

justification to support exclusion of ground water dependent exposure pathways. The 

licensee should also demonstrate the expected persistence of those conditions during the 

operational lifetime of the facility. For a more robust assessment, licensees could also 

evaluate the impacts of reasonable changes to the current conditions based on future climate 

conditions during the post-closure time period being evaluated. For example, evolution of the 

climate in the vicinity of the site may result in greater ground water recharge and lead to an 

adequate ground water source at some time in the future. Section 4.3.1.2 provides guidance 

on using site-specific information to screen exposure pathways.

Example 4.4

A licensee proposes an engineered facility in which all waste is disposed of in reinforced 

concrete trenches (e.g., hot waste cell). The licensee performs an intruder assessment to 

demonstrate that the 10 CFR 61.42 performance objective is met using the default intruderdiscovery receptor scenario.

Conclusion: The default intruder-discovery receptor scenario may be suitable to 

demonstrate that the performance objective is met during the time period when the hot waste 

cell would be distinguishable from the native rocks and sediments that an intruder might 

encounter at the site. The licensee should provide a technical basis, including a 

consideration of cementitious degradation processes and consideration of local geology, to 

support the time period over which the hot waste cell would be distinguishable from native 

rocks and sediments. The licensee should consult Section 4.3.1.2 for guidance on screening 

scenarios based on site-specific information. The licensee should also demonstrate either 

that other site-specific scenarios are not reasonably foreseeable, or that the intruderdiscovery scenario results in a greater radiological exposure than other reasonably 

foreseeable site-specific scenarios.
4-19

1 Because of the uncertainties in projecting human behavior far into the future, there may be 

2 limitless speculation on the types of activities an intruder may engage in, many possibly 

3 unknown at this time. The activities represented by the generic receptor scenarios are 

4 considered normal human activities (e.g., providing shelter, engaging in agriculture, and seeking 

5 natural resources such as water) and avoid excessive speculation about future human activities. 

6 The generic receptor scenarios also can be used to evaluate a nearly comprehensive set of 

7 exposure pathways and are expected to be sufficient to assess the need for additional 

8 measures to mitigate doses to inadvertent intruders. Licensees can use information on the 

9 likelihood of natural processes to support receptor scenario development over the longer term. 

10

11 For example, licensees can assess the effects of potential degradation of the capabilities of 

12 intruder barriers (e.g., concrete degradation or erosion of cover materials) to justify when one of 

13 the generic receptor scenarios can be initiated. However, speculating on future human 

14 disruptive activities beyond a few hundred years should be avoided because there is no 

15 scientifically credible basis to estimate the likelihood of a human disruptive activity so far into the 

16 future. Reviewers should evaluate the licensee’s scenario(s) for demonstrating the inadvertent 

17 intruder performance objective is met for consistency with the types of activities associated with 

18 the generic receptor scenarios described in this section or with the types of activities expected 

19 in and around the disposal site at the time of closure for site-specific scenarios, which are 

20 discussed in more detail in Section 4.3.1.2. Reviewers should also evaluate the technical basis 

21 supporting the long-lived capabilities of intruder barriers and long-term evolution of the site 

22 environmental conditions that may affect receptor scenarios. For example, a licensee should 

23 appropriately consider long-term degradation processes such as corrosion or cement

24 degradation in evaluating the longevity of intruder barrier capabilities. Section 4.2.2 discusses 

25 guidance for evaluating the longevity of intruder barrier capabilities. Section 2.2.3 discusses 

26 considerations in evaluating whether a licensee has provided adequate model support to 

27 provide confidence in the projection of long-term processes. Section 5.0 presents guidance on 

28 long-term site stability (e.g., erosion).

Example 4.5

A licensee proposes to dispose of large quantities of depleted uranium at a disposal facility. 

The licensee demonstrates that the inadvertent intruder performance objective is met using 

the generic receptor scenario with the default exposure pathways that result in the greatest 

radiological exposure to the intruder.

Conclusion: In this example, the licensee should consider information regarding 

site-specific waste characteristics in developing a scenario to demonstrate the performance 

objective is met. The scenario should include credible exposure pathways. The generic 

scenarios do not assess exposure to radon gas, a decay product in the uranium-238 decay 

chain, which can migrate from the disposal cell to the surface of the site. The licensee 

should evaluate potential impacts from radon generation, migration to the surface, and 

potential exposures to an intruder for comparison with the performance objective. If 

necessary, the licensee may need to consider additional disposal requirements, such as 

minimum disposal depths or engineered intruder barriers, to provide assurance that an 

inadvertent intruder is protected.
4-20

1 4.3.1.2 Site-Specific Intruder Receptor Scenarios

2

3 Site-specific receptor scenarios, which are developed by the licensee, give licensees greater 

4 flexibility in developing the scenario(s) to demonstrate that the inadvertent intruder performance 

5 objective is met. In developing a site-specific receptor scenario or modifying a generic receptor 

6 scenario using site-specific information, licensees should provide a technical basis to support

7 the scenario. The receptor scenario(s) used to demonstrate that the performance objective is 

8 met should be a reasonably foreseeable receptor scenario or scenarios that result in estimates 

9 of exposure to the inadvertent intruder that tend to not underestimate potential exposures; other

10 reasonably foreseeable receptor scenarios should not result in higher doses to an inadvertent 

11 intruder than the scenario(s) selected to demonstrate that the performance objective is met. 

12 This does not mean that the scenario with the highest estimated exposure should be selected, 

13 but rather of the reasonably foreseeable receptor scenarios, the scenario that results in the 

14 highest exposure should be selected to demonstrate that the performance objective is met. As 

15 described in Table 4-1, reasonably foreseeable receptor scenarios may consider the capabilities 

16 of intruder barriers, site characteristics, likelihood of contacting certain waste, and trends and 

17 area land use plans. Licensees may consider intruder activities typical of the generic scenarios 

18 that are plausible within the specific time period being evaluated (i.e., compliance period, 

19 protective assurance period, or performance period) assuming the loss of institutional controls 

20 and considering the capabilities of intruder barriers and the natural evolution of site 

21 characteristics are acceptable to develop reasonably foreseeable receptor scenarios. Use of 

22 generic scenarios limits excessive speculation about future human activity. Licensees may also 

23 consider intruder activities that are site-specific and consistent with activities anticipated in and 

24 around the site at the time of facility closure to develop reasonably foreseeable receptor 

25 scenarios.

26

27 The types of site-specific information that a licensee should use to justify selection of a receptor 

28 scenario to demonstrate the inadvertent intruder performance objective is met are broadly 

29 categorized as physical information and cultural information. Physical information includes the 

30 location, climate, topography, geology, soil types, and water availability of the site, including 

31 features of the disposal facility such as waste characteristics, disposal methods, and the use of 

32 intruder barriers. Cultural information is essentially how the human population uses the land. 

33 Physical properties of the site may change over time, particularly long time periods; however, 

34 the change is expected to be slow compared to changes in the cultural use of the land. 

35 Because of the uncertainty in estimating future human disruptive activities, cultural use of the 

36 land is anticipated to be very uncertain over long time periods. Therefore, licensees should not 

37 rely on cultural information as a basis for receptor scenario selection beyond a few hundred 

38 years. Rather, licensees should limit consideration of cultural information to the operating 

39 lifetime of the disposal facility.

40

41 The level of justification and analysis that should be provided by the licensee will depend on the 

42 reasonableness of the physical characteristics the potentially foreseeable land uses, and the 

43 length of time that the radiological hazard persists at the site. Licensees modifying generic

44 receptor scenarios or developing a site-specific intruder receptor scenario should consider the 

45 performance of intruder barriers and the evolution of site characteristics over the duration of the 

46 radiological hazard. Conversely, licensees modifying generic receptor scenarios or developing 

47 site-specific intruder receptor scenarios should limit consideration of potential future uses of the 

48 site and demographic information to the time of facility closure. Such considerations might 

49 include characteristics of the disposed waste, disposal practices, degradation processes of 
4-21

1 intruder barriers, estimates of the evolution of site characteristics, and, when appropriate, the 

2 prevailing and possible future uses of the land, within the operational lifetime of the facility, that 

3 could constrain use. Several potential intruder receptor scenarios may need to be evaluated to 

4 determine the reasonably foreseeable receptor scenario resulting in the greatest exposure. 

5 These scenarios could be based on different combinations of site-specific receptor scenarios 

6 developed from radiological characteristics, disposal practices, evolution of physical 

7 characteristics of the site, and expected land use.

8

9 Selection of site-specific receptor scenarios or the modification of generic exposure pathways 

10 will typically require a technical basis that accounts for the longevity of the hazard. Licensees 

11 should base justifications for modifying generic receptor pathways or developing unique site12 specific receptor scenarios on: (1) waste characteristics and disposal practices; (2) the nature 

13 of the land and reasonable estimates based on physical and geologic characteristics; and (3) 

14 societal uses of land based on past historical information, current uses, and what is reasonably 

15 foreseeable in the near future (i.e., at the time of facility closure). The reviewer should evaluate 

16 the justifications provided by the licensee for the selection of the receptor scenario(s) used to 

17 demonstrate the inadvertent intruder performance objective and the screening of alternate 

18 plausible receptor scenarios considering the following guidance related to the use of site19 specific information.

20 4.3.1.2.1 Site-Specific Physical Information 

21

22 Physical information about the site includes information related to waste characteristics and 

23 disposal practices as well as site physical characteristics. Site-specific physical information can 

24 be used as a basis to modify the generic receptor scenarios and associated pathways. The 

25 physical information should focus on key factors that would impact the likelihood of an intruder 

26 who is engaging in normal activities (e.g., agriculture, dwelling construction, or other pursuits 

27 such as resource exploration) or other reasonably foreseeable pursuits that are consistent with 

28 activities in and around the site at the time of closure in which the person might be unknowingly 

29 exposed to radiation from the waste.

30 4.3.1.2.1.1 Waste Characteristics and Disposal Practices

31

32 Licensees may consider both waste characteristics and disposal practices when developing an 

33 appropriate receptor scenario to demonstrate the performance objective is met. This site34 specific information can affect the level of information needed to support the use of other 

35 physical or cultural information. For example, the hazard from long-lived waste would require a 

36 consideration of the evolution of site characteristics, such as climate, for a longer period than for 

37 shorter-lived waste. Reviewers should consider the effects of this information on selection of an 

38 appropriate receptor scenario.

39

40 A key waste characteristic is the longevity of the hazard from the disposed waste. The longevity 

41 of the hazard should be considered as a factor when using other site-specific information to 

42 develop receptor scenarios. For example, if a facility accepts only shorter-lived waste (e.g., 

43 waste containing only radionuclides with half-lives on the order of the radionuclides listed in 

44 Table 2 in 10 CFR 61.55), then the consideration of the potential evolution of the physical 

45 characteristics of the site can be limited to a shorter time period commensurate with the time it 

46 takes for the shorter-lived waste to decay and result in an acceptable exposure to the 

47 inadvertent intruder. For long-lived waste, consideration of land use information to justify the 
4-22

1 selection of less conservative receptor scenarios may not be appropriate given the uncertainty 

2 in estimating future human behavior. The use of land use information is discussed further in 

3 Section 4.3.1.2.2 below.

4

5 Another important consideration is the use of disposal practices such as the presence of 

6 intruder barriers (e.g., a wasteform designed to be recognizable for long time periods or the 

7 depth of waste emplacement intended to limit direct contact with the waste). Intruder barriers 

8 that are expected to be recognizable over a given timeframe may prevent inadvertent intrusion 

9 or limit potential receptor scenarios over that time period. Licensees should provide an 

10 adequate technical basis for the time period over which a barrier’s capabilities would limit direct 

11 contact with the waste or limit an inadvertent intruder’s radiological exposure. Licensees are 

12 expected to provide a technical basis to support the use of intruder barriers that are expected to 

13 be effective for more than a few hundred years. The analysis should evaluate whether the 

14 barrier precludes or mitigates intruder exposures. The technical basis should also consider the 

15 evolution of the physical conditions to which the intruder barriers would be exposed over the 

16 duration of the hazard.

17

18 For example, a licensee may rely on depth of disposal to limit the consideration of an intruder19 construction type of receptor scenario (see Example 4.6). In this example, the licensee’s 

20 technical basis for exclusion of an intruder-construction scenario should include an assessment 

21 of the effects of erosion and other geomorphologic processes at the site to understand whether 

22 the depth will be sufficient over the duration of the hazard of the disposed waste. Reviewers 

23 should evaluate the adequacy of the technical basis for intruder barrier capability using the 

24 guidance in Section 4.2. 

Example 4.6

A licensee proposes a new disposal cell in which long-lived waste is emplaced beneath an 

intruder barrier at least 5 meters thick. The licensee eliminates from further consideration the 

default scenarios based on the thickness of the cover, and proposes an alternate site-specific 

scenario.

Conclusion: While the alternate site-specific scenario may be appropriate given the cover 

depth, reviewers should confirm that the screening of the default scenarios due to the 

presence of the cover is justified over the portion of the time period analyzed that the longlived hazard persists. The reviewers should confirm that the licensee has adequately 

evaluated the longevity of the cover (e.g., the cover thickness), and the cover’s ability to limit 

contact with the waste by the default excavation scenario over the duration of the long-lived 

hazard. The evaluation should consider geomorphologic processes (e.g., erosion) including 

the impact of any long-term evolution of the site characteristics (e.g., climate) on the rate of 

geomorphism at the site. If the licensee cannot adequately demonstrate that the cover will 

remain thick enough to limit contact with the waste when considering long-term evolution of 

intruder barrier capabilities, then the default scenarios should be evaluated to determine if the 

scenarios result in a greater exposure to radiation from the waste compared to the licensee’s 

alternate scenario. The licensee should consult Section 4.3.1.2 for guidance on screening 

scenarios based on site-specific information.
4-23

1 4.3.1.2.1.2 Characteristics of the Disposal Site

2

3 Licensees may consider the physical characteristics of their disposal site (e.g., the presence of 

4 natural resources such as water and its quality and quantity, soil conditions, topography, and 

5 climate) in justifying an appropriate receptor scenario to demonstrate the inadvertent intruder 

6 performance objective is met. The consideration of physical characteristics often results in 

7 modifications of receptor scenarios rather than complete elimination of the scenario type. For 

8 example, farming may not be supported because of poor soil quality (i.e., not economically 

9 practical), but residential gardening may still be reasonable. The justification should also 

10 include an assessment of the physical characteristics over the period of time a significant 

11 hazard from the disposed waste persists (see Example 4.7). Reviewers should evaluate the 

12 justification for modifying generic receptor scenarios or selecting site-specific receptor scenarios 

13 based on a site’s physical characteristics.

14

15

16

17 The existence of natural resources may result in reasonably foreseeable exploratory activities at 

18 the site, though it should be noted that 10 CFR 61.50(a)(4) requires that areas that have known 

19 natural resources that, if exploited, would result in failure to meet the performance objectives of 

Example 4.7

A licensee proposes a site-specific residential scenario that excludes water pathways because 

of a current lack of water suitable for drinking or irrigation at the site.

Conclusion: While the alternate site-specific scenario may be appropriate given the current 

lack of potable water at the site, reviewers should confirm that the screening of the water 

pathways is justified over the portion of the compliance period that a significant hazard 

persists. The reviewers should confirm that the licensee has adequately evaluated the natural 

evolution of the climate over the compliance period. The reviewer should also confirm that the 

expected evolution of the climate does not result in a change in the potability of the water 

sources available at the site or their availability for use in irrigation. The evaluation should 

consider potential changes in precipitation, evaporation, vegetative cover and its impact on 

transpiration, and their effect on recharge and potability or availability for irrigation at the site. 

If the licensee cannot adequately demonstrate that the water remains nonpotable over the 

duration of the compliance period, then the water pathways should be included in the 

scenario.

Reviewers should also confirm that the screening of the water pathways is justified over the 

portion of the protective assurance period that a significant hazard persists. Because the 

potability of water from the aquifer may be affected by dynamic climatic changes which are not 

expected to be static during the compliance period, licensees would need to provide a 

justification that the continuing evolution of the climate would not lead to a potable water 

source during the protective assurance period. Licensees may be able to simply estimate the 

change in the level of chemical constituents that cause the non-potability based on future 

expected climates and demonstrate that the potability of the water would not change based on 

the estimates of future constituent levels. For instance, a licensee may need to demonstrate 

that under wetter future climates, increased infiltration would not decrease the total dissolved 

solids of a non-potable aquifer sufficiently to change the water source’s potability during the 

protective assurance period according to drinking water standards in effect today.
4-24

1 10 CFR Part 61, Subpart C must be avoided during siting. Because of the uncertainty in

2 estimating future human activity, licensees do not need to speculate on a future society’s 

3 interest in known resources that are not economically viable in the vicinity of the site after 

4 closure. When reviewing the technical basis involving the presence or absence of economically 

5 viable resources at the site, reviewers should limit speculation about natural resources that may 

6 be economically viable to future societies after closure. Rather, reviewers should focus on 

7 natural resources for which exploration is currently being conducted in the region of the site (i.e., 

8 within an 80-kilometer (50-mile) radius of the site). Natural resources not currently being 

9 extracted in the region of the site may still be considered if their presence is currently known to 

10 exist in economically viable quantities in the immediate vicinity of the site.

11

12 Justification to limit receptor scenarios and exposure pathways for intruders based on 

13 groundwater quality and quantity should be based on site conditions rather than local codes and 

14 should be based on classification systems used by the EPA or the State, as appropriate. 

15 Arguments involving depth to water table or well production capacity should have supporting 

16 documentation from either the U.S. Geological Survey (USGS), an appropriate State agency, or 

17 an independent consultant. Tables M.5-M.12 in NUREG-1757, Appendix M (NRC, 2006) 

18 provides additional details on water quality standards.

19

20 Licensees using soil quality as a justification for modifying receptor scenarios should provide 

21 supporting documentation from the U.S. Natural Resources Conservation Service, appropriate 

22 State or local agency, or an independent consultant. Reviewers should carefully consider 

23 whether the state of the soil would reasonably preclude all activities or only certain activities. In 

24 most cases, soil quality can reasonably preclude activities such as crop production, but could 

25 allow grazing or small gardens. 

26

27 Licensees using topography as a justification for modifying receptor scenarios should provide 

28 supporting documentation of the existing topography in the form of pictures, USGS or similar 

29 topographic maps, hand-drawn maps, or a detailed description of how the topography would 

30 limit activities. Licensees may need to conduct landform evolution modeling that might also 

31 include expert elicitation to provide support for reasonably foreseeable landform changes to 

32 local topography. See Section 5.0 for additional guidance for long-term landform evolution.

33

34 When reviewing justifications involving topography, the reviewer should limit speculation about

35 future topographical changes from offsite civil engineering projects, but not topographical 

36 changes from the final closure design of the disposal facility itself. The reviewer should 

37 evaluate the reasonableness of the inadvertent intruder performing activities on the current 

38 topography; for example, a slope, and reasonable evolution of the site due to geomorphologic 

39 processes (e.g., fluvial, eolian, tectonic, biological). Reviewers may wish to perform a site visit 

40 to evaluate the current topography first hand and assess how the disposal design may impact 

41 the current topography. For example, reviewers may wish to assess the effect of runoff from a 

42 licensee’s cover system on erosion of the site topography. 

43 4.3.1.2.2 Site-Specific Cultural Information

44

45 Licensees may consider cultural information in justifying an appropriate receptor scenario or 

46 scenarios to demonstrate the performance objective is met. Cultural information is essentially 

47 how land is used by the human population and describes the types of normal activities an 

48 inadvertent intruder might engage in on the site. Information on land use can be based on past, 
4-25

1 current, or projected land uses. The shorter of either the anticipated operational lifetime or one 

2 hundred years is a reasonable period of interest for future land use projections to provide the 

3 basis for receptor scenarios, depending on the rate of change in the region, and the peak 

4 exposure time. Note that the 100-year timeframe described here is only for estimating future 

5 land uses to justify a receptor scenario; the licensee must evaluate doses that could occur over 

6 the time periods specified in the regulations. However, because of the uncertainty in estimating 

7 future human behavior, cultural information projected beyond the operational lifetime of the 

8 disposal facility should not typically be used as justification for a receptor scenario. Instead 

9 licensees may rely on suitable physical information and should refer to Section 4.3.1.2.1 for 

10 guidance on its use in justifying a scenario to demonstrate that the inadvertent intruder 

11 performance objective is met. 

12

13 A licensee’s assumptions about land use should focus on current practice in the region of 

14 concern, which can be as large as an 80-kilometer (50-mile) radius. To narrow the focus of 

15 current land practices, the licensee can use information on how land use has been changing in 

16 the region and should give more weight to land use practices either close to the site or in similar 

17 physical settings. Licensees should also evaluate land uses that occur in locations outside the 

18 region of concern that share characteristics (temperature, precipitation, topography) expected 

19 for the region of concern over the duration evaluated in a site-specific receptor scenario. 

20 Consideration of environmental-analog regions may help identify whether present-day land uses 

21 have been driven by past socio-economic development. Land uses primarily resulting from 

22 socio-economic development are generally more uncertain over longer time periods than land 

23 uses primarily resulting from physical conditions (e.g., climate).

24

25 Licensees should categorize potential land use as reasonably foreseeable, less likely but 

26 plausible, or implausible. Any land uses that similar real-estate properties in the region currently 

27 have, or may have in the near future (e.g., in approximately 100 years or the operational lifetime 

28 of the disposal facility) should be characterized as reasonably foreseeable. Consideration 

29 should be given to trends and area land use plans in determining the likelihood of potential land 

30 use. Land uses that are plausible, generally because similar land either was used for this 

31 purpose historically in the region of interest, or is used currently in regions with analogous 

32 environmental characteristics, but that are counter to current trends or regional experience 

33 should be characterized as less likely but plausible. Licensees should provide either a 

34 quantitative analysis or qualitative argument discounting the need to analyze all scenarios 

35 generated from the less likely but plausible land uses. If peak doses from the less likely but 

36 plausible land uses are significant, the licensee should provide greater support that the receptor 

37 scenario is unlikely to occur. Implausible land uses are those that, because of physical 

38 limitations, could not occur. Because of the uncertainty in predicting human behavior in the 

39 future, land use information should be consistent with normal activities or other activities that 

40 typically occur in and around the site at the time of closure of the disposal facility.

41

42 Reviewers should evaluate the justification provided for the selection of reasonably foreseeable 

43 scenarios. One goal of the review is to ensure that, if land uses other than the reasonably

44 foreseeable land use were to occur in the future, significant exposure would not result. 

45 Reviewers may wish to involve State and local land use planning agencies in discussions if the 

46 licensee has not already requested their involvement. Additional guidance on potential sources 

47 of land use information is available in Appendix M.5 to NUREG-1757 (NRC, 2006).
4-26

1 4.3.2 Model Abstraction

2

3 Model abstraction is the process of incorporating the significant FEPs into a conceptual model 

4 that can reasonably describe how the facility limits the inadvertent intruder’s radiological 

5 exposures. As discussed in Section 2.7.2, the conceptual model is then abstracted so that it 

6 can be represented in a mathematical model to estimate potential radiological exposure and the 

7 associated uncertainties. In this way, model abstraction for an intruder assessment is similar to 

8 model abstraction for a performance assessment. Because of the similarity in the model 

9 abstraction process for performance assessments and intruder assessments, the guidance in 

10 NUREG-1573, Section 3.3 (NRC, 2000a), may also be generally applicable to the intruder 

11 assessment. The following sections discuss modeling issues that require additional, specific 

12 guidance applicable to the intruder assessment.

13

14 The abstraction process for the intruder assessment may be more stylized than for the 

15 performance assessment depending on the particular receptor scenario used to demonstrate 

16 that the performance objective for protection of the inadvertent intruder is met. Inadvertent 

17 intrusion receptor scenarios involving direct contact with the waste are generally expected to 

18 use more stylized modeling approaches because the release of radionuclides from the disposal 

19 units is primarily affected by the assumed human activities. For example, an intruder 

20 assessment evaluating direct contact with the waste may be limited to abstractions for the 

21 degradation of intruder barriers, the source term, and the resulting biosphere exposures, while 

22 receptor scenarios involving the release and migration of radionuclides through the site 

23 environment as a result of natural processes may use modeling approaches that are more 

24 similar to performance assessments.

25

26 General considerations for performing and reviewing model abstractions of the various technical 

27 analyses are discussed in Section 2.0 of this guidance and may apply to an inadvertent intruder 

28 assessment. Guidance on selecting site-specific input parameters for the models and providing 

29 technical basis can also be found in NUREG-1757, Section I.6 (NRC, 2006). The guidance in 

30 NUREG-1757 is oriented toward decommissioning activities; however, the concepts presented 

31 are also generally relevant to intruder assessment for LLW disposal.

32

33 Reviewers should focus on understanding the importance of various assumptions, models, data, 

34 and uncertainty in the intruder assessment. To review the overall intruder assessment, the 

35 reviewer should recognize that models used by a licensee may range from highly complex 

36 process-level models to simplified models, such as response surfaces or look-up tables. The 

37 reviewer should determine whether uncertainties in the models and parameters are 

38 appropriately accounted for in the intruder assessment. Section 2.2.2 of this document 

39 discusses general considerations for uncertainty and sensitivity analysis.

40

41 An intruder assessment can be a collection of models of varying complexity or it can be an 

42 integrated model. Intruder assessment commonly involves the execution of numerical models

43 that mathematically represent the conceptual model of the contaminated site. The numerical 

44 models used to implement the mathematical equations are usually linked via the conceptual 

45 model and codified in a software package known as “the code.” Reviewers should ensure that 

46 the intruder assessment codes and models and the associated databases are properly 

47 documented and verified in accordance with a QA/QC criterion that is acceptable to the NRC 

48 staff. Chapter 9 of NUREG-1199 and NUREG-1200 provide guidance on the information 

49 needed to comply with the QA requirements specified in 10 CFR 61.12(j) (NRC, 1991a; 1994). 
4-27

1 Further, NUREG-1293 (NRC, 1991c) provides specific guidance on how to meet the 

2 10 CFR Part 61 QA requirements. If site-specific models and codes are used, a justification of 

3 the conceptual model should be provided. Reviewers should also review the source term 

4 models, the transport models, the exposure models, and the overall dose models. Reviewers

5 should assess the QA/QC documentation and the level of conservatism of any alternate code 

6 and model. Section I.5 of NUREG-1757 (NRC, 2006) provides guidance, in the context of 

7 decommissioning activities, which is generally relevant to intruder assessments on the selection 

8 of codes and models and approaches for NRC acceptance of the codes and models. Section 

9 2.7.1.2 of this document provides additional information on quality assurance.

10

11 The following sections discuss guidance on developing specific model abstractions for intruder 

12 assessment and general model development issues.

13 4.3.2.1 Intruder Barriers

14

15 The intent of an intruder barrier is to inhibit contact with waste and help ensure that an 

16 inadvertent intruder’s radiation exposure will be limited, which provides reasonable assurance 

17 that the performance objectives can be met. A variety of intruder barriers may be employed at a 

18 waste disposal facility depending on the nature of the waste, the facility design, and the site 

19 characteristics. Intruder barriers may include a sufficient depth of cover over the waste or 

20 engineered structures that provide protection to the inadvertent intruder (e.g., engineered 

21 covers, concrete vaults, engineered wasteforms, or waste containers). Each intruder barrier will 

22 have a time period over which it will perform its intended functions, which should be justified by 

23 the licensee. Additional guidance on justifying intruder barrier capabilities is available in Section

24 4.2.

25

26 The objective of intruder barrier model abstraction is to establish model representations of the 

27 intruder barriers that are reasonably consistent with their intended capabilities and their 

28 expected behavior with time. A primary outcome of the intruder barrier model abstraction is an 

29 estimate of the time after disposal when intrusion could occur. This output is affected by the 

30 particular intruder receptor scenario, the design of the intruder barrier system, and the impact of 

31 natural processes on the longevity of the intruder barriers’ capabilities. Reviewers should apply 

32 the general guidance in Section 2.7.2 when evaluating the abstraction of intruder barriers in the 

33 intruder assessment. Reviewers should consider the degree to which the licensee relies on the 

34 capabilities of the intruder barriers to demonstrate the performance objective is met. For 

35 intruder assessments for which licensees have demonstrated that intruder barriers have a minor 

36 impact on protection of the intruder, a simplified review should be sufficient.

37

38 The nature of the activities in which an inadvertent intruder might engage will affect the time at 

39 which an intruder might unknowingly be exposed to radiation from the waste. Licensees should 

40 assess the capability of the intruder barrier system to preclude contact with the waste or limit 

41 radiological exposures. For example, construction activities could be limited to the discovery 

42 receptor scenario as long as the wasteform remains stable, and is, therefore, expected to be 

43 distinguishable from soil. Licensees should provide a technical basis for the ability of the 

44 intruder barrier to delay the time of initiation for intruder activities. The level of detail in the 

45 technical basis should be commensurate with the delay time afforded by the capabilities of the 

46 intruder barrier. Barriers providing substantial delay time would require a more robust technical 

47 basis. The technical basis should consider insights gained from scenario analysis and 

48 beneficial as well as deleterious natural processes that may affect the capability of the barrier to 
4-28

1 preclude the intruder activities. Reviewers should focus on those barriers that provide 

2 substantial delay time to the onset of intruder activities; activities that otherwise would result in 

3 direct contact with the waste or other radiological exposures.

4

5 In developing intruder barrier models, the licensee should present information on spatial 

6 relationships among the physical components (e.g., the layout and physical dimensions of a 

7 vault or cover system) and the physical distribution of various types of materials that are used in 

8 the intruder barriers. The intruder assessment should include features of the intruder barriers 

9 that are most important to demonstrating the performance objective is met. Licensees should 

10 ensure that the models for intruder barriers are integrated with related model abstractions. For 

11 example, the conditions and assumptions used in the degradation of intruder barriers should be 

12 consistent with those used for other model abstractions (e.g., climate and infiltration). 

13

14 Reviewers should examine the identified physical components that are important to 

15 demonstrating the performance objective is met and evaluate whether their representation in the 

16 intruder assessment modeling is consistent with their description, the technical basis supporting 

17 their capabilities, and other model abstractions. The review should assess whether the 

18 descriptions are adequate to detail the important design features, capabilities, and properties of 

19 the barriers (e.g., thickness, porosity, or saturation of a cover layer designed to limit gaseous 

20 fluxes to the surface). The modeling of intruder barriers by licensees should reflect the level of 

21 quality that is expected to be achieved in the implementation of the design. Reviewers should 

22 assess whether the level of quality being proposed can be attained, that it is supported by an 

23 acceptable quality assurance program, and that it is adequately represented in the abstraction.

24

25 Licensees should evaluate processes that may limit the effectiveness of the intruder barriers. 

26 Barriers may degrade from internal processes (e.g., interaction between incompatible materials, 

27 interaction with the waste) or external processes (e.g., interaction with biota, erosion, leaching 

28 by infiltrating water, and processes such as seismically induced cracking). Analysis of a barrier 

29 system should be performed in an integrated manner because of potential synergisms between 

30 degradation mechanisms. If the analysis is performed assuming that the degradation 

31 mechanisms are independent, the reviewer should evaluate the information to determine 

32 whether an adequate basis has been provided for the analysis approach (e.g., assuming that 

33 the degradation mechanisms can be evaluated as independent). This may include 

34 demonstrating that the degradation analysis was reasonably conservative or that uncertainty 

35 was adequately characterized and propagated in the model. Reviewers should also verify that 

36 the models for degradation of intruder barriers are consistent with environmental parameters, 

37 material properties, and assumptions implemented in the performance assessment.

38

39 The intruder assessment should account for relevant materials and conditions that could affect 

40 the ability of an inadvertent intruder to contact the waste over time. Licensees should consider 

41 interactions of the components and materials of the intruder barriers. Factors that may need to 

42 be considered include (1) compatibility among materials that may come into contact with each 

43 other, (2) the manner in which construction may affect system behavior (e.g., construction joints, 

44 changes in geometry, penetrations), (3) the effect that failure of a design feature or some 

45 portion of an intruder barrier would have on the overall ability of the intruder barrier system to 

46 limit contact with the waste, and (4) how the degradation of material properties affects barrier 

47 performance over time.

48
4-29

1 Representation of the temporal performance of an intruder barrier may be accomplished by 

2 dividing the barrier performance into three phases. The first phase is the service life during 

3 which the intruder barrier would effectively inhibit contact with the waste and help ensure that 

4 radiation exposures will meet the performance objective. The second phase represents a time 

5 of decreasing performance from ongoing processes of degradation. It should also be 

6 recognized that, for some barriers, the time between initial non-degraded conditions and 

7 completely degraded conditions may be quite short. The third or final phase represents 

8 complete degradation. In this third phase, the barrier is no longer able to limit contact with the 

9 waste. However, barriers that are designed help ensure that radiation exposures will meet the 

10 performance objective, rather than inhibit contact with the waste, may still provide some level of 

11 diminished performance (see Example 4.8). It is expected that the service life periods of 

12 different barriers may vary significantly because of the inherent diversity and variability of the 

13 barrier components. This variability in service life would need to be accounted for in the intruder 

14 assessment.

15

16 Licensees should provide information to support the model estimates of intruder barrier 

17 performance. Section 2.2.3 of this document provides guidance on model support. Model 

18 support can include laboratory experiments, field measurements, previous experience with 

19 similar systems, process modeling of barrier performance (e.g., detailed models of landform 

20 evolution to estimate cover erosion), natural or industrial analogs, independent peer review 

21 (NRC, 1988a), or additional sources of relevant information. Reviewers should examine the 

22 evidence supporting the modeling to confirm that the information is based on similar 

23 environmental parameters, material factors, assumptions, and approximations shown to be 

24 appropriately analogous and that the models are not likely to underestimate actual degradation 

25 and failure of intruder barriers. Reviewers should also examine the procedures that the licensee 

Example 4.8

A licensee proposes a reinforced concrete vault as an intruder barrier for the disposal of 

depleted uranium. The licensee describes the capabilities of the vault to both inhibit contact 

with the depleted uranium and limit release of radon gas to the surface over some expected 

service life.

Conclusion: An appropriate model representation would be to limit an inadvertent 

intruder’s contact with the waste during the service life of the vault, provided that an 

adequate technical basis for the service life is provided. Reviewers should evaluate the 

adequacy of the technical basis for the service life. When the service life has ended, the 

applicant could conservatively assume that the vault would return to constituent sand and 

gravel aggregates and take no credit for the ability of the degraded vault to either inhibit 

contact with the waste or limit radon diffusion to the surface. Alternatively, the licensee may 

choose to still consider the ability of the degraded state to sufficiently reduce diffusion of 

radon to the surface thereby limiting radon exposures to an inadvertent intruder. In this 

alternate case, the reviewer should evaluate the licensee’s basis for the degraded material, 

including degraded material properties (e.g., effective diffusivity), to continue delaying radon 

transport to the surface.
4-30

1 used to construct and test its mathematical and numerical models. Section 4.2.1 provides

2 guidance on a risk-informed approach to model support for engineered barrier capabilities.

3

4 Licensees should assess the uncertainty in model estimates for the longevity of intruder barrier 

5 capabilities. This assessment should also evaluate the sensitivity of the potential inadvertent 

6 intruder exposures to uncertainties in the support for the model estimates. Section 2.2.2 of this

7 document discusses guidance on dealing with uncertainty.

8 4.3.2.2 Source Term

9

10 The objective of the source term abstraction in an intruder assessment is to estimate the 

11 radionuclide concentrations in the environment (i.e., the biosphere) accessible to the inadvertent 

12 intruder. The modeling of the source term can be one of the most important determinants of the 

13 estimated exposures to an inadvertent intruder. Typically, source term modeling includes the 

14 inventory of radionuclides, physical and chemical characteristics, and other properties used to 

15 estimate release rates.

16

17 For the intruder assessment, radionuclides are primarily expected to enter the accessible 

18 environment through inadvertent contact with the waste, though entry may also be possible 

19 through the use of contaminated surface water or groundwater on the site, or as a result of 

20 gaseous releases from the disposed waste to the atmosphere above the site. In direct contact 

21 receptor scenarios, the source term modeling estimates the concentration of radionuclides 

22 contacted and exhumed to the biosphere that the inadvertent intruder inhabits. These 

23 concentrations are then used as input in the biosphere modeling. In indirect receptor scenarios, 

24 the source term modeling typically estimates releases occurring by advective or diffusive 

25 mechanisms, although other mechanisms may be possible (e.g., biointrusion, erosion). The 

26 release rates from the disposal unit are estimated and used as input for transport modeling.

27 4.3.2.2.1 Inventory

28

29 Assumptions about the characteristics of the radionuclide inventory can have a significant effect 

30 on the determination and selection of modeling approaches appropriate for representing the 

31 source term. Radionuclide inventories need to be addressed on a facility-specific basis. The 

32 anticipated distribution of specific radionuclides in the disposal facility inventory should be 

33 estimated by waste class (A, B, and C), waste type, wasteform, waste stream, and waste 

34 container type, as appropriate. This information provides a basis for selecting an approach for 

35 source term modeling. The necessary level of detail in this information will vary with the 

36 modeling approaches under consideration. For example, the source term model for direct 

37 contact receptor scenarios in which a discrete volume of waste is exhumed may be more 

38 stylized than for other scenarios involving release from the disposal units. 

39

40 Licensees should provide a description of the radionuclide inventory in the disposal facility. All 

41 radionuclides should be described by volume, concentration, and location within the disposal 

42 system. The radionuclide inventory should be consistent with the resulting waste acceptance 

43 criteria as well as the inventory used in the performance assessment to assess whether the 

44 requirements of 10 CFR 61.41 are demonstrated. However, in some cases, licensees may use 

45 conservative estimates of inventory in the intruder assessment and more realistic estimates in 

46 the performance assessments. 

47
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1 Reviewers should evaluate the methods used to characterize the radionuclide inventory and 

2 estimate the concentrations of radionuclides not measured to ensure that uncertainty and 

3 variability are appropriately accounted for in the source term model.

4

5 Licensees may use a screening approach to determine which radionuclides in the facility 

6 inventory need to be considered further in the intruder assessment. Licensees may:

7

8 (1) Eliminate radionuclides with half-lives less than 5 years that are not present in 

9 significant activity levels and do not have long-lived daughter products.

10

11 (2) Perform an intruder dose calculation using the generic receptor scenarios discussed in 

12 Section 4.3.1.1 assuming that the intruder inadvertently intrudes into the spatially 

13 averaged peak waste concentration and that all intruder barriers are completely 

14 ineffective in inhibiting contact with the waste. All radionuclides are assumed to be 

15 available for all relevant exposure pathways of each generic receptor scenario. 

16 Radionuclides with an estimated peak dose in each of the receptor scenarios that is less 

17 than 10 percent of the performance objective can be eliminated from further 

18 consideration provided that the sum of their doses is accounted for in demonstrating the 

19 performance objective is met. If a radionuclide has an estimated peak dose greater 

20 than or equal to 10 percent of the performance objective in any of the generic receptor 

21 scenarios, the radionuclide should be retained for the final intruder assessment. 

22 Computer models such as RESRAD (Yu et al., 2001) may be useful to conduct this type 

23 of screening analysis. To ensure that important radionuclides are not inadvertently 

24 screened out of the assessment, it is important to confirm that the dominant pathways 

25 (i.e., those contributing most to estimated dose) in the screening calculation are 

26 consistent with those in the final intruder assessment.

27

28 Reviewers should examine the description of the screening assessment to ensure that the 

29 licensee used reasonably conservative parameters and appropriately screened the 

30 radionuclides.

31

32 For intruder receptor scenarios in which direct contact with the waste occurs, source term 

33 modeling can be as simple as distributing a concentration of radionuclides in the biosphere for 

34 exposure modeling. For receptor scenarios with indirect releases from the disposal cells, such 

35 as through advective-diffusive release mechanisms, licensees may develop source term 

36 abstractions similar to those in the performance assessment (see Section 3.1). Licensees may 

37 also use estimated concentrations from the performance assessment at an appropriate location 

38 and point in time to assess exposures to the inadvertent intruder in the biosphere. 

39

40 Reviewers should verify that licensees using radionuclide concentrations from the performance 

41 assessment have not overestimated releases in the performance assessment to conservatively 

42 estimate doses to the general public, resulting in a potential underestimation of radionuclide 

43 concentrations in the intruder assessment.

44

45 Licensees should also consider the radioactive decay before the time of intrusion or may 

46 conservatively assume no decay. In either case, however, licensees should consider the 

47 impacts of significant progeny on the intruder. Radioactive decay can result in significant 

48 ingrowth of progeny at future times. For example, doses from depleted uranium may increase 

49 for more than one million years due to ingrowth of shorter-lived and more highly mobile decay 
4-32

1 products. Given that the regulations specify that 100 years is the maximum duration over which

2 active institutional controls can be relied upon, radioactive decay is expected to be more 

3 important in direct contact receptor scenarios for shorter-lived radionuclides such as cesium4 137. Reviewers should verify that radioactive decay is appropriately accounted for in the 

5 source-term modeling. 

6

7 Licensees may also account for the time of waste emplacement. Justification should be 

8 provided to support the time of waste emplacement. For example, if a licensee assumes that all 

9 waste is emplaced in the final year of operation, minimal justification would be needed. 

10 Additional justification would be expected to support earlier times of waste emplacement.

11 4.3.2.2.2 Radionuclide Concentrations for Direct Contact Scenarios

12

13 To develop the source term abstraction, licensees should estimate the concentration of 

14 radionuclides in the facility at times after which the direct contact scenarios would be expected 

15 to occur. Estimating the time after which a direct contact scenario could occur is related to the 

16 assumed institutional control period (see Section 4.4), intruder activities envisioned in the 

17 receptor scenario, and the capabilities of intruder barriers to preclude intrusion. Section 4.3.2.1 

18 provides guidance on the capabilities of intruder barriers to preclude intrusion.

19

20 Licensees should consider the following elements when generating concentrations of 

21 radionuclides for the intruder dose assessment:

22

23 • Licensees are permitted to account for dilution of the waste following disposal due to 

24 mixing with uncontaminated materials in the disposal cells.

25 • The amount of mixing should account for methods of waste emplacement such as 

26 disposal depth.

27 • The justification for the amount of mixing may rely on site characteristics.

28 • The justification may include reasonable assumptions about the intruder activities.

29

30 Licensees are permitted to account for dilution of the waste following disposal due to mixing with 

31 uncontaminated materials in the disposal cells. Licensees should provide a technical basis for 

32 dilution of the waste mixed with uncontaminated materials. Mixing should be consistent with the 

33 design of the facility, waste emplacement, site characteristics and assumptions about intruder 

34 activities for a given scenario. The design of the facility may incorporate uncontaminated 

35 materials within the disposal cells that would tend to reduce radionuclide concentrations found 

36 in the waste. Design features may include intruder barriers and stabilization materials such as 

37 backfill or grout. For example, waste may be emplaced beneath an engineered cover with 

38 uncontaminated backfill material to help maintain the stability of the disposal cells. The 

39 uncontaminated cover and backfill would likely reduce radionuclide concentrations found in the 

40 waste when mixed during excavation, as considered in an intruder-construction scenario.

41

42 The amount of mixing should account for methods of waste emplacement. For example, the 

43 intruder-construction scenario typically only excavates a depth necessary to construct a 

44 dwelling. Waste and backfill placed beneath this depth should not be considered in the mixing 

45 assessment. The justification for the amount of mixing may rely on site characteristics. For

46 example, the thickness of uncontaminated soil between the bottom of the disposal cells and the 
4-33

1 underlying aquifer or resource may also be considered in estimating dilution for an 

2 intruder-driller scenario. To justify mixing based on deeper aquifers or resources, licensees 

3 should explain why mixing with the uncontaminated soils between the bottom of the disposal 

4 cells and the depth of the deeper aquifers or resources would be more appropriate than a 

5 smaller mixing volume associated with shallower aquifers or resources. Licensees should limit 

6 this mixing volume to the shallowest aquifer or resource that would be reasonable to access for 

7 water or the natural resource. Licensees should also consider the impact of variability in the 

8 mixing volume associated with the use of shallower aquifers or resources if it is plausible that an 

9 inadvertent intruder might access the shallower aquifer or resources.

10

11 In the absence of knowledge that the site is a disposal facility, it may be considered reasonable 

12 to assume that an intruder will excavate at random locations. In this case, the excavation could 

13 exhume material that was contained within multiple waste packages or disposal cells. 

14 Therefore, the waste concentration for estimating intruder impacts from excavation can be 

15 averaged over the area of the site where waste is disposed, including uncontaminated regions 

16 within the disposal cells (e.g., backfill). Portions of the site beyond the disposal cell footprints, 

17 such as administrative areas or the buffer zone, should not be included in the areal-average of 

18 the waste concentrations. If the excavation area is larger than the disposal cell area, dilution 

19 with uncontaminated soil surrounding the disposal cell may be considered.

20

21 For intruder-driller scenarios, the waste concentrations may be based on the average 

22 concentration in the waste containers (for containerized waste) or the average concentration of 

23 the waste for bulk waste. Uncontaminated material above and below the waste may be credited 

24 in the intruder-driller waste concentration calculations. However, uncontaminated material 

25 within the disposal cell (e.g., backfill) should not be credited because the intruder driller disturbs 

26 a small, discrete area. Operational limits, such as waste emplacement strategies, may be 

27 necessary to limit the impact from wastes with significant heterogeneity that is near class limits.

28

29 Licensees should provide a basis to justify the suitability of the random access assumption. For 

30 example, at an aboveground disposal facility characterized by mounded disposal cells that 

31 create a “ridge and valley” topography in which the disposal mounds are expansive with gentle 

32 terrain and are separated by steep and narrow “valleys” that are uncontaminated, licensees can 

33 argue that the excavation of a dwelling foundation would preferentially occur on the expansive 

34 “ridges” rather than in the steeper, narrower “valleys”. In this case, random access may not be 

35 an appropriate assumption for an intruder-construction scenario. 

36

37 Reviewers should evaluate the reasonableness of the technical basis supporting the dilution of 

38 waste concentrations. The review should consider the facility design, waste emplacement, site 

39 characteristics, and scenario assumptions. Generally, the more mixing that is assumed, the

40 more robust the technical basis should be to support the dilution factor.

41

42 Radionuclides that are released offsite through natural processes before intrusion are generally 

43 not available to be exhumed by an inadvertent intruder. Licensees may consider radionuclide 

44 migration before the commencement of the intruder activities. Licensees may use information 

45 from the performance assessment to estimate radionuclide releases. However, licensees 

46 should exercise caution with this approach if the performance assessment overestimates 

47 releases to conservatively assess exposures to the general public. This may result in an 

48 underestimation of waste concentrations remaining for the intruder to access. 

49
4-34

1 Reviewers should assess the consistency between radionuclide releases estimated in the 

2 intruder assessment and in the performance assessment. Reviewers should ensure that 

3 pessimistic modeling biases and assumptions in the performance assessment do not result in 

4 overly optimistic biases in the intruder assessment.

5

6 The spatial extent over which exhumed waste is deposited at the surface on site will depend on 

7 the activities assumed to occur in the intrusion receptor scenario (e.g., drill cuttings 

8 management practices) and physical arguments about the exhumed volumes (i.e., how widely 

9 can a volume of exhumed waste physically be spread?). The area over which exhumed 

10 material is spread may be an important parameter because it can limit certain exposure 

11 pathways. For example, too small an area may make certain agricultural pathways unviable,

12 while too large an area may unreasonably dilute the concentration of the waste exhumed. For

13 the spatial distribution of the exhumed radionuclides, licensees should use reasonable physical 

14 assumptions consistent with the scenario assumptions. For example, a small volume of drill 

15 cuttings is not likely to be spread over a large area of the site. 

16

17 Reviewers should evaluate the licensee’s basis for the extent of contaminated material in the 

18 biosphere. Reviewers should assess the consistency between the assumptions about spatial 

19 distribution of exhumed radionuclides and the assumed activities of the receptor scenario.

20 4.3.2.2.3 Radionuclide Concentrations for Transport Scenarios

21

22 Reviewers should assess the adequacy of the radionuclide release and transport models in the 

23 intruder assessment consistent with the guidance in Sections 3.2 and 3.3 of this document. 

24 Radionuclide concentrations for intruder transport scenarios typically involve an estimation of 

25 environmental concentrations after release and transport through the disposal site environment. 

26 The release and transport are typically controlled by natural processes (e.g., degradation, 

27 sorption). The performance assessment for demonstrating that the requirements of 

28 10 CFR 61.41 are met also estimates radionuclide releases from the source term of a disposal 

29 facility that are controlled by natural processes.

30

31 There are a few exceptions to the similarity between a performance assessment and an intruder 

32 assessment. First, access points for the intruder to the contaminated environmental media 

33 should all be on site. Offsite exposures are assessed in the performance assessment for 

34 demonstrating that the performance objective for protection of the general population is met. 

35 Second, intruder receptor scenarios are typically independent. For example, the intruder who 

36 contacts the waste during construction of a dwelling may not be the same individual who lives in 

37 the dwelling. The radiological exposures from these receptor scenarios are generally mutually38 exclusive and not additive. However, there may be some cases in which it would be appropriate 

39 for receptor scenarios to be dependent. For example, it would be reasonable to assume that a 

40 well is installed to supply water to a dwelling that is constructed on the site and that the intruder 

41 residing in the dwelling is also exposed to contaminated ground water from the well. When 

42 dependent receptor scenarios are used and one is a direct contact scenario and the other is a 

43 migration scenario, the licensee may provide a basis for including the removal of some of the 

44 waste from the disposal cell at the time of the intrusion event in order to calculate releases from 

45 the buried waste for the transport scenario. Licensees should also assess the transport of 

46 waste that was accessed by the intruder (i.e., the transport of radionuclides from waste that was 

47 excavated by the intruder).

48
4-35

1 Reviewers should verify that the source term modeling is consistent with and appropriately 

2 integrated with other model abstractions (e.g., climate and infiltration for advective-diffusive 

3 release mechanisms). Reviewers should consult Section 3.1 for guidance on reviewing source 

4 term models involving advective-diffusive release mechanisms. Reviewers should evaluate the 

5 assessment of concentrations for the direct contact scenario and transport scenario to ensure 

6 that biases or assumptions in one of the scenarios don’t result in an unreasonably optimistic 

7 approach in the other scenario.

8 4.3.2.3 Transport

9

10 Radionuclides directly contacted by an inadvertent intruder or released from disposal units may 

11 be transported in the general environment by groundwater, surface water, air, and biota (e.g., 

12 rodents). Model abstraction for transport processes in an intruder assessment is generally 

13 similar to model abstraction for transport processes in a performance assessment. Therefore,

14 the guidance in Section 3.2 of this document is generally applicable to an intruder assessment. 

15 One notable exception is that an intruder assessment is focused on exposures on site. 

16 Therefore, model abstraction of transport processes should be focused on onsite transport 

17 processes for an inadvertent intruder assessment. This may include gaseous diffusion of 

18 radionuclides from the disposal unit through the overburden to an intruder at the surface or 

19 leaching from the disposal units and transport via groundwater to the intruder well at the site 

20 boundary. Radionuclides released from the disposal units that are transported beyond the site 

21 boundary (i.e., off site) are generally evaluated in a performance assessment for members of 

22 the public and would not need to be considered in an inadvertent intruder assessment.

23 4.3.2.4 Dose 

24

25 The objective of the dose modeling in an inadvertent intruder assessment is to provide 

26 estimates of potential doses to an inadvertent intruder, in terms of the average member of the 

27 critical group, from direct contact with disposed waste or onsite releases from an LLW disposal 

28 facility after the period of active institutional controls. In this role, dose modeling integrates the 

29 information from the various modeling areas. In general, dose modeling in an intruder 

30 assessment is similar to dose modeling in a performance assessment. Dose modeling consists 

31 of converting radionuclide concentrations in environmental media to dose through various 

32 exposure pathways. Guidance on development of receptor scenarios is discussed in Section 

33 2.2.4.2 of this document, and guidance on the dosimetry parameters is discussed in Section

34 2.2.4.3. Additional guidance applicable to modeling of exposure pathways in intruder 

35 assessment are discussed in the following sections.

36

37 The exposure pathway models link the radiological source, transport of radionuclides within 

38 environmental media, receptor location, and behaviors of the receptor that lead to its exposure 

39 to radionuclides through direct exposure, inhalation, and ingestion of contaminated water, soil, 

40 plants, and animal products. 

41

42 Reviewers should evaluate the conceptual models that describe the human behaviors that lead 

43 or control the amount of receptor exposure. Therefore, the occupational, behavioral, and 

44 metabolic parameters describing these models should be reviewed and compared with the 

45 receptor scenarios and associated parameters.

46
4-36

1 In some cases, either the location of the disposed waste, the physical characteristics of the site, 

2 or the facility design may make the generic exposure pathways inappropriate. In other cases, 

3 the licensee may wish to provide a transparent and traceable development of the receptor

4 scenarios used to demonstrate the performance objectives are met starting with potential land 

5 use and site-specific conditions. Development and review of alternate scenarios may involve 

6 iterative steps, including the development of the conceptual model of the site. For example, the 

7 licensee may (1) develop a generic list of exposure pathways, (2) develop the site conceptual 

8 model to screen the generic list, (3) aggregate or reduce the remaining exposure pathways to 

9 the major exposure pathways, and (4) reevaluate the conceptual model to verify that all the 

10 necessary processes are included.

11

12 A brief summary of the NRC-recommended process for exposure pathway analysis follows:

13

14 (1) Compile a list of exposure pathways applicable to any site containing radionuclides. 

15 There are a number of existing sources of information that can be used;

16 e.g., NUREG/CR-5512, Volume 1 (NRC, 1992); NUREG-1757, Volume 2 (NRC, 2006); 

17 NUREG/CR-5453, “Background Information for the Development of a Low-Level Waste 

18 Performance Assessment Methodology,” Volumes 1 and 2 (NRC, 1989b; NRC, 1989c); 

19 and the International list of Features, Events, and Processes (SSI, 1996).

20 (2) Categorize the general sources of radioactivity at the site (e.g., mixed in sediment or 

21 soil, groundwater).

22 (3) Screen out pathways for each source of radioactivity that do not apply to the site.

23 (4) Identify the physical processes pertinent to the remaining pathways for the site.

24 (5) Separate the list of exposure pathways into unique pairs of exposure media (e.g., source 

25 to groundwater). Determine the physical processes that are relevant for each exposure 

26 media pair and combine the processes with the pathway links.

27 (6) Reassemble exposure pathways for each source type, using the exposure media pairs 

28 as building blocks, thus associating all of the physical processes identified for the 

29 individual pairs with the complete pathway.

30

31 A licensee’s documentation of the decisions made about inclusion or exclusion of the various 

32 pathways should be transparent and traceable. If any of the pathways studied are found to 

33 contribute less than 10 percent of the total dose limit in 10 CFR 61.42, that pathway does not 

34 need to be evaluated in detail. However, the sum of the doses from all the pathways that are 

35 excluded from more detailed evaluation should be accounted for in the demonstration that the 

36 performance objective is met. If there are alternative, equally reasonable receptor scenarios for 

37 a particular site, then the significance of the exposure pathways needs to be screened and 

38 analyzed for each scenario. This approach is needed because pathways determined to be 

39 insignificant based on one scenario may not be insignificant for other equally credible scenarios.

40 4.4 Institutional Controls

41

42 As required in 10 CFR 61.23(g), a licensee must provide reasonable assurance that institutional 

43 controls will be provided for enough time to protect the general population from releases per 

44 10 CFR 61.41, to protect individual inadvertent intruders per 10 CFR 61.42, to assure that 

45 radiation standards in Part 20 will be met per 10 CFR 61.43, and to provide long-term stability of 
4-37

1 the disposed waste and disposal site per 10 CFR 61.44. In the context of inadvertent intrusion, 

2 institutional controls are used to limit intruder access to, and/or use of the site for a period of 

3 time following transfer of the disposal site to the owner and cannot be relied on for more than 

4 100 years under 10 CFR 61.59. An institutional control program includes legal mechanisms 

5 (e.g., land use restrictions), environmental monitoring, periodic surveillance, minor custodial 

6 care, or other requirements as determined by the Commission, as well as the administration of 

7 funds to cover the costs for these activities. For the purposes of the intruder assessment, the 

8 institutional control period is separated into an “active” and “passive” period. During the active 

9 period, which cannot be relied on for more than 100 years, monitoring, surveillance, and 

10 custodial activities may be assumed to be carried out by the site owner. During this period, 

11 licensees may assume that the institutional controls are durable and will preclude inadvertent 

12 intrusion from occurring at the disposal site. The passive period follows the active period, and 

13 during this period it should be assumed that relatively few custodial activities are carried out. 

14 Inadvertent intrusion is expected to be unlikely due to the passive-period legal mechanisms, but 

15 is possible after 100 years, consistent with 10 CFR 61.59.

16

17

18

5-1

1 5.0 SITE STABILITYANALYSES

2

3 The regulations at 10 CFR 61.50 require that LLW disposal sites should not be susceptible to 

4 erosion, flooding, seismicity, or other disruptive events or processes to such a degree or 

5 frequency that compliance with the 10 CFR Part 61 performance objectives cannot be 

6 demonstrated with reasonable assurance. The regulations at 10 CFR 61.44 also include a

7 performance objective for disposal site stability after closure. It states that the disposal facility 

8 must be sited, designed, used, operated, and closed to achieve long-term stability of the 

9 disposal site for the compliance and protective assurance periods and to eliminate, to the extent 

10 practicable, the need for ongoing active maintenance of the disposal site following closure. This 

11 section focuses on the types of information expected to be associated with the stability 

12 demonstration required by the regulations, including consideration of the potential effects of 

13 erosion, flooding, seismicity, and other disruptive processes. This section also addresses 

14 stability of the wasteform as well as other engineered features that might be used at a particular 

15 LLW disposal facility. 

16

17 Stability is the capability of the wasteform, disposal containers, disposal site, and disposal 

18 facility to maintain their shape and properties to an extent that the disposal action will meet the 

19 10 CFR 61.41 and 10 CFR 61.42 performance objectives. Stability is defined in the regulation 

20 as “structural stability”. However, the impact of instability is not solely the change in shape and 

21 properties of the system. The change in shape and properties of the system may affect other 

22 FEPs associated with safety. Stability is important to:

23

24 • minimize changes to the disposal system that may result in increased releases of 

25 radioactivity to the environment as a result of increased infiltration (10 CFR 61.7)

26 • limit erosion and similar processes (10 CFR 61.50)

27 • ensure waste is recognizable by an inadvertent intruder (10 CFR 61.7)

28 • protect an inadvertent intruder by maintaining an appropriate overburden over the waste 

29 (10 CFR 61.7)

30

31 The NRC strategy for waste disposal is to “concentrate and contain” the LLW for as long as it 

32 remains hazardous. A key feature of that regulatory strategy is that components of the LLW 

33 disposal system must maintain stability. Stability ensures that once waste is emplaced and 

34 covered, access to the waste by water, biota, or humans is minimized, reducing the potential 

35 exposure to the public. 

36

37 Many of the early problems associated with LLW disposal arose from site stability issues

38 primarily associated with water (NRC, 2007b). To address these problems, a number of 

39 practical regulatory requirements, such as waste segregation requirements, were introduced to 

40 the regulations to obviate or limit future problems associated with waste stability (NRC, 1982a). 

41 Disposal of long-lived waste can increase the disposal challenge associated with ensuring 

42 waste stability, relative to short-lived LLW. For long-lived waste, it may be difficult to 

43 demonstrate the performance of the engineered features used to ensure waste stability, 

44 because the service life of those features is relatively short compared to the time those wastes 

45 remain hazardous. Site stability, in addition to the physical and chemical stability of the waste 
5-2

1 itself, can affect the performance of the waste disposal facility. Instability can be initiated by 

2 internal or external phenomena. NUREG-1200 provides details about site stability focused on 

3 waste stability, erosion protection, and geotechnical issues (slope stability, settlement, 

4 subsidence) (NRC, 1994). The guidance in this document is not a substitute for the information 

5 found in NUREG-1200; it can be used to supplement NUREG-1200, in particular for the site 

6 stability analyses associated with long-lived LLW. Within this guidance document, the term “site 

7 stability” is used to refer to the overall stability of the LLW disposal system, which includes 

8 stability of the waste, disposal site, and disposal facility and surrounding environment, as 

9 applicable.

10

11 The discussion in this section is divided into the following topics: disruptive processes (i.e., 

12 hazards), technical assessment, and engineered barriers. After a licensee identifies and 

13 characterizes potential disruptive processes, they can use different technical assessment 

14 methods to understand the impact of the disruptive processes on the ability of a disposal site to 

15 meet the performance objectives. Licensees can use technical assessment to evaluate if the 

16 hazards can be reduced or even mitigated. The technical assessment for site stability does not 

17 need to be complex (see Section 5.2.2). Screening analyses may be appropriate. In addition, it 

18 may be possible to mitigate the impact of some hazards using engineered barriers. Long-term 

19 evaluation and monitoring of the site can be used to confirm if site stability can be achieved. If 

20 the monitoring calls into question whether site stability can be achieved, licensees can propose 

21 a mitigating engineering action to enhance stability. The technical assessment may also 

22 determine that the candidate site is not suitable for the disposal of certain concentrations and 

23 quantities of LLW. Consequently, it may be necessary to impose limits (see Section 7.0) on the 

24 concentrations and quantities of LLW suitable for disposal at the proposed site. Similarly, the 

25 material itself may not be suitable for near-surface disposal under the 10 CFR Part 61 

26 regulations. An overview of the main elements of site stability analyses is provided in 

27 Figure 5-1. 

28

29 Stability and uncertainty are interrelated topics. Site stability analyses should be developed and 

30 evaluated in a risk-informed manner. Section 61.44 requires that site stability must be 

31 demonstrated for the compliance and protective assurance periods. As timeframes increase, 

32 the uncertainty associated with site stability increases. In addition, the scope and diversity of 

33 phenomena that may impact the stability of the disposal site increases. As previously noted, 

34 stability is to be applied in the context of meeting the performance objectives for 10 CFR 61.41 

35 and 10 CFR 61.42. However, site stability is an independent performance objective (10 CFR 

36 61.44). Uncertainty associated with high instability may limit the ability of a licensee to 

37 demonstrate that 10 CFR 61.41 and 10 CFR 61.42 can be met. Inventory limits may be used to 

38 mitigate the uncertainties associated with potential instability, or to prohibit disposal of waste 

39 that remains hazardous when the instability is projected to occur.

40

41 Section 61.50 of 10 CFR Part 61 provides the disposal site suitability requirements used in the 

42 site selection and evaluation process. Licensees can use site selection to eliminate or greatly 

43 reduce the number of disruptive processes and events that may need to be considered in the 

44 site stability analysis. For example, alluvial fans that form at the base of mountain ranges tend 

45 to be particularly unstable landforms over the long-term (NRC, 1983b). 

46
5-3

1

2

3 Figure 5-1 Main Elements of Site Stability Analysis

4 The impact of some events may be impossible to design against, given current technology and 

5 understanding (e.g., volcanism), and therefore, should be avoided. Other processes and events 

6 may be unavoidable (e.g., erosion, large precipitation events) and may be mitigated through site 

7 selection and engineered design.

8

Most, if not all, of the requirements listed in 10 CFR 61.50 were developed to ensure site1 9

10 stability, as well as to allow licensees and reviewers to evaluate the performance of the site with 

11 reasonable assurance (NRC, 1982a). Site suitability requirements are separated into 

12 hydrological characteristics (10 CFR 61.50(a)(2) and 10 CFR 61.50(a)(3)) and other 

13 characteristics (10 CFR 61.50(a)(4)). 10 CFR 61.50(a)(2) specifies hydrological characteristics 

14 that a disposal site must have for a 500-year timeframe following closure of the disposal facility 

15 (e.g., not be located in a 100-year flood plain). 10 CFR 61.50(a)(3) specifies that after the 

16 500-year timeframe following closure of the disposal facility, if any of the negative disposal site 

17 hydrological characteristics listed in 10 CFR 61.50(a)(2) are present (e.g., groundwater 

 1 The terminology used in 10 CFR Part 61 is that waste is emplaced in a disposal unit. The disposal 

units, area between units, plus the surrounding buffer zone is the disposal site. The disposal site plus the 

other buildings, structures, equipment, and land used to complete disposal operations is the land disposal 

facility or disposal facility. In this guidance document, the term disposal system is used to refer to the 

disposal facility and surrounding environment
5-4

1 discharge to the surface), that they shall not significantly affect the ability of the disposal facility 

2 to meet the Subpart C performance objectives of the regulations.

3

4 The other site suitability requirements in 10 CFR 61.50(a)(4) specify characteristics of the site 

5 that should not be present such that they significantly affect the ability of the disposal site to 

6 meet the performance objectives of Subpart C (e.g., population growth, tectonic processes). 

7 The requirements in 10 CFR 61.50(a)(4) can exclude a potential disposal site location only if the 

8 FEP in question (e.g., tectonic processes, see Section 2.0) may prevent the performance 

9 objectives from being met, or if defensible modeling is precluded. For FEPs that result in severe 

10 disruption, defensible modeling may be precluded, and licensees, in concert with their 

11 regulators, may need to establish conservative inventory limits to mitigate the associated 

12 uncertainties. Practical solutions, other than numerical modeling, to limiting the impact of 

13 uncertainties associated with the disposal of long-lived waste may be necessary. Requirements 

14 associated with specific FEPs associated with site stability are provided in 10 CFR 61.50, 

15 including, but not limited to, the following:

16

17 • Areas must be avoided where tectonic processes such as faulting, folding, seismic 

18 activity, or volcanism may occur with such frequency and extent to significantly affect the 

19 ability of the disposal site to meet the performance objectives of Subpart C of this part, 

20 or may preclude defensible modeling and prediction of long-term impacts. 

21 [10 CFR 61.50(a)(4)(iii)]

22

23 • Areas must be avoided where surface geologic processes such as mass wasting, 

24 erosion, slumping, land sliding, or weathering occur with such frequency and extent to 

25 significantly affect the ability of the disposal site to meet the performance objectives of 

26 Subpart C of this part, or may preclude defensible modeling and prediction of long-term 

27 impacts. [10 CFR 61.50(a)(4)(iv)]

28

29 Site stability analyses should be tailored to the types of waste disposed. A facility designed for 

30 what might be regarded as traditional LLW streams (i.e., short-lived waste and low 

31 concentrations or quantities of long-lived waste) will have comparatively less complex site 

32 stability analyses than a facility designed for large quantities of concentrated, long-lived waste. 

33 For example, higher quality rock and more detailed testing and assessment of the durability of

34 rock used for erosion protection will be needed for long-lived waste (compared to short-lived 

35 waste). Site stability analyses have three areas of focus: stability of the wasteform, stability of 

36 the engineered disposal facility, and geologic/geomorphic stability of the disposal site. For 

37 disposal of traditional LLW, site stability analyses will likely focus on the former two areas. For 

38 disposal of large quantities of long-lived waste, the focus will likely be on the latter two areas. 

39 The areal extent of the site stability analyses will be strongly influenced by the type of waste to 

40 be disposed. Stability of wasteforms, disposal units, engineered barriers (such as cover 

41 systems), disposal site, disposal facility, and disposal system may all be within the scope of the 

42 stability assessment. 

43

44 The scope of the analyses needed for the site stability evaluation will be defined primarily by the 

45 type of waste and secondarily by the disposal system. The type of waste (i.e., short-lived or 

46 long-lived) will determine the timeframe appropriate for the site stability assessment, and 

47 therefore, what types of FEPs a licensee should evaluate. Waste disposal systems are 

48 generally reliant on passive features of the site geology/hydrology to limit long-term releases of 

49 radioactivity. To limit short-term releases, waste disposal systems may be strongly reliant on 
5-5

1 the performance of engineered systems. Passive systems, in general, will be more resistant to 

2 failure from unlikely natural events than active systems. Figure 5-2 provides an integrated view 

3 of the NRC staff’s perspective on the temporal and spatial scales as well as the relative

4 influence of various processes on site stability for LLW disposal. 

5

6 The NRC staff developed Figure 5-2 based on the experience with assessments of LLW

7 disposal facilities and complex decommissioning sites. Waste disposal facilities have not 

8 experienced frequent unlikely natural events (which are defined relative to the operating 

9 experience base) because the experience base is limited to the last half century. There is not 

10 enough data to quantitatively define the relative importance of the relevant processes. 

11

12 Figure 5-2 is a generalization; however, the relative importance of a particular process at a 

13 specific site may differ from the conceptualization shown in the figure primarily as a result of 

14 location of the site. For instance, based on historical data, glaciation at a southern state would 

15 be expected to be of lower relative importance compared to glaciation at a northern state. 

16 Individual processes may also be dynamic such that the relative importance is not constant over 

17 time. The relative importance of climate could change based on the magnitude of effects of 

18 anthropogenic processes on climate change. With these types of caveats, the general 

19 expectation of the relative influences of various processes on site stability is:

20 1) In the near term (left hand side of the figure), erosion is expected to be the dominant 

21 process impacting stability. Secondary processes are biologic and climate. Igneous and 

22 tectonic phenomena are expected to be insignificant for a properly selected site. 

23 Glaciation is not anticipated in the near term.

24 2) In the medium term (middle of the figure), erosion is still of primary importance, but 

25 changes to climate which can impact erosion or result in glaciation at some sites 

26 become more significant.

27 3) On the long term (right hand side of the figure), most processes are expected to be

28 important to evaluate in the stability assessment. As time increases, the likelihood of a 

29 large, low frequency seismic or volcanic event occurring increases.

30 4) The overall significance of various processes on site stability increases as time 

31 progresses. Over long timeframes, there is a higher overall likelihood that site stability 

32 will be affected by one or more of the processes.

33

34 A variety of resources are available to facilitate the development or review of site stability 

35 analyses. This section attempts to consolidate and summarize some of these resources but 

36 does not attempt to replicate them. The reader of this document should consult the original 

37 documents for more detailed information, as needed.
5-6

Figure 5-2 Spatial and Temporal Scales and NRC Staff Perspective on the Relative Importance of Var

Geomorphological Processes Relevant to LLW Disposal. 

Note: The time on the figure is comparable to the persistence of the hazard of the waste
5-7

1 5.1 Disruptive Processes and Events

2

3 A number of disruptive natural and anthropogenic processes and events could impact the 

4 long-term stability of a waste disposal system. Section 2.5 discusses the FEP screening 

5 process that licensees could use to identify disruptive processes and events that need to be 

6 evaluated in the site stability assessment. This section describes the information that a licensee 

7 should provide and a reviewer should evaluate with respect to identifying potential disruptive 

8 processes and events. The NRC staff expects the disruptive processes important to the 

9 estimation of the stability of an LLW disposal site to be site-specific. 

10

11 The following primary elements are associated with the evaluation of disruptive phenomena:

12

13 • Which disruptive phenomena need to be evaluated?

14 - What screening process will be used?

15 - What criteria will be used for screening?

16

17 • How may the phenomena influence each other (i.e., are they correlated, dependent, 

18 independent)?

19

20 • Can the frequency and consequences of the events or processes impact the 

21 demonstration of compliance with the performance objectives of 10 CFR Part 61, 

22 Subpart C?

23

24 Licensees should describe in the site stability analysis the screening process that will be used to 

25 evaluate which disruptive phenomena to consider and the criteria used for the screening. Some 

26 natural phenomena will be expected to occur while others will be unlikely or very unlikely. The 

27 likelihood that a phenomenon occurs will be related to the timeframe of the evaluation; as 

28 explained in Section 2.3.2, the timeframe evaluated in the analysis is dictated by the type of 

29 waste that is disposed. Longer timeframes will result in higher likelihood that low-frequency 

30 events may occur over the timeframe considered. 

31

32 The NRC staff recommends that licensees evaluate reasonably foreseeable disruptive 

33 conditions from natural events or processes in the site stability analysis. Reasonably 

34 foreseeable disruptive conditions include both those expected to occur and events that are 

35 moderately unlikely to occur (approximately 10 percent or greater chance of occurring) over the 

36 analyses period. The reasonably foreseeable disruptive conditions should be consistent with 

37 the characteristics of the waste. In other words, if the radiological inventory of the disposal site 

38 is primarily short-lived with low concentrations of long-lived radionuclides, it would not be 

39 necessary to look at reasonably foreseeable disruptive events based on the 10,000-year 

40 protective assurance period. A licensee could justify a shorter period, consistent with the 

41 characteristics of the radiological inventory. However, lower frequency events will need to be 

42 considered for higher concentrations of long-lived inventory. NUREG/CR-3964 (NRC, 1989d) 

43 provides examples of techniques that licensees can use to investigate and then determine the 

44 likelihood of certain natural events or processes that might be disruptive to the performance of 

45 an LLW disposal facility over the analyses timeframes. 

46

47 The LLW intruder analysis required by 10 CFR Part 61 (see Section 4.0) often mitigates the 

48 need to consider low frequency disruptive events by bounding their projected dose impacts. 
5-8

1 Intruder analyses typically bound the dose from a disruptive event because less dilution and 

2 dispersion is expected in a LLW intruder analysis, and the resultant contact with disturbed 

3 material is more direct than what would be expected for low-frequency natural events, which are 

4 typically highly energetic (e.g., a volcanic event). Thus, in many cases, the dose to the intruder 

5 may bound the impact from unlikely disruptive events. However, if a licensee does not estimate 

6 the impact to intruders from the generic receptor scenarios (i.e., they use 

7 site-specific scenarios), then they will need to evaluate the impact from unlikely disruptive 

8 events, which may drive overall performance.

9

10 Licensees should use a screening process to determine which processes and events need to be 

11 considered in the stability analysis. Processes and events can be screened out of the 

12 assessment based on probability, consequence, risk-based criteria (i.e., the combination of 

13 probability and consequence), or regulation (see Section 2.5.3.1.2). Screening based on 

14 probability typically establishes a cutoff frequency below which disruptive events are not 

15 considered further. This cutoff frequency is expected to be higher for LLW analyses compared 

16 to HLW analyses because the consequences of disruptive events are likely to be less severe for 

17 less-concentrated waste than for HLW. Screening of disruptive processes and events based on 

18 consequence or risk should consider the integrated effect of the process or event. For example, 

19 the growth of trees and eventual tree fall on a cover may not significantly impact the stability of 

20 the cover from the event itself, but it may create a depression to initiate future gullying. In many 

21 cases, there is limited experience, and therefore, limited understanding of potential combined 

22 phenomena. Although the uncertainty associated with combined phenomena cannot be 

23 reduced, the use of multiple independent peer reviewers with diverse backgrounds can reduce 

24 the likelihood of omitting important combined effects. 

25 5.1.1 Natural Processes

26

27 The near-surface environment is continually evolving, influenced by processes as well as by 

28 discrete events. The processes and events may interact, compounding or reducing the effects 

29 observed from the individual phenomena. The systems are dynamic and may include positive 

30 or negative feedback effects. The phenomena most likely to impact the stability of a disposal 

31 facility, and therefore, the demonstration of compliance with the performance objectives of 

32 10 CFR Part 61, Subpart C, will be specific to the particular site and design. This section covers 

33 different types of natural processes that can impact disposal facility stability. Geomorphic (e.g.,

34 mass wasting, erosion, slumping, land sliding, or weathering), tectonic (e.g., faulting, folding, 

35 seismic activity, or volcanism), biologic (e.g., animal or plant intrusion), or other processes and 

36 events (e.g., flood, fire, or extreme weather) may impact the future stability of waste disposal 

37 facilities. Processes and events may impact stability either directly (e.g., a large flood) or 

38 indirectly (e.g., a fire that disturbs vegetation, leading to increased erosion). In general, 

39 geomorphic processes will be the most likely stressors of long-term performance for most 

40 facilities.

41

42 Geomorphology is multidisciplinary, combining aspects of hydrology, climatology, ecology, and 

43 geology. Landform evolution is the sequence of processes and events that shape a given 

44 landscape. Landforms are built through tectonic, volcanic, and sedimentation processes and 

45 reduced through such processes as erosion and mass wasting. Evolution of landforms is 

46 associated with the balance between additive and subtractive processes. For long-term 

47 assessments, site stability analysis of geomorphological processes will likely entail integration of 

48 the effects of many of the processes described in the following sections.
5-9

1

2 Site characterization and selection will play an integral role in assessing natural processes that 

3 may impact the stability of the disposal facility. For new sites, applicants should characterize 

4 the site to define the geomorphic, tectonic, and other hazards that may significantly impact site 

5 stability. For existing sites that want to accept new waste, additional site characterization may 

6 be needed if the waste is materially different (e.g., more long lived) from previously accepted 

7 waste.

8 5.1.1.1 Geomorphic Processes

9

10 Geomorphic processes, such as mass wasting, slumping, land sliding, erosion, sinkhole 

11 formation, and weathering, may impact the stability of an LLW disposal facility. A description of 

12 the geomorphology of the site, including USGS topographic maps that emphasize pertinent 

13 local geomorphic features, may be useful to identify processes, such as erosion, that may affect 

14 long-term site stability. Erosion and weathering, in particular, are especially applicable to LLW

15 disposal. Fluvial (at more humid sites) and eolian (at more arid sites) erosion of engineered 

16 covers is a common disturbance mechanism for disposal of long-lived waste. To mitigate 

17 certain geologic processes, licensees may achieve long-term erosion protection with robust 

18 engineered designs using a system of erosion controls and durable, appropriately sized rock

19 (NRC, 2002b). Long-term erosion protection is protection for thousands of years to possibly a 

20 few tens of thousands of years. To achieve such long-term protection, the erosion controls 

21 should be independent and redundant, and the rock should be resistant to weathering. 

22 Avoidance of steep slopes and the use of low relief designs can reduce the impact of some 

23 geologic processes (NRC, 2002b). As discussed in Section 5.3, licensees may consider

24 engineered barriers to mitigate the impact of many geologic processes. 

25

26 The NRC staff expects erosive processes (fluvial and eolian) to be the most likely of all of the 

27 disruptive processes to impact the long-term stability of most disposal facilities. Therefore, the 

28 NRC staff recommends licensees develop robust erosion control designs using durable 

29 materials, as discussed in Section 5.3. Robust erosion control designs are usually developed 

30 based on the consideration of low-probability events, such as the PMP and corresponding PMF 

31 (NRC, 2002b). The PMF is defined as the hypothetical flood that is considered to be the most 

32 severe flood reasonably possible, based on (1) comprehensive hydrometeorological application 

33 of the PMP, and (2) other hydrologic factors favorable for maximum flood runoff, such as 

34 sequential storms and snowmelt. The return period of the PMP has been debated in technical 

35 literature because there are few direct observational data. A comparison of PMF peaks with 

36 historic floods in the United States yielded nine floods that exceeded 80 percent of the PMF but 

37 none that exceeded the estimated PMF (Bullard, 1986). Similar data compiled for 

38 approximately 20,000 gauging-station-years did not yield a flood that exceeded the calculated 

39 PMF (Crippen and Bue, 1977). For specific streams, paleo-flood data may be available to 

40 provide confidence that the computed PMF estimate is appropriately conservative. The return 

41 period associated with the PMP has been estimated from one statistical analysis of historical 

42 data at approximately 60,000 years (Koutsoyiannis, 1999). Koutsoyiannis cited the work of 

others that estimated the return period of the PMP to be from 200,000 to 1 x 109 43 years, using 

44 different approaches. The NRC staff considers the PMP to be a very unlikely event with respect 

45 to LLW disposal and that the PMP is appropriate to use to produce a conservative design.

46

47 The general approach used to assess geomorphic processes will involve three general steps: 

48 (1) identification of past geomorphic processes and estimation of their rates from stratigraphic 
5-10

1 and geomorphic records; (2) identification of present geomorphic processes and estimation of 

2 their rates from historic records and field observations (roughly the last 100 years); and (3) 

3 prediction of future processes and rates, accounting for uncertainty. Schumm and 

4 Chorley (NRC, 1983c) identified over 25 geomorphic processes that can create hazards at 

5 waste disposal sites. Typically, only a few of the hazards may be of concern at a particular site. 

6 A geomorphic hazard assessment will typically include the site and the surrounding region. An 

7 assessment of the surrounding region may provide information on processes that are occurring 

8 beyond the boundaries of the site but that may eventually extend onto the site. For example, 

9 the rate of migration of a river channel near a disposal site may give an indication that the river 

10 channel may eventually migrate onto the disposal site. In addition, the presence of a dam 

11 upstream from a facility may influence the frequency or likelihood of flooding, or result in 

12 flooding if the dam were to fail. Therefore, licensees should use information from the 

13 surrounding region to help estimate rates and magnitudes of processes that are not currently 

14 occurring at the disposal site.

15

16 It should be noted that a site does not necessarily need to experience mass loss; a site may be 

17 located in an accreting environment. Accreting environments are more favorable to long-term 

18 stability as nature enhances the isolation of the waste from the environment by covering the 

19 disposal facility over time. However, certain depositional processes may degrade the 

20 performance of some types of erosion controls (such as diversion channels). Licensees may

21 use diversion channels at a site in an accreting environment to limit fluvial erosion and ensure 

22 site stability. Even in a net depositional environment, the site may experience periods of both 

23 deposition and erosion.

24 5.1.1.2 Tectonic Processes

25

26 Tectonic processes that can lead to long-term processes or events such as faulting, seismicity, 

27 or volcanism, may impact the stability of an LLW disposal facility. Nevertheless, a variety of 

28 factors should make tectonic processes generally less significant than the aforementioned 

29 geomorphic processes. First, licensees should use the site selection and characterization 

30 process to avoid areas of high tectonic activity. Second, LLW facilities are generally passive 

31 buried systems that are more susceptible to gradual deterioration from degradation and 

32 weathering than to discrete failure due to unlikely tectonic events. An exception would be 

33 facilities that rely significantly on engineered barriers to meet the performance objectives, 

34 especially resistive engineered barriers. A resistive engineered barrier is one that is reliant on 

35 physical properties to prevent liquid flow or transport, such as a geomembrane or cementitious 

36 barrier. A significant seismic event could result in cracking that would impact the ability of the 

37 engineered barrier to limit water flow. The durability of the engineered barriers would need to 

38 be assessed, as discussed in Section 5.3.

39

40 Licensees should assess the potential for subsurface geologic processes, such as faulting or 

41 seismic activity, to affect the site and proposed disposal facility. They should review historically 

42 recorded seismic information including recurrence intervals, magnitudes, and durations, as well 

43 as factors that contribute to peak ground acceleration, such as underlying geologic structures

44 (e.g., active faults) and the stratigraphy and lithologies of the site. Licensees should evaluate 

45 the predicted effects of seismic events on waste isolation. Reviewers should evaluate any 

46 aspects of the disposal system designed to mitigate the potential effects of seismic events on 

47 waste isolation. Additional guidance on reviewing information related to seismic events in waste 

48 disposal is provided in NUREG-1804 (NRC, 2003c).
5-11

1 5.1.1.3 Other Disruptive Processes

2

3 Other natural processes and events can significantly impact the stability of an LLW disposal 

4 facility. These may include, but are not limited to, climactic processes and climate change, 

5 biological processes, and fires.

6

7 Climate and climate change have the potential to impact the stability of the disposal site. The 

8 impact of natural cycling of the climate could range from minimal to severe, depending on the 

9 location of the disposal facility. The type of waste that is disposed can influence how disruption 

10 from climate and climate change should be included in the assessment. Climate change can 

11 result in changes to the magnitude and duration of precipitation events, modification of the 

12 number of freeze-thaw cycles, or other effects.

13

14 For conventional LLW inventory, licensees should limit unnecessary speculation about severe 

15 climate change, such as glaciation. This type of event is envisioned as broadly disrupting the 

16 disposal site region. For conventional LLW, the hazard from the inventory remaining after 

17 500 years is expected to be relatively low. The dispersion and dilution associated with the 

18 glaciation would further reduce doses. For LLW inventories that have low concentrations of 

19 radioactivity, the impact from waste that has been disturbed by severe climate change would 

20 likely be small compared to the impacts on humans from the event itself (i.e., the non21 radiological consequences should outweigh the radiological consequences). However, it is still 

22 useful to communicate the sensitivity of the traditional LLW disposal facility performance to less 

23 severe changes in climate, such as changes to annual average precipitation and temperatures.

24

25 For LLW inventories containing long-lived isotopes, the hazard from the inventory remaining 

26 after 500 years may be significant. Therefore, licensees may need to evaluate the impact of 

27 climate change on the disposal site taking into account reasonably conservative scenarios. 

28 Climate change may not be gradual and continuous, as evidenced by a wide variety of 

29 published information (Alley, 2000; Fricke, 1993; Jenkyns, 2003). The rate of change may be 

30 just as important to establishing the risk as the magnitude of change. As previously discussed 

31 in Section 2.0, the site suitability requirements do not allow for selection of sites where instability 

32 precludes defensible modeling or assessment of long-term impacts. 

33

34 Licensees should examine plausible scenarios for site evolution and characteristics in the site 

35 stability analysis. For example, the erosion rate under a future, wetter climate may be 

36 significantly larger than the present day. It should be noted that even small erosion rates can 

37 amount to substantial thicknesses in the long term (e.g., a 0.5 mm/yr rate will reduce a cover’s 

38 thickness by half a meter in 1,000 years). However, the NRC staff believes that the magnitude 

39 of the PMP and PMF is not likely to change significantly in a wetter climate, based on the 

40 conservatisms associated with the estimation of the PMP and the PMF.

41

42 Biological processes can impact the stability of the waste and the stability of the disposal 

43 system. Biodegradation can affect the stability of waste containers and result in deterioration of 

44 the wasteform, which can result in instability of the disposal system. The BTP on wasteform 

45 describes test procedures and criteria to evaluate wasteform stability (NRC, 1991b). 

46 Biointrusion into the waste, or into engineered covers over the waste, can increase infiltration to 

47 the waste and contribute to the instability of the disposal system. Plants and animals may 

48 create pathways in resistive barriers. The disruptive effects of biological processes tend to be 

49 gradual but can be significant in some systems. Evapotranspiration or water balance covers 
5-12

1 can depend strongly on vegetation to eliminate water. Therefore, licensees should consider the 

2 impact of invasive species that have different root depths or growing patterns and water usage 

3 on site stability and on the performance assessment. Climate change may affect pedogenesis 

4 (i.e., soil development processes) and the type of cover vegetation. Fires, severe storms, and 

5 ecological succession due to changing temperature and precipitation could influence the ability 

6 of the vegetation to resist wind and water erosion and maintain a sufficient depth of cover over 

7 the waste.

8

9 LLW facilities are generally not highly sensitive to the impact of fires, such as brush fires. 

10 However, as mentioned above, vegetation can play an important role in reducing soil loss and 

11 maintaining site stability, in particular for slopes and covers. Under extreme conditions, fires 

12 may change the physical properties of the site soils and other cover materials. Strong fires 

13 have been shown to impact durability of select rock types (e.g., silica based minerals deteriorate 

14 after exposure to high temperatures) (Dorn, 2003). However, researchers at DOE’s Hanford 

15 Site directly evaluated the impact of fire on an engineered cover system and found the impact to 

16 be minimal for their particular cover materials (Ward et al., 2009).

17 5.1.2 Anthropogenic Processes

18

19 Human activity may influence site stability both directly and indirectly. An example of a direct 

20 influence is construction of a dwelling on the disposal facility. An example of an indirect 

21 influence is acid rain generation, which could impact the durability of erosion control materials. 

22 The intruder assessment examines the direct impact to an inadvertent intruder who unknowingly 

23 disturbs the waste disposal facility. A licensee does not need to evaluate indirect impacts 

24 associated with human activity at the disposal site because they would be difficult to define and 

25 highly speculative. In addition, the direct impacts to the intruder should, in most cases, bound 

26 the impacts to the general public (offsite) from anthropogenic disruption of the disposal facility 

27 because there is comparatively less dispersion and dilution in the assessment of impacts to the 

28 intruder, so the direct impacts will be greater. However, if site-specific intruder scenarios are 

29 used that result in relatively limited exposures, then indirect processes may dominate the risk 

30 and should be examined. A licensee does not need to consider anthropogenic climate change, 

31 but should consider natural cycling of climates, if necessitated by the analysis timeframe for 

32 their waste disposal actions. Section 4.0 provides guidance for performing the intruder 

33 assessment. 

34

35 5.1.3 Subsidence and Differential Settlement

36

37 Subsidence and differential settlement are very important for all types of waste disposal 

38 because instability of the disposal site can result in unacceptable releases, regardless of the 

39 overall stability of the disposal facility. Differential settlement was a key failure mode in earlier 

40 LLW disposal sites, prior to the passage of 10 CFR Part 61 (NRC, 2007b). Subsidence and 

41 differential settlement can lead to unacceptable releases because the processes can adversely 

42 affect barrier performance (e.g., cracking of an engineered cover, creating a depression in the 

43 cover resulting in localized ponding). 

44

45

46
5-13

1 Subsidence and differential settlement can result from a variety of different processes and 

2 events such as:

3

4 • Excessive void space in the waste or backfill

5 • Lack of compaction or improper compaction of waste, backfill, engineered or natural 

6 materials

7 • Degradation of wasteforms, engineered barriers, and other structures

8 • Improper waste emplacement

9 • Alteration of natural materials (e.g., collapse of subsurface zones)

10 • Excessive or uneven loading of the disposal site or engineered cover

11 • Interaction of the disposal system with water

12

13 Some designs may be more susceptible to subsidence and differential settlement than others 

14 (e.g., designs that combine robust cementitious barriers with unconsolidated and uncompacted 

15 natural materials may be particularly sensitive). Subsidence and differential settlement are

16 covered in sufficient detail in NUREG-1199 an NUREG-1200 and additional guidance is not 

17 provided in this document (NRC, 1991a; NRC, 1994). These documents detail relevant site 

18 characteristics, construction and operations phase data, experiment and test data, modeling, 

19 and remedial actions.

20 5.2 Technical Assessment

21

22 Some form of technical assessment will be used to complete the site stability analysis. The 

23 technical assessment should provide an assessment of the stability of the wasteform, the 

24 disposal facility including waste containers and other engineered barriers, and the site. The 

25 level of detail and type of assessment used will be dictated by the specific waste, the complexity 

26 of the site and facility, the hazard being mitigated, and the uniqueness of the problem. 

27 Probabilistic or deterministic analyses may be used by licensees as long as uncertainty and 

28 variability are assessed.

5.2.1 Available Tools and Codes2 29

30

31 A number of tools and codes are available to licensees to support site stability analysis. Recent 

32 advances have resulted in a vast array of technologies available to facilitate analysis of the 

33 near-surface environment. Licensees are encouraged to use the best available information for 

34 their assessments. The field of geomorphology has evolved quite significantly over the last half 

35 century and numerous technologies are available today to facilitate site stability analysis 

36 (Kondolf and Piégay, 2005). Although the preponderance of tools is focused on near-term and 

37 small-scale evaluations, a number of tools and techniques have been developed to look at the 

38 longer-term and large-scale. Tools such as optical dating, image analysis, pollen analysis, 

39 paleomagnetic dating, oxygen isotopes, lithologic analysis, vegetation surveys, tree-ring 

40 analysis, archives, and radioisotopic dating have been used to study the evolution of the 

41 near-surface environment (Anderson and Anderson, 2010).

42

 2 NRC does not endorse or recommend any specific code or model for site stability analysis. Licensees 

are free to select and justify their particular code or model for their application. Regardless of the code 

selected, it is important that the code meet all applicable quality assurance requirements.
5-14

1 Walter and Dubreuilh (2007) evaluated computational approaches used to simulate engineered 

2 cover performance and degradation. They evaluated 21 computer codes, which they 

3 categorized as hydrologic codes, erosion codes, and miscellaneous codes. The erosion codes 

4 were further categorized as generalized erosion codes, localized erosion codes, and mass 

5 wasting codes. The generalized erosion codes generate estimates of the average soil loss due 

6 to water or wind erosion from a plot of land. The localized erosion codes simulate soil loss at 

7 specific locations, and some can simulate landscape evolution that may be important to long8 term site stability. Table 5-1 provides a summary of the erosion and mass wasting computer 

9 codes, including their applicability and limitations for long-term site stability analysis (Walter and 

10 Dubreuilh, 2007). The review by Walter and Dubreuilh (2007) does not represent an exhaustive 

11 list of available computer codes; however, it may be useful to licensees to identify the types of 

12 tools that are available and their limitations.

13

14 The University of Colorado and partners have created the Community Surface Dynamics 

15 Modeling System (CSDMS) that provides (1) a modular modeling environment capable of 

16 significantly advancing fundamental earth-system science, and (2) fully functional and useful 

17 repositories for models, supporting data, and other products for educational and knowledge 

18 transfer use (Syvitski et al., 2011). The CSDMS repository contains more than 160 models and 

19 tools. The system includes 66 codes and 41 tools associated with terrestrial modeling. 

20 Landscape evolution (CHILD, SIBERIA, Caesar, Erode, GOLEM, MARSSIM, and WILSIM), 

21 eolian transport (Eolian Dune Model), and cryosphere (GC2D, ISGR, Ice Ages) codes are 

22 included.

23 5.2.2 Approaches for Assessment

24

25 Licensees may use a variety of technical assessment methods to perform site stability analyses. 

26 Because of the site-, facility-, and waste-specific nature of site stability analysis, the tools and 

27 assessment techniques used may differ considerably from site to site. 

28

29 The approaches may be design-based, model-based, or a combination of the two. Modeling 

30 may play a more significant role for analysis of longer timeframes, due to the limited data to 

31 support long-term performance of stability designs. It is important that licensees provide a 

32 technical basis for the site stability analysis, regardless of the approach used. The technical 

33 basis will likely be more extensive for analysis of longer timeframes than for shorter timeframes. 

34 Model support is essential for site stability analysis. Licensees should develop model support 

35 throughout the process, although the largest effort (comparable to model validation) is usually 

36 completed at the end of the process. Model validation in the traditional usage is not possible for 

37 analyses over long timeframes. Iteration may be necessary.

38

39
5-15

1 Table 5-1 Summary of Applicability and Limitations of Erosion and Mass Wasting 

2 Codes for Long-Term Performance Assessment Evaluated in Walter and 

3 Dubreuilh, 2007

Code Process Representation Calculation of Cover Loss and 

Topographic Change

RUSLE

Empirically-based using correlations from 

test plots, requires empirical data to 

simulate future cover conditions

Computes areally-averaged soil loss, 

independent calculations required to 

compute changes in cover thickness

EPIC

Empirically-based using correlations from 

test plots, requires empirical data to 

simulate future cover conditions, but 

more versatile than RUSLE by

calculating soil water balance

Computes areally-averaged soil loss, 

independent calculations required to 

compute changes in cover thickness

WEPP

Physics-based approach to erosion 

allows flexibility in representing future soil 

cover conditions based on fundamental 

properties

Computes areally-averaged soil loss, 

for multiple hill slopes, independent 

calculations required to compute 

changes in cover thickness

EUROSEM

Physics-based approach applied over 

two-dimensional topographic surface 

allows representation of complex 

topography including future changes in 

topography

Computes distributed soil loss within 

model domain with a fixed 

topography, independent calculations 

required to compute change in 

surface elevation and cover thickness

LISEM-Gullies

Empirically-based approach to soil loss 

and gully formation over two-dimensional 

topographic surface but limited to single 

storm events

Calculates gully formation and 

associated changes in topography 

during single storm events, multiple 

simulations required to evaluate longterm performance

SIBERIA/CHILD

Physics-based approach applied over 

two-dimensional topographic surface 

allows representation of complex 

topography including future changes in 

topography

Calculates changes in elevation and 

topography based on soil loss and 

deposition, including soil creep and 

slumping

WESS Empirically-based wind erosion code with 

limited process documentation

Computes areally-averaged soil loss, 

independent calculations required to 

compute changes in cover thickness

WEPS

Physics-based wind erosion code with 

ability to represent topographic and windbreak effects

Computes areally-distributed soil loss, 

independent calculations required to 

compute changes in cover thickness

LISA Simple, physics-based calculation of 

sliding potential with stochastic output

Computes sliding potential, but not 

topographic change

DLISA Simple, deterministic, physics-based 

calculation of sliding potential

Computes sliding potential, but not 

topographic change

4 The authors of the codes listed can be found in Walter and Dubreuilh (2007)

5

6

7

8
5-16

1

2 The site stability assessment should include the following general steps:

3

4 (1) Site description — Provide a description of the features of the site, facility, and waste 

5 associated with stability. At a minimum, licensees must provide information required by

6 10 CFR Part 61, Subpart D (technical requirements for land disposal facilities). As 

7 previously discussed, 10 CFR 61.50(a)(4)(iii) and 10 CFR 61.50(a)(4)(iv) provide specific 

8 requirements with respect to tectonic and geologic processes. The site description can 

9 include geologic and geomorphic characteristics that contribute to site stability. The site 

10 description may also include geologic, geomorphic, and tectonic hazard assessments.

11

12 (2) Overall initial radiological risk screening — It may be possible to demonstrate, with a 

13 conservative evaluation, that release of radioactive material, as a result of potential 

14 instability, will not result in sufficient radiological risk (i.e., that the 10 CFR 61.41 and 

15 10 CFR 61.42 performance objectives can still be met). If so, site stability analysis may 

16 be limited to providing the screening assessment. In other words, a licensee can satisfy 

17 10 CFR 61.44 if its screening assessment provides reasonable assurance that 10 CFR 

18 61.41 and 10 CFR 61.42 will be met under disturbed conditions. Licensees should 

19 select receptors and scenarios consistent with the guidance provided in Section 2.2.4 

20 and Section 4.0. The screening assessment should be sufficiently conservative to 

21 account for uncertainty.

22

23 (3) Process and event screening — Perform screening of disruptive processes and events 

24 (consistent with guidance provided in Section 2.5). Processes and events can be 

25 screened out of the assessment based on probability, consequence (as indicated in 

26 Step 2 above), or risk-based criteria. Risk-based criteria combine probability and 

27 consequence. Screening of disruptive processes and events based on consequence or 

28 risk should consider the integrated effects of the processes or events, as discussed in 

29 Section 5.1. 

30

31 (4) Define scope of the assessment — The FEPs that cannot be screened out from the site 

32 stability analysis should be used to define the scope of the assessment. Licensees can 

33 compile the processes and events that cannot be screened into scenarios. They can 

34 inform the scope of the assessment by considering the operating experience of 

35 analogous sites and facilities. If there is uncertainty about whether a process or event 

36 should be included, the NRC staff recommends erring on the side of inclusion. It is 

37 generally more difficult to add processes and events at a later date in the analysis 

38 process than it is to remove them from the assessment.

39

40 (5) Characterize information — Use the site description to determine what data are available 

41 to complete the assessment and to identify the significant sources of uncertainty. The 

42 type of information available is likely to depend on the timeframe being analyzed. 

43 Interpretation of the available information, with the characteristics of the waste, disposal 

44 facility, and site, will inform the approach that should be used for the assessment (e.g.,

45 model-based or design-based). The purpose of this step is to determine how much 

46 information is available to support the assessment completed in Step 6.

47

48 (6) Perform assessment — The assessment may be performed with a model-based 

49 approach (generally for long-lived wastes), a design-based approach (for short-lived or 
5-17

1 long-lived wastes), or a combination of the two. The NRC staff expects that licensees 

2 may use modeling to assess and develop designs. The steps provided below may be 

3 completed in a different sequence. However, regardless of the order, most assessments 

4 will include the following steps:

5

6 a. Design-Based:

7

8 i. Define the design objectives.

9 ii. Develop or select the design.

10 iii. Document and provide the basis for assumptions.

11 iv. Characterize or parameterize the design.

12 v. Assess the expected performance of the design.

13 vi. Provide support for the design.

14 vii. Iterate, if necessary.

15

16 b. Model-Based:

17

18 i. Define the model objectives.

19 ii. Develop or select the conceptual model.

20 iii. Document and provide the basis for assumptions.

21 iv. Develop the numerical model.

22 v. Parameterize the model.

23 vi. Calibrate the model.

24 vii. Verify the model.

25 viii. Characterize uncertainty.

26 ix. Provide model support.

27 x. Iterate, if necessary.

28

29 The elements listed above for design-based or model-based approaches are not unique 

30 to site stability assessment for LLW disposal. Many information sources are available to 

31 provide additional guidance, such as Section 2.0 of this document and NUREG-1573. In 

32 addition, licensees may mix elements from design-based and model-based approaches. 

33 For example, Appendix P of NUREG-1757 provides a description of how to risk-inform 

34 the design-based approach. Risk-informing the design-based approach involves 

35 performing technical analyses, such as sensitivity analyses, to test the robustness of the 

36 design to unanticipated events and processes.

37

38 (7) Integrate — The site stability assessment will likely need to be integrated with the 

39 performance assessment required by 10 CFR 61.41 and the intruder assessment 

40 required by 10 CFR 61.42. If stability can be assured, then the level of integration is 

41 minimal. However, if instability is projected, then licensees will need to evaluate the 

42 significance of that instability in the performance assessment and intruder assessment. 

43 For example, degradation of an engineered cover via erosion may lead to increased 

44 water infiltration to waste in a disposal cell. The releases estimated in the performance 

45 assessment should include nominal performance, as well as releases attributed to 

46 instability of the waste, the disposal site, and the disposal facility. When the stability 

47 assessment is based on both design and modeling, it is also important for a licensee to 

48 document that the design has been integrated into the stability modeling.

49
5-18

1 (8) Iterate (if necessary) — It is likely that some amount of iteration will be necessary in the 

2 site stability assessment. There are likely to be many sources of uncertainty, including 

3 some that were unanticipated initially in the design or modeling process. 

4 5.2.2.1 Design-Based Approach

5

6 A licensee may use a design-based approach to site stability analysis to demonstrate site 

7 stability for the associated timeframe. The design-based approach has been used successfully 

8 for the management of uranium mill tailings, though the experience base is only tens of 

9 decades. Section 5.3.2 presents information on analogs that suggests longer-term performance 

10 should be achieved. Appendix E provides an example of the use of the design-based approach 

11 to develop erosion protection measures for the management of uranium mill tailings. Section 

12 5.3 of this document provides guidance on the use of engineered barriers for site stability 

13 assessment. A licensee may use the guidance provided in Section 5.3, or alternatively, may 

14 develop their own approach as long as the technical content of the steps listed in Section 5.3 is 

15 included. Designs may use risk information, however, to mitigate long-term uncertainties, 

16 conservative, robust designs may be beneficial irrespective of projected risk information. The

17 design-based approach may be supplemented with sensitivity analyses in order to better 

18 characterize and understand uncertainties (see NUREG-1757, Volume 2, Revision 1, Sections 

19 3.5.2 and 3.5.3). The following steps outline the design-based approach.

20

21 Define the design objectives. The primary design objectives are the time period over which 

22 stability must be achieved and what instability, if any, is tolerable in order to ensure compliance 

23 with the 10 CFR Part 61 Subpart C performance objectives. Waste characteristics and the risk 

24 to the public, including an inadvertent intruder, will be the drivers of the design objectives. For 

25 example, a licensee that wishes to dispose of only short-lived waste and small quantities of 

26 long-lived waste may achieve protection of public health and safety by ensuring waste stability 

27 for 500 years. The licensee may be able to demonstrate that insufficient radioactivity remains in 

28 the disposal system to pose an unacceptable risk to a member of the public after that time.

29

30 Develop or select the design. After defining the design objectives, the licensee should develop 

31 the design or select a design that has previously been demonstrated to achieve their objectives. 

32 Because of the site-specific nature of LLW disposal, even previously utilized designs are likely 

33 to require modifications for site-specific application. Section 5.3 presents information on rock 

34 durability, for example, that is strongly influenced by local environmental conditions. Section 5.3 

35 provides an acceptable process for selecting erosion protection materials.

36

37 Document and provide the basis for assumptions. Assumptions are a common part of design 

38 and analysis. A licensee should document and provide a basis for assumptions that they have 

39 used in developing or selecting their design. Regulators should review the documentation and 

40 determine if adequate technical bases for the assumptions have been provided. Assumptions 

41 should be internally consistent.

42

43 Characterize or parameterize the design. After a licensee has developed or selected a design, 

44 they may need information in order to characterize the features of the design or provide 

45 parameters that will be used to represent the performance of the design. For example, a design 

46 may use natural and engineered materials. The licensee may need to consider the weathering 

47 or degradation rate of those materials. The licensee may need to perform and document 

48 experiments used to develop the design information, such as the thickness of a reactive wall. 
5-19

1 Sufficient documentation should be provided to allow for independent verification of the results. 

2 If a licensee uses information from generic sources to parameterize the design, they should 

3 demonstrate that the information is relevant to the site-specific application.

4

5 Assess the expected performance of the design. Verification of the design involves technical 

6 assessment of the performance of the design over the range of disruptive processes and events 

7 expected to influence the design. A licensee should clearly identify over what range of 

8 conditions the design is expected to perform and over what range of conditions the design may 

9 fail. This information is useful to regulators and other stakeholders if future conditions are to 

10 occur that may have been unforeseen during the design process. A licensee or regulator may 

11 use independent confirmatory analyses or modeling in order to verify the design.

12

13 Provide support for the design. A licensee must provide adequate support for the design prior 

14 to implementation of the design. Types of support for engineered designs are similar to the 

15 types of support provided for models, as discussed in Section 2.2.3 (e.g., tests, experiments, 

16 field studies, and analogs). In addition, support for designs may also include the experience 

17 base for the particular design, such as the number of locations where it has been used and for 

18 how long. Unique or novel designs, which may be necessary for particular applications, are not 

19 to be discouraged but they will have additional uncertainty as to whether they will achieve the 

20 design goals. A licensee should provide additional support for the performance of a novel 

21 design compared to what would be provided for a standard design that is expected to perform 

22 for a similar time period. The support for the design at the time of implementation will not have 

23 information from the actual performance of the design. Therefore, the overall design process 

24 may need to be iterative as monitoring and performance confirmation data are collected.

25

26 An example of a design-based approach to erosion protection is described in NUREG-1623, 

27 “Design of Erosion Protection for Long-Term Stabilization”. In addition, Appendix P of 

28 NUREG-1757 provides a description of how to risk-inform the design-based approach. 

29 NUREG-1623 was developed to provide methods, guidelines, and procedures that the NRC 

30 staff considers to be acceptable for designing erosion protection at uranium mill tailings sites 

31 (NRC, 2002b). These design approaches are based on technical procedures and design 

32 parameters that are widely used in the engineering community and by other Federal agencies 

33 and have been applied at various disposal sites. 

34 5.2.2.2 Model-Based Approach

35

36 A licensee may also use a model-based approach for performing the site stability analysis for 

37 the associated timeframe. The model-based approach is used to evaluate the FEPs that may 

38 affect the stability of the site and determine if the effects are significant. The model-based 

39 approach may determine that the impacts are:

40

41 • Insignificant - adequate site stability has been achieved irrespective of the FEPs

42 associated with site stability, or 

43 • Significant - site stability has not been achieved. 

44

45 A licensee may undertake a variety of actions if site stability is not demonstrated. They may 

46 refine their modeling and the refined model may demonstrate that the projected FEPs have 

47 insignificant impacts. As a result of their modeling, the licensee could implement design 
5-20

1 changes to achieve stability, or they may conclude that the site does not have adequate stability 

2 to be suitable for the type of waste proposed for disposal at the site. Inventory limits may be 

3 used to mitigate the risks and uncertainties associated with site instability

4

5 The steps involved in the model-based approach to site stability assessment provided in Section 

6 5.2.2 are similar to the steps to develop a performance assessment. Therefore, a licensee 

7 should review the guidance provided in Section 2.0 when using a model-based approach to site 

8 stability assessment. Appendix E provides an example of using a model-based approach to site 

9 stability assessment. As discussed in Section 2.2.3 and Section 5.3.2, model support is 

10 essential to any type of model-based approach. The NRC staff strongly recommends natural 

11 analogs and other forms of evidence of the long-term stability of the site. 

12

13 A number of tools and codes are available to licensees to support site stability modeling. The 

14 field of geomorphology has evolved considerably over the last half century and numerous 

15 technologies are available today to facilitate site stability analysis, including various programs 

16 such as hydrologic codes, erosion codes, and landscape evolution codes (e.g., CHILD and 

17 SIBERIA). Although the uncertainties associated with such landscape modeling are often large,

18 licensees can use modeling to gain insights and perform site stability assessments. 

19

20 Licensees should consider the following technical issues when developing the scope of a 

21 model-based approach to site stability assessment:

22

23 • Changes to nearby stream beds and river channels in unconsolidated material may 

24 impact landform evolution. 

25 • Given sufficient precipitation, large rain events, and time, the watershed of an area can 

26 change considerably. Specific parts of a facility may end up close to stream channels 

27 with an increased gradient, leading to accelerated erosion. 

28 • Drainage patterns may change. For example, if variations in climate circulation patterns 

29 are great enough, regional drainage patterns may change. 

30 • Erosion/deposition rates may vary spatially and temporally across the site. 

31 • Long-term erosion often concentrates in gullies, which do not uniformly erode over their 

32 entire length. Peak erosion depth may translate into a total breach of a portion of the 

33 disposal facility. 

34 • Evidence of the past natural history of the area, or from an appropriate natural analog, 

35 may be an indicator of future surface geomorphic processes. 

36 • Pedogenic processes, biotic activities, and bioturbation are all processes that may 

37 impede or accelerate surface geomorphic processes. Thick root systems from certain 

38 plants are known to greatly reduce erosion; however, it is difficult to rely on the 

39 continuous presence of a specific plant that might be needed to ensure stability over 

40 longer timespans.

41 • A number of factors could influence flora, such as drought, fire, disease, fungi, and 

42 insects. Observed erosion rates may be tied to the presence of flora.

43 • Changes in the water chemistry and other environmental conditions may cause changes 

44 to properties of the rock responsible for physical and chemical stability. 
5-21

1 5.2.3 Uncertainty

2

3 Uncertainty is inherent in the stability assessment of near-surface LLW disposal facilities and 

4 should be accounted for in the evaluation. Probabilistic assessment techniques are generally 

5 more amenable to accounting for uncertainty (see Section 2.7.4), although the NRC staff does 

6 not prescribe a particular technique. The site stability assessment should consider the general 

7 types of uncertainty (e.g., data, model) described in Section 2.0. A variety of techniques are 

8 available for handling uncertainty in multimedia environmental modeling (NRC, 2004c).

9

10 Site stability assessment may involve additional uncertainties that can be particularly 

11 challenging to characterize and understand. Some near-surface processes can display high 

12 sensitivity to initial conditions, such as the sensitivity of erosion rates to initial topography. 

13 There can also be feedbacks, both positive and negative, that can result in complex responses 

14 (Pelletier, 2008). The complex responses in turn translate into uncertainty in interpreting 

15 characterization and observational data. The complex responses can be difficult to understand;

16 therefore, they can be difficult to implement in numerical modeling, which results in model 

17 uncertainty. Environmental systems can also exhibit sensitivity to the initial conditions as well 

18 as the pathway taken to arrive at the initial conditions (i.e., hysteretic phenomena). As 

19 discussed in Section 2.0, one approach to mitigate the impact of uncertainty is to use 

20 conservatism. For example, using the PMP/PMF for the design-based approach to erosion 

21 protection can result in a robust design.

22

23 Incorporation of information at variable temporal and spatial scales is a challenge in site stability 

24 analysis. Some processes or events may span orders of magnitude in temporal and spatial 

25 scales. The key to understanding complex systems often lies in understanding how the 

26 processes on different scales influence each other. Licensees may need to coarsen or upscale 

27 detailed information at finer scales, in order to perform numerical modeling at coarser scales. If 

28 upscaling is used, it is important to perform physical or numerical experiments that demonstrate 

29 that essential information is preserved in the upscaling process. In addition, it is important that 

30 data are representative for the scale and conditions being simulated. For example, a single 

31 point measurement of soil moisture content may not be representative of (1) a sitewide value, 

32 (2) a more global value, or (3) of the distribution of local values needed in a site stability 

33 analysis.

34 5.3 Engineered Barriers for Site Stability

35

36 This section of the guidance document describes the information that a licensee should provide 

37 and a reviewer should evaluate with respect to the use of engineered barriers for site stability. 

38 Engineered barriers are likely to be used for erosion control or may be used for other reasons 

39 such as to mitigate the impact of disruptive processes and events. 

40

41 A surface cover is frequently utilized to provide physical stabilization of the site (10 CFR 61.44). 

42 The components of engineered barrier systems may include liners, covers or caps, and/or 

43 lateral barriers or walls. These systems may use a variety of natural material such as 

44 aggregates, soil, or clay, and synthetic, cementitious, and bituminous materials including 

45 polyethylenes, fabrics, mortar, and asphalt. The regulatory disposal requirements, and the type 

46 of engineered barrier system that a licensee chooses, depend on the waste type. Of all the 

47 components of a disposal system, the engineered surface barrier, or cover, is the most 

48 commonly used barrier and is often considered to be one of the most important components. 
5-22

1 Since engineered surface covers can be significant barriers, they may provide reasonable 

2 assurance that one or more performance objectives will be met. Engineered covers may greatly 

3 contribute to disposal facility performance by minimizing infiltration and slowing degradation of 

4 the stabilized wasteform (10 CFR 61.41) and by providing an intruder deterrent (10 CFR 61.42). 

5 5.3.1 Existing Guidance

6

7 The NRC staff believes that existing guidance developed for analogous programs (e.g., uranium 

8 mill tailings, decommissioning) is applicable to the design of engineered barriers for the stability 

9 of LLW disposal facilities. The processes and events that may disrupt LLW facilities are 

10 essentially identical to those that may impact a uranium mill tailings disposal facility or a 

11 decommissioned site. The main exception is that some LLW disposal facilities could contain 

12 higher concentrations and quantities of long-lived waste. Considerations for the design of 

13 engineered barriers for long-lived waste are provided in Section 5.3.2. Existing guidance is 

14 focused on cover design, particularly material durability. The focus of stability analysis in waste 

15 disposal has been on erosion protection. The NRC staff plans to periodically assess the 

16 sufficiency of guidance in this area and supplement it when necessary.

17

18 Engineered barriers are distinct and separate from institutional controls. Used in the general 

19 sense, an engineered barrier could be one of a broad range of barriers with varying degrees of 

20 durability, robustness, and isolation capability. Generally, engineered barriers are passive, 

21 man-made structures or devices intended to enhance a facility’s ability to meet the performance 

22 objectives. Engineered barriers are usually designed to inhibit water from contacting waste, 

23 limit releases of radionuclides (e.g., through groundwater, biointrusion, or erosion), or to mitigate 

24 doses to inadvertent intruders. Engineered barriers can serve a variety of functions; therefore, 

25 they have markedly different technical considerations and designs. The NRC staff expects that 

26 the main type of engineered barrier used for LLW disposal will be engineered covers. 

27 Engineered covers can be further classified into conventional (resistive) covers and water 

28 balance (evapotranspiration) covers. Each of these types of covers may have erosion control 

29 functions. The following sections further discuss these types of covers.

30

31 Section 3.5 of NUREG-1757, Volume 2, provides guidance on a risk-informed, graded approach 

32 to the design and evaluation of engineered barriers (NRC, 2006). High-level guidance on 

33 engineered barriers is provided in NUREG-1573 for general application to LLW disposal. 

34 NUREG-1573 contains a bibliography organized by different topics, including Appendix C on 

35 engineered and natural barriers. There are a broad range of engineered barriers for stability,

36 with varying degrees of durability and robustness. Engineered barriers for LLW disposal 

37 stability should be passive (i.e., they should perform without reliance on active monitoring and 

38 maintenance). Licensees should provide an appropriate technical basis for engineered barriers 

39 providing significant performance. The following steps are the main steps of an appropriate 

40 technical basis for engineered barriers, as described in NUREG-1757:

41

42 • Describe the design, features, and functionality of the engineered barriers.

43 • Provide the technical basis that the performance of the barriers will allow a licensee to 

44 demonstrate that the performance objective will be met, considering the degradation 

45 mechanisms, including consideration of combined and synergistic effects resulting from 

46 the real-world conditions expected for the barriers.
5-23

1 • Describe uncertainty in parameters and models used in the assessment of barrier 

2 performance and the design of engineered barriers.

3 • Demonstrate the suitability of numerical models for the estimation of engineered barrier 

4 performance.

5 • Perform parametric or component sensitivity analysis to identify how much degradation 

6 of the engineered barrier would result in noncompliance.

7 • Provide model support for the engineered barrier performance (e.g., analogs, 

8 experiments, engineering calculations to demonstrate reasonableness of the results).

9 • Provide QA/QC for the data collection, design, construction, and analysis of engineered 

10 barriers.

11

12 These steps apply to engineered barriers used to support demonstration of compliance with 

13 10 CFR 61.41 and 10 CFR 61.42, as well as for site stability. Model support for engineered 

14 barrier performance is essential. Model support, as discussed in Section 2.2.3, can come in 

15 many different forms, including but not limited to analogs, laboratory experiments, field 

16 experiments, and formal and informal expert judgment. The basis for why a barrier is expected 

17 to perform the desired function is essential for a licensee and other stakeholders to have

18 confidence in the future performance of the barrier. For engineered barriers that are estimated 

19 to have long-term performance (e.g., thousands of years), licensees should consider natural 

20 analogs in order to provide confidence that the estimates are reasonable. Extrapolating short21 term observations to estimate long-term performance can result in a significant uncertainty in 

22 long-term barrier performance. Standard approaches implicitly assume that the initial conditions 

23 persist; however, the actual application of a barrier may be more appropriately viewed as an 

24 evolving component of a larger dynamic system (Waugh and Richardson, 1997). In addition, 

25 inaccurate conceptualization of degradation mechanisms and their interaction can be a source 

26 of significant error. Adequate model support can help reduce the impact of these uncertainties. 

27 NUREG-1757 provides examples of analogs for cement performance (wasteform stability), 

28 durability of earthen covers, and riprap durability (site stability) (NRC, 2006). NUREG-1757 also 

29 summarizes the degradation mechanisms of common engineered barriers, including engineered 

30 covers such as resistive covers, water-balance covers, and erosion control covers. Section

31 3.5.5 of NUREG-1757 provides a summary of reference information related to engineered cover 

32 design and performance.

33

34 One of the most common barriers used to ensure stability in waste disposal facilities, especially 

35 for near-surface disposal of long-lived waste, is an erosion protection cover. As mentioned in 

36 Section 5.2.2.1, NUREG-1623 may be useful to licensees because it provides methods, 

37 guidelines, and procedures that the NRC staff considers to be acceptable for designing erosion 

38 protection at uranium mill tailings sites (NRC, 2002b). The main elements to developing an 

39 engineered barrier for erosion protection in waste disposal are:

40

41 • Selection of proper rainfall and flooding events

42 • Selection of appropriate parameters for determining flood discharges

43 • Computation of flood discharges using appropriate and/or conservative methods

44 • Computation of appropriate flood levels and flood forces associated with the design flood 

45 discharge
5-24

1 • Use of appropriate methods for determining erosion protection needed to resist the 

2 design discharge

3 • Selection of a rock type for the riprap layer that will be durable and capable of providing 

4 the required erosion protection for the required timeframe

5 • Placement of riprap layers in accordance with accepted engineering practice and in 

6 accordance with appropriate testing and quality assurance controls

7

8 The NRC staff considers that the guidance provided for design of erosion protection systems for 

9 management of uranium mill tailings is applicable to the stability of LLW disposal facilities, and 

10 may be used for nearly all types of erosion protection designs. However, for high 

11 concentrations and large quantities of long-lived waste (as discussed in Section 5.3.2),

12 licensees should place greater emphasis on rock durability, and additional considerations may 

13 be necessary. Erosion protection designs may take many different forms. Many designs use a

14 durable rock cover; however, some combine vegetation or employ multiple layers of differing 

15 materials (i.e., a composite cover) or rock mulches. 

16

17 The main design considerations of an erosion protection cover using rock can be separated into 

18 two main areas: (1) appropriate sizing of the rock, and (2) assurance of the durability of the 

19 cover materials. NUREG-1623 provides methods for sizing rock for an erosion protection cover 

20 (design based on the PMP). Licensees should size the rock to ensure it will stay where it is 

21 placed; they should then ensure that the rock will not degrade significantly (i.e., it will stay close 

22 to its initial size). In order to maintain its design size, appropriately sized rock should not 

23 experience significant mass loss from weathering or experience significant cracking.

24

25 Rock durability is defined as the ability of a material to withstand forces of weathering. Factors 

26 that affect rock durability are (1) chemical reactions with water, (2) saturation time, (3) the 

27 temperature of the water, (4) scour by sediments, (5) windblown scour, (6) wetting and drying, 

28 and (7) freezing and thawing. Chemical weathering and mechanical weathering may reduce the 

29 effectiveness of a rock cover for erosion protection. Chemical weathering is generally a slow, 

30 continuous process that usually occurs in the presence of water. Mechanical weathering is a 

31 process that can lead to deterioration of the rock without chemical alteration. The most 

32 prevalent mechanical weathering processes are (1) frost action and freeze-thaw activity, (2) salt 

33 crystallization, migration, and hydration, (3) water sorption, (4) mineral hydration, (5) wetting and 

34 drying cycles, (6) abrasion by wind, water, and mechanical means, and (7) temperature-induced 

35 expansion and contraction of mineral grains (NRC, 1982c). Comparing the latter processes with 

36 the factors that affect rock durability demonstrates that mechanical weathering is a dominant 

37 degradation process. The individual weathering mechanisms are likely to be specific to the rock 

38 type selected and the weathering processes at the site. Weathering processes and rates are 

39 strongly influenced by climatic conditions. Figure 5-3 provides a macroscale relationship of 

40 climatic variables, environments, and rock weathering agents. 

41

42 An engineered cover for LLW disposal may have more than one design goal. For example, in 

43 addition to providing site stability, the engineered cover may be used to reduce infiltration. 

44 Licensee should ensure all design goals are achieved realizing that some decisions may have 

45 competing influences on the design goal. For instance, using large rock may promote site

46 stability but may increase infiltration compared to other erosion protection designs such as using

47 rock mulch.

48
5-25

Figure 5-3 Macroscale Relationship between Climatic Variables, Environments, and Rock Weathering 

Occurrence of Weathering as a Function of Climate (NUREG/CR-2642)
5-26

1 As discussed in Appendix P “Example of a Graded Approach for Erosion” to NUREG-1757, a 

2 licensee should conduct three evaluations of rock durability to provide multiple and 

3 complementary lines of evidence and greater confidence in rock durability. The three 

4 evaluations are for (1) rock durability testing and scoring, (2) absence of adverse minerals and 

5 heterogeneities, and (3) evidence of resistance to weathering. 

6

7 First, licensees should conduct rock durability testing and scoring. They should test and 

8 evaluate potential rock sources to ensure that rock used for erosion protection remains effective 

9 for the required timeframe. NUREG-1623 (NRC, 2002b) presents a procedure for determining 

10 the acceptability of a rock source. In general, rock durability testing is usually performed using 

11 standardized test procedures, such as those developed by ASTM and published and updated in 

12 ASTM’s Annual Book of ASTM Standards. The scoring procedure indicates that rock scores of 

13 80 percent or greater indicate a high-quality rock for most applications (e.g., decommissioning 

14 sites for 1,000 years). Long-lived waste would generally require very high-quality rock, higher 

15 than necessary for most decommissioning applications.

16

17 Second, the absence of adverse minerals and heterogeneities is essential. Licensees should 

18 use analyses (such as petrographic analyses) to establish the absence of adverse minerals that 

19 could cause rapid degradation of the rock, such as clays, olivine, or calcite cement. If adverse 

20 minerals are present, licensees should evaluate their potential effect on the weathering of the 

21 rock and on weathering rates.

22

23 Third, licensees should provide evidence of resistance to weathering, using direct evidence from 

24 the selected rock source whenever possible. For example, weathering rind thickness and 

25 alteration of minerals and rock properties from exposures of the weathered rock source can 

26 provide insights on the extent and nature of future weathering. Table 5-2 provides general 

27 characteristics of rocks related to weathering resistance (NRC, 1982c). Some characteristics 

28 are favorable with respect to chemical weathering and unfavorable with respect to mechanical 

29 weathering, and vice versa. Indirect evidence from natural and archeological analogs should 

30 also be used, as discussed in Section 5.3.2.

31 5.3.2 Long-Term Considerations

32

33 Disposal of long-lived waste introduces additional complexity and uncertainty with respect to site 

34 stability. A disposal facility containing long-lived waste may be exposed to (1) more extreme 

35 conditions, as well as more cycles of extreme conditions, (2) greater uncertainty because of 

36 more limited performance and observational data, and (3) greater difficulty in obtaining relevant 

37 information. Because of the increased uncertainty inherent in longer-term predictions, the 

38 design of engineered barriers for stability of long-lived waste relies on conservative designs. 

39 Licensees should use the following elements in their design of engineered barriers for long-lived 

40 waste:

41

42 • multiple, independent, and redundant barriers

43 • conservative approaches

44 • very high quality and durable materials

45

46
5-27

Table 5-2 Weathering Resistance and Susceptibility Related to Weathering Type and Rock Propertie

Chemical Weathering Physical Weather

Durable Nondurable Durable Non

Mineral 

Composition

- Uniform mineral composition

- High silica content

- Low metal ion content

 (Fe-Mg), low biotite

- High orthoclase, 

 Na feldspars

- High aluminum ion content

- Mixed/variable mineral comp.

- High CaCO3 content

- Unstable primary

 igneous minerals

- Low quartz content

- High calcic plagioclase

- High olivine

- High feldspar content

- Calcium plagioclase

- low quartz content

- CaCO3

- homogeneous composition

- Hig

- Na 

- He

 com

Texture - Fine grained dense rock

- Uniform texture

- Crystalline

- Clastics

- Gneissic

- Coarse-grained igneous

- Variable textural features

- Schistose

- Fine grained (general)

- Uniform texture

- Crystalline, 

 tightly packed clastics

- Gneissic

- Fine-grained silicates

- Co

- Va

- Sc

- Co

 sili

Porosity - Large pore size, 

 low permeability

- Free draining

- Low internal surface area

- Small pore size,

 high permeability

- Poorly draining

- High internal surface area

- Low porosity,

 freely draining

- Low internal surface area

- large pore diameter

 free draining 

- Hig

 poo

- Hig

 sur

- Sm

Bulk

Properties

- Low adsorption

- High compressive and 

 tensile strength

- Fresh rock

- Hard

- High adsorption

- Low strength

- Partially weathered rock

- Soft

- Low adsorption

- High strength with good 

 elastic properties

- Fresh rock

- Hard

- Hig

- Low 

- Pa

 roc

- So

Structure - Strongly cemented, 

 dense grain packing

- Siliceous cement

- Massive

- Poorly cemented

- Calcerous cement

- Thin bedded

- Fractured cracked

- Mixed soluble and 

 insoluble minerals

- Minimal foliation

- Clastics

- Massive formations

- Thick bedded sediments

- Fo

- Fra

- Thi

- Mix

 ins

Representative

Rock

- Igneous varieties (acidic)

- Metamorphics 

 (other than marbles)

- Crystalline rocks

- Rhyolite, granite, 

 quartzite, gneisses

- Granitic gneiss

- Calcerous sedimentary

- Poorly cemented sandstone

- Slates

- Limestones, basic igneous,

 clay-carbonates

- Marble, dolomites

- Carbonates, schists

- Fine-grained granites

- Some limestones

- Diabases, gabbros, some

 coarse-grained granites

- Quartzite (metamorphic)

- Strongly cemented sandstone

- Slates, granitic gneiss

- Co

- Po

 san

- Ma

- Do

- So

- Sc
5-28

1 5.3.2.1 Conventional (Resistive) Covers

2

3 Licensees can use conventional covers for different design functions. However, with respect to 

4 site stability, the most likely use of a conventional cover is for erosion protection. Designs for 

5 sites with relatively short-lived waste are expected to be simpler than designs for sites with 

6 waste that will remain hazardous until or beyond the end of the compliance period. Design 

7 concepts for the end of the compliance period and for the performance period should include 

8 the use of oversized rock and overthickened rock layers, use of the PMF, and consideration of 

9 more stable structures such as low slopes and blending with local topography. By using 

10 multiple, independent, and redundant barriers licensees can reduce the impact from the 

11 unanticipated failure of a single barrier. 

12

13 One example of the use of multiple barriers for an erosion control system could include one or 

14 more of the following types of erosion controls:

15

16 • The riprap layer for the top slopes and side slopes of the closure covers could be 

17 designed to resist the PMP and PMF. 

18 • The top of the covers could be sloped to drain entirely in one direction, minimizing the 

19 flows that would enter gullies that form in areas that are not designed for drainage 

20 downstream of the covers. 

21 • The site could be optimally graded to enhance drainage, and diversion channels could 

22 be constructed to convey runoff to noncritical locations. 

23 • Downstream gullies could be armored with very large rock to prevent further gullying and 

24 nickpoint migration.

25 • Diversion channels could also be constructed upstream of the covers to divert flows 

26 away from the covers or from potential critical gully locations.

27

28 Licensees may need to consider the natural cycling of climates in the design for sites intended 

29 for long-lived waste disposal. In some locations (e.g., more northern), glacial and interglacial 

30 cycles may result in glacier development and migration over a LLW disposal facility. It is 

31 beyond current technology and understanding to design a near-surface facility to withstand such 

32 forces. In more northern locations, the assessment should focus on the risks following 

33 disruption of the design. Licensees may consider natural analogs to estimate the amount of 

34 waste dispersion associated with those processes, and therefore, the risk. Stylized, conditional 

35 dose assessments may also be useful. In more southern geographic locations, the impact of 

36 natural cycling of climates may be less severe and may be confined to effects such as 

37 increased precipitation and cooler temperatures (e.g., more freeze-thaw cycling). Licensees 

38 should examine the durability of erosion control materials over the range of projected future 

39 climate states. The use of the PMP and PMF for erosion protection design may mitigate the 

40 need to consider future climate states because these parameters represent maximum events. 

41 The PMP approach approximates the maximum rainfall that is physically possible, and the PMF 

42 is a hypothetical flood that is considered to be the most severe reasonably possible (NUREG43 1623, p. 10). Climates that are arid now and more likely to be arid in the future are typically 

44 preferable over wetter climates. Low amounts of water reduce chemical and mechanical 

45 weathering.

46
5-29

1 Licensees should consider the following when selecting and justifying the long-term durability of 

2 erosion protection materials:

3

4 • Selection of highly durable rock—Select only a highly durable rock type with a mineral 

5 that is most resistant to chemical weathering, such as quartz. This would favor a 

6 metamorphic quartzite or a sedimentary orthoquartzite with a high percentage of quartz 

7 grains (99 percent) cemented by quartz. Rock types that can easily alter to clay over the 

8 timeframe considered, such as feldspars, should not be used. This may eliminate many 

9 rock types. Locally-available, highly durable rock is preferable because it has a higher 

10 likelihood of being in equilibrium with the disposal environment.

11

12 • Selection of a homogeneous rock source—Select a rock unit that will result in riprap 

13 pieces that are homogeneous and free of heterogeneities, such as bedding planes, thin 

14 shale layers, or joints. Heterogeneities that can allow access of water can contribute 

15 significantly to mechanical weathering, such as freeze-thaw.

16

17 • Reliance on natural analogs and weathering rate studies—Evaluate natural analogs and 

18 obtain applicable weathering rate studies, if available, to justify the durability of the 

19 selected rock.

20

21 - Weathering rate studies: Obtain weathering rate study data over relevant 

22 timeframes, to the extent available, recognizing any uncertainties in the studies. 

23 Weathering rate data may be estimated from laboratory data complemented with 

24 field observations. If laboratory data are generated, use caution in extrapolating the 

25 data to field performance because mineral weathering does exhibit scale 

26 dependence (Drever and Clow, 1995). Weathering rates can also decrease 

27 exponentially with time, as the most vigorous attack usually occurs early in the 

28 weathering process. Comparative data on natural materials may help with material 

29 selection, such as that shown in Table 5-3 (Brookins, 1984) or in Table 6.7 of 

30 NUREG/CR-2642, “Long-Term Survivability of Riprap for Armoring Uranium Mill 

31 Tailings and Covers: A Literature Review” (NRC, 1982c). Comparative data show 

32 the relative durability of different materials in a common test. Various compilations of 

33 research on weathering rates have been developed (e.g., Colman and Dethier, 

34 1986). Use weathering rate data that is representative of the estimated future 

35 exposure conditions because weathering rates can be very sensitive to exposure 

36 conditions. For example, one of Cleopatra’s Needles (a granite obelisk) survived

37 very well in over 3,000 years of exposure in arid conditions in Egypt but weathered 

38 heavily in under 100 years after being moved to New York City. 

39

40

41
5-30

1 Table 5-3 Comparison of Chemical Durability (Soxhlet Test) of Waste Glass and 

2 Common Minerals 

Minerals Wt% Leached1

Quartz crystals 0.41

Milky quartz 0.50

Dolomite 0.55

HLW glass 0.70

Garnet 0.73

Corundum 0.77

Orthoclase 0.90

Granite 1.10

Quartzite 1.20

Felsite 2.10

HLW glass (devitrified) 2.50

Marble (dolomite) 2.90

Calcite 5.80

Basalt 6.10

3 Source: Brookins, 1984

1 4 Represents mass loss in the Soxhlet test

5

6 - Natural analogs: Use natural analogs to provide confidence in the long-term 

7 durability of the materials selected. Use natural analogs for the specific rock type 

8 proposed and from the region of the disposal facility, instead of more distant 

9 examples, because local materials with demonstrated long-term durability will 

10 provide the most direct link to long-term performance. However, more distant 

11 examples may be useful to address the durability of the materials under a more 

12 diverse range of exposure conditions. Quaternary glacial striations on quartzites and 

13 dating of very old rock surfaces may also be useful. Research is ongoing using 

14 cosmogenic dating to estimate the ages of natural materials that have not weathered 

15 significantly over very long time periods. For long-term stability, engineered systems

16 should mimic durable natural systems as much as feasible. The comparison of 

17 engineered systems to natural systems should address material properties as well as 

18 how the materials are emplaced and distributed to achieve stability.

19

20 For a variety of reasons, certain Quaternary glacial features have the potential to be 

21 a very good source of natural analogs. The features of greatest potential value 

22 include glacial striations, polished rock surfaces, and glacial erratics. In particular, 

23 glacial striations (i.e., fine scratches on a bedrock surface that can be less than a 

24 millimeter in depth) and polished rock surfaces are delicate features that could easily 

25 be removed by weathering. Preservation of such vulnerable features over long time 

26 periods demonstrates a significant resistance of the rock to weathering. Glacial 

27 features (found in various climates today) are also fairly common in a range of 

28 different rock types, which makes them reasonably available for use as analogs. 

29 Glacial features have been exposed to a range of climates during the past thousands 

30 of years since their formation. Finally, licensees do not need to determine the 

31 precise age of the features in order to use them as a natural analog for a LLW 

32 facility. In the absence of available specific dating studies, a general assumption can 

33 usually be made that the features are the result of the last glacial period, which 
5-31

1 ended about 10,000 years ago. Fortuitously, the general 10,000 year age 

2 assumption coincides with the 10,000 year protective assurance period for a LLW 

3 facility.

4

5 - Historic analogs: Preliminary analyses and observations by the NRC staff and 

6 consultants have indicated that many manmade sites exist that demonstrate the 

7 long-term stability of both manmade structures and naturally occurring features. 

8 Many of these analog sites are located in the United States and consist of structures 

9 such as Native American earthen burial mounds, Native American ruins, and rock 

10 features. Many of these sites have been dated and have been shown to have 

11 remained intact for thousands of years. Licensees can use studies of the long-term 

12 survivability of such features to demonstrate the potential for manmade sites, such 

13 as LLW waste disposal facilities, to remain intact for very long periods of time without 

14 the need for ongoing active maintenance. For example, the Sarsen stones of 

15 Stonehenge may provide data for orthoquartzites.

16

17 The benefits of historic or archeological analogs are that the ages of rock carvings, monuments, 

18 or buildings are usually fairly well known and can demonstrate preservation under known 

19 climates and time periods. Furthermore, many potential analogs might be available for a range 

20 of rock types. While many of these analogs can demonstrate preservation for hundreds or 

21 thousands of years, the timeframe is less than the 10,000 year protective assurance period. 

22 However, if available, historic or archeological analogs can complement natural analogs by 

23 providing a variety of evidence that together increase the confidence in the ability of a rock type 

24 to resist significant weathering over long time periods.

25

26 Although direct evidence of material durability from the site or site region is preferable, licensees 

27 can also use indirect evidence from other locations where the general rock type is similar to the

28 selected rock source, considering differences in environmental conditions. In some cases the 

29 durability may be sensitive to the exposure conditions while in other cases it may be less 

30 sensitive or insensitive. For example, licensees could use evidence of durability from a diabase 

31 igneous rock-type found in Europe to provide insights on a diabase rock-type source in 

32 Maryland because the general mineralogy of diabase is similar, regardless of the location. This 

33 approach allows the use of datable natural or archaeological and historical rock sites, which

34 could provide general evidence of rock weathering rates or time periods during which rock types 

35 have remained resistant to weathering. For example, a licensee could use many datable 

36 archaeological sites, such as Stonehenge (constructed about 4,000 years ago of diabase and 

37 silica-cemented sandstone); Hadrian's Wall (constructed by the Romans over 2,000 years ago 

38 of primarily diabase); and numerous buildings, monuments, and megoliths in Europe, to 

39 demonstrate that these rock types have been resistant to weathering over time periods that 

40 exceed thousands of years.

41

42 Historical evidence can also provide useful insights on the durability of certain rock types. One 

43 example is the comparison of dated Civil War photographs of diabase outcrops in Devil's Den at 

44 the Gettysburg National Military Park to present-day conditions of the same outcrop. A licensee 

45 could demonstrate with such a comparison that this diabase has been resistant to weathering 

46 for about 150 years. Similarly, dated grave markers or historical buildings made from the 

47 selected rock source or a similar rock type can also provide evidence of resistance to 

48 weathering for 100–200 years. Appendix A to NUREG/CR-2642 provides additional information 
5-32

1 on rock weathering, durability, and examples of analogs that provide insights on general 

2 weathering rates of various rock types (NRC, 1982c).

3

4 A test wall of building stones was constructed in 1948 in Washington, DC and in 1977 it was 

5 moved to the National Institute of Standards and Technology in Gaithersburg, MD (Stutzman

6 and Clifton, 1997). This test wall provides the opportunity to study the effects of weathering on 

7 different types of stones under identical exposure conditions and the durability of different 

8 materials. Imaging and petrological studies have been performed to characterize texture and 

9 mineralogy. Correlation of mineralogical and microstructural features to stone performance 

10 provides information for estimating the long-term durability of natural materials. The wall 

11 contains 2,352 individual samples of stone: 2,032 domestic stones from 47 States and 320 

12 stones from 16 foreign countries. Over 30 distinct types of stones are represented, including 

13 marble, limestone, sandstone, and granite. Data from this project are limited at this time and 

14 are likely to be uncertain but may provide licensees another line of evidence for rock durability.

15 5.3.2.2 Evapotranspiration Covers

16

17 Evapotranspiration (ET) covers, use the natural processes of evaporation and transpiration to 

18 remove water from the cover. Although evaporation and transpiration can remove water from 

19 the conventional covers discussed in Section 5.3.2.1, it is primarily the layers of low permeable 

20 material present in the conventional covers that limit water from reaching the waste. In contrast, 

21 ET covers rely more heavily on evaporation and transpiration to limit water from reaching the 

22 waste. The performance of ET covers depends on many factors, especially the climate, soil 

23 hydrology, fauna, and plant ecology at a site. ET covers may be used in a variety of settings, 

24 but may be most effective in arid or semi-arid climates with high potential evapotranspiration.

25

26 Licensees should develop a design for an ET cover that is effective over the range of expected 

27 natural and ecological conditions. Natural and ecological conditions are inherently variable over 

28 the timeframe of most LLW disposal analyses. With effective design and development, ET 

29 covers may be very effective, especially in arid and semi-arid climates. In humid climates, ET 

30 covers may be effective at managing a substantial fraction of the infiltration but may not achieve 

31 design goals. Infiltration may exceed evapotranspiration in humid climates or in colder climates, 

32 where a large fraction of infiltration may occur as snowmelt when evapotranspiration is low. 

33 Therefore, one of the major lessons learned, albeit not related to physical degradation, is that 

34 design of an ET cover must consider natural and ecological variability over the analyses 

35 timeframe (Benson et al, 2011).

36

37 Licensees should be aware that physical, biological, and chemical processes can induce 

38 changes in the structure, physical, and biological characteristics of covers that are intrinsic to 

39 their proper functioning as barrier systems. Degradation processes occur over a broad range of 

40 time scales and include, but are not limited to, climatic variability, plant succession, geomorphic 

41 processes, pedogenesis, anthropogenic impacts, erosion, microbial processes that affect barrier 

42 materials and drains (e.g., biofouling), and geochemical processes. An example of 

43 unanticipated ecological consequences is the development of deeper rooted plant species that 

44 result in pathways for moisture that are below the design zone for moisture storage and 

45 removal.

46

47 Engineered systems evolve towards a natural equilibrium. Licensees should recognize that soil 

48 properties may change quickly, and therefore, should minimize the consequences of these 
5-33

1 changes by designing and constructing covers that mimic longer-term conditions that are 

2 congruent with nature. Cover degradation attributable to pedogenesis and ecological change 

3 should be recognized as an inevitable, fairly predictable, natural succession. In some cases, 

4 natural pedogenesis and ecological succession can lead to improved system performance over 

5 time. Therefore, performance will be steadier over time when licensees design engineered soil 

6 layers and vegetation to more closely resemble the characteristics of natural systems. A 

7 licensee should determine the function of each ET cover component (e.g., use of plants and 

8 their roots to stabilize the cover of a site). Licensees should develop techniques to understand 

9 the magnitude and direction of natural changes anticipated to occur. One approach is to 

10 evaluate natural analogs. General strategies that a licensee may use to minimize the negative 

11 impacts of degradation processes include: 

12

13 (a) Attention to construction QA; QA is especially important to the successful short-term 

14 performance of the cover

15 (b) Identification of the phenomena that have the greatest impact on total system 

16 performance

17 (c) Analysis of each component within the system context

18

19 In addition, to increase confidence in the long-term stability of the site, the licensee should focus 

20 on: (1) using natural analogs to better understand and evaluate long-term degradation 

21 processes, including both spatial heterogeneity and temporal trajectories of change; (2) 

22 designing covers that mimic the favorable attributes of selected natural analogs; (3) evaluating 

23 effects of soil development and ecological change; (4) evaluating effects of waste subsidence 

24 on long-term cover performance; and (5) predicting and incorporating landform changes in 

25 cover and disposal cell designs (NRC, 2011b). In addition, the performance of an ET cover can 

26 be particularly sensitive to temporal and spatial variability in precipitation and other processes.

27 5.3.3 Monitoring of Engineered Barriers

28

29 Most waste containment facilities require monitoring to verify performance and/or support 

30 predictive modeling (NAS/NRC, 2007). Environmental monitoring of a LLW disposal facility is 

31 required for the duration of the institutional control period. The design of monitoring systems for 

32 engineered barriers has proven difficult as a result of technological challenges and complex 

33 goals; however, progress is being made. Observing the effects of degradation processes is 

34 critical to understanding long-term performance. Performance monitoring of engineered surface 

35 covers provides a licensee confidence that the cover is functioning as predicted in the 

36 performance assessment, intruder assessment, or site stability assessment.

37

38 Monitoring of engineered covers generally is conducted at two levels: direct non-destructive 

39 performance monitoring, and direct or indirect interpretive monitoring. Performance monitoring 

40 consists of directly and continuously monitoring the primary performance variable (e.g., the flux 

41 of water through a cover) using an in-situ device. Interpretative monitoring consists of 

42 measuring secondary variables (e.g., water content) related to the primary performance variable 

43 that can be used to understand or interpret data obtained from primary performance monitoring 

44 (NRC, 2011a). Interpretive monitoring can be conducted directly using embedded sensors or 

45 indirectly using remote sensing methods such as ground penetrating radar or airborne radar 

46 systems. Water content and temperature are the two most commonly measured secondary 

47 variables employed for interpretive monitoring. Interpretive monitoring currently is conducted 
5-34

1 almost exclusively using direct methods. However, indirect remote sensing methods likely will 

2 become more important in the future, especially for long-term monitoring from remote locations 

3 (NRC, 2011a). 

4

5 During the period of institutional controls, licensees must perform environmental monitoring to 

6 ensure continued satisfactory disposal system performance and to develop confidence that the 

7 observed performance is likely to persist after the institutional control period. Licensees should 

8 also perform physical surveillance to restrict access to the site and minor custodial activities 

9 during the institutional control period. Short-term performance of the disposal site can be 

10 physically monitored with various types of onsite instrumentation or by remote sensing. 

11 Licensees can use monitoring to: detect any early significant releases of contaminants, and to 

12 verify the validity of assumptions made and the accuracy of the results of predictive modeling, 

13 thereby, reducing uncertainty. Though not required, the NRC staff strongly recommends that 

14 licensees conduct interpretive monitoring because it can provide observations of performance 

15 problems that are a precursor to releases of radioactivity into the environment. 

16

17 Licensees should include assumptions, parameters, and features that have a large influence on 

18 the disposal facility performance and have relatively large uncertainties as an important part of a 

19 monitoring plan. For example, monitoring plant processes or more generally ecological 

20 processes, can add greatly to understanding cover stability and performance. Even carefully 

21 designed cover systems begin a process of change immediately following construction. These 

22 changes can affect containment system performance both directly and indirectly, and should be 

23 monitored. Additional information gained through various sources can reduce uncertainties and 

24 support previous predictive modeling. Monitoring is considered important in obtaining 

25 confidence that barrier components are performing as intended and is an important tool in 

26 detecting early signs of degrading stability of a disposal system. Because of increased 

27 understanding of potential shortcomings with engineered surface barriers, monitoring of 

28 engineered systems is being recognized as a powerful tool that has the potential to yield 

29 valuable data. A well-conceived monitoring system for engineered surface barriers would 

30 provide information to assess barrier performance including degradation. 

31

32 Airborne and satellite-based remote monitoring techniques are able to efficiently monitor 

33 particular aspects of the engineered surface covers. For example, remote sensing may detect 

34 vegetative change that is dependent on characteristics of water flow. Linear features of heavier 

35 vegetation may be indicative of cracks or other structural features allowing increased contact of 

36 water with the waste and may be indirect signs that the overall stability of a barrier may be 

37 decreasing. Sensor development has rapidly advanced so that sensors are becoming not only 

38 quicker, more reliable, and longer lasting, but also smaller, more automated, wireless, and more 

39 sophisticated. Licensees can obtain changes in vegetation, soil water content and temperature 

40 through multispectral imaging. Ground penetrating radar, LIDAR (or Light Detection and 

41 Ranging technology), and other remote sensing techniques may detect stabilization problems at 

42 the very early stages due to its high resolution output (NAS/NRC, 2007). Licensees may 

43 someday be able to place automated sensors throughout the different components of the cover 

44 to monitor those features and processes demonstrated to be significant. 

45

46 Licensees should not use monitoring as a substitute for the development of an adequate 

47 performance database prior to implementing their system, but rather to support the previous 

48 determination of adequacy considering uncertainty. When there is uncertainty associated with 
5-35

1 the waste disposal system, monitoring can maintain confidence in the performance 

2 demonstration.

3

4 Monitoring and modeling activities are complementary to one another. Modeling can serve to 

5 focus monitoring efforts by identifying key processes and parameters or disconnects between 

6 field observations and model results. Similarly, the results of monitoring provide feedback to

7 refine models and improve the understanding of the system. Licensees should design their 

8 monitoring systems to understand processes and events and identify early indicators of 

9 performance problems.

10

11

6-1

1 6.0 PROTECTIVE ASSURANCE PERIOD ANALYSES

2

3 10 CFR 61.41(b) requires that:

4

5 Concentrations of radioactive material that may be released to the general environment 

6 in groundwater, surface water, air, soil, plants, or animals shall be minimized during the 

7 protective assurance period. The annual dose, established on the license, shall be 

8 below 5 mSv (500 mrem) or a level that is supported as reasonably achievable based on 

9 technological and economic considerations in the information submitted for review and 

10 approval by the Commission. Compliance with this paragraph must be demonstrated 

11 through analyses that meet the requirements specified in 10 CFR 61.13(a).

12

13 10 CFR 61.42(b) requires that:

14

15 Design, operation, and closure of the land disposal facility shall minimize exposures to 

16 any inadvertent intruder into the disposal site at any time during the protective assurance 

17 period. The annual dose, established on the license, shall be below 5 mSv (500 mrem)

18 or a level that is supported as reasonably achievable based on technological and 

19 economic considerations in the information submitted for review and approval by the 

20 Commission. Compliance with this paragraph must be demonstrated through analyses 

21 that meet the requirements specified in 10 CFR 61.13(b).

22

23 The primary purpose of the protective assurance period analyses is to provide information that 

24 demonstrates that releases of radioactivity from a LLW disposal facility are minimized during the 

25 protective assurance period. Minimization is the reduction of doses to as low as reasonably 

26 practical with technical and economic factors taken into consideration. The protective 

27 assurance period is the period from the end of the compliance period through 10,000 years 

28 following closure of the site. This section provides guidance on developing the technical 

29 analyses for the protective assurance period.

30

31 The requirements for the protective assurance period differ from the compliance period in two 

32 respects. First, the requirement for 10 CFR 61.41(b) and 10 CFR 61.42(b) is minimization. As 

33 explained later in this section, minimization should not be interpreted to mean that zero release 

34 or zero risk can be achieved. Few activities are without risk; there must be a balance between 

35 the health and safety measures that are introduced to control risk and the costs arising or 

36 benefits forgone when these health and safety measures are introduced. Depending on the 

37 particular waste and the technical challenges associated with its disposal, the end point of the 

38 minimization process may correspond to different levels of risk or dose. Second, a target to 

39 compare the minimization against is provided in the regulation (i.e., 5 mSv (500 mrem)); 

40 however, other targets may be supported if they are reasonably achievable based on 

41 technological and economic considerations. The process of minimization typically involves a 

42 comparison of alternatives and could involve cost-benefit analysis. Generally, a preferred

43 option is compared to various alternatives to justify that the preferred option minimizes impacts 

44 in a technically and economically practical manner. There are many different technical and 

45 programmatic options (e.g., an enhanced wasteform) that can be used to control risk. If 

46 impractical, not all of the combinations of these options need to be evaluated; however, each of 

47 the main technical options should be evaluated. Section 6.1 provides guidance on developing 
6-2

1 the scope of the analyses for the protective assurance period. Section 6.2 discusses the 

2 conceptual framework for the analyses, presents different types of analyses that may be 

3 considered by licensees, as well as the NRC staff’s recommended approach for applying long4 term discounting to LLW disposal. The minimization process and associated metrics are also 

5 described.

6 6.1 Scope of the Protective Assurance Period Analyses

7

8 Protective assurance analyses are an extension of the technical analyses used to evaluate 

9 disposal system performance during the compliance period. The results of the performance 

10 assessment, intruder assessment, and site stability assessment provide the technical basis to 

11 demonstrate that impacts to the general population and inadvertent intruders have been 

12 minimized.

13

14 The assumptions, data, and models used to develop the compliance period technical analyses 

15 will be sufficient for the protective assurance period analyses unless changes are necessary to 

16 address the uncertainties associated with the longer timeframes. If scientific information is 

17 available, or can be developed in a cost-effective manner, then the compliance period 

18 assessments should be enhanced or modified. The FEPs that were represented in the 

19 compliance period analyses will generally apply to the protective assurance period, although the 

20 frequency and magnitude may vary temporally. Section 2.5 provides guidance on developing 

21 the scope of the technical analyses.

22 6.2 Framework for the Protective Assurance Period Analyses

23

24 The analyses for the protective assurance period will be similar to the technical analyses for the 

25 compliance period. The primary difference will be in the metrics used to evaluate the results of 

26 the analyses. Figure 6-1 provides the framework for the minimization process used for the 

27 protective assurance analyses applied to 10 CFR 61.41(b). The recommended approach is to 

28 treat the minimization process similar to an optimization problem, using the doses estimated 

29 from the technical analyses (e.g., performance assessment, intruder assessment) as the 

30 objective function. The requirements for protective assurance analyses apply to both 

31 10 CFR 61.41(b) and 10 CFR 61.42(b), however, the process for the latter should be simpler.

32

33 Figure 6-1 depicts levels that licensees should consider and regulators should evaluate that 

34 distinguish different degrees of effort and types of analyses that may be performed for the 

35 minimization process. A graded approach to the protective assurance period analyses is 

36 recommended. Licensees should consider uncertainty in the protective assurance analyses. 

37 The absence of the evidence of risk does not prove that risks are not present. For example, a 

38 licensee may claim that no releases from their LLW disposal facility have been observed, 

39 however, they may not have installed any monitoring wells that would be able to detect such 

40 releases (i.e., the absence of the evidence of risk). This claim would have significantly different 

41 meaning than the assertion by a licensee that releases from their LLW disposal facility have not 

42 been observed because monitoring wells were installed and samples were tested (i.e., evidence 

43 that risks are not present).

44
6-3

1

2

3 Figure 6-1 Analyses Framework for the Minimization Process for the Protective 

4 Assurance Period Analyses Applied to 10 CFR 61.41(b)

5

6 Proposed waste disposal is different from a remediation or clean-up activity in that waste 

7 disposal is a potential future action that could result in future harm to the public, whereas a 

8 remediation activity involves evaluating what should be done to remedy a previous action that 

9 could result in future harm to the public. The operator of a waste disposal facility can set waste 

10 concentration and inventory limits that will control future doses to essentially any level desirable 

11 if the associated technical analysis is of high quality and is reasonably accurate. Inventory limits 

12 are quantity- or concentration-based limits on the amount of radioactive isotopes suitable for 

13 disposal at a particular location. Inventory limits can play a key role in managing risk. 

14

15 Uncertainty in projected doses is an important factor for licensees and regulators to consider 

16 when determining the appropriate target to use when evaluating the minimization process. If 

17 there are significant uncertainties, then the minimization process should be biased towards 

18 more conservative levels (i.e., more effort to minimize). The levels a licensee may consider for 

19 minimization under 10 CFR 61.41(b) include:

20

21 Level 0: At hundredths of mSv/yr (a few mrem/yr and below), design, process, waste 

22 acceptance, or other changes are generally not warranted unless they result in a 
6-4

1 cost savings without increasing worker exposures. Therefore, analyses of 

2 alternatives are not necessary for Level 0.

3

4 Level 1: Above a few mrem/yr and up to the compliance period dose limit of 0.25 mSv/yr 

5 (25 mrem/yr), changes to design, process, waste acceptance or other areas may 

6 be warranted if they can be justified based on technological and economic 

7 considerations. The analyses of alternatives may be qualitative or quantitative. 

8 Quantitative analyses are preferred. Inventory limits may be used to limit doses 

9 if there is uncertainty that the doses are Level 1 or that they may be higher. A 

10 licensee may also propose and implement inventory limits for purposes other 

11 than the quantitative dose assessment (e.g., to simplify waste handling and 

12 management).

13

14 Level 2: Above 0.25 mSv/yr (25 mrem/yr) and up to 5 mSv/yr (500 mrem/yr), changes to 

15 design, process, waste acceptance or other areas may be warranted unless they 

16 are shown to be impractical based on technological and economic 

17 considerations. The analyses of alternatives should be quantitative to the extent 

18 practical. Inventory limits should be used to maintain projected future doses 

19 below 5 mSv/yr (500 mrem/yr) and those inventory limits should be reflected in 

20 the associated waste acceptance criteria.

21

22 Level 3: Above 5 mSv/yr (500 mrem/yr), changes to design, process, waste acceptance, 

23 or other areas are expected of the licensee unless they are shown to be 

24 impractical based on technological and economic considerations. For example, 

25 changes to the wasteform or geochemistry of the disposal cells may significantly 

26 reduce projected future doses. The analyses of alternatives should be 

27 quantitative, and include a rationale for why the annual dose cannot be reduced 

28 to 5 mSv/yr (500 mrem/yr) based on technological and economic considerations. 

29 The analyses of alternatives should consider why inventory limits cannot be 

30 established. The licensee and regulator should consider if an alternative facility 

31 could receive the waste and maintain doses below 5 mSv/yr (500 mrem/yr).

32

33 For Level 3, the analyses of alternatives should be broader and more comprehensive compared 

34 to the other levels. One method that is irrefutably capable of reducing risk is to establish 

35 concentration- or quantity-based inventory limits. Inventory limits may be used to account for 

36 potential shortcomings in the engineered or natural components of a LLW disposal system. In 

37 addition, if the results of the analyses demonstrate that doses for 10 CFR 61.41(b) are likely to 

38 be above 5 mSv/yr (500 mrem/yr), the results may indicate that the proposed disposal facility 

39 and disposal site are not suitable candidates for that type of waste. Alternative disposal 

40 facilities may exist that can better manage the particular radiological hazard.

41

42 The minimization process for 10 CFR 61.42(b) is conceptually similar to 10 CFR 61.41(b) with 

43 the exceptions that different levels and a simpler process can be used. A variety of factors 

44 contribute to this recommendation. The inadvertent intruder is a hypothetical construct to 

45 account for the uncertainty with respect to providing long-term control of a closed disposal site, 

46 and intruder doses are usually driven by short-lived waste for most commercial LLW. In 

47 addition, the dose limit for the intruders during the compliance period is significantly higher than 

48 the limit found in 10 CFR 61.41(a). Finally, the long-term performance of engineered barriers to 

49 inhibit intruder doses during the protective assurance period cannot be reliably assured. Each 
6-5

1 of these factors contributes to making the minimization process for the intruders somewhat 

2 different than for 10 CFR 61.41(b). Whereas there may be practical actions that can be taken to 

3 minimize doses to a member of the public for 10 CFR 61.41(b), it is more challenging to identify 

4 actions to minimize doses for a hypothetical intruder scenario. Because it is more difficult to 

5 identify actions, a simpler approach is recommended for minimizing intruder doses. The 

6 recommended approach for 10 CFR 61.42(b) is to minimize intruder doses below a 500 

7 mrem/yr target (analogous to Level 1 above but with the corresponding equivalent threshold) or 

8 to a level that can be justified based on technological and economic considerations.

9 6.2.1 Types of Analyses

10

11 A licensee has flexibility in the type of analyses to use for the protective assurance period. The 

12 analyses should be tailored to the specific waste, disposal facility design, and the projected 

13 future radiological doses. The suitability of particular analyses to support decision-making may 

14 be impacted by projected uncertainty. If projected uncertainty is relatively large, it may not be 

15 clear from the results of analyses which alternatives should be adopted and which can be 

16 rejected. Therefore, the licensee should err on the side of evaluating more alternatives to 

17 ensure defense-in-depth will be provided. In addition, a larger set of alternatives may be 

18 considered because some of those alternatives may help to reduce uncertainty in the projected 

19 performance.

20 6.2.1.1 Alternatives Analyses

21

22 The simplest form of analysis for the protective assurance period is for a licensee to compare 

23 alternatives. This section provides guidance on:

24

25 What alternatives should be considered by a licensee?

26 How much detail should be included in the analyses?

27 Should combinations be evaluated by a licensee?

28

29 In general, in an alternatives analysis, a licensee will have a base case or preferred alternative 

30 with an associated projected impact (i.e., dose). An alternatives analysis can be simpler than a 

31 minimization or cost-benefit analysis because economic variables may be omitted. For the 

32 alternatives analysis, the base case or preferred alternative should be the alternative that is 

33 projected to result in the lowest dose or that calculates a dose comparable to the lowest result 

34 given the uncertainty. If the preferred alternative dose is not comparable to the lowest dose, 

35 then technological and economic practicality can be factored into the decision, as discussed in 

36 Section 6.2.1.2. 

37

38 Performance assessments may be deterministic or probabilistic, but regardless of the type of 

39 analyses all performance assessments should consider uncertainties. The base case or 

40 preferred alternative will usually have the lowest projected doses. However in some cases a 

41 number of alternatives may be comparable when uncertainties in the projected doses are 

42 considered. The projected uncertainties will be reflected in a range of estimated doses. If the 

43 analysis is probabilistic, the appropriate metric to use is the peak-of-the-mean dose. Though 

44 the peak-of-the-mean dose is a scalar output, it will have uncertainty associated with the 

45 estimated value. The uncertainty in the estimates of the peak-of-the-mean doses for different 

46 alternatives may overlap. Alternatively, plots of other percentiles of projected doses can be 

47 compared when selecting an alternative. When uncertainty causes the results to overlap the 
6-6

1 “lowest” result may not be unambiguously identified, and therefore other considerations may be 

2 taken into consideration when selecting the preferred alternative or base case.

3

4 The main variables to consider as alternatives generally include:

5

6 • Design (e.g., different engineered barriers),

7 • Processing (e.g., waste emplacement and configuration), and

8 • Waste acceptance (e.g., waste types, waste streams, waste characteristics).

9

10 The framework for the protective assurance period analyses provides for a risk-informed 

11 approach based on the projected future doses. For projected future doses of a few mrem per 

12 year and less (Level 0), formal evaluation of alternatives is not necessary. Because the doses 

13 are low, essentially the case for minimization has already been established by the licensee in 

14 the base case or preferred alternative. An exception would be if the projected dose is low but 

15 the uncertainty is high. In this case, it may be reasonable to evaluate alternatives to the base 

16 case to see if the uncertainty can be reduced or managed. It should be noted that reduction in 

17 uncertainty may not result in a reduction of risk; rather the risks may be better understood.

18

19 For projected future doses greater than a few mrem per year and up to 0.25 milliSieverts per 

20 year (25 mrem per year) (Level 1), an evaluation of alternatives is recommended. However, 

21 that evaluation should be focused on first-order effects of the main variables (e.g., design, 

22 processing, waste acceptance). A large number of alternatives do not need to be considered; 

23 generally less than ten alternatives should suffice. The analyses may be qualitative, although 

24 quantitative analyses are preferred. The experience base considered for design and process

25 changes should include domestic experience, though licensees may also consider international 

26 experience.

27

28 For projected future doses greater than 0.25 mSv/yr (25 mrem/yr) and up to 5 mSv/yr (500 

29 mrem/yr) (Level 2), an evaluation of alternatives is recommended. Changes to design, process, 

30 waste acceptance or other areas may be warranted unless their adoption cannot be justified 

31 based on technological and economic considerations. The alternatives analysis should be 

32 quantitative to the extent practical. The experience base considered for design and process 

33 changes should focus on domestic experience but may also include international experience. 

34 The number of alternatives considered should be complete without being exhaustive. For 

35 example, the licensee should be able to use the results of the performance assessment to 

36 identify those features of the design that are most important to limiting releases, or that provide 

37 defense-in-depth. Alternatives analyses should include variants of the key design features. 

38 Design alternatives should include those related to the hydrology, geochemistry, site stability, 

39 wasteforms, or other significant characteristics. Depending of the projected doses, a licensee 

40 may wish to consider combinations of alternatives, as the projected responses of performance 

41 assessment models is not always linear and complex responses can occur. Alternatives may 

42 be rejected if they are not technologically or economically practical irrespective of the benefits to 

43 system performance.

44

45 A primary difference between the analyses for Level 2 and Level 1 is the consideration of 

inventory limits to maintain projected future doses to a prescribed level.1 46 Inventory limits should 

 1 This section is focused on 10 CFR 61.41(b). As noted previously, the approach recommended for 

10 CFR 61.42(b) is simpler.
6-7

1 be used to maintain projected future doses below 5 mSv (500 mrem) and those inventory limits 

2 should be reflected in the associated waste acceptance criteria. For waste disposal, inventory 

3 limits can be derived that correspond to different levels of dose to a member of the public. In 

4 many cases, it may be practical for a licensee to set inventory limits that correspond to a fraction 

5 of the 5 mSv (500 mrem/yr) target. A licensee should consider alternative inventory limits in the 

6 minimization process for Level 2.

7

8 For projected future doses to a member of the public greater than 5 mSv/yr (500 mrem/yr) 

9 (Level 3), evaluation of alternatives is necessary. The licensee is expected to make changes to 

10 the design, process, waste acceptance or other areas unless changes cannot be justified based 

11 on technological and economic considerations. The experience base considered should include 

12 domestic and international experience as well as emerging technologies that may become 

13 viable in the near term. The number of alternatives the licensee considers should be complete 

14 and include the identification of relevant combinations of alternatives that may have significantly 

15 increased performance. Design features that act in combination may enhance performance or 

16 provide defense-in-depth. The analyses of alternatives performed by a licensee should be 

17 quantitative, and include a rationale for why the annual dose cannot be reduced to 5 mSv (500 

18 mrem) or lower based on technological and economic considerations. The licensee should 

19 justify in the alternatives analysis why inventory limits cannot be established to lower the 

20 projected future dose. The presence of previous disposals that could result in future doses 

21 greater than 5 mSv (500 mrem) is not a sufficient basis for allowing future LLW disposals to 

22 result in doses of that magnitude. Previous and future disposals may be managed separately 

23 such that the impacts do not overlap, especially with respect to demonstration of compliance 

24 with 10 CFR 61.42(b). The licensee should provide the regulator with a strong basis for why the 

25 waste should be accepted at their facility, despite the higher projected dose, as opposed to 

26 being disposed at an alternate LLW disposal facility. The regulator should consider, through 

27 consultation with peers at other regulatory agencies, if an alternative facility could receive the 

28 waste and maintain doses below 5 mSv (500 mrem). The alternative facilities considered 

29 should be limited to existing facilities or proposed facilities for which technical analyses are 

30 available. In most cases existing facilities will have developed inventory limits for a large 

31 number of isotopes. Therefore, it should be easy to determine the quantity of a particular new 

32 waste stream that an existing facility could suitably dispose. 

33 6.2.1.2 Minimization Analysis

34

35 Whereas the alternatives analysis discussed in the previous section examines different options 

36 based on projected annual doses, the minimization analysis may also include economic 

37 considerations. This section provides guidance for licensees completing analyses and for 

38 regulators reviewing analyses on:

39 • Long-term discounting,

40 • Metrics to use for minimization analysis, and

41 • Consideration of uncertainty.

42

43 A challenge with minimizing the long-term impacts from LLW disposal is that the costs may be 

44 incurred in the near term, whereas the impacts or benefits, such as averted doses, may not be 

45 observed until hundreds or thousands of years in the future. In addition, the size of the group 

46 that may be exposed in the future, especially the distant future, is difficult to estimate with a high 

47 degree of confidence due to the complexity of estimating future land use and demographic 
6-8

1 changes. There is no clear consensus in the technical literature with respect to discounting very 

2 long-term impacts in order to perform cost-benefit analysis or other similar types of analyses. 

3 Discounting has been applied by some researchers, though the timeframes are generally limited 

4 to a few hundred years. Some researchers have inferred effective discount rates from financial 

5 data (e.g., long-term lease data) and have concluded that they are likely to be very low (e.g., 

6 fractions of a percent) with respect to financial decisions (Giglio et al., 2013). The issue of long7 term discounting combines technical and philosophical components; staff favors a pragmatic 

8 solution over theoretical rigor. 

9

10 The NRCs recommended approach to discounting applied to regulatory analysis development 

for backfitting can be found in NUREG/BR-0058, Rev. 4 (NRC, 2004d).2 11 A distinction is made

12 between shorter-term problems (e.g., reactor licensing) and problems involving the long-term 

13 and potential intergenerational considerations (e.g., decommissioning or waste disposal). 

14

15 The approach recommended here (e.g., long time horizons) has four components:

16

17 1. Discount costs of alternatives over the operational life, 

18 2. Do not discount long-term future averted doses (i.e., benefits), and

19 3. Evaluate alternatives based on a) the normalized effectiveness of the alternative and b) 

20 the absolute level of the risk involved.

21 4. Consider uncertainty and sensitivity in the assumptions.

22

23 Discounting of costs to compare design alternatives is a well-established method used in 

24 engineering economics. Consideration of the time value of money is central to most 

25 engineering projects. For example, to compare two alternatives one that involves a capital 

26 expenditure today with one that involves an expenditure that will occur n years in the future, the 

27 present value of the future expenditure is estimated with:

28

PV = FV

1

(1 + m)n

29

30 where r is the annual discount rate.

31

 2 The NRC staff’s recommended approach to discounting for shorter-term problems was initially reflected in NUREG1757, Appendix N for application to ALARA analyses in decommissioning (up to a 1,000 year timeframe). However, 

portions of that appendix, including the recommendations for discounting, were later withdrawn (NRC, 2007d). 

NUREG/BR-0058, Rev 4 discusses the issue: For certain regulatory actions, such as those involving 

decommissioning and waste disposal issues, the regulatory analysis may have to consider consequences that can 

occur over hundreds, or even thousands, of years. The Office of Management and Budget (OMB) recognizes that 

special considerations arise when comparing benefits and costs across generations. Under these circumstances, 

OMB continues to see value in applying discount rates of 3 and 7 percent. However, ethical and technical arguments 

can also support the use of lower discount rates. Thus, if a rule will have important intergenerational consequences, 

one should consider supplementing the analysis with an explicit discussion of the intergenerational concerns such as 

how future generations will be affected by the regulatory decision. Additionally, supplemental information could 

include a presentation of the values and impacts at the time in which they are incurred with no present worth 

conversion. In this case, no calculation of the resulting net value or value-impact ratio should be made. Also, one 

should consider a sensitivity analysis using a lower, but positive discount rate. Finally, as a general principle, 

sensitivity or uncertainty analysis, or both, should be performed.
6-9

1 On the other hand, discounting of benefits (e.g., averted doses) is open to considerable debate. 

2 Over very long timeframes, the mathematics of discounting results in the conclusion that the 

3 current generation should not invest any resources to mitigate impacts that occur to distant 

4 future generations. The NRC staff’s recommendation not to discount long-term, future averted 

5 doses is based on the position that the current generation is responsible for the long-term 

6 impacts from its LLW disposal decisions and that the solution for these impacts should not be 

7 deferred to future generations. 

8

9 In order to compare alternatives, the recommended approach is 1) to normalize the cost of the 

10 alternative to what is considered to be an appropriate amount (cost) to protect the current 

11 generation (e.g., the base cost of the facility), and 2) to scale the normalized result based on 

12 risk. Higher risk should result in a higher threshold to reject an alternative, whereas at very low 

13 risk, all alternatives should be rejected. The use of the term ‘risk’ here is referring to the 

14 projected doses to a member of the public or an inadvertent intruder. A broader interpretation of 

15 risk for the risk-based discounting process may unnecessarily complicate the decision-making 

16 process. However, licensees may choose to consider the balance between future risk from 

17 disposed waste and present risk to workers or the public. For example, a wasteform with much 

18 higher durability could have much lower waste loading, resulting in a transferal of risk but not a 

19 significant reduction (e.g., from disposal to transportation). The metric the NRC staff developed 

20 is to compare the proportional increase in cost for the alternative relative to the base cost of the 

21 facility (e.g., the appropriate cost to protect the current generation) against the fractional change 

22 in long-term impacts. The overall result is then conditional on the absolute level of the risk as 

23 presented in Figure 6-1. This is explained in more detail with an example later in this section.

24

25 Long-term Discounting:

26 The NRC has developed a policy to inform regulatory decision-making with respect to 

27 rulemaking that is subject to NRC’s backfit requirements. A $2,000 per person-rem conversion 

28 factor has been applied to inform regulatory decisions for fuel cycle facilities (NUREG-1530) and 

29 evaluate potential new regulatory requirements (NRC, 1995d; NRC, 2004d). The conversion 

30 factor is also used in regulatory applications, such as in ALARA analyses, to provide a monetary 

31 valuation to collective dose. A number of criticisms have been expressed about the use of 

32 collective dose (NCRP, 1995). Though some have found collective dose still has value in 

33 comparing alternatives (Brock and Sherbini, 2012). The dollar per person-rem conversion factor 

34 attempts to capture the dollar value of the health detriment resulting from radiation exposure. 

35 Health detriments (e.g., excess cancer fatalities) are linked to radiation exposure through 

36 conversion factors (e.g., 0.05/Sv). 

37

38 In NUREG-1530, the NRC staff evaluated different methods to value a statistical life to develop 

39 a dollar per person-rem factor (NRC, 1995d). One method looked at values implied by 

40 government expenditures for many different programs designed to protect human life. The 

41 values in 1990 dollars ranged from $12,000 per statistical life for scoliosis and neuromuscular 

42 disease to $85,000 for regulatory and warning signs. The implied value to limit exposure in the 

43 defense HLW program was $490 million per statistical life. The primary driver of the large range 

44 of results was the differences in the amount spent on each program, not in the methods to 

45 calculate the statistical deaths. The NRC staff’s evaluation resulted in a large range of values of 

46 expenditures per statistical life and the values for nuclear issues, especially nuclear waste 

47 issues, tended to be at the very high end of the range.

48
6-10

1 The range of positions taken on long-term discounting is extremely large (Farber and 

2 Hemmersbaugh, 1993). There are a variety of considerations discussed in the technical 

3 literature that the NRC staff considered when developing this guidance. These include but are 

4 not limited to:

5 • The long-term doses from LLW disposal could be relatively small but persist for a very 

6 long period of time. Without discounting, the cumulative effect could be very large if the 

7 exposed population is large. However, the incremental increase in risk to any individual 

8 would be very small, perhaps below the threshold where individuals take action or 

9 otherwise modify their behaviors.

10 • Because of the timeframes involved, if a licensee were to use discounting to estimate 

11 the present worth of benefits (e.g., averted doses), even at very low discount rates, the

12 licensee may conclude that no amount should be spent today to mitigate long-term 

13 radiological risks (i.e., beyond a few hundred years).

14 • Society incurs the cost of LLW disposal regulation long before the benefits of doses 

15 averted; compound discounting has a greater impact in the calculation of the present 

16 value of benefits than on the costs because the benefits occur over much longer 

timeframes.3 17

18 • Most studies of discounting are focused on the impacts within a generation and not on 

19 the impacts to later generations. Studies that have looked at longer timeframes have 

20 generally been limited to around a hundred years because there are not many longer 

21 term problems.

22 • The NRC staff acknowledges that discounting is based on unstated economic 

23 assumptions that may not be valid over very long timeframes (NRC, 1982b).

24 • Temporal volatility in discount rates can increase future valuations significantly 

25 compared to assuming that rates are constant (Newell and Pizer, 2003).

26 Consistent with guidance provided in NUREG-1854 (NRC, 2007a), future long-term doses 

should not be discounted with time to present day values in order to compare alternatives.4 27 As 

28 discussed below, the recommended approach for a licensee to use is to compare the long-term 

29 costs and reductions in dose to the compliance period costs and effectiveness of the design 

30 while scaling the result with the magnitude of the projected long-term impacts. In other words, 

31 even for a large reduction in long-term doses if the baseline long-term doses are small, the 

32 resource expenditure is likely not warranted. This approach could be described as a normalized 

33 utility-based approach with a floor threshold. The recommended approach is based on 

 3 Although a present benefit from disposal would be that the public is not exposed to radiation if the waste is disposed 

compared to a scenario if the waste is not disposed and control is lost.

4 Some stakeholders have referred to Appendix N of NUREG-1757 as providing guidance on the topic of long-term 

discount rates. However, relevant portions of Appendix N were withdrawn after issuance and are yet to be revised. 

This guidance refers to NUREG/BR-0058, Rev. 4, “Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory 

Commission” (NRC, 2004d). Section 4.3.5 of NUREG/BR-0058 indicates that for certain regulatory actions, such as 

those involving decommissioning and waste disposal, special considerations arise when considering benefits and 

costs across generations. Section 4.3.5 indicates that the analysis should be supplemented with an explicit 

discussion of intergenerational concerns. This could be done by performing the analysis based on costs and impacts 

at the time they are incurred, with no present worth conversion, or by performing a sensitivity analysis using lower 

discount rates.
6-11

1 individual doses, thus avoiding the issues noted above with use of collective dose and 

2 cumulative impacts.

3

4 The recommended approach is to estimate the cost of the alternatives represented by current 

5 net present value (NPV) by applying discounting over the projected operational period of the 

6 facility. A range of discount rates should be considered, such as in the range of 1 to 7 percent, 

7 but biased towards the long-term trend values. Though long periods of deflationary rates have 

8 not been frequent historically, the licensee could consider the sensitivity of the NPV calculations 

9 to a deflationary period or periods. Sensitivity of the NPVs to the uncertainty in discount rates 

10 should be examined. Considering the uncertainty, a licensee should be biased towards 

11 conservatism (i.e., select alternatives that provide a greater reduction in risk). After the 

12 operational period and preparation for closure, the site operator is not expected to be 

13 performing significant activities at the disposal facility; therefore, discounting should not apply 

14 after the operational period.

15 Metrics to Use for Minimization Analysis:

16 The performance metrics a licensee must apply to the assessment of long-term releases during 

17 the protective assurance period is to minimize releases (10 CFR 61.41(b)) and to minimize 

18 exposures to any inadvertent intruder (10 CFR 61.42(b)). In addition, the licensee must 

19 demonstrate stability of the disposal site after closure (10 CFR 61.44). A target to compare this 

20 minimization against is provided in the regulation (i.e., 5 mSv (500 mrem), or to a level that is 

21 supported as reasonably achievable based on technological and economic considerations). 

22 The metrics afford flexibility to a licensee to consider socioeconomic factors when assessing the 

23 long-term protection of public health and safety. 

24

25 These requirements to minimize releases and exposures are intended to be conceptually similar 

to aspects of the ALARA requirement5 26 found in 10 CFR Part 20, which includes the use of 

27 optimization, feasibility analyses, and traditional cost-benefit analyses. The 5 mSv (500 mrem) 

28 value provided in the regulation is a target or dose goal to use in the minimization process. It is 

29 not identical to a dose limit. In most cases, the NRC staff expects that the minimization process 

30 will produce a design that is projected to result in doses to a member of the public much lower 

31 than the target value. The minimization analysis is conceptually similar to ALARA, but it is not 

32 identical. 

33

34 Because of the problems associated with discounting over long timeframes, the NRC staff 

35 recommends that licensees follow the process given below to implement the minimization 

36 process. Other processes may also be suitable and regulators may evaluate any other 

37 processes on a case-by-case basis. Based on the results of their minimization analyses, 

38 licensees may elect to adopt alternatives that are more protective.

39

40 The analyses should be based on best estimates and uncertainties. Uncertainty may impact 

41 whether an alternative should be adopted or not. If the uncertainties are low, and confidence in 

42 the decision to eliminate an alternative is high, then it is practical to not adopt the alternative. If 

 5 ALARA is defined as “making every reasonable effort to maintain exposures to radiation as far below the dose limits 

in this part as is practical consistent with the purpose for which the licensed activity is undertaken, taking into account 

the state of technology, the economics of improvements in relation to the state of technology, the economics of 

improvement in relation to benefits to public health and safety, and other societal and socioeconomic considerations, 

and in relation to utilization of nuclear energy and licensed materials in the public interest." See 10 CFR 20.1003.
6-12

1 the uncertainties in the estimates are comparatively higher, and confidence in a decision to 

2 eliminate an alternative is low, then caution is warranted and the alternative should be adopted. 

3 If the uncertainties are prohibitively large (see Section 6.2.1.4), then inventory limits may be 

4 necessary to mitigate those uncertainties or disposal of certain waste isotopes may be not be 

5 appropriate. Because of the different types of uncertainties and their magnitudes in relation to 

6 the decision metrics, many different approaches to factoring uncertainty into the minimization 

7 process may be appropriate and should be evaluated on a case-by-case basis. A licensee is 

8 not required to, but may consider, performing the minimization process calculations 

9 probabilistically. Uncertainty in the doses, net present values, design effectiveness, and level 

10 scaling factors could be included resulting in probability distributions of the outputs.

11

12 Minimization Process:

13 The following variables are defined:

14

15 “Dx” is a dose for scenario or alternative x in units of mrem/yr

16 “NPVx” is the net present value for scenario or alternative x in units of $

17 “Effx” is the effectiveness of the scenario or alternative x in units of mrem/yr-$

18

19 Dbase = Peak dose from the base design within the protective assurance period

20 Dnatural = Peak dose without the base design within the protective assurance period (natural 

21 barrier capabilities only)

22 Dnatural-int = Peak dose to the intruder without the base design during the protective assurance 

23 period

24 Dbase-int = Peak dose to the intruder with the base design during the protective assurance period

25 NPVbase = NPV of the base design ($)

26 Effbase = the effectiveness of the base design within 10,000 years (dose/$)

27 Dalti = Peak dose from alternative i during the protective assurance period

28 NPValti = NPV of alternative i ($)

29 Effi = the effectiveness of alternative i for the protective assurance period (dose/$)

Lf = Level scaling factor6 30 (dimensionless) 

31 NER = Normalized Effectiveness Ratio

32

33 For evaluation of minimization with respect to 10 CFR 61.41:

34 For Dbase < 25 mrem/year: Lf = 0

For Dbase >= 25 mrem/year: Lf = 0.13 * (DBase) 0.333 35

36

37 For evaluation of minimization with respect to 10 CFR 61.42:

38 For Dbase-int < 500 mrem/year: Lf = 0

For Dbase-int >= 500 mrem/year: Lf = 0.13 * (Dbase-int)

0.333 39

40

41 1) Calculate the effectiveness of the base design using:

42

EEEbbbb = Dnbturbl −Dbbbb

NPVbbbb

43 (6.1)

44

45

46

 6 Development of the expressions for the level scaling factors is discussed later in this section.
6-13

1 2) Calculate the effectiveness of the n protective assurance period alternatives

2 (for i = 1 to n) using:

3

EEEi = Dbbbb −Dblti

NPVblti

4 (6.2)

5

6 3) Compare the results with the expression:

NERi = DEBbbb

DEi

7 > LE (6.3)

If NERi 8 is greater than Lf for all alternatives, then the alternatives are not viable (i.e., 

9 minimization has been achieved). Example 6.1 demonstrates the application of this approach.

10

11 The peak doses used in the minimization process analysis should be consistent with the type of 

12 analyses performed for the performance and intruder assessments. For instance, if probabilistic 

13 analysis is used the recommended metric is the peak-of-the-mean. Uncertainty in the 

14 magnitude and timing of the peak doses should be considered, especially if deterministic 

15 analyses are used.

16

17 Figure 6-2 provides a conceptual representation of the level scaling factor (Lf), which is used to 

18 compare with the normalized effectiveness ratio (NER). The value of the constant and the 

exponent in the following equation (Lf = 0.13 * (Dbase, Dbase-int, Dnatural, or Dnatural-int,) 0.333 19 )

20 were selected to ensure the following:

21 • At very low values of dose, no resources would be expended to further reduce the 

22 doses, 

23 • At a dose of 500 mrem during the protective assurance period, the effectiveness of an 

24 alternative would be compared on an equal basis with the effectiveness of the base 

25 design during the compliance period, and 

26 • At doses greater than 500 mrem, more resources would be practical to apply to reduce 

27 the potentially large impacts. 

28 The use of the level scaling factor, Lf, is a risk-based discounting approach. Figure 6-3 provides 

29 the various Lf values highlighted on a hypothetical performance assessment model output that 

30 would be used to demonstrate minimization for 10 CFR 61.41(b).

31

32 The benefits of this approach are 1) strong base designs are encouraged (i.e., larger Effbase), 

33 2) costs are scaled with doses, 3) resources are not expended for reducing very low doses, and 

34 4) transparency of costs and alternatives is provided to regulators and other stakeholders. 

35 Summary points of the approach are:

36 • It is a benefit to maximize Effbase, This can be done by implementing as effective a 

37 design as possible in a cost-effective manner (i.e., maximize performance, minimize 

38 cost).

39 • It is also a benefit to having a highly-effective natural system (i.e., high-performance 

40 LLW disposal site), because the Lf is scaled with overall dose levels.

41 • It is a benefit to minimize Dbase. Selecting a base design that performs well for the 

42 protective assurance period will reduce the viability of alternatives.
6-14

1

EEEbbbb = (220 − 172)mmmm/y

$30,000,000 = 1.6E-6 

mmmm

$-yr

EEE1 = (172 − 15)mmmm/y

$110,000,000 = 1.42E-6 

mmmm

$-yr

EEE2 = (172 − 137)mmmm/y

$20,000,000 = 1.75E-6 

mmmm

$-yr

EEE3 = (172 − 69)mmmm/y

$1,000,000 = 1.03E-4 

mmmm

$-yr

L = 0.13 ∗ (172)0.333 = 0.72

Example 6.1: A license applicant has developed an application to dispose of commercial 

LLW at the Nerak Site. If approved, the waste will contain a mixture of short- and long-lived 

isotopes. The applicant performed analyses of their base case design and estimated the 

peak dose to be 5 mrem/yr within 1,000 years and 172 mrem/yr within 10,000 years. They 

then performed analyses of the waste in the disposal site without the benefit of the base 

design features (e.g., engineered cover, concrete vaults, and waste packages) and 

estimated the peak dose would be approximately 220 mrem/yr at 10,000 years. The 

applicant’s estimates of costs and doses are based on the median results of probabilistic 

analyses. The applicant considered three alternatives to the design and evaluated the 

effectiveness of those alternatives in minimizing projected doses for the protective 

assurance period. The results of various analyses developed by the applicant are:

Description Time NPV Cost Peak dose 

 (yr) ($) (mrem/yr)

Base case – compliance 1,000 10,000,000 5

Site effectiveness – 10,000 NA 220

Base case – protective assurance 8,437 30,000,000 172

Alternative 1 – improved wasteform 10,000 110,000,000* 15

Alternative 2 – improved cover 8,708 20,000,000* 137

Alternative 3 – chemical barrier 10,000 1,000,000* 69

* Increase in cost over the base case, NPV = net present value

NER1 = 1.13, NER2 = 0.91, and NER3 = 0.02. Since NER3 is less than the Lf, then the base 

design does not adequately minimize releases (in the case of releases to the general 

population). The exposures to inadvertent intruders would be evaluated with a similar 

approach using the pertinent information. The license applicant proposes in their 

application that a chemical barrier will be added to the base case design in order to 

minimize releases for the protective assurance period.

Conclusion: After examining the sensitivity of the results to the uncertainties, the regulator 

concludes that with the chemical barrier added (Alternative 3) the license applicant has 

demonstrated that releases would be minimized for the protective assurance period.
6-15

1

2 Figure 6-2 Level Scaling Factors as a Function of the Base Case Dose in the 

3 Protective Assurance Period (For 10 CFR 61.41 and 10 CFR 61.42)

4

5 • By setting the Lf to 0 below a threshold, design alternatives will not be implemented, and 

6 therefore, costs will not be incurred by current generations to reduce doses that are 

7 already low from a holistic risk perspective.

8 • Inventory limits can be used to avoid costs associated with implementing alternatives to 

9 reduce large doses that may be incurred from a large radionuclide inventory.

10

11 In addition to the quantitative information discussed above, the licensee may provide regulators 

12 with the proportion of resources that they propose to use to achieve the protection of public 

13 health and the environment for different timeframes. This information can help identify 

14 disproportionate costs and benefits in the proposed approach, and provide transparency to 

15 stakeholders. It can also help communicate the proposed approach in terms of the commitment 

16 to short- and long-term environmental protection. These resource estimates should only be 

17 discounted up to the closure of the facility, as no long-term design changes are expected to be 

18 implemented following closure. 

19

20

0

0.5

1

1.5

2

2.5

3

0.01 0.1 1 10 100 1000 10000

Lf

Base PAP Dose (mrem/yr)

61.41

61.42
6-16

1 

2

3 Figure 6-3 Hypothetical Performance Assessment Model Output with the Minimization 

4 Parameters

Note: For Case1, the dose for the 

base case during the protective 

assurance period is below 25 mrem/yr 

for 10 CFR 61.41; therefore, 

minimization has been achieved
6-17

1 The alternatives evaluated under the minimization process should be limited to those that are 

2 currently technically feasible. It is not necessary to attempt to project the development of future 

3 practices. The cost estimates should reflect the uncertainties associated with the maturity of the 

4 technology by considering different ranges of effectiveness for a unit cost or a range of costs for 

5 a given effectiveness.

6 6.2.1.3 Other Decision Analyses

7

8 Other decision analyses techniques may be applied by licensees and regulators should review 

9 each type of analyses on a case-by-case basis. For instance, cost-benefit analyses are used to 

10 evaluate alternatives and facilitate decision-making. NRC has developed guidance for cost11 benefit analyses applied to environmental reviews (NRC, 2003d, Section 6.7) and waste 

12 incidental to reprocessing waste determinations (NRC, 2007a). However, direct application of 

13 cost-benefit analyses to the very long timeframes associated with LLW disposal is tenuous 

14 because discounting can be difficult, as well as accurately estimating the population surrounding 

15 a disposal site thousands of years into the future. In previous staff guidance, NRC has 

16 recommended that the monetary value associated with averted future doses should not be 

17 discounted in the analyses for application to 10 CFR Part 61 (NRC, 2000c).

18 6.2.1.4 Other Considerations

19

20 According to 10 CFR 61.50(a)(1), to the extent practicable, the disposal site shall be capable of 

21 being characterized, modeled, analyzed, and monitored. In addition, a licensee must account 

22 for uncertainty and variability in the performance assessment analyses. Over the protective 

23 assurance period, stability of the disposal site may be difficult to demonstrate if the disposal site 

24 is in an unfavorable location. In some circumstances, a licensee may not be able to adequately 

25 reflect uncertainties in the long-term technical analyses. In these cases, the licensee or 

26 regulator should impose inventory limits as a method to manage the uncertainties that may not 

27 be adequately assessed with technical analyses. Example 6.2 provides information on how 

28 other considerations can be used to ensure protection of public health and safety during the 

29 protective assurance period.

30
6-18

1

2

3

4

5

6

7

Example 6.2

The Radala disposal site is located in a semi-arid environment with a relatively stable 

present day environment. A license applicant proposes to dispose of a waste stream 

comprised of large quantities of long-lived waste in the disposal facility. The projected future 

environment is anticipated to be unstable due to geomorphological processes (e.g., high 

erosion associated with landform evolution). The performance assessment from the 

compliance period is used to estimate potential future doses during the protective assurance 

period. Projected doses to a member of the public are estimated to be approximately 100 

mrem/yr. The regulator reviews the analyses and determines that the licensee did not 

adequately incorporate the uncertainty in erosion rates driven by variability in future climate 

states. If uncertainty in erosion rates is considered, the projected future doses can be on 

the order of many thousands of mrem per year. In response to a request from the regulator, 

the license applicant evaluates different design alternatives and determines that none can 

be relied upon to significantly reduce the projected future doses or that the alternatives 

would be prohibitively expensive.

Conclusion: The regulator approves of the application with conditions. The disposal of 

either limited quantities or concentrations of the waste stream is approved. The regulator 

agrees with the licensee that no technologically or economically practical alternatives exist 

to prevent disturbance of the disposal facility by the geomorphological processes. The 

regulator examined other operating LLW disposal facilities and determined that other 

facilities have been analyzed for disposal of the waste stream. Other facilities, due to 

favorable site conditions for long-lived waste disposal, could dispose of the waste stream 

and are projected to result in a radiation dose to a member of the public of a few mrem per 

year. Considering the uncertainty and the fact that the license applicant cannot demonstrate 

long-term stability of the disposal site, the regulator determines that inventory limits should 

be developed. The regulator provides inventory limits as a license condition that limit the 

inventory to a fraction of the inventory originally proposed by the licensee. The licensee can 

determine whether it is economically viable to dispose of the waste stream at the inventory 

limits.
7-1

1 7.0 PERFORMANCE PERIOD ANALYSES

2

3 10 CFR 61.13(e) requires performance period analyses to:

4 (1) Assess how the disposal facility and site characteristics limit the potential long-term 

5 radiological impacts, consistent with available data and current scientific understanding. 

6

7 (2) Identify and describe the features of the design and site characteristics that will 

8 demonstrate that the performance objectives set forth in 10 CFR 61.41(c) and 

9 10 CFR 61.42(c) will be met. 

10 The primary purpose of the performance period analyses is to provide information that 

demonstrates that releases of long-lived1 11 radioactive waste from a disposal facility are 

12 minimized to the extent reasonably achievable. The protective assurance period is defined in 

13 the regulations as the period from the end of the compliance period through 10,000 years 

14 following closure of the site. The performance period is defined in the regulations to be the 

15 period of time following the protective assurance period. Performance period analyses are 

16 required only if the disposal facility is accepting long-lived waste that has disposal site-averaged 

17 concentrations of long-lived radionuclides greater than the values provided in Table A of 

18 10 CFR 61.13(e), or if necessitated by site-specific conditions. Table 7-1 lists long-lived 

19 isotopes that may be present in LLW inventories.

20

21 The disposal system consists of the disposal units, disposal site, land disposal facility, and 

22 surrounding environment. The assessment should evaluate natural and engineered 

23 characteristics of the disposal system and describe how those characteristics will reduce long24 term impacts. Over the long timeframes of regulatory concern, performance of the disposal 

25 system is likely to be driven by the features of the natural system rather than by man-made 

26 engineered barriers. The level of detail in the assessment should be risk-informed. The 

27 licensee should calculate the expected concentrations of long-lived waste remaining in the 

28 disposal site after the protective assurance period to risk-inform the longer-term performance 

29 period analyses. In general, the amount of resources and effort devoted to the assessment for 

30 the performance period will increase in proportion to the magnitude of the longer-lived 

31 radioactive waste inventory, considering both the initial inventory and ingrowth. Table 7-2 

32 provides examples of how performance period analyses may be risk-informed, taking into 

33 account the longevity of the inventory and the duration of the hazard it poses. 

 

1 Long-lived waste means waste containing radionuclides (1) where more than ten percent of the initial 

activity of a radionuclide remains after 10,000 years (e.g., long-lived parent), (2) where the peak activity 

from progeny occurs after 10,000 years (e.g., long-lived parent – short-lived progeny), or (3) where more 

than ten percent of the peak activity of a radionuclide (including progeny) within 10,000 years remains 

after 10,000 years (e.g., short-lived parent – long-lived progeny). The first part of the definition represents 

radionuclides with approximately a 3,000 year or longer half-life. Examples of isotopes that are shortlived but produce long-lived progeny are provided in Table 7-1 (e.g., Am-241, Cm-242).
7-2

Table 7-1 Long-lived Isotopes Potentially Present in LLW Performance Assessment Inventories

1 

Any isotope that is to be disposed of in sufficient quantities should be considered as part of the LLW PA inventory. Howev

“Yes” are expected to more commonly be a significant isotope in a LLW PA inventory based on past analyses as of the dat

progeny important for the radiological dose calculations should be considered in the technical analyses. For example, Rn-2

short-lived progeny of Ra-226. 

2 Only the first long-lived progeny encountered in decay chains are listed in this column.

Isotope Half-life 

(yr)

Long-lived LLW PA 

Inventory1 Isotope Half-life (yr)

Long-lived 

Parent Progeny2 Parent Pro

Al-26 7.17 x 105 X U-233 1.59 x 105 X T

C-14 5,730 X Yes U-234 2.45 x 105 X T

Cl-36 3.01 x 105 X Yes U-235 7.038 x 108 X P

K-40 1.3 x 109 X U-236 2.342 x 106 X T

Ni-59 7.5 x 104 X Yes U-238 4.468 x 109 U

Se-79 1.1 x 106 X Np-237 2.14 x 106 X U

Zr-93 1.53 x 106 X Pu-238 87.7 U

Nb-94 2.0 x 104 X Pu-239 2.41 x 104 X U

Tc-99 2.14 x 105 X Yes Pu-240 6.54 x 103 X U

Pd-107 6.56 x 106 X Pu-241 14.4 N

Sn-126 1 x 105 X Pu-242 3.76 x 105 X U

I-129 1.6 x 107 X Yes Pu-244 8.26 x 107 X P

Cs-135 3 x 106 X Am-241 432 N

Sm-146 1 x 108 X Am-242m 16 hr U

Pm-147 2.62 Sm-147 Am-243 7.38 x 103 X P

Sm-147 1.06 x 1011 X Cm-242 0.446 U

Eu-152 13.3 Gd-152 Cm-243 28.5 Am

Gd-152 1.08 x 1014 X Cm-244 18.1 P

Ra-226 1,600 X Yes Cm-245 8.5 x103 X N

Th-229 7.3 x 103 X Yes Cm-247 1.56 x 107 X Am

Th-230 7.7 x 104 X Ra-226 Yes Cm-248 3.39 x 105 X P

Th-232 1.41 x 1010 X Yes Cf-249 351 Cm

Pa-231 3.28 x 104 X Cf-251 898 Am

U-233 1.59 x 105 X Th-229 Yes Cf-252 2.64 Cm
7-3

1 Table 7-2 Expected Level of Review Effort Associated With Performance Period 

2 Analyses

Radiation Hazard 

And Duration

Level Of 

Review Effort

Example

Short-lived, any 

concentrations

NA Long-term performance period analyses are 

not necessary.

Short-lived and low 

concentrations of 

long-lived or limited 

quantities of 

concentrated 

long-lived

Low Using undisturbed concentrations during the 

performance period, provide analyses 

showing that the dose to inadvertent intruders 

meets the compliance period performance 

objective (e.g., assume no dilution and 

perform an intruder assessment).

Moderate 

concentrations of 

long-lived or 

moderate quantities 

of concentrated 

long-lived

Moderate Provide analyses showing that the disposal 

system will limit releases from natural 

processes and plausible disruptive events1

. 

Estimate the range of doses that may result to 

intruders and members of the public and 

demonstrate that they are minimized to the 

extent reasonably achievable2

. Include 

uncertainty and variability. Formal peer 

review of the analyses and results should be 

considered.

High-concentrations 

and quantities of

long-lived

High Provide analyses showing that the disposal 

system will limit releases from natural 

processes and plausible disruptive events. 

Estimate the range of doses that may result to 

members of the public and demonstrate that 

they are minimized to the extent reasonably 

achievable. Include uncertainty and 

variability. Support for the range of impacts 

should include model support, such as that 

derived from natural analogs of long-term site 

evolution. Independent, formal peer review of 

the analyses, results, and model support 

should be performed.

1 3 Discussed in Section 7.3

2 4 Discussed in Section 7.4.1.1.3

5

6
7-4

1 Licensees should provide model support for the performance period analyses; however, that 

2 support will likely be less quantitative and involve more expert judgment than model support that 

3 would be required for the compliance period and the protective assurance period. Table 7-2 

4 shows how the performance period analyses may be risk-informed by licensees. Review 

5 methods other than those suggested by the examples in the table may be suitable. The level of 

6 review effort should be higher when risks for the performance period are larger. If at all 

7 possible, simple, conservative analyses should be used, especially when projected risks are 

8 low. For higher hazard and longer-lived wastes, expected scenarios as well as less likely, but 

9 plausible, disruptive scenarios (discussed in Section 7.3) should be addressed in the analyses. 

10 Licensees should provide model support, as discussed in Section 2.2.3, especially for higher 

11 hazard and longer-lived wastes. In order to determine what hazard is posed, it may be useful 

12 for licensees to estimate the doses to intruders and public receptors with conservative scenarios 

13 and compare those estimates to their expected scenarios. A hazard may or may not translate 

14 into risk, but high hazard problems should have more support and independent review relative 

15 to low hazard problems.

16

17 The NRC staff developed the concentration values provided in 10 CFR 61.13(e) as a risk18 informed screening device to help determine if performance period analyses are necessary for a 

19 particular disposal action. The challenge associated with developing criteria for when to 

20 perform the performance period analyses, such as these concentration values, is that there are 

21 many variables that can influence the magnitude of the impact from the disposal of a particular 

22 type of waste. Site-specific design and hydrogeology as well as potential disruptive processes 

23 and events will influence which pathways are important and the magnitude of risk associated 

24 with the different pathways. In most circumstances, the variability in the analyses results will be 

25 larger for calculations supporting demonstration of compliance with 10 CFR 61.41 than those 

26 associated with 10 CFR 61.42. 

27

28 A key driver of variability in the risk associated with long-lived, mobile isotopes is variability in 

29 hydrogeology. The disposal of wastes containing long-lived isotopes near or at the Class A 

30 limits can result in drinking water doses that exceed 0.25 mSv/yr (25 mrem/yr) at certain sites. 

31 Depending on the site-specific hydrogeology, these water pathway impacts may occur during 

32 the compliance period, during the protective assurance period, or during the performance 

period. For example, it was previously found that the isotopes 129I, 99Tc, 36Cl, and 14 33 C are most 

34 problematic because of their relatively high mobility in the environment (NRC, 1982b, p. 5-43). 

35 Though the drinking water pathway is discussed in the text above, the concept is not limited to 

36 the drinking water pathway. Various site-specific conditions can drive the risk from waste 

37 disposal at a site even for waste at the Class A limits (i.e., waste that is generally perceived by 

38 most licensees and stakeholders to be fairly benign). The assumption that disposal of Class A 

39 waste is inherently compliant with the performance objectives may not always be correct, 

40 particularly for waste classified using 10 CFR 61.55(a)(6).

41

42 Some sites, designs, and waste streams may require performance period analyses even though 

43 the average concentrations of long-lived radionuclides are below those provided in 

44 10 CFR 61.13(e). The types of site-specific conditions that could trigger the need to conduct 

45 performance period analyses include, but are not limited to:

46

47 • Limited dilution or dispersion (e.g., sites with high infiltration and low groundwater

48 velocities) at a site with a potable groundwater pathway and long travel times.
7-5

1 • Highly-soluble wasteforms combined with resistive engineered barriers that fail 

2 discretely.

3 • Ingrowth of progeny that increases the radiotoxicity of the waste significantly during the 

4 performance period compared to the radiotoxicity at the end of the compliance period.

5 • Erosion rates and engineered barrier performance that result in limited protective cover 

6 remaining over the waste during the performance period.

7 • Gaseous releases from the ingrowth of radon being a significant exposure pathway 

8 during the performance period.

9 The importance of the conditions described above is conditional on the timing (i.e., delay) of a 

10 potential radioactive release. However, delay by itself is generally not enough to result in a 

11 significant impact during the performance period from waste disposal at the Class A limits. One 

12 or more of the conditions listed above must also be present. Although the travel time concept 

13 presented here is focused on the time from release to exposure of the public, it is not limited to 

14 hydrologic transport through groundwater. Transport through surface water or the atmosphere

15 could also be considered though the travel times through those pathways are generally much 

16 shorter. Performance period analyses are required when the isotopic concentrations exceed 

17 the values specified in 10 CFR 61.13(e), or if site-specific conditions warrant confirmation that 

18 the performance objectives will be met (see Example 7.1).

19 7.1 Estimation of Disposal Site-Averaged Isotopic Concentrations

20

21 Table 7-3 in this document is the same as Table A in 10 CFR 61.13(e). It provides the 

22 concentrations values that a licensee must use to determine if performance period analyses are 

23 necessary for their proposed disposal action. These concentration values, for radionuclides 

24 other than the long-lived alpha-emitting non-transuranic isotopes, are the Class A waste 

25 concentrations provided in Table 1 of 10 CFR 61.55. Long-lived alpha-emitting non-transuranic 

26 isotopes are included at the same concentrations as the long-lived transuranic isotopes. During 

27 the original development of the 10 CFR 61.55 waste classification tables, long-lived alpha28 emitting non-transuranic isotopes were not included because it was expected that LLW would 

29 not contain those isotopes in sufficient quantities and concentrations to impact public health and 

30 safety from their disposal (NRC, 1982b). However, there is no compelling reason for the long31 lived non-transuranic isotopes to be treated differently in the technical analyses if both 

32 transuranic and non-transuranic isotopes are included in the wastes proposed for disposal. The 

33 radiological risk will be determined in part by the dose conversion factors of individual isotopes 

34 and the concentration of those isotopes. There is variability in dose conversion factors from 

35 isotope to isotope (EPA, 1988). NRC decided to reduce this variability when deriving the 

36 10 nanocuries per gram (nCi/g) concentration value for all transuranic isotopes in Class A waste

37 (NRC, 1982b). The dose conversion factors for non-transuranic isotopes are generally 

38 comparable to the transuranic isotopes, and NRC believes it is appropriate to simplify the 

39 consideration of variability. The concentrations provided in Table A of 10 CFR 61.13(e) are only 

40 to determine if performance period analyses are necessary.
7-6

1

2

3 A licensee should estimate the disposal site-averaged isotopic concentrations of radionuclides 

4 to determine if performance period analyses are necessary. Disposal site-averaged 

5 concentrations can include the volume of the waste, uncontaminated materials used to stabilize 

6 waste or reduce void space within waste packages, the volume of uncontaminated materials 

7 placed within the disposal cells, and the volume of engineered or natural materials used to 

8 construct the disposal cells. For the purpose of determining if performance period analyses are 

9 necessary, the licensee should base the disposal site-averaged concentrations on the total 

10 amount of waste averaged over the total volume of all disposal cells. Though the disposal site 

11 definition includes a buffer zone, the buffer zone should not be included in the calculations. For 

12 radionuclides where the concentration shown in Table 7-3 is based on mass and not volume, 

13 the licensee can use the total mass of the different materials within the disposal cells.

14

15 A reviewer should consider the variability in radionuclide concentrations over the disposal site. 

16 The significance of variability will need to be interpreted in the context of the particular technical 

17 analysis. For the purposes of determining if performance period analyses should be performed 

18 by a licensee, the variability in concentrations of radionuclides over the disposal site should 

19 translate into a significant change in a performance metric (e.g., dose, environmental 

20 concentration, flux rate to the environment). Some metrics will be more sensitive to inter-site 

21

22

Example 7.1 

A disposal site is located in a humid, oxidizing environment with a shallow water table and 

potable groundwater. Infiltration that will flow through the waste to the saturated zone is 

not expected to experience significant dilution. The groundwater flow velocity is relatively 

slow, such that most of the discharge from the aquifer is balanced by recharge from 

infiltration. In order to control infiltration, the licensee intends to use geomembranes with a 

design-life of approximately 1,500 years. The water table is located within a geologic unit 

that is mostly clay with good properties with respect to slowing radionuclide transport. The 

waste streams being disposed of are dominated by Tc-99 and material contaminated with 

soluble forms of DU.

Conclusion: The licensee develops waste acceptance criteria that specify isotopic 

concentrations for anticipated waste streams such that the sum of fractions on a disposal 

site-averaged basis is ≤ 0.5 of the Table A values in 10 CFR 61.13(e), thereby limiting the 

types of waste that can be received to concentrations less than the Table A values in 

10 CFR 61.13(e). The licensee develops compliance period calculations that show the 

performance objectives are likely to be met for the next 10,000 years, primarily as a result 

of the long travel times from the waste to a potential receptor location. Their intruder 

assessment shows that potential impacts to intruders are well within the established limits. 

Because the site has a number of characteristics that could result in significant 

performance period impacts (e.g., resistive engineered barriers, soluble waste streams, 

limited dilution, waste with ingrowth of progeny, and long travel times), the licensee 

develops performance period analyses to demonstrate that 10 CFR 61.41(c) is met. The 

licensee chooses to extend the compliance period analyses with conservative parameters 

to provide a comparison of the estimated impacts during the performance period with those 

for the compliance period.
7-7

1 variability in radionuclide concentrations than others, as a result of different averaging volumes 

2 when calculating the metric. For instance, the concentrations in an aquifer may have more 

3 variability compared to a drinking water dose metric because the aquifer concentrations are 

4 averaged over some volume, due to extraction of the aquifer water in a well, in order to 

5 calculate the drinking water dose (i.e., wellbore dilution).

6

7 Most disposal facilities are expected to dispose of waste containing a mixture of different 

8 isotopes. In order to determine if performance period analyses are necessary, a sum of 

9 fractions approach must be used, as required by 10 CFR 61.13(e). Licensees should estimate 

10 the disposal site-averaged concentrations of each isotope. They should divide the resultant 

11 values by the concentrations found in Table 7-3 to estimate a fraction for each isotope, and then 

12 sum the fractions over all isotopes. If the total is greater than 1.0, then licensees are required to 

13 develop performance period analyses (10 CFR 61.13(e)). Examples 7.2 and 7.3 provide sum of 

14 fractions (SOF) calculations.

15

16 Table 7-3 Disposal Site-Averaged Isotopic Concentrations that Require 

17 Performance Period Analyses

Radionuclide Concentration (Ci/m3

)

C-14 0.8

C-14 in activated metal 8

Ni-59 in activated metal 22

Nb-94 in activated metal 0.02

Tc-99 0.3

I-129 0.008

Long-lived alpha-emitting nuclides 1 10

Pu-241 1 350

Cm-242 1 2,000

1 18 Units are nCi/g

19

20 7.2 Disposal Site Characteristics that Enhance Long-Term Isolation

21

22 The remainder of this section focuses on the long-term analyses that licensees should 

23 complete. The regulatory requirements are not prescriptive with respect to the type of analyses 

24 that must be performed. Licensees may use any analyses (e.g., screening, quantitative 

25 probabilistic) considered sufficient to demonstrate that the regulatory requirements will be met. 

26 However, there are limits to the analyses of projected performance over very long timeframes 

27 (e.g., tens of thousands of years) and how much confidence a licensee should place in the 

28 results of the analyses. Disposal sites and near-surface disposal facility designs that have more 

29 disposal site characteristics that enhance long-term isolation may be more likely to achieve 

30 long-term isolation of waste from the accessible environment if they demonstrate one or more of 

31 the characteristics cited in Table 7-4.

32

33
7-8

1

2

3 Table 7-4 describes the characteristics that will enhance isolation at most, but not all, disposal 

4 sites. Individual characteristics may not apply at a specific site. For example, low porosity in a 

5 cementitious wasteform generally reduces the potential for subsidence and reduces leaching. 

6 However, use of a low porosity cementitious wasteform in a cold climate may result in freeze7 thaw damage that could contribute to release over the long-term. While use of robust, low8 porosity wasteforms is generally favorable, in this specific example it may not be. Some of the 

9 characteristics listed in Table 7-4 apply to the design of disposal facilities while others apply to 

10 the site characteristics.

11 7.3 Scope of the Performance Period Analyses

12

13 The performance period analyses should provide information about the performance of the 

14 disposal system under a range of conditions that represent expected scenarios, as well as less 

15 likely, but plausible, scenarios that may have significant consequences. Licensees should 

16 consider the range of conditions consistent with the site suitability analysis (described in 

17 Section 5.0), including the FEPs analysis (Section 2.0). Less likely but plausible scenarios 

18 include those that are unlikely to be observed (e.g., as low as a 10 percent chance of 

19 occurrence over the analyses timeframe), as well as those that are expected to be observed 

20 over the analysis timeframe. Performance period analyses for various sites may have different 

21 timeframes associated with them owing to differences in radionuclide inventories as well as 

differences in geologic settings. Therefore, a single event frequency (i.e., 10-5 22 /yr) cannot be 

23 defined for the purposes of this particular regulatory requirement.

24

Example 7.2

The total volume of disposal cells for existing waste is 400,000 m3

. The inventory of waste 

located in the facility is comprised of: 50,000 m3 of C-14 containing waste at 0.2 Ci/m3

, 200,000 

m3 of waste containing C-14 at 0.1 Ci/m3 and I-129 at 0.002 Ci/m3

, and 50,000 m3 of Tc-99 

containing waste at 0.01 Ci/m3

. The uncontaminated fill and material used to construct the 

cells represents 100,000 m3

.

Conclusion: The licensee uses the values in Table A to calculate the volume-averaged SOF 

per the following equation. This equation is used to calculate the SOF for n waste streams 

containing m isotopes. V is the volume, C is the concentration on a volumetric basis, and CA

is the value from Table A for the particular isotope.

Because the SOF is less than 1, performance period analyses are not required.
7-9

1

2

Example 7.3

The licensee in Example 7.2 would like to dispose of a new waste stream but they are unsure 

if they would need to develop performance period analyses. 

The new waste stream is a particulate waste. The raw particulate waste contains average 

concentrations of 0.7 Ci/m3 Tc-99, 1.2 Ci/m3 C-14, and 90 nCi/g of long-lived, alpha-emitting 

radionuclides. The total volume of raw waste is 100,000 m3

. In order to reduce the potential 

for dispersion, the particulate waste is solidified in grout prior to disposal. The ratio of grout 

to waste inside the disposal package is 3 to 1 (stabilizer ratio). Because the density of the 

grout and this particular waste stream are similar, the 3 to 1 ratio holds for both a mass and 

volume basis.

The drums of waste will be stacked within the disposal cells achieving a disposal cell packing 

efficiency of 67% (i.e., the ratio of cell volume occupied by waste packages to the total 

internal volume of the cell – Cell Eff.). In addition, the disposal cells are constructed of a 

variety of natural materials to provide structural stability, to reduce water inflow to the waste, 

and to provide chemical retention of the waste. The volume of material comprising the 

disposal cells is 30% of the total internal cell volume available for disposal (i.e., Inert frac.).

Conclusion: First the licensee calculates the cell volume required for the new waste, VTn:

Next the licensee calculates the SOF for the new waste stream (SOFn) containing m

isotopes, where Cm, V, and CAm are the concentration, volume, and long-lived waste 

concentration (for isotope m from Table A of 10 CFR 61.13(e)). Only one new waste stream 

is considered, so the equation from Example 6.2 is simplified:

Then the licensee calculates the average sum of fractions for both old and new waste where 

VT is the combined volume of old and new waste (total disposal cell volumes) and SOFo is 

the sum of fractions for the old waste:

In this case the SOF is greater than 1 on a disposal site-averaged basis when the new waste 

stream is combined with the existing waste stream. Therefore, performance period analyses 

are required to determine if the new waste stream can be safely disposed of in the facility.
7-10

1 Table 7-4 Disposal Site Characteristics that Could Enhance Long-Term Isolation

Characteristic1 Description

Simple, passive designs Simple designs are less likely to experience 

unforeseen failure mechanisms; passive designs do 

not rely on active monitoring and maintenance

Designs that mimic natural features Stable natural features may provide indication of 

design characteristics that may help achieve longterm isolation

Low relief designs Designs with low relief (e.g., buried) will experience 

lower rates of erosion

Low water contact with waste Release and instability are generally associated with 

mass transfer. Limited water contact reduces rates of 

aqueous phase mass transfer

Robust, low-porosity wasteforms Durable, low-porosity wasteforms enhance long-term 

stability by limiting consolidation and subsidence

Geochemical compatibility of the waste 

and disposal environment

Waste that is geochemically compatible with the 

disposal environment is less likely to experience 

significant release into the environment

Stable disposal environment conditions Physically and chemically unstable environmental 

conditions contribute to long-term instability. For 

example, leaching from waste can be highest in zones 

of water table fluctuation

Accreting environments Disposal systems that are gaining mass over time 

contribute to waste isolation by working with the 

natural processes instead of against them

Large distance to water table and 

homogeneous natural materials

The unsaturated zone can provide a significant barrier 

to releases to an aquifer, especially if the natural 

materials are relatively uniform which contributes to 

confidence in the performance of sorptive materials

Deep disposal Many disruptive processes are more dynamic, 

complex, and more likely for shallow disposal 

compared to deeper disposal

Limited natural resources A disposal system with limited natural resources 

decreases the likelihood of anthropogenic processes 

or events impacting the disposal facility or site

Stable climate Disposal systems located in a more stable climate are 

less likely to experience impacts from climate 

variation

Low frequency of geologic and tectonic 

events

Over the long term, disposal systems that are located 

in areas of low geologic and tectonic activity are more 

likely to achieve waste isolation from the environment

 1 2 Highlighted characteristics are associated with 10 CFR 61.50 site suitability characteristics

3
7-11

1 FEPs defining the natural and engineered systems as well as assumptions about future human 

2 behavior will be needed for the performance period analyses. The challenge for performance 

3 period analyses is to provide credible assessments of the future evolution of the disposal 

4 system while avoiding open-ended speculation. Near-surface disposal introduces specific 

5 challenges over the long-term because environmental processes can have complex, dynamic, 

6 and nonlinear responses. The NRC staff believes that the approach recommended in the 

7 following sections is suitable for defining the scope of the performance period analyses for near8 surface disposal. The goal of the long-term analyses is to understand the safety implications of 

9 the type of waste being disposed in the near surface and not to precisely estimate the future 

10 evolution of the surface of the earth. The goal is to provide a perspective on how the hazard 

11 may evolve over time (e.g., persistence of long-lived radionuclides and potential for ingrowth of 

12 risk significant daughters) and implications for near surface disposal.

13

14 Disposal of high-specific activity waste, if improperly managed by a licensee, poses the greatest 

15 radiological risk to public health and safety. It would be very difficult for a licensee to 

16 demonstrate with reasonable assurance that public health and safety is protected from the 

17 disposal of the high-specific activity waste at a site with unfavorable site characteristics because 

18 the margin for error is small. For example, accidental release of a relatively small quantity of Sr19 90 into an aquifer at the West Valley Demonstration Project resulted in a significant ground 

20 water plume requiring remediation (NYSDEC, 2008). Only a small amount of high-specific 

21 activity waste released into the environment can cause significant problems. For the 

22 performance period, the margin for error is not as small because the high-specific activity 

23 fraction of the waste has decayed. The specific activity of the material remaining in the disposal 

24 site is much lower compared to the waste when first disposed. Furthermore, because of the 

25 long timeframes involved, a licensee may consider the performance objectives in Subpart C 

26 when evaluating whether their site meets the site suitability requirements in 10 CFR 61.50 for 

27 the performance period. For example, a disposal site might have some projected seismic 

28 activity (10 CFR 61.50(a)(4)(iii)) sometime after the 10,000-year protective assurance period. 

29 However, because of the uncertainty with events at long timeframes, future seismic activity

30 would only disqualify the site if the licensee was unable to demonstrate that the performance 

31 objectives would be met assuming this seismic activity. Therefore, it is acceptable for the 

32 performance period for a licensee to evaluate the significance of the site characteristics using 

33 technical analyses. 

34 7.3.1 Features, Events, and Processes

35

36 The objective of the performance period analyses is to provide information to decision-makers 

37 about disposal system performance under various scenarios. Licensees should assess the 

38 uncertainties, because they are likely to be large, and present the results of the assessment 

39 using a balanced approach. Reviewers should not hold the performance period analyses to a 

40 level of proof that is not attainable. In comparison with the compliance period, the performance 

41 period analyses will be more susceptible to bias because objective supporting information will 

42 be more limited.

43

44 Different near-surface LLW disposal facilities may have significantly different characteristics and 

45 may contain different wastes. The FEPs for one disposal site may be substantially different 

46 from those at a different site. Identification of the FEPs relevant to the performance period will 

47 be site-specific. Section 2.5 describes the FEP process that may be used by a licensee to 

48 develop the scope of the technical analyses (e.g., performance assessment). This section of 
7-12

1 the guidance document does not reiterate the general information relevant to FEPs analysis 

2 found in Section 2.5.

3

4 A licensee may extend compliance period or protective assurance calculations without 

5 modification provided that the calculations are complete with respect to including key FEPs

6 relevant to the performance period. The compliance or protective assurance period calculations 

7 may not be complete with respect to the scope of the performance period analyses. It will be 

8 necessary for the licensee to communicate the additional uncertainties associated with events 

9 and processes that may occur in the long-term performance period if they are not represented in 

10 the compliance or protective assurance period analyses. The analyses that are developed for 

11 the compliance period or protective assurance period may not be sufficient for the performance 

12 period analyses if (1) disruptive processes are expected to occur during the performance period

13 that have not been included in the compliance period or protective assurance period analyses,

14 or (2) if the cumulative impact from repetitive events over the longer timeframes is not included 

15 and the repetition of those processes and events could lead to significant impacts. It is 

16 appropriate for a licensee to consider potentially beneficial natural processes (from a risk 

17 reduction perspective) such as dispersion and dilution in addition to detrimental processes. In 

18 general, the greater the geological and geomorphological stability a potential disposal site 

19 possesses, the greater the likelihood that FEPs that may occur in the performance period will 

20 have already been represented in the licensee’s compliance period or protective assurance 

21 period analyses. However, the representation of a particular FEP in the compliance period or 

22 protective assurance period analyses may be different in the performance period analyses. 

23 Even if the same set of FEPs may be appropriate for all analyses, they may be represented 

24 differently in each analysis.

25

26 As discussed in Section 2.5.3.1, 10 CFR Part 61.50 provides the disposal site suitability 

27 requirements for the land disposal of LLW. The process to determine if some of these criteria

28 will be met is complementary to the FEPs process (discussed in Section 7.3.2). The criteria

29 from 10 CFR Part 61.50 that best lend themselves to FEPs analysis are 

30 10 CFR 61.50(a)(2)(i-iv) and 10 CFR 61.50(a)(4)(ii-iv). Some of the 10 CFR 61.50 regulatory 

31 requirements list FEPs that a disposal facility must have (e.g., sufficient depth to water table), 

32 whereas other requirements list FEPs or conditions that a disposal facility must not have (e.g., 

33 exploitable natural resources). Since these are regulatory requirements, the FEPs process for 

34 the performance period should analyze the FEPs related to 10 CFR 61.50(a)(2)(i-iv) and 

35 10 CFR 61.50(a)(4)(ii-iv). 

36

37 Many potential scenarios involving certain forms of flooding, landslides, earthquakes, and 

38 volcanoes will not be evaluated in the performance period. Potential sites containing FEPs that 

39 occur with such frequency and extent to significantly affect the ability of the disposal site to meet 

40 the performance objectives of 10 CFR Part 61 Subpart C for the compliance period would not 

41 be considered for LLW disposal. These scenarios may also preclude defensible modeling and 

42 prediction of long-term impacts.

43

44 The number of possible scenarios that can be developed will be reduced after screening 

45 potential disposal systems based on the site suitability FEPs. However, FEPs that have been 

46 screened from further consideration in the compliance period or protective assurance period

47 may not be able to be screened from further consideration for the performance period analyses. 

48 For example, the rate of erosion may be estimated to be sufficiently low over the compliance 

49 period, and as a result, the FEP of erosion is not within the scope of the compliance period 
7-13

1 analyses. However, over the analyses timeframe for the performance period the rate of erosion 

2 may be significant such that the FEP should be included within the scope of the performance 

3 period analyses. Therefore, the compliance period and protective assurance period FEP 

4 processes will reduce the FEPs applicable to the performance period but some of the FEPs 

5 eliminated in the compliance period or protective assurance period may apply for the 

6 performance period. 

7 7.3.2 Screening of Features, Events, and Processes Based on the Requirements 

8 in 10 CFR 61.50 

9

10 A licensee should complete the FEP screening and scenario development process for the 

11 performance period in a risk-informed manner. Whereas some of the site characteristics for the 

12 compliance period are either required to be present or to be absent (i.e., hydrological site 

13 characteristics for 500 years) because they are precursors of poor long-term performance, all of 

14 the site suitability characteristics for the performance period may be evaluated considering 

15 radiological risk. 

16

17 Appendix B presents hazard maps that the NRC staff created related to the features and 

18 phenomena of the 10 CFR 61.50 criteria. The hazard maps provide an illustration of the FEPs

19 associated with the site suitability requirements. The maps cannot be displayed in this 

20 document at sufficient size to be used to determine if any specific location would be impacted by 

21 one of these phenomena. The figures only provide an illustration of potentially impacted areas. 

22

23 Regulators should not use the hazard maps in Appendix B to prohibit disposal because the 

24 resolution of the maps and the precision and accuracy of the techniques used to generate them 

25 may not be sufficient for site-specific evaluations. However, regulators should use the maps to 

26 determine when greater review effort and more technical basis should be expected for a 

27 licensee’s site-specific evaluation. In addition, the data used to produce these maps could be 

28 used, via Geographic Information System (GIS) software, to perform screening-level FEPs 

29 analyses. However, the data used to produce these figures in Appendix A were not collected at 

30 a scale of resolution sufficient to perform detailed site-specific evaluations by either regulators 

31 or applicants. 

32

33 For the performance period analyses, FEPs screening based on the requirements in 

34 10 CFR 61.50 does not need to be resource intensive relative to the other steps in completing 

35 the technical analyses. Background information and knowledge of the geologic history of the 

36 site can be used by qualified specialist(s) to evaluate the likelihood of a FEP being present at a

37 potential disposal site during the performance period. This evaluation is a qualitative exercise 

38 using information on the past history of a disposal system. The phenomena and features that 

39 have occurred there in the past can be used to make judgments about the future and to help 

40 make screening determinations. For example, a disposal system may be near a current 

41 floodplain but outside of the influence of its associated processes. A qualified specialist may 

42 examine the location, nearby topography, and the past history and events, and be able to 

43 provide information on the potential for future floodplain formation at the disposal site. With 

44 diverse information drawn together from various sources, a licensee may be able to create a 

45 sufficient technical basis that supports the exclusion of a floodplain or a near-surface water table 

46 forming at or near the proposed disposal site for at least 10,000 years. However, for the 

47 performance period, information from the disposal system may point to renewed flooding of the 
7-14

1 area or it may simply be insufficient to form a basis to screen the process out. In such a case, 

2 the licensee would include floodplain formation in the assessment for the performance period.

3 7.3.3 Future Human Behavior

4

5 FEPs that describe future human behavior in the performance period should be consistent with 

6 present knowledge of the conditions in the region surrounding the disposal system. It is not 

7 necessary for a licensee to project changes in society, changes in human biology, or increases 

8 or decreases in human knowledge or technology. The selection of receptor scenarios and 

9 exposure pathways in the performance period analyses should be limited to the consideration of 

10 the natural variability in conditions, processes, and events. 

11

12 Over long timeframes, the future environmental conditions may be significantly different from the 

13 present day. If shorter term analyses limited the behavior, characteristics, and pathways of 

14 receptors based on present-day conditions, the licensee should assess the impact from

15 significant changes in climatic or environmental conditions using stylized receptor scenarios 

16 (see Sections 2.2.4.1 and 2.2.4.2). As necessary, the behavior of potential receptors over the 

17 performance period should be modified using present-day climatic analog locations. For 

18 example, ground water may not be potable in the present day, but the long-term analyses could 

19 examine whether that site characteristic is robust under future climate states. If licensees were 

20 to provide a side-by-side comparison of the assumed characteristics of receptors (e.g.,

21 pathways, consumption rates, and exposure times) with those of generic “screening” receptors 

22 used in the dose analyses, the reviewer could determine the importance of the assumed 

23 receptor characteristics. This sort of comparison is strongly recommended. Generic receptor 

24 characteristics are found in a variety of documents (NRC, 1992; NRC, 1981a; NRC, 1982b). 

25 This type of comparison is also good practice for compliance period and protective assurance 

26 period assessments. Estimates of future disposal site performance that are primarily based on 

27 the engineered design, waste characteristics, and site characteristics are likely to be less 

28 speculative than estimates that rely on assumptions about future human behavior.

29 7.4 Analyses for Long-Lived Waste

30

31 The goal of the performance period analyses is to demonstrate how the disposal facility has 

32 been sited and designed to minimize long-term impacts and to provide an indication of the 

33 potential long-term performance. The term “analyses” is used here to describe different types of 

34 evaluations that may be performed, some of which may be quantitative and others that may be 

35 more qualitative. A description of barrier capabilities and design considerations, in addition to a 

36 conservative screening evaluation, may be sufficient for lower-risk systems (e.g., limited 

37 quantities and low concentrations of long-lived waste). Quantitative risk assessment of long38 term performance may be necessary for higher-risk systems (e.g., large quantities of 

39 concentrated, long-lived waste). Figure 7-1 is a diagram of the recommended analyses 

40 approach for the performance period.
7-15

1

2

3

4 Figure 7-1 Recommended Approach to Conducting Performance Period Analysis

5
7-16

1 The analyses for the performance period are not intended to be a prediction concerning future 

2 system states of the LLW disposal system. The analyses associated with the performance 

3 period should represent a credible technical effort using available data and current scientific 

4 understanding to assess the long-term performance of the waste disposal facility, including 

5 consideration of uncertainties.

6

7 Long-term analyses may have limited data available or data that is highly variable spatially and 

8 temporally. Licensees should consider two types of data representation in their assessment of 

9 long-term impacts: expected values (e.g., central tendency) and bounding values. Use of 

10 expected values, such as the median, can convey the most likely outcome. When expected 

11 value calculations are complemented with bounding value calculations, the potential impact of 

12 uncertainty on the expected outcome can be conveyed. For the expected value calculations to 

13 be useful, the expected values must be representative of the site-specific conditions and 

14 features. Expected value calculations must have adequate supporting information to be of utility 

15 to regulators and other stakeholders. Bounding values may be necessary when data are not 

16 available or are very sparse, or formal expert judgment may be necessary. 

17

18 Although the use of bounding values may be necessary, the NRC staff recommends that a 

19 licensee use caution in the use of bounding values for performance assessment or other 

20 analyses used to assess long-term performance. Many environmental systems can have 

21 complex and nonlinear responses, making selection of a bounding value for a specific 

22 parameter challenging, if not intractable. An appropriate bounding value may not be intuitive 

23 and it may only be bounding locally and not globally. In addition, the compound effect of 

24 selecting numerous bounding values in the analyses can result in non-physical results. For 

25 example, it would be unreasonable for a licensee to evaluate a high-energy disruptive process 

26 that destroys the engineered barriers of the waste disposal facility without resultant dispersion 

27 and dilution of the waste. Even in hypothetical bounding calculations, it is useful to provide 

28 some context for the reasonableness of the calculations.

29 7.4.1 Types of Analyses

30

31 The analyses for the performance period may be quantitative, semiquantitative or qualitative in 

32 nature, depending on the waste characteristics or other factors. As discussed previously, the 

33 performance period analyses should be risk-informed. A number of approaches are acceptable 

34 for providing information on long-term performance. The NRC staff recommends that licensees 

35 perform two sets of analyses for the performance period: 

36

37 • Performance analyses to demonstrate that releases from disposal of long-lived waste 

38 will be minimized to the extent reasonably achievable for the performance period

39 • Barrier analyses to understand the performance of engineered and natural barriers

40 7.4.1.1 Performance Analyses

41

42 The NRC staff recommendation for conducting performance period analyses is to first employ 

43 simple, conservative screening analyses. The screening analyses may identify that the 

44 radiological risks are acceptable or that allowable limits (see Section 9.0) or other controls may 

45 be necessary. If the results of the screening analyses are not acceptable, a licensee may limit 

46 disposal of certain types of waste. In addition, quantitative performance period analyses may 
7-17

be performed to determine if the expected radiological risks 2 1 to the public from the disposal 

2 action are acceptable. The results of refined performance period analyses may demonstrate 

3 that the radiological risks are acceptable whereas the results of the conservative, screening 

4 analyses may not.

5 7.4.1.1.1 Screening Analyses

6

7 The recommended first step for performance period analyses is to perform simple, conservative 

8 screening analyses. The benefit of screening analyses is that they are relatively easy to 

9 perform and document, therefore, they are easier for stakeholders to review and understand, 

10 often facilitating decision-making. Screening analyses will be significantly less resource 

11 intensive for a licensee compared to full probabilistic multi-physics simulations. Screening 

12 performed with conservative parameters and calculations are not radiological risk calculations 

13 and should not be interpreted as such. They should be clearly described as hypothetical and 

14 pessimistic with the objective of identifying whether the potential for unacceptable radiological 

15 risk to a member of the public and inadvertent intruder in the performance period exists. Many 

16 beneficial features, processes, and characteristics may be purposely ignored in the calculations. 

17 If the estimated doses to the public and intruder from the screening analyses are below the 

18 limits provided in the 10 CFR 61.41(a) and 10 CFR 61.42(a) performance objectives, additional 

19 performance period analyses are not necessary.

20

21 The NRC staff cannot determine a priori the appropriate conservative screening analyses for all 

22 potential designs, waste streams, and disposal sites. Conservatism may be defined differently 

23 for different decisions. However, for all sites, licensees should provide and reviewers should 

24 evaluate:

25

26 • a list of the potential radiation exposure pathways to the public and intruder from 

27 disposed waste

28 • a description of the pathways expected to be most significant for releases

29 • the technical basis for the conservatism of the screening analyses

30 • a discussion of how the parameterization and representation of the screening analyses 

31 has accounted for uncertainty and variability

32 • a description of the barriers and processes that reduce or mitigate releases

33 For many disposal sites, a potable groundwater pathway will be a primary pathway for releases 

34 to the environment. A conservative screening analysis for the groundwater pathway would be 

35 one for which all waste inventory is available for release, solubility limits are not applied or are 

36 set at conservative values, and delays due to sorption during transport are eliminated by setting 

37 distribution coefficients to zero or very small values. However, the physical limitations on mass 

38 transport processes would still be included in waste release modeling. For example, if a waste 

39 container had a pore volume of X and the volumetric flow rate into the container was a small 

40 fraction of X per unit time, the exchange process should still be included in the simulation. In 

 2 Other metrics such as fluxes of radionuclides in the environment or concentrations of radionuclides in 

the environment may also be used. However, radiological doses are used in the compliance period and 

provide an apples-to-apples comparison for regulatory analysis.
7-18

1 addition, the dilution during transport arising from the geometry of the waste, hydrogeological 

2 system, and infiltration processes should be included in the screening assessment. 

3 7.4.1.1.2 Quantitative Analyses

4

5 If screening analyses have been performed and the projected results are not acceptable (see 

6 Section 7.4.1.1.3), then a licensee may modify their facility design, develop limitations on the 

7 types of waste that are acceptable for disposal, or perform additional analyses. A licensee may 

8 develop performance period analyses to demonstrate that the 10 CFR 61.41(c) and 

9 10 CFR 61.42(c) requirements will be met.

10

11 In many respects, the analyses that a licensee may perform for the performance period will be 

12 similar to the performance assessment and intruder assessment completed for the compliance 

13 period and protective assurance period. The guidance provided in Sections 2.0, 3.0, and 4.0 for 

14 the compliance period and in Section 6.0 for the protective assurance period is applicable to the 

15 performance period analyses. One primary difference is that licensees will need to consider the 

16 additional uncertainties that the long timeframes associated with the performance period 

17 introduce. In addition, the metric licensees must use to determine if the 10 CFR 61.41(c) and 

18 10 CFR 61.42(c) requirements will be met is to minimize releases to the extent reasonably 

19 achievable. Unlike the requirements presented in 10 CFR 61.41(b) and 10 CFR 61.42(b) for the 

20 protective assurance period, there is no dose goal associated with the performance period 

21 analyses.

22

23 Uncertainties associated with the performance period may be larger than those associated with 

24 the compliance or protective assurance periods. As the timeframe for the analyses is extended, 

25 temporal processes that have a rate that is insufficient to result in a significant change to 

26 performance prior to 10,000 years may be significant when evaluated for the longer 

27 performance period. For example, carbonation of a cementitious barrier may be sufficiently 

28 slow such that the passivation of rebar is not affected over the 1,000 year compliance period, 

29 and even the 10,000 year protective assurance period. However, the rate of carbonation could 

30 be sufficient enough to reduce the protective passivation of the rebar after the protective 

31 assurance period, leading to deterioration or failure of the cementitious barrier during the 

32 performance period. 

33

34 Figure 7-2 is an example of the scope of the technical analyses for the compliance period, 

35 protective assurance period, and performance period for a hypothetical site. Figure 7-2 has 

36 shaded areas to show: that some processes and events will only be applicable to the 

37 compliance period or protective assurance period, that some will be only applicable to the 

38 performance period, and that others will need additional analyses to determine their 

39 applicability. The scope of the analyses for the protective assurance period may include both 

40 short- and long-term processes and events depending on the type of waste that is disposed. A 

41 licensee will assess some processes in the performance assessment for the compliance period 

42 and/or the protective assurance period, whereas others may clearly be applicable only to the 

43 performance period. In the quantitative analyses for the performance period, a licensee should 

44 assess those FEPs that were determined to be insignificant for the compliance period and/or the 
7-19

1

2

3 Figure 7-2 Scope of the Technical Analyses for the Compliance Period, Protective 

4 Assurance Period, and Performance Period for a Hypothetical Site

5 protective assurance period based on rates that were too slow or likelihoods that were too low. 

6 The licensee should determine if those FEPs are relevant to the performance period. It may be 

7 useful for a licensee to perform an iterative assessment to determine the appropriate scope of 

8 the performance period analyses. 

9 7.4.1.1.3 Minimize Radioactive Releases to the Extent Reasonably Achievable

10

11 The performance metrics a licensee must apply to the assessment of long-term releases during 

12 the performance period is to minimize releases to the extent reasonably achievable 

13 (10 CFR 61.41(c)) and to minimize exposures to any inadvertent intruder to the extent 

14 reasonably achievable (10 CFR 61.42(c)). The metrics afford flexibility to a licensee to consider 

15 socioeconomic factors when assessing long-term protection of public health and safety. The 

16 requirements to minimize releases and exposures to the extent reasonably achievable are 

17 intended to be conceptually similar to different aspects of the ALARA requirement found in 

18 10 CFR Part 20, optimization, and traditional cost-benefit analyses (see Section 6.0). 

19

20 Dose limits are not established for the performance period in 10 CFR 61.41(c) and 

21 10 CFR 61.42(c). A quantitative ALARA analysis cannot be completed in the traditional sense 

22 (i.e., estimate the cost for further reducing doses below a dose limit). Instead, the requirement 

23 of the performance period quantitative analyses is for a licensee to demonstrate that releases 

24 will be minimized to the extent reasonably achievable.

25

26 The NRC has developed a policy to inform regulatory decision-making (see Section 6.2.1.2). 

Transport of nonsorbing

 radionuclides

Corrosion of carbon 

steel

Initial defects in 

reinforced

 concrete

Durability of erosion

 control materials

Present day climate

Radioactive decay –

short-lived

Compliance Period Scope

Natural climate cycling: 

 -Increased precipitation

 -Higher fluvial erosion

 -Changes to flaura/fauna

 -Changes to hydrogeology

Landform evolution

Transport of moderately 

 sorbing radionuclides

Corrosion of stainless steel

Carbonation of 

reinforced 

 concrete

Degradation of erosion 

 control materials

Radioactive decay –

 long-lived

Potentially Both

Performance Period Scope

Protective Assurance 

Performance Period Scope Period Scope Compliance Period Scope
7-20

1 Because of the problems associated with discounting over long timeframes, the NRC staff 

2 recommends that licensees should provide the proportion of resources that are proposed to be 

3 used to achieve protection of public health and the environment for different timeframes. These 

4 resource estimates should be provided in present day values. Timeframes to consider may 

5 include:

6

7 • the institutional control period (100 years)

8 • the Class C waste intruder barrier period (500 years)

9 • the compliance period (1,000 years)

10 • the protective assurance period (10,000 years)

11 • the performance period (site-specific values > 10,000 years)

12

13 The regulations specify individual dose limits (for the compliance period) rather than collective 

14 dose to a population. A licensee should estimate the projected individual doses for the 

15 performance period and the resources used to reduce those impacts to certain dose values or 

16 the projected cost to achieve different dose values. Dose values that a licensee may consider 

17 for comparison purposes may include the 10 CFR 61.41 public annual dose limit for the 

18 compliance period (25 mrem/yr), the 10 CFR 61.42 intruder annual dose limit for the compliance 

19 period (500 mrem/yr), the 10 CFR Part 20 public annual dose limit (100 mrem/yr), and 

20 background radiation values for the site, or other limits for which a licensee provides technical 

21 bases. Radiological doses at very long timeframes are based on many unstated

22 assumptions. However, radiological doses provide a present day metric for stakeholders to 

23 consider. Other metrics may be appropriate for a licensee to consider, such as concentrations 

24 in the environment and flux rates. The analyses should demonstrate that a reasonable attempt 

25 has been made through site selection, facility design, and waste acceptance to minimize 

26 releases to the public to extent reasonably achievable for the performance period (see Example 

27 7.4).

28

29 In summary, the NRC staff recommended approach to performance period analyses entails the 

30 following primary elements:

31

32 • A summary of present day resources, with no use of discounting, used to limit releases 

33 for different regulatory timeframes

34 • A description of the additional resources needed to achieve a greater reduction in 

35 releases during the performance period, or why further reductions are not possible

36 • A discussion of why additional resource usage is not warranted

37

38

39

40

41
7-21

1

Example 7.4: A disposal site is located in a semi-arid environment in an area of low-relief and 

long-term accretion. The waste streams proposed for disposal have a SOF of 4.3 when 

evaluated against the Table A concentrations in 10 CFR 61.13(e). Therefore, performance

period analyses are required. Because the compliance period analyses demonstrated that the 

performance objectives would be met by a significant margin, the licensee elects to perform a 

screening analysis for the performance period by extending the compliance period calculations 

beyond the protective assurance period to the performance period with conservative 

parameters.

Conclusion: The licensee provides the regulator with a list of the potential radiation exposure 

pathways to the public. The dominant release pathway in the compliance period analysis was 

via ground water. It is anticipated that this may also be the dominant exposure pathway for the 

performance period analyses. The licensee performs an assessment of FEPs that were 

screened out of the compliance period assessment to determine if any of those phenomena are 

potentially significant to the performance period analyses. The only significant process 

identified is natural cycling of the climate. Because the site is located in the Southern US, the 

impact of natural cycling of climate is represented in the screening analyses by assuming a 

wetter and cooler climate (e.g., greater infiltration). Conservatism introduced in the screening 

analyses for the performance period include elimination of sorption during transport, assuming 

the engineered cover provides no reduction in natural recharge rates, and assuming the 

primarily carbon steel waste packages provide no barrier to release or transport. Because 

solubility limits were not applied in the compliance period analyses they are not adjusted for the 

performance period analyses. The receptor characteristics are adjusted to be consistent with 

the climate state. 

The screening analyses for the performance period results in an estimated peak all pathways 

dose of 40 mrem/yr at 30,000 years. A sensitivity analysis on the long-term infiltration rates is 

included to address future climate state uncertainty. The licensee includes a comparison of the 

flux (g/yr) of naturally-occurring radionuclides from the disposal facility with those originating 

from natural sources, in a nearby river. The fluxes from the facility are less than those from 

natural sources.

The licensee also develops a cost comparison of some engineered alternatives to the disposal 

facility and how they could impact performance period doses. Only technologies that would 

result in a significant increase in cost or are unproven result in a significant decrease in 

projected impacts. Because the projected doses from the conservative screening analysis do 

not significantly exceed the compliance period dose limit, only a first-order assessment of 

technologies and their impacts is warranted.

The licensee performs barrier analyses to determine the most significant components of the 

system that are reducing releases. For the performance period, the licensee determines that 

dilution and dispersion during transport are very significant. In addition, solubility limits and 

sorption during transport could be very important, though they are not credited in the 

conservative screening analyses.

Because the compliance period scope was supplemented and conservatism was used in the 

analyses, the performance period analyses should be sufficient to demonstrate that releases for 

the performance period have been minimized to the extent practical even if the estimated doses 

exceed the compliance period dose limit.
7-22

1 7.4.1.2 Barrier Analyses

2

3 Licensees should use barrier analyses to identify and describe the capabilities of barriers, the 

4 challenges and stresses expected to be imposed on barriers, and the contribution of barriers to 

5 limiting or delaying releases of long-lived waste into the environment. Licensees can use 

6 different types of analyses, ranging from qualitative to quantitative, to demonstrate how the 

7 barriers of the disposal facility limit long-term impacts. Barrier and component analyses can be 

8 used to satisfy 10 CFR 61.13(e) to illustrate the long-term performance of barriers and 

9 components of an LLW disposal system. Barrier and component analyses involve decomposing 

10 the performance of the system into the performance of the components under assumed 

11 scenarios or configurations.

12

13 Licensees should provide a discussion of the capabilities of engineered and natural barriers to 

14 reduce releases or exposures. The discussion can be useful for various stakeholders to 

15 develop understanding of the disposal system performance. Events and processes that may 

16 impact those capabilities should also be discussed. A discussion of the expected persistence 

17 and durability of the barriers, and the basis for the expected durability, will be useful for many 

18 stakeholders. At long timeframes, the performance of an engineered barrier is likely to be 

19 diminished. A challenge with taking a qualitative approach to describing barrier performance is 

20 determining the actual barrier performance rather than the potential barrier performance. The 

21 potential barrier performance may not be realized because (1) the performance is deteriorated 

22 or eliminated by processes and events, or (2) because the performance is masked by the

23 performance of other barriers, even though it is favorable to have independent, redundant 

24 barriers (see Section 8.0). In addition, as-built performance can differ from as-designed 

25 expectations. Quantification of the performance of the engineered and natural barriers is useful 

26 because the estimated performance of the barrier is represented in the calculation regardless of 

27 whether the performance is close to potential or has significantly deteriorated. Quantitative 

28 barrier analyses can reduce some of the challenges and provide estimated performance based 

29 on available information.

30

31 Semi-quantitative analyses may involve estimating the performance of individual components or 

32 materials in the disposal system and providing the basis for the performance of the component 

33 or material. For example, if a robust engineered cover using durable rock is provided for 

34 erosion protection, estimation of the durability of the rock over the long-term may provide 

35 confidence in future performance without detailed landform evolution modeling. Estimation of 

36 the ages and stability of surrounding analogous landforms may be useful in providing inferential 

37 information about the long-term stability of the waste disposal site.

38 Quantitative analyses, such as extension of technical analyses calculations from the compliance 

39 period and protective assurance period to the performance period, can provide estimates of the 

40 ability of the disposal system to limit long-term impacts, as long as the scope of the compliance 

41 period and protective assurance analyses is sufficient for the performance period analyses.

42 7.4.1.2.1 Methods

43

44 Different methods are available to perform barrier and component analyses, including, but not 

45 limited to, one-off analyses, one-on analyses, and factorial designs (Esh et al., 2001; NRC, 

46 2004a, Eisenberg and Sagar, 2000). Typically, the term “one-off” analysis is used to refer to 

47 varying one parameter  in this case, one barrier  at a time. Barrier and component 
7-23

1 analyses are usually performed by isolating the performance of the particular barrier or 

2 component. The biggest challenge in performing these analyses is usually communicating what 

3 the results mean and how they should and should not be interpreted, because the calculations 

4 provide a hypothetical situation that may never arise (e.g., elimination of a geologic unit, or 

5 failure of all waste packages at a single instant). 

6

7 Barrier and component analyses can be performed at different levels of resolution. Different 

8 levels of resolution can provide important detail to help focus the regulatory review. For 

9 example, representation of a disposal system as its components — an engineered cover, 

10 disposal vaults, wasteform, unsaturated zone, and saturated zone — can convey broadly which 

11 areas are contributing to performance in mitigating risks. Refinement of that analysis, such as 

12 looking at individual layers in a multilayer engineered cover, may identify specific areas of 

13 performance. 

14

15 Barriers and components in the disposal system (e.g., engineered barriers and disposal site) 

16 may reduce the magnitude of doses or change the timing of when doses could occur. The 

17 barrier and component analyses look at changes to both the magnitude of projected doses and

18 the timing of when those doses are projected to occur. Some barriers may impact both metrics,

19 while others may only impact a single metric.

20

21 Environmental system models may include a range of coupling of components from weak to 

22 strong. The licensee will need to clearly identify how processes that may affect multiple 

23 components have been treated in the barrier and component analyses. In addition, disruptive 

24 processes and events may impact multiple barriers or components. Disruptive processes and 

25 events may be more important to consider during the performance period compared to the 

26 compliance period or protective assurance period because of the longer time during which they 

27 could occur. A licensee can perform barrier and component analyses after a disruptive event 

28 has been assumed to occur, in order to understand how the components of the disturbed 

29 system may be contributing to limit the impacts from the disruptive event.

30

31 One-off analyses are analyses in which the performance of a single barrier or component is 

32 neglected in order to understand the contribution of the barrier to performance when the system 

33 is operating under the anticipated range of conditions. Each barrier or component is analyzed in 

34 this manner and the relative performance, such as the change in peak dose, is compared. For 

35 example, the contribution of an engineered cover to performance could be evaluated by setting 

36 the infiltration rate into the disposal system to a value that represents a natural recharge rate in 

37 the region of the disposal facility. If the engineered cover had other contributions to 

38 performance (e.g., reducing radon fluxes) those should also be eliminated in this type of 

39 analysis. The results are best expressed on a relative basis, such as percent change, as the 

40 analyses may be unphysical. But these types of barrier analyses calculations can have value 

41 because they clearly convey which elements of the system are providing the most contribution 

42 to performance, and therefore, which elements should have the most technical basis and most 

43 rigorous review effort. 

44

45 Individual components or barriers may have a redundant performance function with other 

46 components or barriers. A one-off analysis result that indicates the performance did not change 

47 when a component was “turned off” could represent that the barrier or component truly does not 

48 contribute significantly to overall performance. However, the resulting lack of estimated 

49 performance could indicate that a different barrier is providing a redundant functionality. A 
7-24

1 licensee can use different analyses, such as one-on and factorial designs, to complement the 

2 one-off type of barrier and component analyses to reveal this type of redundant functionality. 

3

4 “One-on” analyses are analyses in which only a single barrier or component is represented in 

5 order to determine the potential performance of the barrier. A benefit of the one-on analysis is 

6 that it identifies the hypothetical maximum consequence that the waste could produce. 

7 However, the usefulness of this type of analysis is reduced when the likelihood for the 

8 hypothetical consequences becomes overly remote. One-on analyses can be useful in 

9 identifying when different barriers may be providing redundant performance functions. A 

10 disadvantage of one-on analyses is that they do not address the likelihood of the hypothetical 

11 result ever being achieved. As long as it is understood that the purpose of the analyses is to 

12 understand how barriers could contribute, misinterpretation of the results can be avoided. The 

13 base case analysis, with all barriers and components present, represents the best estimate of 

14 disposal system performance, assuming that the level of performance assigned has an 

15 adequate technical basis.

16

17 A licensee can use factorial designs to provide a more complete picture of the contribution of 

18 various barriers and components to the overall system performance. The factorial design is one 

19 in which all combinations of barriers being “on” or “off” are generated. This type of analysis can 

20 require significant resources, depending on the number of barriers and components in the 

21 system being evaluated and the computational expense of the models being evaluated. When 

22 a full factorial assessment is not practical, a licensee can consider a partial factorial 

23 assessment. The compilation and interpretation of results from a factorial barrier assessment is 

24 not straightforward. The relative change in performance will be much different depending on the 

25 number of barriers or components that may be active in the calculation. One way to overcome 

26 this problem is simply to rank the barrier contributions for each similar calculation. Example 7.5

27 provides additional detail on how this may be accomplished.

28

29 Barrier addition analysis is a process in which the hypothetical maximum consequence of the 

30 waste is generated. After the licensee estimates the maximum consequence, barriers or 

31 components can be added one by one until the full system is represented. A challenge with 

32 barrier addition analyses is that the sequence in which the barriers are added may influence the 

33 results prescribed to any one particular barrier. Barriers added early in the sequence are more 

34 likely to show large performance benefits than barriers added later in the sequence.

35

36 In order for barrier and component analyses to be most useful, they should be carefully 

37 performed by analysts that understand all of the components and barriers of the overall system 

38 performance. The level of underperformance assigned to a barrier or component, which is 

39 subjective, can influence the results and interpretation of the importance of that barrier or 

40 component. The level of underperformance ascribed may represent both the amount of 

41 degradation expected as well as pessimism in the estimate of performance. Licensees should 

42 clearly explain the level of underperformance and, if possible, should assign this level of 

43 underperformance based on the amount of confidence in the understanding of the performance 

44 of the barrier or component. A barrier with a strong technical basis for its performance should 

45 be much less likely to not perform than one for which the technical basis is weak or limited. 

46 Although barrier and component analyses can provide useful information to understand how a 

47 system may perform, the analyses may not provide a correct representation of how the system 

48 is expected to perform.
7-25

1 7.4.1.2.2 Design, Site, and Overall Performance

2

3 Differentiation between barriers that are engineered (i.e., design) and those that are natural (i.e., 

4 site) classes or types can be useful in understanding overall performance and explaining to 

5 stakeholders why the system is expected to protect public health and safety. Generally, 

6 engineered components provide greater benefit at earlier times and natural system components

7 provide greater benefit at later times. The engineered design should be integrated into the 

8 natural site, which can make a clear separation less obvious. In addition, the NRC staff expects 

9 that natural system conditions will have a strong influence on the performance of the engineered 

10 design. In some cases, the engineered design could influence the performance of the natural 

11 system (e.g., leaching of cement that impacts sorption in the unsaturated zone or erosion of 

12 surface soils at the toe of a slope). Engineering judgment can be used to classify barrier and 

13 component types as long as the analysis is transparent and traceable. Classification is the 

14 separation of the different barriers into classes based on the type of barrier (i.e., engineered or 

15 natural). Some phenomena may be difficult to classify, for example the chemical environment 

16 inside a waste container. The goal is to provide the classification that provides the clearest 

17 understanding of overall system performance.

18
7-26

1

Example 7.5 – A disposal system has five primary barriers, #1 through #5. A factorial barrier 

assessment is completed. The simulations are as follows:

#1 #2 #3 #4 #5

off on on on on

on off on on on

on on off on on

on on on off on

on on on on off

off off on on on

off on off on on

 [full matrix not shown]

off off off on off

off off off off on

An overall performance measure is generated by calculating the rank of the analysis result 

relative to the rest of the results of that class (i.e., analyses with the same number of 

components turned “on” or “off”), then generating the average rank over all classes.

For example the results for the one-off class are as follows:

#1 #2 #3 #4 #5

+5% +30% +1% +200%+17%

This results in the following ranks for the one-off class:

#1 #2 #3 #4 #5

4 2 5 1 3

The average change for each two-off combination is the following (any two-off that includes a 

#1 in the combination goes into the average change for #1):

#1 #2 #3 #4 #5

+57% +89% +132%+310%+37%

This results in the following ranks for the two-off classes:

#1 #2 #3 #4 #5

4 3 2 1 5

The average rank of each barrier averaged over each class provides a barrier importance 

measure:

#1 #2 #3 #4 #5 

4 2.5 3.5 1 4 (average rank for each barrier)
8-1

1 8.0 DEFENSE-IN-DEPTH ANALYSES

2

3 The core of the NRC’s safety philosophy has long 

4 included the concept of defense-in-depth. The 

5 regulations at 10 CFR 61.2 define defense-in-depth 

6 as “the use of multiple, independent, and redundant 

7 layers of defense so that no single layer, no matter 

8 how robust, is exclusively relied upon”. The ultimate 

9 purpose of defense-in-depth is (1) to compensate for 

10 uncertainty in the type and magnitude of safety 

11 challenges, and (2) to compensate for uncertainty in 

12 the performance of the measures that are taken to 

13 ensure safety. Consistent with the NRC’s regulatory 

14 philosophy, the regulations at 10 CFR 61.13(f) 

15 require that land disposal LLW facilities demonstrate 

16 that defense-in-depth protections are included to provide reasonable assurance that the 

17 10 CFR Part 61 performance objectives can be met.

18

19 Defense-in-depth protections are required by 10 CFR Part 61 to prevent, contain, or mitigate 

20 exposure to radioactive material according to the hazard present, the relevant scenarios, and 

21 the associated uncertainties. Defense-in-depth protections also ensure that the risks resulting 

22 from the failure of some or all of the established barriers and controls, including human errors, 

23 are maintained at an acceptably low level. These two aims help to provide reasonable 

24 assurance that the 10 CFR Part 61 performance objectives can be met, in light of the 

25 uncertainties in projecting the behavior of the land disposal facility over both the operational and 

26 post-closure periods. 

27

28 To demonstrate that 10 CFR 61.13(f) is met, licensees should describe the layers of protection 

29 that ensure that the risks are properly managed. The description should identify the use of 

30 multiple layers of protection and describe how the various layers maintain independence and 

31 provide redundancy. The description of the layers of protection can be principally drawn from 

32 risk insights derived from the results of other 10 CFR Part 61.13 technical analyses (e.g., 

33 performance assessment, intruder assessment, stability analyses, and performance period 

34 analyses), although licensees may develop separate analyses for demonstrating defense-in35 depth.

36

37 This chapter describes the information that a licensee should provide and a reviewer should 

38 evaluate with respect to demonstrating that a land disposal facility includes defense-in-depth 

39 protections. Sections 8.1 and 8.2 discuss NRC’s defense-in-depth philosophy and elaborate on 

40 key concepts of the defense-in-depth regulatory philosophy as they apply to LLW disposal 

41 facilities, respectively. Section 8.3 provides guidance on demonstrating defense-in-depth 

42 protections during the operational and post-closure phases of the land disposal facility lifecycle.

43 8.1 Background on Defense-in-Depth

44

45 Defense-in-depth is a regulatory philosophy or concept that has been used since at least the 

46 1960s in the context of ensuring nuclear reactor safety. The philosophy is intended to deliver a 

Defense-in-Depth:

The use of multiple, 

independent, and 

redundant layers of 

defense so that no single 

layer, no matter how 

robust, is exclusively 

relied upon for safety.
8-2

1 design that compensates for uncertainties in knowledge of facility behavior, component 

2 reliability, or operator performance that might compromise safety. 

3

4 In the context of nuclear reactor safety, defense-in5 depth has traditionally focused on layers of protection 

6 to prevent accident initiators, contain radioactivity, and 

7 mitigate exposures through safety systems. The 

8 defense-in-depth concept has evolved from its early 

9 narrow application in the context of nuclear reactor 

10 safety to a more expansive application as an overall 

11 safety strategy for radioactive materials that includes 

12 the multiple barrier approach.

13

14 In the 1995 Policy Statement on the Use of 

15 Probabilistic Risk Assessment Methods in Nuclear 

16 Regulatory Activities (NRC, 1995a), the NRC 

17 recognized that complete reliance for safety cannot be 

18 placed on any single element of the design, maintenance, or operation of a facility. The policy 

19 statement highlighted the need for redundancy in active safety systemsand a multiple barrier 

20 approach to protect against releases. An essential property of defense-in-depth is the concept 

21 of successive barriers or layers. These barriers or layers are commonly represented within the 

22 NRC’s regulatory framework in two different ways (Sorensen, et al., 1999). First, the NRC’s 

23 framework requires the use of high-level layers of protection, such as the prevention of accident 

24 initiators, the quick termination of accident sequences, and the mitigation of accidents that are 

25 not successfully terminated. Second, the NRC’s framework requires the use of multiple physical 

26 barriers, which are specified for particular facilities or material uses.

27

28 In 1999, the NRC’s Staff Requirements Memorandum for SECY-98-144, “White Paper on Risk29 Informed and Performance-Based Regulation,” approved descriptions of many terms including 

30 defense-in-depth (NRC, 1999d). The SRM describes defense-in-depth as an element of the 

31 NRC’s safety philosophy that employs successive compensatory measures to prevent accidents 

32 or mitigate damage if a malfunction, accident, or naturally caused event occurs at a nuclear 

33 facility. The philosophy ensures that safety will not be wholly dependent on any single element 

34 of the design, construction, maintenance, and operation of a nuclear facility. The net effect of 

35 incorporating defense-in-depth into design, construction, maintenance, and operation is that the 

36 facility or system in question tends to be more tolerant of failures, external challenges, and 

37 uncertainty in the behavior of the facility.

38

39 More recently, in NUREG-2150, the NRC has characterized defense-in-depth protections as 

40 part of the development of a holistic vision for all facilities regulated by the NRC, including land 

41 disposal facilities (NRC, 2012c). The NRC’s risk-informed, performance-based characterization 

42 indicates that defense-in-depth protections (1) ensure appropriate barriers, controls, and 

43 personnel prevent, contain, and mitigate exposure to radioactive material according to the 

44 hazard present, the relevant receptor scenarios, and the associated uncertainties; and (2) 

45 ensure that the risks resulting from the failure of some or all of the established barriers and 

46 controls, including human errors, are maintained acceptably low.

47

48 As stated above, the regulations in 10 CFR Part 61.13 (f) require licensees to explicitly describe 

49 how the proposed disposal facility includes defense-in-depth protections. The regulations in 

The Defense-in-Depth 

philosophy is intended to 

deliver a design that 

compensates for 

uncertainties in knowledge 

of facility behavior, 

component reliability, or 

operator performance that 

might compromise safety.
8-3

1 10 CFR Part 61 also implicitly incorporate the concept of defense-in-depth into the regulatory 

2 framework. Implicit defense-in-depth provisions of the regulations include multiple performance 

3 objectives, as well as requirements for site suitability, site design, facility operation, site closure, 

4 environmental monitoring, waste acceptance, land ownership and institutional control, and 

5 financial assurance.

6 8.2 Defense-in-Depth Concepts for a Land Disposal Facility

7

8 The NRC’s use of the defense-in-depth philosophy is intended to deliver a design that is can 

9 handle uncertainties in knowledge of facility behavior, component reliability, or operator 

10 performance that might compromise safety. While justifiably important for nuclear facilities with 

11 large potential risks, it is similarly important for land disposal facilities for radioactive wastes, 

12 which can contain relatively large radiological hazards. Disposal facilities, being the endpoint of 

13 the nuclear fuel cycle, are responsible for containing and isolating the radioactivity for time 

14 periods far into the future. As with any estimation far into the future, uncertainty grows the 

15 farther out in time one attempts to estimate future performance as the system moves away from 

16 what is known today. The use of defense-in-depth protections is, therefore, a prudent approach 

17 to managing the uncertainty associated with estimating performance far into the future to ensure 

18 the safe disposal of radioactive wastes.

19

20 As identified in the definition of defense-in-depth at 10 CFR Part 61.2, the philosophy relies 

21 upon multiple, independent, and redundant layers of defense so that no single layer, no matter 

22 how robust, is exclusively relied upon. Multiple layers provide confidence that if an individual 

23 layer fails or underperforms, other layers of defense would be available to protect health and 

24 safety and the environment. Redundant layers provide confidence that should an individual 

25 layer fail or underperform, another layer will be available to provide similar capabilities as the 

26 individual layer to protect health and safety and the environment. Independent layers enhance 

27 confidence that the layers of defense are less likely to fail or underperform by common-cause 

28 modes in which a single event or process is able to defeat or diminish the capabilities of each 

29 layer simultaneously. In the end, multiple, independent, and redundant layers are intended to 

30 provide a margin of safety to account for uncertainty in the evolution of the land disposal facility 

31 over time. The margin of safety needed is dependent upon the hazard presented by the waste 

32 emplaced in the land disposal facility and ultimately by the potential risk of harm to health and 

33 safety and the environment. The different types of layers complement each other and when 

34 used together develop a more comprehensive approach to defense-in depth protections.

35 8.2.1 Multiple Layers

36

37 Multiple layers of defense provide confidence (1) that accidents can be prevented, (2) that the 

38 effects of an accident can be lessened should a malfunction or accident occur, and (3) that 

39 there is adequate protection should a layer of defense underperform due to uncertainty in its 

40 expected behavior. Reliance on multiple layers of defense ensures safety will not be wholly 

41 dependent upon any single element of the design, construction, maintenance, or operation of 

42 the land disposal facility. Multiple layers of protection use numerous, diverse protection 

43 mechanisms or actions to ensure safety. Layers of defense can consist of a number of 

44 attributes including physical or chemical barriers to radionuclide release, appropriate controls 

45 that help ensure the physical barriers perform as intended, and trained and qualified personnel 

46 who are focused on safety (see Figure 8-1). 

47
8-4

1

2

3 Figure 8-1 Multiple Layers of Defense 

4 Each of the layers has an associated capability or safety function that is intended to mitigate 

5 releases and exposures to workers and the public during both normal operations and accidents. 

6 The function performed by an individual layer may be active, passive, or in some cases, a layer 

7 may provide both active and passive safety functions. Active safety functions are those that 

8 require activity or energy by the licensee to monitor and maintain protection. For example, an 

9 air filtration system requires continual maintenance, as well as an available energy source to 

10 ensure that its functionality to remove radioactive particulates is available when needed. 

11 Passive safety functions do not require ongoing activity or external energy inputs from the 

12 licensee to provide protection. For instance, a wasteform, once created, typically would not 

13 require ongoing maintenance to perform its safety function (i.e., limit the release of 

14 radionuclides), though its safety function would likely degrade over time. The role of barriers, 

15 controls, personnel, and their associated safety functions in demonstrating defense-in-depth for 

16 a land disposal facility are discussed in more detail below. For land disposal of LLW, passive 

17 layers of defense are more appropriate for the post-closure period because they do not require 

18 ongoing maintenance and monitoring. Active layers of protection are more appropriate during 

19 the operational period.

20

21 When identifying barriers, controls, or personnel relied upon for safety, licensees should clearly 

22 describe the functionality or capability provided by the barrier, control, or personnel to achieve 

23 the performance objectives and provide defense-in-depth protections. The description of the 

24 safety function should include a technical basis for the function and associated uncertainty in 

25 the function of the barrier, control, or personnel. In some cases, the safety function may only be 

26 necessary for a specific timeframe. For example, a licensee identifies a waste container as a 

27 barrier to waste release for short-lived radionuclides. The licensee should specify the time 

28 period over which the safety function provided by the waste container is necessary to 

29 demonstrate that the performance objectives are met and defense-in-depth protections are 
8-5

1 provided. Figure 8-2 depicts approximate time periods over which safety functions for barriers, 

2 controls, and personnel may be appropriate.

3

4

5 8.2.1.1 Barriers

6

7 For the land disposal of LLW, barriers can take a number of forms, such as a container, a 

8 wasteform, a wall, or a restricted area of land. Barriers can be either engineered or natural. 

9 Barriers are intended to improve the land disposal facility’s ability to meet the 10 CFR Part 61 

10 performance objectives and add to defense-in-depth. In the context of disposal, barrier safety 

11 functions, which are also described as barrier capabilities, are typically intended (1) to prevent 

12 or limit the contact of water with the waste, (2) to limit the release or rate of release of 

13 radioactivity from the waste, or (3) to prevent or limit biotic contact with the waste (for 

14 inadvertent intrusion). For normal operations and abnormal events (e.g., accidents), barrier 

15 safety functions are typically designed to prevent, contain, and mitigate exposure to radioactive 

16 material.

17

18 Engineered barriers at LLW disposal facilities are generally man-made features that are 

19 designed to mitigate the effect natural processes could have on the performance of the disposal 

20 facility and also limit human activities that may expose individuals to radiation or initiate or 

21 accelerate release of radioactivity from the waste through environmental pathways. The intent 

22 of engineered barriers is to improve the land disposal facility’s ability to meet the performance 

23 objectives in 10 CFR Part 61 and add to the defense-in-depth provided by the facility design and 

24 construction. Examples of engineered barriers include (1) closure caps, which are designed to 

25 limit infiltration into disposal units, and (2) waste containers and wasteforms, which may provide 

26 shielding to site workers during emplacement and preclude or limit the release of radionuclides 

27 from the disposal units once emplaced.

28

29 Natural barriers are generally barriers inherent to the disposal site that limit exposures to the 

30 waste. Examples of natural barriers include the climatic or the hydrogeologic layers of the 

31 disposal site. Limited infiltration associated with an arid climate may result in long travel times 

Figure 8-2 Land Disposal Facility Lifecycle and Timeframes for Defense-in-Depth 

Layers (Duration of lifecycle timeframes are not to scale. Dark blue 

timeframes are considered the post-closure period.)

OPERATIONS

OBSERVATION 

& 

MAINTENANCE

INSTITUTIONA

L CONTROL

ACTIVE BARRIERS

PASSIVE BARRIERS

PERSONNEL

CONTROLS
8-6

1 of radionuclides from the waste to underlying groundwater. Likewise, hydrogeological layers 

2 may retard the movement of radionuclides from the disposal site as a result of sorption 

3 processes.

4 8.2.1.2 Controls

5

6 A control can be an apparatus or mechanism that, through its manipulation or administration, 

7 serves to protect public health and safety and the environment around the land disposal facility. 

8 During operations, controls are intended to prevent, contain, and mitigate exposure to 

9 radioactive material at or in the vicinity of the land disposal facility. Operators also employ 

10 controls to ensure the performance of barriers during either the operational period or after 

11 closure of the land disposal facility. 

12

13 Controls can largely be classified as either engineering or administrative controls. Engineering 

14 controls include any man-made apparatus that is designed to ensure the performance of a 

15 barrier or provide a level of protection independent of a barrier. For example, an engineering 

16 control may include something as simple as a visible or audible alarm as part of access control 

17 to restricted areas or more complex systems such as a fire alarm and suppression system to 

18 limit the damage from a potential fire. Administrative controls encompass a wide array of 

19 managerial mechanisms. Administrative controls include provisions related to organization and 

20 management, procedures, record keeping, material control and accounting, waste acceptance, 

21 and management review. Administrative controls can also include legal mechanisms such as 

22 land ownership, as required by 10 CFR 61.59(a).

23 8.2.1.3 Personnel

24

25 Properly trained and qualified personnel are necessary to ensure proper design, installation, 

26 operation, or administration of barriers and controls that prevent, contain, or mitigate exposure 

27 to radioactive materials. Properly trained and qualified personnel also ensure that human errors 

28 that can lead to failure of some or all of the established barriers and controls are maintained 

29 acceptably low and safety is maintained. Personnel may also be able to mitigate exposures in 

30 an accident. For example, properly trained emergency responders can mitigate potential 

31 releases of radioactivity from the land disposal facility in the case of an accident or an 

32 emergency. Because reliance on personnel inherently relies upon activity, personnel always 

33 provide active safety functions. Designated personnel may be an appropriate layer of protection 

34 during operations; however, the regulations at 10 CFR Part 61 intend that active maintenance 

35 and monitoring of the site after closure is not relied upon for safety. Therefore, the reliance on 

36 personnel for defense-in-depth protections after closure is not appropriate.

37 8.2.2 Independent Layers

38

39 In addition to multiple layers of protection, which are discussed in Section 8.2.1, the definition of 

40 defense-in-depth specified in 10 CFR Part 61 requires that the land disposal facility incorporate 

41 independent layers of protection. Independence applies to the safety function in that a given 

42 layer should provide a safety function that is not dependent upon other layers of defense to 

43 perform their respective safety function. For instance, a licensee may use an engineered 

44 closure cap as one barrier to limit the flow rate of water contacting the waste and an engineered 

45 wasteform as another barrier to limit the solubility of certain radionuclides in water contacting 

46 the waste. If the closure cap were to fail, resulting in a higher flow rate of water into the disposal 
8-7

1 unit, the wasteform would still be expected to maintain low concentrations of solubility-limited 

2 radionuclides in the water leaving the disposal units. In some cases, multiple layers of defense 

3 may appear to be independent, but the safety functions performed by each layer may actually 

4 be dependent. For example, a land disposal facility may dispose of waste in a metallic waste 

5 container whose safety functions are to limit the contact of water with the wasteform and release 

6 of radionuclides. In addition, the facility disposes of the waste in a metallic wasteform that is 

7 also relied upon to limit the release of radionuclides. The waste container and wasteform may 

8 appear to provide independent functionality; however, certain chemical environments could 

9 cause degradation to both the waste container and waste form simultaneously. Further, the 

10 degradation of the waste container may directly result in more rapid degradation of the 

11 wasteform, resulting in a dependency between the waste container and wasteform to limit 

12 releases of radionuclides from a disposal unit.

13 8.2.3 Redundant Layers

14

15 In addition to the multiple and independent layers of protection discussed above, the definition 

16 of defense-in-depth specified in 10 CFR Part 61 requires that the land disposal facility 

17 incorporate redundant layers of protection. The use of redundant layers increases confidence 

18 that safety is not reliant upon any single layer. Redundancy is the duplication of layers of 

19 defense, in order to prevent the failure of the entire disposal system if a single item or 

20 component relied upon for safety fails or provides a safety function that is less than expected. 

21 Licensees should demonstrate that the land disposal facility has redundancy for key functions 

22 relied upon for safety. 

23

24 Redundancy goes beyond implementing multiple layers. Redundancy requires that the safety 

25 function performed by a layer be duplicated or even triplicated or more for safety-critical 

26 components in high-risk scenarios. The duplication of a safety function may occur within the 

27 same layer (e.g., as a redundant component) or in other layers incorporated into the siting, 

28 design, construction, maintenance, or operation of the disposal facility. For instance, different 

29 stratigraphic units beneath a disposal facility may each sufficiently retard the migration of key 

30 radionuclides, thereby, providing redundancy should the licensee’s understanding of the primary 

31 unit’s sorptive capabilities prove incorrect. In this example, licensees would also need to 

32 provide reasonable assurance that groundwater flow occurs through the stratigraphic units and 

33 that preferential flow pathways that could minimize the water contact with the stratigraphic units 

34 would not be limited.

35 8.2.4 Safety Margin

36

37 Safety margin is the excess functionality remaining to provide safety after the demands of a 

38 particular scenario are placed on the functionality of the layer or system. In some high-risk 

39 scenarios, additional controls on the safety functions of a layer may be necessary in order to 

40 ensure an adequate safety margin. For instance, licensees may impose a design requirement 

41 on an engineered barrier that provides confidence that the barrier’s capability exceeds the 

42 demand imposed by the scenario (e.g., by using a pre-determined safety factor). This excess 

43 capability would provide greater confidence that safe conditions are maintained during normal 

44 operations in light of uncertainties or in the event of abnormal occurrences, accidents, or 

45 disruptive events. Conversely, licensees may overestimate the demands imposed by a 

46 scenario when determining the required safety function needed from the layer of defense. The 

47 concept of the safety margin is to ensure that safety functions are sufficient to account for 
8-8

1 uncertainty in the characterization of the demands, as well as the robustness of the functionality. 

2 Safety margins can be determined for an individual layer or the entire disposal facility, but the 

3 margin for the disposal facility should be the primary interest for licensees and regulators.

4

5 Meeting the performance objectives, as demonstrated by the other 10 CFR 61.13 technical 

6 analyses, demonstrates a level of safety margin for a LLW disposal facility because the dose 

7 limits established in the performance objectives are purposefully established below the public 

8 dose limit specified in 10 CFR Part 20 (to account for the possibility of exposure from multiple 

9 facilities). Understanding the safety margin for abnormal occurrences, accidents, or disruptive 

10 events is important to provide confidence that the performance objectives can be met, even if 

11 less likely, but plausible, scenarios occur. Because abnormal occurrences, accidents, and 

12 disruptive events are plausible, but generally less likely, licensees may account for the likelihood 

13 of these events or processes occurring to determine the expected safety margin. For the post14 closure periods that are concerned with projected consequences, the licensee should 

15 demonstrate that for plausible abnormal occurrences, accidents, or disruptive events the

16 potential doses would remain below those for which intervention would be necessary if they 

17 were to occur today. Guidance on these levels is discussed in more detail in Section 8.3.3.2 for 

18 each post-closure period. Section 2.5.4 discusses the terms used to describe scenarios to be 

19 included in technical analyses (i.e., reasonably foreseeable, less likely, but plausible, and 

20 implausible). These terms and types of scenarios can also be used to demonstrate defense-in21 depth protections.

22 8.2.5 Risk-Informed Approach

23

24 Defense-in-depth should be applied in a risk-informed manner. As the hazard from the waste 

25 increases, more robust layers of protection may be needed to account for uncertainty in the 

26 performance of the barriers, controls, and personnel used to maintain safety. Alternatively, less 

27 robust layers of defense may make additional redundancy necessary, depending upon the 

28 uncertainty in the safety functions provided by the layer(s) and the risk posed by the waste. The 

29 timeframe that the safety functions are provided by the layers of protection should be 

30 appropriate for the time period over which significant risks are presented by the waste. Also, as 

31 uncertainty in the functionality and reliability of barriers, controls, or personnel grows, additional 

32 layers of protection may be needed to provide confidence that safety can be maintained. For 

33 instance, for significant concentrations of long-lived waste, additional layers of protection may 

34 be needed to account for the uncertainty associated with projecting performance over very long 

35 time periods. Whereas, for shorter-lived hazards, fewer layers of protection or layers that 

36 provide safety functions confidently over an appropriate time period may be sufficient.

37 8.3 Defense-in-Depth Analyses

38

39 The regulations at 10 CFR 61.13(f) require licensees to demonstrate that defense-in-depth 

40 protections are provided for the land disposal facility. The licensee must demonstrate that 

41 defense-in-depth protections are provided and should be performed in the context of the 

42 Subpart C performance objectives, namely protection of the general population, protection of 

43 inadvertent intruders, protection of individuals during operations, and stability of the disposal site 

44 after closure. Therefore, licensees should demonstrate in a risk-informed manner that multiple, 

45 independent, and redundant layers of defense are included in the disposal system to provide 

46 confidence that no single layer will be exclusively relied upon. Demonstrating defense-in-depth 

47 also provides greater confidence that each of the performance objectives can be met. In
8-9

1 providing layers of defense, licensees should consider the risks posed by the waste. Therefore, 

2 more layers or more robust layers may be needed for demonstrating safety will be maintained 

3 for higher risk scenarios. This section describes the role of defense-in-depth analysis required 

4 by 10 CFR Part 61 and acceptable approaches licensees may take to demonstrate that 

5 defense-in-depth protections are provided.

6

7 At a minimum, licensees should identify the defense-in-depth protections, describe the safety 

8 functions the protections perform, provide a technical basis for the safety function provided by 

9 each protection, and estimate the margin available to maintain safety. As described in Section 

10 2.3 of this guidance, licensees should identify the regulatory requirement to demonstrate that 

11 defense-in-depth protections are included at the land disposal facility as part of the assessment 

12 context process. Thus, the defense-in-depth protections will typically be represented in the 

13 system description (see Section 2.4) and modeled in one or more of the 10 CFR 61.13(a) 

14 through (e) technical analyses, such as the performance assessment or intruder assessment, in 

15 order to demonstrate that the performance objectives will be met. Therefore, licensees should 

16 be able to draw, principally, upon the results and risk insights gained from those other analyses 

17 to identify and describe defense-in-depth protections at the land disposal facility rather than 

18 developing separate analyses for demonstrating defense-in-depth. 

19

20 In some cases, licensees may need to consider whether additional features, events, and 

21 processes or alternative scenarios might be appropriate to consider solely for demonstrating 

22 that defense-in-depth protections are included (see Section 2.5). For example, a licensee may 

23 not expect a certain scenario to be reasonably foreseeable for the purposes of demonstrating 

24 that the performance objectives are met and would therefore not include such a scenario in the 

25 demonstration. However, for the purpose of demonstrating that adequate defense-in-depth 

26 protections are provided, the licensee should consider the less likely, but plausible scenario. 

27 Licensees would not need to consider scenarios that are sufficiently unlikely and can be 

28 considered implausible.

29 8.3.1 Identification of Defense-in-Depth Protections

30

31 Licensees should identify the defense-in-depth protections that are included at the land disposal 

32 facility. The identification of these protections, or layers of defense, should demonstrate that 

33 multiple barriers, controls or personnel are used at the land disposal facility. Section 8.2.1 

34 provides descriptions of barriers, controls, and personnel in the context of defense-in-depth 

35 protections for a land disposal facility. Licensees should also clearly indicate when layers are 

36 included for redundancy.

37

38 The specific defense-in-depth protections for the operational versus the post-closure time 

39 periods are likely to be different because active maintenance of the site after closure is not 

40 anticipated. Following closure of the land disposal facility, defense-in-depth protections shift 

41 from a collection of active and passive barriers and controls that are used during operations to 

42 reliance on more passive barriers and controls to provide reasonable assurance that the 

43 performance objectives will be met for timeframes far into the future. This shift is necessary 

44 because the regulations at 10 CFR Part 61 specify that active maintenance of the disposal site 

45 beyond monitoring, surveillance, and minor custodial activities cannot be relied upon after the 

46 period of post-closure observation and maintenance, and without ongoing maintenance, active 

47 barriers and controls cannot be relied upon indefinitely. Defense-in-depth protections for the 

48 post-closure period may include, but are not limited to, engineered features (e.g., closure caps, 
8-10

1 wasteforms, and containers) and natural characteristics (e.g., hydrogeology) of the disposal site 

2 that are intended to contain and isolate the waste, as well as controls such as institutional 

3 controls, which are designed to limit access to the disposal site for a limited period of time, and 

4 waste acceptance requirements, which are designed to limit the radionuclide inventory in the 

5 disposal site.

6

7 The identification should be generally consistent with the key barriers, controls, or personnel 

8 relied upon in the other 10 CFR 61.13 technical analyses. In some cases, the layers identified 

9 to describe defense-in-depth may not be represented in the other 10 CFR 61.13 analyses. This 

10 may often be the case for personnel identified as a defense-in-depth protection. In these cases, 

11 licensees should provide a basis for identifying the layer as a defense-in-depth protection. In 

12 other cases, there may be less significant barriers, controls, or personnel that provide 

13 demonstrable safety functions in the other 10 CFR 61.13 technical analyses. If more significant 

14 layers of defense provide multiple layers that can be shown to be sufficiently independent and 

15 redundant, licensees would not need to identify the less significant layers as demonstrated by 

16 the other 10 CFR 61.13 analyses. In other words, a licensee does not need to identify a 

17 comprehensive list of layers of defense. Rather, a licensee only needs to identify that there are 

18 multiple, independent, and redundant layers of defense, so that no single layer is exclusively 

19 relied upon for safety.

20

21 As part of the identification, licensees should also identify which layers of defense are included 

22 for redundancy. Licensees should identify the primary layer and any associated redundant 

23 layers that are expected to be relied upon in the event the primary layer degrades or fails early. 

24 Licensees may associate a secondary layer as a redundancy for multiple primary layers and do 

25 not need to limit the association of a redundant layer with a single primary layer. In some 

26 cases, multiple layers can provide redundancy without subordinating one of the layers in the 

27 identification. The classification of primary versus secondary in this case may be in name only.

28

29 Reviewers should confirm that the licensee has identified multiple layers of defense. While 

30 licensees do not need to identify a comprehensive list, reviewers should confirm that the layers 

31 of defense identified by the licensee are generally consistent with the layers represented in the 

32 other 10 CFR 61.13 technical analyses. As part of this confirmation, reviewers should evaluate 

33 whether the most significant barriers, controls, or personnel that are relied upon for meeting the 

34 performance objectives are identified as defense-in-depth protections.

35

36 Reviewers should also confirm that the licensee has identified at least one redundant and one 

37 independent layer of defense, though there may be different layers that provide the redundancy 

38 and independence. Reviewers should confirm that the identification clearly associates a 

39 redundant layer with each primary layer of defense. A single layer may be able to perform a 

40 safety function that is redundant to several primary layers. As discussed in the next section, the 

41 safety function provided by the redundant layer would need to be comparable, but not identical,

42 to the safety function provided by the primary layer and that often the designations of primary 

43 and secondary, or redundant, layers are in name only for the purposes of identification. 

44 Reviewers should also confirm that the licensee has identified which layers ensure 

45 independence. Independence should be assured for the disposal site as a whole. In other 

46 words, sufficient independence is demonstrated when common-cause failure scenarios are 

47 implausible or result in consequences that are acceptable. The next section also provides 

48 guidance on ensuring the safety functions for each independent layer are not subject to 

49 common-cause failures.
8-11

1 8.3.2 Description of Safety Functions

2

3 Licensees should describe the safety function(s) provided by each layer that is identified as a 

4 defense-in-depth protection for 10 CFR 61.13(f). Licensees should either qualitatively or 

5 quantitatively describe the capability of the individual layers identified to maintain safety. In 

6 addition, the description should include a technical basis supporting for the safety function. In 

7 general the technical basis supporting the safety function can be drawn from the technical basis 

8 used to support the layer’s representation in the other 10 CFR 61.13 analyses. General 

9 guidance on developing support for the technical analyses is provided in Section 2.2, and 

10 specific guidance for each of the other 10 CFR 61.13 analyses is provided in their 

11 respective sections of this document. 

12

13 Often, for a land disposal facility, the safety functions focus on (1) limiting the contact of water or 

14 individuals with the waste, (2) minimizing the release of radionuclides from the waste, or (3)

15 minimizing the rate of radionuclide transport through the site environment. In addition, safety 

16 functions cover other considerations such as maintaining structural stability or limiting 

17 exposures to radioactivity. The safety function(s) may vary depending upon which performance 

18 objective the layer of defense is focused. In addition, a layer of defense may provide more than 

19 one safety function and these different safety functions may be aimed at providing confidence 

20 that different performance objectives will be met. Thus, licensees should clearly identify the 

21 capabilities the layer performs and the performance objective(s) the safety function is focused 

22 on in their description of the safety function for an individual layer.

23

24 If a layer of defense is included for redundancy, the licensee should clearly indicate other layers 

25 of defense the redundant layer is intended to support. Also, licensees should describe whether 

26 the safety function is independent or dependent upon the safety functions of other layers. If a 

27 layer’s safety function is dependent upon another layer, the licensee should identify the other 

28 layer(s) in each layer’s safety function description. If a layer’s safety function is independent of 

29 other layers’ safety functions, the licensee should provide justification that reasonably 

30 foreseeable and plausible common-cause failure scenarios would not result in significant 

31 consequences should they occur. Alternatively, licensees could identify additional layers of 

32 defense which would render the likelihood of the common-cause failure scenario as implausible.

33

34 Licensees should ensure that the description of the safety function is consistent with the 

35 representation of the layer of defense in the other 10 CFR 61.13 analyses. For instance, if a 

36 waste container is identified as a layer of defense aimed toward providing confidence that the 

37 performance objective for protection of the general population can be met, the description of its 

38 safety function should be consistent with its representation in the performance assessment. If 

39 the waste container is relied upon to limit water contacting waste and were to be modeled as 

40 degrading over time in the performance assessment, the licensee should identify the 

41 degradation of its safety function as part of the description for defense-in-depth. 

42

43 In some cases, layers of defense may not be amenable to representation in one of the other 

44 10 CFR 61.13 analyses. For instance, representing the response of personnel to an emergency 

45 such as a fire may not be amenable to representation in one of the other 10 CFR 61.13 

46 analyses because the behavior of the personnel may vary depending upon the specific 

47 circumstance of the fire and would be difficult to simulate. In these cases, licensees would need 

48 to document the safety functions performed by the layers for demonstrating defense-in-depth. 
8-12

1 For the emergency personnel example, this may include written procedures for training and 

2 emergency response, staffing plans, or agreements with local emergency responders.

3

4 Each layer of defense is likely to have a time period over which it will be designed to perform its 

5 intended safety function. For instance, the safety functions of engineered barriers would 

6 typically be expected to degrade and fail at some point in the future. However, some site 

7 characteristics may continue to provide safety functions indefinitely. The licensee should 

8 describe and justify this time period in its technical basis supporting the safety function of the 

9 layer. If the time period over which the layer is intended to perform its safety function 

10 significantly exceeds relevant experience, the licensee should provide additional support that 

11 the layer will likely achieve its goal. Additional support could include the use of additional 

12 redundant layers of defense, though support would be needed that the principal layer and any 

13 redundant layers in combination would perform the safety function over the intended time 

14 period.

15

16 Licensees should also describe the uncertainty in each layer’s ability to perform its intended 

17 safety function and the time period over which it is expected to perform. The description of the 

18 uncertainty may be either qualitative or quantitative. The description of the uncertainty should 

19 be generally consistent with the layer’s representation in the other 10 CFR 61.13 analyses. In 

20 describing the uncertainty associated with the safety function of a barrier, control, or personnel, 

21 licensees should identify heterogeneity and variability in the behavior or reliability of the barrier, 

22 control, or personnel as well as challenges to the functionality provided by a barrier, control, or 

23 personnel that result from plausible scenarios. Challenges should include both reasonably 

24 foreseeable scenarios expected during normal operations as well as less likely but plausible 

25 scenarios such as abnormal conditions, accidents, or disruptive events.

26

27 Reviewers should evaluate the description of the safety function of each layer to ensure that it 

28 clearly describes how the layer provides defense-in-depth protections and which performance 

29 objective the safety function supports. Reviewers should also confirm the description includes 

30 whether the safety function is included for redundancy, and whether the safety function is 

31 independent of or dependent upon another layer’s safety function.

32

33 Reviewers should confirm that the safety function for a layer identified as redundant is clearly 

34 linked to the layer it is intended to support. Reviewers should confirm that the redundant layer’s 

35 safety function provides a comparable capability to the primary layer’s safety function. In some 

36 cases redundant layers that are identical to the primary layer are not possible. However, 

37 licensees could identify another barrier that may provide a comparable safety function. 

38 Reviewers should confirm that the safety functions are comparable. For instance, different 

39 hydrostratigraphic layers of the site may provide similar, but not identical safety functions. 

40 Likewise, an engineered barrier, such as a permeable reactive barrier, and a hydrostratigraphic 

41 layer of the site may both retard the movement of radionuclides as their safety functions. In this 

42 example, the safety functions are similar but the layers are not identical. 

43

44 In addition to evaluating the licensee’s basis for a redundant safety function, reviewers should 

45 also evaluate whether the layers identified as independent are subject to common-cause 

46 failures based on reasonably foreseeable as well as less likely, but plausible scenarios. 

47 Plausible common-cause failures may result from a common initiating event such as a seismic 

48 event that fails two barriers simultaneously. Plausible common-cause failures may also result 

49 from a cascade of processes such as when a geochemical environment leads to the 
8-13

1 degradation of a barrier whose degradation produces an environment that leads to the 

2 degradation of a different barrier, which may not have been affected by the initial aggressive 

3 environment.

4

5 Reviewers should also confirm the licensee’s safety function descriptions include a discussion 

6 of the time period over which the layer’s safety function is intended to perform and the 

7 uncertainty in whether the layer can perform the safety function. For layers of defense which 

8 are expected to perform for time periods that significantly exceed relevant experience, reviewers 

9 should confirm that the licensee has provided sufficient justification. Reviewers should also 

10 coordinate the review of the safety function descriptions with reviews of the other 10 CFR 61.13 

11 analyses to ensure that the layers and their safety functions are implemented in those other 

12 analyses consistent with the licensee’s description of the safety function for defense-in-depth. If 

13 redundant layers are expected to perform over different time periods than the primary layers, 

14 reviewers should confirm that the licensee has identified other layers to ensure that redundancy 

15 is provided for the entire time period of interest or that the consequences would remain 

16 acceptable should redundancy not be possible for the entire time period of interest.

17

18 Reviewers should also confirm the licensee has included a description of the uncertainty in each 

19 layer’s ability to perform its intended safety function and the time period over which it is 

20 expected to perform as part of the safety function description. Reviewers should coordinate 

21 their review of the description of uncertainty with the reviews of the other 10 CFR 61.13 

22 analyses to ensure that the uncertainty descriptions are generally consistent and describe the 

23 heterogeneity and variability in the behavior or reliability of the barrier, control, or personnel as 

24 well as challenges to the functionality provided by a barrier, control, or personnel that result from 

25 plausible scenarios.

26 8.3.3 Demonstrating Safety Margin

27

28 After describing the safety function provided by each layer identified as a defense-in-depth 

29 protection, licensees should demonstrate that the layers will maintain safety and that no single 

30 layer will be relied upon exclusively for safety. The level of detail provided by the licensee in 

31 describing the technical basis should be risk-informed. Namely, for layers of defense that 

32 provide one or more significant safety functions, licensees should provide a more detailed 

33 description of the basis for the safety function provided by the layer of defense.

34

35 Licensees can use a variety of methods to demonstrate that safety functions will perform 

36 adequately and that no single layer will be relied upon for safety in light of the uncertainties in 

37 the performance of the layers and the evolution of the disposal site over long periods of time 

38 such as those associated with the post-closure period. Generally, the methods may be 

39 quantitative, qualitative, or a combination of the two approaches. Quantitative approaches

40 attempt to assign numerical values to the safety functions and the resulting safety margin 

41 provided by the layers of defense relied upon for safety. Qualitative approaches are less 

42 numerical and narratively describe the safety functions and the resulting safety margin of the 

43 various layers of defense relied upon for safety.

44

45 Licensees may use different approaches to demonstrate layers of defense for different stages or 

46 even within a stage of the land disposal facility’s lifecycle. For instance, while some layers of 

47 defense used during operations may be amenable to quantification other layers of defense may 

48 not be, such as the safety margin provided by an emergency response plan to mitigate the 
8-14

1 consequences of an accident. Typically, licensees should be able to use the results of the other 

2 10 CFR 61.13 analyses to demonstrate that the layers will ensure the performance objectives 

3 are met for reasonably foreseeable scenarios and that the consequences from less likely but 

4 plausible scenarios would not be so large to require intervention if they were to occur today. For 

5 instance, for the post-closure period, licensees should draw risk insights from the performance 

6 assessment (see Section 3.0), intruder assessment (see Section 4.0), and site-stability analyses 

7 (see Section 5.0) to demonstrate that no single layer is relied upon exclusively for safety over 

8 the various time periods.

9

10 Rather than developing new analyses, licensees may use uncertainty analyses conducted for 

11 the other 10 CFR 61.13 analyses such as barrier analyses, including one-off or what-if types of 

12 analyses, to demonstrate adequate independence and redundancy is provided and that no 

13 single layer is relied upon for safety for both reasonably foreseeable and less likely, but 

14 plausible scenarios over the time period of interest. The results of these uncertainty analyses 

15 can be used to demonstrate that if any single barrier fails during the time period of interest, 

16 another barrier is available to provide a similar and adequate level of protection. The results of 

17 the other 10 CFR 61.13 analyses can also be used to demonstrate that common-cause failures 

18 would not result from reasonably foreseeable or less likely, but plausible scenarios or if 

19 common-cause failure were to occur for plausible scenarios, the consequences would not be so 

20 large as to require intervention today. Barrier analyses are described in more detail in 

21 Section 7.4 for analyses for long-lived waste. Although the description in Section 7.4 relates to 

22 performance period analyses, the barrier analysis techniques are also generally appropriate for 

23 any of the post-closure time periods of interest. Consequences that may require intervention 

24 today are discussed in the following sections for each time period with due consideration for the 

25 uncertainty associated with projecting safety functions, human activities, and the behavior of the 

26 disposal site environment far into the future.

27

28 The lifecycle of a land disposal facility can be divided into two broad phases: the operational 

29 phase and the post-closure phase. The operational phase is the time period during which the 

30 facility is being constructed, is receiving waste for disposal, or is preparing for closure. The 

31 post-closure phase extends from the cessation of operations far into the future depending upon 

32 the type of waste accepted for disposal. At a minimum the post-closure phase would include 

33 the compliance and protective assurance periods. For facilities disposing of significant 

34 quantities of long-lived waste, the post-closure phase would also include the performance 

35 period. Because of the differences in length of the time periods for the operational and post36 closure phases, the role of defense-in-depth analyses may be markedly different for 

37 each phase.

38

39 The following two sections describe considerations for licensees and reviewers for the two 

40 major lifecycle phases of a land disposal facility and guidance on demonstrating a land disposal 

41 facility’s safety margin provided by the defense-in-depth protections and demonstrating that no 

42 single layer, not matter how robust, will be exclusively relied upon for safety.

43 8.3.3.1 Operational Period

44

45 During the operational phase of the land disposal facility, defense-in-depth protections may be 

46 similar to those used at other nuclear facilities that present similar hazards. Defense-in-depth 

47 protections during operations may include, but are not limited to: (1) the selection and use of 

48 facility capabilities, such as functions, structures, systems, and components of the facility 
8-15

1 design; (2) programmatic processes, such as decisions regarding the processes of constructing, 

2 operating, maintaining, testing, and inspecting the plant, as well as processes that ensure 

3 facility safety through its operational lifetime; and (3) risk-informed strategies that manage the 

4 risks of accidents, including the strategies of accident prevention and mitigation.

5

6 During the operational period, licensees can draw upon risk insights gained from the analyses 

7 used to demonstrate compliance with the performance objectives for operations including 

8 protection of the general population, protection of inadvertent intruders, and protection of 

9 individuals during operations (i.e., 10 CFR 61.41 through 10 CFR 61.43) to demonstrate that 

10 defense-in-depth protections are included. The defense-in-depth analyses for an operating land 

11 disposal facilty are expected to be similar to demonstrations of defense-in-depth for other 

12 operating facilities that use radioactive materials (e.g., nuclear reactors). For instance, the land 

13 disposal facility would use procedures and engineering controls as part of a radiation control 

14 program, required by 10 CFR 20.1101, to minimize occupational doses and doses to members 

15 of the public. In this example, the licensee would need to identify the specific procedures and 

16 controls that are relied upon for safety should plausible scenarios that challenge safety functions 

17 occur, describe the safety functions associated with the procedures and controls and when 

18 those safety functions are expected to be necessary, as well as describe the uncertainty in the 

19 safety functions to perform adequately.

20

21 Licensees can use the results of analyses that are similar to those used at other nuclear 

22 facilities (e.g., an integrated safety analysis as described in NUREG-1513 (NRC, 2001) for 

23 licensing of special nuclear material) to demonstrate that no single layer of defense will be 

24 exclusively relied upon for safety at an operating land disposal facility. The amount of detail 

25 needed for a land disposal facility may be markedly different than for other facilities depending 

26 upon the hazards present and the risks involved in the use of the radioactive material at the 

27 disposal facility or the handling of waste for disposal. 

28

29 Licensees should examine whether the layers of defense, their associated safety functions, and 

30 uncertainty ensure safety during operations by limiting the likelihood or consequences during 

31 reasonably foreseeable and less likely, but plausible scenarios. For facilities or activities with 

32 more significant hazards (e.g., emplacement of high activity waste), which could result in 

33 significant potential exposures to workers or the public during operations, additional redundancy 

34 and a stronger basis for independence among the layers may be necessary. Reviewers should 

35 consider potential exposures from plausible scenarios during operations of a land disposal 

36 facility as significant for the purposes of defense-in-depth when they are expected to exceed the 

37 occupational dose limits or the dose limits for members of the public specified in 10 CFR 

38 Part 20, Subparts C and D, respectively. For plausible scenarios with significant exposures, 

39 licensees should consider additional layers of defense, further redundancy, improved 

40 independence, or reduced uncertainty in the safety function of the layers to ensure that the 

41 likelihood of significant exposures is reduced so that the scenario becomes implausible or that 

42 the consequences are sufficiently reduced.

43 8.3.3.2 Post-Closure Period

44

45 Following closure of the land disposal facility, defense-in-depth protections shift from a 

46 collection of active and passive barriers and controls that are used during operations to reliance 

47 on controls and passive barriers. Defense-in-depth protections for the post-closure period may 

48 include, but are not limited to, engineered features (e.g., closure caps, wasteforms, and 
8-16

1 containers) and natural characteristics (e.g., hydrogeology) of the disposal site that are intended 

2 to contain and isolate the waste, as well as controls such as institutional controls, which are 

3 designed to limit access to the disposal site for a limited period of time, and waste acceptance 

4 requirements, which are designed to limit the radionuclide inventory in the disposal site.

5

6 As a result of increasing uncertainty in the behavior of the layers of defense, particularly 

7 barriers, and the disposal site environment, the associated uncertainty in the margin of safety 

8 provided by the layers of defense is also expected to increase. In addition to the barriers 

9 themselves, licensees may also need to consider additional controls to ensure that the barriers 

10 relied upon for safety will perform adequately. These controls may include quality assurance 

11 controls during the design, construction, operation, and maintenance of engineered barriers 

12 (e.g., engineered closure caps, wasteforms, or containers) or, more importantly, given the long 

13 timeframes involved, additional inventory controls to limit the amount of waste disposed at the 

14 disposal site. Development of waste acceptance criteria is described further in Section 9.0.

15

16 For the post-closure period, licensees can draw upon risk insights gained from the results of the 

17 other 10 CFR 61.13 post-closure analyses (i.e., performance assessment, intruder assessment, 

18 and stability analyses) rather than developing specific analyses for demonstrating that defense19 in-depth protections are included. Those other 10 CFR 61.13 analyses are expected to focus 

20 on protections that can maintain safety, in the context of the post-closure performance 

21 objectives, for the disposed waste over the various regulatory time periods that are pertinent. 

22 The post-closure period is subdivided into three timeframes: the compliance period, the 

23 protective assurance period, and the performance period. Depending upon the waste received, 

24 the performance period may not apply to the land disposal facility and defense-in-depth 

25 analyses may not be needed for the longer-term period (i.e., analyses that extend beyond 

26 10,000 years). Because 10 CFR Part 61 does not envision ongoing active maintenance and 

27 monitoring at the disposal site, the layers of defense for these three time periods are expected 

28 to be primarily barriers and inventory limits after 100 years following closure, when the 

29 institutional controls are assumed to no longer be effective.

30

31 Licensees can use results from the uncertainty analyses performed for the other 10 CFR 61.13 

32 analyses to demonstrate that adequate defense-in-depth protections are included. For 

33 instance, Section 7.4 describes how to conduct barrier analyses for demonstrating compliance 

34 during the performance period. Barrier analyses are common uncertainty analyses performed 

35 as part of a performance assessment, and though the guidance in Section 7.4 is focused on the 

36 performance period, it is generally applicable to conducting barrier analyses for any of the post37 closure time periods. By examining the consequences of early degradation or failure of one or 

38 more of the layers of defense, licensees can demonstrate that safety can be maintained or 

39 determine that additional layers of defense, further redundancy, improved independence, or 

40 reduced uncertainty in the safety function of a layer may be needed.

41

42 To demonstrate that the layers of defense are adequate, licensees should examine not only the 

43 reasonably foreseeable scenarios considered for demonstrating that the performance objectives 

44 would be met, but also less likely, but plausible scenarios (e.g., accidents, disruptive events, 

45 abnormal occurrences). If the performance objectives are met, significant consequences are 

46 generally not expected for reasonably foreseeable scenarios. However, significant 

47 consequences may be possible for less likely, but plausible scenarios. The following sections 

48 provide guidance on when additional layers of defense, further redundancy, improved 
8-17

1 independence, or reduced uncertainty in the safety function of a layer may be needed to 

2 demonstrate that adequate defense-in-depth protections are included for each of the three post3 closure time periods.

4 8.3.3.2.1 Compliance Period

5

6 During the compliance period, uncertainty in the behavior of the layers of defense and the 

7 evolution of the disposal site environment at a well-sited disposal facility is expected to be more 

8 manageable than for longer time periods. In terms of the disposal site and design, licensees 

9 should demonstrate that the margin of safety determined by the layers of defense provides 

10 confidence that the limits specified in 10 CFR 61.41(a) and 10 CFR 61.42(a) are not expected to 

11 be exceeded for the central scenario. To demonstrate that an adequate margin of safety is 

12 available, licensees could compare probabilistic dose curves derived from the analyses used to 

13 demonstrate that the performance objectives are met below the designated limit. At a minimum, 

14 unless otherwise authorized, licensees must demonstrate that the specified limit is not 

15 exceeded for the mean doses during the compliance period to demonstrate that the 

16 performance objectives are met. If the peak of the mean dose curve exceeds the specified limit, 

17 then additional layers of defense or added redundancy are necessary for engineering design of 

18 the facility or the disposal site to be appropriate for disposal. Guidance on demonstrating that 

19 the performance objectives are met is described in Section 3.0 for protection of the general 

20 population and Section 4.0 for protection of inadvertent intruders. Guidance on demonstrating 

21 the long-term stability of the disposal site is discussed in Section 5.0.

22

23 Once licensees demonstrate that the performance objectives are met, they should demonstrate 

24 that an adequate margin of safety is provided by the layers of defense such that significant 

25 exposures, which would require future intervention to mitigate, would not be expected to occur. 

To do this, licensees should demonstrate that the 95th 26 percentile of annual doses from 

27 probabilistic analyses of the central scenario at each discrete time during the compliance period 

28 is less than the dose limits for members of the public (i.e., 1 mSv [100 mrem]), as described in 

29 Section 3.2.4.3 of NUREG-1573 (NRC, 2000a), and less than 20 mSv (2 rem) for protection of 

30 inadvertent intruders, an exposure level for human intruders above which alternative options for 

31 waste disposal are to be considered (IAEA, 2011). 

32

33 Additionally, if the peak annual dose from any single or subset of realizations of the central or 

34 alternative scenarios exceeds 50 mSv (5 rem) for either protection of the general population or 

35 inadvertent intruders, licensees should examine those realizations (or alternative scenarios) to 

36 determine whether additional layers of defense, independence, or redundancy would be 

37 beneficial. The 50 mSv (5 rem) guidance threshold provides confidence that any plausible 

38 projected scenario would not be expected to exceed a level that would almost always justify 

39 intervention in an emergency situation. The ICRP advises that intervention would almost 

40 always be justified for existing exposures that would exceed 100 mSv (10 rem) (ICRP, 2007). In 

41 this case, additional layers of defense, independence or redundancy should ensure that doses 

42 for any plausible scenario are maintained well below levels where future intervention to mitigate 

43 significant exposures would be necessary or that the likelihood of the realization would be 

44 reduced by the additional layers of defense to render it implausible.

45

46 If a licensee performs deterministic rather than probabilistic analyses, the licensee should 

47 demonstrate that all annual doses from reasonably foreseeable or less likely, but plausible 
8-18

1 scenarios are less than 1 mSv (100 mrem) for protection of the public and less than 20 mSv 

2 (2 rem) for protection of inadvertent intruders, in order to ensure doses would be maintained 

3 well below levels where future intervention to mitigate significant exposures would be 

4 necessary.

5

6 Disposal facilities that could result in potential doses from reasonably foreseeable or less likely 

7 but plausible scenarios that exceed those mentioned in the previous paragraphs during the 

8 compliance period are considered higher-risk disposal facilities. Licensees should employ 

9 additional layers of protection or ensure that sufficient independence and redundancy will be 

10 provided to ensure that common-cause failures are minimized and adequate redundancy in the 

11 safety function is provided so that a significant exposure is highly unlikely to occur.

12 8.3.3.2.2 Protective Assurance Period

13

14 During the protective assurance period, uncertainty in the behavior of the layers of defense and 

15 the evolution of the disposal site environment is expected to increase, due to lack of knowledge 

16 about the key properties of the layers of defense and the disposal site environment, as well as 

17 FEPs and human activities that may occur in the future at or near the disposal site. In terms of 

18 the disposal site and design, licensees should demonstrate that the margin of safety determined 

19 by the layers of defense provides confidence that the exposures will be below a reasonably 

20 achievable level, as required in 10 CFR 61.41(b) and 10 CFR 61.42(b), for the central scenario. 

21 The performance objectives for the protective assurance period require the minimization of 

22 exposures, with a goal of limiting the annual dose below 5 mSv (500 mrem) or a level that is 

23 reasonably achievable based on technological and economic considerations. Therefore, at a 

24 minimum, licensees must demonstrate that the specified minimization goal for the disposal site 

25 is met. If the minimization goal is not met, licensees would need to add additional barriers or 

26 impose additional controls to demonstrate that the minimization goal will be met. Guidance on 

27 developing minimization targets and demonstrating that minimization is met is provided in 

28 Section 6.0.

29

30 Once licensees demonstrate that the performance objectives are met, licensees should 

31 demonstrate that an adequate margin of safety is provided by the layers of defense for the 

32 protective assurance period such that significant consequences are minimized for plausible 

33 scenarios. Licensees should be able to draw risk insights from the results of the performance 

34 assessment (see Section 3.0), intruder assessment (see Section 4.0), and site stability analyses 

35 (see Section 5.0) to estimate safety functions and their associated uncertainty during the 

36 protective assurance period. 

37

38 To demonstrate that an adequate margin of safety is provided for the protective assurance 

39 period, licensees should demonstrate that the layers of defense maintain the peak annual dose 

40 from any single or subset of realizations (or from a less likely, but plausible alternative scenario 

41 for a deterministic analyses) below 50 mSv (5 rem) for either protection of the general 

42 population or inadvertent intruders. The 50 mSv (5 rem) guidance threshold provides 

43 confidence that any plausible projected scenario would not be expected to exceed a level that 

44 would almost always justify intervention in an emergency situation. The ICRP advises that 

45 intervention would almost always be justified for existing exposures that would exceed 100 mSv 

46 (10 rem) (ICRP, 2007). Disposal facilities that could result in potential doses from reasonably 

47 foreseeable or less likely, but plausible scenarios that exceed 50 mSv (5 rem) during the 

48 protective assurance period are considered higher-risk disposal facilities.
8-19

1

2 At higher-risk facilities, licensees should employ additional layers of protection or ensure that 

3 sufficient independence and redundancy will be provided to ensure that common-cause failures 

4 are minimized and adequate redundancy in the safety function is provided so that a significant 

5 exposure is highly unlikely to occur. In deciding whether additional layers of defense would be 

6 needed, licensees may consider technological or economic limitations to employing additional 

7 barriers. However, the additional cost of limiting inventory is typically not expected to be overly 

8 burdensome from either a technical or economic consideration and is expected to provide the 

9 greatest certainty that the performance objectives can be met. Therefore, licensees and 

10 reviewers should strongly consider additional inventory limits as added controls or alternative 

11 siting to ensure the performance of the existing barriers when other additional layers of defense 

12 may not be practical for technological or economic reasons.

13 8.3.3.2.3 Performance Period

14

15 During the performance period, uncertainty in the behavior of the layers of defense and the 

16 evolution of the disposal site environment is expected to be significantly larger than other 

17 analysis time periods. The increased uncertainty is due to lack of knowledge about the key 

18 properties of the layers of defense and the disposal site environment as well as FEPs and 

19 human activities that may occur in the future at or near the disposal site. As a result of 

20 increasing uncertainty in FEPs that could be expected to occur, the associated uncertainty in

21 the margin of safety provided by the layers of defense is also expected to increase. 

22

23 In terms of the disposal site and design, licensees should demonstrate that the the layers of 

24 defense provide confidence that releases and exposures will be minimized to the extent 

25 reasonably achievable, as specified in 10 CFR 61.41(c) and 10 CFR 61.42(c), respectively. 

26 Guidance on developing demonstrating that minimization is met for the performance period is 

27 provided in Section 7.0.

28

29 Once licensees demonstrate that the performance objectives are met, licensees should 

30 demonstrate that an adequate margin of safety is provided by the layers of defense for the 

31 performance period such that significant consequences are minimized for plausible scenarios. 

32 To demonstrate that an adequate margin of safety is maintained during the performance period, 

33 licensees should demonstrate that the layers of defense maintain releases from the disposal 

34 site that result in exposures to the general population or exposures to an inadvertent intruder at 

35 or below 50 mSv (5 rem). The 50 mSv (5 rem) guidance threshold provides confidence that any 

36 plausible projected scenario would not be expected to exceed a level that would almost always 

37 justify intervention in an emergency situation. The ICRP advises that intervention would almost 

38 always be justified for existing exposures that would exceed 100 mSv (10 rem) (ICRP, 2007). 

39 Disposal facilities that could result in potential doses that exceed 50 mSv (5 rem) for plausible 

40 scenarios during the performance period are considered higher-risk disposal facilities. 

41 Licensees may also be able to develop alternative measures of safety margin. For instance, 

42 licensees could compare concentrations in media to concentrations that would require 

43 intervention today should a less likely, but plausible scenario occur.

44

45 Disposal facilities that could result in potential doses from reasonably foreseeable or less likely 

46 but plausible scenarios that exceed those mentioned in the previous paragraphs during the 

47 compliance period are considered higher-risk disposal facilities. Licensees should employ 
8-20

1 additional layers of protection or ensure that sufficient independence and redundancy will be 

2 provided for the disposal site to ensure that common-cause failures are minimized and 

3 adequate redundancy in the safety function is provided so that a significant exposure is highly 

4 unlikely to occur.

5

6 In deciding whether additional layers of defense would be needed, licensees may consider 

7 technological or economic limitations to employing additional barriers. However, the additional 

8 cost of limiting inventory is typically not expected to be overly burdensome from either a 

9 technical or economic consideration and is expected to provide the greatest certainty that the 

10 performance objectives can be met. Therefore, licensees and reviewers should strongly 

11 consider additional inventory limits as added controls or alternative siting to ensure the 

12 performance of the existing barriers when other additional layers of defense may not be 

13 practical for technological or economic reasons.

14

15

16

17

18
9-1

1 9.0 WASTE ACCEPTANCE

2

3 Section 61.23 of 10 CFR Part 61 specifies standards that must be met to receive a license to 

4 operate a land disposal facility for LLW. The standards require licensees to demonstrate that 

5 the waste acceptance criteria and other components of the licensee’s proposal are adequate to 

6 protect public health and safety and provide reasonable assurance that the performance 

7 objectives specified in Subpart C will be met. The regulations at 10 CFR 61.58 specify the 

8 requirements for waste acceptance. The regulations require licensees to identify (i) criteria for 

9 the acceptance of waste for disposal, (ii) acceptable methods for characterizing the waste, and 

10 (iii) a program to certify that waste meets the acceptance criteria prior to receipt at a land 

11 disposal facility for radioactive waste. Figure 9-1 illustrates the main components of waste 

12 acceptance under 10 CFR Part 61. The regulations also require licensees to review the content 

13 and implementation of the waste acceptance criteria, characterization methods, and certification 

14 program at least annually.

15

16

17

18

19 Figure 9-1 Waste Acceptance Components

20

21 This section describes the information that a licensee should provide and a reviewer should 

22 evaluate with respect to the waste acceptance requirements. First, the waste acceptance 

23 criteria identify the following for waste generators: (1) the allowable limits on radionuclides, 

24 (2) acceptable wasteforms and container specifications, and (3) restrictions or prohibitions in 

25 order for waste to be accepted for disposal as LLW. Section 9.1 describes information that 

26 licensees should provide to demonstrate that the waste acceptance criteria will be adequate. 

27 Second, in order to demonstrate that the waste meets the acceptance criteria, waste must be 

28 adequately characterized. Section 9.2 provides guidance to licensees on defining acceptable 

Waste 

Acceptance

Characterization 

Criteria

Certification
9-2

1 methods for waste characterization. Finally, waste must 

2 be certified to ensure that waste meets the acceptance 

3 criteria and is, therefore, suitable for disposal in a land 

4 disposal facility. Section 9.3 describes information that 

5 licensees should include in a certification program to 

6 ensure that waste received at the disposal facility is 

7 acceptable for disposal.

8 9.1 Waste Acceptance Criteria

9

10 Section 61.52 of 10 CFR Part 61 requires that all waste 

11 disposed in the disposal site must meet approved waste 

12 acceptance criteria. Section 61.58 of 10 CFR Part 61 

13 requires licensees to submit proposed waste 

14 acceptance criteria for approval. Once the disposal site 

15 regulator approves the waste acceptance criteria, 

16 licensees wishing to make modifications to the criteria 

17 must request an amendment. This section provides 

18 guidance on developing or modifying waste acceptance 

19 criteria and discusses information that licensees should 

20 include in order to allow a regulator to evaluate whether 

21 the proposed waste acceptance criteria provide 

22 reasonable assurance that the performance objectives 

23 will be met.

24

25 Section 61.58 of 10 CFR Part 61 specifies the 

26 requirements for waste acceptance at a disposal facility 

27 for LLW. Waste acceptance criteria are intended to 

28 provide reasonable assurance that the performance objectives of 10 CFR Part 61 will be met. 

29 The regulations require licensees to identify the criteria for the acceptance of waste for disposal 

30 (i.e., waste acceptance criteria). Specifically, the regulations require that the waste acceptance 

31 criteria specify, at a minimum, allowable activities and concentrations of specific radionuclides, 

32 acceptable wasteform characteristics and container specifications, and restrictions or 

33 prohibitions on waste, materials, or containers. Figure 9-2 depicts the minimum components of 

34 the waste acceptance criteria. 

35

36 Licensees may need to specify other criteria beyond those required by 10 CFR Part 61 to satisfy 

37 other regulatory requirements. For instance, land disposal facilities may also be required by 

38 other Federal or State regulations to limit certain non-radiological materials because of their 

39 impact on public health and safety and the environment. This guidance document focuses only 

40 on the waste acceptance criteria required by 10 CFR Part 61.

41 9.1.1 Allowable Activities and Concentrations

42

43 The waste acceptance requirements of 10 CFR Part 61 allow licensees the option to develop 

44 allowable limits from either the technical analyses required in 10 CFR 61.13 for any land 

45 disposal facility or the waste classification limits in 10 CFR 61.55 for a near-surface disposal 

46 facility. For instance, licensees disposing of waste that is similar to the waste streams 

47 considered in the development of the waste classification limits may wish to rely on those limits. 

Figure 9-2 Components of 

10 CFR Part 61 

Waste Acceptance 

Criteria
9-3

1 Whereas, licensees disposing of waste streams beyond those considered for the waste 

2 classification requirements may wish to determine limits from the results of their technical 

3 analyses. Likewise, licensees with facility designs, operational practices, or site characteristics 

4 that differ significantly from those considered to develop the waste classification limits, may also 

5 wish to develop site-specific waste acceptance criteria. The unique characteristics of the waste 

6 in concert with the disposal site and design are the primary determinants of risk from near7 surface disposal. Thus, not all radioactive waste streams may be suitable for near-surface 

8 disposal. 

9

10 This section of the guidance document describes the information that a licensee should provide 

11 and that a reviewer should evaluate with respect to the development of allowable activities and 

12 concentrations for radionuclides. Reviewers may want to consult Standard Review Plan 6.1.1 of 

13 NUREG-1200 (NRC, 1994) to ensure that the allowable limits specified by the licensee’s waste 

14 acceptance criteria are reasonable, given the types and quantities of radionuclides projected for 

15 disposal.

16 9.1.1.1 Allowable Limits Derived from Technical Analyses

17

18 Radioactivity disposed in a land disposal facility may need to be limited to ensure (i) protection 

19 of the general public from releases during operations and after operations have ceased, (ii) 

20 protection of individuals who may inadvertently intrude into the disposal site after active 

21 institutional controls are removed, (iii) protection of individuals during operations, and (iv) 

22 stability of the disposal site after closure. Limits on radioactivity disposed in a land disposal 

23 facility can also provide defense-in-depth. The limits may vary widely from site-to-site 

24 depending on the waste streams proposed for disposal, facility design, and site characteristics. 

25

26 Licensees who elect to develop limits on radionuclide activities or concentrations from the 

27 results of the analyses required in 10 CFR 61.13 should document how the proposed limits are 

28 developed from the analyses performed to satisfy the requirements of 10 CFR 61.13. The 

29 proposed limits should focus on radionuclides that may affect meeting the performance 

30 objectives. These limits may be unique for specific waste streams or total limits for the disposal 

31 site. Proposed limits for specific waste streams may be more appropriate when unique factors 

32 may affect meeting one or more of the 10 CFR Part 61 performance objectives such as the 

33 anticipated release from a particular wasteform, the concentrations of radionuclides in the 

34 waste, the potential for criticality due to the presence of special nuclear material in the waste, 

35 the radiation fields emanating from the waste, or the heat generated by the decaying waste. 

36

37 Licensees should develop allowable limits, either total activity or concentration, from the 

38 resulting peak doses from a unit activity of each radionuclide for the performance objective 

39 which is the most limiting for each radionuclide. When evaluating each performance objective, 

40 the licensee should use the scenario(s) used to demonstrate that the performance objectives 

41 are met to establish the limits. Selection of the scenario(s) for demonstrating compliance with 

42 the performance objectives is described in Sections 2.0, 3.0, and 4.0 of this document. To 

43 develop limits, licensees should compare the peak dose for each radionuclide with the limits 

44 specified by the performance objective. 

45

46

47

48
9-4

1 Licensees may calculate a limit for each radionuclide using:

2

3 (9.1)

4 where

Limiti,s is the total activity [Bq] or activity concentration [Bq/m3 5 or Bq/g] limit of 

6 radionuclide i in waste for scenario s;

7

8 Dose Limits is the dose limit [mSv/yr] for scenario s;

9

Activity(0)i is the initial total activity [Bq] or activity concentration [Bq/m3 10 or Bq/g] of 

11 radionuclide i in the waste; and

12

13 Peak Dosei,s is the annual peak dose [mSv/yr] resulting from the Activity(0)i of radionuclide i

14 for scenario s.

15

16 Licensees should clearly identify the technical basis for each allowable limit specified in the 

17 waste acceptance criteria. The basis for each proposed limit should include the performance 

18 objective(s) that the limits are designed to support. For instance, proposed limits for some 

19 radionuclides may be more relevant to protection of facility workers during operations than to 

20 protection of the general public from releases. The basis should also emphasize why each 

21 proposed limit is sufficient to demonstrate that the performance objectives will be met. 

22 Licensees will also need to demonstrate that the individual limits, when taken together, will 

23 ensure that the performance objectives continue to be met.

24

25 Licensees should describe in the technical basis supporting the allowable limits how they 

26 account for variability and uncertainty in anticipated inventories when developing the allowable 

27 limits. One method to mitigate uncertainty is to manage it through controls or restrictions. A key 

28 control to account for variability and uncertainty is imposing limits on the radionuclide inventory 

29 of disposed waste. The inventory of radionuclides in the waste disposed can be readily 

30 controlled, whereas uncertainty associated with the natural system or engineered barriers, for 

31 example, may be difficult to define or, if understood, difficult to reduce. The following general 

32 guidelines are useful for the development of allowable limits under 10 CFR 61.58:

33

34 • Allowable limits should be established conservatively so as to reduce the need for future 

35 mitigation.

36 • The analysis used to develop allowable limits should be as complete as practicable and 

37 include uncertainties.

38

39 The allowable limits will be affected by the complexity of the disposal site and its environment, 

40 the conservatism in the analyses, and the amount of information available to support the 

41 assessment. Figure 9-3 provides the relative change in the safety factor with changes in the 

42 relative value of these variables. For example, when the amount of information available to 

43 support the analyses is relatively large, licensees may apply a small margin of safety to the 

44 results of the analyses to establish the limits. On the other hand, if the disposal site 

45 environment is complex, licensees may need to apply a large margin of safety in order to 
9-5

1 provide reasonable assurance the performance objectives will be met. In other words, the 

2 uncertainty in the risk will be a primary driver of the values for the limits. There is no absolute 

3 value (e.g., an inventory that corresponds to 10 percent of the 0.25 mSv/yr or 25 mrem/yr dose 

4 limit) that is appropriate to use to establish limits for all sites. Allowable limits can be viewed as 

5 one of several safety factors or defense-in-depth protections used to mitigate uncertainty in 

6 other (i.e., non-inventory) areas of the technical analyses. 

7

8 Reviewers should coordinate reviews of the allowable limits with the reviews of the technical 

9 analyses required to meet 10 CFR 61.13 (see Sections 3.0, 4.3, and 8.0 of this document). 

10 Reviewers should verify that the licensee’s proposed limits on activities and concentrations are 

11 adequate, considering the source term inventory used in the performance and intruder 

12 assessments as well as assessments of expected and accidental occupational exposures 

13 during handling, storage, and disposal of waste. 

14

15 In general, the proposed limits should be consistent with the source term evaluated in the 

16 technical analyses. However, licensees may use a variety of approaches to develop allowable 

17 limits. For example, a licensee may derive a limit from the results of one or more of the 

Figure 9-3 Influence of Key Variables on the 

Required Relative Safety Factors
9-6

1 analyses for a source term with a unit concentration (e.g., 1.0 nCi/g) that is then scaled to the 

2 limit specified by the relevant performance objective. In this example, the proposed limits would 

3 not be expected to be consistent with the source term employed in the analyses. Rather, the 

4 limits should be consistent with the outcome of the analyses that meets the performance 

5 objective. Regardless of the approach, the reviewer should evaluate whether the approach is 

6 appropriate. Assuming a source term with unit concentrations of disposed waste may not be 

7 appropriate in circumstances where the radiological exposures are non-linearly correlated to 

8 source term concentrations.

9

10 Reviewers should also assess the spatial extent and distribution of the radionuclide inventory 

11 evaluated in the analyses to determine whether the allowable proposed limits are appropriate 

12 (e.g., when considering the disposal facility’s operating procedures for emplacement). In some 

13 cases, depending on the scenarios evaluated in the technical analyses to demonstrate the 

14 performance objectives are met, the spatial extent or distribution of radionuclide inventory in the 

15 disposal site may also need to be controlled as an allowable limit. For example, a licensee 

16 develops a total average activity concentration limit for a particular radionuclide from the intruder 

17 assessment based on an expected volume to which the intruder may be inadvertently exposed. 

18 Reviewers should confirm that the facility’s operating procedures would reasonably ensure that 

19 the proposed limit would not be expected to be exceeded for any volume of the disposal unit 

20 equivalent to the intrusion volume assumed by the licensee in the assessment. 

21

22 Reviewers should evaluate how stability is considered when establishing the allowable limits. 

23 First, stable wasteforms or containers may allow the disposal of waste with higher radioactivity 

24 than unstable waste. Therefore, reviewers should evaluate the estimated performance of the 

25 wasteforms, containers, or other design features that provide stability. Reviewers should 

26 coordinate this review with the review of acceptable wasteforms and containers, which is 

27 described Section 9.1.2 of this document. Reviewers should confirm that licensees are 

28 establishing waste acceptance criteria for wasteforms, containers, or design features that 

29 provide assurance the expected performance can be achieved. Second, there may be cases 

30 where the radionuclide inventory could significantly affect the stability of the disposal site after 

31 closure. For example, reviewers may need to evaluate whether radiation fields or thermal 

32 output from radioactive decay for certain higher activity waste streams would significantly 

33 compromise structural stability of the waste containers or degradation of the wasteforms and 

34 lead to stability issues for the disposal unit. Reviewers should coordinate their review of the 

35 development of allowable limits from the technical analyses with their review of acceptable 

36 wasteform characteristics and container specifications (see Section 9.1.2).

37

38 In general, reviewers should focus their review of allowable limits on those radionuclides 

39 expected to contribute most significantly to risk to the public, workers, and the environment. 

40 Typically, radionuclides with relatively high solubility, low sorption, high dose conversion factors, 

41 and/or significant in-growth are of particular significance. However, the importance of 

42 radionuclides may vary based upon the specific performance objective under consideration. For 

43 instance, non-mobile radionuclides may be more significant for protection of inadvertent 

44 intruders.

45

46 Reviewers should also evaluate whether the technical basis provided for each proposed limit 

47 will provide assurance that all the performance objectives will be met. For example, if an 

48 allowable limit is set for a particular radionuclide based on the results of the performance 

49 assessment, reviewers should confirm that the limit would not preclude licensees from 
9-7

1 demonstrating that the other performance objectives (e.g., protection of inadvertent intruders via 

2 the intruder assessment) are met. This review should also ensure that the allowable limits are 

3 comprehensive and that all necessary limits are included. Reviewers may elect to conduct 

4 independent analyses to inform the review. 

5 9.1.1.2 Allowable Limits Derived from Waste Classification

6

7 Licensees who elect to develop allowable limits for radionuclides from the 10 CFR Part 61 

8 waste classification requirements may simply report the concentration limits reported in 10 CFR

9 61.55. The limits specified in 10 CFR 61.55 shall be applied on a per package basis. 

10 Licensees may also need to develop alternative limits for radionuclides not listed in the tables, 

11 particularly for waste that is significantly different than what was considered in the analyses 

12 used to develop the tables (see NRC, 1981a). Tables 4.1 and 4.2 of NUREG-0945, Volume 1 

13 (NRC, 1982b) list the waste streams and radionuclides considered in the analyses to develop 

14 the tables. Guidance on developing limits on radionuclides not listed in the waste classification 

15 tables is also provided in this section. 

16

17 The concentration limits reported in 10 CFR 61.55, together with the requirements for waste 

18 characteristics, 10 CFR 61.56, and segregation, 10 CFR 61.52(a)(1) and (2), provide 

19 reasonable assurance that an intruder would be protected should they inadvertently be exposed 

20 to the waste that has been disposed in a facility. Classification involves consideration of both 

21 (1) long-lived radionuclides, whose potential hazard will persist long after precautions such as 

22 institutional controls, improved wasteform, and deeper disposal have ceased to be effective;

23 and (2) shorter-lived radionuclides, for which such precautions can be effective. 

24

25 Classification is also used to determine which waste characteristics requirements in 

26 10 CFR 61.56 are necessary. Further, classification is used to determine the waste segregation 

27 requirements in 10 CFR 61.52 that the disposal facility must meet during operations. Waste 

28 segregation provides assurance that waste is disposed of in a manner that limits potential 

29 exposures, including those to an inadvertent intruder, based on the hazard and stability of the 

30 waste. Figure 9-4 illustrates the waste classification and segregation requirements for LLW. 

31 Thus, licensees relying on the waste classification system to develop allowable limits must also 

32 ensure that criteria for both acceptable wasteform characteristics and facility operating practices 

33 are consistent with the related requirements for waste characteristics and segregation. Further 

34 guidance on developing criteria for acceptable wasteform characteristics is provided in Section 

35 9.1.2 of this document.

36

37 Licensees can demonstrate compliance with the classification requirements for selected 

38 radionuclides by comparing radionuclide concentrations in LLW to the values listed in the tables 

39 of long- and short-lived radionuclides in 10 CFR 61.55. The radionuclide concentrations listed 

40 in the classification tables were developed, in part, from two considerations: direct contact with 

41 the disposed waste (i.e., intrusion) and potential consumption or use of contaminated 

42 groundwater (i.e., migration).
9-8

1

Y

N

Conduct

technical analyses

to determine if additional 

limits are needed to meet 

performance objectives.

Waste is not 

required to be 

segregated per 

10 CFR 

61.52(a)(1).

Demonstrate 

compliance w ith 

minimum 

requirements 

specified at 

10 CFR 61.56(a).

Demonstrate 

compliance w ith 

stability 

requirements 

specified at 

10 CFR 61.56(b).

Does w aste 

meet stability 

requirements 

at 10 CFR

61.56(b)?

Class A w aste 

is required to 

be segregated 

per 10 CFR 

61.52(a)(1).

Demonstrate 

compliance w ith 

intruder barrier 

requirement 

specified at 

10 CFR 61.52(a)(2).

Determine w aste class per

10 CFR 61.55.

Determine concentrations of 

radionuclides in w aste.

Class A Class B Class C Greater

Than

Class C

Generally not 

acceptable for 

near-surface 

disposal per 

10 CFR 61.55.

Evaluate on a 

case-by-case 

basis.

Figure 9-4 Waste Classification and Segregation for Waste Classes
9-9

1 The approach, established in NUREG-0782 (NRC, 1981a), first considered protection of the 

2 individual inadvertent intruder. The approach then identified a limited set of radionuclides that 

3 are significant from the standpoint of migration. Radionuclides significant from the standpoint of 

4 migration are generally more important for protection of the general population; therefore, the 

5 NRC staff expected these radionuclides to be evaluated as part of the demonstration that the 

6 performance objective for protection of the general population (i.e., 10 CFR 61.41) is met. 

7 Therefore, the waste classification requirements were primarily derived from the analysis for 

8 protection of the inadvertent intruder. 

9

10 For near-surface disposal, 10 CFR 61.55 specifies the three classes of waste, A, B, and C, that 

11 resulted from the analysis. Upper concentration limits are also defined for Class C waste. 

12 Wastes containing radionuclide concentrations higher than the upper limits would be generally 

13 unacceptable for near-surface disposal. However, there may be instances where these wastes 

14 would be acceptable for near-surface disposal with special processing or design. These 

15 instances would be evaluated on a case-by-case basis.

16

17 Wastes for which there are no stability requirements, but which must be disposed of in a 

18 segregated manner from other wastes, are termed “Class A.” These wastes are defined in 

19 terms of maximum allowable concentrations of certain isotopes and certain minimum 

20 requirements on wasteform that are necessary for safe handling. The minimum requirements 

21 are specified in 10 CFR 61.56(a). Waste designated as Class A must be segregated from other 

22 waste as required in 10 CFR 61.52(a)(1), unless a licensee can demonstrate that the waste also 

23 meets the stability requirements specified in 10 CFR 61.56(b). The technical analyses required 

24 in 10 CFR 61.13 could demonstrate based on site-specific conditions that additional disposal 

25 practices or waste characteristics—such as those required for Class B or C wastes—would be 

26 necessary to demonstrate that the performance objectives are met.

27

28 Wastes that need to be placed in a stable form and disposed of in a segregated manner from 

29 unstable wasteforms are termed “Class B.” These stable wastes are also defined in terms of 

30 allowable concentrations of isotopes and requirements for a stable wasteform as well as 

31 minimum handling requirements. The minimum requirements are specified at 10 CFR 61.56(a), 

32 and the stability requirements are specified at 10 CFR 61.56(b). Further, the technical analyses 

33 could demonstrate that additional disposal practices or waste characteristics, such as those 

34 required for Class C waste, would be necessary, based on site-specific conditions, to 

35 demonstrate that the performance objectives are met.

36

37 Wastes that need to be placed into a stable form, disposed of in a segregated manner from 

38 unstable wasteforms, and disposed of so that a barrier is provided against potential inadvertent 

39 intrusion after institutional controls are no longer assumed to be effective are termed “Class C.” 

40 These intruder wastes are also defined in terms of allowable concentrations of isotopes and 

41 requirements for disposal by deeper burial or some other intruder barrier, as well as minimum 

42 stability requirements. The minimum requirements for waste characteristics are specified at 

43 10 CFR 61.56(a), and the stability requirements are specified at 10 CFR 61.56(b). 

44 Requirements for near-surface disposal by deeper burial or some other barrier are specified at 

45 10 CFR 61.52(a)(2). The technical analyses could demonstrate that additional disposal 

46 requirements, such as the longevity of the intruder barriers, or limits on the radionuclide 

47 inventory, are necessary to provide reasonable assurance that the performance objectives 

48 would be met.

49
9-10

1 Because the classification limits were developed primarily from an analysis that examined 

2 inadvertent intrusion, the limits are not intended to provide reasonable assurance that all the 

3 performance objectives of 10 CFR Part 61 will be met. Therefore, licensees using the waste 

4 classification requirements may need to develop additional limits based on the results of the 

5 technical analyses used to demonstrate that the remaining performance objectives would be 

6 met. For example, radionuclides that are prone to migration may require additional limits to 

7 ensure that the 10 CFR 61.41 performance objective will be met.

8

9 As specified in 10 CFR 61.55(a)(6), waste that does not contain any radionuclides listed in 

10 either Table 1 or 2 is considered Class A. As such, this waste is subject to the same 

11 requirements as Class A waste that does contain radionuclides listed in either Table 1 or 2. 

12 However, the technical analyses could demonstrate that additional disposal practices, such as 

13 those required for Class B or C wastes, are necessary to provide reasonable assurance that 

14 public health and safety will be protected. Therefore, licensees may also need to specify limits 

15 for those radionuclides that are not specifically identified in the waste classification requirements 

16 in 10 CFR 61.55 if the radionuclides significantly affect the demonstration that the performance 

17 objectives would be met. In these cases, licensees should refer to Section 9.1.1.1 for guidance 

18 on developing allowable limits from technical analyses.

19

20 NRC has developed guidance on classifying waste according to the waste classification 

21 requirements. The guidance includes a BTP, “Final Waste Classification and Waste Form 

22 Technical Position Papers,” dated May 11, 1983, which presents guidance on classifying waste 

23 (NRC, 1983d). The NRC staff has also developed guidance on acceptable methods of 

24 concentration averaging in a BTP on concentration averaging and encapsulation (NRC, 1995b) 

25 and recently published a revised draft for comment (NRC, 2012a). Guidance is also available in 

26 Standard Review Plan 4.1 of NUREG-1200 (NRC, 1994) to assist reviewers in determining 

27 whether licensees have adequate procedures to ensure that waste disposal is conducted in 

28 compliance with 10 CFR 61.55 and 10 CFR 61.56. 

29 9.1.1.3 Insignificant Radionuclides

30

31 Licensees may choose not to develop limits for radionuclides that contribute, in aggregate, a 

32 projected dose of no greater than 10 percent of the limits prescribed in the performance 

33 objectives. That is, the sum of the contributions from all radionuclides considered insignificant 

34 should be no more than 10 percent of the limit for a particular performance objective. However, 

35 radionuclides that could be excluded based on comparison to the limit for one performance 

36 objective (e.g., protection of the public from releases) may need to have limits to comply with 

37 another performance objective (e.g., protection of inadvertent intruders). Once a licensee has 

38 demonstrated that radionuclides are insignificant, the dose from the insignificant radionuclides 

39 must be accounted for in demonstrating that the performance objective is met, but insignificant 

40 radionuclides may be excluded from further detailed evaluation in the technical analyses. 

41

42 When radionuclides are considered insignificant and eliminated from further consideration, 

43 licensees should justify the decision to consider them insignificant. However, licensees should 

44 be aware that these decisions may need to be revisited if warranted by new information. For 

45 instance, if a disposal facility proposes to accept new waste streams with significantly different 

46 radionuclide inventories than previously considered, the licensee may need to assess whether 

47 radionuclides that were previously considered insignificant continue to remain so. Further, if a 

48 licensee obtains updated information suggesting that its understanding of the behavior of a 
9-11

1 radionuclide in the site environment has significantly changed, they should reassess the 

2 significance of the radionuclide to meeting the performance objectives.

3 9.1.1.4 Application of Allowable Limits

4

5 In general, licensees will need to develop the allowable limits on a radionuclide-by-radionuclide 

6 basis. Because the performance objectives are based on the total contribution of all 

7 radionuclides to the dose limits, licensees will need to perform a sum-of-fractions calculation to 

8 demonstrate the allowable limits are met. Sum-of-fractions calculations are also performed for 

9 waste classification under 10 CFR 61.55(a)(7). In general form, licensees can use the following 

10 relationship to sum the ratio of the activity to its corresponding allowable limit for each 

11 radionuclide present in the waste:

12

13 (9.2)

14 where

15

Activityi is the total activity [Bq] or activity concentration [Bq/m3 (Ci/m3 16 )or Bq/g (Ci/g)] of 

17 radionuclide i; and

18

Limiti is the allowable limit [Bq (Ci), Bq/m3 (Ci/m3 19 )or Bq/g (Ci/g)] of radionuclide i from 

20 the most restrictive scenario used to demonstrate the performance objectives 

21 would be met.

22

23 Licensees should also develop a mechanism to report the inventory that has been disposed of 

24 and identify when action may be needed. Types of actions that could be taken may include the 

25 following:

26

27 • Notify the regulator that a certain percentage of the allowable limit has been received.

28 • Evaluate whether any updates to the technical analyses are needed.

29 • Update the technical analyses (establish new allowable limits).

30 • Modify the disposal facility design, including acceptable wasteforms (see Section 9.1.2).

31 • Limit or prohibit further disposal of a certain type of waste (see Section 9.1.3).

32

33 When significant changes to the operations of the disposal facility occur, licensees should 

34 consider updating the allowable limits. Some operational changes may simply require that a 

35 licensee performs an assessment of the impact on the allowable limits; however, no change to 

36 the allowable limits is required. Other operational changes may require the licensee to re37 examine the allowable limits. These changes may include, but are not limited to, a proposal to 

38 receive new material, receipt of significant new information on the site characteristics or 

39 engineering design, changes to intruder barriers, changes to the understanding of the 

40 performance of key components of the disposal system, and updating of the technical analyses. 

41 It is important for licensees to establish early in the process the criteria for updating and 

42 revision. If a licensee uses a process to determine the significance of a potential change to the 

43 established allowable limits, they should maintain a comprehensive list of all items that were 
9-12

1 screened from impacting the allowable limits. The cumulative impact from many small changes 

2 can be additive such that, in total, the allowable limits would require revision to avoid future 

3 mitigation.

4

5 If a licensee revises allowable limits, they should provide regulators a clear basis for the 

6 revisions that includes a side-by-side comparison of changes to parameters or models and the 

7 basis for those changes. The side-by-side comparison will provide transparency for the 

8 revisions and will facilitate the regulator’s review of the revised allowable limits. It may also 

9 have the complementary benefit of enhancing stakeholder confidence in the revisions. If limits 

10 are relied upon to demonstrate defense-in-depth protections, the licensee should provide a 

11 basis that defense-in-depth protections remain adequate in light of the proposed change.

12 9.1.2 Acceptable Wasteform Characteristics and Container Specifications

13

14 Section 61.58(a)(2) of 10 CFR Part 61 requires licensees to specify acceptable wasteform 

15 characteristics and container specifications as part of the waste acceptance criteria. Acceptable 

16 wasteform characteristics and container specifications, together with the other waste 

17 acceptance criteria (e.g., radionuclide limits), provide reasonable assurance that the 

18 performance objectives will be met and may provide defense-in-depth protections. Acceptable 

19 wasteform characteristics and container specifications include properties that facilitate handling 

20 of the waste at the disposal facility, promote stability of the waste to minimize subsidence and 

21 water contact with the waste, minimize release and migration of the radionuclides from the 

22 disposal site, deter or preclude inadvertent intrusion into the waste, or limit exposures during 

23 operations.

24

25 The regulations require that all waste meet the minimum requirements specified in 10 CFR

26 61.56(a). The minimum requirements are designed to facilitate handling of waste and protect

27 the health and safety of personnel at the disposal facility. Licensees may also identify additional 

28 minimum criteria to facilitate handling and protect facility personnel depending on the particular 

29 operational practices and environmental conditions at a disposal facility. Additional minimum 

30 criteria might include acceptable limits for waste package external surface dose rate and heat 

31 generation, necessary labeling and marking to be applied to waste packages, container 

32 specifications, or specific requirements for acceptance of bulk waste. Licensees should provide 

33 a rationale for inclusion of the additional minimum criteria which should include a demonstration 

34 that they are comprehensive. The basis, as appropriate, should identify why the additional 

35 criteria were developed and include the performance objective(s) that the additional criteria 

36 support and whether the criteria are considered defense-in-depth protections.

37

38 Certain waste streams will need to meet more rigorous requirements on wasteform than the 

39 minimum requirements to ensure stability after disposal and demonstrate compliance with the 

40 performance objectives. The requirements to ensure stability are specified in 10 CFR 61.56(b). 

41 For licensees relying on the waste classification requirements to develop waste acceptance 

42 criteria, Class B and Class C waste must meet the requirements to ensure stability. Licensees 

43 developing waste acceptance criteria from the results of the technical analyses will need to 

44 identify the wastes that will need to meet stability requirements. 

45

46 Licensees may demonstrate that the stability requirements would be met by using one or more 

47 of the following approaches - a stable wasteform, a container that provides stability, or facility 

48 design. The approach, or combination of approaches, that a licensee uses to demonstrate 
9-13

1 stability will determine the waste acceptance criteria needed to ensure stability and defense-in2 depth protections. For example, if a licensee constructs a reinforced concrete vault and 

3 demonstrates that it provides adequate stability for the required timeframe, the waste 

4 acceptance criteria may not need to address stable wasteforms or container specification for 

5 stability. In general, stable wasteforms or container specifications for stability are necessary 

6 when degradation of the waste may significantly affect meeting the performance objectives. For 

7 example, stable wasteforms or containers would be necessary for wasteforms with sufficient 

8 radioactivity such that its release from the disposal site would significantly affect public health 

9 and safety. Likewise, stable wasteforms may be necessary for higher activity waste to minimize 

10 exposures to inadvertent intruders since stable wasteforms tend to provide a recognizable and 

11 non-dispersible waste. 

12

13 Licensees should provide a technical basis for the criteria for allowable wasteform 

14 characteristics and container specifications. Licensees relying on the waste classification 

15 requirements to develop waste acceptance criteria may use the requirements of 10 CFR 61.56 

16 as a basis. Licensees developing waste acceptance criteria from the results of the technical 

17 analyses should clearly identify how the acceptable wasteform characteristics and container 

18 specifications are consistent with the assumptions, modeling approaches, and results of the 

19 technical analyses. The basis, as appropriate, should also identify which performance 

20 objective(s) the criteria support and whether the criteria provide defense-in-depth protections. 

21 Further, the basis should identify the time period over which the criteria are intended to protect 

22 public health and safety. For example, some acceptable wasteform characteristics may only 

23 need to be relied upon to protect facility personnel during operations; other acceptable

24 wasteform characteristics may be important to protect the general population from releases of 

25 radioactivity from longer-lived waste or intruders from inadvertent exposures to longer-lived 

26 waste farther into the future. The criteria should provide reasonable assurance that the 

27 longevity required of the wasteform and container capabilities is and can be adequately 

28 demonstrated. In some cases this may involve criteria to document traditional engineering tests 

29 or specifications for waste containers. In other cases, the criteria may be more rigorous and 

30 include laboratory testing, predictive modeling, or comparison to analogs. In still other 

31 instances, the duration of time that stability of the wasteform or containers is required may be so 

32 long that the uncertainty in material behavior may preclude a reasonable demonstration of 

33 stability. In these instances, licensees should not solely rely upon wasteform characteristics or 

34 container specifications to ensure safety. Rather, other limits or defense-in-depth protections 

35 may need to be imposed such as allowable limits. 

36

37 NRC has developed guidance in the form of a BTP (NRC, 1991b) for waste generators on 

38 wasteform test methods and results that are considered acceptable for complying with the 

39 10 CFR Part 61 stability requirements. The guidance also identifies conditions that stable 

40 wasteforms should meet to demonstrate that the stability requirements are met. The guidance 

41 is applicable for licensees who develop waste acceptance criteria from the 10 CFR Part 61 

42 waste classification requirements. The guidance is also considered generally applicable to 

43 licensees who develop waste acceptance criteria from the results of the technical analyses 

44 required in 10 CFR 61.13. However, there may be specific cases where the approaches need 

45 to be amended due to site-specific conditions. In these cases, licensees should provide a 

46 technical basis for the divergence from the BTP on wasteform test methods (NRC, 1991b). 

47 Licensees may also consult IAEA-TECDOC-864 (IAEA, 1996), which provides considerations 

48 for establishing container specifications. Section 3.2.4 of this document provides guidance 

49 associated with wasteforms.
9-14

1

2 Reviewers should coordinate reviews of the acceptable wasteform characteristics and container 

3 specifications with the reviews of the technical analyses required to meet 10 CFR 61.13 (see 

4 Sections 3.2.4, 4.3, and 8.0). Reviewers should verify that the licensee’s proposed criteria for 

5 wasteform characteristics and container specifications are reasonably consistent with the 

6 wasteforms and containers assessed in source terms for the performance assessment and 

7 intruder assessment, as well as assessments of expected and accidental occupational 

8 exposures during handling storage, and disposal of waste. In other words, licensees should 

9 establish criteria for wasteforms and containers that ensure the wasteforms and containers will 

10 provide the expected performance relied upon in the technical analyses including defense-in11 depth capabilities.

12

13 In general, reviewers should focus their review of wasteform characteristics and container 

14 specifications on waste streams that are expected to contribute most significantly to risk to the 

15 public, workers, and the environment. The significant wasteform characteristics and container 

16 specifications are likely to vary depending upon the disposal site characteristics and engineering 

17 design. Reviewers should evaluate the licensee’s technical analyses to understand which 

18 wasteform characteristics and container specifications are important for demonstrating that the 

19 performance objectives are met. These may include mechanical properties to ensure stability or 

20 limit the likelihood or consequences of potential accidents during operations. They may also 

21 include durability or leaching characteristics to minimize releases to the general environment. 

22 Further, reviewers should evaluate whether the allowable wasteform characteristics and 

23 container specifications would significantly affect the stability of the disposal site after closure. 

24 For example, reviewers may need to evaluate whether waste containers could withstand 

25 anticipated mechanical loads after disposal. Reviewers should coordinate their review of the 

26 development of acceptable wasteform characteristics and container specifications with their 

27 review of allowable limits from the technical analyses (see Section 9.1.1.1). 

28

29 Reviewers should also evaluate whether the technical basis provided for each criteria will 

30 provide assurance that all the performance objectives will be met. For example, if a particular 

31 wasteform characteristic or container specification is based on the results of the performance 

32 assessment, reviewers should confirm that the limit would not preclude licensees from 

33 demonstrating that the other performance objectives (e.g., protection of inadvertent intruders via 

34 the intruder assessment) are also met. Reviewers should also evaluate whether the criteria 

35 ensure that the wasteform characteristics and container specifications are expected to persist 

36 for the duration needed to demonstrate compliance with the performance objectives. Reviewers 

37 may elect to conduct independent analyses to inform the review. 

38 9.1.3 Restrictions or Prohibitions

39

40 Section 61.58(a)(3) of 10 CFR Part 61 requires licensees to specify restrictions or prohibitions 

41 on waste, materials, or containers that might affect meeting the performance objectives. 

42 Licensees should identify any specific radionuclides, chemical or hazardous materials, or 

43 specific containers or types of containers that are restricted or prohibited from acceptance at the 

44 facility. The restrictions and prohibitions should adequately reflect those identified in the 

45 minimum waste characteristic requirements specified in 10 CFR 61.56(a). 

46

47 Reviewers should assess the licensee’s list of restrictions or prohibitions to ensure it is 

48 comprehensive and adequately considers the minimum waste characteristics requirements 
9-15

1 specified in 10 CFR 61.56(a). Reviewers should also assess whether the licensee’s list is 

2 consistent with the assumptions, modeling approaches, and results of the technical analyses 

3 used to demonstrate that the performance objectives would be met. Reviewers may perform 

4 independent modeling to assist this review.

5 9.2 Waste Characterization Methods

6

7 Licensees are required, per 10 CFR 61.58(b), to provide methods for characterizing waste for 

8 acceptance. The methods shall identify the parameters to be characterized and the level of 

9 uncertainty in the characterization data that is considered acceptable. The regulations specify 

10 that, at a minimum, the following information must be required to adequately characterize waste 

11 for acceptance:

12

13 • Physical and chemical characteristics. Information on the physical and chemical 

14 characteristics of the waste support handling, the determination of compatibility with the 

15 container and other waste, as well as any potential treatment or conditioning processes. 

16 Physical characteristics may include a description of the material including its density, 

17 consistency, and appearance. Chemical characteristics may include pH, reactivity, 

18 chemical compounds present, and the presence of hazardous or toxic constituents. 

19

20 Volume, including the waste and any stabilization or absorbent media. Information 

21 on volume supports waste handling decisions. The information is also important to 

22 determine or verify the concentration of radionuclides for comparison with the waste 

23 acceptance criteria. Volume information should include container volume, actual waste 

24 volume, and the container utilization factor. The container utilization factor represents 

25 the portion of the container value that is filled with waste, including stabilization or 

26 absorbent media. Information on the container volume should represent the volume of 

27 the disposal site that will be occupied by the container. Information on the actual waste 

28 volume should include stabilization or absorbent media. If used, stabilization or 

29 absorbent media should be identified. 

30

31 • Weight of the container and contents. Information on weight should include container 

32 weight (or mass) that would have to be handled. Weight information may be important 

33 for meeting stability criteria as well as transportation requirements. This information is 

34 also important to determine or verify the concentration of radionuclides for comparison to 

35 the waste acceptance criteria.

36

37 • Identities, activities, and concentrations of radionuclides. This information may 

38 include the total activity in a container, the identities and activities of the significant 

39 radionuclides per unit volume or mass, radiation dose levels at the surface of the 

40 container, and external contamination levels on the surface of the container. Significant 

41 radionuclides are primarily those that affect the demonstration that the performance 

42 objectives would be met. Significant radionuclides also include radionuclides important 

43 for waste classification.

44

45 • Characterization date. The characterization date helps determine the validity of the 

46 characterization documentation.

47
9-16

1 • Generating source. Identification of the generating source helps determine the validity 

2 of the characterization documentation. Information on the generating source may 

3 include packaging date, generator site, location of the process which generated the 

4 waste, and information on conditioning, if applicable.

5

6 • Any other information needed to characterize the waste to demonstrate that the 

7 waste acceptance criteria are met. This information includes any additional data 

8 about the waste that are important to the facility’s ability to protect public health and 

9 safety. This information should be identified from waste acceptance criteria which are 

10 drawn from either the results of the technical analyses that are used to demonstrate that

11 the performance objectives or the waste classification requirements are met. For 

12 example, data on mechanical properties of wasteforms or containers may be needed to 

13 ensure that any criteria for stability, as necessary, can be met.

14

15 The purpose of these requirements is to ensure that knowledge of the waste’s characteristics is

16 (1) commensurate with the assumptions and approaches employed in the technical analyses 

17 used to develop the proposed waste acceptance criteria and is, thus, (2) sufficient to 

18 demonstrate that the waste acceptance criteria are met.

19

20 For waste acceptance criteria developed from the waste classification requirements specified in 

21 10 CFR 61.55, waste characterization methods should be commensurate with the assumptions 

22 and approaches employed to develop the waste classification requirements. For waste 

23 acceptance criteria developed from the technical analyses, waste characterization methods 

24 should be consistent with the approaches employed in the analyses. In other words, for each of 

25 these two approaches to develop waste acceptance criteria, there will have been assumptions 

26 and approaches employed to derive the criteria. The methods should ensure that significant 

27 assumptions or approaches are characterized sufficiently to provide assurance that the criteria 

28 can be met. For example, the limits for Class B and C waste, per the 10 CFR 61.55 waste 

29 classification requirements, were developed from an analysis that assumed Class B and C 

30 waste would be stable. As a result, 10 CFR Part 61 includes requirements that Class B and C 

31 waste must be disposed in a stable form. Therefore, licensees may need to specify waste 

32 acceptance criteria to ensure that Class B and C waste are stable. In this example, licensees 

33 would also need to specify acceptable methods to characterize the waste to demonstrate that 

34 the stability criteria will be met.

35

36 Regardless of the method used to develop waste acceptance criteria, licensees should specify 

37 acceptable methods to characterize waste, criteria for determining an acceptable level of 

38 uncertainty in the characterization data, and documentation required to ensure sufficient detail is 

39 available to demonstrate that the waste acceptance criteria of the land disposal facility are met.

40 9.2.1 Acceptable Waste Characterization Methods

41

42 Licensees shall specify methods for adequately characterizing waste for the purposes of 

43 demonstrating that the disposal facility’s waste acceptance criteria are met. These 

44 specifications should identify methods for characterizing the radionuclide content of the waste,

45 as well as any significant waste characteristics and container specifications. The intent of the 

46 methods should be to ensure that generators provide reasonably realistic representations of the 

47 radionuclide content of their waste and the necessary waste characteristics for comparison with 

48 the waste acceptance criteria. In general, the characterization methods would be specific to 
9-17

1 each individual waste stream, and would consider the different radiological and other 

2 characteristics of the waste streams destined for disposal at a land disposal facility. Ideally, the 

3 disposal facility operator should ensure that the generator’s characterization is near in time to 

4 the demonstration that waste meets the acceptance criteria. When proximate characterization 

5 is not possible and the time interval from characterization to disposal may significantly affect 

6 meeting the waste acceptance criteria, the disposal facility operator should ensure that a basis 

7 supports why earlier characterization remains acceptable for demonstrating that the waste 

8 acceptance criteria are met. IAEA has developed guidance on strategies and methodologies for 

9 radioactive waste characterization that licensees may find applicable to develop acceptable 

10 waste characterization methods (IAEA, 2007).

11 9.2.1.1 Acceptable Methods for Characterizing Activities and Concentrations

12

13 The first step in characterizing the waste to meet the waste acceptance criteria is to determine 

14 the activities and concentrations of significant radionuclides in the waste. Licensees may use a

15 variety of methods to determine radionuclide activities or concentrations in LLW. Acceptable 

16 methods would likely include either direct measurement of individual radionuclides or indirect 

17 methods that infer activities or concentrations of radionuclides from other measurements or 

18 knowledge of the waste to enhance confidence that the waste acceptance criteria are met. 

19 These methods are described in more detail in the BTP on radioactive waste classification and 

20 wasteforms (NRC, 1983d). 

21

22 Direct measurement of individual radionuclides generally provides the most confidence that the 

23 allowable activities and concentration limits identified in the waste acceptance criteria are met. 

24 However, the NRC staff recognizes that direct measurement may not always be necessary or 

25 warranted. For example, activities or concentrations of certain radionuclides may be overly 

26 difficult to measure with current technology (e.g., below minimum detection capabilities) or 

27 personnel safety considerations may limit direct measurement of specific waste streams. In 

28 these cases, licensees are permitted to accept other methods (e.g., indirect or material 

29 accountability methods) to demonstrate that the allowable limits are met.

30

31 Indirect methods infer radionuclide activities or concentrations from a number of approaches 

32 which include materials accountability, characterization by source, and the use of scaling 

33 factors. Radionuclide material accountability relies on inferences from the difference between 

34 the quantity of radioactive material entering and exiting a given process. Characterization by 

35 source is similar to material accountability and involves determining the radionuclide content 

36 through knowledge and control of the source of the waste. Indirect methods often rely on the 

37 use of scaling factors to relate the inferred activity or concentration of one radionuclide to 

38 another radionuclide or gross radioactivity that is measured. 

39

40 Indirect methods may be appropriate to determine activities or concentrations of difficult-to41 measure radionuclides provided there is reasonable assurance that the indirect methods can be 

42 correlated with actual measurements. Licensees should require that generators develop 

43 correlations between measured or known quantities and the inferred quantity on a generating 

44 facility and waste stream basis. NRC guidance on the use of indirect methods to determine the 

45 inventory of radionuclides will be summarized in a Regulatory Issue Summary in 2015. The 

46 BTP on radioactive waste classification provides additional guidance on acceptable uses of 

47 indirect methods for use in waste classification (NRC, 1983d). This guidance may also be 

48 useful for identifying methods to demonstrate that the allowable limits developed from technical 
9-18

1 analyses are met. Licensees should also consider the issues identified by the NRC staff in 

2 Information Notice 86-20 (NRC, 1986b) when specifying criteria for application of indirect 

3 methods to meeting the allowable limits. Further, NUREG/CR-6567 (NRC, 2000b) and IAEA 

4 (2009) provide information on scaling factors that licensees may wish to consider when 

5 identifying criteria for the use of indirect methods. Although previous NRC guidance is focused 

6 on determining concentrations for demonstrating compliance with allowable limits developed 

7 from the waste classification requirements of 10 CFR Part 61, they may also be applicable, on a 

8 case-by-case basis, for determining limits for demonstrating that allowable limits developed from 

9 the technical analyses are met. Land disposal facility operators should require generators to 

10 provide information as part of certification that details how scaling factors are derived and 

11 whether periodic re-analysis of the scaling factors resulted in a revision to the scaling factors. If 

12 a generator determines that scaling factors need to be revised, a determination should be made 

13 whether the revision affects previous shipments of waste to the facility. Land disposal facility 

14 operators should assess any impacts to acceptable inventories, both previous disposals and 

15 future acceptable waste acceptance criteria that may result from revisions to scaling factors.

16

17 Each of these methods is subject to various sources of uncertainty (Figure 9-5), including, but

18 not limited to, the following:

19 • a sample’s degree of representativeness of the whole, due to temporal and spatial 

20 variability in: 

21 - concentrations of directly sampled radionuclides

22 - samples used to establish scaling factors

23 - samples used to establish concentrations of process inputs

24 • analytical uncertainty in sampled radionuclides

25 • uncertainty in dose rate scans

26 • uncertainty in any scaling factors for unsampled containers

27 • uncertainty in radionuclide concentrations in inputs and input volumes, if the radionuclide 

28 concentrations in the product are based on the inputs and are not independently 

29 measured

30 9.2.1.1.1 Characterization Methods for Criteria Based on the Waste Classification 

31 Requirements

32

33 The BTP on radioactive waste classification and wasteforms provides guidance on the use of 

34 various methods to determine concentrations to demonstrate that allowable concentrations 

35 developed from the 10 CFR Part 61 waste classification requirements are met (NRC, 1983d). 

36 The BTP indicates that the NRC staff considers a reasonable target for determining measured 

37 or inferred radionuclide concentrations to be that concentrations are accurate to within a factor 

38 of 10. However, more precision may be required in certain cases to demonstrate that the 

39 performance objectives will be met. In general, licensees should reflect uncertainty in 

40 radionuclide activities and concentrations in the technical analyses.
9-19

1

2 Section 61.55(a)(8) of 10 CFR Part 61 provides acceptable methods for determining 

3 radionuclide concentrations for comparison with allowable activities and concentrations that are 

4 developed from the waste classification requirements. A licensee may average the 

5 concentration of a radionuclide over the volume of the waste, or the weight of the waste, if the 

6 concentration units are expressed as nanocuries per gram. The NRC staff has developed 

7 guidance on acceptable methods of concentration averaging in a BTP on concentration 

8 averaging and encapsulation (BTP CA) (NRC, 1995b) and recently published a revised draft for 

9 comment (NRC, 2012a). The revised draft BTP CA is based on many of the same methods for 

10 performing an intruder assessment that were used to develop this guidance. 

11

12 For example, the revised draft BTP CA considers the intruder receptor scenarios described in 

13 Section 4.3.1.1 of this document in developing guidance on acceptable averaging approaches

14 (NRC, 2012a). However, the revised draft BTP CA also considers receptor scenarios in which 

15 an individual may be exposed to discrete items (NRC, 2012a). For instance, the NRC staff 

16 considered a waste-handling receptor scenario to develop the revised draft BTP CA for disposal 

17 of discrete sources, which is not discussed in this guidance for intruder assessments (NRC, 

18 2012a). The NRC staff used these receptor scenarios to develop the revised draft BTP CA for 

19 limiting the range of waste concentrations that can be mathematically averaged in a single 

20 container, as well as specific guidance on appropriate waste volumes over which concentrations 

21 should be averaged for waste classification by waste generators or processors (NRC, 

22 2012a). This guidance—as opposed to the guidance in the revised draft BTP CA (NRC, 

23 2012a)—focuses on determining appropriate concentrations to use in site-specific intruder 

24 analyses performed by land disposal facility licensees. Irrespective of any concentration 

25 averaging used to determine waste classification, radionuclide concentrations should be 

26 representative of the actual waste distribution (see Section 4.3.2.2.2 of this guidance), although 

27 conservative assumptions are, in general, appropriate. 

Materials

Accounting

Scaling

Direct Sampling

Analytical 

Uncertainty

Temporal 

and 

Spatial 

Variability

Scaling factor 

uncertainty

Uncertainty in 

input 

concentrations

Uncertainty in 

dose scans

Uncertainty in 

input volumes

Uncertainty in Radionuclide 

Activity/Concentrations

Uncertainty in Dose

Figure 9-5 Sources of Uncertainty in Various Methods of Determining Radionuclide 

Activities or Concentrations in Waste Packaged for Disposal
9-20

1 9.2.1.1.2 Characterization Methods for Criteria Based on the Results of the Technical 

2 Analyses

3

4 Licensees developing waste acceptance criteria from the results of the technical analyses must 

5 identify acceptable methods for characterizing the waste to demonstrate that the waste 

6 acceptance criteria are met. The appropriate method for characterizing the waste will depend 

7 on the specific parameter being measured, the hazards associated with acquiring the 

8 information, and the amount and quality of the data needed to adequately characterize the 

9 waste. 

10

11 The specific parameters and the quantity and quality of the data should be consistent with the 

12 intended use of the information, namely to demonstrate that the waste acceptance criteria are 

13 met. In this case, the criteria are developed from the results of the technical analyses. 

14 Therefore, the parameters and the data developed to characterize the parameters should be 

15 consistent with the analyses. In other words, characterization parameters should focus on 

16 those parameters of the analyses which are significant for a licensee’s demonstration that the 

17 performance objectives are met and that defense-in-depth protections are provided. Licensees 

18 should identify significant parameters of the analyses as criteria for waste acceptance. 

19 Likewise, the quantity and quality of data should be commensurate with the parameter’s 

20 importance to meeting the performance objectives. Licensees may use a graded approach in 

21 defining the level of quality for the data. Therefore, characterization data for parameters that 

22 are more significant for demonstrating that the performance objectives would be met should 

23 generally require more robust pedigree than data for parameters of lesser significance. 

24 Likewise, characterization data for parameters that are considered defense-in-depth protections 

25 should generally require more robust pedigree than data for parameters that are not relied upon 

26 for defense-in-depth. 

27

28 As discussed in Section 9.2.1.1, adequate waste characterization may include a combination of 

29 both direct and indirect methods. Direct methods may include sampling and laboratory analysis 

30 as well as certain non-destructive evaluation techniques. Indirect methods may use non31 destructive evaluation techniques as well as acceptable knowledge to supplement or provide 

32 data that might otherwise be collected by direct methods. The BTP on radioactive waste 

33 classification provides guidance on the use of various methods to determine concentrations to 

34 demonstrate that allowable concentrations developed from the 10 CFR Part 61 waste 

35 classification requirements are met (NRC, 1983d). The revised draft BTP CA is also available 

36 to licensees (NRC, 2012a). The methods discussed in these documents may also be 

37 appropriate for use in characterizing waste for meeting waste acceptance criteria developed 

38 from the results of the technical analyses. Licensees should provide a basis for inclusion or 

39 exclusion of the methods discussed in the BTP on radioactive waste classification and 

40 wasteform (NRC, 1983d) for characterizing data to meet waste acceptance criteria developed 

41 from the results of the technical analyses. The basis should include either a description of why 

42 the method from the BTP on radioactive waste classification and wasteform (NRC, 1983d) is 

43 appropriate or inappropriate depending upon whether it is included or excluded.

44 9.2.1.2 Acceptable Methods for Characterizing Wasteform and Containers

45

46 Section 61.56 of 10 CFR Part 61 specifies requirements for waste characteristics, which apply 

47 to all waste classes, as well as stability requirements, which are required only for Class B and C 

48 wastes because of their higher radioactivity. Waste stability helps to limit inadvertent intrusion 
9-21

1 exposures and minimize water infiltration into the disposal units. Wastes that are stable, and 

2 thus recognizable after the active institutional control period has ended, ensure that the impacts 

3 of inadvertent intrusion remain limited to discovery-type receptor scenarios. To the extent 

4 practical, Class B and C wastes should maintain their gross physical properties and identity over 

5 a 300-year period to be consistent with the concepts in 10 CFR 61.7(b)(2). However, certain 

6 waste may need to meet the stability requirements for longer periods of time in order for a 

7 licensee to demonstrate that the performance objectives would be met. 

8

9 The NRC staff has developed guidance on wasteforms to comply with waste characteristic 

10 requirements in its BTP on wasteforms (NRC, 1991b). The guidance also applies to Class A 

11 waste that is not segregated from Class B and C wastes. Additional requirements, specified as 

12 license conditions, may be necessary for waste, including that categorized as Class A by 

13 10 CFR 61.55(a)(6). Regulators can specify additional requirements in license conditions based 

14 on the need to mitigate potential exposures as demonstrated in the technical analyses.

15

16 Licensees should specify methods for characterizing the wasteform and container. 

17 Characterizing the waste to demonstrate that acceptable wasteform characteristics and 

18 container specifications are met is generally a two-staged process. First, the licensee should 

19 require generators to define the wasteform characteristics and container attributes, which 

20 includes performance data (e.g., compressive strength, load bearing capability, resistance to 

21 impact, corrosion, fire resistance, etc.). The disposal facility operator should identify which 

22 quality-related parameters need to be controlled, including any necessary details of the 

23 arrangements for controlling them, in order to provide confidence that the acceptance criteria 

24 are met. Second, the licensee should require that generators confirm that the wasteform or 

25 container conforms to the applicable specifications. The disposal facility operator should ensure 

26 that the generator’s confirmation of the applicable specifications meets the quality requirements 

27 for the characterization. In some instances, this may include the timeliness of the generator’s 

28 confirmation. For instance, if a waste container’s structural stability is important (e.g., a 

29 defense-in-depth protection) and the waste is planned to be stored for extended periods prior to 

30 shipment for disposal, the environment in which it is stored may need to be controlled to ensure 

31 that an earlier characterization of the container’s stability is adequate to meet the acceptance 

32 criteria at the time of disposal.

33 9.2.2 Data Quality Objectives Process

34

35 Demonstrating that the waste acceptance criteria are met is a process that is supported by 

36 waste characterization data. For most waste, this decision is supported by statistical tests 

37 based on the results of one or more direct samples. The initial assumption, or null hypothesis,

38 that a licensee should use is that each parameter to be characterized exceeds the allowable 

39 limits specified in the waste acceptance criteria. The characterization should be designed to 

40 provide information to reject this initial assumption. The NRC staff recommends that licensees 

41 use the Data Life Cycle as a framework for planning, implementing, and evaluating 

42 characterization results prior to making a decision. Licensees and generators should coordinate 

43 to apply the framework or a similar methodology to the waste characterization activities. Figure 

44 9-6 summarizes the major activities associated with each phase of the Data Life Cycle for waste 

45 characterization.
9-22

1 One aspect of the planning phase of the Data Life Cycle is the Data Quality Objectives (DQOs) 

2 process. The DQO process is a series of seven planning steps for establishing criteria for data 

3 quality and developing characterization plans:

4 1. State the problem;

5 2. Identify the goals of the study;

6 3. Identify inputs to the decision;

7 4. Define the study boundaries;

8 5. Develop the analytic approach;

9 6. Specify performance or acceptance criteria; and

10 7. Develop the plan for obtaining data

11

12 The process should use a graded approach to data quality requirements. A graded approach is 

13 one in which the level of effort required to develop data quality objectives should be 

14 commensurate with the importance of the data for demonstrating that the performance 

15 objectives would be met. This approach facilitates more effective characterization planning with 

16 consideration of how the data will be used. Thus the process should be a flexible planning tool 

17 that can be varied from simple to complex depending on the specific situation.

18

19 DQOs should be qualitative or quantitative statements that satisfy all of the following:

20

21 • Clarify the characterization objective;

22 • Define the most appropriate type of data to collect;

23 • Determine the most appropriate conditions for collecting the data; and

24 • Specify limits on decision errors that will be used as the basis for establishing the 

25 quantity and quality of data needed to demonstrate the waste acceptance criteria are 

26 met.

Figure 9-6 Data Life Cycle Framework for Waste Characterization
9-23

1 Using the DQO process can help ensure that the type, quantity, and quality of data will be 

2 appropriate to determine that the waste acceptance criteria are met. Additional guidance on the 

3 Data Life Cycle and Data Quality Objectives process is provided in an EPA guidance report 

4 (EPA, 2006). The Multi-Agency Radiation Survey and Assessment of Materials and Equipment

5 (NRC, 2009) provides guidance for applying the Data Life Cycle to disposition surveys of 

6 materials and equipment to ensure the surveys are adequate to meet the disposition 

7 requirements. Likewise, the Multi-Agency Radiation Laboratory Analytical Protocols Manual

8 (NRC, 2004b) provides guidance for the application of the Data Life Cycle to projects that 

9 require the laboratory analysis of radionuclides to ensure that the laboratory data will meet the 

10 data requirements. These guidance documents may be useful to licensees to develop data 

11 quality objectives for generator characterization data that demonstrates the waste acceptance 

12 criteria are met.

13 9.2.3 Documentation

14

15 Licensees should require waste generators to provide sufficient waste characterization 

16 documentation to ensure that the waste is adequately characterized to demonstrate that the 

17 acceptance criteria are met. The level of documentation that licensees require may vary across 

18 the waste streams accepted for disposal depending on both the complexity of the waste 

19 streams and the importance of the waste streams to demonstrating that the performance 

20 objectives are met or that defense-in-depth protections are provided. The elements of the 

21 documentation should typically include the following:

22

23 • Organization and Responsibilities. Organizations and personnel responsible for 

24 waste characterization should be identified. Personnel responsible for collecting and 

25 managing characterization data should be properly trained to recognize the significance 

26 of the data. Qualifications of personnel should be included to provide assurance that the 

27 data are properly collected and managed. For instance, acceptable methods for waste 

28 characterization may specify the use of a certified laboratory. In this case, the 

29 documentation should include the laboratory’s accreditation.

30

31 • Quality Assurance. Waste characterization data should be collected according to an 

32 acceptable QA program. The documentation should identify the QA program. Standard 

33 Review Plan 9.1 of NUREG-1200 provides guidance on acceptable QA programs (NRC, 

34 1994).

35

36 • Procedures. Procedures formalize the process for characterizing the waste. The 

37 procedures should describe the steps followed to characterize the waste as well as the 

38 administrative processes for ensuring the type, quantity, and quality of data is 

39 appropriate to adequately characterize the waste. Procedures should include processes 

40 for sampling, packaging, transportation, laboratory analysis, and data control, as 

41 appropriate. Standard Review Plan 8.6 of NUREG-1200 provides additional guidance 

42 on administrative and operating procedures (NRC, 1994).

43

44 • Records. Waste characterization records should include those that are necessary to 

45 meet the disposal facility’s waste acceptance criteria as specified by the waste 

46 certification program (see Section 9.3). Records that contain characterization 

47 procedures, data, and specifications, including the QA program, should be controlled 

48 documents that are subject to review, approval, and distribution procedures, as well as a 
9-24

1 process for making revisions. Existing record control programs may be adequate to 

2 provide the necessary controls.

3 9.3 Waste Certification Program

4

5 Licensees, per 10 CFR 61.58(c), must develop a program to certify that waste meets the 

6 acceptance criteria prior to receipt at a disposal facility. Certification of waste also provides 

7 assurance that a disposal facility operates within the limits established to demonstrate that the 

8 10 CFR Part 61 performance objectives would be met. Once certified to meet a disposal 

9 facility’s waste acceptance criteria, waste must then be managed to maintain its certification 

10 until its emplacement in a disposal unit.

11

12 The regulations specify that the certification program must: 

13

14 • Designate the authority to certify and receive waste for disposal at the disposal facility; 

15 • Provide procedures for certifying that waste meets the waste acceptance criteria; 

16 • Specify documentation required for waste characterization, shipping and certification; 

17 • Identify records, reports, tests, and inspections that are necessary to maintain and 

18 provide criteria for auditing; and 

19 • Provide approaches for managing certified waste to maintain its certification status. 

20 9.3.1 Certification Program

21

22 Licensees should develop a certification program that defines administrative procedures to 

23 provide assurance that waste and its packaging meets the waste acceptance criteria of the 

24 disposal facility prior to receipt of the waste at the disposal facility. The program should also 

25 provide a traceable and verifiable record of and basis for certification. The certification program 

26 should address the following questions:

27

28 • Who is responsible for certifying that waste is acceptable for disposal and what are their 

29 required qualifications?

30 • How and when shall waste be certified as acceptable for disposal?

31 • What documentation is required to provide a traceable and verifiable record for 

32 certification and how will certification be audited?

33 • How shall waste that has been certified be managed to maintain its certification?

34

35 The following sections provide guidance on information to adequately address these questions 

36 in order to meet the requirements of 10 CFR 61.58.

37

38 The principal documents that constitute the certification program should be subject to controls. 

39 Therefore, the certification program should identify which documents are to be controlled such 

40 as the waste certification program description, certification procedures, and QA program 

41 documentation. Document control includes review and approval, distribution to designated 

42 recipients, and a controlled process for making revisions to the documents. Existing document 

43 control programs at a disposal facility may provide the necessary controls for the documents 

44 that are part of the waste certification program.
9-25

1 9.3.1.1 Organizations and Responsibilities for Certification

2

3 The land disposal facility operator is responsible for developing the requirements of and 

4 managing the certification program. Waste certification is typically performed by the waste 

5 generator because they are the most knowledgeable about the waste and can most effectively 

6 characterize it as it is generated. As waste progresses toward disposal, characterization to 

7 meet the acceptance criteria can become more challenging and expensive to perform. 

8 However, in some cases, such as an infrequent generator of small quantities of waste, a waste 

9 collector, processor, or the land disposal facility operator may be more qualified to perform the 

10 certification on behalf of the generator. The organization responsible for certification must 

11 certify that waste is acceptable for disposal at the land disposal facility according to the disposal 

12 facility’s waste certification requirements and obtain authorization from the land disposal facility 

13 operator to transfer the waste for disposal.

14

15 A certification program must identify the designated individuals or organizations that are 

16 responsible for the certification process. These designees include representatives of the land 

17 disposal facility who are responsible for managing the certification program, and representatives 

18 of organizations responsible for complying with the land disposal facility’s certification program 

19 to certify that the waste meets the acceptance criteria. These individuals or organizations may 

20 include waste generators, waste collectors, waste processors, the land disposal facility operator, 

21 or other organizations or individuals qualified to certify that the waste meets the acceptance 

22 criteria. The program should require that personnel who are designated to certify waste be 

23 identified, qualified, and approved by the disposal facility operator’s designated authority.

24

25 The certification program should also identify the training requirements needed for the various 

26 individuals who are involved in the program. At a minimum, the program should require training 

27 of the official who certifies that the waste meets the acceptance criteria of the disposal facility. 

28 In addition, individuals should be trained in the procedures that control the part of the 

29 certification process with which they are involved.

30 9.3.1.2 Certification Procedures

31

32 Licensees should implement the certification program through the use of documented 

33 processes and procedures. The certification program should formalize the processes and 

34 procedures for certifying waste and for maintaining certification until the waste is emplaced in a 

35 disposal unit. 

36

37 The procedures should describe the administrative process that designated certification officials 

38 should follow to ensure that waste is certified prior to receipt at the land disposal facility. The 

39 procedures should require a signed statement certifying that the waste meets the disposal 

40 facility’s waste acceptance criteria and, therefore, is acceptable for disposal. The signature on 

41 the certification statement confirms that the waste has been characterized adequately and 

42 necessary shipping requirements have been met.

43

44 Waste must be certified prior to shipment to the disposal facility. This requirement ensures that 

45 the waste certification program is effective in preventing the transfer of waste that does not meet 

46 the waste acceptance criteria of the disposal facility. The requirement also prevents potential

47 hazards associated with managing the waste rejected by the disposal facility to which it is 

48 transferred. Requiring certification before waste is transferred also reduces the likelihood of 
9-26

1 having to recall a waste shipment due to a discovery by the certification official, after the waste 

2 is in transit, that the waste does not comply with the waste acceptance criteria. Certification that 

3 the waste is ready for transfer and meets the waste acceptance criteria and the applicable 

4 transportation regulations is a control point in the transfer process. The procedures controlling 

5 waste transfer should not allow transfer to occur unless the certification statement has been 

6 signed. Once signed, the certification statement becomes part of the record for the transfer of 

7 the waste. Once the waste is certified as acceptable for disposal, the land disposal facility can 

8 authorize transfer of the waste for receipt and disposal. The certification program should 

9 describe the administrative process for attaining authorization from the disposal facility to 

10 transfer the waste for disposal once the waste has been certified.

11

12 The procedures should require characterization of the waste, as well as inspection of the 

13 characterization process to demonstrate that it meets the acceptance criteria. Guidance on 

14 acceptable characterization is discussed in Section 9.2. For waste that does not meet the 

15 acceptance criteria when inspected, the procedures should specify the administrative process 

16 that a waste generator would need to follow to gain acceptance and properly certify that the 

17 waste is acceptable for disposal.

18

19 The procedures should also document the necessary steps for complying with the applicable 

20 transportation requirements for the transfer of certified waste to the land disposal facility, 

21 including those specified by the Department of Transportation and in 10 CFR Part 71. These 

22 requirements include the requirements for transfer of waste intended for disposal at a licensed 

23 land disposal facility that are found in Appendix G to 10 CFR Part 20.

24

25 The procedures should clearly describe the process for maintaining the waste certification until 

26 the waste has been placed in a disposal unit at the land disposal facility. Guidance on 

27 procedures for maintaining certification is provided in Section 9.3.2. As part of certification 

28 maintenance, the certification program should also identify adequate procedures for receipt and 

29 inspection of waste at the disposal facility to ensure that arriving waste shipments are in 

30 compliance with applicable Federal regulations and the waste acceptance criteria. Standard 

31 Review Plan 4.1 of NUREG-1200 (NRC, 1994) provides guidance on developing adequate 

32 procedures for receipt and inspection of waste arriving at a disposal facility. Standard Review 

33 Plan 8.6 of NUREG-1200 (NRC, 1994) also discusses guidance on administrative and operating 

34 procedures that may be applicable. The IAEA has also published guidance on inspection and 

35 verification of waste packages for near-surface disposal that may be applicable to developing 

36 adequate procedures (IAEA, 2000).

37

38 Finally, the procedures should clearly describe the process for restricting access to disposal for 

39 waste generators that are not meeting the requirements of the certification program. The 

40 procedures should identify conditions that would warrant restriction of generator access to 

41 disposal including, for example: radiological contamination; wasteform or container integrity 

42 deficiencies; improper characterization; improper manifesting; transportation violations; 

43 inadequate nuclear safety limits; and improper certification maintenance. These procedures 

44 may include suspension of access to disposal capacity or possible heightened oversight by the 

45 disposal facility operator and its regulatory authority, and should describe corrective actions 

46 necessary to restore access to the disposal facility following suspension.

47
9-27

1 9.3.1.3 Certification Documentation

2

3 The key document in a waste certification is the certification statement. The certification 

4 statement is the documentation signed by a designated official that certifies that the waste 

5 meets the waste acceptance criteria of the disposal facility. The certification statement should 

6 also include information required by the certification program, including radiological properties, 

7 wasteform characteristics, and container specifications. Licensees should use the waste 

8 acceptance criteria to identify key elements to include as part of the waste certification 

9 statement. In addition to the certification statement, documentation should also include 

10 confirmation that an official from the disposal facility to which the waste is to be transferred has 

11 authorized transfer of the waste to the disposal facility.

12

13 The documentation supporting the waste certification statement may include or reference the 

14 following information. The land disposal facility may use a graded approach to determine which 

15 of the following information is necessary for generators to provide prior to granting authorization 

16 to transfer waste to the disposal facility. A graded approach would focus on information that is 

17 necessary for generators to provide that is significant for demonstrating the waste acceptance 

18 criteria have been met and the waste can be certified as acceptable.

19

20 • Waste Stream Profile. The waste stream profile is a description of the waste stream, 

21 generally identifying the source, physical and chemical description, and upper limits on 

22 radionuclides.

23

24 • Radionuclide Content. Radionuclide content includes the concentration and inventory 

25 of radionuclides determined from waste characterization. See Section 9.2 for guidance 

26 on waste characterization.

27

28 • Radiological Surveys. Survey results include the determination of the surface 

29 contamination of the waste container and the external dose rate if necessary for waste 

30 certification.

31

32 • Waste Container Attributes. Container attributes include information about the 

33 physical attributes (e.g., dimensions) of the container as well as any necessary 

34 procurement information relevant to certification. Disposal facility operators may require 

35 generators to provide container specifications, particularly if they are relied upon by the 

36 land disposal facility operator for defense-in-depth. Each container specification should 

37 include a description of the specification’s purpose, procedures for complying with the 

38 requirements of the specification, description of the container and manufacturing 

39 specification, and results of tests to assess the integrity of the container.

40

41 • Uniform Low-Level Radioactive Waste Manifest. The manifest is required by 

42 Appendix G to 10 CFR Part 20 for transfers of waste intended for disposal at licensed 

43 land disposal facilities (NRC, 1998).

44

45 • Quality Assurance Records. QA records may include documentation of testing or 

46 inspections required for waste certification, particularly for defense-in-depth protections. 

47 The records may also include a statement ensuring access for designees of the land 

48 disposal facility to perform audits and inspections. This may include assurance of 
9-28

1 access to the providers of procured items or services that are significant to certify that 

2 the waste acceptance criteria are met.

3

4 • Certification Maintenance Procedures. Certification maintenance procedures include 

5 the processes and controls required to maintain waste certification. Guidance on 

6 certification maintenance is provided in Section 9.3.2.

7

8 The waste certification program should also identify which records need to be maintained and 

9 how they are to be maintained. The certification program may detail specific records 

10 management requirements, or may simply invoke an existing acceptable records management 

11 program such as one that complies with the requirements of 10 CFR 61.80.

12 9.3.1.4 Audits of Certifications

13

14 A certification program should also formalize procedures for independent audits of individuals or 

15 organizations designated to certify waste as acceptable for disposal by the land disposal facility 

16 operator. Standard Review Plan 8.5 of NUREG-1200 (NRC, 1994) provides guidance on plans 

17 for conducting reviews and audits of operational activities important to safety that may also be 

18 applicable to audits of the certification process. The periodic audits should provide an 

19 independent verification of the implementation of the certification program. The audit 

20 procedures should describe the principle documents of the waste certification record that will be 

21 audited and the frequency of audits.

22

23 The principal documents that should be subject to inspection may include the waste certification 

24 statement, procurement or purchasing documents (e.g., for approved containers), radiological 

25 survey data, and laboratory testing data for characterization required to demonstrate 

26 conformance with the waste acceptance criteria. Audits may also include observation of testing 

27 and characterization that are significant to demonstrating the waste acceptance criteria are met 

28 as well as how the certification process is implemented by the certifying organization. 

29 Licensees may use a graded approach to determine the documents subject to audit and the 

30 frequency of the audits for a given certifying organization. The certification program should 

31 identify the records required to adequately document the audits and the management 

32 requirements for the audit records. Licensees should maintain records of the certification audits 

33 for inspection by the NRC.

34 9.3.2 Certification Maintenance

35

36 Waste that has been certified as meeting the waste acceptance criteria for a land disposal 

37 facility must be controlled so that the certification remains valid until disposal at the facility. 

38 Otherwise, the waste will need to be re-certified. The waste certification program should identify 

39 the requirements for protecting the certification status of the waste. These requirements may be 

40 specific to a waste stream or applicable to all waste streams. The certifying organization should 

41 develop procedures for maintaining the waste certification that comply with the certification 

42 program’s requirements. Certification maintenance may be especially important for waste that 

43 will be stored for long periods of time or significantly treated or conditioned prior to disposal.

44

45 Requirements for maintaining the certification status include protecting the waste container, 

46 preventing unauthorized introduction of material into the waste, protecting the data marked on 

47 the waste container, and protecting any other capabilities relied upon for defense-in-depth. 
9-29

1 Requirements for protection of the waste container may include sufficient protection from the 

2 environmental conditions during storage, conditioning, or transport (e.g., precipitation, heat, ultra 

3 violet) or designated limits for damage, should it occur. Waste may also need to be controlled in 

4 a manner that prevents modifying the contents. These controls may include requirements for 

5 tamper indication devices and secured storage depending on the waste stream. It is also 

6 important to be able to relate each container to information about the certification of the 

7 container. Therefore, licensees may need to have requirements regarding container markings 

8 to protect from defacement or removal. Also, licensees should safely store records regarding 

9 certification.

10 9.4 Periodic Review

11

12 The regulations at 10 CFR 61.58(f) require disposal facility licensees to review the content and 

13 implementation of their waste acceptance program at least annually. The purpose of this review 

14 is to ensure that the content of the waste acceptance program continues to be adequate and 

15 that the program is being implemented in a way that continues to protect public health and 

16 safety. As part of this annual review of the waste acceptance program, disposal facility 

17 licensees should also evaluate and document whether waste acceptance criteria continue to be 

18 protective of public health and safety. If the evaluation indicates that the waste acceptance 

19 criteria continue to provide reasonable assurance that the performance objectives will be met

20 and adequate defense-in-depth will be maintained, the documentation should include the basis 

21 for relying on the existing waste acceptance criteria. If the evaluation finds that the waste 

22 acceptance criteria no longer provide reasonable assurance that the performance objectives will 

23 be met or that defense-in-depth protections are inadequate, the criteria should be updated. The 

24 licensee should submit the amended criteria and supporting technical analyses that 

25 demonstrate the performance objectives will be met as part of a request for amendment to the 

26 license.

27

28 Periodic reviews should incorporate the following features to assess procedural compliance, 

29 technical performance, implementation, and effectiveness of the facility waste acceptance 

30 program:

31

32 • Waste acceptance supervisory reviews. Onsite waste acceptance supervisors should 

33 periodically perform and document reviews of the effectiveness of the waste acceptance 

34 personnel in such areas as development of waste acceptance criteria, characterization 

35 adequacy, and procedural compliance.

36

37 • Quality assurance audits. Quality assurance audits should be performed by the onsite 

38 auditing group. Personnel in the auditing group should have sufficient waste acceptance 

39 training or experience so they can determine whether waste acceptance functions (e.g., 

40 characterization or certification) are being performed as required. These audits should 

41 also be performed periodically at generators.

42

43 • Corporate or contract audits. Offsite (corporate or contract) audits and evaluations 

44 should be performed to determine whether the waste acceptance program complies with 

45 the regulations and other requirements and whether objectives are being met as well as 

46 to identify needed program improvements.

47
9-30

1 Periodic review records should contain the following information to be acceptable: date of the 

2 review, name of person(s) who conducted the review, persons contacted by the reviewer(s), 

3 areas reviewed, review findings, corrective actions, and follow-up. The licensee is not required 

4 to submit documentation of its periodic review to the NRC. Rather, licensees should maintain 

5 the records as required in 10 CFR 61.80. However, if a licensee identifies during its periodic 

6 review a significant implication for public health and safety or common defense and security, the 

7 licensee shall notify the NRC as required by 10 CFR 61.9a(b).

8 9.5 Mitigation

9

10 In some cases, a land disposal facility may learn of new information that indicates that 

11 previously disposed of waste may present an unreasonable risk to public health and safety or 

12 the environment. For example, the new information may indicate a significant reduction in the 

13 expected performance of engineered or intruder barriers or the site characteristics to limit 

14 radionuclide release and migration. In these cases, licensees may need to consider mitigation 

15 to reduce the impact to humans or the environment to ensure that waste disposal continues to 

16 meet the performance objectives with reasonable assurance and provides defense-in-depth 

17 protections. Mitigation could take many different forms, such as but not limited to, modification 

18 of the disposal facility design or remediation of the disposed waste. This section of the 

19 guidance document describes when licensees should consider mitigation, what information they 

20 should provide to the regulator to demonstrate that mitigation has been implemented, and what 

21 a reviewer should evaluate to verify that mitigation has occurred.

22

23 As licensees periodically update the waste acceptance criteria, new information may be learned 

24 that, when considered in the technical analyses, could indicate that waste previously accepted 

25 for disposal may present an unreasonable risk to public health and safety or the environment. 

26 Additionally, prior to final closure of the disposal site, the licensee, per the requirements of 

27 10 CFR 61.28, shall submit an application to amend the license for closure upon which the 

28 Commission shall make a determination if there is reasonable assurance that the performance 

29 objectives of 10 CFR Part 61 will be met and that defense-in-depth protections have been 

30 provided. A component of the application for closure should include a final set of technical 

31 analyses and a final revision of the safety case to demonstrate that the performance objectives 

32 will continue to be met after closure. If a land disposal facility determines during updates to the 

33 waste acceptance criteria or the closure process that the facility is no longer meeting the 

34 performance objectives, mitigation is one method of bringing the facility into compliance.

35

36 Before engaging in mitigation, a licensee should provide information describing the actions they 

37 propose to take, including the basis for those actions. The proposed actions could include 

38 modification of the disposal facility design or remediation. Design modifications could include 

39 installation of a higher performance engineered cover or the use of permeable treatment walls, 

40 diversion ditches, sheet piling, and so forth. The licensee’s design modifications should be 

41 developed through consideration of the results of the technical analyses. Remediation of the 

42 disposed waste may involve actions such as in situ stabilization of the wasteforms (e.g., 

43 grouting) and removal of a portion of or the entire unacceptable waste inventory for offsite 

44 disposal. The licensee should provide a technical basis that demonstrates 10 CFR 61.43 (i.e., 

45 protection of individuals during operations) will be met during remediation. They should develop 

46 a cost-benefit analysis to inform the selection of remedial actions.

47
9-31

1 A reviewer should evaluate the information provided by the licensee, including the technical 

2 basis for the proposed actions. The reviewer should evaluate the alternatives considered and 

3 the basis for the action selected. If design modifications are proposed, the reviewer should 

4 evaluate the technical basis for the performance of engineered barriers. The reviewer should 

5 determine if the licensee’s desired performance of the engineered barriers is likely to be 

6 achieved. The reviewer should determine if the design modifications were developed in 

7 consultation with the results of the technical analyses. If remediation is selected, the reviewer 

8 should evaluate the basis for how much waste the licensee would remove and how the licensee 

9 would remove the waste. The technical basis demonstrating that 10 CFR 61.43 will be met 

10 should be reviewed. Finally, the reviewer should evaluate cost-benefit analyses that have been 

11 developed to ensure the mitigation activities are justified.

12

13

10-1

1 10.0 PERFORMANCE CONFIRMATION

2

3 Performance confirmation is the program of tests, experiments, and analyses that licensees 

4 conduct to evaluate and verify the accuracy of information used to demonstrate that the 

5 10 CFR Part 61 performance objectives are met before disposal site closure. Prior to final 

6 closure of the disposal site, licensees are required by 10 CF 61.28 to submit an application to 

7 amend the license for closure. The closure application must include the specific details of the 

8 site closure plan. The plan must include additional geologic, hydrologic, and other disposal site 

9 data pertinent to the long-term containment of emplaced radioactive wastes obtained during the 

10 operational period. In addition, the plan must include the results of tests, experiments, or other 

11 analyses pertinent to the long-term containment of emplaced waste within the disposal site. 

12 Licensees must update the technical analyses for 10 CFR 61.13 using details of the final 

13 closure plan and the waste inventory. Although the terminology “performance confirmation” is 

14 not used in the regulation, as discussed below, the NRC staff believes that the elements of a 

15 performance confirmation program are supported by 10 CFR Part 61. Elements of performance 

16 confirmation may be completed during active operation as well as during the institutional control 

17 period. The following are the main elements of a performance confirmation program:

18

19 • verification that site conditions encountered during construction were within limits

20 assumed during licensing

21 • verification that engineered barriers and other defense-in-depth protections were 

22 constructed as designed and will perform within limits assumed during licensing

23 • verification of the performance of natural barriers that were relied upon by the licensee in 

24 licensing to achieve compliance with the performance objectives

25 • monitoring of facility performance

26 • verification of the safety case

27 Performance confirmation is integrated with disposal system design, development, and 

28 construction to provide confidence that the disposal system will perform as intended. 

29 Performance confirmation can be used to supplement information satisfying the requirements of 

30 10 CFR 61.28, 10 CFR 61.52, and 10 CFR 61.53. 

31

32 The NRC staff expects that licensees may obtain additional geologic, hydrologic, or other 

33 disposal site data pertinent to the long-term containment of emplaced radioactive wastes during 

34 the operational period. In addition, monitoring of the performance of the disposal facility and 

35 minor custodial care is required for the duration of the institutional control period. Additional 

36 data with respect to facility performance and site conditions may be obtained during the 

37 institutional control period. Under 10 CFR 61.28 licensees are required to update technical 

38 analyses; the Commission will issue an amendment authorizing closure if the updated analyses 

39 demonstrate that the 10 CFR Part 61 performance objectives are met (e.g., performance 

40 assessment, intruder assessment, site stability evaluation, defense-in-depth analyses, 

41 protective assurance period analyses, and performance period analyses). A performance 

42 confirmation program can be used to proactively generate information to support updating the 

43 technical analyses for closure. The performance confirmation program can be designed to 

44 produce information to support the most risk-significant and uncertain elements of the technical 

45 analyses.

46
10-2

1 Table 10-1 Regulatory Requirements Supportive of Performance Confirmation

Section Requirement

10 CFR 61.7(c)(3) Post-closure monitoring and maintenance

10 CFR 61.12(g)

A description of the disposal site closure plan, including 

those features that facilitate closure and eliminate the 

need for maintenance

10 CFR 61.28 Contents of application for closure

10 CFR 61.52 Land disposal facility operation and disposal site closure

10 CFR 61.53(c) Environmental monitoring during construction and 

operation

10 CFR 61.53(d) Environmental monitoring, post operational surveillance

2

3

4 It is good practice for a licensee to update the technical analyses supporting licensing (e.g., 

5 performance assessment and intruder assessment) at regular intervals. Upon amending waste 

6 acceptance criteria to accept new waste, proposing changes to the design of the disposal site, 

7 or when new information about site characteristics or design properties becomes available, the 

8 licensee should determine if the site will continue to comply with the Subpart C performance 

9 objectives and, if necessary, update the analyses. While a regular interval for updating the 

10 technical analyses is not specified in the regulation, 10 CFR 61.58(f) requires licensees to 

11 annually review the content and implementation of their waste acceptance program. The 

12 purpose of this review, as described further in Section 9.4 of this document, is to ensure that the 

13 content of the waste acceptance program continues to be adequate and that the program is 

14 being implemented in a way that continues to protect public health and safety. As part of this 

15 annual review, licensees should also evaluate and document whether the waste acceptance 

16 criteria continues to provide reasonable assurance of compliance with the Subpart C 

17 performance objectives. If the evaluation finds that the waste acceptance criteria do not provide 

18 reasonable assurance of compliance with the Subpart C performance objectives, the licensee 

19 should submit the revised criteria and supporting technical analyses that demonstrate that the 

20 proposed criteria meet the performance objectives as part of a request for amendment to the 

21 license.

22

23

24
10-3

1 Updating technical analyses can ensure that appropriate operational practices (e.g., allowable 

2 limits) are identified and implemented to reduce the potential need for more challenging 

3 mitigation activities (e.g., installing new barriers, removing waste). Conditions that could trigger 

4 a decision to update waste acceptance criteria, and therefore to update technical analyses, may 

5 include but are not limited to the following:

6

7 (1) substantially different inventory than anticipated (e.g., quantity, concentration, form)

8 (2) new site information (e.g., environmental conditions, characterization data)

9 (3) new information on engineered barrier performance or other defense-in-depth

10 protections

11 (4) monitoring data that are inconsistent with current analyses

12 (5) substantial changes to relevant scientific understanding

13 (6) use of updated dosimetry

14

15 Licensees should document the basis for their decision on whether the waste acceptance 

16 criteria and supporting technical analyses should be updated. The availability of new 

17 information may not always prompt a decision to update the waste acceptance criteria and 

18 supporting technical analyses. The operating period of a disposal facility may extend over 

19 multiple decades, and the NRC staff expects that new information will be developed. Regular 

20 updating is advised because it will allow a licensee to evaluate relevant information generated 

21 since the previous update. Regular updating will also reduce the likelihood that something 

22 unforeseen develops that calls into question the performance of the disposal facility. In some 

23 cases, significant information may become available to a licensee. The licensee should 

24 determine, in consultation with the pertinent regulator, if the new information warrants an update 

25 to the technical analyses. 

26

27 Monitoring of environmental media is required by 10 CFR 61.53. Monitoring is required to 

28 provide early warning of release of radionuclides from the disposal site before they leave the 

29 site boundary. Most monitoring systems focus on sampling environmental media, such as 

30 groundwater or the atmosphere, some distance from the facility, such as within the buffer zone 

31 surrounding the facility or at the site boundary. It may be useful for a licensee to identify and 

32 use performance indicators. A performance indicator is a measure of the performance of 

33 subsystems of the disposal system that may be a precursor to the overall performance of the 

34 disposal system. Performance indicators may be a less direct measure of overall performance 

35 but have the advantage of providing early warning of changes in system performance. For 

36 example, monitoring of soil moisture underneath an engineered cover may provide an indication 

37 of increased infiltration that may lead to increased release of radioactivity from the disposal 

38 facility.

39

40 Licensees can use technical analyses supporting demonstration of compliance with the 

41 performance objectives to help determine what types of information would be most useful to 

42 monitor and when that information is significant. Monitoring data typically have moderate to 

43 significant variability. If monitoring data are used in performance confirmation, the NRC staff 

44 recommends that a licensee provide a description of the anticipated variability of monitoring 

45 data to help prevent misinterpretation of observational data. In some cases, a monitoring 

46 observation may appear to be an outlier. Licensees should not reject data as outliers without a 

47 statistical or physical basis. If a statistical basis cannot be provided, additional information 

48 should be collected. Reviewers should evaluate the technical basis for the treatment of outliers 

49 and determine if it is adequate. In some cases, a technical explanation may be available (e.g., 
10-4

1 the sample was contaminated). It is possible that initial information may appear to represent an 

2 outlier, however, in reality the observed behavior is not due to measurement error, for example, 

3 but rather represents complex, unanticipated phenomena. Licensees should use caution in 

4 dismissing outliers in observational data. 

5

6
11-1

11.0 USE OF OTHERNRC GUIDANCE DOCUMENTS

The following tables are intended to provide references that may be useful to licensees in developing their techn

Tables 11-1, 11-2, and 11-3 present references according to the performance objectives in 10 CFR Part 61. Ta

list of general topics and associated references. NUREG-1573 includes a bibliography of technical references a

disposal (as of 2000) in its Appendices B and C (NRC, 2000a) that may also be useful to licensees and reviewe

Table 11-1 Guidance Crosswalk for Performance Objective 10 CFR 61.41, Performance As

Document Description

NUREG-1573, “A Performance Assessment Methodology for 

Low-Level Radioactive Waste Disposal Facilities” (NRC, 2000a)

Section 2, regulatory framework for 10 

Section 3.1, performance assessmen

NUREG-1200, “Standard Review Plan for the Review of a License 

Application for a Low-Level Radioactive Waste Disposal Facility”

(NRC, 1994)

Review procedures for technical analy

NUREG-1757, “Consolidated Decommissioning Guidance” (NRC, 

2006)

Volume 2, Section 3.5, evaluation of e

barriers

NUREG/CR-5512, “Residual Radioactive Contamination from 

Decommissioning” (NRC, 1992) Volume 1, Appendix E, Table E.6, sol

NUREG-1854, “NRC Staff Guidance for Activities Related to 

U.S. Department of Energy Waste Determinations” (NRC, 2007a)

Section 4.2, guidance on review of pe

assessments for waste determination

technical review procedures
11-2

Table 11-2 Guidance Crosswalk for Performance Objective 10 CFR 61.42, Inadvertent Intruder As

Document Description

NUREG-1854, “NRC Staff Guidance for Activities Related to 

U.S. Department of Energy Waste Determinations” (NRC, 2007a)1

Guidance on review of intruder analys

determinations

NUREG-0782, “Draft Environmental Impact Statement on

10 CFR Part 61 Licensing Requirements for Land Disposal of 

Radioactive Waste” (NRC, 1981a)

NUREG-0945 “Final Environmental Impact Statement on

10 CFR Part 61 Licensing Requirements for Land Disposal of 

Radioactive Waste” (NRC, 1982b)

NUREG/CR-1759, “Data Base for Radioactive Waste 

Management” (directly supports NUREG-0782) (NRC, 1981b)

Describes the intruder assessment m

to develop waste classification tables 

including generic scenarios

Revised Draft Branch Technical Position on Concentration 

Averaging and Encapsulation, Rev. 1, May 2012 (NRC, 2012a) Acceptable methods of concentration 

“Final Waste Classification and Waste Form Technical Position 

Papers,” Rev. 0, May 11, 1983 (NRC, 1983d)

Guidance on classifying waste

Various methods to determine radionu

concentrations

“Technical Position on Waste Form (Revision 1)”, 

January 18, 1991 (NRC, 1991b)

Guidance on wasteforms to comply w

characteristics requirements in 10 CF

NUREG-1573, “A Performance Assessment Methodology for 

Low-Level Radioactive Waste Disposal Facilities” (NRC, 2000a)

Section 3.3, model abstractions for a p

assessment, generally applicable to in

assessment

 1 Many of these references also apply to performance objective 10 CFR 61.41.
11-3

Table 11-2 Guidance Crosswalk for Performance Objective 10 CFR 61.42, Inadvertent Intruder As

Document Description

NUREG-1200, “Standard Review Plan for the Review of a License 

Application for a Low-Level Radioactive Waste Disposal Facility”

(NRC, 1994)

Section 6.1, evaluation of pathways in 

calculation

NUREG-1757, “Consolidated Decommissioning Guidance” (NRC, 

2006)

Volume 2, Section 3.5.4, degradation 

capabilities of engineered barriers

Volume 2, Section 3.5.5, summary of 

and reference information for applicati

barriers at disposal facilities

Appendix M, Table M.5-M.12, water q

Appendix M.5, Potential sources of lan

information

Section I.6, selecting site-specific inpu

models and providing a technical basi

Section I.5, selection of codes/models 

for NRC acceptance of the codes/mod

NUREG/CR-4370, “Update of Part 61 Impacts Analysis

Methodology” (NRC, 1986a)

Provides scenarios and calculation ap

estimate intruder doses
11-4

Table 11-3 Guidance Crosswalk for Performance Objective 10 CFR 61.44, Site Stability A

Document Description

NUREG-1623, “Design of Erosion Protection for Long-Term 

Stabilization” (NRC, 2002b)

Design of erosion protection at uranium 

sites 

Procedure for determining the suitabilit

source

NUREG-1804, “Yucca Mountain Review Plan” (NRC, 2003c ) Seismic events in waste disposal

NUREG-1200, “Standard Review Plan for the Review of a License 

Application for a Low-Level Radioactive Waste Disposal Facility”

(NRC, 1994)

Site stability analysis for long-lived wa

NUREG-1757, “Consolidated Decommissioning Guidance” (NRC, 

2006)

Section 3.5, risk-informed approach to 

barriers

Analogs for wasteform stability

Durability of earthen covers

Section 3.5.5, reference information re

engineered cover design and performa

Appendix P, evaluations of rock durab

NUREG/CR-2642, “Long-Term Survivability of Riprap for 

Armoring Uranium Mill Tailings and Covers” (NRC, 1982c)

Table 6.7, comparative data on natura

Appendix A, information on rock weath

examples of analogs

Technical Position on Waste Form (Revision 1) (NRC, 1991b)

Specific test procedures and criteria to 

wasteform stability

NUREG-0902, “Site Suitability, Selection and Characterization, 

BTP – Low-Level Waste Branch” (NRC, 1982e)

Provides additional information on proc

avoided that may affect site stability
11-5

Table 11-4 Guidance Crosswalk by Topic

Topic Document

Analysis Timeframe

NUREG-1573, “A Performance Assessment M

for Low-Level Radioactive Waste Disposal Fa

2000a)

“Technical Analysis Supporting Definition of P

Performance for Low-Level Waste Disposal” (

Radon Diffusion and Barriers

Regulatory Guide 3.64, “Calculation of Radon 

Attenuation by Earthen Uranium Mill Tailings 

1989a)

NUREG/CR-4370, “Update of Part 61 Impacts 

Methodology” (NRC, 1986a)

Sensitivity/Uncertainty Analysis

NUREG-1757, “Consolidated Decommissioni

Vol. 2, Appendix I, Section 1.7 (NRC, 2006)

NUREG-1573, Section 3.3.2 (NRC, 2000a)

Expert Elicitation and Judgment

NUREG-1563, “Branch Technical Position on 

Expert Elicitation in the High-Level Radioactiv

Program” (NRC, 1996)

Modelling Climate and Infiltration

NUREG-1573, Section 3.3.3 (NRC, 2000a)

NUREG-1854, Section 4.3.1 (NRC, 2007a)
11-6

Table 11-4 Guidance Crosswalk by Topic

Topic Document

Waste Acceptance Criteria

Revised Draft Branch Technical Position on C

Averaging and Encapsulation, Rev. 1, May 20

2012a)

Final Waste Classification and Waste Form T

Position Papers,” Rev. 0, May 11, 1983 (NRC

Technical Position on Waste Form, Rev. 1 (N

NUREG-1200, SRP 4.1, SRP 6.11 (NRC, 199

Waste Characterization

See Waste Acceptance Criteria (NRC, 1983d

2012a); (NRC, 1991b)

NUREG/CR-6567, “Low-Level Radioactive W

Classification, Characterization, and Assessm

Streams and Neutron-Activitated Metals.” (NR

NUREG-1575, “Multi-Agency Radiation Surve

Assessment of Materials and Equipment (MA

Supplement 1 (NRC, 2009)

NUREG-1576, “Multi-Agency Radiological Lab

Analytical Protocols Manual (MARLAP)” (NRC

NUREG-1200, SRP 8.6, SRP 9.1 (NRC, 1994
11-7

Table 11-4 Guidance Crosswalk by Topic

Topic Document

Waste Certification

NUREG-1200, SRP 8.5 (NRC, 1994)

NUREG/BR-0204, “Instructions for Completin

Uniform Low-Level Radioactive Waste Manife

2, July 1998 (NRC, 1998)

Source Term Modelling

NUREG-1573, Section 3.3.5 (NRC, 2000a)

NUREG-1854, Section 4.3.3 (NRC, 2007a)

Inventory Abstraction

NUREG-1573, Sections 3.3.5.1, 3.3.5.2, and 

2000a)

NUREG-1854, Sections 3.1 and 4.3.3.1.1 (NR

Chemical Environment and Abstraction

NUREG-1573, Section 3.3.5.6 (NRC, 2000a)

NUREG-1854, Section 4.3.3.1.4 (NRC, 2007a

NUREG-1804, “Yucca Mountain Review Plan

Section 4.2.1.3.3 (NRC, 2003c )

Waste Containers, Wasteform, and Waste Type

Wasteforms and Degradation

NUREG-1573, Section 3.3.5.4 (NRC, 2000a)

NUREG-1854, Section 4.3.3.1.2 (NRC, 2007a

Engineered Barriers

NUREG-1573, Section 3.3.4 (NRC, 2000a)

NUREG-1854, Section 4.3.2 (NRC, 2007a)

NUREG-1804, Sections 4.2.1.3.1 and 4.2.1.3
11-8

Table 11-4 Guidance Crosswalk by Topic

Topic Document

2003c )

Aqueous Release Models

NUREG-1854, Section 4.3.3.2 (NRC, 2007a)

NUREG-1804, Section 4.3.1.2.4 (NRC, 2003c 

Gaseous Release Screening, Processes Generating 

Gases

NUREG-1573, Sections 3.3.5.7.1 and 3.3.5.7

2000a)

Climate and Infiltration Modelling

NUREG-1573, Section 3.3.3 (NRC, 2000a)

NUREG-1854, Section 4.3.1(NRC, 2007a)

Biosphere Characteristics and Dose Modelling

NUREG-1573, Section 3.3.7 (NRC, 2000a)

NUREG-1854, Section 4.3.5 (NRC, 2007a)

Transport Modelling

NUREG-1573, Section 3.3.6 (NRC, 2000a)

NUREG-1854, Section 4.3.4 (NRC, 2007a)

Groundwater Transport NUREG-1573, Section 3.3.6.1.2 (NRC, 2000a

Surface Water Transport

NUREG-1573, Section 3.3.6.2 (NRC, 2000a)

NUREG-1854, Section 4.3.4.1.2 (NRC, 2007a

Atmospheric Transport NUREG-1573, Sections 3.3.6.3.2.1 and 3.3.6

2000a)
11-9

Table 11-4 Guidance Crosswalk by Topic

Topic Document

Development of a License Application for an LLW 

Disposal Facility

NUREG-1199, “Standard Format and Conten

Application for a Low-Level Radioactive Wast

Facility,” Revision 2 (NRC, 1991a)

Regulatory Guide 4.18, “Standard Format and 

Environmental Reports for Near-Surface Disp

Radioactive Waste” (NRC, 1983e).

NUREG-1300, “Environmental Standard Revi

the Review of a License Application for a Low

Radioactive Waste Disposal Facility” (NRC, 1

Environmental Monitoring

NUREG-1388, “Environmental Monitoring of L

Radioactive Waste Disposal Facility” (NRC, 1

Alternate Disposal Methods

NUREG-1241, “Licensing of Alternative Metho

Disposal of Low-Level Radioactive Waste” (N

Site Selection

NUREG-0902, “Site Suitability, Selection and 

Characterization, BTP – Low-Level Waste Bra

1982e)

Regulatory Guide, 4.19, “Guidance for Select

Near-Surface Disposal of Low-Level Radioac

(NRC, 1988b)

Scenario Development

NUREG/CR-5927, Vol. 1, (SAND91-2802), “E

Performance Assessment Methodology for Lo

Radioactive Waste Disposal Facilities: Evalua

Modeling Approaches,” (NRC, 1993a)
11-10

Table 11-4 Guidance Crosswalk by Topic

Topic Document

NUREG-1200, Section 2.4.1, Appendix A (NR

NUREG-1199, “Standard Format and Conten

Application for a Low-Level Radioactive Wast

Facility,” Revision 2 (NRC, 1991a)

NUREG-1623, “Design of Erosion Protection 

Stabilization,” (NRC, 2002b)
12-1

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Survivability of Riprap for Armoring Uranium Mill Tailings and Covers: A Literature Review,” 

NUREG/CR-2642 (PNL-4225), Washington, DC, 1982, Public Legacy Library Accession No. 

8207140120, Microform 13845:347-13846:124.

NRC, 1982d. U.S. Nuclear Regulatory Commission (McKenzie, D.H., L.L. Cadwell, C.E. 

Cushing, Jr., R. Harty, W.E. Kennedy, Jr., M.A. Simmons, J.K. Soldat, and B. Swartzman), 

“Relevance of Biotic Pathways to the Long-Term Regulation of Nuclear-Waste Disposal: A 
12-10

Report on Tasks 1 and 2 of Phase 1,” NUREG/CR-2675, Vol. 1, National Technical Information 

Service, Springfield, VA, 1982, Public Legacy Library Accession No. 8208180126, Microform 

14391:289-14391:361.

NRC, 1982e. U.S. Nuclear Regulatory Commission, “Site Suitability, Selection and 

Characterization, BTP – Low-Level Waste Branch,” NUREG-0902, April 1982, ADAMS 

Accession No. ML053010325.

NRC, 1983a. U.S. Nuclear Regulatory Commission, “Influence of Cover Defects on the 

Attenuation of Radon with Earthen Cover,” NUREG/CR-3395, Public Legacy Library Accession 

No. 8312020113, Microform 21367:292-21367:326.

NRC, 1983b. U.S. Nuclear Regulatory Commission (Vorhees, L.D., M.J. Sale, J.W. Webb, and 

P.J. Mulholland), “Guidance for Disposal of Uranium Mill Tailings: Long-Term Stabilization of 

Earthen Cover Materials,” NUREG/CR-3199 (ORNL/TM-8685), Public Legacy Library Accession 

No. 8310260423, Microform: 20926:016-20926:123.

NRC, 1983c. U.S. Nuclear Regulatory Commission (Schumm, S.A., and P.J. Chorley), 

“Geomorphic Controls on the Management of Nuclear Waste,” NUREG/CR-3276RW, Public 

Legacy Library Accession No. 8311080172, Microform 21091:001-21091:151.

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Form Technical Position Papers,” Rev. 0, Washington, DC, May 11, 1983, ADAMS Accession 

No. ML033630755.

NRC, 1983e. U.S. Nuclear Regulatory Commission, “Standard Format and Content of 

Environmental Reports for Near-Surface Disposal of Radioactive Waste,” Regulatory Guide

4.18, June 1983, ADAMS Accession No. ML003739515.

NRC, 1984. U.S. Nuclear Regulatory Commission, “Radon Attenuation Handbook for Uranium 

Mill Tailings Cover Design,” NUREG/CR-3533, ADAMS Accession No. Public Legacy Library 

Accession No. 8405210555, Microform 24529:119-24529:207.

NRC, 1986a. U.S. Nuclear Regulatory Commission, “Update of Part 61 Impacts Analysis 

Methodology,” NUREG/CR-4370, 2 Vols., Washington, DC, January 1986, ADAMS Accession 

Nos. ML100251399, ML100250917.

NRC, 1986b. U.S. Nuclear Regulatory Commission, “Low-Level Radioactive Waste Scaling 

Factors, 10 CFR Part 61,” Information Notice No. 86-20, March 28, 1986, ADAMS Accession 

No. ML031220625. 

NRC, 1986c. U.S. Nuclear Regulatory Commission, “Licensing of Alternative Methods of 

Disposal of Low-Level Radioactive Waste,” NUREG-1241, December 1986, ADAMS Accession 

No. ML053010322.

NRC, 1987. U.S. Nuclear Regulatory Commission, “Environmental Standard Review Plan for 

the Review of a License Application for a Low-Level Radioactive Waste Disposal Facility,” 

NUREG-1300, April 1987, ADAMS Accession No. ML053010347.
12-11

NRC, 1988a. U.S. Nuclear Regulatory Commission, “Generic Technical Position on 

Peer-Review for High-Level Nuclear Waste Repositories,” NUREG-1297, Washington, DC,

February 1988, ADAMS Accession No. ML033160259.

NRC, 1988b. U.S. Nuclear Regulatory Commission, “Guidance for Selecting Sites for NearSurface Disposal of Low-Level Radioactive Waste,” Regulatory Guide 4.19, August 1988, 

ADAMS Accession No. ML003739520.

NRC, 1989a. U.S. Nuclear Regulatory Commission, “Calculation of Radon Flux Attenuation by 

Earthen Uranium Mill Tailings Covers,” Regulatory Guide 3.64, Washington, DC, 1989, ADAMS 

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NRC, 1989b. U.S. Nuclear Regulatory Commission (Shipers), “Background Information for the 

Development of a Low-Level Waste Performance Assessment Methodology, Identification of 

Potential Exposure Pathways,” NUREG/CR-5453, Vol. 1, Public Legacy Library Accession No. 

9001100240, Microform 52229:171-52229:200.

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Information for the Development of a Low-Level Waste Performance Assessment Methodology, 

Assessment of Relative Significance of Migration and Exposure Pathways,” NUREG/CR-5453, 

Vol. 2, Public Legacy Library Accession No. 9001100237, Microform 2231:340-52232:037.

NRC, 1989d. U.S. Nuclear Regulatory Commission (Hunter, R.L. and Mann, C.J.), “Techniques 

for Determining Probabilities of Events and Processes Affecting Performance of Geologic

Repositories,” NUREG/CR-3964, Vol. 1, June 1989, Public Legacy Library Accession No. 

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NRC, 1989e. U.S. Nuclear Regulatory Commission, “Environmental Monitoring of Low-Level 

Radioactive Waste Disposal Facility,” NUREG-1388, December 1989, ADAMS Accession No. 

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NRC, 1990a. U.S. Nuclear Regulatory Commission, "Uncertainties Associated with 

Performance Assessment of High-Level Radioactive Waste Repositories,” NUREG/CR-5211, 

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of Radioactive Waste: Scenario Selection Procedure,” NUREG/CR-1667, (SAND80-1429), 

Washington, DC, April 1990. https://ntrlr3.ntis.gov/view.php?pid=NTIS:NUREGCR1667

NRC, 1991a. U.S. Nuclear Regulatory Commission, “Standard Format and Content of a 

License Application for a Low-Level Radioactive Waste Disposal Facility,” NUREG-1199, 

Rev. 2, Washington, DC, January 1991, ADAMS Accession No. ML022550605. 

NRC, 1991b. U.S. Nuclear Regulatory Commission, Technical Position on Waste Form, Rev. 1, 

Washington, DC, January 18, 1991, ADAMS Accession No. ML033630746.

NRC, 1991c. U.S. Nuclear Regulatory Commission, “Quality Assurance Guidance for a 

Low-Level Radioactive Waste Disposal Facility,” Office of Nuclear Material Safety and 
12-12

Safeguards, Division of Low-Level Waste Management and Decommissioning, NUREG-1293, 

Rev. 1, April, 1991, ADAMS Accession No. ML11242A180.

NRC, 1992. U.S. Nuclear Regulatory Commission, “Residual Radioactive Contamination from 

Decommissioning, Technical Basis for Translating Contamination Levels for Annual Total 

Effective Dose Equivalent,” NUREG/CR-5512 (PNL-7994), Washington, DC, October 1992, 

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NRC, 1993a. U.S. Nuclear Regulatory Commission, “Evaluation of a Performance Assessment 

Methodology for Low-Level Radioactive Waste Disposal Facilities: Evaluation of Modeling 

Approaches,” NUREG/CR-5927, Vol. 1, (SAND91-2802), Washington, DC, August 1993, 

ADAMS Accession No. ML11126A295. 

NRC, 1993b. U.S. Nuclear Regulatory Commission (Wierenga, P.J., M.H. Young, G.W. Gee, 

C.T. Kincaid, R.G. Hills, T.J. Nicholson, and R.E. Cady), “Soil Characterization Methods for 

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Methods in Nuclear Regulatory Activities: Final Policy Statement,” Federal Register, Vol. 60, No. 

158, pp. 42622– 42630, August 16, 1995.

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Concentration Averaging and Encapsulation, Revision in Part to Waste Classification Technical 

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12-13

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NRC, 1999b. U.S. Nuclear Regulatory Commission (Young, M.H., P.J. Wierenga, 

A.W. Warrick, L.L. Hofmann, S.A. Musil, M. Yao, C.J. Mai, Z. Zou, and B.R. Scanlon), “Results 

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Models and Parameters,” April 12, 2002, ADAMS Accession No. ML021050104.
12-14

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Washington, DC, April 2004, ADAMS Accession No. ML040560162.

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Technology, July 2004, ADAMS Accession No. ML042310547, ML042310738, ML042320083. 

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of the International Workshop on Uncertainty, Sensitivity, and Parameter Estimation for 

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Characterization, Survey, and Determination of Radiological Criteria—Final Report,” 

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to U.S. Department of Energy Waste Determinations: Draft Final Report for Interim Use,” 

NUREG-1854, Washington, DC, August 2007, ADAMS Accession No. ML072360184.
12-15

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13-1

1 13.0 GLOSSARY

2

3 Alternative conceptual model: An additional and different model on how the system might 

4 work that is consistent with available supporting information. For example, a scenario may have 

5 a matrix flow conceptual model and an alternative fracture flow conceptual model; the model 

6 outputs from each may yield significantly different results. 

7

8 Alternative scenario: In addition to the central scenario, possible future evolution of the 

9 disposal site. Alternative scenarios may include disruptive events if those FEPs are relevant at 

10 a particular site.

11

12 Analysis timeframe: The timeframe over which a licensee should assess the projected 

13 performance of the disposal facility factoring in the characteristics of the waste, engineered 

14 barriers, disposal site, and associated uncertainties. The analysis timeframe is divided into two 

15 phases: a compliance period and a performance period. 

16

17 Assessment Context: The assessment context provides a framework for performance 

18 assessment and covers the following key aspects: purpose; regulatory framework; assessment 

19 end-points; assessment philosophy; disposal system (or facility) characteristics; and 

20 timeframes. 

21

22 Buffer zone: Portion of the disposal site that is controlled by the licensee and that lays under 

23 the disposal units and between the disposal units and the boundary of the site. 

24

25 Central scenario: The scenario that the licensee can best support as to the expected future 

26 dynamic evolution of the disposal site. As a result of the site selection process for low-level 

27 waste disposal, the central scenario generally will not include disruptive events.

28

29 Code: A set of software commands used to solve mathematical equations representing 

30 phenomena of the conceptual model. 

31

32 Compliance period: The period of time over which a licensee must demonstrate with 

33 reasonable assurance that the disposal facility will meet the performance objectives found in 

34 10 CFR 61.41(a), 10 CFR 61.42(a), and 10 CFR 61.44. A quantitative assessment should be 

35 performed. The compliance period is defined by 10 CFR 61.2 to be the time out to 1,000 years 

36 after closure of the disposal facility.

37

38 Computational model: See Numerical model.

39

40 Conceptual model: A well-defined, connected sequence of phenomena describing the 

41 behavior of the system of concern.

42

43 Critical group: A group of individuals reasonably expected to receive the greatest exposure to 

44 releases over time, given the circumstances under which the analysis would be carried out. 

45 The average member of the critical group is that individual who is assumed to represent the 

46 most likely exposure situation, based on cautious but reasonable exposure assumptions and 

47 parameter values.
13-2

1

2 Defense-in-depth: The use of multiple independent and redundant layers of defense such that 

3 no single layer, no matter how robust, is exclusively relied upon. Defense-in-depth for a land 

4 disposal facility includes, but is not limited to, the use of siting, waste forms and radionuclide 

5 content, engineered features, and natural geologic features of the disposal site.

6

7 Degradation: A process of gradual reduction in the capability of materials used in the 

8 construction of low-level waste disposal facilities to limit water infiltration and the release of 

9 radionuclides; the decline of an engineered barrier following the service life, when important 

10 characteristics of an engineered barrier progress from an expected design value to the 

11 degraded condition. 

12

13 Degraded barrier: An engineered barrier that has fully undergone the process of degradation 

14 resulting in reduced material and performance characteristics: a degraded barrier could still 

15 perform a function based on the properties of the remaining durable constituent materials. 

16

17 Deterministic analysis: An analysis using a single set of values for key assumptions or 

18 parameters to calculate a single value of model output. 

19

20 Disposal: Placement of waste in a facility designed to isolate waste from the accessible 

21 environment without an intention to retrieve the waste. 

22

23 Disposal site: That portion of a land disposal facility which is used for disposal of waste. It 

24 consists of disposal units and a buffer zone. 

25

26 Disposal unit: A discrete portion of the disposal site into which waste is placed for disposal. 

27 For near-surface disposal the unit is usually a trench. 

28

29 Distribution coefficient (Kd): An empirical constant employed in mathematical expressions 

30 representing sorption isotherms that relate the mass of solute on the solid phase to the 

31 concentration of solute in solution as a function of temperature and pressure. The distribution 

32 coefficient (Kd) represents an empirical constant for a linear sorption isotherm, the validity of 

33 which requires that the reactions that cause the partitioning are fast and reversible (e.g., 

34 chemical equilibrium is achieved) and the sorption isotherm is linear.

35

36 Dose: Generically refers to radiation dose, absorbed dose, dose equivalent, effective dose 

37 equivalent, committed dose equivalent, committed effective dose equivalent, or total effective 

38 dose equivalent.

39

40 Dosimetry: The process or method of measuring the dosage of ionizing radiation. 

41

42 Engineered barrier: A man-made feature that is intended to improve the land disposal facility's 

43 ability to meet the performance objectives in Subpart C. Examples of engineered barriers 

44 include intruder barrier, resistive cover, evapotranspiration or water balance cover, and clay 

45 cap.

46

47 Exposure pathway: The route by which radioactivity travels through the environment to 

48 produce radiation exposure to a person or group. 

49
13-3

1 Exposure scenario: See Receptor scenario.

2

3 Event: A qualitative or quantitative phenomenon or change that has the potential to affect the 

4 performance of the disposal system and that occurs during an interval that is short compared to 

5 the analyses timeframe. Examples of events that cause relative rapid change are earthquakes, 

6 floods, storms, well drilling, and excavation. 

7

8 Feature: An object, structure, or characteristic that has a potential to affect the performance of 

9 the disposal system. Examples include rocks within an erosion layer of an engineered cover or 

10 a drainage layer of an engineered cover. 

11

12 FEP: Feature, event, or process that has a potential to affect the performance of the disposal 

13 system.

14

15 FEP categorization: The process of organizing individual FEPs into categories of similar 

16 properties to facilitate FEP screening. For example, FEPs related to natural, human, or waste 

17 phenomena may be grouped into separate categories.

18

19 FEP screening: The process of using regulatory, probability, and consequence criteria to 

20 eliminate FEPs from further consideration that will not significantly impact the performance of 

21 the disposal system or are otherwise excluded by regulation.

22

23 Hazard: A feature, event, or process that is capable of causing harm. In waste disposal, the 

24 radiological inventory represents a hazard but if contained does not present risk to the public.

25

26 Inadvertent intruder: Any person who might occupy the disposal site after closure and engage 

27 in normal activities, such as agriculture, dwelling construction, resource exploration or 

28 exploitation (e.g., well drilling) or other reasonably foreseeable pursuits that might unknowingly 

29 expose the person to radiation from the waste. 

30

31 Institutional controls: Measures to control access to a site and minimize disturbances to 

32 engineered measures established by the licensee to control the residual radioactivity. 

33 Institutional controls include administrative mechanisms (e.g., land use restrictions) and may 

34 include, but are not limited to, physical controls (e.g., signs, markers, landscaping, and fences). 

35

36 Intruder assessment: An analysis that (1) assumes an inadvertent intruder occupies the site 

37 or contacts the waste and engages in normal activities or other reasonably foreseeable pursuits 

38 that might unknowingly expose the person to radiation from the waste; (2) examines the 

39 capabilities of intruder barriers to inhibit an inadvertent intruder’s contact with the waste or to 

40 limit the inadvertent intruder’s exposure to radiation; and (3) estimates an inadvertent intruder’s 

41 potential annual dose, considering associated uncertainties. Intruder assessments are 

42 generally constrained to a limited set of receptor scenarios to avoid excessive speculation about 

43 future human behavior. An intruder assessment is used to demonstrate compliance with 

44 10 CFR 61.42(a) and 10 CFR 61.42(b).

45

46 Intruder scenario: See Receptor scenario.

47

48 Land disposal facility: The land, building, and structures, and equipment which are intended 

49 to be used for the disposal of radioactive wastes. 
13-4

1

2 Licensee: A person possessing a license to dispose of waste in a land disposal facility. In this 

3 document the term “licensee” is meant to include both persons possessing a 10 CFR Part 61 

4 license as well as applicants who are applying to obtain a 10 CFR Part 61 license. 

5

6 Long-lived waste: Waste containing radionuclides (1) where more than 10 percent of the initial 

7 activity of a radionuclide remains after 10,000 years (e.g., long-lived parent), (2) where the peak 

8 activity from progeny occurs after 10,000 years (e.g., long-lived parent – short-lived progeny), or 

9 (3) where more than 10 percent of the peak activity of a radionuclide (including progeny) within 

10 10,000 years remains after 10,000 years (e.g., short-lived parent – long-lived progeny).

11

12 Low-level (radioactive) waste (LLW): Items that have become contaminated with radioactive 

13 material or have become radioactive through exposure to radiation. The radioactivity in these 

14 wastes can range from just above natural background levels to much higher levels, such as 

15 seen in parts from inside the reactor vessel in a nuclear power plant. Low-level radioactive 

16 waste is defined by what it is not, so that an understanding of the definitions of high-level 

17 radioactive waste, spent nuclear fuel, transuranic waste, byproduct material, and naturally 

18 occurring radioactive material is necessary to determine whether a subject waste is low-level 

19 waste. 

20

21 Mathematical model: A representation of a conceptual model in mathematical terms (i.e., a 

22 governing equation or set of equations intended to represent important processes). 

23 Mathematical models can be solved analytically or numerically. 

24

25 Member of the public: An individual in a controlled or unrestricted area. However, an 

26 individual is not a member of the public during any period in which the individual receives an 

27 occupational dose. 

28

29 Model: A conceptual or mathematical representation of a system used to project future 

30 performance. 

31

32 Model abstraction: The process of abstracting a conceptual model representing a dynamic 

33 site in the physical world into a mathematical model governed by equations that is implemented 

34 within a numerical model.

35

36 Model integration: The connection of models, submodels, and abstractions at the level of 

37 detail necessary to represent the conceptual model. For example, a model simulating 

38 precipitation may be integrated with models of infiltration and erosion.

39

40 Model simplification: The process of simplifying a complex numerical model into a reduced 

41 numerical model while still maintaining the validity of the simulation results.

42

43 Model support: The technical basis that demonstrates the validity and appropriateness of the 

44 results of the numerical model, and by extension provides support for the conceptual model. 

45 The basis may include comparisons made with outputs of models (e.g., detailed process-level 

46 models) and/or empirical observations (e.g., laboratory testing, field investigations, and natural 

47 analogs). 

48
13-5

1 Model uncertainty: The uncertainty in the conceptualization of the system, the uncertainty in 

2 its mathematical representation, and the uncertainty in the solution of the mathematical 

3 representation. 

4

5 Monitoring: Observing and making measurements to provide data to evaluate the performance 

6 and characteristics of the disposal site. 

7

8 Near-surface disposal facility: A land disposal facility in which radioactive waste is disposed 

9 of in or within the upper 30 meters of the earth's surface. 

10

11 Numerical model: A model to solve the equations of the mathematical model using codes or 

12 modeling software. The results of the simulations can represent, for example, potential 

13 radiological exposures and their associated uncertainties. 

14

15 Parameter uncertainty: Uncertainty associated with the input to the numerical model being 

16 used in the analysis including uncertainty in the actual values and the statistical and spatial 

17 distributions of data use to infer model parameters. Parameter uncertainty is highly dependent 

18 on the quality of the data. 

19

20 Pathway: Route or means of release of contaminants from a disposal facility, transport of 

21 contaminants in the environment, or exposure of humans. 

22

23 Performance assessment: An analysis that (1) identifies the features, events, and processes 

24 that might affect the disposal system; (2) examines the effects of these features, events, and 

25 processes on the performance of the disposal system; and (3) estimates the annual dose to any 

26 member of the public caused by all significant features, events, and processes. A performance 

27 assessment is used to demonstrate compliance with 10 CFR 61.41(a) and 10 CFR 61.41(b).

28

29 Performance period: The period of time over which a licensee evaluates the ability of the 

30 disposal system to contain long-lived waste and demonstrates that releases are minimized to 

31 the extent reasonably achievable. The performance period begins at the end of the protective 

32 assurance period and extends as long as necessary to demonstrate that the metric of the 

33 performance period can be met. 

34

35 Performance period analyses: Analyses for certain types of waste for the timeframe after the 

36 compliance period, which assess how the disposal facility and site characteristics minimize the 

37 potential long-term impacts. Performance period analyses are required if the disposal facility is 

38 accepting long-lived waste that has disposal site-averaged concentrations of long-lived 

39 radionuclides greater than the values provided in Table A of 10 CFR 61.13(e). Performance 

40 period analyses may be conservative screening analyses or a probabilistic risk assessment. 

41

42 Phenomenon: Either a process or an event. Typically, a phenomenon acts upon a feature. 

43

44 Probabilistic analysis: Refers to computer codes or analyses that use a sampling method to 

45 select parameter values from a distribution. Results of the calculations are also in the form of a 

46 distribution of values or time series of different values. 

47

48 Process: A qualitative or quantitative phenomenon or change that has the potential to affect the 

49 performance of the disposal system and that occurs during all or a significant part of the 
13-6

1 analyses timeframe. Examples of processes that cause relative gradual change are 

2 radionuclide transport, differential settlement, leaching, and erosion. 

3

4 Protective assurance period: The period from the end of the compliance period through 

5 10,000 years following closure of the site.

6

7 Qualified specialist: A person, by reason of training or experience, who possesses expertise 

8 in a particular field or scientific study (e.g., geomorphologist, seismologist, or chemist). 

9

10 Radionuclide inventory: The isotopic distribution of radioactive materials by waste class, 

11 wasteform, and waste container disposed of in the facility and potentially available for release to 

12 the environment. 

13

14 Receptor: The exposed individual relative to the exposure pathway considered. 

15

16 Receptor scenario: A type of scenario that describes the FEPs associated with people 

17 becoming exposed to radiation.

18

19 Reviewer: This document uses the term “reviewer” to include NRC staff reviewers as well as 

20 Agreement State reviewers. 

21

22 Risk: The combined answer to the three questions that consider (1) what can go wrong, (2) 

23 how likely it is, and (3) what the consequences might be. In the context of radioactive waste 

24 disposal risk refers to probability-weighted radiological doses.

25

26 Safety assessment: A systematic analysis of the ability of the site and design to provide the 

27 safety functions and meet technical requirements. 

28

29 Safety case: A collection of information that demonstrates the assessment of the safety of a 

30 waste disposal facility. This includes technical analyses, such as the performance assessment 

31 and intruder assessment, but also includes information on defense-in-depth and supporting 

32 evidence and reasoning on the strength and reliability of the technical analyses and the 

33 assumptions made therein. The safety case also includes descriptions of the safety relevant 

34 aspects of the site, the design of the facility, and the managerial control measures and 

35 regulatory controls. 

36

37 Safety function: Defined qualitatively as a function through which a component of the disposal 

38 system contributes to safety and achieves its safety objective throughout the analyses 

39 timeframe. 

40 Scenario: A future evolution of the disposal site resulting from a subset of FEPs. 

41

42 Scenario development: The process of incorporating a site's current and future features, 

43 events, processes, and their interactions into a scenario. Frequently, a top-down or bottom-up 

44 approach is used, or a mixture of the two. 

45

46 Scenario uncertainty: Uncertainty about the future of the site due to the inherent lack of 

47 knowledge about how the site will evolve in time. 

48
13-7

1 Sensitivity analysis: An examination of how the behavior of a system varies with change, 

2 usually in the values of the governing parameters. An analysis to investigate the dependencies 

3 of the result of the assessment on the alternative input elements (i.e. data, assumption, etc.). 

4

5 Site characterization: Studies that enable the licensee to sufficiently describe the conditions of 

6 the site to evaluate the acceptability of the decommissioning plan. 

7

8 Site closure and stabilization: Those actions that are taken upon completion of operations 

9 that prepare the disposal site for custodial care and that assure that the disposal site will remain 

10 stable and will not need ongoing active maintenance. 

11

12 Site stability analyses: Analyses considering the potential effects of erosion, flooding, 

13 seismicity, and other disruptive processes and events on the ability of the disposal facility to 

14 meet the performance objectives. In addition, such analyses consider the potential effects of 

15 degradation of mechanical properties of containers or other stabilizing man-made features. 

16 Stability analyses may be design-based or model-based and may or may not be based on risk 

17 considerations.

18

19 Solubility limit: The maximum amount of a radionuclide (solute) that can be dissolved per unit 

20 of liquid (solvent) under specified conditions (e.g., temperature, pH).

21

22 Source term: A conceptual representation of the radionuclide inventory in a disposal site. The 

23 quantity of radionuclides expected to be released over time out of a clearly identified boundary 

24 (such as the wasteform, container, disposal unit, or facility). 

25

26 Stability: A term that refers to the ability of the waste and the disposal site to maintain their 

27 physical characteristics so that once waste is emplaced, backfilled, and covered, water access 

28 to the waste and release of radioactivity is minimized. 

29

30 Surveillance: Observation of the disposal site for purposes of visual detection of need for 

31 maintenance, custodial care, evidence of intrusion, and compliance with other license and 

32 regulatory requirements. 

33

34 System description: A description of the characteristics and interactions, including features 

35 and phenomena, of the disposal site and surrounding area to ensure information used to 

36 develop the technical analyses and describing the overall disposal system performance have 

37 been adequately described. 

38

39 Technical analyses: Analyses associated with the performance assessment, the intruder 

40 assessment, the stability evaluation, and the performance period needed to demonstrate 

41 compliance with the Subpart C performance objectives. 

42

43 Total effective dose equivalent (TEDE): The sum of the deep-dose equivalent (for external 

44 exposures) and the committed effective dose equivalent (CEDE) (for internal exposures) (see 

45 10 CFR 20.1003). 

46

47 Upscaling: The modification of data for use at a different scale. Most commonly upscaling

48 transforms data from fine-scale observations for use at a much coarser scale. 

49
13-8

1 Uncertainty analysis: A method of formally assessing, reducing or managing, and 

2 documenting the inherent uncertainties of a system. The uncertainties include model 

3 uncertainty (which spans conceptual model uncertainty and mathematical model uncertainty), 

4 uncertainty about the future of the site, and parameter uncertainty (i.e., uncertainty in values 

5 used in the numerical model). 

6

7 Validation (model): The process of determining the degree to which a model is an accurate 

8 representation of the real world from the perspective of the intended uses of the model. 

9

10 Verification (software): Comparison of the numerical solution generated by the computational 

11 model with one or more analytical solutions or with other numerical solutions. Verification of the 

12 code ensures that the computer program accurately solves the equations that constitute the 

13 mathematical model. Verification of the governing equation demonstrates that it accurately 

14 describes the physical processes that occur. 

15

16 Waste acceptance criteria: Administrative limits, required by 10 CFR Part 61.58 that provide 

17 reasonable assurance of compliance with the performance objectives of Subpart C. The criteria 

18 include allowable activities and concentrations of specific radionuclides, acceptable wasteform 

19 characteristics and container specifications, and restrictions or prohibitions on waste, materials, 

20 or containers that might affect the facility’s ability to meet the performance objectives in 

21 Subpart C. 

22

23 Waste incidental to reprocessing (WIR): Wastes that are incidental to the reprocessing of 

24 nuclear fuel that can be managed as LLW. 

25

26 Waste stream: The origin of a low-level waste type or combination of waste types with a 

27 particular radionuclide content and distribution independent of its physical characteristics. 

28

29 Waste type: Radioactive materials such as cloth, wood, plastic, glass, or metal, or other 

30 substances obtained from radioactive waste treatment systems, industrial processes, or 

31 research experiments. Some examples of waste types are dry solids, dry active waste, ion 

32 exchange resins, sorbed liquids, filter cartridges, and activated metals.

33

34 Wasteform: Radioactive waste in its physical and chemical form including any stabilizing or 

35 encapsulating material within which it is incorporated.

36
A-1

APPENDIX A

CHANGES TO 10 CFR PART 61 MADE IN 2015 RULEMAKING

Table A-1 Comparison Table of Current and Proposed 10 CFR Part 61 Regulations

Protection of the 

general 

population from 

releases of 

radioactivity 

(10 CFR 61.41)

Protection of 

individual from 

inadvertent 

intrusion 

(10 CFR 61.42)

Stability of the 

disposal site 

after closure

Long-term 

analyses 

(10 CFR 61.44)

Defense-inDepth

Current

10 CFR Part 61 

regulations

- Pathway 

analysis

- Undefined 

period of 

performance

- 0.25 mSv (25 

mrem) annual 

whole body dose 

limit for the 

protection of the 

general population 

- ALARA concept

- Comply with 

10 CFR 61.55 

- Provide 

adequate barriers 

to inadvertent 

intrusion

- Undefined 

period of 

performance

- No annual dose 

limit

Analyses of 

active natural 

processes that 

demonstrate 

that there will 

not be a need 

for ongoing 

active 

maintenance of 

the disposal 

site following 

closure.

Implicit in 

Subpart D 

technical 

requirements.

Proposed 

10 CFR Part 61 

regulations

Within 1,000 Years Following Closure of Disposal Facility

(Compliance Period)

- Performance 

assessment that 

estimates peak 

annual dose that 

occurs within 

1,000 years 

following closure 

- 0.25 mSv (25 

mrem) annual 

dose limit for the 

protection of the 

general 

population from 

the releases of 

radioactivity that 

occurs within 

1,000 years 

- ALARA 

concept

- Comply with 

LLW acceptance 

criteria

- Provide 

adequate barriers 

to inadvertent 

intrusion

- Intruder 

assessment that 

estimates peak 

annual dose that 

occurs within 

1,000 years 

following closure 

of disposal facility

- 5 mSv (500 

mrem) annual 

dose limit

Analyses of 

active natural 

processes that 

demonstrate 

that long-term 

stability of the 

site can be 

ensured and 

that there will 

not be a need 

for ongoing 

active 

maintenance of 

the disposal 

site following 

closure.

Analyses that 

demonstrate the 

proposed disposal 

system includes 

defense-in-depth 

protections.
A-2

Table A-1 Comparison Table of Current and Proposed 10 CFR Part 61 Regulations

Protection of the 

general 

population from 

releases of 

radioactivity 

(10 CFR 61.41)

Protection of 

individual from 

inadvertent 

intrusion 

(10 CFR 61.42)

Stability of the 

disposal site 

after closure

Long-term 

analyses 

(10 CFR 61.44)

Defense-inDepth

Between 1,000 and 10,000 Years Following Closure of Disposal Facility 

(Protective Assurance Period)

- Performance 

assessment that 

estimates peak 

annual dose that 

occurs between 

1,000 and 10,000 

years following 

closure of disposal 

facility

- Annual dose 

shall be below 5

mSv (500 mrem) 

or a level that is 

reasonably 

achievable based 

on technological 

and economic 

considerations for 

the protection of 

the general 

population from 

releases of 

radioactivity that 

may occur 

between 1,000 

and 10,000 years 

following closure 

- Intruder 

assessment that 

estimates peak 

annual dose that 

occurs between 

1,000 and 10,000 

years following 

closure of 

disposal facility

- Annual dose 

shall be below 5 

mSv (500 mrem) 

or a level that is 

reasonably 

achievable based 

on technological 

and economic 

considerations for 

the protection of 

the inadvertent 

intruders from 

exposures that 

may occur 

between 1,000 

and 10,000 years 

following closure 

Analyses of 

active natural 

processes that 

demonstrate 

that long-term 

stability of the 

site can be 

ensured and 

that there will 

not be a need 

for ongoing 

active 

maintenance of 

the disposal 

site following 

closure.

Analyses that 

demonstrate the 

proposed 

disposal system 

includes defensein-depth 

protections.

After 10,000 Years Following Closure of Disposal Facility 

(Performance Period)

- Analyses for 

10,000 or more 

years following 

closure of disposal 

facility that 

demonstrates 

releases will be 

- Analyses for 

10,000 or more 

years following 

closure of 

disposal facility 

that demonstrates 

exposures will be 

Analyses that 

demonstrate the 

proposed 

disposal system 

includes defensein-depth 

protections.
A-3

Table A-1 Comparison Table of Current and Proposed 10 CFR Part 61 Regulations

Protection of the 

general 

population from 

releases of 

radioactivity 

(10 CFR 61.41)

Protection of 

individual from 

inadvertent 

intrusion 

(10 CFR 61.42)

Stability of the 

disposal site 

after closure

Long-term 

analyses 

(10 CFR 61.44)

Defense-inDepth

minimized to the 

extent reasonably 

achievable for the 

protection of the 

general population

- Analyses only 

apply for disposal 

sites containing 

long-lived 

radionuclides 

exceeding 

concentrations 

listed in table A of 

10 CFR 61.13(e), 

or if necessitated 

by site-specific 

conditions

- Analyses that 

demonstrate how 

the facility has 

been designed to 

limit long-term 

releases.

minimized to the 

extent reasonably 

achievable for the 

protection of 

inadvertent 

intruders

- Analyses only 

apply for disposal 

sites containing 

long-lived 

radionuclides 

exceeding 

concentrations 

listed in table A of 

10 CFR 61.13(e), 

or if necessitated 

by site-specific 

conditions

- Analyses that 

demonstrate how 

the facility has 

been designed to 

limit long-term 

exposures to an 

inadvertent 

intruder.

B-1

1 APPENDIX B

2 HAZARD MAPS

3

4 The NRC staff created hazard maps related to the features and phenomena of the 

5 10 CFR 61.50 criteria. The hazard maps presented in this appendix provide an illustration of 

6 features, events, and processes (FEPs) related to 10 CFR 61.50 site suitability criteria. The 

7 maps cannot be displayed in this document at sufficient size to be used to determine if any 

8 specific location would be impacted by one of these phenomena. The figures provide an

9 illustration of potentially impacted areas. 

10

11 The figures should not be used by regulators to prohibit disposal because the resolution of the 

12 maps and the precision and accuracy of the techniques used to generate them may not be 

13 sufficient for site-specific evaluations. However, regulators should use the maps to determine 

14 when greater review effort and more technical basis should be expected for the licensee’s site15 specific evaluation. In addition, the data used to produce these maps could be used, via 

16 Geographic Information System (GIS) software, to perform screening-level analyses of the 

17 FEPs.

18

19 Preparation of the hazard maps:

20

21 ArcGIS was used to process the data from the data sources and produce all of the maps. 

22 ERDAS Imagine was used to process image data used for the groundwater depth (B-4) and 

23 erosion (B-8) maps.

24

25 Figure B-1 – The source is elevation data compiled from various sources and provided by 

26 Environmental Systems Research Institute (ESRI). The NRC staff created an indicator plot for 

27 areas less than 5 m above the current sea level using data from ESRI (2008b).

28

29 Figure B-2 – Figure is based on wetlands land use classes from USGS land use/land cover data

30 (USGS, 2011).

31

32 Figure B-3 - Where available, one percent annual chance flood event risk zones (100-year 

33 floodplain) from the FEMA Digital Flood Insurance Rate Map Database (DFIRM) are shown 

34 (FEMA, 2012). When DFIRM data was not available, one percent annual chance flood event 

35 risk zones (100-year floodplain) from the FEMA National Flood Insurance Program Q3 Flood 

36 Data are shown (FEMA, 1998). When DFIRM or Q3 FEMA data were not available, the source 

37 is NRC staff calculations performed on data compiled and provided by ESRI (ESRI, 2008a; 

38 ESRI, 2008b). A slope model (a grid where each cell is assigned maximum slope between it 

39 and the neighboring cells) was created from a digital elevation model (DEM) of the continental 

40 US. From the slope model, a flow direction grid was generated using the direction of maximum 

41 slope out of the cell. From the flow direction grid, a flow accumulation grid was generated 

42 based on how many cells lay upstream of the grid cell. Cells with a very low slope that 

43 accumulated flow over a certain threshold were displayed as black (prone to flooding), all others 

44 were white. On top of this image a hydrology layer was added that showed ponds, lakes, 

45 reservoirs, and large rivers as black. 

46
B-2

1 Figure B-5 – The source is hydrology data compiled from various sources and provided by 

2 ESRI. The figure is based on the categories provided in the referenced data sources. There

3 could be other data categories not in the ESRI data source that might be areas of previous 

4 flooding (ESRI, 2008a; ESRI, 2008b).

5

6 Figure B-6 through B-9 - The data were available for download from the websites listed in the 

7 references cited on the figure caption. Thirty five tiles of data that covered the continental U.S.

8 were used. Each tile of data contained files representing the extent of glaciation at various 

9 times during the quaternary. There were four general time periods covered in most cases: the 

10 Younger Dryas, the Late Weichselian (Wisconsinian), the Early-Middle Weichselian, and the 

11 maximum limit of Pleistocene glaciation. For some tiles there were separate files for various 

12 features: ice sheets, mountain glaciers, and basin glaciers. Sometimes they were all combined 

13 into one file with a field in the attribute table which signified which type of feature it was. 

14 Sometimes there were separate data files for the work of from more than one author. Not all 

15 authors studied the entire continental U.S. The files representing like datasets were merged for 

16 all of the tiles and those files were clipped to the boundary of the continental U.S. plus Great 

17 Lakes. Areas in each file that represent ice covered areas for that particular time stamp are 

18 displayed as black on the map. By stacking all of the files from various authors and features on 

19 top of each other in the map, the maximum extent of glaciation for the period covered by these 

20 files is represented in the figure.

21

22

23

24

25

26

27

28

29

30
B-3

Figure B-1: Approximate area of land that might flood if mean sea level rose by 5 m. Proposed sites located 

may require additional analysis and evaluation (ESRI, 2008b) 
B-4

Figure B-2: Approximate area of current wetlands. Proposed sites located near these areas may require add

characterization and analysis. Wetland areas in the future may change (USGS, 2011)
B-5

Figure B-3: Areas of potential flooding that may require additional site characterization and analysis (FEMA, 

ESRI, 2008a; ESRI, 2008b)
B-6

Figure B-4: Approximate area of simulated current water tables shallower than 30 m. Proposed sites located 

require additional site characterization and analysis. Future near-surface groundwater areas ma

al, 2012)
B-7

Figure B-5: Approximate areas that may have frequently flooded in the past (e.g., dry lake beds, salt flats, ar

Proposed sites located in these areas may require additional site characterization and analysis (E

2008b)
B-8

Figure B-6: Approximate locations of Holocene volcanic activity. Proposed sites located near these locations 

additional analysis and evaluation (Siebert and Simkin, 2002)
B-9

Figure B-7: Current approximate areas of higher potential seismic hazard. Proposed sites located near these 

additional analysis and evaluation (Petersen et al, 2011)
B-10

Figure B-8: Current approximate areas of higher vulnerability to water erosion. Proposed sites located in the

additional site characterization and analysis. Areas of high vulnerability to water erosion in the fu

(USDA, 1988)
B-11

Figure B-9: Approximate area covered by glaciers during the last three glacial periods of the current Quatern

Wisconsin, Illinoisan, and Pre- Illinoisan glacial periods. Glaciers can cause very disruptive surfa

processes and potential sites located in areas created by previous glacial processes could requir

and careful evaluation (Ehlers et al., eds., 2011)
B-12

REFERENCES

Ehlers et al., 2011. Ehlers J., P.L. Gibbard, and P.D. Hughes, eds., “Quaternary Glaciations -

Extent and Chronology: A closer look", Vol. 15, 1st Ed., ISBN: 9780444534477. 

[http://booksite.elsevier.com/9780444534477/DCW_North_America_wgs84_1.php and 

http://booksite.elsevier.com/9780444534477/DCW_North_America_wgs84_2.php] 

ESRI, 2008a. Environmental Systems Research Institute, Inc., “Data & Maps and StreetMap –

North America DVD,” U.S. National Atlas Water Feature Areas, ArcGIS 9, ESRI Data & Maps 

9.3, April 1, 2008. 

ESRI, 2008b. Environmental Systems Research Institute, Inc., “Data & Maps 9.3, Elevation and 

Image Data - World DVD,” U.S. Geological Survey Center for Earth Resource Observation and 

Science (EROS), ArcGIS 9, ESRI, April 1, 2008. 

FEMA, 1998. Federal Emergency Management Agency, National Flood Insurance Program Q3 

Flood Data, September 1998, Washington D.C.

FEMA, 2012. Federal Emergency Management Agency, National Flood Hazard Layer on DVD, 

Washington D.C.

Kreakie et al., 2012. Kreakie, B.J., Y. Fan, and T.H. Keitt, ”Enhanced Migratory Waterfowl 

Distribution Modeling by Inclusion of Depth to Water Table Data,” PLoS ONE 7(1): e30142, 

doi:10.1371/journal.pone.0030142, 2012. 

[http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030142] 

Petersen et al., 2011. Petersen, M.D., A.D. Frankel, S.C. Harmsen, C.S. Mueller, K.M. Haller, 

R.L. Wheeler, R.L. Wesson, Yuehua Zeng, O.S. Boyd, D.M. Perkins, Nicolas Luco, E.H. Field, 

C.J. Wills, and K.S. Rukstales, “Seismic-hazard maps for the conterminous United States,” U.S. 

Geological Survey, 2008 Scientific Investigations Map 3195, 2011. 

[http://pubs.usgs.gov/sim/3195/]

Siebert and Simkin, 2002. Siebert L. and T. Simkin, “Volcanoes of the World: an Illustrated 

Catalog of Holocene Volcanoes and Their Eruptions,” Smithsonian Institution, Global Volcanism 

Program Digital Information Series, GVP-3, 2002.

USDA, 1998. U.S. Department of Agriculture, “Water Erosion Vulnerability Map,” Natural 

Resources Conservation Service (NRCS), 1998. 

USGS, 2011. U.S. Geological Survey, Multi-Resolution Land Characteristics Consortium 

(MRLC), National Land Cover Database (NLCD), National Land Cover Database 2006 

(NLCD2006), 20110216, 2011. [http://www.mrlc.gov/nlcd06_data.php]
C-1

1 APPENDIX C

2 GENERIC FEATURES, EVENTS, AND PROCESSES LIST FOR

3 NEAR-SURFACE DISPOSAL OF LOW-LEVEL RADIOACTIVE 

4 WASTE

5

6 The NRC staff has developed a generic FEPs list that can be used by reviewers, applicants, 

7 licensees, and other interested stakeholders involved with preparation and review of technical 

8 analyses conducted to support licensing of a near-surface low-level radioactive waste (LLW) 

9 disposal facility. The NRC staff consulted numerous references to develop (1) a comprehensive 

10 list of FEPs, and (2) a smaller set of FEPs (or “starter list”) that need to be analyzed and 

11 screened based on requirements in 10 CFR Part 61 and are usually considered to be essential 

12 to development of performance assessment (PA) analyses. The starter FEP list should not be 

13 considered a complete FEP list. Rather, it is a core list of FEPs that any LLW PA should 

14 consider for screening. There are various methods of screening FEPs as described in Section 

15 2.0. The level of technical analysis needed to justify exclusion of a required FEP or needed to 

16 evaluate the impact of an included (or “screened in”) FEP (or set of FEPs) can vary. The level 

17 of effort expended on disposition or evaluation of the FEP(s) should be commensurate with the 

18 expected risk-significance of the FEP (or group of FEPs that form a central or alternative 

19 scenario). A complete set of site-specific FEPs can only be developed from an adequate 

20 understanding of the disposal system that is gained through site characterization and review of 

21 detailed facility designs. Development and evaluation of FEPs is considered an iterative 

22 process, beginning first with evaluation of a central scenario that incorporates all of the key 

23 FEPs that best represent the dynamic system being studied. Following initial analyses, 

24 alternative scenarios that might include potential disruptive events should also be considered, 

25 as appropriate; to ensure that potential vulnerabilities in disposal system performance are 

26 identified and adequately addressed. As-emplaced conditions (including final engineered 

27 barrier configurations and waste allocation), monitoring, and other data developed following 

28 initial PA preparation, should also be considered to ensure that the results of the PA adequately 

29 assess the risk of the disposal facility. The starter list of FEPs that should be addressed in LLW 

30 PAs is provided in Table C-1.

31

32
C-2

Table C-1 Starter FEPs List for Near-Surface Disposal in the United States

Type FEP

Feature

Radiological inventory

Waste inventory

Wasteform (e.g. design, properties, characteristics)

Waste container

Free liquids

Colloids

Backfill

Disposal unit

Disposal site

Buffer zone

Engineered barriers (e.g. intruder barriers, engineered cover):

Presence of agents in waste that may increase mobility (e.g., chelating 

agents) or lead to degradation of engineered barriers (e.g., corrosive 

agents)

Material defects

Geologic units and materials:

 Surface soils and sediments

 Stratigraphy and lithology

 Hydrogeologic units

Surface water

Preferential pathways (anthropogenic, natural)

Perched water

Wetlands

Biosphere:

 Humans

 Ecology

 Flora and fauna (including insects)

Receptors – surrounding population

Exposure pathways

Land use

Institutional control

Natural resources

Process

Climate and meterology

Natural climate cycling (e.g. glaciation)

Radioactive decay and in-growth

Waste interactions

Gas generation

Radon emanation

Waste release (e.g. leaching, dissolution)

Pyrophoricity

Criticality

Degradation:

 Corrosion – all forms

 Creep

 Fatigue

 Abrasion (wind, water)

 Temperature cycling or extremes

 Freeze thaw cycling
C-3

Table C-1 Starter FEPs List for Near-Surface Disposal in the United States

Type FEP

 Frost action

 Wet/dry cycling

 Salt action

 Oxidation

 Acid attack

 Weathering

 Fracturing (via mechanical or chemical/reaction)

 Cementitious material degradation

 Geosynthetic degradation processes

 Plugging of drainage layers

 Degradation of clays (e.g. dessication)

 Polymer degradation

 Seismic-induced degradation

Internal degradation processes, to waste and disposal units:

 Biodegradation

 Reaction – incompatible materials

 Thermodynamic instability

 Excessive void space – subsidence

 Radiation damage

 Dissolution

 Internal stress generation

External degradation processes, interaction of system with environment

Geochemical evolution (disposal unit, disposal site, surrounding 

environment)

Geochemistry – speciation, solubility, sorption, etc.

Interactions of environment and disposal system

Biological driven release (plant uptake, burrowing animals, biotrubation, etc.)

Ecological succession

Water balance processes (e.g. evapotranspiration, runoff, recharge)

Infiltration

Near-field flow processes (e.g. flow bypassing, film flow)

Episodic flow

Groundwater flow:

 Advection

 Dispersion

 Water table fluctuation

 Discharge to surface

 Matrix diffusion

 Dilution

 Diffusion

 Density-driven flow

 Capillary rise

Erosion

Deposition

Instability

Geomorphology

Surface geologic processes – mass wasting, subsidence, slope failure, etc.

Dynamic change to geology (e.g. sinkhole formation)

Loading and differential settlement
C-4

Table C-1 Starter FEPs List for Near-Surface Disposal in the United States

Type FEP

Pedogenesis

Tectonic processes

Groundwater transport

Gas transport

Soil transport (fluvial, aeolian)

Colloid transport

Dose assessment processes (e.g. drinking water consumption, soil build-up, 

resuspension)

Dynamic processes (e.g. natural temporal variability, seasonal effects, 

episodic changes, barometric pumping)

Event

Explosion

Fire

Inadvertent human intrusion (habitation, drilling, resource exploration)

Accident – operational or external

Dam failure

Aircraft crash

Tectonic events:

 Seismic (including earthquakes)

 Volcanic

 Tsunami

Tornado

Hurricane

1

2 As discussed above, a comprehensive generic FEPs list appropriate for near-surface disposal 

3 of low-level radioactive waste was developed for use by reviewers, applicants, licensees, and 

4 other interested stakeholders. A number of references were consulted during development of 

5 this list (NEA, 2000; NEA, 2006; IAEA, 2003, BIOMOVS II, 1996; Arlt, 2013; Neptune, 2011; 

6 SRS, 2012a; SRS, 2012b). Some of the reference FEPs are not expected to be risk-significant 

7 during the time periods of interest for disposal of most LLW or are considered outside the scope 

8 of the 10 CFR Part 61 regulatory framework. An effort was made to identify classes of these 

9 FEPs. For those FEPs sources that screened FEPs in or out (e.g., BIOMOV and site-specific 

10 FEPs lists), the rationale for exclusion of the FEP was considered in determining whether to list 

11 the FEP in the generic FEPs list. It is important to note that although a project-specific FEP may 

12 have been “screened out”, if the FEP was considered potentially applicable to near-surface LLW 

13 disposal, it was included in the generic FEP list. Although screening approaches and the results 

14 of screening are summarized below, it is important to note that no effort was made to evaluate 

15 the adequacy of FEP screening processes discussed, nor the completeness of project-specific 

16 FEP lists.

17

18 The comprehensive, generic FEPs list is structured after the Improvement of Safety 

19 Assessment Methodologies (ISAM) and Nuclear Energy Agency (NEA) FEPs lists described in 

20 more detail below. FEPs that may be considered unlikely during the compliance period but may 

21 become increasingly more likely over time are flagged as “long-term” FEPs. If a FEP is 

22 designated a “long-term” FEP the applicability of the FEP for a specific site should be 

23 considered by licensees when performance period analyses are required. Although some FEPs 

24 from the reference sources are not explicitly listed in the generic FEP list, one can assume that 

25 the FEP should be considered unless it is clearly linked to a category of FEPs that are not 

26 considered important for near-surface waste disposal. 
C-5

1 Reference sources used to create the generic FEPs list include databases developed by 

2 international standards setting organizations, as well as several site-specific applications for 

3 low- and high-level radioactive waste disposal facilities (or repositories) in the United States and 

indirectly, Europe1 4 . A brief description of each of these data sources is provided first below.

5

6 BIOMOVs II

7 A structured, generic biosphere FEP list was developed by the BIOMOVS II Reference 

8 Biospheres Working Group. BIOMOVS is an international study to test models designed to 

9 predict the environmental transfer and bioaccumulation of radionuclides and other trace 

10 substances. The BIOMOVS II FEP list was developed specifically for application to the 

11 calculation of annual individual doses arising at an inland site from long-term release of 

12 radionuclides to groundwater. The list is, nonetheless, relevant to a wide range of 

13 assessments. The developers recognized that the list may not include sufficient detail for any 

14 specific project or assessment. Additionally, definitions may not be universally applicable. The 

15 structure of the FEPs list is provided in Figure C-1. The expanded FEPs list includes 

16 approximately 140 FEPs.

17 Figure C-1 Structure of BIOMOVs FEPs List

18

 1 Several domestic FEP lists incorporate FEPs from European assessments.
C-6

1 Nuclear Energy Agency

2

3 The Nuclear Energy Agency (NEA) developed an international FEPs list (2000) relevant to the 

4 post-closure safety of repositories for solid radioactive waste. The NEA FEPs list was intended 

5 to (i) provide a list of FEPs to be considered when determining the scope of a new assessment;

6 (ii) provide a list of FEPs against which completed assessments could be audited or reviewed;

7 and (iii) provide confidence in the comprehensiveness of a completed assessment. A list of 134 

8 FEPs were provided in the NEA FEPs list (2000).

9

10 A database was created to facilitate use of the international FEPs list, consisting of two parts:

11

12 1. The International FEP List—the structured list of factors, or FEPs, relevant to the 

13 assessment of the long-term safety of nuclear waste repositories.

14 2. Project Databases—a collection of FEPs lists and databases from specific project 

15 studies, along with their references.

16

17 The database was developed by the NEA FEP Database Working Group that included 

18 representatives from seven, Organization for Economic Cooperation and Development 

(OECD)/NEA countries. Version 2.1 of the database contains over 1650 project-specific FEPs 2 19

20 from 10 projects (2006). Two additional project-specific FEPs lists were added: (i) SCK-CEN 

21 Catalogue of Events, Features and Processes for the Mol Site in Belgium and (ii) Encyclopedia 

22 of FEPs for the Swedish SFR and Spent Fuel Repositories. Additional details on the project 

23 specific FEPs lists and additional functionality were added to the database. The NEA 

24 international FEPs list can be considered a generic, high-level FEPs list from which project25 specific FEPs could be developed or categorized. Project-specific FEPs available in the 

26 database are cross-walked back to the categorical FEPs comprising the international FEPs list. 

27 The major categories of the NEA international FEPs list are provided in Figure C-2. The 

28 international FEPs list has 4 layers with the 3 inner layers in Figure C-2 further subdivided into 

29 additional categories. Not listed in Figure C-2 are individual FEPs for each subcategory. 

30

31 IAEA ISAM

32

33 In 1997, the IAEA launched a coordinated research project on Improvement of Safety 

34 Assessment Methodologies for Near Surface Disposal Facilities (ISAM) to critically evaluate, 

35 enhance, and provide confidence in the approaches and tools used for post-closure safety 

36 assessments of near-surface radioactive waste disposal facilities. As part of the ISAM project, 

37 the NEA international FEPs list was modified for near surface disposal facilities. For example, 

38 some of the NEA FEP definitions and comments associated with the FEPs were altered to be 

39 more representative of near surface conditions. The ISAM FEPs list was intended to be a user40 friendly list. The ISAM FEPs list was also intended to be a comprehensive, initial list from which 

41 FEPs applicable to any specific site could be developed. Because the NEA list was extensively 

42 reviewed for completeness for geologic systems and the ISAM FEPs lists was based on the 

43 NEA FEPs list, the developers of the ISAM FEPs list reasoned that users should have additional 

44 confidence in the comprehensiveness of the ISAM FEPs list. Additionally, experience with both 

45 specific near surface disposal facilities and FEPs lists developed and applied in the ISAM test 

46 cases was used in the development of the ISAM FEPs list. The hierarchy of the ISAM FEPs list 

is similar to that of the NEA international FEPs list in Figure C-23 47 . 

 2 These project FEPs are not unique and many of them overlap. 3 This is true with the exception that “1.5 Other External Factors” is not included in the ISAM FEPs list.
C-7

1

2 Figure C-2 Structure of the NEA FEPs list

0. Assessment Basis

1. External Factors

1.1 

Repository 

Issues

1.2 

Geological 

Processes 

and 

Events

1.3 

Climatic 

Processes 

and 

Events

1.4 

Future 

Human 

Actions

1.5 

Other

2. Disposal System Domain: 

Environmental Factors

2.1 Wastes 

and 

Engineered 

Features

2.2 

Geological 

Environment

2.3

Surface 

Environment

2.4

Human 

Behavior

3. Radionuclide/Contaminant Factors

3.2 

Release/

Migration 

Factors

3.1

Contaminant

Characteristics

3.3

Exposure

Factors

Impact
C-8

1 BIOMASS

2

3 Changes were made to the organization of the BIOMOVS II list described above in developing 

4 the BIOMASS FEPs list. These changes include the following:

5

6 • A clearer distinction between FEPs related to basic elements of the assessment context 

7 and those related to the biosphere system, radionuclide transport and radiation 

8 exposure.

9 • The expression of intrinsic phenomena relating to the biosphere system in terms of 

10 characteristics of the system, rather than the behavior of radionuclides within the 

11 system. Those FEPs that relate to radionuclide behavior can (where necessary) be 

12 incorporated in the respective definitions. FEPs that are related solely to the presence 

13 of radionuclides within the system (e.g. radiation exposures) are then clearly identified 

14 under a separate heading.

15 • Experience gained with the application of the reference biosphere methodology since 

16 BIOMOVS II, which has helped to amplify certain details of the original list and led to the 

17 incorporation of additional FEPs.

18

19 Department of Energy (DOE) Hanford site

20

The Hanford Site is an approximately 586 mi2 21 area north of the city of Richland within the 

22 semiarid Pasco Basin of the Columbia Plateau in southeastern Washington State. Hanford

23 occupies a relatively undeveloped area of shrub-steppe (a drought-resistant, shrub and 

24 grassland ecosystem) that contains a rich diversity of plant and animal species. The area has 

25 been protected from disturbance, except for fire, over the past 60 years. This protection has 

26 allowed plant species and communities that have been displaced by agriculture and 

27 development in other parts of the Columbia Basin to thrive at the Hanford Site. The Columbia 

28 River flows eastward through the northern part of the Hanford Site and then turns south, forming 

29 part of the eastern site boundary. Other important rivers near the Hanford Site are the Yakima 

30 River to the south and southwest and the Snake River to the east. The Cascade Mountains, 

31 which are about 160 km (100 mi) to the west, have an important effect on the climate of the 

32 area.

33

34 In 1943, the U.S. Army Corps of Engineers created the Hanford Site from small farming areas 

35 along the Columbia River to locate facilities used to produce nuclear weapon materials for 

36 World War II. Since then, the major activities on the Hanford Site have been controlled by DOE 

37 and its predecessors. Current major programs at the Hanford Site are dedicated to waste 

38 management, environmental restoration, long-term stewardship, and research and 

39 development. Fuel reprocessing, plutonium and uranium separation, plutonium finishing, and

40 waste management, including treatment, storage, and disposal activities have been conducted 

41 in the 200 Area of the site. 

42

43 There are a total of 177 underground tanks located in the 200 Area of the site used to store 

44 reprocessed, liquid high-level waste from reactor operations and other site activities. DOE has 

45 initiated the process of retrieving, treating, and disposing of radioactive mixed waste from 149 

46 underground single-shell tanks that do not have secondary containment. DOE Hanford 

47 prepared a performance assessment (PA) to support closure of these tanks. In 2009, DOE 

48 Hanford initiated a scoping process to assist with updating the PA to support tank closure. An 

49 extensive FEPs list was developed as part of this scoping process. Due to lack of funding the 

50 scoping process was curtailed; however, the draft list of FEPs developed during Hanford PA 

51 scoping was considered in this study.
C-9

1

2 Clive, Utah Site

3

4 The Clive, Utah low-level waste disposal facility is operated by Energy Solutions. Energy 

5 Solutions prepared a PA to evaluate the risk of disposal of depleted uranium (DU) waste. To 

6 support PA development, Energy Solution developed a list of FEPs. Neptune (2011) 

7 documents and examines the universe of FEPs that may apply to the disposal of DU waste at 

8 the Clive Facility. The identification of FEPs for use in the Clive facility PA was an iterative 

9 process that began with compilation of an exhaustive list of candidate FEPs that could affect the 

10 long-term performance of the low-level waste disposal facility. Table C-2 lists the reference 

11 sources considered in developing the initial list of FEPs. As an initial step, all potentially 

12 relevant FEPs from a variety of reference sources were collected (e.g., Yucca Mountain Project, 

13 the Waste Isolation Pilot Plant, and several foreign radioactive waste projects). The initial list 

14 from external sources was modified as additional FEPs were identified that are specific to the 

15 Clive facility. Approximately 980 FEPs were identified.

16

17 This exhaustive compilation of FEPs led to significant redundancy across the original sources. 

18 Redundancy was addressed by the modification of the candidate list of FEPs through 

19 normalization (removal of redundant FEPs) and assignment of FEPs categories (groupings of 

20 common FEPs). This consolidation process reduced the total number to 135 unique FEP 

21 groupings. These 135 unique FEP groupings were binned into 18 major categories.

22

Of the 135 FEP groupings, 674 FEP groupings were retained for further consideration and 685 23

24 FEP groupings were dismissed from inclusion in the PA model. All FEP groupings considered 

25 and retained for inclusion in the conceptual site model (CSM) and scenarios are reported in 

26 Table C-3 (see unshaded FEPs). FEPs that were dismissed from consideration in the PA 

27 include those that do not fall within the scope of the PA, were characterized as extremely 

28 unlikely to occur or having a low magnitude of consequence of affecting the performance of the 

29 disposal facility, or were dismissed based on site-specific considerations. FEP groupings that 

30 were excluded from the PA are also listed in Table C-3 (see grey shaded FEPs).

31

32

 4 Neptune (2011) indicates in text that 90 FEP grouping were retained but only 67 FEP groupings are 

actually listed in Table B of the same document.

5 Neptune (2011) indicates in text that 45 groupings were excluded but 68 FEP groupings are actually 

listed in Table C of the same document.
C-10

1 Table C-2 List of Clive Facility FEPs Reference Sources

Project or Facility Reference

SKI/SKB

Andersson, J., T. Carlsson, T.F. Kautsky, E. Soderman, and S. 

Wingefors, 1989. The Joint SKI/SKB Scenario Development Project. 

SKB-TR8 9-35, Swedish Nuclear Fuel and Waste Management Co., 

Stockholm, Sweden.

WIPP

Burkholder, H.C., 1980. Waste Isolation Performance Assessment—A 

Status Report, in Scientific Basis for Nuclear Waste Management, Ed. 

C.J.M. Northrup, Jr., Plenum Press, New York, NY, Vol. 2, p. 689-702.

WIPP

Guzowski, R.V., 1990. Preliminary Identification of Scenarios That May 

Affect the Escape and Transport of Radionuclides From the Waste 

Isolation Pilot Plant, Southeastern New Mexico, SAND89-7149, Sandia 

National Laboratories, Albuquerque, NM.

NTS

Guzowski, R.V., and G. Newman, 1993. Preliminary Identification of 

Potentially Disruptive Scenarios at the Greater Confinement Disposal 

Facility, Area 5 of the Nevada Test Site, SAND93-7100, Sandia National 

Laboratories, Albuquerque, NM.

NTS

Hertzler, C.L., and C.L. Atwood, 1989. Preliminary Development and 

Screening of Release Scenarios for Greater Confinement Disposal of 

Transuranic Waste at the Nevada Test Site, EGG-SARE-8767, EG&G 

Idaho, Inc., Idaho Falls, ID.

Hypothetical 

Columbia Plateau 

Repository

Hunter, R.L., 1983. Preliminary Scenarios for the Release of 

Radioactive Waste From a Hypothetical Repository in Basalt of the 

Columbia Plateau, SAND83-1342 (NUREG/CR-3353), Sandia National 

Laboratories, Albuquerque, NM.

WIPP

Hunter, R.L., 1989. Events and Processes for Constructing Scenarios 

for the Release of Transuranic Waste From the Waste Isolation Pilot 

Plant, Southeastern New Mexico, SAND89-2546, Sandia National 

Laboratories, Albuquerque, NM.

HLW Repository

IAEA, 1983. Concepts and Examples of Safety Analyses for 

Radioactive Waste Repositories in Continental Geological Formations, 

Safety Series Report No. 58, IAEA, Vienna. 

HLW Repository

Koplik, C.M., M.F. Kaplan, and B. Ross, 1982. The Safety of 

Repositories for Highly Radioactive Wastes, Reviews of Modern 

Physics, Vol. 54, No. 1, p. 269-310.

HLW Repository 

(Canada)

Merrett, G.J., and P.A. Gillespie, 1983. Nuclear Fuel Waste Disposal: 

Long-Term Stability Analysis, AECL-6820, Atomic Energy of Canada 

Limited, Pinawa, Manitoba.

NEA Working Group

NEA (Nuclear Energy Agency), 1992. Systematic Approach to Scenario 

Development. A report of the NEA Working Group on the Identification 

and Selection of Scenarios for Performance Assessment of Radioactive 

Waste Disposal, Nuclear Energy Agency, Paris, France.

NEA Working Group

NEA, 2000. Features, Events, and Processes (FEPs) for Geologic 

Disposal of Radioactive Waste. An International Database. Nuclear 

Energy Agency, Organization for Economic Cooperation and 

Development.

1 2 References for Andersson et al. (1989), Burkholder (1980), Guzowski (1990), Hertzler and Atwood 

3 (1989), Hunter (1983), Hunter, (1989), IAEA (1983), Koplik et al. (1982), Merrett and Gillespie, NEA 

4 (1992) and Prij et al. (1991) were found in Guzowski and Newman (1993).

5
C-11

Table C-3 Clive Facility FEP Groupings and FEPs

ID FEP Category FEP Groupings

1

Model Settings

Model Parameterization

2 Period of Performance

3 Regulatory Requirements

4 Spatial Domain 

5

Geological

Diagenesis

6 Gas or Brine Pockets

7 Landslide

8 Local Subsidence

9 Geochemical Geochemical Effects

10

Hydrogeological

Denudation

11 Erosion

12 Erosional Transport

13 Hydrogeological Effects

14 Sedimentation

15 Subrosion

16

Hydrology

Groundwater Transport

17 Hydrological Effects

18 Inundation

19 Flooding

20 Surface Water Transport

21

Meteorology

Frost Weathering

22 Meteorology

23 Resuspension

24 Atmospheric Dispersion

25 Tornado

26

Marine

Coastal Processes

27 Hurricanes

28 Insolation

29 Marine Effects

30 Tsunami

31

Climate Change

Climate Change

32 Lake Effects

33 Wave Action

34 Glacial Effects

35 Permafrost

36

Tectonic/Seismic/Volcanic

Geophysical Effects

37 Breccia Pipes

38 Diapirism

39 Discontinuities

40 Earthquake

41 Faulting

42 Fracturing

43 Geological Intrusion

44 Hydraulic Fracturing
C-12

Table C-3 Clive Facility FEP Groupings and FEPs

ID FEP Category FEP Groupings

45 Instrusion Into Accumulation Zone in the 

Biosphere

46 Isostatic Effects

47 Lava Tubes

48 Orogeny

49 Regional Subsidence

50 Seismic Effects

51 Tectonic Effects

52 Volcanism

53 Celestial Meteorite Impact

54

Natural Processes

Microbial Effects

55 Radiological Effects

56 Wildfire

57

Other Natural Processes

Ecological Changes

58 Gas Generation

59 Pedogenesis

60 Radioactive Decay and In-growth

61 Radon Emanation

62 Reconcentration

63

Human Processes

Anthropogenic Climate Changes

64 Community Development

65 Excavation

66 Explosions

67 Human-Induced Processes

68 Human-Induced Transport

69 Inadvertent Human Intrusion

70 Inhabitation

71 Institutional Control

72 Land Use

73 Post-Closure Subsurface Activities

74 Accidents During Operations

75 Climate Control

76 Closure Failure

77 Fire

78 Fisheries

79 Geothermal Energy Production

80 Injection Wells

81 Intentional Intrusion

82 Investigation

83 Irrigation

84 Monitoring

85 Nuclear Testing

86 Operational Effects

87 Operational Error
C-13

Table C-3 Clive Facility FEP Groupings and FEPs

ID FEP Category FEP Groupings

88 Quality Control

89 Resource Extraction

90 Sabotage

91 Unplanned Events

92 War

93 Waste Recovery

94 Water Resource Management

95 Weapons Testing

96 Containerization Containment Degradation

97 Corrosion

98

Engineered Features

Compaction Error

99 Engineered Features

100 Material Properties

101 Repository Design

102 Source Release

103 Subsidence of Repository

104 Waste

105 Waste Nuclear Criticality

106 Other Waste

107 Source Release Electrochemical Effects

108 Explosions

109

Contaminant Migration

Biotically-Induced Transport

110 Colloid Transport

111 Contaminant Transport 

112 Diffusion

113 Dilution 

114 Dispersion

115 Dissolution

116 Dust Devils

117 Gas Transport

118 Infiltration

119 Local Geology

120 Preferential Pathways

121 Gas Intrusion

122 Convergence of Opening

123 Design Error

124 Material Defects

125 Mechanical Effects

126 Release of Stored Energy

127 Repository Seals

128

Exposure

Animal Ingestion

129 Dosimetry

130 Exposure Media

131 Human Behavior
C-14

Table C-3 Clive Facility FEP Groupings and FEPs

ID FEP Category FEP Groupings

132 Human Exposure

133 Ingestion Pathways

134 Inhalation Pathways

135 Agriculture

1

2 DOE Savannah River Site (SRS)

3

SRS is a 780 km2 (300 mi2 4 ) DOE facility located approximately 12 miles south of Aiken, South 

5 Carolina, and 15 miles southeast of Augusta, Georgia. The SRS region is characterized as a

6 humid subtropical climate with relatively short, mild winters and long, warm, and humid 

7 summers. Summer-like conditions typically last from May through September, when the area is 

8 frequently under the influence of a western extension in the semi-permanent Atlantic subtropical 

9 anticyclone (i.e., the ‘Bermuda’ high). The influence of the Bermuda high begins to diminish 

10 during the fall as continental air masses become more prevalent, resulting in lower humidity and 

11 more moderate temperatures. Less than one-third of winter days have minimum temperatures 

12 below freezing on average, and days with temperatures below 20°F are infrequent. Measurable 

13 snowfall occurs an average of once every two years. 

14

15 Operation at SRS began in 1951. The primary use for the site was the production of nuclear 

16 material for national defense. Between 1954 and 1986, DOE generated significant quantities of 

17 radioactive waste from the reprocessing of spent nuclear fuel and to a lesser extent from the 

18 production of targets for nuclear weapons and material for space missions. This waste was 

19 stored in 51 underground tanks located in two tank farms: F-Tank Farm (FTF) contains 22 

20 tanks and H-Tank Farm houses 29 tanks. DOE plans to clean the tanks and stabilize the waste 

21 residuals in a cementitious wasteform. DOE also plans to dispose of relatively low-activity salt 

22 waste retrieved from the tanks in the saltstone disposal facility. DOE has prepared PAs to 

23 demonstrate that the stabilized waste remaining in the tank farms and the waste disposed of in 

24 the saltstone disposal facility can meet performance objectives for low-level waste disposal. 

25 DOE prepared an ex post facto FEPs analysis to provide support for its compliance 

26 demonstrations.

27

28 The initial, SRS FEPs list included 245 FEPs drawn from five different reference sources listed 

29 in Table C-4. The initial 245 FEPs were then binned into six categories listed in Table C-5.

30

31
C-15

1 Table C-4 SRS FEP List Reference Sources

FEPs 

List

Source Document Total Number 

of FEPs

ISAM

Safety Assessment Methodologies for Near-Surface 

Disposal Facilities, Results of a Coordinated Research 

Project, Volume 1 [ISBN 92-0-104004-0]

141

UFD

Features, Events, and Processes for the Disposal of Low 

Level Radioactive Waste FY2011 Status Report [FCRDUSED-2011-000297]449

YMP

Features, Events, and Processes for the Total System 

Performance Assessment: Analyses [ANL-WIS-MD000027 REV 0] 374

DGR

Deep Geologic Repository for OPG's Low and 

Intermediate Level Waste, Post-Closure Safety 

Assessment (Volume 1): Features, Events and Processes

[NWMODGR-TR-2009-05]

299

SKI

Encyclopedia of Features, Events and Processes (FEPs) 

for the Swedish SFR and Spent Fuel Repositories [SKI 

Report 02:35] 

120

2

3
C-16

1 Table C-5 SRS FEPs List Categories

Category Group

1.0 Assessment Basis

1.1 General

1.2 Regulations and Controls

1.3 Models and Calculations

1.4 Other Assessment Factors

2.0 External Factors

2.1 Human Characteristics

2.2 Land and Water Management

2.3 Future Human Activity

2.4 Biological Factors

2.5 Geologic Features

2.6 Geologic Processes

2.7 Climate

2.8 Water Cycle

3.0 Closure System

3.1 General Closure System

3.2 Pre-Closure Activities

3.3 Closure System Components

3.4 Closure System Hydrology

3.5 Chemical Processes

3.6 Thermal Processes

3.7 Material Degradation

3.8 Other Closure System Factors

4.0 Contaminant Factors

4.1 Contaminant Description

4.2 Contaminant Properties

4.3 Concentrations

4.4 Exposure Factors

4.5 Other Contaminant Factors

5.0 Flow and Transport

5.1 Flow Factors

5.2 Hydraulic Effects on Flow

5.3 Release and Transport

6.0 Disruptive Events

6.1 Intrusions

6.2 Seismic Events

6.3 Igneous Events

6.4 Other Events

2

3 In addition to consolidation of the 5 FEPs list reference sources, DOE included 17 additional 

4 FEPs evaluated in SRS PAs leading to a total of 262 FEPs. Next, the list of 262 FEPs was 

screened6 5 . The SRS FEPs screening team performed screening in two phases. During the first 

phase, team members independently applied the FEPs screening criteria7 6 via survey. The 

7 independent survey results were collected and a subset of FEPs was “screened in” or “out” 

8 based on the results. Those FEPs with relatively more ambiguous results (i.e., survey results 

9 indicated that FEP should be considered further) were discussed in the second phase. At the 

 6 This is true except for 46 programmatic FEPs that would not be subject to screening. These FEPs are 

listed in Table 4.0-1 of SRR-CWDA-2012-00011, Revision 0.

7 Criteria were based on the perceived probability of occurrence within 10,000 years and the perceived 

consequence relative to final PA results.
C-17

1 start of Phase 2, 142 FEPs remained. The FEPs that were “screened out” in Phase 1 and 2 are 

2 listed in Table C-6.

3

4 A total of 230 FEPs remained after Phase 2 screening. SRR-CWDA-2012-00022 crosswalks 

5 remaining FEPs to the FTF PA to ensure all relevant FEPs were addressed.

6

7
C-18

1 Table C-6 FEPs Excluded from SRS PAs

FEP ID Phase

Large Scale Salt Processes (Diapirism, 

Dissolution, and Creep)

2.6.11 Phase 1

Ashfall 6.3.03 Phase 1

Extraterrestrial Events 6.4.08 Phase 1

Changes in the Earth's Magnetic

Field

6.4.09 Phase 1

Changes to Earth's Tidal Processes 6.4.10 Phase 1

Pollution (Soil, Groundwater, Air, etc.) 2.2.07 Phase 2

Ozone Layer Failure 2.3.07 Phase 2

Species Evolution 2.4.05 Phase 2

Stress Regimes 2.5.09 Phase 2

Orogeny 2.6.03 Phase 2

Diagenesis and Pedogenesis 2.6.05 Phase 2

Sedimentation 2.6.06 Phase 2

Creeping of the Rock Mass 2.6.10 Phase 2

Acid Rain 2.7.04 Phase 2

Costs of Construction, Operation, Closure 3.2.04 Phase 2

Chelating Agent Effects 3.5.12 Phase 2

Thermal Processes and Conditions the 

Engineered System

3.6.01 Phase 2

Thermal Processes and Conditions the Natural 

System

3.6.02 Phase 2

Thermo-Mechanical Stresses Alter 

Characteristics of Engineered Barrier System 

Components

3.6.04 Phase 2

Recrystallization of Vitrified Wastes 3.6.05 Phase 2

Effects of System Heat on the Biosphere 3.6.06 Phase 2

Creep of Metallic Materials in the Engineered 

System

3.7.04 Phase 2

Oxygen Embrittlement of Engineered System 

Metals

3.7.05 Phase 2

Localized Interactions Between Emplaced 

Wastes

4.2.05 Phase 2

Nuclear Criticality 4.5.01 Phase 2

Chemically-Induced Density Effects on 

Groundwater Flow

5.1.06 Phase 2

Hydrothermal Activity 5.2.02 Phase 2

Seismicity Associated with Igneous Activity 6.2.04 Phase 2

Igneous Intrusion Into the Closure Facility 6.3.01 Phase 2

Volcanic Eruptions and Magmatic Activity 6.3.02 Phase 2

Releases Prior to Closure 6.4.01 Phase 2

Impacts from Meteorites or Space Debris 6.4.07 Phase 2

Extraterrestrial Events 6.4.08 Phase 2

Changes in the Earth's Magnetic Field 6.4.09 Phase 2

Changes to Earth's Tidal Processes 6.4.10 Phase 2

2
C-19

1 Generic FEPs List

2

3 The ISAM and NEA international FEP list structure illustrated in Figure C-2 above was retained 

4 in constructing a comprehensive, generic FEP list applicable to near-surface disposal of LLW. 

5 Because the ISAM FEP list modified the NEA FEP list to be more applicable to near-surface 

6 disposal facilities, the ISAM FEP category titles are specifically listed in the generic FEP list. 

7 However, some FEPs of the ISAM FEPs are not expected to be risk-significant during the time 

8 periods of interest for disposal of most LLW in the United States or are considered outside the 

9 scope of the 10 CFR Part 61 regulatory framework. These FEPs that are considered less 

10 relevant for near-surface disposal of LLW are listed in Table C-7. The remaining ISAM FEPs 

11 represent the major FEP groupings in the generic FEPs list (see column 1). 

12

13 The generic FEPs list comprises three separate tables: (i) assessment context or operational 

14 factors (see Table C-8) used to develop FEPs presented in Tables C-9 and C-10, (ii) FEPs to 

15 analyze or screen to construct central and alternative scenarios presented in Table C-9, and (iii) 

16 FEPs to analyze or screen for receptor scenarios presented in Table C-10. Table C-8 factors 

17 are not FEPs per se, but assessment context (e.g., purpose of the assessment), operational 

18 factors, and site characterization or monitoring activities may dictate the types and scope of 

19 FEPs considered in a technical assessment. Therefore, Table C-8 factors are listed and 

20 expected to be valuable considerations in the development of project-specific FEPs. 

21

22 FEPs that may be considered unlikely during the compliance period or protective assurance 

23 period but may become increasingly more risk-significant over time are flagged “long-term” in 

24 column 3 of Tables C-9 and Table C-10 (no assessment context factors are marked long-term). 

25 If the “long-term” column is marked in column 3, the applicability of the FEPs for a specific site 

26 should be considered when performance period analyses are required for that site. Although 

27 some FEPs from the reference sources are not explicitly listed in Tables C-7 through C-10, one 

28 can assume that the FEP should be considered unless it is clearly linked to a category of FEPs 

29 that are not considered applicable for near-surface waste disposal listed in Table C-7. 

30 Reviewers, applicants, licensees, and other interested stakeholders should find the level of 

31 detail provided in tables sufficient to understand the scope of FEPs that should be considered 

32 for any site with many examples provided in the last column (column 4); however, the generic 

33 FEP list should not be considered an exhaustive list that would encompass every potentially 

34 applicable FEP that may be important for a particular site. Likewise, not every FEP listed in 

35 Tables C-8 through C-10 would need to be considered by a licensee.

36

37
C-20

Table C-7 FEPs Less Relevant for Near-Surface Disposal of LLW in the United States

Excluded FEP ISAM ID Rationale

Future human action assumptions1 0.05

The scope of this FEP is limited. 

Future societal and technology 

development will occur, but is difficult 

to predict. Therefore, unnecessary 

speculation about future human 

actions should be avoided. The 

uncertainty associated with future 

human actions is accounted for in the 

technical analyses by using reasonably 

conservative receptor scenarios.

Future human behavior (target 

group) assumptions1 0.06

The scope of this FEP is limited. 

Future human behavior is difficult to 

predict. Unnecessary speculation 

about future human behavior should be 

avoided.

Retrievability 1.1.13

LLW disposal facilities in the United 

States are not designed for 

retrievability.

Orogeny and related tectonic 

processes at plate boundaries 1.2.01

Orogeny and related tectonic 

processes at plate boundaries are only 

expected to be important for nearsurface disposal facilities if very long 

performance periods are evaluated.

Anorogenic and within-plate tectonic 

processes (Deformation, elastic, 

plastic, and brittle)

1.2.02

Anorogenic and within-plate tectonic 

processes are only expected to be 

important for near-surface disposal 

facilities if very long performance 

periods are evaluated.

Metamorphism 1.2.05

Metamorphism is not expected to be 

important for near-surface disposal 

facilities or in the timeframes of 

interest.

Diagenesis2 1.2.08

Diagenesis is not expected to be 

important for near-surface disposal 

facilities or in the timeframes of 

interest.

Human influences on climate

including ozone depletion, global 

warming, and greenhouse effect.

1.4.01

Future anthropogenic impacts on 

climate change are difficult to predict 

and are covered under natural climate 

change.

Motivation and knowledge issues 

(inadvertent/deliberate human 

actions)

1.4.02

The scope of this FEP is limited. 

Advertent intruders are not protected 

under 10 CFR Part 61.
C-21

Table C-7 FEPs Less Relevant for Near-Surface Disposal of LLW in the United States

Excluded FEP ISAM ID Rationale

Pollution (as it impacts site 

performance, radionuclide mobility, 

or monitoring)

1.4.07

The scope of this FEP is limited. 

Unnecessary speculation about future 

pollution should be avoided. Current 

and reasonably foreseeable pollution 

should be considered.

Social and institutional 

developments 1.4.11

The scope of this FEP is limited. 

Unnecessary speculation regarding 

future human actions and behavior 

should be avoided such as changes in 

demography, land use, controls, and 

regulatory requirements that may not 

need to be evaluated. Loss of control 

of a site due to loss of records or 

societal memory should be included 

and is considered in the inadvertent 

intruder analysis.

Technological developments 1.4.12

The scope of this FEP is limited. 

Technological developments are likely 

to occur but difficult to predict. 

Unnecessary speculation regarding 

future technological advances should 

be avoided (e.g., cure for cancer, 

technological advances in food 

production).

Explosions and crashes 1.4.14

The scope of this FEP is limited. For 

example, deliberate or malicious 

human actions may not need to be 

considered.

Meteorite impact3 1.5.1 Considered unlikely.

Species evolution3 1.5.2

Expected to be of limited significance 

in the timeframes of interest and with 

unknown impact.

Miscellaneous and FEPs of 

uncertain relevance3 1.5.3

Items in this category are not 

considered likely or significant to nearsurface, LLW disposal (e.g., 

extraterrestrial activity, dust, changes 

in magnetic field, change in tidal 

processes).

Non-radiological toxicity/effects 3.3.08 Non-radiological effects are not 

considered. 1 FEP of limited scope. 2 Table C-9 includes a portion of this FEP (pedogenesis) that may need to be considered. 3

NEA FEPs that are excluded from the ISAM FEPs list.

1

2

3
C-22

Table C-8 Assessment Context, Site Characterization, or Operational Factors Considered in D

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example Factors

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Assessment endpoints 

(including intermediate 

outputs or results)

0.01

Points of assessment (e.g., well location, horizontal distance from so

Annual Individual Dose (BMA 2.1.2., 2.1.2.1., BMO 1.1.2.1., H

Radionuclide Flux or Concentration

(BMO 1.1.2.8, BMA, 2.1.2.8., SRS 1.1.09, SRS 5.3.

HAN 0.4.06.08, HAN 0.4.06.09, HAN 0.4.06.10)

Results, Presentation of (e.g., multiple lines of reason, barrier analysis, 

simpler models) (HAN 0.4.08)

Timescales of concern 0.02

Timeframes (BMA 2.1.7)

Post-closure period (HAN 0.2.02.03)

Assessment Timeframe (e.g., institutional control period, compliance p

peak impact) (HAN 0.4.01, SRS 1.1.06, CLV 2)

Safety Effects Beyond Periods of Control (beyond institutional control 

Spatial domain of 

concern

8

) 0.03 Assessment Domain/Spatial Domain of Concern (HAN 0.4.02, SR

Facility 9 assumptions

(e.g., assumptions 

regarding the success of 

facility closure and any 

changes to design, 

construction, or waste 

emplacement)

0.04

Repository System (BMA 2.1.4 )

Site, Context (BMA, 2.1.5, BMO 1.1.4)

Facility Type (BMO 1.1.3)

Disposal Facility Assumptions (HAN 0.4.09)

Facility Factors (life cycle of disposal facility) (SRS 3.

Dose response 

assumptions 10 0.07 Dose Response Assumptions (HAN 0.4.07)

Assessment purpose 0.08 Assessment Purpose (e.g., site selection and characterization, designcorrective action, confidence) (BMA 2.1.1, BMO 1.1.1, HAN 0.1

 8 Spatial extent over which disposed waste presents a risk to human health.

9 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to te

10 For example, linear relationship of human health effect to dose with no threshold dose below which no effects are observ
C-23

Table C-8 Assessment Context, Site Characterization, or Operational Factors Considered in D

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example Factors

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Assessment Audience (HAN 0.4.05)

Regulatory requirements 

and exclusions 0.09

Regulatory Requirements and Criteria (e.g., radiological protection sta

(HAN 0.2, CLV 3)

Protection of Human Health and the Environment (SRS 

Performance Requirements and Criteria (SRS 1.2.0

ALARA (SRS 1.2.05)

Functional and Technical Requirements (e.g., containment, isolation, ch

construction, quality assurance (QA), waste acceptance criteria (WAC

peer review) (HAN 0.2.03, SRS 1.2.04)

Waste Acceptance Criteria (SRS 1.2.07)

Model and data issues 

(e.g., uncertainty, model 

abstraction)

0.10

Uncertainties, or confidence (BMA, 2.1.2.10)

Confidence, Model (e.g., calibration, verification, validation) (HAN 0

Uncertainties, treatment of (e.g., subjective, future, conceptual, ma

parameter) (HAN 0.3.02, SRS 1.3.10)

Sensitivity Analysis, Performance of (HAN 0.3.03, SRS 

Model and Data Issues (including conceptual and mathematical m

discretization, boundary conditions, coupled processes; parameter 

correlations; and scale issues) (SRS 1.3.01, CLV 

Software Codes (SRS 1.3.02)

Assessment Philosophy 

or Approach New

Assessment Philosophy (e.g., assessment and modeling approach, tre

sensitivity analysis, and confidence building) (BMA 2.1.3, 

Assessment Approach (e.g., iterative, systemic, realistic, conserva

(HAN 0.3.01, SRS 1.3.04, SRS 1.3.05, SRS 1.3.06, SRS 1.3.07, SRS 

Modeling Approach (e.g., screening, bounding, deterministic, 

(HAN 0.3.05, SRS 1.3.03)

Alternative Simplified Modeling Approach (SRS 1.3.
C-24

Table C-8 Assessment Context, Site Characterization, or Operational Factors Considered in D

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example Factors

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Transparency of Assessment Approach (SRS 1.1.0

Documentation and Presentation of Results (SRS 1.1

Site investigation 1.1.01

Technical Requirements (e.g., site characterization) (HAN 

Investigations, Site (e.g., geology, hydrogeology, geochemistry, tectonic 

environment, meteorology and climatology, geography and demography

land use) (HAN 1.1.01, CLV 82)

Site Characterization and Investigations (SRS 3.1.0

Schedule and planning 1.1.09 Schedule and Planning (e.g., construction, operation, and closure s

alternative schedule) (HAN 1.1.03, SRS 3.2.02)

Administrative control, 

facility site11 1.1.10

Administrative control, Disposal facility (from pre- to post-closur

(HAN 1.1.08, SRS 1.2.06)

Institutional control (CLV 71)

Monitoring of facility 12 1.1.11

Operation, Disposal Facility (e.g., monitoring) (HAN 1.1.05

Radionuclide Fluxes to the Biosphere (to monitor barrier performan

Releases Prior to Closure (SRS 6.4.01)

 11 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to t

12 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to t
C-25

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Design, facility 13 1.1.02

Design, Disposal Facility (e.g., description, documentation, functional r

alternative designs) (HAN 1.1.02, CLV 101)

Design error (CLV 123)

Construction, facility 14 1.1.03

Construction, Disposal Facility (e.g., process, performance and veri

conditions) (HAN 1.1.04)

Construction (includes factors related to the excavation, stabilization, a

assembly of structural elements) (SRS 3.2.05)

Material defects (CLV 124)

Emplacement of wastes 

and backfilling 1.1.04

Operation, Disposal Facility (e.g., waste emplacement and repackaging

handling and emplacement) (HAN 1.1.05)

Operation (waste emplacement, backfilling, monitoring and surveillanc

(SRS 3.2.06)

Void Space Formation (SRS 3.8.06)

Waste Type Classification (as it impacts disposal requirements for diffe

(SRS 4.1.01)

Waste Form Characteristics (SRS 4.1.02)

Waste allocation and emplacement (SRS 4.1.04)

Localized Interactions Between Emplaced Wastes (SRS 

Closure, facility 15 1.1.05

Closure, Disposal Facility (e.g., closure plan, performance requirements

construction, confirmation, remedial alternatives) (HAN 

Disposal Unit and/or Facility Closure (includes activities undertaken to p

into and limit the migration of contaminants from the individual waste 

 13 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to t

14 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to t

15 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to t
C-26

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Closure System Features and Materials (SRS 3.3.0

Compaction Error (CLV 98)

Waste allocation 

(projected inventory, 

waste acceptance 

criteria)

1.1.07

Operation, Disposal Facility (e.g., waste acceptance, waste allocat

Radionuclide inventory disposed and remaining in the disposal faci

Activity limits in disposed waste (HAN 0.4.6.12, SRS 4

Waste allocation and emplacement (SRS 4.1.04)

Homogeneity (SRS 4.1.05)

Waste Acceptance Criteria (SRS 1.2.07)

Quality control 1.1.08

Quality control (CLV 88)

Technical Requirements (e.g., QA) (HAN 0.2.03)

Quality Assurance (e.g., all stages including research and developm

manufacturing, siting, design, construction, commissioning, operation

QA/QC failures) (HAN 1.1.07)

Procurement of Items and Services (quality assurance) (SR

Manufacturing and Commissioning of Components (including defe

Inadequate Quality Assurance/Control and Deviations from Desi

Accidents and unplanned 

events 16 1.1.12 Accidents and unplanned events (e.g., human-induced and natu

(HAN 1.1.09, SRS 6.4.05, CLV 99, CLV 91)

Seismicity 1.2.03

Seismicity and effects (e.g., soil liquefaction) (HAN 1.2.03, SRS 

Seismicity Associated with Igneous Activity (SRS 6.2

Earthquakes (CLV 40)

Tsunami (CLV 30)

 16 For example, earlier than expected engineered barrier system failure, unexpected event, or unexpected waste.
C-27

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Faulting (CLV 41)

Volcanic and magmatic 

activity 1.2.04 Longterm

Volcanic and magmatic activity (HAN 1.2.04, SRS 6.3.02, 

Igneous Intrusion Into the Closure Facility (SRS 6.3

Geological intrusion (CLV 43)

Hydrothermal activity 1.2.06 Longterm Hydrothermal activity (HAN 1.2.06, SRS 5.2.02, CLV 

Erosion and 

sedimentation 1.2.07

Denudation and Deposition (large-scale, including erosion, corrasion

aeolian, glacier, coastal, mass-wasting, sedimentation, deposition, ev

wasting) (HAN 1.2.07, CLV 10, CLV 11)

Geomorphologic response to geological changes (e.g, structural, weat

depositional landforms) (HAN 1.2.13, CLV 13)

Depositional Environments and Landforms (including beaches, deltas, f

moraines) (SRS 2.5.03)

Sedimentation (SRS 2.6.06, CLV 14)

Pedogenesis 17 1.2.08 Pedogenesis (factors related to the development and origin of soils, m

(HAN 1.2.09, SRS 2.6.05, CLV 59)

Salt diapirism and 

dissolution 1.2.09 Longterm

Salt diapirism and dissolution (intrusion or upwelling of a salt formation 

(such as salt domes). Salt dissolution can occur when any soluble m

flowing water) (HAN 1.2.10, SRS 2.6.11, CLV 9, CLV 

Hydrological/

hydrogeological response 

to geological changes

1.2.10 Longterm

Hydrological/hydrogeological response to geological changes (e.g., 

conditions, surface water flow path, geochemical properties, and hydr

preferential pathways) (HAN 1.2.12, SRS 5.2.01, CLV 13, 

Unconsolidated soft zones (as it affects site stability and contaminan

(SRS 2.5.07)

 17 Diagenesis was originally included with this FEP; however, diagenesis is excluded in Table C-6.
C-28

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Climate change, global18 1.3.01 Longterm

Description of Climate Change (BMA 2.2.1.1, BMO 1.

Climate-driven changes (BMO 2.1.1.2)

Climate change, global (e.g., climate reconstruction, climate ch

(HAN 1.3.01, SRS 2.7.07, CLV 31)

Isostatic effects (CLV 46)

Climate change, regional 

and local 1.3.02

Description of Climate Change (BMA 2.2.1.1, BMO 1.3.1.2, S

Climate-driven changes (BMO 2.1.1.2)

Climate change, regional and local (e.g., climate fluctuations, volcanic e

induced changes, lake effects) (HAN 1.3.02, SRS 2.7.07, CLV 

Sea level change 1.3.03 Longterm

Sea level changes (HAN 1.3.03)

Isostatic Effects (CLV 46)

Periglacial effects 

(physical processes in 

cold but ice sheet free 

environments)

1.3.04 Longterm

Cold weather effects (permafrost, freeze/thaw cycles, frost heav

(SRS 2.7.06, CLV 35)

Periglacial (HAN 1.3.04, CLV 34)

Solifluction (CLV 12)

Glacial and ice sheet effects, local (HAN 1.3.05, CLV 

Glacial and ice sheet 

effects, local 1.3.05 Longterm Warm climate effects (HAN 1.3.06, SRS 2.7.05)

Warm climate effects 

(tropical and desert)19 1.3.06 Hydrological/hydrogeological response to climate changes (e.g., change and flow) (HAN 1.3.07)

Hydrological/

hydrogeological response 1.3.07 Longterm Climate Driven Changes (BMO 2.1.1.2)

 18 Does not include anthropogenic impacts on climate change. 19 Facilities in tropical climates may experience extreme weather patterns such as monsoons, hurricanes, flooding, storm s

arid climates may be dominated by infrequent storm events.
C-29

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

to climate changes Ecological changes (BMA 2.3.1.1.3, CLV 57)

Ecological response to 

climate changes 1.3.08

Climate Driven Changes (BMO 2.1.1.2)

Ecological response to climate changes (e.g., soil, water, atmosphere, 

organisms, ecological adaptation) (HAN 1.3.08, CLV 

Other geomorphological 

changes 1.3.10

Geomorphologic responds to climate changes (e.g., periglacial landfo

(HAN 1.3.10)

Surface excavations (e.g., construction, remediation, geotechnical invest

(HAN 1.4.07, CLV 65)

Pollution (as it impacts 

site performance, 

radionuclide mobility, or 

monitoring)20

1.4.07

Artificial soil fertilization (BMA 2.3.2.1.1, BMO 2.2.3

Chemical pollution (BMA 2.3.2.1.2, BMO 2.2.1.1)

Acid rain (BMA 2.3.2.1.3, BMO 2.2.1.1.1, SRS 2.7.0

Pollution (e.g., soil, groundwater, and air that may increase mobilit

(HAN 1.4.12, SRS 2.2.07)

Inventory, radionuclide 

and other material 2.1.01

Inventory, waste (e.g., waste stream, volume, homogeneity, radiolo

radiological content (as it impacts radionuclide mobility), classifica

(HAN 2.1.01, SRS 4.1.03)

Source Term/Radionuclide Content (BMA 2.1.6, BMO 1

Wasteform materials, 

characteristics and 

degradation processes

2.1.02

Wasteform, characteristics and degradation processes (HAN 2.

Removal or Stabilization of Waste (stabilization of waste) (S

Waste characteristics (e.g., metallic, inorganic/non-metallic, organic, 

(SRS 4.1.07, 4.1.08, 4.1.09, 4.1.10, CLV 77, CLV 1

Waste degradation (inorganic and organic) (SRS 4.2.06, S

Container materials, 

characteristics and 2.1.03 Container materials, characteristics and degradation processes (HAN 2.1.03, CLV 96, CLV 97, CLV 100)

 20 Unnecessary speculation about future pollution should be avoided. Current and reasonably foreseeable pollution should 
C-30

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

degradation processes Waste tank, container, or package characteristics (physical, chemic

properties) (SRS 3.3.04)

Waste Container, Package, or Over-Pack Failure (e.g., corrosion, ma

improper seal) (SRS 3.7.06)

Buffer/backfill materials, 

characteristics and 

degradation processes

2.1.04

Buffer/backfill, characteristics and degradation processes (HAN 2

Closure System Buffer (Closure Cap, Backfill, and Near-Field Soil) Prop

of zeolites, mineraological dehydration, geothermal fluid impacts, sorptio

vermiculite) (SRS 3.3.05, SRS 3.3.06)

Swelling of Backfill and Emplacement Materials (e.g., bentonite and ve

(SRS 3.3.06, SRS 3.7.08)

Engineered barriers 

system (EBS), 

characteristics and 

degradation processes

2.1.05

Other engineered features, characteristics and degradation processes 

covers or multi-layer cap designs) (HAN 2.1.05, CLV 

Multi-Barrier Safety Function (combination of natural and engineered b

Design Basis for Engineered Components (SRS 3.2

Consolidation of System Components (consolidation of engineered barr

that may affect the chemical environment and release) (SR

Chemical Degradation of Engineered System Metals (SR

Corrosion (e.g., stress corrosion cracking, hydride cracking) (SRS 3

Creep of Metallic Materials in the Engineered System (SR

Oxygen Embrittlement of Engineered System Metals (SR

Concrete Shrinkage/Expansion (may impact hydraulic properties

Cementitious material degradataion (e.g., sulfate and chloride att

(SRS 3.7.10, SRS 3.7.11)

Seismic-Induced Damage or Changes to System Components 

Frost Weathering (engineered cover degradation mechanism

Geophysical effects on degradation (CLV 36)
C-31

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Other engineered 

features materials, 

characteristics and 

degradation processes

2.1.06

Other engineered features, characteristics and degradation processes (e

cut off walls or fire breaks, engineered drainage system) H

Engineered Barrier Thickness and Other Material Properties (e.g., closu

(SRS 3.3.07, SRS 3.3.08, SRS 3.3.09, CLV 100)

Ancillary Equipment and Piping/Transfer Lines (SRS 3

Degradation of Non-Metal Solids: Backfill, Rock, Grout, Cement, 

Polymer Degradation (SRS 3.7.12)

Material Volume Changes (e.g., expansion may lead to cracking of ma

Mechanical processes 

and conditions (in wastes 

and EBS)

2.1.07

Mechanical Effects at EBS Component Interfaces (e.g., mechanical 

(SRS 3.8.09)

Effects of Subsidence (including increased infiltration) (SRS 6.2.03

Cave-In, Collapse, or Rockfall (SRS 6.4.04)

Compaction Error (CLV 98)

Hydraulic/hydrogeological 

processes and conditions 

(in wastes and EBS)

2.1.08

Hydrological Processes and Conditions (processes affecting flow through 

features) (SRS 3.4.01, CLV 99)

Hydrostatic Pressure on the Closure System (hydrostatic pressure (or 

waste and engineered system components) (SRS 3.4

Condensation on Closure System Surfaces (SRS 3.4

Resaturation and Desaturation (equilibration of engineered barriers with 

may cause hydraulic, thermal, chemical, mechanical changes such as 

corrosion) (SRS 3.4.04)

Groundwater Flow and Movement (Near-Field) (e.g., preferential f

Focusing of Flow Along Preferred Flow Paths (Fingers, Weeps, Fau

(SRS 5.1.05)

Episodic Or Pulse Flow and Release (SRS 5.1.03

Water Influx at the Closure Facility (SRS 5.1.04)
C-32

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Flow Diversion and Bypass Flow (focused flow around cap, flow throug

(SRS 5.1.07)

Film/Laminar Flow (through waste zone) (SRS 5.1.0

Perched Water (SRS 5.2.06)

Contaminant Release Pathways (SRS 5.3.02, CLV 1

Multi-Phase Transport Processes (SRS 5.3.03)

Fast Transport/Preferential Pathways (SRS 5.3.13, CLV 

Flooding or drainage system failure (SRS 6.4.02)

Mechanical effects (CLV 125)

Chemical/geochemical 

processes and conditions 

(in wastes and EBS)

2.1.09

Chemical/geochemical-mediated processes, effects on contaminant r

(including dissolution, precipitation, speciation, solubility, sorption/desorp

complexing agents, and reconcentration; interface between near- and f

sorption and permeability) (HAN 3.2.02, SRS 3.5.01, SRS 5.3.01, 

Evolving water chemistry in wasteform, containment system, or near-fie

cementitious materials and impacts on contaminant mobility; impacts o

transport; pH and Eh changes; colloid generation

(SRS 3.5.02, SRS 3.5.03, SRS 3.5.05, SRS 3.5.06, SRS 3.5.07, 

Chemical Effects of Waste-Rock Contact (direct contact of waste with 

waste package) (SRS 3.5.08)

Rind (Chemically Altered Zone) Forms in the Near-Field (thermal-chemi

precipitation, condensation, and re-dissolution could alter the proper

materials) (SRS 3.5.09)

Reaction Kinetics (non-equilibrium conditions) (SRS 3

Contaminant Release from the Waste Form and Engineered B

(SRS 5.3.04, CLV 99)

Long-Term Release of Radionuclides (SRS 5.3.14

Electrochemical effects (CLV 107)
C-33

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Biological/biochemical 

processes and conditions 

(in wastes and EBS)

2.1.10

Microbial/biological-mediated processes, effects on contaminant releas

biological activity that may change radionuclide mobility and micr

(HAN 3.2.03, SRS 2.4.02, CLV 54, CLV 102)

Thermal processes and 

conditions (in wastes and 

EBS)

2.1.11

Thermal Processes and Conditions the Engineered System (as from 

radioactive decay) (SRS 3.6.01)

Thermo-Chemical Alteration, Near-Field (e.g., effects on solubilit

Thermo-Mechanical Stresses Alter Characteristics of Engineered Barrie

(e.g., thermal cracking of cementitious materials) (SRS 

Recrystallization of Vitrified Wastes (SRS 3.6.05)

Gas sources and effects 

(in wastes and EBS) 2.1.12

Waste Form Characteristics(e.g., gas generation) (HAN 2.1.02

Gas generation (e.g., radon) (CLV 58)

Radiation effects (in 

wastes and EBS) (e.g., 

radiolysis, material 

degradation)

2.1.13

Radiation Effects on the Waste Closure System (SRS 4

Radiolysis Effects (SRS 4.5.07)

Radiological effects (CLV 55)

Nuclear criticality 2.1.14 Nuclear Criticality (HAN 1.1.11, SRS 4.5.01, CLV 1

Extraneous materials 2.1.15 No examples were identified.

Disturbed zone, host 

lithology 2.2.01 Disturbed zone, host lithology (e.g., formation of cracks, interface, hydbackfilling, contaminant migration) (HAN 2.2.03)

Host lithology 2.2.02

Host lithology (e.g., geological properties, physical charac

(HAN 2.2.02, CLV 119, CLV 100)

Stratigraphy and Host Lithology (including description, flow and transp

homogeneity, and potential changes) (SRS 2.5.04

Lithological units, other 2.2.03 Stratigraphy (e.g., stratrigraphic record, formation) (HAN 2.2.0

Discontinuities, large 

scale (in geosphere) 2.2.04 Discontinuities, large scale (in geosphere) (e.g., faults, folds, dykesdiscontinuities affecting boundary conditions) (HAN 2.2.04, SRS 

Contaminant transport 

path characteristics (in 2.2.05 Contaminant migration path characteristics (in geosphere) (e.g., hydinterstitial geometry, bypass flow, fracture infill, weathering) (HAN 
C-34

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

geosphere) (e.g., fracture 

flow, porous flow, 

fracture/matrix 

interactions)

Unconsolidated soft zones (SRS 2.5.07)

Fast Transport Pathways (SRS 5.3.13)

Mechanical processes 

and conditions (in 

geosphere)

2.2.06

Mechanical processes and conditions (in geosphere) (e.g., changes in s

load, mechanical rapture, changes in rock properties) (HAN 2.20

Stress Regimes (caused by coupled thermal-hydro-mechanical effects

isostatic rebound (such as when glaciers recede), salt creep, etc., and 

flow/directions). (SRS 2.5.09)

Hydraulic/hydrogeological 

processes and conditions 

(in geosphere)

2.2.07

Hydraulic/hydrogeological processes and conditions (in geosphere) (e.

groundwater flow, saturated/unsaturated flow, water table fluctua

conditions/variability, gradients, hydraulic properties/variability, salinity

(HAN 2.2.07, SRS 2.5.11, SRS 2.6.12, SRS 5.1.02, SRS 5.1.09, SRS 5.

Hydrological Regime and Water Balance (Near-Surface) (S

Aquifer Properties (including vadose and saturated zone thicknesses) 

Capillary rise (SRS 2.8.05)

Discharge Zones Within and Outside the Assessment Domain (including 

or remote areas) (SRS 2.8.07, SRS 2.8.08, SRS 5.3

Interfaces Between Different Waters (SRS 5.2.04

Chemically-Induced Density Effects on Groundwater Flow (S

Unconsolidated soft zones (SRS 2.5.07)

Hydrological Effects (CLV 17)

Flooding (CLV 19)

Chemical/geochemical 2.2.08 Properties of the Groundwater Plume (BMO 1.2.3.
C-35

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

processes and conditions 

(in geosphere) Chemical/geochemical processes and conditions (in geosphere) (e.g., 

and evolution; geochemistry factors, evolution and geochemical inter

recharge; geothermal effects, solubility controls) (HAN 2.2.

Evolving Water Chemistry in the Far-Field (e.g., perturbations due to clim

cause infiltration of sea-water or glacial melt waters) (SRS 

Complexation in the Natural System (SRS 3.5.10

Alteration and Chemical Weathering Along Flow Paths (SR

Biological/biochemical 

processes and conditions 

(in geosphere)

2.2.09

Biological/biochemical processes and conditions (in geosphere) (e.g., bi

generation of chelating agents, microbial species, influence on pH and 

changes in microbial populations) (HAN 2.2.09, CLV 

Thermal processes and 

conditions (in geosphere) 2.2.10

Thermal processes and conditions (in geosphere) (e.g., sources and dis

heat, geothermal gradient, temperature, and geothermal induced cha

fracture displacement, solubility changes, or changes in flow direction

(HAN 2.2.10, SRS 3.6.02)

Temperature and Thermal Gradient Effects on the Geosphere (temperat

of chemical and microbiological processes, stress field, groundwater flo

radionuclide transport) (SRS 3.6.07)

Gas sources and effects 

(in geosphere) 2.2.11

Gas sources and effects (in geosphere) (e.g., effects of natural gases, g

flow) (HAN 2.2.11)

Gas or brine pockets (CLV 6)

Undetected features (in 

geosphere) (e.g., 

boreholes, abandoned 

mines, gas or brine 

pockets, geological 

features [faults, dykes, 

lava tubes, breccia pipes, 

lava tubes])

2.2.12

Undetected features (in geosphere) (HAN 1.2.11, SRS 

Breccia pipes (CLV 37)

Lava tubes (CLV 47)

Gas intrusion (CLV 121)
C-36

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Geological resources 2.2.13

Geological resources (in geosphere) (e.g., methane, water, and other 

surface and deep deposit exploration) (HAN 2.2.12, CL

Natural and Geological Resources and Land Use (resources including w

minerals, geothermal energy; land use including reclamation, logging, a

and waste disposal) (SRS 2.2.01, CLV 79)

Hydraulic fracturing (CLV 44)

Resource extraction (CLV 89)

Topography and 

morphology 2.3.01 Topography and morphology (e.g., landform, changes in landform, topclimate) (HAN 2.3.01, SRS 2.5.02)

Soil and sediment 

(physical, chemical and 

biological properties; 

sedimentation; evolution)

2.3.02

Chemical changes (e.g., chemical changes to soil) (BMA 

Geosphere/Biosphere Interface (e.g., deep soil) (BMO 

Environmental Components (e.g., top soil, deep soil) (BMO 

Alkalinization (BMO 2.1.1.1.2.1)

Soil conversion (BMO 2.1.1.1.3.1)

Soil and sediment (composition, structure, profile, type, depth, and im

mobility) (HAN 2.3.03, SRS 2.5.10)

Radionuclide Accumulation (Recycling) in Soils (SRS 4

Sedimentation (CVL 14)

Aquifers and waterbearing features, near 

surface

2.3.03

Environmental Components (e.g., deep soil, biosphere aquifer) 

Aquifers and water-bearing features, near surface (including water tab

geometry [fractures], recharge and discharge zones, alkali flats, se

(HAN 2.3.04, SRS 2.5.06)

Lakes, rivers, streams 

and springs 2.3.04 Geosphere/Biosphere Interface (e.g., river) (BMO 1.

Environmental Components (e.g., river water, river sediment) (
C-37

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Terrestrial surface water bodies (lakes, dams, rivers, streams, springs, 

recharge/discharge zones) (HAN 2.3.05)

Surface water bodies (characteristics of surface-water bodies such as 

and springs, and their evolution in time) (SRS 2.8.02, C

Lake Effects (including appearance, disappearance of large lake le

sedimentation, wave action, erosion/inundation, isostasy) (CLV 

Wave Action (from large lake) (CLV 33)

Regional subsidence (effect on lake levels) (CLV 4

Coastal features 2.3.05 Coastal features and processes (HAN 2.3.06, CLV 

Marine features 2.3.06 Marine features and effects (HAN 2.3.07, CLV 29

Atmosphere (e.g., 

physical transport of 

gases, chemical and 

photochemical reactions, 

aerosols and dust)

2.3.07

Environmental Components (e.g., atmosphere) (BMO 1

Atmosphere (transport of contaminants in gas, vapor, or particulat

contamination of atmosphere may occur due to water evaporation, de

water, transpiration from plants, suspension due to wind erosion, 

(HAN 2.3.08, SRS 2.7.01)

Effects related to air and vapor flow and evaporation within the sys

Gas Transport (CLV 117)

Resuspension (CLV 23)

Atmospheric dispersion (CLV 24)

Tornado (CLV 25)

Vegetation 2.3.08 Vegetation (including such factors as vegetation type, contamination pro

root intrusion, evolution, and hydroponics) (HAN 2.3.09, SRS 2

Animal populations (e.g., 

diets, external 

contamination of)

2.3.09

Living components of ecosystems (BMA 2.2.3.2.1

Animal populations (including animal types, burrowing animals, scaveng

pets) (HAN 2.3.10, SRS 2.4.04)
C-38

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Meteorology 2.3.10

Identification and Characterization of Climate Categories (B

Differentiation of Climate Categories (BMO 1.3.1.1

Diurnal variability (BMA 2.3.1.2.1)

Meteorology (HAN 2.3.11, CLV 22)

Precipitation (control on the amount of runoff and infiltration, flow in the 

groundwater recharge) (SRS 2.7.03)

Events (e.g., flooding, tornados, hurricanes) (CLV 10, CLV 2

Insolation (CLV 28)

Hydrological regime and 

water balance (near 

surface)

2.3.11

Interannual and longer timescale variability (e.g., seasonal water t

(BMA 2.3.1.2.3)

Hydrological regime and water balance (near-surface) (HA

Water (characteristics of water and its evolution) (SRS 

Evapotranspiration, Surface Runoff, Infiltration and rec

(SRS 2.8.03, SRS 2.8.04, SRS 2.8.06, CLV 118)

Hydrological Effects (CLV 17)

Erosion and deposition 2.3.12

Physical changes (e.g., erosion, sea level change) (BMA 2.3.1.1.1

Erosion (BMA 2.3.1.4.2.10, BMO 2.1.1.1.3.2, CLV 

Erosion and deposition (HAN 2.3.13)

Erosion and weathering (SRS 2.6.08)

Deposition (SRS 2.6.07)

Mass Wasting (soil movement as a result of gravitational forces

Creeping of the Rock Mass (slow movement of the rock along pre-exist

the rock matrix due to differential stress fields; may affect hydraulic pr

(SRS 2.6.10)
C-39

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Subrosion (CLV 15)

Ecological/biological/micr

obial systems (e.g., 

features, microbial 

activity, chemical 

changes). The NEA 

database also has the 

following examples: fire, 

ecological succession.

2.3.13

Ecosystems (BMA 2.2.3.2)

Non-living components of ecosystems (BMA 2.2.3.2

Interannual and longer timescale variability (e.g., natural succession aft

Burning (BMO 2.1.2.1.3)

Chemical Changes Caused by Micro-Organisms (BMO 2.

Ecological/biological/microbial systems (HAN 2.3.1

Biomes (e.g., desert, grassland, forest, and mountain biomes) (HAN 

Insolation, effects of (CLV 28)

Animal/plant intrusion 

leading to vault/trench 

disruption (e.g., root 

uptake, burrowing 

animals)

2.3.14

Transport mediated by flora and fauna (e.g., root uptake, transpiration, 

fauna, bioturbation, burrowing, root development, translo

(BMA 2.3.1.3.1, BMO 2.1.2.7, CLV 109)

Animal/Plant intrusion (HAN 2.3.15, SRS 6.1.05)

Radioactive decay and ingrowth 3.1.01

Radioactive decay and in-growth (HAN 3.1.01, SRS 4.2.01

Radionuclide properties, other (HAN 3.1.02)

Chemical/organic toxin 

stability 3.1.02 Chemical/organic toxin stability (HAN 3.1.04)

Inorganic solids/solutes 3.1.03 Inorganic solids/solutes (HAN 3.1.05)

Volatiles and potential for 

volatility 3.1.04 Volatiles and potential for volatility (HAN 3.1.06)

Organics and potential for 

organic forms 3.1.05 Organics and potential for organic forms (HAN 3.1.0

Noble gases 3.1.06 Noble gases (HAN 3.1.07)

Dissolution, precipitation 3.2.01 Release Mechanisms (BMA 2,1.6.2, CLV 102)
C-40

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

and crystallisation, 

contaminant

Source Term Mechanisms (BMA 2.1.6.3)

Dissolution/precipitation (BMA 2.3.1.4.4.1, BMO 2.1.3.1.1, S

Chemical/geochemical-mediated processes, effects on contaminant rele

dissolution, precipitation, and crystallisation) (HAN 3.

Dissolution (CLV 115)

Speciation and solubility, 

contaminant 3.2.02

Source Term/Release Mechanisms (BMA 2,1.6.2, BMA 2.1.6

Chemical/geochemical-mediated processes, effects on contaminant rele

speciation and solubility) (HAN 3.2.02, CLV 9)

Contaminant Solubility, Solubility Limits, and Speciation (S

Reduction-Oxidation Potential (Redox Fronts) (SRS 4

Solubility and Sorption Changes From Chemical and Temperature Inte

Sorption/desorption 

processes, contaminant 3.2.03

Source Term/Release Mechanisms (BMA 2,1.6.2, BMA 2.1.6

Adsorption/desorption (BMA 2.3.1.4.4.2, BMO 2.1.3.1.2, SRS 5

Soil and sediment (e.g., sorptive capability) (HAN 2.3

Chemical/geochemical-mediated processes, effects on contaminant rele

sorption/desorption processes) (HAN 3.2.02, CLV 

Electrochemical Effects in the Closure System (Including Anion Excl

Solubility and Sorption Changes From Chemical and Temperature Inte

Radionuclide Interaction with Corrosion Products (SRS 

Colloids, contaminant 

interactions and transport 

with

3.2.04

Colloid formation (BMA 2.3.1.4.4.3)

Transport of Colloids (BMO 2.1.2.3.6.2)

Colloids mediated migration of contaminant (HAN 3.2.10, 
C-41

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Chemical/complexing 

agents, effects on 

contaminant 

speciation/transport

3.2.05

Source Term Content of Other Hazardous Materials 21 (BM

Chemical/geochemical processes and conditions (in geosphere) (e.g

complexing agents or complexing agents formed in the near-field) (H

Chelating Agent Effects (SRS 3.5.12)

Microbial/biological/plantmediated processes, 

contaminant

3.2.06 Microbial/biological-mediated processes, effects on contaminant rel

(HAN 3.2.03, SRS 2.4.02, CLV 54, CLV 102)

Water-mediated transport 

of contaminants 3.2.07

Water-borne transport (e.g., surface run-off, infiltration, percolation, mul

capillary rise, groundwater transport, discharge) (BMA 2

Geosphere Aquifer Discharge (BMO 1.2.2.1.1)

Porous Media Aqueous Transport Processes (e.g., infiltration, percola

diffusion/dispersion, dual flow systems, capillary rise, groundwa

(BMO 2.1.2.3, CLV 118)

Physical Processes (e.g., rainfall, snowfall, evaporation, evapotranspira

Surface Water Aqueous Transport Processes (e.g., surface water run-

bodies) (BMO 2.1.2.2, CLV 17, CLV 111)

Water-mediated migration of contaminants (including groundwater ad

dilution, imbibition, diffusion, multi-phase, unsaturated flow; surface wa

and sea spray)

(HAN 3.2.04, SRS 5.3.07, SRS 5.3.08, SRS 5.3.09, CLV 16, CLV 

Dilution of Radionuclides in Groundwater (SRS 4.3.

Contaminant Release Pathways (water-mediated release from infiltration

barriers or wasteform, and leaching) (HAN 3.2.01, CLV 118

Leaching (SRS 3.5.14)

Solid-mediated transport 

of contaminants 3.2.08 Solid-phase transport (e.g., landslides, rock fall, wash out, sedimentatisplash) (BMA 2.3.1.4.3, BMO 2.1.2.5, CLV 7)

 21 This FEP is included to the extent that chemical constituents impact the mobility of radionuclides. Non-radiological impac
C-42

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Solid-mediated migration of contaminants (including from erosion, denu

sediment migration, saltation, wet deposition, and mass 

(HAN 3.2.05, CLV 10, CLV 11, CLV 12)

Contaminant Release Pathways (e.g., solid-mediated release from intru

or animal action) (HAN 3.2.01, CLV 102)

Solid-Mediated Migration of Contaminants (SRS 5.3

Gas-mediated transport 

of contaminants 3.2.09

Gaseous Release (BMO 1.2.2.2)

Gaseous Transport (BMO 2.1.2.1.1.)

Gas-mediated migration of contaminants (including expelling groundwate

and multi-phase flow effects) (HAN 3.2.06)

Contaminant Release Pathways (gas-mediated release from barometr

controlled release of gas, and dissolution; gas release models, transpor

entry points, and monitoring may also be discussed) (HAN 3.2

Gas-Mediated Migration of Contaminants (SRS 5.3.

Atmospheric transport of 

contaminants 3.2.10

Atmospheric transport (e.g., gas, aerosols, evaporation, wet/dry depos

Aerosol Transport (BMO 2.1.2.1.2)

Wet Deposition (BMO 2.1.2.5.4.5)

Atmospheric migration of contaminants (including convection, diffusion, 

saltation, burning, showers and humidifiers) (HAN 3.2.07, 

Dust devils (CLV 116)

Animal, plant and 

microbe mediated 

transport of contaminants 

(e.g., microbial-enhanced 

contaminant transport)

3.2.11

Recycling of bulk solid materials (e.g., recycling materials for com

(BMA 2.3.2.3.5, BMO 2.2.3)

Fodder products (BMA 2.4.1.1.3)

Animal, plant and microbe mediated migration of contaminants (includ

external contamination, carcasses, root uptake, microbes, translocat

(HAN 3.2.08, SRS 4.4.04, CLV 54, CLV 111)
C-43

Table C-9 Generic FEPs List for Near-Surface Disposal in the United States (FEPs independent of 

exposure assumptions)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Radiological 

toxicity/effects 3.3.06 Radiological toxicity/effects (somatic, genetic, stochastic, non-stoch

Scope of FEP limited. See Table C-7 for more information on what scope can be excluded from the FEP.

Additional FEPs not included in the NEA international FEPs database.
C-44

Table C-10 Generic FEPs List for Near-Surface Disposal in the United States (FEPs related to human 

assumptions, etc.)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Future human action 

assumptions 0.05 Future Human Action Assumptions (e.g., present day technology, past 

(HAN 0.4.03, SRS 2.3.01, CLV 67)22

Future human behavior 

(target group) 

assumptions

0.06 Future Human Behavior (target group) Assumptions (HAN 0.4.

Human Behavior and Habits (Non-Diet Related) (SRS 2.1.03

Records and markers, 

facility 24 1.1.06

System of Records (HAN 0.2.03.09)

Loss/degradation of societal memory (HAN 1.4.02.0

Loss of archives/records (HAN 1.4.02.07)

Natural climate cycling 

(e.g. glaciation) 1.3.09 Longterm Human behavior response to climate changes (e.g., irrigation rate

Motivation and 

knowledge issues 25

(inadvertent human 

actions)

1.4.02

Motivation and knowledge issues (inadvertent intrusion) (HA

Inadvertent Intrusion (SRS 6.1.01, CLV 69)

Loss/degradation of societal memory (HAN 1.4.02.0

Loss of archives/records (HAN 1.4.02.07)

Future Knowledge of the Facility (loss of records) (SRS 

 22 The Hanford FEP includes three sub-categories of future human actions that are explicitly excluded in Table C-7: (i) cure 

malicious acts or acts of war, (iii) deliberate intrusion. If a cure for cancer is assumed, then future generations will not be p

level as current generations. Additionally, credit cannot be given for an assumed future mitigative action to meet regulatory 

regulations in 10 CFR Part 61 do not protect deliberate acts or advertent intrusion. In general, analysts should avoid unnec

about future human actions that tend to reduce potential dose impacts. See Table C-7 for additional details.

23 Use of dose estimates for the average member of the critical group, or that group of individuals reasonably expected to r

based on current or reasonably foreseeable (e.g., within the next 100 years) future practices, is an acceptable approach for 

This FEP should not include unsupported assumptions regarding future human species evolution or changes to radiosensit

additional details.

24 The ISAM FEP name was changed from “repository assumptions” to “facility assumptions” to avoid confusion related to t

25 The original ISAM FEP (and Hanford FEP) was named “Motivation and knowledge issues (inadvertent/deliberate human 

renamed “Motivation and knowledge issues (inadvertent human actions)”. This FEP does not include advertent or deliberat

disposal facility. See Table C-6 for additional details regarding exclusion of deliberate or advertent intrusion.
C-45

Table C-10 Generic FEPs List for Near-Surface Disposal in the United States (FEPs related to human 

assumptions, etc.)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

Igneous or Seismic Event Precedes Human Intrusion (making it difficult 

wasteform) (SRS 6.1.06)

Drilling activities (human 

intrusion) 1.4.03 Water extraction by pumping (BMA 2.3.2.2.2, BMO 2.

Water recharge by pumping (BMA 2.3.2.2.3)

Mining and other 

underground activities 

(human intrusion)

1.4.04

Drilling activities (e.g., exploration, studies, site characterization, explo

water-supply well, reuse of boreholes, waste disposal, remedial act

geothermal) (HAN 1.4.04, SRS 6.1.03, CLV 73, CLV 

Mining and other underground activities (e.g., mining, underground c

exploitation, exploration, excavation, solution mining, waste disposal, 

testing) (HAN 1.4.05, SRS 6.1.04, CLV 65, CLV 73, CL

Un-intrusive site 

investigation 1.4.05 Un-intrusive site investigation (HAN 1.4.06)

Surface excavations 1.4.06

Dredging (BMA 2.3.2.3.7, BMO 2.2.4.4.1)

Ploughing (BMA 2.3.2.3.1, BMO 2.2.3.1)

Earth Works (BMO 2.2.4.4.2)

Site Development 1.4.08

Land reclamation (BMA 2.3.2.2.5, BMO 2.2.2.3)

Dam building (BMA 2.3.2.2.4, BMO 2.2.2.2)

Construction (BMA 2.3.2.2.1)

Site Development (e.g., construction, road building, dam building, d

topography, change in land use) (HAN 1.4.08, SRS 3

Community development (e.g., establishment of residences) (CL

Land use (CLV 72)

Archaeology 1.4.09 Archaeology (HAN 1.4.09)

Water management 1.4.10 Well supply (BMA 2.3.2.3.2, BMO 2.2.4.1.1)
C-46

Table C-10 Generic FEPs List for Near-Surface Disposal in the United States (FEPs related to human 

assumptions, etc.)

IAEA ISAM FEP List

IAEA 

ISAM 

ID

LongTerm 

Example FEPs

(Project IDs)

BMA=BIOMASS, BMO=BIOMOVS, CLV=Cliv

HAN=Hanford, SRS=Savannah River Site

(wells, reservoirs, dams) Dam building (BMA 2.3.2.2.4)

Other water supply (BMA 2.3.2.3.3)

Irrigation (BMA 2.3.2.3.4, BMO 2.2.3.1.2, CLV 83

Geosphere/Biosphere Interface (e.g., biosphere aquifer with we

Environmental Components (e.g., well) (BMO 1.3.2

Water management (including construction of dams, reservoirs, canals, 

for flooding) (HAN 1.4.10, SRS 2.2.02, CLV 94)

Flooding or drainage system failure (SRS 6.4.02)

Social and institutional 

developments 26 1.4.11

Loss/degradation of societal memory (HAN 1.4.02.0

Loss of archives/records (HAN 1.4.02.07)

Social and Institutional Developments (loss of records or society me

Technological 

developments 27 1.4.12 Technological development assumptions (e.g., including retrograde d

technological development) (HAN 1.4.14, SRS 2.3.04, SR

Remedial actions 1.4.13 Remedial actions (e.g., remediation, additional engineered barrier const

(HAN 1.4.13, SRS 3.8.05)

Explosions and crashes 28 1.4.14 Longterm

Explosions and crashes (e.g., accidental human acti

(HAN 1.4.11, SRS 6.4.06, CLV 66, CLV 108)

Scope of FEP is limited. See Table C-7 for more information on what scope can be excluded from the FEP.

 26 This FEP is limited in scope to include loss of records and societal memory. Other social and institutional developments 

they are difficult to predict. See Table C-7 for additional details.

27 Several aspects of this FEP are excluded from consideration as they are difficult to predict and may be considered specu

for cancer. See Table C-7 for additional details.

28 Acts of war, terrorism, and sabotage may be excluded.
C-47

REFERENCES

BIOMOVS, 1996. BIOMOVS II, Technical Report 6, Development of a Reference Biospheres 

Methodology for Radioactive Waste Disposal, Final Report of the Reference Biospheres 

Working Group of the BIOMOVS II Study, September 1996.

Arlt, 2013. Arlt, Hans, “Note to File: Hanford Site Waste Management Area C Features, Events, 

and Processes List,” March 8, 2013, ADAMS Accession No. ML13066A730.

IAEA, 2003. International Atomic Energy Agency, "Reference Biospheres for Solid Radioactive 

Waste Disposal: Report of BIOMASS Theme 1 of the BIOsphere Modelling and ASSessment 

(BIOMASS) Programme," BIOMASS, IAEA-BIOMASS-6, Vienna, July 2003.

Neptune, 2011. Neptune and Company, Inc., FEP Analysis for Disposal of Depleted Uranium at 

the Clive Facility, May 2011.

NEA, 2000. Nuclear Energy Agengy, Features, Events, and Processes (FEPs) for Geologic 

Disposal of Radioactive Waste, An International Database, Radioactive Waste Management, 

OECD (Organisation for Economic Co-operation and Development), AEN NEA, 2000.

NEA, 2006. Nuclear Energy Agengy, The NEA International FEP Database : Version 2.1 User 

Notes, Nuclear Energy Agency, Safety Assessment Management Ltd., http://www.oecdnea.org/rwm/igsc_coprojectactivities.html, 2006.

SRS, 2012a. U.S. Department of Energy Savannah River Site, Features, Events, and 

Processes for Liquid Waste Performance Assessments, SRR-CWDA-2012-00011, Rev. 0, 

February 2012.

SRS, 2012b. U.S. Department of Energy Savannah River Site, Evaluation of Features, Events, 

and Processes in the F-Area Tank Farm Performance Assessment, SRR-CWDA-2012-00022, 

Rev. 0, February 2012.

D-1

1 APPENDIX D

2 ADDITIONAL APPROACHES TO CONSTRUCT SCENARIOS AND 

3 CONCEPTUAL MODELS

4

5 This appendix presents techniques for constructing scenarios and conceptual models. Many of 

6 the techniques discussed may be from non-LLW examples (NEA, 1992; IAEA, 2004; NRC, 

7 1995; SKB, 2008); however, the NRC staff believes the techniques and associated references

8 could be of value to a licensee who is looking for possible approaches to construct alternative 

9 scenarios and alternate conceptual models. Licensees have the flexibility to use any 

10 documentable technique and are not required to use the techniques presented in this appendix. 

11

12 The techniques presented here can provide a logical structure for the comprehensive 

13 documentation of the relevant processes and their representation in models or scenarios. The 

14 techniques are not mutually exclusive and may be used in combination. Whatever techniques 

15 are used, the judgment of performance assessment modelers and scientific subject experts is 

16 important to successfully completing this portion of the performance assessment (PA) process. 

17

18 Event tree analyses 

19

20 Event trees are one of the oldest techniques used to assess the operational safety of nuclear 

21 reactors. Probabilities can be systematically treated but the variations are mainly binary, (fault -

22 no fault). This method describes system behavior as an event or series of events leading to 

23 system failure or loss of function. Application of the technique yields a number of combinations 

24 of basic events whose occurrence causes system failure or loss of function. These event

25 combinations are then evaluated by various screening techniques to determine high risk 

26 scenarios. This technique is not used extensively in the context of radioactive waste disposal 

27 for several reasons including: (1) most processes are generally slow and difficult to define as 

28 abrupt events, and (2) the tree methods are not suitable to handle interaction and feedback 

29 between features, events, and processes (FEPs). In addition, the sheer number of possible 

30 combinations in an event tree can very rapidly become unmanageable. 

31

32 Logic diagrams 

33

34 Another technique to assist in scenario development involves the use of logic diagrams. The 

35 development of scenarios by taking combinations of the various release and transport 

36 phenomena is illustrated by the following example in Figure D-1 (NRC, 1990). Two release 

37 phenomena (R1, R2) and three transport phenomena (T1, T2, T3) create 32 possible 

38 combinations or scenarios. The use of a logic diagram, as illustrated in Figure D-1, ensures that 

39 all possible combinations of these phenomena are identified. The central scenario represents 

40 the initial conceptualization of the disposal system. All components of the engineered barrier 

41 system are assumed to perform as designed. The other scenarios are perturbations to these 

42 central conditions (e.g., assuming less than 100 percent barrier performance). 

43
D-2

1

2

3 Figure D-1 Potential combinations of two release and three transport phenomena 

4

5 In another example, alternative scenarios were characterized as discrete events (NRC, 2007). 

6 Climate change, floods, and introduction of irrigated agriculture are examples of discrete events 

7 affecting the hydrologic conditions at a site. These events are often not mutually exclusive (e.g., 

8 the occurrence of irrigated agriculture does not preclude the occurrence of climate change). By 

9 defining scenarios as possible combinations of alternative events, the scenarios can be made 

10 mutually exclusive. An example for three events is shown in Figure D-2. A “1” in the figure

11 signifies the occurrence of the event in a scenario and a “0” indicates the absence of that event. 

12 Scenario 1 in Figure D-2 has none of the events occurring and might be referred to as a central 

13 scenario, perhaps characterized by the continuation of current hydrologic conditions into the 

future. For n events, this procedure will result in 2n 14 scenarios; some of these scenarios may be 

15 discarded because of an insignificant probability or because they are not of regulatory concern. 

16 Scenario development for LLW disposal is typically qualitative; exact probability values for 

17 events characterizing scenarios will usually not be available. Qualitative science-based 

18 appraisals, such as large, medium, and small, are workable substitutes. Numerical equivalents 

19 for high, medium, and low (e.g., 0.75, 0.5, and 0.25, respectively) could be used to derive 

20 estimates on the likelihood of a scenario. 
D-3

1

2

3 Figure D-2 Example formulation of mutually exclusive scenarios from three 

4 scenario-characterizing events 

5

6 Interaction matrices

7

8 The interaction matrix methodology starts with a top-down approach to dividing the system into 

9 constituent parts. The main components are identified and listed in the leading diagonal 

10 elements of the matrix. The interactions between the leading diagonal elements are then noted 

11 in the off-diagonal elements. The convention is to allocate off-diagonal elements in the direction 

12 of contaminant migration (see Table D-1 and Figure D-3). This allows FEP interactions and 

13 pathways to be mapped, which is an important step in developing and defining a conceptual 

14 model. Moreover, the systematic process of examining how the system components relate to 

15 one another may help to identify new, previously unrecognized relevant characteristics of the 

16 system. 

17

18 When using a reference list of FEPs for populating the matrix, some processes may not be 

19 allocated to any of the defined off-diagonal elements. In this case it might be necessary to 

20 subdivide some of the leading diagonal elements in particular if these processes are considered 

21 important. Introducing more divisions of the leading diagonal elements should result in a more 

22 detailed matrix and associated conceptual model (Avila, 2012). Table D-1 provides an example 

23 of a simple interaction matrix. 

24

25
D-4

1 Table D-1 Example of a simple interaction matrix

Component A LEADING DIAGONAL ELEMENT

1,1 

Influence of A on B

 OFF-DIAGONAL ELEMENT

1,2 

Influence of B on A

 OFF-DIAGONAL ELEMENT

2,1 

Component B LEADING DIAGONAL ELEMENT

2,2 

2

3 IAEA (2004) describes this method in more detail (also see Figure D-3). The first step of the 

4 procedure of constructing an interaction matrix is to identify and define its diagonal elements. 

5 This is done by exploring how the state of the system can be described in terms of physical 

6 components and spatial and temporal extension of the system. Usually, the diagonal elements 

7 represent system state variables (such as chemical composition of water or rock stress) and the 

8 off-diagonal elements represent processes affecting the state. For example, a typical diagonal 

9 element could be “groundwater composition,” which in turn can be divided into concentrations of 

10 various constituents, colloid content, and other components. 

11

12 As the diagonal elements of the matrix are filled in with features or components, the interactions 

13 between them are identified and introduced into the appropriate off-diagonal elements of the 

14 matrix. All interactions should be binary (i.e. they should be direct interactions between 

15 variables in two diagonal elements and not a path via a variable in a third diagonal element). 

16 For interactions described in off-diagonal elements, the variable in the diagonal element on the 

17 row should affect the variable in the diagonal element on the column. Each off-diagonal 

18 element should be checked for interactions, and where no interaction is found marked “none.” 

19

20 In the process of identifying interaction, plausible interacting mechanisms are first considered 

21 without making any evaluation of the probability of occurrence or the significance of the effect. 

22 Only fully irrelevant or totally unreasonable interactions should be discarded. For each off23 diagonal element the question is asked whether there are any potential events or processes that 

24 are affecting any of the variables assigned to the target diagonal element (found on the column) 

25 and at the same time are affected by any of the variables in the source diagonal element (found 

26 on the row). If the answer was yes, a short description of the interaction should be documented 

27 together with the variables in the two diagonal elements that are involved in the interaction. 

28

29 In addition to the interaction matrix example, IAEA (2004) also contains Appendix B, titled 

30 “Generation of Scenarios for Near Surface Disposal Systems”.

31

32

33
D-5

1

2

3 Figure D-3 Example of an interaction matrix for a central scenario including the 

4 bathtub effect (IAEA, 2004) 

5

6 Influence diagrams 

7

8 Influence diagrams are one of several methods to systematically evaluate and visualize FEPs 

9 that influence the disposal facility performance. The aim with these methods is to systematically 

10 identify and review all FEPs interactions and combinations that can influence the performance 

11 of the disposal system. Each process is dealt with in detail and described as an influence 

12 between features or parameters (nodes) in the system. As the number of nodes quickly rises, 

13 these influence diagrams tend to become complicated, but are usually still helpful.

14

15 In the influence diagram approach, the direction of the influence or the interactions between 

16 FEPs is shown by the use of arrows: one arrow per direct influence and one box per FEP. An 

17 example influence diagram is given by Chapman et al. (1995) for the deep repository 

18 performance assessment project in Sweden (see Figure D-4). 
D-6

1

2

3 Figure D-4 Example of an influence diagram for proposed repository PA in Sweden. 

4 Many of the specific phenomena reflected in this diagram are not expected 

5 be relevant to LLW (Chapman et al., 1995)

6

7
D-7

1 The main steps to building the influence diagram can be summarized as follows (IAEA, 2004): 

2

3 • Definition of the system barriers and selection of FEPs relevant to the defined system. 

4 The FEPs can be sorted into FEPs belonging to the system and those external scenario 

5 initiating FEPs to the system. 

6 • Representation of the system FEPs in boxes. If a FEP is relevant for several disposal 

7 components, then it should be represented by one box for each of the disposal 

8 components.

9 • Identification and representation of the influences between selected FEPs. Each 

10 influence in the diagram is marked with a unique code. There are no restrictions on the 

11 number of influences between two FEPs.

12 • Documentation of FEPs and influences. A more comprehensive description of each 

13 FEP and influence is needed to clarify the representation.

14

15 The influence diagram should not include large disruptive events that would alter the system 

16 features since this would produce a separate, alternative scenario and would require a separate 

17 conceptual model of its own. For the influence diagram approach, process to process 

18 influences should be avoided, since it is assumed that the processes do not influence each 

19 other directly. Influences of this type should be broken in feature-process or process-feature 

20 influences. The development of influence diagrams is an iterative process. Two FEPs may be 

21 combined into one and one FEP may be split into several FEPs in order to obtain an improved 

22 representation of the system. New influences between FEPs can be identified and classified 

23 using a significance scale. This can be used to build a reduced influence diagram by removing 

24 the influences with a lower significance than a defined level. Thereafter, various influence 

25 diagrams can be developed based on the significance of the FEPs. 

26

27 Judgmental approaches 

28

29 Scenario formation can be made in several ways and human judgment is an important element 

30 of scenario formation. After systematic screening, the number of retained FEPs is normally too 

31 large to allow any detailed consideration of all possible combinations of them. There are, 

32 however, scenario formation procedures that allow the judgment and knowledge of qualified 

33 specialists to be integrated with quantitative considerations so that they result in a manageable 

34 number of representative scenarios. 

35

36 In the judgmental method, the assessment team or invited experts select the phenomena or 

37 conditions that they believe are most important, and define possible release situations. A list of 

38 phenomena or FEPs can be used as a starting point. Documentation and transparency should 

39 not be lacking for the sake of expediency; formal and rigorous scenario development 

40 procedures are encouraged. Documents of this procedure should be sufficiently detailed to 

41 withstand detailed scrutiny by the regulatory authorities and public. Comprehensiveness and 

42 traceability in both the assessment and documentation is very important when using a 

43 judgmental approach. 

44
D-8

1 REFERENCES

2

3 Avila, 2012. Avila, R., "Conceptual Model Development: Approaches, Assumptions and Level of 

4 Detail," Presentation for the "Workshop on Performance Assessments of Near-Surface Disposal 

5 Facilities: FEPs Analysis, Scenario and Conceptual Model Development, and Code Selection," 

6 U.S. Nuclear Regulatory Commission, Rockville, MD, August 29 –30, 2012. 

7

8 Chapman et al., 1995. Chapman N.A, J. Andersson, P.C. Robinson, K. Skagius, C-O. Wene, 

9 M. Wiborgh and S. Wingefors, "Systems Analysis, Scenario Construction and Consequence 

10 Analysis: Definition for SITE-94," Swedish Nuclear Power Inspectorate Report No. 95:26, 

11 Stockholm, Sweden, 1995.

12

13 IAEA, 2004. International Atomic Energy Agency, "Safety Assessment Methodologies for Near 

14 Surface Disposal Facilities (ISAM), Vol. I – Review and Enhancement of Safety Assessment 

15 Approaches and Tools; Vol. II – Tests Cases," Vienna, Austria, 2004. 

16

17 NEA, 1992. Nuclear Energy Agency, "Safety Assessment of Radioactive Waste Repositories: 

18 Systematic Approaches to Scenario Development." "Report of the NEA Working Group on the 

19 Identification and Selection of Scenarios for the Safety Assessment of Radioactive Waste 

20 Disposal." OECD Nuclear Energy Agency, Paris, 1992.

21

22 NRC, 1990. U.S. Nuclear Regulatory Commission, "Risk Methodology for Geologic Disposal of 

23 Radioactive Waste: Scenario Selection Procedure, “NUREG/CR-1667, (SAND80-1429), 

24 Washington, D.C. April 1990. https://ntrlr3.ntis.gov/view.php?pid=NTIS:NUREGCR1667

25

26 NRC, 1995. U.S. Nuclear Regulatory Commission, “Review of Scenario Selection Approaches 

27 for Performance Assessment of High-Level Waste Repositories and Related Issues,” 

28 NUREG/CR-6351, (CNWRA 94-002), Washington, D.C, August 1995, ADAMS Accession No. 

29 ML040080567.

30 

31 NRC, 2007. U.S. Nuclear Regulatory Commission, NUREG/CR-6940, (PNNL-16396), 

32 "Combined Estimation of Hydrogeologic Conceptual Model, Parameter, and Scenario 

33 Uncertainty with Application to Uranium Transport at the Hanford Site 300 Area," NRC: 

34 Washington, D.C. July 2007. https://ntrlr3.ntis.gov/view.php?pid=NTIS:NUREGCR6940

35

36 SKB, 2008. Svensk Kärnbränslehantering (SKB), "Project SFR 1 SAR-08: Update of priority of 

37 FEPs from Project SAFE," SKB Rapport R-08-12, ISSN 1402-3091, Stockholm, March 2008. 

38

39

40
E-1

1 APPENDIX E

2 SITE STABILITY ASSESSMENT EXAMPLES

3

4 As discussed in Section 5.0, a licensee may use a design-based, model-based, or a combined 

5 approach to site stability assessment. The combined approach incorporates elements of both 

6 the design-based approach and the model-based approach. The licensee may use modeling to 

7 assess and improve a design. In addition, they can use a risk-based approach to the site 

8 stability assessment; if modeling demonstrates that the risks are not acceptable, the licensee 

9 can modify the disposal site design to mitigate the risk. 

10

11 The NRC staff selected the West Valley Demonstration Project (WVDP) site in this appendix as 

12 an example of a model-based approach because it is presently the only example of using 

13 landform evolution modeling to evaluate site stability in an NRC-regulated or NRC-reviewed 

14 waste disposal or site decommissioning application. By use of the WVDP site example, the 

15 NRC is not approving the approach. The Department of Energy (DOE) has not yet completed 

16 its work, nor has it selected its approach to the final decommissioning of the WVDP site. 

17 However, the NRC staff believes much of the content (modeling, data collection, development of 

18 analogs) for the WVDP site is reasonably representative of what may be expected for a LLW

19 disposal facility, and therefore, may be useful to a 10 CFR Part 61 licensee.

20

21 This appendix also provides an example of a design-based approach from the Moab, UT 

22 uranium mill tailings disposal site. A licensee of a LLW disposal facility can use the selected 

23 examples to develop an understanding of the type of information and actions involved in using 

24 the different approaches. A licensee should not use the information in these examples to justify 

25 site-specific stability assessments for LLW disposal. 

26
E-2

1 Model-Based Approach: West Valley Erosion Modeling

2

3 Background

4

5 West Valley is a complex decommissioning site located in western New York State, about 50 

6 km (30 miles) south of Buffalo (see Figure E-1). The New York State Energy Research and 

Development Authority (NYSERDA) holds the license and title to the 3300 acre (13.5 km2 7 ) 

8 Western New York Nuclear Service Center (WNYNSC), originally developed as the first and 

9 only commercial spent fuel reprocessing plant to operate in the United States. The 1980 West 

10 Valley Demonstration Project (WVDP) Act gave the United States Department of Energy (DOE) 

11 exclusive possession of a 200-acre portion of the larger WNYNSC which includes the former 

12 reprocessing facility, a land disposal facility, and HLW tanks to allow DOE to carry out a number 

13 of activities, most notably, the solidification of high-level waste that had been generated as a 

14 result of spent fuel reprocessing. This 200-acre portion of the WNYNSC is referred to as the 

15 WVDP, or project premises. In conjunction with NYSERDA, DOE issued a draft environmental 

16 impact statement (DEIS) in 2008 evaluating various alternatives to decommissioning and long17 term stewardship. DOE and NYSERDA selected the Phased Decision-making alternative as 

18 the preferred alternative in the final environmental impact statement (FEIS) issued in 2010

19 (DOE, 2010). Under the Phased Decision-making alternative, decommissioning would proceed 

20 in two phases. Phase 1 involves near-term decommissioning work (e.g., removal of the main 

21 plant process building and contaminated subsurface soils) and studies that could facilitate future 

22 decision-making for the remaining facilities and areas (see Figure E-1). A phased approach 

23 would allow additional time for technical and programmatic uncertainties to be addressed prior 

24 to making a final decision regarding decommissioning of the site. Consistent with the Phase 

25 Decision-making alternative, DOE finalized the Phase 1 Decommissioning Plan for West Valley 

26 in 2009 (DOE, 2009). NRC reviewed the plan and found it to be generally acceptable (NRC, 

27 2010). 

28

29 Decommissioning Site Characteristics

30

31 WVDP is located on the west shoulder of a steep-sided, glacially-scoured bedrock valley that is 

32 filled with a sequence of glacial sediments. These glacial deposits are comprised primarily of 

33 clays and silts separated by coarser-grained layers created during periods of glacial retreat. 

34 The site is bordered by two streams, Franks Creek to the east and Quarry Creek to the north. 

35 The WVDP is bisected by Erdmann Brook that divides the site into the North Plateau and South 

36 Plateau. Franks Creek is a tributary of Buttermilk Creek. Figure E-2 below shows major site 

37 facilities and features, as well as source areas to be addressed in Phase 1 and 2 

38 decommissioning. Figure E-3 below shows site streams and a portion of the Buttermilk Creek 

39 watershed.

40
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1

2

3 Figure E-1 Location of West Valley Site. 

4 Image Credit: Figure F-2 (DOE, 2010)
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1

2

3 Figure E-2 Layout of West Valley Site and Source Areas to Be Addressed in Phase 1/2 

4 Areas to be addressed in Phase 1 are marked with a symbol. Areas to be 

5 addressed in Phase 2 are marked with a symbol. Image Credit: Figure ES-5 

6 (DOE, 2009)

7

8
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1

2 Figure E-3 West Valley Site Streams and Buttermilk Creek

3 Image Credit: Slide 3, Page 23, “Erosion Working Group Recommendations for 

4 Phase 1 Studies”, August 22, 2012, Agenda/Presentation at 

5 http://westvalleyphaseonestudies.org/index.php/public-meeting.

6

7 As a result of site operations, WNYNSC and project premises soils, groundwater, and surface 

8 water/sediments are radiologically contaminated. Contamination includes what is referred to as

9 the North Plateau Groundwater Plume that is characterized by high concentrations of relatively 

10 mobile and short-lived Sr-90 (see Figure E-2). The North Plateau Groundwater Plume resulted 

11 from the accidental leak of radioactive nitric acid recovered from spent fuel reprocessing 

12 operations that traveled through a floor expansion joint into soils beneath the southwest corner 

13 of the Main Plant Process Building in 1968. DOE is in the process of remediating the North 

14 Plateau Groundwater Plume including the recent installation of a permeable reactive barrier to 

15 remove Sr-90 from WVDP groundwater prior to its seepage or discharge to surface water. In 

16 Phase 2, DOE and NYSERDA must also make decisions related to closure of four tanks used to 

17 store liquid high-level waste and two radioactive waste disposal facilities, as well as final 

18 decisions regarding clean-up of radiologically contaminated areas on-site (see Figure E-2). 

19

20 Decisions regarding the amount of residual radioactivity that may safely remain in contaminated 

21 surface and subsurface soils, and sediments, high-level waste tanks, treatment lagoons, and 

22 disposal areas is complicated by several technical and programmatic uncertainties that must be 

23 addressed prior to making a final decision regarding the disposition of the site. For example, 

24 one of the key technical uncertainties is the expected evolution of the landscape of the actively 

25 eroding site and the performance of engineered barriers used to minimize or mitigate the 

26

Quarry Creek

Erdmann Brook

Buttermilk 

Creek

Franks Creek
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1 release of residual radioactivity to the environment. Phase 1 studies are proposed to address 

2 these technical uncertainties as described below.

3

4 Key Site Stability Technical Issue: Erosion

5

6 As stated above, erosion is a key technical issue affecting West Valley site stability. Major 

7 erosion processes affecting WNYNSC, including the WVDP, include stream channel 

8 downcutting, stream valley rim widening, gully advance, and in disturbed areas, sheet and rill 

9 erosion. Figure E-4 shows site erosion features, while Figure E-5 shows larger Buttermilk 

10 Creek watershed erosion features. Development of the current topography and stream 

11 drainage patterns began with the glaciation and retreat process that ended approximately 

12 17,000 years ago. Erosion processes have affected site topography due to gravitational forces 

13 and water flow within the Buttermilk Creek watershed. Buttermilk Creek flows in a northwesterly 

14 direction along the central axis of the WNYNSC at an elevation of approximately 60 meters 

15 below the plateau on which the WVDP is located (see Figure E-1 and E-3). At WVDP, Franks 

16 Creek flows along the eastern boundary and drains to Buttermilk Creek. Franks Creek 

17 downcutting rates reflect base level lowering of Buttermilk Creek at the confluence of Franks 

18 Creek and Buttermilk Creek. Buttermilk Creek downcuttting rates are, in turn, affected by base 

19 level lowering of Cattaraugus Creek at the confluence of Buttermilk and Cattaraugus Creeks.

20

Erdmann Brook Knickpoint Franks Creek Knickpoint

WVDP Site Gully

21 Figure E-4 WVDP Erosion Features

22
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Buttermilk Creek Landslide

1 Figure E-5 Buttermilk Creek Watershed Erosion Features

2

3 The FEIS (DOE, 2010) includes Channel Hillslope Integrated Landscape Development (CHILD) 

4 long-term erosion modeling predictions used to evaluate the impact of erosion on site 

5 performance and the ability of various alternative end states to meet decommissioning criteria. 

6 The FEIS also provides details regarding the results of numerous other studies (e.g., erosion 

7 frame measurements, age dating of terraces to estimate stream down-cutting and stream valley 

8 rim-widening rates, aerial photography comparisons to estimate gully migration, and various 

9 short-term modeling exercises) that also help to evaluate the reasonableness of CHILD 

10 modeling predictions. However, the CHILD model was not used in earlier EIS analyses. Years 

11 of erosion work culminated in the selection of CHILD as the primary tool used by DOE in the 

12 FEIS and recommended by a DOE and NYSERDA-convened erosion working group (EWG) 

13 that made recommendations for Phase 1 studies to address the long-term effects of erosion at 

14 the site. The WVDP EWG indicated that CHILD is “the current state-of-the-art of predictive 

15 numerical landscape evolution models and embodies significant advantages and refinements 

16 compared with other generally-accepted numerical models” (2012a). 

17

18 West Valley CHILD Model

19

20 The FEIS CHILD model domain consists of the Buttermilk Creek watershed area (see Figure 

21 E-6). Prior to predictive modeling, the CHILD model was calibrated against site data. 

22 Calibration modeling simulations begin approximately 17,000 years in the past using remnant 

23 terrace elevations and valley slope data to estimate the post-glacial (pre-incised) Buttermilk 

24 Creek watershed surface. To minimize the “butterfly effect” in which small perturbations in the 

25 initial conditions lead to notable differences in simulated drainage patterns, the existing drainage 

26 network was etched on the initial topography. Boundary conditions included the elevation 

27 history of Cattaraugus Valley at the outlet of Buttermilk Creek. As a modeling simplification, and 

28 in the absence of more detailed information about climate variation over the past 17,000 years, 

29 a constant climate was assumed. Three primary material types were selected including (i) 

30 Paleozoic bedrock, (ii) thick but unlithified glacial sediments, and (iii) shallow surface 

31 soils/sediments. Five discrete values that reflect the range of each parameter value were 

32
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1 assigned based on site-specific measurements (e.g., water balance data), site-specific material 

2 correlations (e.g., detachment capacity related parameters), values taken from the literature for 

3 similar or analog sites (e.g., creep coefficients), or commonly accepted values (e.g., critical 

4 slope parameter). Selected calibration metrics reflect characteristics of the present-day 

5 Buttermilk Creek watershed including the (i) creek longitudinal profile, (ii) hypsometric curve 

6 (area below a certain elevation), (iii) slope-area diagram (gradient versus upstream contributing 

7 areas), (iv) width function (frequency distribution of catchment flow-path length), (v) cumulative 

8 area distribution (rate of flow aggregation), and (vi) strath terrace positions (pass/fail terrace 

9 elevation criterion considering measurement uncertainty). 

10

11 Monte Carlo methods were used in the calibration process. One thousand sets of parameter 

12 values were generated by sampling the discrete parameter distributions. These 1000 sets of 

13 parameter values were run through the Buttermilk Creek watershed model described above. 

14 The results of 1000 simulations were evaluated against the calibration metrics. The scores for 

15 the first five calibration metrics were normalized to allow equal weighting of each calibration 

16 metric for subsequent averaging. Four final calibration criteria were established to select the 

17 most likely sets of parameters to be used in forward modeling projections: (i) the total average, 

18 normalized score must be greater than 0.5 (considering the first five calibration metrics listed 

19 above), (ii) the longitudinal profile score, by itself, must be greater than 0.7, (iii) the intermediate 

20 strath terrace elevation metrics should be met within a given tolerance and within a given time 

21 span, and (iv) visual agreement between the model simulation results and the current 

22 topography should be achieved. Only 5 runs passed the final four calibration criteria with the 5 

23 sets of parameter values subsequently used in forward modeling projections to predict future 

24 erosion at WVDP. 

25

26 The observed topography and “best-fit” model run topography are presented in Figure E-6. 

27 Sensitivity runs were also conducted that considered (i) wetter climates and less permeable 

28 soils, and (ii) wetter climate parameters with fast creep (for the South Plateau only). For the 

29 forward modeling projections, a digital elevation model of the current topography with a 10 m 

30 resolution was used. Additionally, a second set of model simulations were run to estimate 

31 erosion for the Sitewide-Close-In-Place alternative in which three burial mounds would remain 

32 on the North and South Plateaus. The modeling grid was refined in the area of the North 

33 Plateau and South Plateau with a grid resolution of approximately 3 m to facilitate simulation of 

34 smaller scale erosion features such as gullies. Due to the computation demands of a finer grid 

35 resolution, only one of the plateaus at the finer grid resolution could be modeled at a time, 

36 leading two separate models reflecting the mesh refinements on the North and South Plateau. 

37 With respect to boundary conditions, the final base level lowering rate from the corresponding 

38 calibration run was applied at the outlet of Buttermilk Creek. The results of the 26 modeling 

39 simulations were presented and discussed in the FEIS. The FEIS modeling results provided a 

40 reasonable approach to evaluating erosion impacts at the WVDP. However, DOE and 

41 NYSERDA acknowledged limitations of the modeling approach and recognized several areas 

42 where additional information could be collected to improve and refine erosion predictions for 

43 future Phase 2 decision-making.

44
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1

2 Figure E-6 Observed (left) and “Best-fit” (right) Model Run Topography 

3 Image Credit: Figure F-8 (DOE, 2010)

4

5 Potential Additional Data Collection Efforts

6

7 DOE and NYSERDA commissioned an erosion working group to develop recommendations for 

8 studies that could be conducted during Phase 1 to further reduce uncertainty associated with 

9 the impact of erosion on site performance. The purpose of the additional studies would be to (i) 

10 fill data gaps, (ii) produce converging lines of evidence, (iii) improve scientific defensibility, and 

11 (iv) strengthen confidence in long-term erosion projections. DOE and NYSERDA are in the 

12 process of evaluating the recommendations and will determine which Phase 1 studies will be 

13 sponsored to provide additional support for erosion modeling predictions. A description of 

14 recommended studies follows. 

15

16 Phase 1 studies may include the following (EWG, 2012b):

17

18 1. Terrain Analysis

19

20 The purpose of terrain analysis would be to build on previous work cited in the FEIS to better 

21 understand the post-glacial geomorphic history of the site and larger Buttermilk Creek 

22 watershed. This would provide calibration information for the numerical model and constrain 

23 important modeling parameters. For example, the geomorphic evolution of the Buttermilk Creek 

24 watershed is likely more complicated than reflected in simplified calculations of stream 

25 downcutting and valley rim widening rates based on limited age data. Stream downcutting may 

26 be slowing over time due to a slowing of glacio-isostatic rebound since recession of the 

27 Laurentide ice sheet. A better understanding of the geomorphic history of Buttermilk Creek may 

28 enable better definition of critical parameter values used for erosion modeling. The terrain 

29 analysis could include the following:
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1

2 • Identification of elementary landforms or “land elements” using ArcMap (see Figure 

3 E-7a)

4 • Construction of geomorphic (land element) maps of WVDP, Buttermilk Creek, and 

5 potentially a companion basin site

6 • Performance of field reconnaissance to justify and verify potentials of mapped land 

7 elements

8 • Evaluation of available materials for age dating

9 • Development of a conceptual framework for geomorphic history of Buttermilk Creek and 

10 its base level

11 2. Age Dating and Paleoclimate

12

13 The purpose of age dating and paleoclimate study would be to provide additional age data to 

14 better define and constrain past rates of stream downcutting and valley rim widening for the 

15 WVDP, Buttermilk Creek watershed, and potential companion drainages; and to better 

16 understand post-glacial climate cycles and their effects on erosion processes. Data collected in 

17 this study may help constrain key modeling parameters or boundary conditions, such as the 

18 base-level history for Buttermilk Creek. The study would also provide data to improve model 

19 calibration or constrain parameter ranges. The Age Dating and Paleoclimate study could

20 include the following:

21

22 • Excavation or examination of mapped “land elements” for age dating (Figure E-7b)

23 • Examination of landslide toes in channel walls or tributary gullies for buried debris to 

24 determine timing of landslide activity (landsliding mapping activities are illustrated in 

25 Figure E-7c)

26 • Coring of tree rings to determine times of tree deformation from landslide movements 

27 and for locate climate proxy (drought)

28 • Dating of post-glacial erosional and depositional features

29 • Evaluation of age data for possible correlation with Late Wisconsin glacial or postglacial 

30 climatic events

31 3. Recent Erosion and Depositional Processes

32

33 The purpose of the recent erosion and depositional process study would be to better quantify 

34 and characterize recent rates of surface and near-surface erosion and temporary sediment 

35 storage occurring on hillslopes, in regions of concentrated flow, and in stream channels at and 

36 near the facility. The scope of previous studies and measurements would be expanded to 

37 obtain more useful and complete information to inform erosion predictions at the site. This 

38 study would also collect data at a finer spatial and temporal scale than represented by the 

39 efforts in proposed studies 1 and 2. Areas of initial focus would include the two licensed 

40 disposal areas, the rim of the North Plateau, and potentially in Buttermilk Creek watershed. Key 

41 attributes of this study would include the following:

42

43 • Hillslope stability including characterization of rates and mechanisms of mass-wasting 

44 and landsliding
E-11

1 • Rill and gully characterization including the mapping of locations and a determination of 

2 the erodibility and erosivity of concentrated flow channels of critical concern, monitoring 

3 of flow and sediment transport (if possible)

4 • Stream characterization including monitoring of flow and sediment transport (if possible), 

5 assessment of knickpoint development and migration, and channel evolution (Figure 

6 E-7d)

7 • Surface features including identification of erosional and depositional surface forms

8 4. Model Refinement, Validation and Improved Erosion Predictions

9

10 The purpose of the model refinement, validation and improved erosion prediction study would 

11 be to (i) improve confidence in erosion modeling predictions through independent validation, (ii) 

12 to reduce CHILD conceptual model and parameter uncertainty, and (iii) improve CHILD model 

13 calibration. All of these activities would serve to increase confidence in the erosion modeling 

14 predictions. Study 4 activities could include the following:

15

16 • Refine CHILD model parameters, structure, and calibration using data and information 

17 collected in studies 1-3.

18 • Perform an independent validation test using calibrated model parameters to simulate a 

19 second (companion) drainage basin that is comparable in dimensions to similar 

20 landforms at WVDP.

21 • Project future erosion at the WVDP using refined and re-calibrated CHILD model; 

22 perform sensitivity and uncertainty analysis, including evaluation of the sensitivity of the 

23 results to climate.

24 How Does the West Valley Erosion Modeling Example Relate to 10 CFR Part 61 Guidance?

25

26 Although the West Valley erosion modeling example described above applies to a 

27 decommissioning site and has not been fully executed by DOE and NYSERDA for NRC review, 

28 the example illustrates many of the steps listed in Section 5.2.2 of this document, and 

29 reproduced below for ease of reference, that may be taken to assess performance of a 

10 CFR Part 61 facility1 30 . 

31

32

 1 Inclusion of this example does not provide any tacit or implied approval of the erosion modeling as a 

basis for demonstrating compliance with the radiological criteria for license termination. Such 

determinations will be made at the appropriate time following review of the Phase 2 decommissioning 

plan. The example is provided because the current approach nicely illustrates the components of a 

model-based stability assessment. 
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(a) (b)

(c) (d)

1 Figure E-7 Potential Phase 1 Erosion Studies 

2 Image Credit: “Erosion Working Group Recommendations for Phase 1 Studies”, 

3 August 22, 2012, Agenda/Presentation at 

4 http://westvalleyphaseonestudies.org/index.php/public-meeting.

5

6

7 These steps include the following:

8

9 • Define model objectives

10 • Develop or select the model

11 • Document and provide the basis for assumptions

12 • Parameterize the model

13 • Calibrate the model

14 • Verify the model

15 • Characterize uncertainty

16 • Provide model support

17 • Iterate, if necessary

18
E-13

1 Data collection and modeling of WVDP erosion has proceeded in an iterative fashion with 

2 erosion analyses building on previous work and erosion modeling continuing to improve over 

3 time as additional information is obtained. The code selection process began with the initial 

4 evaluation of various shorter-term, smaller scale models that evaluated one or two key erosion 

5 processes operable at the WNYNSC and WVDP but that were limited in their ability to simulate 

6 multiple, coupled erosion processes over larger time and spatial scales. DOE also attempted 

7 more sophisticated landscape evolution modeling using codes such as SIBERIA in earlier EIS 

8 analyses. All of these modeling exercises led to the ultimate selection of the CHILD model that 

9 is considered the state-of-the-art in landscape evolution modeling. Due to the complexity of the 

10 site, landscape evolution modeling was considered necessary to provide technically defensible 

11 erosion modeling predictions to facilitate consensus decision-making. 

12

13 DOE used site-specific data or measurements, literature information from analog sites and 

14 finally generic information sources, if more relevant data were not available to assign CHILD 

15 modeling parameters. DOE used a calibration process to identify the most likely set of 

16 parameter values to predict future erosion at the site. DOE also used site data and other 

17 shorter-term modeling results to evaluate the reasonableness of CHILD modeling results. DOE 

18 performed Monte Carlo analysis to evaluate the impact of parameter uncertainty on the results 

19 of the analysis. DOE attempted to reduce uncertainty in the erosion predictions through 

20 selection of “best-fit” parameter values in the calibration process that were able to produce 

21 current day topography. DOE performed sensitivity analysis to evaluate “what-if” scenarios 

22 such as a wetter climate or faster creep coefficients. DOE documented the results of its erosion 

23 analyses in the FEIS including a discussion regarding potential model limitations. DOE 

24 provided information on areas for potential improvement, including the collection and use of 

25 additional data or additional analyses that might be conducted as code capabilities matured. 

26 Phase 1 studies have been recommended by the WVDP erosion working group and will be 

27 considered by DOE and NYSEDA to further reduce uncertainty and provide additional support 

28 for erosion modeling predictions. 

29

30 As indicated in Section 5.2.2.2, a model-based approach, similar to what was performed for 

31 WVDP erosion modeling analyses, is typically used for longer-term analyses. A design-based 

32 approach may also be justified for relatively shorter time periods. An example of a design33 based approach is provided in the next section. A hybrid approach may also be used for 

34 assessments of site stability and could help provide multiple lines of evidence that the 

35 performance objectives in 10 CFR Part 61, Subpart C will be met. For example, support may be 

36 provided for the performance of a particular design for some period of time that mitigates the 

37 risk of relatively short-lived waste. Over longer time periods, the performance of the design may 

38 be less certain. Sensitivity analyses could be performed to evaluate the impact of various levels 

39 of underperformance of the design over time to evaluate the acceptability of the design and the 

40 ability of the site to meet performance objectives, lending confidence to the compliance and 

41 performance period analyses. As illustrated in the next example, the model- and design-based 

42 approaches share many common features (NRC, 2008).

43

44
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Design-Based Approach: Moab UT Example:
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E-16
E-17
E-18
E-19
E-20
E-21
E-22
E-23
E-24
E-25
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1

2

3

4 Figure E-8 Example of an Erosion Protection Cover for Uranium Mill Tailings

5

6 REFERENCES

7

8 DOE, 2009. U.S. Department of Energy, “Phase 1 Decommissioning Plan for the West Valley 

9 Demonstration Project,” Revision 2, December 2009.

10

11 DOE, 2010. U.S. Department of Energy, “Decommissioning and/or Long-Term Stewardship at 

12 the West Valley Demonstration Project and Western New York Nuclear Service Center,” 

13 DOE/EIS-0226, January 2010.

14

15 NRC, 2008. U.S. Nuclear Regulatory Commission, “Final Technical Evaluation Report for the 

16 Proposed Remedial Action of the Moab, Utah Uranium Mill Tailings Site,” Washington, DC, 

17 2008, ADAMS Accession No. ML082060216.

18

19 NRC, 2010. U.S. Nuclear Regulatory Commission, “Technical Evaluation Report on the U.S. 

20 Department of Energy Phase 1 Decommissioning Plan for the West Valley Demonstration 

21 Project,” February 25, 2010, ADAMS Accession No. ML100400099.

22

23 EWG, 2012a. WVDP Erosion Working Group (EWG), “Recommendations for Phase 1 Erosion 

24 Studies,” Prepared for U.S. Department of Energy, and New York State Energy Research and 

25 Development Authority, July 20, 2012, ADAMS Accession No. ML13072B210.

26

27 EWG, 2012b. WVDP Erosion Working Group (EWG), “Erosion Working Group 

28 Recommendations for Phase 1 Studies,” August 22, 2012, ADAMS Accession No. 

29 ML13071A284. http://westvalleyphaseonestudies.org/index.php/public-meeting.




NUREG

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Draft

Guidance for Conducting Technical Analyses for 10 CFR Part 61