Risk-Informed Decisionmaking 

for Nuclear Material and Waste Applications 

Revision 1 

February 2008 

Prepared by: 

Office of Nuclear Regulatory Research 

Office of Nuclear Material Safety and Safeguards 

Office of Federal and State Materials and Environmental Management Programs 

U.S. Nuclear Regulatory Commission 

Washington, DC 
 iii 

ABSTRACT

The Office of Nuclear Material Safety and Safeguards and the Office of Federal and State Materials 

and Environmental Management Programs of the U.S. Nuclear Regulatory Commission have 

developed a risk-informed decisionmaking (RIDM) framework. The staff can use this framework 

to supplement its approach to decisionmaking on regulatory relief, safety considerations, and 

additional requirements. This report outlines the RIDM framework and explains its applicability 

to a wide range of nuclear material and waste regulatory issues. This report also provides a 

step-by-step procedure with specific examples in various nuclear material and waste areas to 

help the staff consistently and effectively apply a risk-informed approach on a case-by-case 

basis for regulatory decisionmaking. 
 v 

FOREWORD 

This document contains guidance on how to make appropriate risk-informed regulatory 

decisions for nuclear material and waste applications. This risk-informed decisionmaking 

(RIDM) framework can be used on a case-by-case basis for activities conducted by the 

U.S. Nuclear Regulatory Commission (NRC) Office of Nuclear Material Safety and Safeguards 

(NMSS) and Office of Federal and State Materials and Environmental Management Programs 

(FSME). Periodic modifications can be made to this guidance to incorporate lessons learned by 

the staff. Proposed modifications can be reviewed by persons or committees representing a 

wide range of technical and application areas. Thus, this report should be a living document 

with future additions and changes to be incorporated as appropriate. 

Staff should apply a process to identify, evaluate, and recommend specific regulatory decisions 

that would benefit from a risk-informed approach. The selection of candidate applications 

should be based on the benefits and feasibility of applying such an approach. 

Candidates for risk assessments are expected to be regular, ongoing regulatory tasks and not 

special additional studies. The staff usually responsible for the regulatory task will conduct the 

assessment, but the assessment can be supplemented by risk experts familiar with this 

document. These experts can use information from the performing organization to carry out the 

specialized analyses associated with the process. The responsibility for documenting the 

normal regulatory outcome of the application, such as a safety evaluation report, would reside 

with the regular line organization performing the application. If the risk information forms a part 

of the basis for regulatory actions, it should be documented as part of that action. All personnel 

involved will be expected to contribute changes or supplements to the method, as necessary. 

Regulatory situations that would most benefit from the methods in this guidance document are 

those for which an understanding of risk and other factors is needed to support a good decision, 

but for which such information may not be readily available. For example, complex situations 

involving competing tradeoffs among a number of factors, such as various risks and costs, could 

benefit. The guidance suggests two specific decision algorithms. These two algorithms apply 

to changes to regulatory requirements (i.e., rules or license conditions). Staff could apply these 

algorithms to a variety of ongoing regulatory work, such as the following: 

$ proposed rulemaking to impose new requirements directed at improving safety 

$ proposed additional license conditions directed at improving safety 

$ proposed rules or license conditions directed at increasing effectiveness, while 

continuing to maintain safety 

$ proposed exemptions or changes to existing regulations or license conditions that might 

cause small increases in risk 

$ new information raising a question as to whether a specific activity or design is 

acceptably safe 
 vi 

$ events or information for which the safety significance is unclear 

$ license applications or amendments for facilities, devices, activities, or processes 

involving hazards or safety designs that differ significantly from those currently regulated 

$ risk information relevant to license applications for which existing guidance is silent 

The NMSS/FSME staff has already incorporated any available risk information in making 

regulatory decisions. This document provides a structured guidance to risk assessment and 

also serves as a reference to other existing guidance. The RIDM process considers many 

factors in addition to risk. This guidance attempts to provide an integrated consideration of most 

of these factors. An effort has been made to make these methods, to the extent they apply to 

NMSS/FSME, consistent with existing applicable NRC policies and guidance, in general, and 

those pertaining to regulatory analysis, in particular. 

Together, NMSS and FSME regulate a wide variety of activities. The methods in this guidance 

have been tested in pilot studies on a few of these activities. Thus, to ensure continual 

improvement, these methods and guidelines would benefit from additional testing and 

modification through a process of candidate applications. 

Staff from the Office of Regulatory Research (RES) considered the comments and 

recommendations provided in the related letter from the Advisory Committee on Nuclear Waste 

and Materials (ACNW&M), dated May 2, 2006, in Revision 1 of this guidance. As a result, the 

RES staff made the following modifications in this revision of the RIDM document: 

• provided the logic description for using the screening criteria to identify an applicable 

regulatory issue 

• converted the probability of quantitative health guideline (QHG) for latent cancer fatality 

to dose per year for an individual public 

• converted the probability of QHG for latent cancer fatality to dose per year for a worker 

• added an example and a discussion on risk-informing activities for fuel cycle facilities 

This revision also reflects feedback received as a result of reviews by staff from RES, NMSS, 

FSME, and the Office of Nuclear Reactor Regulation. As a result, this revision of the RIDM 

document includes substantial changes to the following sections: 

• Executive Summary 

• Section 4.2.2, “Consideration of New Requirements” 

• Section 6, “Risk Communication” 

• Appendix J, “Applying the RIDM Methodology to Fuel Cycle Facilities” 

• Appendix K, “Issue Characterization” 
 vii 

• Appendix L, “Defining Alternative Actions” 

• Appendix M, “Initial Risk Assessment” 

• Appendix N, “Assessing the Impact of the Issue on Defense in Depth” 

• Appendix O, “Assessing the Impact of the Issue and Alternative Actions on Safety 

Margins” 

• Appendix P, “Reaching Consensus on a Recommendation” 

Since NMSS and FSME intend to use the revised RIDM document for future applications on a 

case-by-case basis, it is important that the document remains a living document and that it is 

updated periodically to factor in any additional insights gained from its application. Future 

revisions of the RIDM document should consider the following potential topics: 

• an algorithm for performing risk-informed inspections 

• an algorithm for license reviews 

• detailed risk assessments in support of rulemaking activities (e.g., development of 

regulatory requirements for reprocessing and nonreactor facilities) 

• lessons learned from operating events 

• comments from the NMSS/FSME staff for revising the RIDM document 

• comments and recommendations from ACNW&M, the Commission, and other NRC 

management 

• comments and recommendations from the Office of Management and Budget 
 viii 

CONTENTS 

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

FOREWORD.............................................................................................................................. v 

EXECUTIVE SUMMARY........................................................................................................... xi 

ACKNOWLEDGMENTS ...........................................................................................................xiii 

ABBREVIATIONS..................................................................................................................... xv 

1 OVERVIEW OF A FRAMEWORK FOR RISK-INFORMED DECISIONMAKING 

IN NMSS/FSME..............................................................................................................1-1 

1.1 Risk-Informed Decisionmaking Concepts.................................................................1-2 

1.2 Summary of the Risk-Informed Decisionmaking Process .........................................1-3 

1.2.1 Overall Risk-Informed Decision Process..........................................................1-3 

1.2.2 Step 1: Define the Regulatory Issue and Alternative Actions. .........................1-4 

1.2.3 Step 2: Decide Whether To Use a Risk-Informed Approach. ..........................1-4 

1.2.4 Step 3: Perform Risk Assessment. .................................................................1-5 

1.2.5 Step 4: Apply Risk-Informed Decision Method................................................1-6

1.2.6 Decision Algorithms.........................................................................................1-6 

1.3 References...............................................................................................................1-8 

2 DECIDING WHETHER NMSS/FSME SHOULD USE A STRUCTURED RISK-INFORMED 

PROCESS ......................................................................................................................2-1 

2.1 Introduction ..............................................................................................................2-1 

2.1.1 Applying Risk Analysis to Nuclear Material and Waste Activities .....................2-1 

2.1.2 Risk Information and Regulatory Decisionmaking............................................2-1 

2.2 Defining the Regulatory Issue and Alternatives ........................................................2-2 

2.3 Screening Considerations ........................................................................................2-3 

2.3.1 Discussion of Screening Considerations Based on Benefits ............................2-3 

2.3.2 Discussion of Screening Considerations Based on Feasibility .........................2-7 

2.4 Disposition of Screened-Out Regulatory Issues .......................................................2-9 

2.5 References...............................................................................................................2-9 

3 GUIDANCE FOR CONDUCTING RISK ASSESSMENTS FOR NUCLEAR MATERIAL 

AND WASTE APPLICATIONS .......................................................................................3-1 

3.1 Introduction ..............................................................................................................3-1 

3.2 Approach..................................................................................................................3-1 

3.3 Grouping NMSS/FSME-Regulated Facilities, Sites, and Devices .............................3-2 

3.3.1 Overview of Licensed Facilities........................................................................3-2 

3.3.2 Description of Facility Grouping .......................................................................3-3 

3.4 Risk Assessments....................................................................................................3-4 

3.4.1 Why Perform Risk Analyses? ..........................................................................3-4 

3.4.2 Potential Benefits from Performing Risk Analyses ...........................................3-5 

3.4.3 The Use of Expert Judgment in Risk Analysis .................................................3-5 
 ix 

3.5 Tools to Assess Risks ..............................................................................................3-6 

3.5.1 Probabilistic Risk Analysis (PRA) ....................................................................3-7 

3.5.2 Integrated Safety Analysis (ISA) ......................................................................3-9 

3.5.3 Performance Assessment (PA)......................................................................3-11 

3.5.4 Barrier/Hazard Analysis (BHA) ......................................................................3-14 

3.6 References.............................................................................................................3-16

4 RISK-INFORMED DECISIONMAKING METHODS ........................................................4-1 

4.1 Introduction ..............................................................................................................4-1 

4.1.1 Decisionmaking Methods in the Overall Process .............................................4-1 

4.1.2 Decision Methods and Algorithms ...................................................................4-2 

4.1.3 Attributes Considered in RIDM ........................................................................4-2

4.1.4 Risk Guidelines..................................................................................4-6 

4.2 Decisionmaking Processes ......................................................................................4-6 

4.2.1 Exemptions and Changes to the Licensing Basis ............................................4-9 

4.2.2 Consideration of New Requirements .............................................................4-13 

4.2.3 Consideration of Risk-Related Issues in Implementing Above Algorithms......4-16 

4.3 References.............................................................................................................4-18

5 RISK GUIDELINES FOR USE IN NMSS/FSME .............................................................5-1 

5.1 Introduction ..............................................................................................................5-1 

5.2 Three-Tier Approach to Risk Guidelines...................................................................5-2 

5.2.1 Qualitative Risk Guidelines..............................................................................5-2 

5.2.2 Quantitative Health Guidelines (QHGs)—Base Approach................................5-3 

5.3 Rationale for Qualitative and Quantitative Risk Guidelines.......................................5-4 

5.4 Data on the Background Risks Used To Develop Base QHGs.................................5-7 

5.4.1 Public—QHGs 1, 2, 3 ......................................................................................5-7 

5.4.2 Workers—QHGs 4, 5, 6...................................................................................5-8 

5.5 Population at Significant Risk.................................................................................5-10 

5.5.1 Estimation of Risk Metrics for Selected Materials Systems............................5-12 

5.5.2 Estimation of Risk for Fuel Cycle Facilities ....................................................5-16 

5.5.3 Summary.......................................................................................................5-16 

5.6 Draft Risk Guidelines—Other Approaches for Consideration..................................5-17 

5.7 International Activities on Safety Goals and Risk Guidelines..................................5-17 

5.8 References.............................................................................................................5-18

6 RISK COMMUNICATION ...............................................................................................6-1 

6.1 Introduction ..............................................................................................................6-1 

6.2 Document the Decision ............................................................................................6-1 

6.3 Communicate the Decision ......................................................................................6-2 

6.4 References...............................................................................................................6-2 
 x 

Appendices

A Glossary .........................................................................................................................A-1 

B Value-Impact Analysis ....................................................................................................B-1 

C Examples of the Application of Screening Considerations.............................................. C-1 

D Supplemental Information on Performing Risk Assessments for NMSS/FSME-Licensed 

Facilities and Applications.............................................................................................. D-1 

E Summary of NMSS/FSME Risk Issues and Status of Risk Information ...........................E-1 

F (Reserved).......................................................................................................................F-1 

G Example of the Use of Risk Guidelines .......................................................................... G-1 

H (Reserved)..................................................................................................................... H-1 

I Key Issues Associated with the Proposed Approach and Implementation of Guidance 

for Risk-Informed Decisionmaking ...................................................................................I-1 

J Applying the RIDM Methodology to Fuel Cycle Facilities ................................................ J-1 

K Issue Characterization ....................................................................................................K-1

L Defining Alternative Actions ............................................................................................L-1 

M Initial Risk Assessment .................................................................................................. M-1 

N Assessing the Impact of the Issue on Defense in Depth ................................................ N-1 

O Assessing the Impact of the Issue and Alternative Actions on Safety Margins ............... O-1 

P Reaching Consensus on a Recommendation .................................................................P-1 

Figures

1.1 Overall Risk Assessment Process ..................................................................................1-5 

3.1 Grouping of NMSS/FSME-Licensed Activities.................................................................3-3 

3.2 Components of a Total System PA ...............................................................................3-13 

4.1 Risk Assessment Process...............................................................................................4-5 

4.2 Three-Region Risk Diagram............................................................................................4-9 

Exhibits

2.1 Screening Considerations...............................................................................................2-4 

2.2 NRC Strategic Plan (FY 2008–2013) ..............................................................................2-5 

Tables

4.1 Risk Acceptance Logic for Changes .............................................................................4-12 

4.2 Risk Reduction Decision Logic for NMSS/FSME Facilities ............................................4-15 

5.1 Numbers and Rates of Fatal Occupational Injuries by Industry Division, 2000................5-8 

5.2 Public Accident Risks from Byproduct Materials Systems in the Case Study 

 (Based on data from NUREG/CR-6642) .......................................................................5-12 

5.3 Average Public Individual Risk of Latent Cancer Based on Different Approaches 

 for Estimating the Population at Risk ............................................................................5-14 

5.4 Worker Accident Risks from Byproduct Materials Systems in the Case Study 

 (Based on data from NUREG/CR-6642) .......................................................................5-15 

5.5 Average Individual Risk of Latent Cancer for Workers ..................................................5-15

5.7 ICRP 64 Framework for Potential Exposures................................................................5-18 
 xi 

EXECUTIVE SUMMARY 

The objective of this risk-informed decisionmaking (RIDM) guidance document is to provide 

guidance to the staff of the U.S. Nuclear Regulatory Commission (NRC) Office of Nuclear Material 

Safety and Safeguards (NMSS) and Office of Federal and State Materials and Environmental 

Management Programs (FSME) on making appropriate risk-informed decisions for nuclear 

material and waste. The NRC staff designed this guidance document specifically for 

NMSS/FSME applications. All relevant attributes should be considered and communicated, 

including risk, when making risk-informed regulatory decisions. This guidance document aims 

to achieve the following objectives: 

(1) Provide a step-by-step procedure on how to make risk-informed regulatory decisions. 

(2) Suggest quantitative health guidelines (QHGs) for acute and latent fatality and injury for 

both public individuals and workers. The QHGs define the negligible risk levels, below 

which resources generally should not be used to reduce a risk contributor. 

(3) Provide three regions of risk (unacceptable, tolerable, and negligible) for consideration in 

analyzing risk information and making regulatory decisions. 

(4) Discuss how to apply this RIDM methodology to nuclear material and waste regulatory 

applications. 

Since this guidance document is only intended to familiarize NMSS/FSME staff with the riskinformed process and when it should be applied, actual application of the process to complex 

situations will normally require the assistance of specialists in risk assessment and 

decisionmaking techniques. 

Appendix A provides a glossary of the terms presented in this report. 
 xiii 

ACKNOWLEDGMENTS 

This document was produced as a joint effort by staff from the U.S. Nuclear Regulatory 

Commission (NRC) Office of Nuclear Material Safety and Safeguards (NMSS) Risk Task Group 

(RTG) (2000–2004), the Office of Nuclear Regulatory Research (RES), and Brookhaven 

National Laboratory (BNL). Staff from all divisions of NMSS reviewed and provided extensive 

comments on the earlier drafts of this document. The NMSS Risk Steering Group, which 

includes representation from each of the NMSS divisions, as well as from other NRC offices, 

including the Office of the General Counsel and the Office of Nuclear Reactor Regulation, 

offered insightful direction and input. 

 

The following staff participated in preparing the draft risk-informed decisionmaking (RIDM) 

document dated May 11, 2005: 

NMSS RTG

Christiana H. Lui, Dennis R. Damon, James A. Smith, Albert Wong, Jessica Shin, 

Kimberly A. Gruss, Raeann M. Shane, Terry Brock, Mary Adams, Michael Raddaz, and 

Shamica Walker 

RES

 

Alan Rubin and Edward Chow 

BNL

M. Ali Azarm, Robert Bari, Edward J. Grove, Charles Meinhold, and Vinod Mubayi 

Lead Reviewers from NMSS Staff

Patti Silva, Mike Waters, Chris McKenney, John Lubinski, Kevin Ramsey, Hans Arlt, 

Dennis Galvin, Chris Grossman, Lydia Chang, Joe DeCicco, Patricia Rathbun, and Kim Hardin 

In response to a user need request from NMSS, RES produced Revision 1 of the draft RIDM 

guidance document. To obtain important risk insights and diverse views from those who have 

been using the risk-informed approach for regulatory decisionmaking, staff members from 

various NRC offices reviewed and commented on the RIDM guidance document. As a result, 

comments from the reviewers were incorporated during the revision process. 

The following NRC staff participated in preparing Revision 1 of the draft RIDM document: 

RES

Edward Chow, Michelle González, Mary Drouin, and Ronaldo Jenkins 
 xiv 

NMSS

Dennis Damon, Keith Compton, and Kevin Morrissey 

Office of Federal and State Materials and Environmental Management Programs

Joseph DeCicco and Chris McKenney 

Office of Nuclear Reactor Regulation

Steven Laur 
 xv 

ABBREVIATIONS

ACNW&M Advisory Committee on Nuclear Waste and Materials 

ACRS Advisory Committee on Reactor Safeguards 

AIChE American Institute of Chemical Engineering 

ALARA as low as is reasonably achievable 

BNL Brookhaven National Laboratory 

BEIR Biological Effects of Ionizing Radiation (Committee on) 

BHA barrier/hazards analysis 

CCDF complementary cumulative distribution function 

CDF core damage frequency 

CFR Code of Federal Regulations

DOE U.S. Department of Energy 

EPA U.S. Environmental Protection Agency 

FMEA failure modes and effects analysis 

FSME Office of Federal and State Materials and Environmental Management 

Programs 

HAZOP hazard and operability analysis 

HLW high-level waste 

HRA human reliability analysis 

IAEA International Atomic Energy Agency 

ICRP International Commission on Radiological Protection 

IE initiating event 

IPEEE individual plant examination of external events 

ISA integrated safety analysis 

ISFSI independent spent fuel storage installation 

ISL in situ leach 

km kilometer 

LCF latent cancer risk 

LERF large early-release frequency 

LLW low-level waste 

LTR lower tolerable risk region or License Termination Rule 

LTS Licensing Tracking System 

mrem millirem 

MSHA Mine Safety and Health Administration 

NMED Nuclear Material Events Database 
 xvi 

NMSS Office of Nuclear Material Safety and Safeguards 

NPP nuclear power plant 

NRC U.S. Nuclear Regulatory Commission 

NRR Office of Nuclear Reactor Regulation 

PA performance assessment 

PGE Portland General Electric Company 

PHA process hazards analysis 

PRA probabilistic risk assessment 

PRAM preclosure risk assessment methodology 

PSA probabilistic safety assessment 

QHG quantitative health guideline 

QHO quantitative health objective 

RES Office of Nuclear Regulatory Research 

RG regulatory guide 

RIDM risk-informed decisionmaking 

RTG Risk Task Group of the Office of Nuclear Material Safety and Safeguards 

SER safety evaluation report 

SGPS Safety Goal Policy Statement for the Operation of Nuclear Power Plants 

SSC structure, system, or component 

UF6 uranium hexaflouride 

UTR upper tolerable risk region 

WB whole body 

WISQARS Web-based Injury Statistics Query and Reporting System 
 1-1 

1 OVERVIEW OF A FRAMEWORK 

FOR RISK-INFORMED DECISIONMAKING IN NMSS/FSME 

Risk-informed decisionmaking (RIDM) is the use of risk insights, along with other important 

information, to help in making decisions. This document aims to put this concept into practice in 

the materials and waste arenas of the U.S. Nuclear Regulatory Commission (NRC). The 

document provides a framework of structured decision methods that, when applied properly, 

can help staff achieve risk-informed (Ref. 1.1) regulation. The basic principles of this framework 

are (1) that staff should apply fully quantitative risk-informed methods only to appropriate 

regulatory decisions, and (2) once the appropriate regulatory decision is identified, staff should 

follow a structured process when performing a quantitative risk analysis. The guidance 

presented here defines this structured process. Use of this guidance will ensure that all relevant 

attributes, including risk, are considered in an effective and consistent manner when making 

risk-informed regulatory decisions for nuclear material and waste applications. This document 

focuses on supplementing existing NRC guidance on the use of risk information, particularly 

guidance on regulatory analysis (Refs. 1.2, 1.3). This document is intended to help the staff of 

the Office of Nuclear Material Safety and Safeguards (NMSS) and the Office of Federal and 

State Materials and Environmental Management Programs (FSME) identify when a structured, 

quantitative, risk-informed process should be followed and to provide a general familiarity with 

such a process. Actual application of the process to complex situations will normally require the 

assistance of specialists in risk assessment and decisionmaking techniques. 

Protecting public health and safety is a primary NRC objective. Challenges to public health and 

safety include both normal and accidental exposures to radiation and other regulated hazards. 

Normal exposures can be planned, minimized, and monitored, but information on the probability 

and consequence of accidents must come from statistics or risk assessments. Risk information 

is a technical description of things that can go wrong, the likelihood of those things happening, 

and the consequences if they were to happen. This information can be both quantitative and 

qualitative. Risk information can provide a structured presentation of public health and safety 

knowledge. Methods of risk assessment have become highly developed. Since public health 

and safety are a primary concern to the agency, risk information is useful for agency 

decisionmaking. However, risk information can be complex, and regulatory decisions must 

consider many inputs, such as normal exposures, accident risk, costs, defense in depth, safety 

margins, and security. When a decision affects multiple attributes, use of a structured process 

can be helpful. 

This chapter provides an overview of the RIDM process for NMSS/FSME applications. 

Subsequent chapters describe in greater detail the major steps in the process. Section 1.1 

briefly discusses the basic concept of RIDM and some of the terminology used, while Section 1.2 

contains an overview of the process itself. Section 1.3 provides a list of references. 
 1-2 

1.1 Risk-Informed Decisionmaking Concepts

The intent of this section is to briefly discuss what is meant by RIDM. The NRC has defined the 

phrase “risk-informed decisionmaking” (Ref. 1.1) as follows: 

A “risk-informed” approach to regulatory decisionmaking represents a philosophy 

whereby risk insights are considered together with other factors to establish 

requirements that better focus licensee and regulatory attention on design and 

operational issues commensurate with their importance to health and safety. 

The concept of a risk-informed approach first evolved in the reactor context. When the term 

“risk” is used in that context, it typically refers to risk of core damage and to the offsite public 

from accidents. However, in the materials and waste arenas of NMSS/FSME, risk is often 

defined more broadly, taking into account both normal and accidental exposures to workers and 

the general public. Thus, in the NMSS/FSME context, risk-informed guidance involves 

consideration of normal and accidental exposures to the public and workers. 

An important distinction exists between normal and accidental exposures. Normal exposures 

are those that occur as planned or are expected to occur during the lifetime of a facility. 

Accidents are unintentional, typically rare, episodic events. Normal exposures may be directly 

monitored as part of a radiation protection program. Chronic future exposures and rare 

accidents require some form of risk assessment, usually quantitative. Subsequent chapters will 

focus, to a certain extent, on accident risk because 10 CFR Part 20, “Standards for Protection 

against Radiation,” regulates normal exposures. However, the proposed algorithms for making 

risk-informed decisions address both types of risk. 

This document uses the term “accident” to refer to all unexpected or unintended exposures. 

There is no implication that, because an event is referred to as an “accident,” the licensee is not 

accountable for preventing or mitigating it. Furthermore, in this context, the term “accident” 

does not refer to “design-basis accidents” or other regulatory designations. The term “accident 

risk assessment” is an attempt to make realistic estimates in support of a risk-informed 

approach to regulation. 

NMSS/FSME regulates a wide variety of nuclear material uses. The risk-informed decision 

methods presented are intended to be generic—hence, they are widely applicable across the 

diverse activities regulated by NMSS/FSME. To make the methods generic, these decision 

methods consider all relevant attributes and not just risk. In any particular category of licensed 

activity, only a few of these attributes may be important. This guidance document encompasses 

all so as to be inclusive and generally applicable. The guidance identifies the types of risk to 

calculate, the criteria to consider, and the methods of integrating risk with other information to 

provide the kind of insight that is relevant to a particular situation. Such risk-informed decisions 

should help NRC staff formulate more effective regulatory programs. 
 1-3 

1.2 Summary of the Risk-Informed Decisionmaking Process

The staff developed the methods described in this document to facilitate the application of a 

risk-informed approach to certain regulatory actions. This current guidance continues to be 

updated as appropriate. Some criteria and methods have been specified fully, whereas others 

are in a less developed state. However, users can expect the overall process and most of the 

decision logic to continue basically in the same form. The methods presented herein are 

applicable to a number of important decision situations encountered in regulatory activities, but 

they do not cover all possible situations. 

Criteria and considerations included in RIDM are not additional regulatory requirements. 

Rather, the methods for making risk-informed decisions are useful tools to consider in certain 

situations, for example, a proposed change to regulatory requirements. Such changes can 

affect a number of important factors, including individual and societal risk. If these impacts are 

expected to be significant, the staff may find a risk-informed analysis useful. COMSAJ-97-008, 

“Discussion on Safety and Compliance,” dated August 25, 1997 (Ref. 1.2), states the following: 

Since some requirements are more important to safety than others, the 

Commission should use a risk-informed approach wherever possible when 

adding, removing, or modifying NRC regulations, as well as when applying NRC 

resources to the oversight of licensed activities (this includes enforcement). 

1.2.1 Overall Risk-Informed Decision Process

The guidance in the subsequent chapters is based on an overall risk-informed decision process. 

This process consists of four major steps: 

(1) Define the regulatory issue and preliminary alternative actions. 

(2) Decide whether to risk-inform. 

(3) Perform risk assessment as needed. 

(4) Apply a RIDM method. 

Such a process should be carried out as a coordinated team effort involving a number of 

disciplines and responsibilities. The first two steps, identifying the issue and deciding whether 

to use a risk-informed approach, should have substantial involvement by staff in the specific 

regulatory area. Chapter 2 of this document discusses the screening considerations and how 

they are applied. Step 3, or risk assessment (discussed in Chapter 3) and step 4, 

decisionmaking (discussed in Chapter 4), often require assistance from specialized risk 

analysts. Step 3 generates the risk information needed to support the decision methods in 

step 4. Step 4 applies logic algorithms that consider risk and a number of other factors to 

develop input to a decision. The output of these algorithms is information indicating which 

proposed action(s) should be rejected and which one(s) considered. However, step 4 is 

intended to be in the spirit of a risk-informed approach. That is, even though the logic of the 

algorithm may indicate a preferred action, it is intended only to inform, not dictate, the decision. 

Figure 1.1 illustrates the overall process. The process appears as a simple, single-pass 
 1-4 

sequence of steps but, in practice, iterations will occur. Information from the risk analysis often 

helps identify refinements in the proposed action that are more effective than the original. The 

following sections summarize further the four major steps in the RIDM process. 

1.2.2 Step 1: Define the Regulatory Issue and Alternative Actions

The first step in any decisionmaking situation is to clarify the regulatory question or problem. 

Given a clear statement of the problem, the next typical step is to propose one or more actions 

that might solve the problem. A typical regulatory situation might be a lack of sufficient 

information about the safety of a new type of design or facility. Another typical situation might 

be a proposed change to regulatory requirements or practices to improve effectiveness. These 

situations may give rise to questions about risk, such as whether the risk is low enough, or what 

effect a particular action may have on risk. Devising actions that are the best solution to a 

problem may not be easy. Risk, cost, and other impacts often involve tradeoffs. The challenge 

is to find the best alternative. To do this, it is often useful to consider a number of alternatives. 

Appendices K and L provide additional guidance on the considerations related to issue 

characterization and defining alternative actions.

1.2.3 Step 2: Decide Whether To Risk-Inform

After step 1, in which the issue is defined and a list of alternative actions formed, step 2 

introduces screening considerations. The purpose of screening is to decide whether a riskinformed approach should be used. 

There are four screening considerations, which can be placed in two groups. The first group of 

screening considerations asks whether risk information would be useful or is necessary in 

making the decision to support the strategic objective or one or more of the goals in the 

agency’s Strategic Plan (Ref. 1.4). If risk insights cannot be used to support the strategic 

objective of providing excellence in regulatory actions that are open, effective, realistic, and 

timely, a risk-informed approach is not necessary. The second group of considerations asks 

whether developing and using the risk information is feasible, cost effective, and not precluded 

by other considerations. If risk information is relevant, but cannot be provided because of 

feasibility or cost, the decision still must be made using another framework—perhaps a less 

structured or quantitative process. Chapter 2 provides detailed guidance for applying the four 

screening considerations. 
 1-5 

1.2.4 Step 3: Perform Risk Assessment As Needed

Step 3, performing the risk assessment, consists of producing the risk information needed to 

apply the applicable decision method in step 4. The initial task in this step is to determine the 

scope of the risk assessment. This amounts to identifying what risk metrics need to be 

calculated and what level of quality and detail needs to be provided. Some of the needed risk 

information may already exist or a simple risk analysis may be sufficient. The risk assessment 

must be forward looking, in that the decision method to be used in step 4 must be identified in 

advance. This is because different decision methods in step 4 require the calculation of 

different risk metrics in the risk assessment. 

The degree of completeness, applicability, and scientific detail in the risk assessment is often 

referred to as risk assessment quality. Quality should be commensurate with the purposes of 

the decision to be made and may also be influenced by the scope and the level of detail 

associated with the risk assessment. 

Regulations, risk 

guidelines, criteria, 

other data

Yes 

4. Apply riskinformed decision 

method 

3. Evaluate risk 

information 

2. Decide 

whether to risk 

inform 

Decision or Action 

Apply other 

data decision method 

data 

No 

1. Define regulatory 

issue and preliminary 

alternative actions 

Initial risk and 

cost information

Figure 1.1. Overall Risk Assessment Process
 1-6 

It is usually important to quantify the uncertainty in the risk results and to assess qualitative 

uncertainties in step 3, as well. The risk and uncertainty information generated in step 3 is then 

used in step 4 to assist in making the decision and evaluating its robustness. Chapter 3 of this 

document provides guidance on this step of performing risk assessments applicable to 

NMSS/FSME-regulated activities. 

Appendix M provides additional guidance on the considerations related to initial risk 

assessment.

1.2.5 Step 4: Apply a RIDM Method

The objective of step 4 of the risk assessment process is to provide potentially useful guidance 

for decisionmaking by considering risk and other relevant factors. The guidance in Chapter 4 

provides a structure and logic by which this is done in a consistent manner. The logic, referred 

to here as “algorithms,” does not dictate the decision, but simply provides insight to assist the 

decisionmaker. There are many different situations in which such a risk-informed decision 

method could be applied. The current guidance in Chapter 4 addresses the following two 

specific decision situations and provides an algorithm for each: 

(1) exemptions or changes to regulatory requirements that might slightly increase risk 

(2) imposition of a new regulatory requirement to reduce risk 

NRC guidance documents related to regulatory analysis (Refs. 1.2, 1.3) also address these two 

types of regulatory decisions. However, when addressing accident risk, this existing guidance is 

highly oriented to reactors. The guidance in Chapter 4 therefore primarily acts as a supplement 

to the existing guidance by providing accident risk criteria in a form applicable to NMSS/FSME 

activities. Chapter 4 also discusses certain factors other than risk that should be addressed 

when making these decisions. 

 

While additional regulatory situations might also benefit from a risk-informed approach, this 

document does not provide specific algorithms for these situations. However, the basic 

principles discussed in Chapter 4 and in References 1.2 and 1.3 can be expanded to devise 

risk-informed approaches for other situations. For example, these principles might be applied 

when determining risk significance and using risk insights to prioritize licensing or inspection 

activities. Chapter 4 does not address guidance for the use of qualitative risk information. 

Some methods currently in use in NMSS/FSME, such as integrated safety analyses (ISAs), 

produce qualitative risk information. Such information is also useful in applying a risk-informed 

approach to regulatory actions. 

1.2.6 Decision Algorithms 

This discussion provides a general perspective on the decision algorithms used in step 4 of the 

RIDM process (Figure 1.1). Chapter 4 offers a detailed description of the actual decision 

algorithms. Decision algorithms are simply a set of rules that can be followed to determine a 

preferred set of decision alternatives. Decision alternatives are typically different regulatory 

actions that could be taken in a given situation. In the simplest case, two alternatives exist—to 

take a single proposed regulatory action or to take no action. More often, there are several 

reasonable alternative actions, including the no-action alternative. 
 1-7 

In most situations, the proposed actions affect multiple factors. Such factors might include risk 

to individual members of the public, risk to individual workers, defense in depth, safety margins, 

total health impacts on affected populations, total cost impacts, security impacts, environmental 

impacts, regulatory requirements, and Commission policies. 

The decision algorithms described in step 4 treat these factors as either (1) mandatory 

conditions or (2) individual qualitative contributors to a net total impact. In the first case, certain 

factors, including risk to individuals, are evaluated and compared directly to any and all 

regulatory requirements and Commission policies that must be met. These criteria are thus 

mandatory conditions. Examples include adequate defense in depth and adequate safety 

margins. For other factors that may involve nonquantifiable considerations, regulatory

requirements reveal the applicable criteria. Reference 1.3 provides some guidance for 

mandatory conditions, but definitive quantitative criteria are not always provided. Chapter 4 also 

discusses some of these conditions. 

The annual limits on dose to the public and workers from routine exposures as outlined in 

10 CFR Part 20 are one example of a set of mandatory conditions applicable to individual 

routine risk. Analogous limits for accident risk are not common in the regulations. However, as 

indicated in Chapter 4, a logic similar to the annual limits in 10 CFR Part 20 can apply. For 

example, if a regulatory action were to result in an accident risk to individuals that is too great, 

that may be sufficient grounds not to take the action. Chapter 4 refers to this concept as 

“unacceptable risk.” 

Another criterion indicating that no action should be taken is negligible risk to individuals. 

Chapter 5 discusses accident risk guidelines defining such levels of risk. These values 

represent very low risk, relative to U.S. average fatality risks. The decision algorithm in 

Chapter 4 for new regulatory requirements uses these risk guidelines to indicate when new 

requirements are unlikely to be justified. That is, if the staff proposes a new requirement to 

lower accident risk, but its effect on risk would be negligible, the agency might choose not to 

pursue the proposal. The staff provided guidelines for both workers and the general public. In 

some areas regulated by NMSS/FSME, worker accident risk is a primary concern. The base 

values for these guidelines, for negligible accident risk, proposed for evaluation, are as follows: 

(1) Public individual risk of acute fatality (QHG 1) is negligible if less than or equal to 

5x10-7 fatality/yr. 

(2) Public individual risk of latent cancer fatality (LCF) (QHG 2) is negligible if less than or 

equal to 2x10-6 fatality/yr or 4 mrem/yr. 

(3) Public individual risk of serious injury (QHG 3) is negligible if less than or equal to 

1x10-6 fatality/yr. 

(4) Worker individual risk of acute fatality (QHG 4) is negligible if less than or equal to 

1x10-6 fatality/yr. 

(5) Worker individual risk of LCF (QHG 5) is negligible if less than or equal to 

1x10-5 fatality/yr or 25 mrem/yr. 
 1-8 

(6) Worker individual risk of serious injury (QHG 6) is negligible if less than or equal to 

5x10-6 fatality/yr. 

In response to a recommendation by the Advisory Committee on Nuclear Waste and Materials 

(ACNW&M), the latent cancer QHG values, which are in units of fatalities per year, can be 

converted into equivalent doses per year by using the International Commission on Radiological 

Protection (ICRP) conversion factors of 5x10-4 fatalities per rem for the public and 

4x10-4 fatalities per rem for workers. The ACNW&M provided its review findings of the RIDM 

guidance document in a letter dated May 2, 2006, to the Commission Chairman (Ref. 1.3). 

The staff is considering other approaches for expressing such guidelines as part of the 

case-by-case evaluation process. To draw risk insights for decision considerations, the staff 

needs a full range of reference levels for various NMSS/FSME-regulated areas because each 

area poses different types of safety issues. These issues may include routine doses, chronic 

exposures, and potential accident risk. 

The second use of risk and other attributes occurs in value-impact analysis. Value-impact 

analysis does not compare each attribute individually to its criterion. Instead, it evaluates the 

net impact of all attributes together. The attributes include costs as well as public health and 

safety. The output is a net impact score for each proposed alternative, relative to the no-action 

alternative. The score indicates which action would produce the highest benefit. This analysis 

is often helpful when proposed actions have both beneficial and adverse impacts. 

References 1.5 and 1.6 provide NRC guidance for performing value-impact analysis, including a 

list of attributes to consider. Appendix B provides a brief summary of this type of analysis. 

The approach for regulatory decisionmaking used in NMSS/FSME for nuclear material and 

waste applications is similar to the approach used by the Office of Nuclear Reactor Regulation 

(NRR) for reactor applications (Refs. 1.7, 1.8). For regulatory decisionmaking in reactor 

applications, NRR has been using three-region risk diagrams with acceptable guidelines for 

core damage frequency (CDF) and large early release frequency (LERF). For regulatory 

decisionmaking in NMSS/FSME applications, the NRC is using three-region risk diagrams with 

quantitative risk guidelines for a public individual and a worker. 

To translate a risk-informed decision to an effective action, a risk communication plan is needed 

to be developed as one of the early activities. Chapter 6 provides a discussion on how to 

develop a risk communication plan, document the risk-informed decision, and communicate the 

decision to the internal and external stakeholders. 

1.3 References

1.1 U.S. Nuclear Regulatory Commission, “White Paper on Risk-Informed and PerformanceBased Regulation,” SECY-98-144, June 22, 1998. 

1.2 U.S. Nuclear Regulatory Commission, “Discussion on Safety and Compliance,” 

COMSAJ-97-008, August 25, 1997. 

 
 1-9 

1.3 U.S. Nuclear Regulatory Commission Letter from Michael T. Ryan, Advisory Committee 

on Nuclear Waste to Chairman Diaz, “Risk-Informed Decisionmaking for Nuclear 

material and Wastes,” May 2, 2006. 

1.4 U.S. Nuclear Regulatory Commission, “FY 2008-2013 Strategic Plan," NUREG-1614 

Volume 4, February 2008. 

1.5 U.S. Nuclear Regulatory Commission, “Regulatory Analysis Guidelines of the 

U.S. Nuclear Regulatory Commission,” NUREG/BR-0058, July 2000. 

1.6 U.S. Nuclear Regulatory Commission, “Regulatory Analysis Technical Evaluation 

Handbook,” NUREG/BR-0184, January 1997. 

1.7 U.S. Nuclear Regulatory Commission, “An Approach for Using Probabilistic Risk 

Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing 

Basis,” Regulatory Guide 1.174, Revision 1, November 2002. 

1.8 U.S. Nuclear Regulatory Commission, “Integrated Risk-Informed Decisionmaking 

Process for Emergent Issues,” LIC-504, Revision 2, February 12, 2007. 
 2-1 

2 DECIDING WHETHER NMSS/FSME SHOULD USE A STRUCTURED 

RISK-INFORMED PROCESS 

2.1 Introduction 

This chapter focuses on identifying regulatory issues that would benefit from the application of 

risk-informed process and the screening considerations that can help staff determine whether 

they should use such an approach. The screening considerations as documented in Ref. 2.1 

were tested on a spectrum of NMSS/FSME activities and were found to be effective for 

determining the appropriateness of applying a risk-informed approach to regulatory issues. 

The screening considerations are a series of four questions to help determine whether to use a 

structured risk-informed approach. These questions examine both the benefits and feasibility of 

implementing such an approach. They are intended to prompt thought and elicit professional 

judgment on whether to apply a risk-informed approach. They are not intended as a strict 

“yes/no” checklist to a decision. 

2.1.1 Applying Risk Analysis to Nuclear Material and Waste Activities 

The NRC policy on implementing risk-informed regulation was expressed in the 1995 Policy 

Statement (Ref. 2.2) on the use of Probabilistic Risk Assessment (PRA) methods in regulatory 

activities. This statement called for an increase in the use of PRA technology in all regulatory 

matters, consistent with state-of-the-art PRA methods and data. The use of PRA technology 

should complement NRC’s deterministic approach and support NRC’s traditional defense in 

depth philosophy. The concept of “risk-informed regulation” is the one advanced in the NRC 

White Paper on the subject (Ref. 2.3). 

The Commission recognized that a single approach for incorporating risk insights into the 

regulatory process would not be appropriate, given the widely varying use of nuclear material in 

reactors, industrial applications, and waste disposal facilities. Accordingly, several activities in 

the then NMSS between 1998 and 2001 (Refs. 2.4 – 2.11) led to the development of the 

screening considerations discussed here. 

2.1.2 Risk Information and Regulatory Decisionmaking

The screening considerations, taken together, make up one important element in the process 

associated with risk-informed regulatory decisionmaking. Figure 1.1 illustrates the process to 

be used in the nuclear material and waste arenas. 

To begin, staff should identify the regulatory issues or action alternatives for a risk-informed 

approach. The next step is to develop or collect the information needed to address each of the 

screening considerations; that is, information needed to address the suitability of pursuing the 

RIDM approach. 
 2-2 

After applying the screening considerations, staff should make a feasibility check. If, at this 

point, the issue is screened out for a structured risk-informed approach, then the outcome 

should be recorded and the reasons documented (for further information, refer to Chapter 4). 

If the issue is screened in for a structured risk-informed approach, the next step is to carry out or 

adapt an appropriate risk assessment suitable to the regulatory issues in question and to 

document the reasons for doing so. We will discuss suitable assessments in more detail in 

Chapter 3. At this juncture, it is important for staff to determine the depth and scope of the 

analysis required, identify the population at risk, and determine risk metrics. If approved risk 

guidelines relevant to the risk metrics are available, staff should identify those guidelines in 

order to compare the risk results to the guidelines in arriving at the decision. In using risk 

results to make the decision, staff should identify competing risks and, to the extent possible, 

review other related cases and studies. 

Applying a risk-informed approach to the regulatory framework may involve changing 

regulations through rulemaking. However, it will more likely provide additional insights to aid the 

decisionmaking in licensing, inspection, and enforcement. Risk information is also used in 

performing regulatory analysis of an alternative action, as documented in NUREG/BR-0184, 

“Regulatory Analysis Guidelines of the U. S. Nuclear Regulatory Commission” (Ref. 2.12). 

Further development of risk applications in the materials and waste arenas can help bring risk 

insights to bear on the process of regulatory analysis. 

2.2 Defining the Regulatory Issue and Alternatives 

As part of this screening process, it is useful to clarify what regulatory question, problem, issue, 

or decision needs to be made. A clear regulatory issue statement would also help define the 

type, scope, and depth of the analysis needed. It is often useful to identify a preliminary set of 

alternative actions the agency may take. Insights derived from risk assessment and other 

information may clarify which are the more effective alternatives. Eventually, it becomes 

necessary to evaluate the risk and other impacts of each of these alternatives to provide insight 

as to a preference. 

Actions to address a regulatory problem can have multiple impacts. Some risks may decrease, 

others increase; the actions may also affect licensed activities in other ways. Hence, identifying 

actions that would be most effective may not be simple and may require management guidance. 

Additional guidance on the considerations related to issue characterization and defining 

alternative actions are provided in Appendices K and L, respectively. 
 2-3 

2.3 Screening Considerations

This section discusses the RIDM screening considerations and identifies the factors that need to 

be addressed when applying them to regulatory issues or action alternatives. Examples of the 

application of screening considerations are provided in Appendix C. 

The four questions used to examine both the benefits and feasibility of implementing a riskinformed approach are listed in Exhibit 2.1. 

The screening considerations were first issued in 2000 as draft screening criteria and tested 

through a series of case studies (Refs. 2.1, 2.11). These case studies were retrospective looks 

at a spectrum of activities in the materials and waste arenas that had elements of a RIDM 

process. One notable change that resulted from the case study program was that the criteria

became considerations. This meant that a strict yes/no logic need not be followed in using the 

outcomes from the questions posed in the considerations. Rather, the screening considerations 

should be seen as guidelines to facilitate decisions by NRC management and staff. Ref. 2.1 

provides a detailed discussion of how the criteria became considerations. 

The four screening considerations can be separated into two groups. The first group, 

consideration 1, relates to the benefits (as embodied in the Commission’s Strategic Plan) that 

can be derived from using the RIDM approach. The second group, considerations 2 through 4, 

addresses whether the risk-informed approach is feasible. A regulatory application is screened 

in if the RIDM process could be both beneficial and feasible. 

To use this screening method, it is necessary to state clearly the regulatory decision or action 

alternatives under consideration. This will permit a clearer evaluation of the possible benefits of 

the risk information with respect to consideration 1. One should answer all the screening 

consideration questions, regardless of whether the issue is screened in before all the questions 

are addressed. A thoughtful response to all the questions could help communicate all the 

benefits of implementing a proposed risk-informed regulatory action such as the RIDM process. 

The amount of time spent on answering the screening questions should be measured in hours, 

not days. 

2.3.1 Discussion of Screening Considerations Based on Benefits

The screening considerations related to the benefits of a risk-informed approach are based on 

NRC’s strategic objective of providing excellence in regulatory actions that are open, effective, 

realistic, and timely as described in the Strategic Plan. Some of the factors that need to be 

addressed and the important questions that should be considered when responding to the 

screening considerations are discussed in Exhibit 2.1. The questions are intended to prompt 

thought and elicit professional judgment and are not to be used as a strict dichotomous “yes/no” 

checklist guiding the decision. 
 2-4 

Exhibit 2.1. Screening Considerations 

 BENEFIT OF IMPLEMENTING A RISK-INFORMED REGULATORY APPROACH 

(1) Could a risk-informed regulatory approach help address the Commission’s strategic objective in 

the NRC’s Strategic Plan? 

FEASIBILITY OF IMPLEMENTING A RISK-INFORMED REGULATORY APPROACH 

(2) Do data and/or analytical models of sufficient quality exist, or could they reasonably be developed 

to permit the application of risk information to a regulatory activity? 

(3) Can startup and implementation of a risk-informed approach be realized at a reasonable cost 

to the NRC, an applicant or licensee, or the public, and provide a net benefit? 

(4) Would any other existing factor(s) limit the utility of implementing a risk-informed approach? 

If the RIDM approach is both beneficial and feasible for a regulatory application, it is screened in.

Screening Consideration 1 - Strategic Plan Compliance

Could a risk-informed regulatory approach help address the Commission’s strategic objective in 

the NRC’s Strategic Plan (Ref. 2.15)?

In applying this screening consideration, one should assess whether risk information can help 

meeting the NRC’s strategic objective of providing excellence in regulatory actions that are 

open, effective, realistic, and timely as described in the Strategic Plan. Exhibit 2.2 outlines the 

major components of this plan that are applicable to NMSS/FSME. 

Exhibit 2.2. NRC Strategic Plan (FY 2008-2013) 

Strategic Objective

Enable the use and management of radioactive materials and nuclear fuels for beneficial civilian 

purposes in a manner that protects public health and safety and the environment, promotes the 

security of our nation, and provides for excellence in regulatory actions that are open, effective, 

efficient, realistic, and timely. 

Strategic Goals

Safety - Ensure adequate protection of public health and safety and the environment. 

Security - Ensure adequate protection in the secure use and management of radioactive materials. 
 2-5 

Application of screening consideration 1 does not imply that the regulatory activity must be 

applicable to each individual strategic objective and goal. Applicability to only one can be 

deemed sufficient. For regulatory activities related to safety, there are three main factors or 

questions: 

(1) Does the activity's current safety level need improvement? 

(2) If the activity is safe enough, could other aspects of the regulatory framework be 

improved while still maintaining safety? 

(3) Would risk information be useful in assessing a new activity's safety level? 

The first question relates to whether the current facility or activity complies with established 

standards or limits. There is a general presumption that adherence to the rules and regulations 

of Title 10 of the Code of Federal Regulations assure adequate protection of the public. 

However, the following issues can give rise to a safety question: 

• An unresolved question, as to physical behavior or data, that bears on safety 

performance; 

• A concern that there may be unrecognized or unevaluated hazards or potential 

accidents; 

• New information (for example, a failure event) indicating that the risk from known 

hazards is greater than previously assessed; and 

• The uncertainty in risk, based on current analyses and information, is too large 

and needs to be reduced. 

The second question arises if an action or a change modifies the current situation or activity. 

These situations can involve changing the existing regulations or their implementation and may 

lead to cases that challenge the presumption of adequate protection. Regulatory Guide(RG) 

1.174, "An Approach for Using Probabilistic Risk Assessment In Risk-Informed Decisions on 

Plant-Specific Changes to the Licensing Basis" (Ref. 2.13) addresses one class of such events; 

namely, requests for changes to the current licensing basis for nuclear power reactors that 

might result in increased risk. In such situations, risk assessments may be needed to address 

the criteria given in RG 1.174 for assessing the acceptability of the proposed change. Of more 

immediate interest to NMSS/FSME is the intriguing situation that is illustrated by the Trojan 

Reactor Vessel transport exemption (Ref. 2.14). In that case, an exemption from the 

regulations resulted in an activity that was a substantial reduction in the overall risk as well as 

cost. This benefit might not have been realized had staff not performed a systematic 

quantitative risk assessment. Applying risk information can help evaluate whether safety will 

continue to be maintained after the change is implemented. 

The third type of question can arise from the introduction of new safety technologies, or the 

application to license a new type of nuclear device or facility not anticipated in the current 

regulatory system. Such situations may benefit from a review using systematic risk assessment 

methods to see if new types of hazards or accidents exist, and to provide assurance that all are 

adequately addressed. Improved risk assessment, including quantification of uncertainty, may 

help to resolve such issues. 
 2-6 

In applying this screening consideration, one should assess whether risk information can help to 

streamline processes and improve consistency, while still keeping the focus on safety. The 

main factors to be addressed in this screening consideration are as follows: 

• Are there aspects of the regulatory framework that could be streamlined through the use 

of risk information? 

• Could the use of risk information produce more consistent decisions among NRC staff? 

The first question ensures that regulations are open, effective, realistic, and timely. For 

example, the case study on the Trojan reactor vessel package shipment (Ref. 2.14) showed that 

applying risk information to the review of an exemption from the regulations streamlined the 

NRC staff’s decisionmaking process. 

The second question ensures consistency across different applications, while keeping the focus 

on safety. By identifying all relevant risk metrics, and by setting risk-based standards and goals, 

a RIDM approach may help to enhance consistency and completeness. Such standards and 

goals can facilitate rapid staff decisionmaking and thus improve efficiency and response time. 

Finally, risk information can be used to optimize the allocation of staff effort and maintain a 

focus on those features and issues most important to safety. 

If the regulatory application under review may reduce the regulatory burden, the following points 

should be considered: 

• Could risk information be used to change regulations or policy so that the regulatory 

burden is more consistent with the magnitude of the risk? 

• Could risk information be used to change NRC licensing or inspection policies to focus 

the most effort on those areas that have the biggest safety impact, while still maintaining 

overall safety? 

The first question relates to the use of risk information to assess whether the regulatory burden 

is consistent with the hazard of the activity. This may be a simple or complex question in the 

case of specific proposals to lessen or eliminate a requirement. Licensees are often able to 

provide applicable data on the impact of regulatory burden. On the other hand, some proposed 

changes result in a mixture of both added and reduced burdens that require analytical effort to 

determine net impact. It is important to recognize that a burden could also be placed on States, 

other entities, and the general public (taxpayers) (e.g., clean-up of a contaminated site when the 

risk is low). 

The second question relates to using risk information in such a way that NRC licensing or 

inspection policies are focused on those areas that have the largest safety impact. Proper risk 

allocation would result in a graded approach that allocates resources to licensing and inspection 

of systems, structures, components, and procedures in a way that optimizes averted risk while 

maintaining overall safety. 

In evaluating applications where risk information can be used to clarify decisions, implement a 

transparent process, and make defensible decisions, several factors should be considered: 

• What is the existing public perception and acceptance of risk in the subject area? 
 2-7 

• Could risk information be used to provide a better understanding of the basis for a 

decision? 

• Would risk information make staff decisions clearer or more defensible, or provide 

greater transparency to our process? 

Generally, a risk-informed approach provides data and information about a set of options and an 

objective basis for choosing a particular option. There is a reasonable assumption that this 

would provide more and better information to support the underlying rationale of an agency 

decision. In principle, it could thus lead to decisions that are more transparent and more 

defensible. 

Risk assessment results often clarify why the system of regulatory requirements provides 

adequate safety. They not only state how safe a situation is, but identify how various features 

limit the risk and provide assurance that all paths to adverse consequences have been 

adequately addressed. Clarity and completeness in the staff’s knowledge of the safety basis 

can enhance its communications even with those less informed as to the technical details. 

2.3.2 Discussion of Screening Considerations Based on Feasibility 

The remaining three screening considerations essentially relate to the feasibility of applying risk 

information to the proposed regulatory activity. 

Screening Consideration 2 - Availability of Data and Models

Are there data and/or analytical models of sufficient quality, or could they be reasonably 

developed, to support the application of a risk-informed approach to a regulatory activity?

The factors involved in this screening consideration all relate to the amount and quality of risk 

information available: 

• What risk information is currently available in the subject area? 

• Have risk studies been done? 

• Do the studies cover the relevant hazards of the activity (e.g., the isotopes involved, the 

quantities, physical form, and chemical hazards of those isotopes)? 

• Are the studies complete and up-to-date (i.e., do they reflect the current configuration of 

the facility and/or activity)? 

• Do the studies reflect the current regulatory environment? 

• What additional studies would be needed to support decisionmaking and at what cost? 

• If computer codes are necessary, are those codes available, could they be modified, or 

would they have to be developed? 

These factors are concerned with the inventory and pedigree of the information that is available 

for the activity under consideration. Some judgment has to be made regarding its applicability, 
 2-8 

completeness, and suitability in order to decide whether further information is needed, such as 

data, analytical models, and computer codes, as well as the time frame required and the costs. 

Screening Consideration 3 - Reasonable Implementation Costs

Can startup and implementation of a risk-informed approach be realized at a reasonable cost to 

the NRC, an applicant or licensee, and/or the public, and provide a net benefit?

In applying this consideration, one should consider the net benefit of a risk-informed approach; 

(i.e., whether the benefits exceed the cost). The factors involved in this screening consideration 

are: 

• What is the societal benefit of the regulated activity? 

• What is the net benefit from applying a risk-informed approach to the activity; in other 

words, would it improve public health and safety, improve protection of the environment, 

improve communication, and improve regulatory efficiency. Would it do all this while 

assuring the same safety level, and would it also result in a cost-savings? 

The concept of net benefit is defined in NRC’s Regulatory Analysis Guidelines (Ref. 2.12) as the 

algebraic difference between the benefits of an action, or alternative, and the costs of its 

implementation when both are expressed in the same units. For instance, if the benefits are 

expressed in averted doses, they are converted to monetary units at $2000 per avoided personrem. It is useful to realize that screening considerations 2 and 3 are, in one sense, closely 

related. If insufficient data and models are currently available and new information has to be 

developed, the resources that are needed to obtain this information have to be compared to the 

benefits that the information will provide. Thus, net benefit is the criterion for implementing a 

risk-informed approach that can consist of various methods, differing in cost and quality and 

ranging from a scoping study to a detailed risk assessment. However, it is not expected that a 

full cost-benefit analysis be performed during the screening. 

Screening Consideration 4 - Existence of Other Precluding Factors

Would any other existing factor(s) limit the utility of implementing a risk-informed approach?

This screening consideration determines whether other precluding factors, such as legislative or 

judicial decisions, long-standing agency policy issues, or social considerations would limit the 

implementation of a risk-informed approach. 

Legislation (proposed or enacted) or judicial decisions might preclude alteration of the current 

regulatory approach. For example, other agencies, such as Agreement States, may be involved 

in regulating the licensees. The licensee could also fall under multiple jurisdictions. There 

could also be issues of agency policy, or current or emerging social considerations, that may 

become an obstacle to using risk information. Similarly, another potential obstacle is significant 

adverse stakeholder reaction, because of a real or perceived safety concern regarding the 

proposed risk-informed approach. These factors need to be evaluated for their impact on 

NMSS/FSME-regulated activities. 
 2-9 

2.4 Disposition of Screened-Out Regulatory Issues

If an issue is screened out after the determination that a RIDM approach is not appropriate, 

other decisionmaking means should be used. These could include a review of license 

requirements and the impact of the proposed change to these (i.e., dose, public health and 

safety, environmental protection), or impact on regulatory oversight effectiveness. Additional 

discussion on the risk-informed decision methodology is provided in Chapter 4 to guide those 

issues that are screened in to follow the RIDM process. 

2.5 References

2.1 U.S. Nuclear Regulatory Commission, “Risk-Informing the Material and Waste Arenas: 

Integration of Case Studies and Related Risk Assessments in the Nuclear Material and 

Waste Arenas,” December 2001. 

2.2 U.S. Nuclear Regulatory Commission, “Use of Probabilistic Risk Assessment in Nuclear 

Regulatory Activities: Final Policy Statement,” Federal Register, Vol. 60, No.158, August 

16, 1995. 

2.3 U.S. Nuclear Regulatory Commission, “Staff Requirements - SECY-98-144 - White 

Paper on Risk-Informed and Performance-Based Regulation,” March 1, 1999. 

2.4 U.S. Nuclear Regulatory Commission, “Risk-Informed, Performance-Based and RiskInformed, Less-Prescriptive Regulation in the Office of Nuclear Material Safety and 

Safeguards,” SECY-98-138, June 11, 1998. 

2.5 U.S. Nuclear Regulatory Commission, “Framework for Risk-Informed Regulation in the 

Office of Nuclear Material Safety and Safeguards,” SECY-99-100, March 11, 1999. 

2.6 U.S. Nuclear Regulatory Commission, Draft Screening Criteria, Federal Register, 65 FR 

14323, March 16, 2000. 

2.7 U.S. Nuclear Regulatory Commission, Case Study Plan and Revised Draft Screening 

Criteria, Federal Register, 65 FR 54323, September 7, 2000. 

2.8 U.S. Nuclear Regulatory Commission, “Staff Requirements Memorandum - SECY-99-

100- Framework for Risk-Informed Regulation in the Office of Nuclear Material Safety 

and Safeguards,” June 28, 1999. 

2.9 U.S. Nuclear Regulatory Commission, “Transcript of the Public Workshop on RiskInforming NMSS,” Washington, DC, April 25-26, 2000. 

2.10 U.S. Nuclear Regulatory Commission, “Transcript of the Public Meeting on the Draft 

Case Study Plan,” Rockville, MD, September 21, 2000. 
 2-10 

2.11 U.S. Nuclear Regulatory Commission, “Plan for Using Risk Information in the Materials 

and Waste Arena,” Case Studies Plan, prepared by the Risk Task Group, ADAMS 

Accession Number ML003759642, October 27, 2000. 

2.12 U.S. Nuclear Regulatory Commission, “Regulatory Analysis Guidelines of the U. S. 

Nuclear Regulatory Commission,” NUREG/BR-0184, January 1997. 

2.13 U.S. Nuclear Regulatory Commission, “An Approach for Using Probabilistic Risk 

Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing 

Basis,” Regulatory Guide 1.174, July 1998. 

2.14 U.S. Nuclear Regulatory Commission, “Risk-Informing the Materials and Waste Arenas: 

Integration of Case Studies and Related Risk Assessments,” ADAMS Accession 

Number ML022130067, February 2002. 

2.15 U.S. Nuclear Regulatory Commission, “FY 2008-2013 Strategic Plan," NUREG-1614 

Volume 4, February 2008. 
 3-1 

3 GUIDANCE FOR CONDUCTING RISK ASSESSMENTS 

FOR NUCLEAR MATERIAL AND WASTE APPLICATIONS 

3.1 Introduction

This chapter provides an overview of methods to perform a risk analysis that would be 

appropriate to the specific regulatory issue or licensing action. The performance of a risk 

analysis is one important element in the risk-informing process associated with regulatory 

decisionmaking. Figure 1.1 in Chapter 1 illustrates the complete RIDM process for the nuclear 

material and waste arenas. Details on common aspects for all risk assessment methods (e.g., 

project team, hazard identification/screening, delineation of accident sequences) are discussed 

in Appendix D. 

3.2 Approach

There is no single methodology for performing a risk analysis. Each particular methodology 

offers specialized schemes and tools for analyzing facilities or processes. However, all 

methodologies are systematic and tend to provide a disciplined approach to the evaluation of 

safety or risk. The materials and waste arenas present a wide range of technologies for risk 

assessment. Some technologies are comparatively complex, whereas others are rather 

straightforward. This chapter surveys the range of methodologies available and provides 

guidance on how to select an evaluation approach in a particular problem area. The four broad 

categories given in SECY-99-100 (Ref. 3.1) are used to define the areas of interest to 

NMSS/FSME. Some adaptation of these methods to particular cases may be required. 

The assessment of risk involves an estimate of both frequency and consequences. Compared 

with power reactors, materials and waste activities offer a much wider variety, as well as 

combinations, of frequencies and consequences. Some activities involve continuous exposures 

with low consequences; others involve low-frequency events with fatal consequences, but to 

only a very few persons. Only a few facilities are similar to reactors, involving low-frequency 

events with large numbers of persons potentially exposed to high consequences. Identifying the 

population at significant risk is a critical part of assessing the risk from NMSS/FSME-regulated 

applications. NMSS/FSME activities also involve a variety of safety design approaches, many 

relying on human control. This variability in risk, and in safety design, results in a variety of riskanalysis approaches. However, the fundamental considerations of frequency and consequence 

are always involved, if only implicitly. 

Central to performing a risk assessment is determining the scope and depth of the analysis that 

would support decisionmaking related to the regulatory issue or licensing action. In some 

cases, a simplified risk assessment would be warranted. In other situations, sufficient 

information on risk might already be available in reports and papers. A screening procedure is 

presented that will help to focus the more in-depth analysis on the events and situations that 

pose a higher risk. 
 3-2 

This chapter provides guidance on a top-level approach to performing a risk assessment in the 

materials and waste arenas. However, it does not give specific recommendations on how to 

perform the detailed analysis. Particular methodology options are cited and a recent 

International Atomic Energy Agency (IAEA) report (Ref. 3.2) contains a discussion of evaluation 

needs for applying a risk-informed approach to the materials and waste arenas. Particular 

attention is given to the products of an analysis, as they will be vital to the end-uses to which 

they will be applied. The IAEA report is a collaborative effort of several countries with an 

interest in risk-informed approaches to nuclear technology. It is a state-of-the-art exposition and 

represents convergence of thought on methodological approaches. The present guidance relies 

heavily on the information contained in the IAEA report. 

3.3 Grouping NMSS/FSME-Regulated Facilities, Sites, and Devices

3.3.1 Overview of Licensed Facilities

The materials and waste arenas regulated by NMSS/FSME encompass a wide range of 

technologies, types of facilities, and risks. Not surprisingly, the risks associated with these 

varying technologies also vary widely. SECY-99-100 (Ref. 3.1) recognized these variations and 

categorized the NMSS/FSME-regulated activities into four groups (Figure 3.1): (1) activities 

involving the long-term presence of nuclear material at a planned acceptable level; (2) activities 

involving the use of casks to safely store and transport nuclear material under both normal and 

off-normal conditions; (3) activities that involve the physical and chemical processing and 

possession of nuclear material; and (4) activities involving the use of sealed or unsealed

byproduct material. 

This categorization is representative of the current NMSS/FSME organization and its regulatory 

oversight. Although the groups are organized by similar facility types, significant differences 

also exist. As discussed in SECY-99-100, these differences include the facilities, systems, or 

devices employed; potential exposure pathways; risks; accident initiators and frequencies; 

accident consequences; and populations or individuals at risk. Not surprisingly, these 

differences are the major modeling considerations in the risk evaluation process for each facility 

group.
 3-3 

Figure 3.1 Grouping of NMSS/FSME-licensed activities

3.3.2 Description of Facility Grouping

Group 1 comprises facilities designed for the safe disposal of radioactive wastes, as well as 

those sites that will have residual radioactivity after operations cease. The primary issues and 

facilities included in this grouping are: the geologic repository for high-level waste (HLW); land 

disposal of low-level waste (LLW); reclamation of mill tailings disposal sites; and other general 

site cleanup issues, such as residual site contamination after the decommissioning process. 

Group 2 is concerned primarily with issues pertaining to the risks associated with the safe 

transportation and storage of spent fuel, including dry cask storage. 

Group 3 comprises those facilities and issues associated with fuel fabrication, uranium 

enrichment, and the milling of source material. Uranium mining is not included, since this is not 

regulated by NRC. Group 3 also encompasses non-nuclear risks, primarily chemical risks, 

associated with the processes and facilities employed for uranium processing and fuel 

fabrication. Until recently, these risks were not a main NRC concern, and regulation and 

oversight were accomplished by working agreements with other agencies such as the 

Occupational Safety and Health Administration (OSHA), the Mining Safety and Health 
 3-4 

Administration (MSHA), and State agencies. Recent studies at Brookhaven National Laboratory 

(BNL) and the Center for Nuclear Waste Regulatory Analysis (CNWRA) (Ref. 3.4) have 

highlighted the importance of understanding and managing chemical risks associated with the 

extraction, processing, and storage of nuclear material. 

Group 4 is very broad in nature and encompasses the use of sealed (and unsealed) sources for 

industrial radiography, irradiators, nuclear medicine, and well logging. This group includes the 

greatest diversity in terms of the number of sources and users and presents its own unique set 

of risks, which tend to be dominated by human error. Continuing progress in many fields that 

rely on these devices highlights the importance of identifying and controlling the unique risks 

associated with them. 

3.4 Risk Assessments

When the need for a risk assessment is identified, various methodology options exist for 

performing the risk assessment. The choice of method will need to be tailored to the facility or 

process being assessed, and the desired results (e.g., quantitative, qualitative). This Chapter 

will describe four specific types of assessments: the PRA, the Integrated Safety Analysis (ISA), 

the performance assessment (PA), and the barrier/hazard analysis (BHA). 

3.4.1 Why Perform Risk Analyses?

Risk analyses may be performed in response to technical and/or regulatory requirements. 

Regardless of the reason, risk assessments estimate the type and amount of damage or 

personal injury that may be anticipated from exposure to a specific risk (Ref. 3.5). An example 

of a risk analysis performed in response to a regulatory requirement would be an Integrated 

Safety Analysis (ISA). As discussed in NUREG-1513, “Integrated Safety Analysis Guidance 

Document” (Ref. 3.6), NRC promulgated a revised 10 CFR Part 70, “Domestic Licensing of 

Special Nuclear Material” (Ref. 3.7), on September 18, 2000, that addressed requirements for 

facilities using special nuclear material. This rulemaking included a requirement that these 

licensees conduct a specific type of risk assessment, an ISA. The ISA (discussed in Section 

3.5.2) would form the basis for the facility's safety program, to ensure adequate controls and 

systems are in place for continued safe operation. These regulations are applicable to all 

licensees authorized to possess greater than a critical mass of special nuclear material and 

engaged in the following: enriched uranium processing, fabrication of uranium fuel or fuel 

assemblies, uranium enrichment, fabrication of mixed-oxide fuel or fuel assemblies, scrap 

recovery of special nuclear material, or any other activity that could significantly affect public 

health and safety. The 10 CFR Part 70 regulations do not apply to gaseous diffusion plants or 

decommissioned facilities. 

In addition to regulatory requirements, a facility operator or regulator may decide to conduct a 

risk assessment purely for technical or economic reasons, such as for improved design or 

optimized operation. Such analysis would assist the regulators in assessing the significance 

and the effects anticipated from these changes. From this information, staff can evaluate 

various responses to reduce the overall significance of these risks; these could include new or 

revised regulations, changed inspection and enforcement oversight, process or equipment 

modifications, or a change in emphasis on operator performance. Risk assessments can be 
 3-5 

limited in scope and address a unique or specific process or be all-encompassing and address 

all the special hazards present at a facility, including nuclear and chemical risks. 

3.4.2 Potential Benefits from Performing Risk Analyses

In addition to the specific reason for performing a risk analysis (e.g. regulatory compliance) and 

the specific tool chosen (e.g, PRA, ISA, Hazard and Operability Analysis (HAZOP), there are 

many other potential benefits. Regulatory Guide 1.174, “An Approach for Using Probabilistic 

Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis,” 

(Ref. 3.8) encourages the increased use of PRAs to improve safety decisionmaking and 

regulatory efficiency for reactors. Similar results could be realized for specific NMSS/FSME 

applications. NRC inspectors may also benefit from such studies by focusing their oversight on 

the most important aspects of a facility's operation. 

Licensees can use risk analyses to support decisions to modify a facility's licensing basis, for 

example through license amendments or technical specification changes. Licensees and staff 

can obtain specific information on whether a facility meets current regulations, maintains a 

defense in depth philosophy, provides sufficient safety margins; and develops performance 

measurement strategies to monitor change over time. Chapter 4 of this document provides 

additional information in these areas. When performing the detailed review necessary to 

support a risk analysis, the licensee may identify regulatory requirements or commitments that 

may be deemed overly restrictive or unnecessary, or may discover a lack of compliance in some 

area. Similarly, such an analysis may identify aspects of a design or operational process that 

require enhanced safety measures. Defense in depth is an important component of power 

reactor design that, until recently, was not commonly thought of in the NMSS/FSME arena. In a 

letter to the Commission (Ref. 3.9), the chairmen of the Advisory Committee for Nuclear Waste 

and Materials (ACNW&M) and the Advisory Committee for Reactor Safeguards (ACRS) 

observed: 

• The treatment of defense in depth for transportation, storage, processing, and fabrication 

should be similar to its treatment for reactors. Defense in depth for industrial and 

medical applications can be minimal and addressed on the basis of actuarial information. 

• Defense in depth for protecting the public health and environment from HLW repositories 

is both a technical and policy issue that requires a reasonable balance. 

• Since the balancing of compensatory measures to achieve defense in depth depends 

the acceptability of the risk posed by the activity or facility, risk-acceptance criteria 

should be developed for all NMSS/FSME activities. 

3.4.3 The Use of Expert Judgment in Risk Analysis

Judgment is important on at least two levels of the RIDM process. First, judgment is used in the 

development of the risk analysis itself; second, it is used by the decision-maker in the 

decisionmaking process. The decision-maker may not be the analyst and thus, if a decision is 

being made on the basis of risk analysis information, the decision-maker may not have a good 

sense of the degree of judgment that was used in the analysis. 
 3-6 

Judgment is used in many aspects of the qualitative and quantitative developments of any risk 

analysis. At present, there is no unambiguous indicator of the degree to which any given risk 

analysis rests on judgment and of the degree to which it rests on empirical data. The computed 

risk indices contain quantified tolerance bands. However, these uncertainty bands usually 

incorporate an imprecisely specified mixture of contributors to uncertainty, including inherent 

randomness and lack of knowledge. Although it is important for the decision-maker to be aware 

of the degree to which a risk analysis was dependent on judgment, there is currently no precise 

way to measure it. 

One would expect that the more aware the decision-maker is of the technical nuances of 

judgment in risk analysis, the more likely it would be that his or her judgment in the 

decisionmaking process would reflect the judgment used by the analysts. Ideally, results of 

analysis that are based on judgment should be presented to a decision-maker in a format that is 

compatible with his or her level of understanding of the technical details. Results of alternative 

models, assumptions, etc., should be presented in a similar fashion. This information would 

allow the decision-maker to assess the impact of subjective information on results. 

Risk analyses should be performed on a best-estimate, realistic basis. Staff should avoid both 

conservatism or optimism should be in arriving at the final results. Ideally, all parameters and 

models that make up the risk analyses would be characterized by uncertainty distributions that 

would encompass the range of physically realizable situations. These uncertainties would be 

continued throughout the analysis and would be included in the end results. It is important that 

staff fully understand uncertainties and sensitivities before a decision is reached. 

Conservatism (or optimism) can then be applied by the decision-makers in drawing conclusions 

and taking actions based on the knowledge of the results of the risk analysis. 

3.5 Tools to Assess Risks

The purpose of this section is to provide a brief description and overview of several 

representative risk-assessment techniques. After reading this section, the reader will hopefully 

understand why a particular assessment methodology is applied to a specific group of facilities. 

This section does not intend to provide detailed instructions on applying the techniques in the 

assessment of NMSS/FSME-regulated facilities. 

Attachments 2 and 3 to SECY-99-100 identified suggested risk assessment methods for each of 

the four groups and the potential regulatory use of these methods for specific NMSS/FSMEregulated activities. NRC's PRA Policy Statement provides general guidance on the regulatory 

uses of risk assessment. Staff can implement this general guidance in many ways. However, 

as discussed, in each case, staff should address two principal considerations: 

(1) What specific use is the staff expected to make of risk insights and risk assessment in 

the development of regulations and guidance for licensing, inspection, assessment, and 

enforcement? 

(2) What specific use is the licensee expected to make of risk insights and risk assessment 

in planning and conducting its operations? 
 3-7 

In answering these questions, staff must consider numerous facility- and activity-specific factors, 

such as the hazard and complexity of the activity, the amount of human involvement in the 

activity, and the technical sophistication of the licensee. For any given facility or activity, staff 

may find more than one risk-assessment approach suitable. For some, a qualitative use of risk 

assessment will suffice, whereas others will require a quantitative approach. The authors do not 

anticipate extensive probabilistic risk studies by each licensee and for each facility. Previously 

completed risk assessments may be used if available (e.g., NUREG/CR-6642, “Risk Analysis 

and Evaluation of Regulatory Options for Nuclear Byproduct Material Systems” and 

NUREG/CR-6672 for spent fuel transportation). The majority of material licensing applications 

are not expected to require sophisticated risk analyses. It should be noted, however, that, 

compared to operating power reactors, the application of risk assessment to the very diverse set 

of NMSS/FSME-regulated activities and facilities is in a relatively early stage. 

The methods that will be briefly described in this chapter are Probabilistic Risk Analysis (PRA), 

Integrated Safety Analysis (ISA), Performance Assessment (PA), and Barrier/Hazard method 

(BHA). 

3.5.1 Probabilistic Risk Analysis (PRA)

PRA is an analytic tool that can be used by both regulators and industry to protect public health 

and safety. The term “PRA” is used synonymously with Probabilistic Safety Assessments 

(PSAs). However, a PSA generally deals with safety-related issues, whereas a PRA may deal 

with both safety and non-safety issues. The scope, level of detail, tools, and methods needed to 

perform a PSA or a PRA are the same. In Europe, regulators prefer the term PSA because of 

how they use the results from the analysis with regard to safety decisions. In the US, the NRC 

prefers the term PRA, because the analysis is a calculation of the “risk” (likelihood and 

consequences) and not a calculation of safety (even though the results from the PRA may be 

used in safety decisions). In the remainder of this paper, the term “PRA” will be used to denote 

both concepts. Staff can use PRAs to compute the probability of health, environmental, and 

economic consequences of events caused by equipment or human failure. 

The birth of the PRA methodology is frequently traced to NUREG-75/014, “Reactor Safety 

Study” commonly known as WASH-1400 (Ref. 3.10). The most significant result of this work 

was that it provided a pattern for using PRAs at nuclear power plants, introduced the fault-tree 

concept to a large audience, and developed a reliability database. Experience with applications 

of this method showed that risk contributors can come from many areas associated with facility 

operation, not just catastrophic events, as previously thought. This study also discussed the 

importance of human performance, test and maintenance, and the concept of common-mode 

interactions. 

With respect to the use of a PRA by NMSS/FSME, staff can define the following general 

objectives: 

• Identify initiating events and event sequences that may contribute to risk. 

• Provide realistic quantitative measures for the likelihood of the risk contributors. 

• Provide a realistic evaluation of the potential consequences associated with hypothetical 

accident sequences. 
 3-8 

• Provide a reasonable risk-informed framework for making regulatory decisions regarding 

facility design, operation, and siting. 

One outcome of a PRA is a list of plant or facility responses to initiating events, and the 

sequences of events that can be expected. Through analysis of the significance of each 

identified risk contributor, it is possible to identify the high-risk sequences and then implement 

actions to minimize or mitigate them. These evaluations may include estimates of latent 

cancers, immediate fatalities, facility or plant damage, or other consequence measures. 

To accomplish these objectives, a PRA relies on several important steps. One critical step is 

the collaboration of experts from different disciplines. This group may characterize the normal 

and off-normal modes for the facility; develop and codify scenarios for the off-normal events; 

gather reliability data; examine the transport of effluents throughout the facility and offsite in the 

event of an off-normal event; and provide an understanding of other health, environmental, or 

economic impacts. 

A PRA incorporates a logic model that identifies events that can lead to failures. The logic 

model incorporates two types of logic trees, the event-tree and the fault-tree. Fault-tree analysis 

is a deductive procedure for determining failures and human errors that could result in the 

occurrence of specified undesired events (also referred to as top events). The main purpose of 

the fault-tree, through deductive logic, is to identify the different failure pathways of the top 

event. Statistical methods are used to estimate the failure probabilities and unavailabilities of 

the basic events identified in the fault tree. Boolean algebra is used to estimate the failure 

probability of the top event. These calculations involve quantitative reliability and maintenance 

information, such as failure probability, failure rate, and repair rate. The event-tree analysis, by 

comparison, is inductive by nature. This analysis moves forward in time to delineate events 

generally through two-level logic trees: yes/no and fail/success. Overall descriptions of the 

methodologies and tools to perform a PRA are provided in NUREG/CR-2300, “PRA Procedures 

Guide - A Guide to the Performance of Probabilistic Risk Assessments for Nuclear Power 

Plants” (Ref. 3.11) and NUREG/CR-2815, “Probabilistic Safety Analysis Procedures Guide” 

(Ref. 3.12). These sources, while useful, are dated. Much work has been accomplished in 

continuing development of PRA standards. Staff has endorsed the PRA standards in RG 1.200, 

“An Approach for Determining the Technical Adequacy of Probabilistic Risk Assessment Results 

for Risk-Informed Activities.” One of the most critical parts of such an analysis is reliability data 

for various systems and components. Depending on the particular facility or process being 

modeled, data sources may include published data or actual facility data. Commercially 

available software is also available to assist with fault-tree and event-tree setup and analysis, as 

well as reliability and maintenance analyses. 

The understanding and modeling of human behavior related to accident evolution is important in 

PRA. This area has been the focus of extensive research, and NUREG/CR-1278, “Handbook of 

Human Reliability Analysis” (Ref. 3.13) provides examples of the results. Software models for 

the evaluation of human performance are also available. Staff must also understand external 

events (such as fires, floods, hurricanes, and seismic events) when quantifying risk for a given 

facility. To model these events, the likelihood and severity of each event, and the possible 

impact on the plant, must be evaluated. This includes an assessment of the impact of the 

external event on emergency planning. 
 3-9 

Summary of PRA Method

A PRA analysis allows for a holistic and quantitative assessment of a given facility, process, or 

piece of equipment. Historical analysis has demonstrated the usefulness of the technique in 

modeling chemical plant accidents, the effect of accidents at these facilities on onsite/offsite 

risks and consequences, including latent cancers, and the financial implications of such events. 

However, past PRAs have shown that, depending on the intended use, modeling and evaluation 

techniques, quality of reliability data, assessment of the impact of human failure, external 

events, or consequence analysis may need to be further developed. Over the last 30 years, 

PRAs were mostly performed for light water reactors. The steps performed as part of the PRAs 

vary depending on the specific applications involved. 

3.5.2 Integrated Safety Analysis (ISA)

Consistent with 10 CFR Part 70, “Domestic Licensing of Special Nuclear Material” and as 

defined in NUREG-CR1513, “Integrated Safety Analysis Guidance Document” (Ref. 3.6), an ISA 

is a systematic examination of a facility's processes, equipment, structures, and personnel 

activities, to ensure that all relevant hazards that may result in unacceptable consequences 

have been evaluated and appropriate protective measures implemented. The chemical industry 

also refers to ISA as a Process Hazard Analysis. 

The regulatory basis for performing an ISA is contained in 10 CFR Part 70, which includes a 

requirement for certain licensees and applicants to conduct an ISA. The ISA is expected to 

form the basis of a safety program that requires adequate controls and systems to ensure safe 

facility operation. When applied to nuclear fuel cycle facilities, ISA techniques must address all 

applicable hazards and their potential for causing criticality incidents and radiological and 

chemical releases. 

ISAs provide: 

• A description of the structures, equipment, and process activities at the facility. 

• An identification and systematic analysis of hazards at the facility. 

• A comprehensive identification of potential accident or event sequences that would 

result in unacceptable consequences, and the expected likelihood of those sequences. 

• An identification and description of controls that are relied upon to limit or prevent 

accidents and mitigate the consequences. 

• An identification of measures taken to ensure the availability and reliability of the 

identified safety systems. 

For NMSS/FSME-regulated facilities, unacceptable consequences include those that result in 

radiological and chemical exposure to workers or the nearby population. Exposure could be the 

result of a radioactive release, criticality incident, explosion, or other unplanned event. 
 3-10 

ISA techniques use either an inductive or deductive analysis approach. The inductive analysis 

is a bottom-up approach that identifies possible accident sequences by examining deviations 

from normal operating conditions. The deductive, or top-down approach, is better suited for 

identifying the combinations of equipment failures and human errors that can result in an 

accident. The deductive approach identifies a top event that is usually a severe consequence 

and explains the various ways this event can occur. By determining root causes, the deductive 

approach provides assurance that common-mode failures are understood and addressed. One 

effective approach for implementing an ISA program is to combine both approaches. Staff can 

first use the inductive approach to identify the broad range of potential accidents and then turn 

to the deductive approach analyze a potential accident in detail. 

Staff can use a number of ISA hazard evaluation methods: 

$ Safety review 

$ Checklist analysis 

$ Relative event ranking; 

$ Preliminary hazard analysis 

$ What-if analysis 

$ What-if/checklist analysis 

$ HAZOP 

$ Failure Modes and Effects Analysis (FMEA) 

$ Fault-tree analysis 

$ Event-tree analysis 

$ Cause-consequence analysis 

$ Human reliability analysis 

Compared to the previously described PRA and its quantitative results, the insights from an ISA 

are qualitative or semi-quantitative, depending on the hazard evaluation technique used. Some 

of these methods (e.g., HAZOP, FMEA, fault tree, and event tree) may be used to provide input 

to a quantitative risk assessment. 

The choice of the particular method will depend on the reason for conducting an analysis, the 

results needed, the information available, the complexity of the process, the availability of 

experienced personnel, and the perceived risk of the process. Appendix A of NUREG-1513 , 

“Integrated Safety Analysis Guidance Document” provides detailed flow charts that guide the 

choice of a particular method. Regardless of the method chosen, a team that is knowledgeable 

and experienced is essential to ensure the success of the effort. 

Before conducting the ISA, the scope of the analysis must be defined, including the 

consequences of concern. For NMSS/FSME, these include radiological events, criticality, and 

chemical consequences that can affect public and worker health and safety. High-consequence 

events are those that have an acute effect on workers and offsite populations. Intermediate 

consequences are similar, but have lower exposure levels. To ensure an acceptable level of 

risk, 10 CFR Part 70.61, “Performance requirements” requires controls to ensure that the 

occurrence of any credible high-consequence event is highly unlikely and that the occurrence of 

an intermediate-consequence event is also unlikely. The ISA should also include site 

characteristics, structures on site, equipment and processes, and operating personnel, as well 

as credible external events. 
 3-11 

Detailed and accurate process information is essential. This is one of the main reasons that a 

knowledgeable and experienced team must be assembled. To identify hazards in a facility, 

information on the materials used at the facility should be available. Staff should identify all 

materials that pose radiological risks, including what conditions would be necessary to support a 

self-sustaining reaction. For chemicals, staff should collect information on toxicity, flammability, 

reactivity, and other pertinent properties that may affect safety. The analysis should account for 

any interactions between the various hazards. 

The results of the ISA provide several types of risk information: 

(1) A list of the potential accident sequences 

(2) The consequences of each sequence 

(3) Information showing compliance with the performance requirements, which include 

qualitative likelihood requirements; e.g., likely, highly unlikely, and unlikely 

(4) A description of the controls designed to prevent or mitigate accidents ("items relied on 

for safety") 

Documenting the ISA lends credibility to its results by making the basis scrutable and by 

demonstrating the robustness of the analysis. Subpart H, “Additional Requirements for Certain 

Licensees Authorized To Possess a Critical Mass of Special Nuclear Material,” of 10 CFR Part 

70 specifies documentation requirements for ISAs, and guidance on this subject is provided in 

NUREG-1520, “Standard Review Plan (SRP) for the Review of a License Application for a Fuel 

Cycle Facility,” and NUREG-1513, “Integrated Safety Analysis Guidance Document.” In 

addition to the NRC staff guidance on ISAs, the Nuclear Energy Institue has also provided 

guidance on the contents of ISA Summaries to NMSS/FSME Iicensees (Ref. 3.14). 

Summary of ISA 

The ISA is a form of analysis required by the NRC of specified licensees to support their overall 

safety program and to provide a documented safety basis. It is proving suitable for that 

purpose. A variety of analysis techniques can be used in an ISA. Different techniques may be 

used for different processes, to attain the appropriate regulatory results. These techniques 

include FMEA, HAZOP, fault trees, and others. These same techniques may be useful for 

general risk-informed decisionmaking, to address regulatory activities other than compliance 

with the ISA requirements. In fact, these techniques are widely used in industries other than 

nuclear industry. Guidance on the appropriate use of these techniques is found in NUREG1513, “Integrated Safety Analysis Guidance Document.”. 

3.5.3 Performance Assessment (PA)

As discussed in Attachment 2 to SECY-99-100, a PA is the risk assessment method of choice 

for Group 1 facilities. These types of facilities conduct HLW and LLW disposal, 

decommissioning (residual contamination), and mill tailings reclamation. For these types of 

facilities, the main concern is the potential exposure of receptors to radionuclides that are 

released by natural transport processes through various media, such as rocks, soils, and water. 

The main threat is environmental damage from long-term leaching of contaminants. The typical 

components comprising a PA are shown in Figure 3.2 (Ref. 3.15). 
 3-12 

A PA is a systematic safety analysis that assesses what can happen, how likely it is, what the 

potential impacts are, and how these impacts compare to the regulations. For NMSS/FSME 

waste management facilities, the essential elements of a PA include: 

• Description of the site 

• Analysis of the events that would affect long-term facility performance 

• Analysis of the movement of radionuclides from the place of storage to the environment 

(pathway analysis) 

• Estimates of resultant doses to workers and the general public 

• Evaluation of the existing uncertainties 

The PA is a quantitative assessment method tailored to meet the risk assessment needs of an 

individual waste facility. For a simple disposal question, a bounding type of analysis may be 

appropriate, whereas for more complex facilities, a probabilistic analysis is called for. The PA is 

designed to be an iterative process. Initial screening calculations can provide insights on site 

performance. The initial screening process should also provide information on issues and data 

needs that should be evaluated during site characterization. As additional data and information 

become available, the modeling assumptions and input data can be re-evaluated and revised 

accordingly. To provide a robust result, staff must include uncertainty and sensitivity analyses. 

The most common sources of uncertainty in PAs are conceptual and process models, 

scenarios, data, parameters, and other coefficients. The purpose of a sensitivity analysis is to 

identify important parameters by determining the relative effect of each variable on the result. 
 3-13 

As with all risk assessment models, documentation of the results and the assumptions used is 

critical. This requires information detailing the qualitative and quantitative data used, the 

conceptual models employed, the mathematical models used to represent the conceptual 

models, the computer codes used, sensitivity analyses, and a complete interpretation of the 

results. 

With regard to NMSS/FSME regulation, a PA has several main uses. It can provide insights for 

rulemaking or the development of regulatory guidance. It allows for the review and 

understanding of licensee-submitted documentation, demonstrating compliance with regulations 

or providing justifications for requesting exemptions. A PA can also support the efficient 

expenditure of resources (money and personnel) and the identification of needed research. 

Summary of PA 

The quantitative aspects of the PA technique mirror the PRA methodology. It can be performed 

in an iterative fashion, incorporating new data and site characteristics as they become available. 

It identifies important issues and data needs that must be incorporated into the investigation and 

characterization of a waste disposal site. To provide defensible results, uncertainty and 

sensitivity analyses must be part of the PA process. Commonly cited sources of uncertainty 

focus on the model, process, and data uncertainties. Sensitivity studies identify the important 

parameters by determining the relative effect of each variable on the total system. For a PA to 

be successful, staff must be confident that the models used are performing as designed and are 

Iteration as Required 

Facility System Description 

• Source Term/Waste form 

• Eng. Description 

• Site Description

Scenario Analysis 

• Identification 

• Screening 

• Probability

Consequence Modeling 

• Source Term/ Waste form 

• Eng. Barriers 

• Geo-sphere 

• Biosphere

Performance Calculation 

• Distribution of Doses

• Other Measures 

• Sensitivity 

• Uncertainty 

• Importance Analysis

• Comparison to Regulatory 

Standards; 

• Documentation 

Figure 3.2 Components of a total system PA
 3-14 

capable of capturing all relevant features and processes of the disposal system being modeled 

(Ref. 3.16). Gauging this through formal validation exercises is often not practical because of 

the time required. 

Chronic doses and their implied health effects are assessed for a period of time into the future. 

It is virtually impossible to predict, with any degree of certainty, beyond several decades in the 

future. Staff may need to evaluate probabilistically the likelihood of a resident farmer scenario, 

a building-occupancy scenario, and other potential scenarios, as well as the nature of the critical 

group. The authors recognize that there are great uncertainties in this area, and these 

uncertainties could be expressed in an analysis. Model and parameter uncertainties for the 

dose assessments could also be reflected in the quantitative assessment of risk. Staff would 

also have to assess the probabilities for the occurrence of natural events that may disrupt a site 

over a long time period of time. Staff would calculate the consequences of an event in terms of 

transport of radionuclides to the environment and their potential health effects. Staff would also 

assess human intrusions to the site. 

It is difficult for the staff to assess the long-term human and environmental condition of the site. 

Current stylized scenarios may be unduly conservative. The IAEA sponsored the BIOMASS 

Program (Ref. 3.17), one attempt to assess the long-term radiological impact on a site. One 

objective of this program was to develop the concept of "reference biospheres," to assess the 

long-term safety of repositories for radioactive waste. This would also have implications for the 

residual radiological impact of a site that has been decommissioned. The basic idea was to 

develop a subset of sample biospheres that can provide a useful point of reference as broadly 

applicable indicators of potential radiological releases. Difficulties arose with the "reference 

biosphere," when researchers considered a wide variety of climates and topographies. The 

researchers produced a process to promote transparency and traceability on the biosphere 

modeling effort. Application guidance for this method is provided in NUREG-1757, 

"Consolidated NMSS Decommissioning Guidance--Characterization, Survey, and Determination 

of Radiologic Criteria." 

3.5.4 Barrier/Hazard Analysis (BHA)

As discussed in SECY-99-100, staff can analyze the risks associated with the use of either 

sealed or unsealed byproduct material for a wide variety of industrial and medical applications 

(identified as Group 4) by using several different risk assessment techniques. In the past 

decade, PRA was used to study the risk associated with specific medical procedures. Though 

the result was positive, it was costly and had some specific limitations. Although analysts 

identified human error as the principal accident initiator, they found that the fault-tree/event-tree 

methodology was an inadequate tool for analyzing such accidents. SECY-99-062 (Ref. 3.18) 

discusses NRC progress in risk assessment methods for this group. Staff has identified the 

BHA method as being most useful for this group. 

NUREG/CR-6642., “Risk Analysis and Evaluation of Regulatory Options for Nuclear Byproduct 

Material Systems,” (Ref. 3.19) discusses this assessment technique and ranks byproduct 

material systems according to risk (similar to the various programs identified in Inspection 

Manual Chapter 2800) (Ref. 3.20). This report evaluated the various scenarios that may result in 

worker or public dose, and their probability of occurrence. This ranking was based on 
 3-15 

quantitative measures of several risk measures addressing individual worker dose and normal 

use versus accident scenarios. 

Similar to all risk assessment methods, the first step is a comprehensive understanding of the 

nature and uses of byproduct materials throughout the life cycle of the radioactive material, from 

receipt by the user to final disposal. These topics include: 

• System scope, including specific byproduct material used 

• Basic process, equipment, and devices involved 

• Review of current byproduct regulations 

• Identification of hazards 

• Tasks associated with the handling and use of byproduct material 

• Personnel exposures 

• Radioactive barriers and controls during normal and off-normal conditions 

• Potential accidents and occurrences that could result in inadvertent exposure 

The information required to complete this data gathering can be obtained from a variety of NRC, 

industry, and independent sources. NRC databases of value include the Licensing Tracking 

System (LTS); Sealed Source and Device Registry (Ref. 3.21); and the Nuclear Material Events 

Database (Ref. 3.22). The LTS is an internal NRC database that contains information regarding 

general and specific licenses issued; it can be accessed only by NRC staff. Other databases 

are maintained by Agreement States (Ref. 3.23), and can also provide occupational exposures 

and event descriptions. 

For each system, analysts have identified he barriers and controls that could prevent or mitigate 

radiological hazards from byproduct material Barriers or controls are anything that limit or 

reduce the "likelihood," "magnitude," or "radiation exposure" (occupational or public), under both 

normal and off-normal conditions. Barriers and controls may be either physical barriers or 

administrative controls. Then the analysts performed a preliminary hazard analysis. This 

approach is very similar to the HAZOP technique. The analysts used results of this in the risk 

assessment to develop accident scenarios. 

In performing the radiation-risk assessment, several exposure pathways are explicitly analyzed. 

These include external exposure from the source, inhalation of radioactive material released to 

the air, ingestion of radioactive material, and external exposure resulting from submersion in an 

airborne radioactive material cloud. As part of the analysis, staff should also considered the 

dose attributed to ground contamination from airborne transport and deposition to an offsite 

location. Appendix A of NUREG/CR-6642, “Risk Analysis and Evaluation of Regulatory Options 

for Nuclear Byproduct Material Systems,” provides equations to evaluate the consequences of 

each of these pathways. 

The next step in the process is for the analysts to develop the potential scenarios and end 

states that could lead to radiation exposure. The analysts developed a set of credible scenarios 

based on actual experiences. The analysts determined the estimated frequency of each 

scenario and used it to quantify the risk. Although the approach is to quantify the risk, valid data 

for all scenarios do not exist. For these instances, the analysts made a qualitative assessment 

of the likelihood. Historical data, where available, offers the best source of risk quantification. 
 3-16 

The Byproduct Material System Risk Assessment Database (Ref. 3.24) is a software program 

that performs the calculation of byproduct material risk, given specific normal and off-normal 

conditions. A description of this method is provided in NUREG/CR-6642. 

Summary of the Barrier/Hazard Analysis Method 

The BHA method is a relatively straightforward risk assessment method that staff can be apply 

to a wide variety of types and uses of byproduct material. The overall risk is provided in 

exposure (mrem) per event. It is a quantitative method, and like all analytical methods, is 

limited by the robustness of the assumptions underlying the modeling of off-normal events and 

accidents and the variability of the data contained in the various databases. Analysts can use 

Its results to assess the adequacy of the applicable regulatory requirements. For example, with 

these applications, one of the most commonly invoked barriers is public access control and 

source control. 

3.6 References

3.1 U.S. Nuclear Regulatory Commission, “Framework For Risk-Informed Regulation in the 

Office of Nuclear Material Safety and Safeguards,” SECY-99-100, March 31, 1999. 

3.2 IAEA Technical Document, “Procedures for Conducting Probabilistic Safety Assessment 

(PSA) For Non-Reactor Nuclear Facilities,” IAEA-TECDOC (draft), 2001. 

3.3 U.S. Nuclear Regulatory Commission, Bari, R. A, and E. Grove, “A Case Study on RiskInforming Uranium Recovery,” Vol. 2, Appendix E of Risk-Informing the Materials and 

Waste Arenas: Integration of Case Studies and Related Risk Assessments, February 

2002, ADAMS Accession Number ML022130067. 

3.4 U.S. Nuclear Regulatory Commission, Mackin, P., et aI., “A Baseline Risk-Informed 

Performance-Based Approach for In situ Leach Uranium Extraction Licensees,” 

NUREG/CR-6733, June 2001. 

3.5 U.S. General Accounting Office, “Chemical Risk Assessment: Selected Federal 

Agencies’ Procedures, Assumptions, and Policies,” GAO-01-810, August 2001. 

3.6 U.S. Nuclear Regulatory Commission, “Integrated Safety Analysis Guidance Document,” 

NUREG-1513, May 2001. 

3.7 U.S. Code of Federal Regulations, “Domestic Licensing of Special Nuclear Material,” 

Part 70, Title 10, “Energy.” 

3.8 U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research 

Regulatory Guide 1.174, “An Approach for Using Probabilistic Risk Assessment In RiskInformed Decisions on Plant-Specific Changes to the Licensing Basis,” July 1998. 
 3-17 

3.9 U.S. Nuclear Regulatory Commission, Letter from B.J. Garrick and D.A Powers to R.A 

Meserve, “Use of Defense in Depth In Risk-Informing NMSS Activities,” May 25, 2000. 

3.10 Atomic Energy Commission (U.S.), “The Reactor Safety Study,” WASH-1400, NUREG75/014, U.S. Nuclear Regulatory Commission, October 1975. 

3.11 U.S. Nuclear Regulatory Commission, “PRA Procedures Guide - A Guide to the 

Performance of Probabilistic Risk Assessments for Nuclear Power Plants,” NUREG/CR2300, 1983. 

3.12 U.S. Nuclear Regulatory Commission, “Probabilistic Safety Analysis Procedures Guide,” 

NUREG/CR-2815, August 1985. 

3.13 U.S. Nuclear Regulatory Commission, “Handbook of Human Reliability Analysis,” 

NUREG/CR-1278, August 1978. 

3.14 Nuclear Energy Institute, “Industry Guidance Document on Preparation of an ISA 

Summary,” November 2000. 

3.15 Eisenberg, N. A, et aI., “Development of a Performance Assessment Capability in the 

Waste Management Programs of the U.S. Nuclear Regulatory Commission,” Risk 

Analysis, Vol.19, No.5, 1999. 

3.16 Vasseur, D., et aI., “International Survey on PSA Figures of Merit,” Reliability 

Engineering and System Safety, 66 (3), 261-274, 1999. 

3.17 IAEA, “Long-Term Releases from Solid Waste Disposal Facilities: the Reference 

Biosphere Concept,” Draft TECDOC, BIOMASS Theme 1, BIOMASS/T1/WD03, 1999. 

(ADAMS Accession Number MLO03698639). 

3.18 U.S. Nuclear Regulatory Commission, “Nuclear Byproduct Material Risk Review,” SECY99-062, March 1, 1999. 

3.19 U.S. Nuclear Regulatory Commission, “Risk Analysis and Evaluation of Regulatory 

Options for Nuclear Byproduct Material Systems,” NUREG/CR-6642, 2000. 

3.20 U.S. Nuclear Regulatory Commission Inspection Manual, Chapter 2800, "Materials 

Inspection Program," September 25,2003. 

3.21 Oak Ridge National Laboratory, “Index of Radioactive Sealed Sources and Devices,” 

http://www.hsrd.ornl.qov/nrc/sources/index.cfm. 

3.22 Idaho National Engineering and Environmental Laboratory, "Nuclear material Event 

Database (NMED)," http://nmed.inel.qov/. 

3.23 Bureau of Radiation Control, Texas Department of Health, 2001. 

3.24 Arcieri, W., “BMS-RISK Database Users Manual,” Scientech, Inc., December 1999. 
 4-1 

4 RISK-INFORMED DECISIONMAKING METHODS 

4.1 Introduction

4.1.1 Decisionmaking Methods in the Overall Process

Figure 1.1 illustrates the overall RIDM process for NMSS/FSME-regulated activities and 

facilities. Figure 4.1 is an expanded version of Figure 1.1. It involves four basic steps, of which 

the final one is to apply structured algorithms. The decision-maker can use the structured 

algorithms to identify which alternatives appear preferable, based on risk insights and other 

considerations. This chapter suggests two such structured decision algorithms that will cover 

most, though not all, regulatory decision situations. 

Like Figure 1.1, the starting point in Figure 4.1 is the identification of the regulatory issue or 

question. After the particular issue is identified, the next step is to apply the screening 

considerations. Staff should apply screening consideration number 4 (Exhibit 2.1) to assess if 

there are judicial, statutory, policy, or similar considerations that would preclude the 

implementation of a risk-informed decision process. After applying screening consideration 

number 4, staff can consider the content of the issue to identify whether risk information is 

needed. For example, if the issue is purely administrative and does not affect safety, it is 

screened out. 

If the issue is not screened out, then staff should gather information to address the risk-related 

aspects of the issue or action alternatives. Staff can then apply screening considerations 2 and 

3 to assess the amount and quality of available quantitative risk information and, if it is not 

available, whether the cost of obtaining it is reasonable and commensurate with the issue under 

consideration. If it is not, then a qualitative risk assessment is performed, or adapted, to assess 

the change in risk qualitatively, as shown in Figure 4.1. If adequate quantitative risk information 

is available, staff can perform or adapt a quantitative risk assessment. In performing or 

adapting the risk assessment, it is important to ensure that the scope and depth of the analysis 

are appropriate to the issue; the risk metrics relevant to the issue, and their associated 

uncertainty, are reasonably well-characterized; and the recipients of the risk are identified. If 

applicable, staff can then apply the decision logic for various types of regulatory actions 

(Ref. 4.2), as shown in Figure 4.1. 
 4-2 

4.1.2 Decision Methods and Algorithms

This document describes specific decision algorithms in Section 4.2. The first two algorithms 

address a very general situation where changes are proposed to existing requirements. To be 

applicable to the wide variety of situations and regulated activities in NMSS/FSME, these two 

algorithms address a full list of attributes that may affect the decision. The first algorithm 

focuses on exemptions or changes to existing license requirements that might result in 

significant impacts to cost, risk, security, or the environment. The second algorithm is 

applicable to proposed new regulatory requirements, especially those proposed for the 

purposes of reducing risk. This algorithm is closely related to the type of regulatory analysis 

(Ref. 4.1) applied to justify backfit. However, staff can apply it to any proposed requirement that 

might have significant impacts. Both of these first two algorithms apply criteria that consider risk 

to individual workers and members of the public. If the proposed regulatory change(s) meet 

these risk criteria, further criteria are applied in accordance with regulatory analysis guidance 

(Refs. 1.2 and 1.3). If all these criteria are satisfied, staff can perform a quantitative valueimpact analysis. A value-impact analysis evaluates the effect of each of the proposed changes 

on costs, public health, security, and the environment. These impacts are considered jointly 

(summed if quantitative), to judge which of the alternatives has the highest net positive impact. 

The information in this chapter is supplemented with two Appendices. Appendix B provides an 

overview of value-impact analysis and also includes examples of generic tables that can be 

used by staff to perform simple value-impact assessments. Appendix E provides a brief 

discussion of available information and reference sources for risk information in the materials 

and waste arenas. 

4.1.3 Attributes Considered in RIDM

A risk-informed approach, as defined by NRC, considers both risk insights and other 

information. In fact, there are a number of different risk criteria to apply, and a large number of 

other attributes that need to be considered in making regulatory changes. Some of these will be 

listed here for information, but detailed guidance and a more extensive list of these attributes 

are found in NUREG/BR-0184, “Regulatory Analysis Technical Evaluation Handbook.” The 

main purpose of this chapter is to provide risk criteria for accident risks applicable to nonreactor 

accidents. For reactors, risk criteria are found in NUREG/BR-0058, “Regulatory Analysis 

Guidelines of the U.S. Nuclear Regulatory Commission,” applicable to decision situation 2, new 

requirements (backfit), or in Regulatory Guide (RG) 1.174, for decision situation 1, exemptions 

and changes. No analogous risk criteria exist for accident risks that are generically applicable 

to these same decision situations in the nuclear material and waste arenas. Risk insights 

addressing routine or chronic doses for materials and waste can generally be obtained from 

existing guidance based on 10 CFR Part 20, “Standards for Protection Against Radiation” 

annual dose limits and other requirements. 

Staff developed draft quantitative risk guidelines to address the lack of generic guidance on the 

risk of accidents to individuals. Staff needed these guidelines to apply the structured decision 

algorithms. Chapter 5 describes these risk guidelines and the basis for them. Section 4.2 

describes their use. The draft risk guidelines represent an accident-risk reference level that can 

be regarded as a negligible additional risk, compared to the risk faced by workers or the public 

from operations. If the estimated risk is greater than the risk guidelines, the decision-maker 
 4-3 

needs to decide if additional analysis (e.g., a value-impact analysis) is necessary to determine 

whether a further reduction in accident risk is needed. These guidelines are not regulatory 

requirements, nor do they exempt any licensee from compliance with requirements. 

Staff should evaluate each proposal for a change to a licensing basis in a similar fashion, to 

ensure that all attributes for which the agency is responsible are addressed. This of course 

includes public health, safety, and environmental protection. The following discusses some of 

the attributes that are assessed when applying the methods in Section 4.2, together with 

regulatory analysis. 

When considering a proposed change to a regulatory requirement, the decision to proceed or 

grant the change should be based on the evaluation of all relevant attributes. Following are 

some of the more important attributes: 

(1) Staff should consider the effect of the proposed change on regulatory requirements 

other than the one being changed. Compliance must be maintained with all other 

requirements. One cannot just focus on the desired effect of the proposed change but 

must assess its overall effects as well. 

(2) Staff should consider the effect of the proposed change on the defense in depth 

philosophy. Defense in depth guards against over-reliance on any one safety feature. 

For example, defense in depth may be provided by additional barriers, operating 

procedures, and limits, or by redundant and diverse equipment design. Staff must 

evaluate any changes that result in the elimination of a layer of protection and fully 

understand the consequences. 

(3) Staff should consider the effect of the proposed change on safety margins. Quantitative 

risk models may not directly capture the effect of reduced margins. Staff should 

therefore be aware of the need to maintain margins, consider the possibility of reduced 

margins, and assess any impact. 

(4) Staff should consider the effect of the proposed regulatory action on normal doses and 

accident risks to workers and the public. 

(5) Staff should consider the effect of a change to the licensing basis, if approved. Changes 

that might result in increased risk to the public, but nevertheless are found to be 

acceptable in terms of this guidance, may benefit from a program to monitor risk-related 

performance in the area affected by the change. 

The following offers a discussion of adequate protection and how risk, defense in depth, and 

safety margins are involved in this question. Regulatory analysis guidance (Ref. 1.3) indicates 

that when a regulatory action is proposed, it must be evaluated to assess whether adequate 

protection of public health and safety will be maintained and whether the action is consistent 

with the common defense and national security. 

A licensed activity that complies fully with the applicable NRC rules and regulations is presumed 

to meet the “adequate protection” standard. The term adequate protection is introduced in the 

Atomic Energy Act (Ref. 4.2) and, more recently, in various regulations (e.g., in 10 CFR Part 

50.109). The regulatory analysis guidelines NUREG/BR-0058, "Regulatory Analysis Guidelines 

of the U.S. Nuclear Regulatory Commission," (Ref. 4.1) define adequate protection as follows: 

“The risk level associated with adequate protection is that level above which continued 

operation would not be allowed.” and “The level of protection constituting ‘adequate protection’ 
 4-4 

is that level which must be assured without regard to cost. It is to be determined on a case-bycase basis. The determination should be based on plant- and site-specific considerations and 

the body of NRC's regulatory requirements.” 

The concept of adequate protection has also been discussed in RG 1.174, "An Approach for 

Using Probabilistic Risk Assessment In Risk-Informed Decisions on Plant-Specific Changes to 

the Licensing Basis," (Ref. 4.3). Although there is no single quantitative risk value associated 

with the notion of adequate protection, the risk level implied by the quantitative safety goals is 

more stringent (i.e., lower) than that implied by adequate protection. 

Defense in depth is an element of NRC’s safety philosophy that employs successive 

compensatory measures to prevent accidents or mitigate damage if a malfunction, accident, or 

naturally caused event occurs. Diverse and redundant barriers and safety systems serve to 

reduce the failure probability and increase the chance of success. The ACNW&M and the 

ACRS have jointly recommended to the Commission that risk-acceptance criteria be developed 

for all NMSS/FSME-regulated activities, to achieve defense in depth by balancing compensatory 

measures (Ref. 4.5). 

Defense in depth can be achieved by a variety of different measures such as passive 

containment systems (e.g., multiple barriers), active systems (e.g., ventilation systems), and 

administrative procedures. Redundancy and diversity can be used to manage uncertainties 

associated with system reliability. Hence, a minimal level of defense in depth may be 

necessary, despite very low quantitative risk estimates. 

A safety margin is a measure of the conservatism that is employed in a design or process to 

assure a high degree of confidence that it will perform a needed function. It can be defined as 

the probability or level of confidence that a design or process will perform an intended function. 

Sufficient safety margins should be maintained under any proposed regulatory change that 

relies on a risk-informed decision framework. This is typically done by demonstrating that 

sufficient conservatism is preserved in the design parameters, such that reliability and 

effectiveness are reasonably ensured against the most demanding challenge. An alternative 

approach often used is to demonstrate adherence to the acceptable Codes and Standards. 

Similar considerations are applicable to NMSS/FSME facilities. 

Defense in depth and safety margins are both concepts that are used to address the impact of 

uncertainty on safe design and performance. Effective use of defense in depth and safety 

margins increases the likelihood of success in response to challenges. 
 4-5 

• Assess Change in Risk 

Qualitatively 

• Apply Decision Logic 

Similar to Quantitative 

Case 

Identify Regulatory Issue or 

Action Alternative 

Is any statutory 

consideration that 

precludes Risk 

Informing?

Does issue 

impact public, 

worker health or 

environment? 

Screened Out: 

Decision based on 

Other Decision

Approaches

Screened Out 

Develop 

Information and 

Data Pertinent to 

Issue/ Alternatives

Yes 

No 

Identify: 

• Risk Data/Model 

availability 

• Cost Considerations 

• Scope and Depth 

• Risk Metrics 

• Recipients of risks 

Apply SC 2 &3 

No 

Yes 

Perform/Adapt 

Qualitative Assessment 

Apply Quantitative 

Risk Guidelines 

Make Decision Perform or Adapt Risk 

Assessment Quantitatively

Assess Changes in Risk 

Yes 

No 

Apply Risk 

Decision Logic 

Example Outcomes: 

• Perform Cost-Benefit Analysis 

for New Requirements 

• Exemption Request Needs 

Further Examination 

• Inspection Findings Significant

Make Decisions; 

Communicate Results 

• Competing Risks 

• Other Studies 

Maintain: 

• Defense in depth 

• Adequate Safety Margins 

Non-Negligible Risk 

Change 

Negligible Risk 

Change 

Other Decision 

Approach 

Figure 4.1 Risk Assessment Process

Example Outcomes: 

• Exemption Approved 

• New Requirement not Needed 

• Insignificant Inspection Finding 
 4-6 

4.1.4 Risk Guidelines 

Different types of public health and safety impacts may result from regulated nuclear material 

and waste activities. These impacts might include routine radiation exposures in the course of 

normal operations, residual chronic exposures from decommissioned or waste disposal sites, 

and accident risk. For routine and chronic exposures 10 CFR Part 20 provides regulatory limits 

and constraints that must be considered in the decisionmaking process. For accident risk, a few 

regulations covering certain specified activities provide qualitative and quantitative risk 

requirements. Since not all activities involving accident risk are covered by these few 

regulations, this document provides quantitative accident risk guidelines to inform decisions. 

These guidelines, as described in the subsequent chapter, were developed as reference levels 

of risk to individuals that are negligible relative to risks normally experienced. Section 4.2.2 of 

this chapter uses these guidelines to aid in screening out proposed new safety requirements 

that are unlikely to be justified because the level of risk is already negligible. 

In using accident risk and expected routine radiation exposures to make a decision, competing 

risks, cost impacts, and other factors should be considered jointly. Risk-informed decisions are 

not made on the basis of comparison with risk guidelines alone. All factors, including 

requirements in the regulations, the philosophy of defense in depth, safety margins, risk to 

individuals, and regulatory analysis should inform appropriate decisions. 

4.2 Decisionmaking Processes

The types of algorithms needed in the overall decisionmaking process depend to some extent 

on the particular issue involved. In the context of NMSS/FSME decisionmaking, the first 

decision algorithm (i.e., a value-impact analysis of a specific regulatory issue or action 

alternative) is broadly applicable. In fact, NUREG/BR-0184,“Regulatory Analysis Technical 

Evaluation Handbook” (Ref. 4.6), provides a number of examples of the use of value-impact 

analysis in nonreactor nuclear facilities. 

In value-impact analysis, the costs of implementing the proposed action are compared with the 

benefits. The value-impact analysis identifies all the attributes (i.e., the risk metrics affecting 

public health and safety, occupational health and safety, and the environment/property) that are 

potentially affected by the issue or action, and estimates the change in their values after 

implementation of the proposed action. If the net present value is positive, then the action is 

deemed beneficial and passes the value-impact test. NUREG/BR-0058, the regulatory analysis 

guidelines document mentions that for nuclear power plants, regulators should use an 80 km 

distance from the plant site to estimate changes in public health and safety from radiation 

exposure and futhermore that “the appropriate distance for other types of licensed facilities 

should be determined on a case-by-case basis." The same document also recommends that 

“...health risks should be estimated for both routine operations and accidents." For 

NMSS/FSME-regulated facilities and activities, determination of the appropriate population at 

significant risk from implementing a proposed action is an important element in estimating the 

impact on public and worker health and safety. Risk assessments of the diverse facilities and 

activities have provided important information in this regard. Appendix B discusses the main 

attributes that need to be assessed in a value-impact analysis. This document also provides 
 4-7 

generic tables to illustrate how the information would be assembled to document the analysis. 

The exact format for the tables would depend on the specific application. 

NUREG/BR-0058, "Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory 

Commission," addressed another decision algorithm in relation to reactor safety goal evaluation. 

The authors of NUREG/BR-0058 designed the safety-goal-evaluation algorithm to assess 

imposition of generic regulatory requirements (e.g., backfitting), and to determine whether an 

action meets the substantial added protection standard of 10 CFR Part 50.109(a)(3). The 

quantitative objectives of the reactor safety goals, to which this algorithm refers, are applicable 

only to accidents at NPPs. The authors based the safety-goal evaluation decision algorithm of 

NUREG/BR-0058 on fractional reductions in the CDF (and the conditional probability of earlycontainment failure, given core damage). 

As accident risk guidelines do not officially exist for NMSS/FSME, there is currently no analog of 

the reactor safety goal decision algorithm in NUREG/BR-0058 for NMSS/FSME activities. 

However, the recently amended 10 CFR Part 70, “Domestic Licensing of Special Nuclear 

Material,” and 10 CFR Part 76, “Certification of Gaseous Diffusion Plants” include one important 

element—requirements related to backfitting. In particular, consideration of the substantial 

increase in the protection standard, along lines very similar to those found in 10 CFR Part 50, 

“Domestic Licensing of Production and Utilization Facilities.” If the NRC adopted risk guidelines 

for NMSS/FSME-regulated facilities and activities, an important application would be in testing 

the substantial added protection standard, as is done for power reactors in NUREG/BR-0058. 

However, the technical details of the procedure would be quite different, since there are no 

analogs to core damage frequency (CDF) and large early release frequency (LERF) in the 

activities, devices, and processes regulated by NMSS/FSME. 

Figure 4.1 shows two possible outcomes, depending on the results of applying the risk decision 

logic. One outcome is for a result that shows a non-negligible risk change, and the other, a 

negligible risk change (while maintaining defense in depth and adequate safety margins). For 

the non-negligible risk changes, sample results may be used to perform a cost-benefit analysis, 

recommend further examination of the exemption request, or make a decision based on 

significant inspection findings. For those exemptions that staff find to represent a negligible risk 

change, specific outcomes may be the approval of the exemption, a finding that a new 

requirement is not needed, or a decision based on the absence of significant inspection 

findings. In general, burden reduction on the licensee would tend to increase risk, imposition of 

new requirements would tend to reduce risk, and decisions based on inspection findings would 

tend to maintain safety at existing levels. 

The following sections present two possible algorithms for decisionmaking in each of the 

application areas discussed above. For exemptions and changes to the licensing basis of a 

facility that would tend to increase risk, very general guidance is taken from the example in RG 

1.174. The procedure suggested here shows how specific requirements may be relaxed if the 

initial risk is already low and the incremental increases from a change are also small. The 

second algorithm relates to the potential introduction of new generic requirements. The general 

philosophy of the “Regulatory Analysis Guidelines,” NUREG/BR-0058, is followed and a "safety 

goal screening step" is introduced into the analysis process. 
 4-8 

The decision algorithms presented below relate to six health metrics. The base values for these 

guidelines for negligible accident risk proposed for evaluation are: 

(1) Public individual risk of acute fatality (QHG 1) is negligible if ≤ 5x10-7 /yr; 

(2) Public individual risk of latent cancer fatality (QHG 2) is negligible if ≤ 2x10-6 /yr or 

4 mrem/year; 

(3) Public individual risk of serious injury (QHG 3) is negligible if ≤ 1x10-6 /yr; 

(4) Worker individual risk of acute fatality (QHG 4) is negligible if ≤ 1x10-6 /yr; 

(5) Worker individual risk of latent cancer fatality (QHG 5) is negligible if ≤ 1x10-5 /yr or 25 

mrem/year; and 

(6) Worker individual risk of serious injury (QHG 6) is negligible if ≤ 5x10-6 /yr. 

In response to an Advisory Committee on Nuclear Waste and Materials (ACNW&M) 

recommendation, the latent cancer QHG values, which are in units of fatalities, can be 

converted into equivalent doses by using the ICRP conversion factors of 5x10-4 fatalities/rem for 

the public, and 4x10-4 fatalities/rem for workers. 

In most specific applications thus far encountered in the materials and waste arenas, only one 

or two measures have been pertinent to the particular safety issue. This tends to keep the 

problem manageable in terms of decisionmaking. However, for rare situations in which several 

of the six health measures are important to an issue, there are decision-theoretical approaches, 

such as a multi-attribute theory, that can aid staff when considering several decision variables 

simultaneously (Ref. 4.7). 

Accident risk is generally dealt with qualitatively in the rules and regulations. The quantitative 

risk guidelines are only guidance and are not requirements. The metric suggested here is the 

annual frequency of an undesirable health effect from an accident1

. The risk guideline (the 

lower line in Figure 4.2) provides a benchmark for assessing what level of risk is regarded as 

negligible and is a universal concept, independent of any particular application. The upper line, 

in Figure 4.2 corresponds to the concept of reasonable assurance of adequate protection. 

Commission guidance on this issue has refrained from ascribing a numerical value to the 

concept of adequate protection that is evaluated on a case-by-case basis. For purposes of 

decisionmaking, however, as discussed in Section 4.2.1, trial values of a frequency or 

probability that would correspond to the upper line in Figure 4.2 can be used on an interim 

basis. 

As pointed out in Chapter 1, staff must consider both routine and accident risks in risk-informed 

decisions. Rules and regulations deal with routine risk or exposure quantitatively. The metric 

for assessing routine risk is annual exposure and 10 CFR Part 20 prescribes what is legally 

unacceptable (i.e., exceeding the public and worker annual limits). However, Part 20 also 

makes ALARA (as low as is reasonably achievable) (i.e., to reduce exposures to as low as 

reasonably achievable) a legal requirement in planning radiation-related activities. However, 

ALARA is not a one-size-fits-all concept, since what is possible (i.e., achievable and 

 

1 Note that there are several approaches under consideration for possible accident-risk guidelines 

for NMSS/FSME regulatory applications. For illustration purposes, the metric of annual frequency 

of an undesirable health effect is used in the suggested decision algorithms. 
 4-9 

reasonable) will vary, depending on the particular application, technology involved, and costs 

and benefits of implementation. A diagram similar to Figure 4.2, focused on routine risk, can 

help in the decisionmaking process. In the diagram that applies to routine risk, the upper line 

separating the unacceptable from the tolerable would be the appropriate regulatory limit from 10 

CFR Part 20 (100 mrem/yr for the public and 5 rem/yr for workers). If a specific limit applies to a 

specific source or practice (e.g., 10 CFR Part 63), that specific limit is to be used for that activity. 

Regulatory Limit

UNACCEPTABLE RISK (UR)

TOLERABLE RISK (TR)

Negligible Risk

Reference Level

NEGLIGIBLE RISK (NR)

Risk

SAFETY MARGIN

Figure 4.2 Three-region risk diagram 

4.2.1 Exemptions and Changes to the Licensing Basis 

RG 1.174 provides guidance to reactor licensees on using risk information to support licenseeinitiated changes to the licensing basis of an NPP that require review and approval by NRC. 

RG 1.174 indicates that "...consideration of the Commission's Safety Goal Policy Statement is 

an important element in regulatory decisionmaking. Consequently, this regulatory guide 

provides acceptance guidelines consistent with this policy statement." RG 1.174 remarks, 

however, that in the context of integrated decisionmaking, the acceptance guidelines should not 

be interpreted as being overly prescriptive. They are approximate values that provide an 

indication of what is generally acceptable, rather than being strict numerical criteria. 

The Reactor Safety Goals have two tiers: qualitative and quantitative. In addition, there are 

subsidiary objectives, below the quantitative goals, that are useful for more direct computation 

of risks. This third tier is not formally part of the Reactor Safety Goals. The risk acceptance 

guidelines recommended in RG 1.174 are based on estimating the changes to the subsidiary 

objectives, CDF and LERF, from the proposed change in the licensing basis. The licensee's 

plant-specific PRA is used to estimate the change in CDF and LERF and the needed 

sophistication of the analysis, including the scope of the PRA (i.e., whether it covers only fullpower operation and internal events, or whether it extends to lower and shutdown operation and 

also to external events), is stated to depend on the contribution the PRA makes to integrated 

decisionmaking, and on the extent of the potential impact on risk of the proposed action or 

change. Numerical guidelines are recommended for ΔCDF and ΔLERF that indicate which 

changes will be considered broadly acceptable, which will be considered unacceptable, and 

which will require further technical review. These numerical guidelines are derived from, and 
 4-10 

are based on, the values of CDF and LERF (1 x10-4 and 1x10-5 per reactor-year, respectively) 

that are consistent with the reactor safety goal quantitative health objectives (QHOs). 

One difference between light water reactors and NMSS/FSME applications is that currently 

there is no readily identifiable and universal analog of the Tier 3 reactor subsidiary objectives 

CDF and LERF for NMSS/FSME-regulated facilities and activities. NMSS/FSME-regulated 

facilities and activities vary widely with respect to technology, quantity of radioactive material 

involved, and operation. They range from large, stationary facilities, such as enrichment or fuel 

fabrication plants, to small sealed sources. Because of this variety, subsidiary objectives may 

not be warranted for each facility and activity. Regulators will not determine the need and 

identification of suitable Tier 3 subsidiary guidelines for this diverse set of activities until more 

experience is gained. 

A different procedure from RG 1.174 for reactors has been developed here. The suggested 

procedure is to use the quantitative health guidelines (QHGs) directly in the risk-guideline 

decisionmaking algorithm. Unlike reactors, where the endpoints are often CDF and LERF, in 

many risk assessments of NMSS/FSME facilities, radiation doses to the affected population are 

frequently the endpoint of assessment. This is particularly true for industrial and medical 

applications, where the population potentially affected by an accident or during a routine 

operation is often located in the immediate vicinity of the source or device (see NUREG/CR6642). 

The three-region risk-acceptance diagram shown schematically in Figure 4.2 presents the 

suggested procedure. This diagram divides the risk space for any of the applicable risk metrics 

(public acute fatality, worker acute fatality, public latent cancer, etc.) into three regions: an 

unacceptable-risk region, a tolerable-risk region, and a negligible-risk region. The lower bound, 

shown in Figure 4.2, that separates the negligible-risk region from the tolerable risk region (TR) 

is the risk-guideline QHG value. The upper bound corresponds to the risk implication of the 

regulatory or safety limit that constitutes what is implied by "adequate protection" and separates 

the unacceptable-risk from the tolerable-risk region. The TR range is divided into two halves, an 

upper-TR (UTR) range and a lower-TR (LTR) range. 

To see how the risk-acceptance diagram would apply in decisionmaking, consider the risk 

space pertinent to the worker prompt-fatality risk metric. The lower boundary separating LTR 

and NR corresponds to a frequency of 1 E-6/facility-yr (corresponding to the proposed worker 

prompt-fatality QHG 4). Staff can assume the upper boundary to have some higher value. One 

approach would be to have each region in Figure 4.2 separated by one order of magnitude. 

Thus, for the worker prompt fatality risk metric, the line separating LTR from NR corresponds to 

1 E-6/ year, the line separating LTR from UTR corresponds to 1 E-5/ year, and the line 

separating UTR from UR corresponds to 1 E-4/year. (This is somewhat similar to the algorithm 

discussed in RG 1.174, where the corresponding adjacent regions for ΔCDF (and ΔLERF) differ 

by one order of magnitude.) The approach suggested here is just that: an approach for 

discussion. Staff should consider and evaluate other approaches as well, especially those that 

define the upper boundary separating the UR and TR regions. Just as the risk guidelines 

themselves were derived based on percentages of the corresponding background risks faced by 

the public or workers [see Chapter 5), the upper boundary could be based on higher 

percentages of the same background risks. 
 4-11 

Consider another case, where the decision involves the worker latent-cancer fatality risk metric. 

In this case, one approach, for the boundary separating the UR and TR regions, is to assume 

that the upper limit for annual fatality risks from accidental exposures is the same as the annual 

dose limit from normal operation converted via the BEIR-V conversion factor to probability of 

fatality. Hence, the 5 rem annual dose limit for workers would translate into a 2E-3 annual 

probability of latent cancer for workers. If a decision involves the public latent cancer metric, the 

upper bound separating the UR from the TR region could be taken as the 10 CFR Part 20 limit 

of 100 mrem/yr (that translates into a 5E-5 /yr risk of latent cancer fatality using the BEIR-V 

conversion factor). The lower bound in this case would be the draft risk guideline QHG 2 of 2E6 /yr. The line separating the UTR and LTR regions could then be assigned an intermediate 

value depending on the issue under consideration, value-impact analysis results, and other 

relevant factors. The suggested numerical values corresponding to the risk levels of the upper 

bound separating the region of UR from that of TR and separating the upper region of the 

tolerable risk from the lower region need considerable further input and deliberation from 

existing risk assessments, pilot studies, and stakeholders. 

The risk-guideline decision algorithm process would then focus on the calculated mean values 

of the risk metrics before and after the action or issue under consideration is implemented. As 

in the case of reactors, the upper and lower bounds are ones considered broadly acceptable 

and not to be interpreted as strict numerical criteria; in other words, they are indicative of the 

risks that are considered acceptable. 

There are six risk guidelines: three for risk to individual members of the public and three for risk 

to individual workers. For any given situation, the risk of each guideline lies in one of the four 

risk regions from unacceptable to negligible. The risk region to which the overall situation 

belongs is the highest one in which any of the six guidelines falls. 
 4-12 

Table 4.1 Risk Acceptance Logic for Changes 

RISK REGION AFTER CHANGE 

Negligible Lower 

Tolerable

Upper 

Tolerable

Unacceptable

Unacceptable Consider other 

factors, valueimpact? 

4 

Consider other 

factors, valueimpact? 

4

Consider other 

factors, valueimpact? 

4 

Find a way to 

make risk 

tolerable 

1

Upper Tolerable Consider other 

factors, valueimpact? 

4 

Consider other 

factors, valueimpact? 

4

Change not 

normally 

considered 

3 

Change not 

implemented 

2

Lower Tolerable Consider other 

factors, valueimpact? 

4 

Consider other 

factors, do 

value-impact 

5

Change not 

normally 

considered 

3 

Change not 

implemented 

2

B 

E 

F 

O 

R 

E 

C 

H 

A 

N 

G 

E

Negligible Consider other 

factors, valueimpact? 

4 

Consider other 

factors, do 

value-impact 

5

Change not 

normally 

considered 

3 

Change not 

implemented 

2

Table 4.1 is a matrix showing all the possible situations regarding the risk regions before and 

after a proposed exemption or regulatory change. The risk region before the change is shown 

in the left column, and the risk region after the change is shown in the top row. Thus, each box 

in this matrix represents one of the 16 possibilities. These 16 possibilities are placed in five 

categories indicated by the number in each box. These five categories are discussed below. 

(1) The risk after the change is still in the unacceptable region. This means both that the 

current situation is unacceptable and that the proposed change will not fix it. Therefore, 

a different regulatory change should be devised to lower the risk out of this region. 

(2) In these cases, the proposed change causes the risk to increase from tolerable (or 

below) up into the unacceptable region. Such changes should not be implemented, but 

the current situation falls in the acceptable range. 

(3) In the case of those changes labeled 3, the risk has increased into the UTR. This 

means that individual risk is being increased to high values. Normally, such changes 

should not be allowed unless outweighed by other considerations, or by exceptional net 

benefits as shown through value-impact analysis. In the one case where the risk starts 

and remains in the upper tolerable region, if there is actually a decrease in risk, this 

situation may be categorized as a 5. 
 4-13 

(4) The boxes labeled 4 (gray) are those where the change has resulted in a decrease in 

the overall risk region, or has remained in the negligible region. This does not 

necessarily mean that all risk metrics have decreased, but only that the highest one is 

now in a lower-risk region. Other risk metrics may actually have increased. However, in 

practice, changes in these boxes may represent a decrease in all risk metrics. If it is 

also clear that the change will decrease costs, it is probably unnecessary to perform a 

quantitative value-impact analysis in order to draw the conclusion that the change should 

be implemented. This is what is meant by the phrase “value impact” in these boxes. 

Note also that, even when all risk metrics decrease, a change may be rejected based on 

other considerations, such as inadequate defense in depth. These “other 

considerations” are, in part, discussed in this Chapter, but also in guidance on regulatory 

analysis (Refs. 1.2 and 1.3). 

(5) These category 5 situations are the typical ones where the change results in an increase 

in the risk region, but remains at a lower tolerable or negligible level. Such increases in 

individual risk may be tolerable but need to be justified. The justification may involve an 

improvement in some “other consideration” or a clear net benefit shown by the valueimpact analysis. 

In addition, other qualitative considerations, such as those considered in Appendix B, may also 

have a bearing on the decision. Regulatory efficiency, safeguards and security, and 

improvements in knowledge are some of these factors that are often difficult to quantify but are 

nevertheless considered in the decisionmaking process. Other important factors to consider in 

the RIDM process, and that are shown in Figure 4.1, are those that address uncertainty (e.g., 

defense in depth and safety margins). It is important to comply with requirements that address 

the latter issues, particularly when considering possible action alternatives and their risk 

impacts. 

4.2.2 Consideration of New Requirements

NRC requires a regulatory analysis for backfit (see Glossary) or whenever new generic 

requirements, guidance, or staff positions would effect a change in the expenditure of resources 

by licensees. The “Regulatory Analysis Technical Evaluation Handbook” and “Regulatory 

Analysis Guidelines of the U.S. Nuclear Regulatory Commission” provide guidance to the 

regulatory analyst on the preparation of such an appropriate evaluation. The guidance provided 

here is intended as a supplement to the above guidance that provides risk criteria applicable to 

nuclear material and waste. The guidance here is for trial use. 

When considering new regulatory requirements in a risk-informed manner, there are several 

steps in the process, as explained in Ref. 4.6: 

(1) Define the problem or issue. 

(2) Define a number of alternative regulatory actions that might resolve the problem. 

(3) Screen these proposed alternatives based on a number of attributes to eliminate those 

that fail to meet established criteria, or are clearly inferior to other alternatives. 

(4) For the remaining alternatives perform value-impact analysis to aid in selecting the one 

appears to be best. 
 4-14 

The attributes to be considered in Step 3, Screening, are basically those discussed in Section 

4.1.3 and in the regulatory analysis guidance. We will supplement them here with criteria based 

on risk to individuals. These attributes include regulatory requirements or policy that are to 

remain in effect, defense in depth, safety margins, the effect on routine doses to workers and 

public, effect on regulatory efficiency, safeguards and security, and the environment. For each 

of these attributes there may be constraints that either eliminate a possible alternative, or which 

require that some element be included. These constraints may be explicit if they are a 

regulation or policy statement, but for others such as defense in depth and safety margins, there 

is no explicit generic guidance. The discussion in Section 4.2.3.1 implies that for defense in 

depth there may be a minimum level that should be met. As indicated in this section, the 

minimum level is not the same for all situations, but varies depending on a number of factors. 

Proposed alternative requirements not having this minimum level would be screened out. 

A similar logic applies for screening out alternatives based on insufficient safety margins. 

One idea that is fundamental to regulatory analysis for backfit or new requirements is that the 

new requirement be justified. That is, there must be legitimate reasons for the requirement and 

its net impact on society, considering all impacts, should be positive. There are several different 

criteria that can justify a requirement. Some of these are listed in the exceptions in the backfit 

rules themselves. Another is what Reference 4.6 calls regulatory efficiency. In order to 

function, a regulatory agency must have sufficient information to make findings required by 

statute. Thus new requirements that request certain information or analysis in order to make 

such findings may require no further justification. 

For reactors, the Regulatory Analysis Technical Evaluation Handbook states that the process of 

screening proposed new requirements includes a “safety goal evaluation.” This safety goal 

evaluation is described in Ref. 4.1. The guidelines in this reference indicate that one should not 

proceed with a regulatory analysis if the reduction risk is small when benchmarked against the 

subsidiary objectives for the risk guidelines. The rationale is that, if a new requirement is to be 

justified on the basis of its risk-reduction benefit, yet that reduction is very small, then it is not a 

substantial benefit. The proposed new requirement would also be unlikely to be cost beneficial, 

since the cost impact of most requirements on reactors is substantial. Non-reactor nuclear 

activities lack a single risk metric, such as large early release frequency (LERF), that can be 

used for such screening. Instead there are six risk metrics, with Quantitative Health Guidelines 

listed, each of which must be considered in the context of the three risk regions shown in Figure 

4.2. Based on these concepts, a tentative decision algorithm, analogous to the reactor safety 

goal screening step for reactors, as described in NUREG/BR-0058, is provided in Table 4.2 

below. 

For proposed regulatory actions whose primary justification is to reduce risk from its present 

value, the reductions in individual risk should be estimated for each of the six risk metrics used 

in the Quantitative Risk Guidelines (QHG). That is, the six risk values are measured in the units 

of the pertinent QHG: worker - public; acute fatality - latent fatality - injury. Table 4.2 provides 

the necessary decision algorithm for considering the magnitude of the change in risk to be 

produced by the proposed regulatory action. Note that the absolute level of the six risk metrics 

should also be considered. When the purpose of the regulatory action is to reduce one of these 

six metrics, yet its absolute value is already below the negligible level of individual risk 

expressed by the QHG, then further reduction would not normally be warranted. Note also that 

the proposed action may have purposes other than reduction of these six accident risk metrics 
 4-15 

which would justify proceeding with the remainder of the regulatory analysis. Examples would 

be actions to provide defense in depth or to provide needed information to the regulator. In 

such cases, the fact that the action also provides only a negligible risk reduction may not be the 

decisive consideration. 

Table 4.2. Risk-Reduction Decision Logic for NMSS/FSME Facilities

Risk-Reduction of Proposed Regulatory Action Staff Response 

Δ Risk for any of the 6 risk metrics > QHG Proceed with regulatory analysis 

10% QHG < Δ Risk for any of the 6 risks metrics< 

QHG 

Conduct appropriate staff management 

reviews of other factors to determine whether 

to proceed with a regulatory analysis. 

Δ Risk for all 6 risks metrics < 10% QHG Action is not recommended. 

The final step in the evaluation of proposed alternative requirements is to perform a net valueimpact analysis to aid in selecting the best alternative. If staff is considering the proposed new 

requirements in order to decrease risk, a value-impact analysis should be to evaluate the total 

collective risk of the affected facilities. This is the baseline risk. It includes risk to the offsite 

public and to workers, both routine exposures and accident risk. This risk is summed over all 

persons at risk, then converted to dollars at $2000/person-rem or $3 million per fatality. If the 

proposed new requirements completely eliminated all risk, this dollar value would be the benefit 

of the requirement in averting risk. Thus this dollar value is the maximum possible risk 

reduction benefit. Any of the proposed alternative sets of requirements that exceed this value is 

thus screened out as not justified; unless justified on other grounds discussed above. 

For alternatives which make it through the screening process, a quantitative value-impact 

analysis for each alternative is needed. In this analysis, staff estimates all effects on costs and 

risks of each proposed alternative set of requirements. Staff then adds these cost and risk 

changes (relative to the baseline case) to obtain a measure of net benefit. The alternative with 

the highest net benefit is thus the preferred one, provided that this value is positive. Proposed 

alternatives whose net benefit is negative fail the justification test. This value impact analysis 

process is described in Appendix B. 
 4-16 

4.2.3 Consideration of Risk-Related Issues in Implementing Above 

Algorithms 

The RIDM process frequently occurs in the context of changing requirements that may have 

been in place for many years. This is particularly so in the context of reducing requirements that 

may have been routinely imposed in the past; their removal may encounter significant 

opposition from staff who championed them earlier. The issues that have to be carefully 

considered are those that involve safety margins and defense in depth and those that have to 

be applied generically, rather than on a facility-specific basis. In the former case, the issue is to 

determine the correct level of safety margin (i.e., determine the right amount of conservatism in 

the design or process, by maintaining the desired level of safety while avoiding excessive 

defense in depth). In the case of generic versus facility-specific issues, how reasonable is it to 

extend the results of risk studies that may have been performed only for a specific design or a 

specific facility to other, perhaps similar, designs at other locations? In short, what are the 

implications of generalizing the results of studies on specific facilities to a generic class of 

facilities? 

Although there are no simple or straightforward answers to these questions that arise frequently 

in the RIDM process, staff may find it useful to consider a few relevant factors. 

4.2.3.1 Defense in Depth and Safety Margins

In the case of safety margins, staff should consider whether, for example, more than one level 

of safety margin is being applied in a particular case so that the end result is layer upon layer of 

conservatism. As an example, consider an accidental leak of radioactive material through one 

or more barriers, its subsequent transport under atmospheric conditions to the site boundary, 

and the possible dose given to a receptor. If staff uses the most conservative bounding value of 

the accident stress to calculate the probability of barrier rupture and the most conservative 

weather condition to estimate the site boundary dose, this approach can lead to an excessive 

safety margin. It is not a risk-informed approach. It is preferable to do each calculation on a 

best-estimate basis and to then apply conservatism, as warranted, to the overall result. That 

said, consideration of uncertainty is important. If reasonably complete uncertainty calculations 

are available, the degree of needed conservatism can be estimated and additional conservatism 

is not needed. For example, an hourly set of annual weather data is used in probabilisticconsequence-assessment codes to estimate the uncertainty in dose, given a release. The less 

such information regarding uncertainty is available, the more the staff may need to adopt 

conservative approaches. 

In the decision algorithms, risk information needs to be used in a fashion consistent with the 

Commission’s overall defense in depth philosophy. This philosophy helps ensure that key 

safety functions do not depend on a single element of design or operation and that uncertainties 

are taken into account. The extent of defense in depth can vary depending on the nature of the 

risk and/or uncertainty. The application of the defense in depth philosophy is, in fact, aided by 

the use of a risk-informed decision process, in that the risk-informed process provided generally 

employs quantitative guidelines that can be used in deciding on the need for, extent, nature, and 

effectiveness of defense in depth measures. In general, the relation between defense in depth 

and a risk-informed process can be summarized as follows: 
 4-17 

• For low-risk/consequence activities, where uncertainties are also low, defense in depth 

measures can be reduced. 

• For medium-risk/consequence activities, defense in depth measures should be 

considered to ensure that the levels of safety can be met with a specified level of 

confidence. The defense in depth measures considered should include: 

3⁄4 Ensuring key safety functions do not depend on a single element of design or 

operation; 

3⁄4 Using redundancy, diversity, and independence to improve reliability and/or avoid 

common mode failure, when necessary, to ensure safety is maintained; 

3⁄4 Providing safety margins to address uncertainties in modeling or equipment 

performance; 

3⁄4 Conducting regulated activities at locations that facilitate protection of public and 

worker safety; and 

3⁄4 Providing time for recovery operations. 

• For high-risk/consequence activities, defense in depth measures similar to the above 

should be considered, as well as: 

3⁄4 Ensuring the design and operation have both accident prevention and mitigation 

measures; and 

3⁄4 Ensuring the design includes at least two independent barriers to the 

uncontrolled release of radioactive material. 

Accordingly, in making risk-informed decisions, one needs to consider whether defense in depth 

measures are needed (or could be relaxed). If the defense in depth measures are needed, 

consider the degree to which they are needed, based on the application of this process. In all 

cases, staff should monitor regulated activities to ensure that key assumptions used in the risk 

analysis remain valid and adjustments are made to reflect operating experience where 

necessary. In general, low-risk/consequences mean doses are in the range of 10 CFR Part 20 

limits. High-risk/consequences mean doses can be large enough to cause one or more early 

fatalities; medium-risk/consequences correspond to the range between low and high. 

Please note that risk information can only provide defense in depth insights on the known 

uncertainties. However, risk information cannot provide defense in depth insights on the 

unknowns. 

In Appendices N and O, this document provides additional guidance on the considerations 

related to defense in depth and safety margins. 
 4-18 

4.2.3.2 Generic vs. Facility-Specific Issues

Regarding generic versus design- or facility-specific issues, one can take the example of the dry 

storage cask PRA that RES recently carried out for a specific design at a specific location (Ref. 

4.8). Ask how valid and applicable its results are on a generic basis. In other words, can these 

results be applied generically? Two factors are involved. One is cask design and the 

applicability of the failure modes and effects and potential releases from a specific design to all 

designs. The other is site location and whether the results obtained from a specific site are 

broadly applicable everywhere. 

In general, the key contributors to risk should be identified from a specific study. Then, in 

considering the applicability of that risk study more generically, the differences in the other 

system’s design and operation should be assessed (at least qualitatively) with regard to the key 

contributors. 

4.2.3.3 Graded Approach 

In overall application of the RIDM process, it is useful to emphasize the benefits of a graded 

approach. In general, if the risks are very low, both in terms of frequency and consequence, it is 

not very important to refine the calculations beyond a first-cut estimate. However, if the 

situation is the opposite, and the risks are high in terms of both frequency and consequence, 

then expenditure of additional resources is warranted. Risk guidelines, of course, offer valuable 

guidance in framing what risks can be considered “low.” Value-impact analysis can offer 

valuable guidelines here, as it provides a measure of the relative importance of the change in 

risk, just as the risk guidelines do. 

4.2.3.4 Reaching Consensus on a Recommendation 

The technical staff involved with the analysis will summarize the results for each decision option. 

The principal analysts in each discipline (e.g., PRA, engineering, licensing, etc.) will participate 

in the integration process with other technical staff through a facilitated discussion. 

Additional guidance on the considerations related to reaching consensus on a recommendation 

is provided in Appendix P. 

4.3 References

4.1 U.S. Nuclear Regulatory Commission, "Regulatory Analysis Guidelines of the U.S. 

Nuclear Regulatory Commission," NUREG/BR-0058, July 2000. 

4.2 “The Atomic Energy Act of 1954," August 30,1954 as amended in NUREG-0980, 

“Nuclear Regulatory Legislation.” 

4.3 U.S. Nuclear Regulatory Commission, Regulatory Guide 1.174, "An Approach for Using 

Probabilistic Risk Assessment In Risk-Informed Decisions on Plant-Specific Changes to 

the Licensing Basis," July 1998. 
 4-19 

4.4 U.S. Nuclear Regulatory Commission, Letter from Messrs. Garrick and Powers to 

Chairman Meserve, "Use of Defense in Depth In Risk-Informing NMSS Activities," May 

25, 2000. 

4.5 U.S. Nuclear Regulatory Commission Letter from Messrs. Garrick and Powers to 

Chairman Meserve, "Implementing a Framework for Risk-Informed Regulation in the 

Office of Nuclear Material Safety and Safeguards," November 17, 1999. 

4.6 U.S. Nuclear Regulatory Commission, "Regulatory Analysis Technical Evaluation 

Handbook," NUREG/BR-0184, January 1997. 

4.7 Hammond, J.S., et al., Smart Choices: A Practical Guide to Making Better Decisions, 

Harvard University Press, Cambridge, MA, 1998. 

4.8 U.S. Nuclear Regulatory Commission, “A Pilot Probabilistic Risk Assessment of a Dry 

Cask Storage System at a Nuclear Power Plant,” NUREG-1864, September 2006. 
 5-1 

5 RISK GUIDELINES FOR USE IN NMSS/FSME 

5.1 Introduction

This chapter provides the technical basis for the draft risk guidelines used in the RIDM 

algorithms, discussed in Chapter 4. Risk guidelines represent an accident risk reference level 

regarded as a negligible additional risk, compared to the normal risks faced by workers or the 

public. The regulatory activity is generally considered safe enough when estimated accident 

risks to individuals are less than these risk guidelines. However, there are other considerations 

involved in making regulatory decisions. In addition to accident risk, risk from routine exposures 

to individuals must also be considered. Additional regulatory measures may also be needed to 

address safety considerations other than accident risk. For example, defense in depth, safety 

margins, and other programmatic elements may not be adequately addressed, even when the 

estimated accident risk is negligible. Further regulatory action may also be warranted to 

address nonsafety factors, such as environmental protection, the Administrative Procedures Act 

(Part 1, Chapter 5, Sections 511-599), or other statutory requirements. Although attaining 

negligible risk levels does not absolutely preclude a need for further efforts to reduce risk, the 

value of further reduction in risk is negligible; hence, such efforts will have difficulty meeting 

cost-benefit criteria. Cost cannot, however, be a consideration if a particular measure is found 

to be necessary, to attain adequate protection of public health and safety, or to be in accord with 

the common defense and security. 

The multiple attributes and considerations that bear on decisions include risk but also include 

many other things (i.e., decisions should be risk-informed, not risk-based). It is the goal of this 

guidance document to provide a structured framework that ties together existing guidance 

addressing all these considerations, as a convenience to the staff in making decisions that 

change the regulatory process. This chapter focuses on one of these considerations; i.e., 

guidelines as to what level of accident risk to individuals should be regarded as negligible. 

Chapter 4 discusses how these guidelines may be used in specific decisionmaking situations 

and refers the reader to existing guidance documents, addressing the other considerations 

mentioned above. 

Risk guidelines represent an accident-risk reference level that can be regarded as a negligible 

additional risk, compared to the risk faced by workers or the public from normal operations. If 

the estimated accident risk to an individual is less than, or equal to, the risk guidelines, it 

generally indicates that the regulated activity is safe enough and the regulatory resources can 

be used to focus on higher-risk activities. NRC staff can apply risk guidelines to rulemaking, 

licensing, inspection, or enforcement actions. 

Appendix G provides one example of the use of risk guidelines in NMSS/FSME. 
 5-2 

5.2 Three-Tier Approach to Risk Guidelines

The reactor safety goals developed in the “Safety Goal Policy Statement for the Operation of 

Nuclear Power Plants” (Ref. 5.1) (SGPS) have a hierarchical structure composed of three tiers: 

a top, qualitative tier, followed by a quantitative tier which is, in turn, supported by subsidiary 

objectives. Qualitative goals (Tier I) are stated in a language that is readily understandable to 

the general public and express the high-level safety aspirations of the agency. Quantitative 

goals (Tier II) are expressed in terms of health objectives related to the risk of an undesirable 

health effect from the regulated activity. Subsidiary objectives (Tier III) are expressed in terms 

of risk parameters related to specific engineering features of the technology ( e.g., engineered 

barriers), whose failure(s) could lead to an undesired health effect. 

For NMSS/FSME-regulated facilities and activities, the proposed draft risk guidelines can be 

expressed in a similar three-tiered hierarchical structure composed of high-level qualitative 

guidelines followed by quantitative health risk guidelines. Subsidiary guidelines may or may not 

add to a RIDM approach to NMSS/FSME regulatory activities. Such determinations would not 

be made until more experience has accumulated on the use of the draft risk guidelines. 

Therefore, subsidiary guidelines are not discussed further in this chapter. Note that the term 

“guideline” has been chosen to underscore that the quantitative values are not limits or 

standards, but rather guidelines that provide an indication of what can be regarded as an 

insignificant level of risk to assist risk-informed decisionmaking. Further, it is important to 

distinguish these guidelines from the “goals” and “objectives” of the SGPS, which have their 

own particular history and motivation and subsequent implementation in risk-informed 

decisionmaking in the reactor program. 

5.2.1 Qualitative Risk Guidelines

As shown from the case studies in nuclear material waste and disposal (Ref. 5.2), the workers 

are the population that bears a significant portion of the risk, and SECY-99-100 (Ref. 5.3) has 

indicated that the risk metrics and goals should address the safety of workers. In many 

NMSS/FSME-regulated applications, worker risk exceeds public risk, in some cases by several 

orders of magnitude. Thus, ignoring worker risk would be ignoring the major recipient of the 

risk. 

Accordingly, the qualitative risk guidelines for nuclear material and waste activities proposed 

below address both the public and the worker and are expressed in terms of individual risks. 

The guideline is: 

Individual: Nuclear material and waste activities should pose a negligible 

additional risk to the life and health of individual members of the public and to 

workers associated with these activities. 
 5-3 

5.2.2 Quantitative Health Guidelines (QHGs)- Base Approach

In the reactor SGPS, the Commission specified the risk measures in which the qualitative safety 

goals would be couched and then defined what "no significant additional risk" meant in 

quantitative terms. In so doing, the qualitative safety goals were transformed into QHOs. A 

similar approach has been followed in developing the NMSS/FSME QHGs for the public and the 

workers, as stated below. 

The three QHGs for individual members of the public apply to the population at significant risk 

(i.e., people in the vicinity of NMSS/FSME-regulated facilities or activities). The QHGs indicate 

what level of accident risk posed by these facilities should be regarded as “negligible additional 

risk” to the public, compared to everyday risks to which the average member of the public in the 

United States is exposed. In other words, when comparing or evaluating the results of 

quantitative risk assessments in making risk-informed decisions, the quantitative risk guidelines 

are simply meant to indicate what numerical level of risk will be regarded as “negligible.” The 

six QHGs proposed for RIDM are given below. The rationale used to develop the QHGs is 

discussed in Section 5.3. 

Individual Public Acute (QHG 1): A risk to an individual member of the public of a 

prompt fatality, due to inadvertent or accidental exposure from nuclear material and 

waste activities should not exceed one-tenth of one percent (0.1 percent) of the sum 

of prompt fatality risks resulting from other accidents to which members of the U.S. 

public are generally exposed. This could be regarded as “negligible additional risk” 

in risk-informed decisions. The draft QHG1 value is 5x10-7 /yr. 

Individual Public Latent (QHG 2): A risk to an individual member of the public of a 

latent cancer fatality due to inadvertent or accidental exposure from nuclear material 

and waste activities should not exceed one-tenth of one percent (0.1 percent) of the 

sum of latent cancer fatality risks resulting from other accidents to which members of 

the U.S. public are generally exposed. This could be regarded as “negligible 

additional risk” in risk-informed decisions. The draft QHG2 value is 2x10-6 /yr or 

4 mrem/yr.

Individual Public Injury (QHG 3): A risk to an individual member of the public of 

severe injury, due to inadvertent or accidental exposure from nuclear material and 

waste activities should not exceed one-tenth of one percent (0.1 percent) of the sum 

of severe injury risks resulting from other accidents to which members of the U.S. 

public are generally exposed. This could be regarded as “negligible additional risk” 

in risk-informed decisions. The draft QHG3 value is 1x10-6 /yr.

Individual Worker Acute (QHG 4): A risk to a worker of a prompt fatality due to 

inadvertent or accidental exposure from nuclear material and waste activities should 

not exceed one percent of the prompt fatality risk in all higher risk industries. This 

could be regarded as “negligible additional risk” in risk-informed decisions. The draft 

QHG4 value is 1x10-6 /yr.

Individual Worker Latent (QHG 5): A risk to a worker of a latent cancer fatality due to 

inadvertent or accidental exposure from nuclear material and waste activities should 

not exceed one percent of the latent cancer fatality risk in all higher risk industries. 
 5-4 

This could be regarded as “negligible additional risk” in risk-informed decisions. 

The draft QHG5 value is 1x10-5 /yr or 25 mrem/yr.

Individual Worker Injury (QHG 6): A risk to a worker of severe injury due to 

inadvertent or accidental exposure from nuclear material and waste activities should 

not exceed one percent of the severe injury risk in all higher risk industries. This 

could be regarded as “negligible additional risk” in risk-informed decisions. The draft 

QHG6 value is 5x10-6 /yr.

5.3 Rationale for Qualitative and Quantitative Risk Guidelines

In developing the above qualitative and quantitative guidelines, careful attention was given to 

the following: 

• The spectrum of NMSS/FSME-regulated facilities and activities and the nature of the 

risks they represent; and 

• The methods NMSS/FSME used, first to understand the Commission’s use of the logic 

and reasoning in the reactor SGPS to define and outline the basis for the reactor safety 

goals and QHOs, and then to appropriately adapt and modify these goals and objectives 

to derive QHGs for the diverse NMSS/FSME-regulated facilities. 

The principle underlying the development of qualitative risk guidelines for the public is the 

notion that nuclear facilities and activities should not impose a significant additional risk on the 

public beyond the risks that people experience in their daily lives. In the SGPS, the 

Commission expressed this qualitatively as “...such a level of safety that individuals living or 

working near nuclear power plants should be able to go about their daily lives without special 

concern by virtue of their proximity to these plants.” Translated to NMSS/FSME-regulated 

facilities and activities, this principle would imply that individual members of the public who live 

or work or find themselves in proximity to such facilities, or places where such activities are 

conducted, should face a negligible additional risk by virtue of their proximity. 

The Commission went further, in the SGPS, by providing a quantitative dimension to the 

definition of “insignificant” and “proximity.” In this way, the qualitative goal of negligible 

additional risk to the public from NPP operation was transformed into two quantitative risk 

objectives, one for the risk of a prompt fatality (that could potentially occur from large accidental 

radioactive releases) and the other for the risk of a latent-cancer fatality (that could occur from 

accidental releases of all magnitudes). 

The Commission defined negligible as one-tenth of one percent (0.1 percent) of the background 

mortality risk, either of prompt fatality or of fatal latent cancer, which people face in the U.S. 

The Commission felt that the ratio of 0.1 percent to other risks commonly faced in society is 

“...low enough to support an expectation that people living or working near nuclear power plants 

would have no special concern due to the plant’s proximity.” 
 5-5 

The same definition of negligible additional risk has been adopted for QHG 1 and QHG 2 (i.e., 

0.1 percent of the background risk that represents, respectively, the risk of prompt fatality and 

latent cancer fatality for members of the public from accidents at NMSS/FSME-regulated 

facilities and activities). The Commission has already defined this level in the SGPS, as 

representing a level of safety such that individual members of the public should have no special 

concern about being in the vicinity of nuclear facilities or activities that achieve this level. 

Hence, in the interest of consistency and to create a “level playing field” for applying RIDM to all 

NRC activities for the metric of public health and safety, it was deemed appropriate to choose 

one-tenth of one percent of the relevant background risk as a standard for judging “negligible” 

additional risk in the formulation of QHG 1 and QHG 2. 

A qualitative deterministic radiation injury goal for the general public was not defined in the 

SGPS. This is a level of exposure to ionizing radiation that is severe enough to cause radiation 

burns, or other symptoms of severe radiation injury, but lower than the threshold dose needed 

to cause a prompt fatality. Toxic chemical effects of accidents involving licensed materials can 

also result in serious injury. Such deterministic exposures will also greatly increase the risk of a 

latent cancer fatality. To set a guideline for this accidental outcome, the same approach was 

adopted as used in the prompt fatality and latent cancer fatality outcomes (viz., that the risk of a 

deterministic exposure leading to a severe radiation (or chemical) injury from accidents at 

NMSS/FSME-regulated facilities and activities should be a negligible additional risk for 

individual members of the public). Transforming this into the QHG 3 is provided again by the 

criterion of 0.1 percent of background risk of severe injury to the U.S. population from all other 

causes. 

The notion of the aforementioned “proximity” is defined in relation to both the risk metric under 

consideration and the population that is at significant risk, with regard to the consequence 

encompassed by that particular metric. 

For the individual risk of prompt fatality, the Commission defined proximity in the SGPS in terms 

of the population within 1.6 km (1 mile) of the plant boundary, based on the following reasoning 

(Ref. 5.1): 

In applying the objective for individual risk of prompt fatality, the Commission has 

defined the vicinity as the area within 1 mile of the nuclear power plant site boundary, 

since calculations of the consequences of major reactor accidents suggest that 

individuals within a mile of the plant site boundary would generally be subject to the 

greatest risk of prompt death attributable to radiological causes.

For latent cancer fatality, the Commission chose the population at risk as the population within 

16 kms (10 miles) of the plant by reasoning that (Ref. 5.1): 

The bulk of significant exposures of the population to radiation would be concentrated 

within this distance [i.e., 10 miles), and thus this is the appropriate population for 

comparison with cancer fatality risks from all other causes. This objective would ensure 

that the estimated increase in the risk of delayed cancer fatalities from all potential 

radiation releases at a typical plant would be no more than a small fraction of the yearto-year normal variation in the expected cancer deaths from non-nuclear causes. 
 5-6 

Based on the above reasoning, it is clear that the Commission chose to define “proximity” (i.e., 

the population at risk) by taking into account two factors, one factor being causation (viz., that 

the death should be “attributable” to radiological causes), and the other being the population 

subject to the “greatest” or most “significant” risk, depending on the risk metric involved. The 

Commission specifically referred to the experience gained from “...calculations of the 

consequences of....accidents,” which suggests that the distances (and the populations at risk) 

were not chosen arbitrarily but were selected on the basis of reactor probabilistic risk 

assessments or at least consequence assessments. 

The above reasoning can be applied to NMSS/FSME risk guidelines, to determine what factors 

are important in influencing the population most at risk from an accident at the different and very 

diverse NMSS/FSME-regulated facilities and activities. It is clear that the population at risk will 

be very different, depending on the particular facility and activity involved. The nature and kind 

of risk posed by large stationary nuclear facilities, such as fuel fabrication plants, where the 

general public is excluded from routine access by a boundary or exclusion zone, must be 

distinguished from the risk of small, sealed calibration and check sources in laboratories or 

industrial facilities that emit radiation to which the public can potentially be exposed. The factors 

that are important in determining the population at significant risk for various NMSS/FSMEregulated facilities and activities are discussed further in Section 5.5. 

The proposed draft risk guidelines for NMSS/FSME-regulated facilities and activities apply to 

workers as well as the public. There are no worker-risk guidelines and corresponding QHGs for 

workers in the reactor SGPS. Therefore, there is no analog of Commission prior guidance that 

could be considered, in principle, to develop a quantitative risk guideline similar to the guideline 

of 0.1 percent of the background risk of prompt fatality and latent cancer fatality for the general 

public stated above. Accordingly, the development of a basis for defining quantitative risk 

guidelines for workers has taken into account the following elements: (1) the actuarial data 

across all industries and occupations of the risk of fatality; (2) health risk guidelines developed 

by the United Kingdom Safety and Health Executive for workers in the nuclear industry 

(Ref. 5.4); (3) the existence of regulations embodied in 10 CFR Part 20 and the principle of 

ALARA that serve to strictly regulate worker exposure; (4) the safety record achieved by the 

U.S. nuclear industry in protecting its employees from the risks of acute radiation exposures; 

and (5) the notion of voluntary risk that industry workers embrace in return for compensation as 

contrasted with the involuntary risk faced by the public. 

Based on these considerations, a suitable criterion for establishing a worker risk guideline is 

believed to be as follows: that the additional risk of prompt fatality from accidents involving 

acute exposure to ionizing radiation faced by a worker in the nuclear industry should be “small” 

in comparison to the risks of prompt fatality faced by U.S. workers generally (i.e., not so small 

as in the case of members of the public who usually are not properly trained in radiation 

protection, but still small), since all radiation workers are required to be properly trained. This 

concept also acknowledges the generally good safety record of the regulated nuclear industry 

and the aspiration of maintaining it at a high level. 

A quantitative dimension to the concept of “small,” that transforms the worker prompt fatality 

guideline into a QHG, is determined to be about 2 percent of the background fatality risk faced 

by workers across all industries and occupations or, equivalently, 1 percent of the fatality risk in 

the higher-risk industries, such as mining and transportation. This serves to define QHG 4. 
 5-7 

Worker exposure to stochastic doses of radiation is regulated under 10 CFR Part 20, which 

establishes annual limits on exposure from licensed operation and also mandates the principle 

of ALARA, taking into account technology, cost, and related considerations. As stated above, 

the guideline for worker latent cancer fatality is only meant to apply to exposures that are 

unplanned, inadvertent, or accidental. The criterion used for establishing the worker risk 

guideline in this area is that the additional risk of latent cancer fatality should be “small” in 

comparison to the annual risk of cancer faced by the U.S. population. 

A quantitative dimension to the concept of “small” that transforms the worker latent cancer 

fatality guideline into a QHG is estimated to be about 0.5 percent of the background latent 

cancer fatality risk faced by the U.S. population. Because there is no precedent for this figure, it 

is proposed for further discussion and use. This serves to define QHG 5. 

The deterministic radiation injury risk goal for workers is also established on the basis that the 

risk of accidental exposure to a dose of ionizing radiation that could result in a severe injury, but 

not a prompt fatality, should be “small.” The criterion for establishing a quantitative value of this 

risk is that it should lie between the risk of prompt fatality, which is caused by very large 

radiation exposure, and the risk of a latent cancer fatality, caused by a stochastic dose of 

radiation; namely, that the deterministic exposure risk guideline should be set at a more 

stringent level than the stochastic exposure risk guideline, but at a less stringent level than the 

prompt fatality risk guideline. This helps to define the proposed value for QHG 6. 

5.4 Data on the Background Risks Used to Develop Base Quantitative 

Health Guidelines

5.4.1 Public – Quantitative Health Guidelines 1, 2, 3

As stated above, the values of the QHGs for the general public (QHG 1, QHG 2, and QHG 3) 

have been selected based on the concept of “negligible additional risk,” defined to be 0.1 

percent of the background risk of prompt fatality, latent cancer fatality, and severe deterministic 

injury, respectively, that is faced by the U.S. population. 

Data for the year 2000 on the background risks of prompt fatality and severe injury have been 

taken from the WISQARSTM (Web-based Injury Statistics Query and Reporting System) website 

of the National Center for Injury Prevention and Control of the Centers for Disease Control and 

Prevention (Ref. 5.5). Data on the background risk of a latent cancer fatality have been taken 

from publications of the National Cancer Institute of the Centers for Disease Control and 

Prevention (Ref. 5.6). 

Accidental fatalities in the U.S. for the year 2000, for all races and sexes and from all causes, 

amounted to 148,209 out of a total population of 275 million, to provide a rate of approximately 

5.3x10-4, rounded off to 5x10-4 /yr. QHG 1 is 0.1 percent of this background rate, which yields a 

figure of 5x10-7 /yr. 

Cancer fatalities across all races and sexes in the year 2000 occurred at a rate of about 

2x10-3 /yr. QHG 2 is 0.1 percent of the background rate, which gives a value of 2x10-6 /yr which 

is equivalent to 4x10-3 rem/yr. 
 5-8 

Severe, nonfatal injury is defined in the database of the National Center for Injury Prevention 

and Control to be a bodily harm resulting from severe exposure to an external force or 

substance (mechanical, thermal, electrical, chemical, or radiant) or a near-drowning. 

WISQARSTM bases its definition of a nonfatal injury on data from hospital emergency 

departments and contains information on injuries by various causes and intent, and by sexes, 

races, and disposition. The last category refers to the status of the injured person at the time of 

release from the emergency department and includes: (1) treated and released; (2) hospitalized; 

(3) transferred to a specialized trauma center or a rehabilitation unit; (4) transferred and 

hospitalized; and (5) observed or unknown. Category (3) was selected to serve as the 

appropriate background rate for severe injury (comparable to a deterministic radiation injury that 

could result in a permanent disability). The background rate for the year 2001 for all sexes and 

ages in this category is 1.1x10-3 /yr. QHG 3 is 0.1 percent of the background rate, which gives a 

value of 1x10-6 /yr. 

5.4.2 Workers – Quantitative Health Guidelines 4, 5, 6

The quantitative health objective (QHO) for limiting worker exposure that could lead to an early 

fatality is based on examining data on worker fatalities in industry generally. Industry data on 

the rate of worker fatalities across all industries and occupations in the U.S. were obtained from 

the website of the Bureau of Labor Statistics (Ref. 5.7) and are shown in Table 5.1. 

Focusing on Table 5.1, it is seen that the average fatality rate across all occupations in the U.S. 

is 4.4x10-5 /yr, or approximately 5x10-5 /yr. The higher-risk occupations such as transportation, 

manufacturing, and construction have a rate that is about twice the average rate (7.5x10-5), or 

about 1x10-4 /yr. 

Table 5.1. Numbers and Rates of Fatal Occupational Injuries by Industry Division, 2000 

Industry Group Number of Fatalities 2,3 Fatality Rate 1 

(per 100,000 employed)

Construction 1,154 12.9 

Transportation 957 11.8 

Services 768 2.0 

Agriculture 720 20.9 

Manufacturing 668 3.3 

Retail Trade 594 2.7 

Government 571 2.8 

Wholesale Trade 230 4.3 

Mining 156 30.0 
 5-9 

Finance 79 0.9 

TOTAL 5,915 4.4

Agriculture, Mining, 

Construction, 

Transportation, 

Manufacturing 

3655 15.8 

Source: Refs. 5.7, Circa 2000. 

Notes: 1. Rate = (Fatal work injuries/Employment) x 100,000 workers. Employment data extracted from 

the 2000 Current Population Survey. 

 2. Total for categories includes subcategories that are not included in the category amount. 

 3. The fatality rates were calculated using employment as the denominator; employment-based 

rates measure the risk for those employed during a given period of time, regardless of 

exposure hours.

The proposed QHG 4 for the risk of early fatality is set at 1x10-6 /yr; this is approximately 2 

percent of the risk in all industry (it is about 1 percent of the risk in the higher-risk industries of 

agriculture, mining, transportation, manufacturing, and construction industries, combined). 

Compared to the public early fatality QHG 1 of 5x10-7 /yr, QHG 4 is about twice the proposed 

value of QHG 1. This feature is consistent with the notion of voluntary risk; namely, that the 

worker, by virtue of his/her employment, should bear a somewhat higher risk than a member of 

the general public, who is not being compensated for the risk and has not received any training 

in averting the risk. 

QHG 5 is based on the worker risk of developing latent cancer from accidental exposure. As 

stated above, the proposed guideline is to limit the worker risk to 0.5 percent of the background 

spontaneous cancer risk of 2x10-3 /yr, to give a latent cancer health guideline of 1x10-5 /yr which 

is equivalent to 2.5x10-2 rem/yr. 

Data on the background risk of severe injuries to workers in industry are under development at 

this time. Preliminary indications are that the background risk may be in the range of 1 in 1000 

per year and the proposed guideline of 0.5 percent of background would imply a risk of 

5x10-6 /yr. 

The basis on which worker QHGs should be developed is an issue that needs more discussion 

and is ultimately a policy decision. One dimension of the policy issue in this area is whether 

NRC guidelines might conflict with the safety approach of OSHA or MSHA. 

On nonradiological (hazardous chemical) risk in facilities regulated by NMSS/FSME, it should 

be noted that a significant component of the risk in facilities governed by 10 CFR Part 70, 

“Domestic Licensing of Special Nuclear Material” comes from chemicals, as does the risk in 

activities, such as uranium recovery, licensed under 10 CFR Part 40. Uranium recovery is also 

subject to MSHA regulation. In fact, NRC has interagency agreements with a number of other 

agencies (both Federal and State) in activities subject to NMSS/FSME regulation. One issue is 

whether adoption of risk guidelines that include chemical risk might lead to problems of 

overlapping jurisdiction. However, NRC does impose safety limits and standards, and the 
 5-10 

adoption of guidelines may enhance the coherence of the regulatory process by focusing 

attention on the risks that are more significant in any particular area. 

Similar to the public injury QHG 3, the objective of the worker injury QHG 6 is to provide a risk 

guideline for accidental exposure to levels of ionizing radiation (or toxic chemical effects of 

licensed material) that can cause severe injury (e.g., radiation burns) but are below the 

threshold for early fatality. There is a significant amount of actuarial data on worker injuries 

from exposure to radiation in applications such as radiography. The exposures arise mainly 

from accidents involving loss of source control from human error or equipment failure. 

QHG 6, for worker injury, is set at 5x10-6 /yr. This proposed value lies between the worker early 

fatality QHG 4, of 1x10-6 /yr, and the proposed worker latent cancer QHG 5, of 1x10-5 /yr. This 

value allows for a consistent set of worker QHGs based on the magnitude of exposure levels. 

In other words, the early fatality dose levels (e.g., > 200 rem doses) have the most stringent 

QHG 4 of 1x10-6/yr, the injury dose levels (e.g., > 50 rem) have a QHG 6 of 5x10-6/yr, and lower 

stochastic doses have a QHG 5 of 1x10-5 /yr. 

5.5 Population at Significant Risk

In the preceding section, six QHGs were defined in terms of benchmark values of risk metrics. 

These risk metrics can be calculated, in principle, via a probabilistic risk assessment of a 

particular facility or activity and then compared with the QHGs to establish the risk of a particular 

facility or activity with respect to the value of the QHG. The following offers a discussion of the 

factors affecting the calculation of the risk metrics in particular facilities, applications, and/or 

areas shown in Table 5.1, to compare with the values of the corresponding QHGs. The 

objective of the calculation would be to determine, within the context of a PRA of a particular 

facility or area, the baseline value of a particular risk metric with respect to the value defined by 

the QHG. The general approach taken for evaluating individual risk is similar, in principle, to 

that specified by the Commission in its SGPS for calculating the values of the commercial power 

reactor QHGs. This approach basically consists of identifying the relevant “population at 

significant risk” that is appropriate to the risk metric under consideration and then evaluating the 

risk to an "average individual" within that population. 

For the purpose of comparing the risk from a specific NMSS/FSME-regulated application with 

the QHGs, it is important to recognize that the risk should be calculated on a realistic basis and 

not with conservative assumptions regarding the parameters entering the risk calculation. Such 

parameters would be the phenomenology of the accident scenarios, the frequency of their 

occurrence, the estimation of consequences (i.e., radiation doses or chemical exposures 

leading to acute fatalities, latent cancers, and severe injuries), and the population at significant 

risk for various consequences. In this regard, it is important to distinguish between the 

estimation of risk for purposes of comparison with the risk guidelines and the estimation of 

consequences for other regulatory purposes, such as licensing actions that involve the 

evaluation of compliance with various types of regulatory dose limits. In the latter case, 

conservative assumptions are rightly used to establish compliance with regulations, and the 

calculation is carried out in a prescriptive manner. It is noted that the regulations consider 

accidents in a prescriptive fashion (e.g., regulations pertaining to design-basis accidents). 

A risk assessment should consider all realistic accident scenarios. 
 5-11 

The purposes of carrying out a risk assessment, to compare the result with the risk guidelines 

and thus gain risk insight, is different from establishing regulatory compliance. The purpose is 

to understand the magnitude of the risk presented by an operating system and compare it to a 

reference level represented by the risk guideline. For this reason, it is important to carry out the 

calculation in as realistic a manner as possible, with, of course, allowance for incorporating all 

the uncertainties. The purpose of estimating this risk for decisionmaking is to evaluate the 

significance of changes or exemptions, to determine if further risk reduction is warranted, or to 

arrive at a basis for regulatory resource allocation decisions. 

The full-risk spectrum has to be considered, along with the population that is at significant risk 

for a defined consequence. The analogy with the QHOs that were defined in the (reactor) 

SGPS is to estimate the risk to the average individual, within the population, that is at significant 

risk for that particular health outcome. 

Hence, 

 The average individual risk of an acute fatality is: 

= ∑ (Freq. of accident I) * (number of acute fatalities from accident i)/Total 

population at significant risk for acute fatality... (1) 

 The average individual risk of a severe injury is: 

= ∑ (Freq. of accident I) * (number of severe injuries from accident i)/Total 

population at significant risk for severe injury.... (2) 

 And the average individual risk of latent cancer is: 

= ∑ (Freq. of accident I) * (number of latent cancers from accident i)/Total 

population at significant risk for latent cancer..... (3)

Since an acute fatality and a severe injury both involve dose thresholds (approximately 200 rem 

whole body (WB) in the case of acute fatality, and, 50 rem WB (proposed) in the case of severe 

injury), it is possible, in principle, although it may be difficult in practice, to establish the 

denominator for [1) and [2) above, for calculating the average individual risk within the 

population at significant risk. However, given the linear, no-threshold model for latent cancer, 

some criterion has to be developed for establishing a denominator for [3). Such a criterion can 

be based on criteria similar to the reasoning given by the Commission in the (reactor) SGPS for 

choosing the population out to 1.6 km (1 mile) as the population at risk for purposes of 

calculating the early-fatality QHG and the population to 16 kms (10 miles) as the population at 

significant risk for calculating the latent-cancer QHG. Hence, both the numerators and 

denominators in [1) and [3) were taken to 1.6 and 16 kms (1 and 10 miles), respectively, and the 

SGPS risks were understood as the average risks to individuals within 1.6 and 16 kms (1 and 

10 miles), respectively. This does not mean that the populations outside these distances had 

zero risk. However, the estimated individual risk within the 1.6 and 16 kms (1 mile and 10 mile 

zone) represents the maximum level. 
 5-12 

Section 5.5.1 provides some examples of risk assessment results using various quantitative risk 

metrics for comparison with corresponding risk guidelines. These estimates were based on 

readily available information, and may benefit from a more refined treatment consistent with the 

principles discussed above. However, they provide examples of the issues involved in the 

calculation, in particular, the selection of the population at significant risk, to estimate average 

individual risk. The data on which these preliminary calculations are based are of varying 

vintage and quality. Section 5.5.1 is focused on calculating risk for three materials 

facilities/activities that were assessed in the Integration Study Report (Ref. 5.2). 

5.5.1 Estimation of Risk Metrics for Selected Materials Systems

A detailed assessment of risk from 40 materials subsystems was carried out in NUREG/CR6642 (Ref. 5.8). The results from that study have been used to explore various approaches for 

calculating risk for comparison with the proposed QHGs for the three materials device/systems 

that were also analyzed in the case study. These three device/systems are: gas 

chromatographs, static eliminators, and fixed gauges (gamma and beta). As far as public risks 

are concerned, there were no accidents identified that can lead to early fatality (i.e., no 

accidents where the dose would exceed the threshold for early fatality). Hence, the public risk 

of acute fatality from all three devices is zero for purposes of comparison with QHG 1, the risk 

guideline for early fatality, with a value of 5x10-7 /yr. Data on accidents where the dose could 

exceed the severe-injury threshold (tentatively identified as 50 rem WB) were not available; 

hence, the public risk of severe injury for comparison with QHG 3 was not calculated. 

For comparison with QHG 2, the latent cancer risk guideline with a value of 2x10-6 /yr, data are 

available from NUREG/CR-6642 on the public risk of dose on both a per-facility and an industrywide basis. This is presented as an expected value (assumed to be a product of frequency and 

dose summed over all accidents) in mrem/yr. This is shown in Table 5.2 for the three devices in 

the case study. 

Table 5.2. Public Accident Risks from Byproduct Materials Systems in the Case Study 

(Based on data from NUREG/CR-6642)

Public Risk (mrem/year)

Material System

Per-Facility Industry-wide

Dominant Accident 

Cause

Gas Chromatographs 2.6x10-3 1.2x101

Lost or stolen sources; 

approx. 120,000 

devices 

Fixed gauges (gamma) 2.4x10-2 2.9x103 Lost or stolen sources; 

approx. 40,000 devices 

Fixed gauges (beta) 5.7x10-4 3.1x101

Lost or stolen sources; 

approx. 100,000 

devices 

Static eliminators 

(assumed to be about 

10% of all small sealed 

sources) 

1.7x10-4 1.2x102

Lost or stolen sources; 

approx. 75,000 staticeliminator devices 
 5-13 

By using the BEIR-V conversion factor, which for the public is 5x10-4 latent cancers per rem, the 

expected value of dose can be converted into latent cancers per year. However, for purposes 

of comparison with QHG 2, a figure for the population at significant risk is needed. If the 

accidental exposure of the public is from a lost or stolen source, as it is, in fact, for all the 

devices studied, then some basis has to be developed for estimating the population that is at 

risk of exposure. The related question, of course, is what are the assumptions underlying the 

figure for public risk presented in Table 5.1? Does this represent average individual risk, in 

some sense, or is it calculated on the basis that only one individual receives (or can receive) the 

accidental dose? In other words, if the figures presented in NUREG/CR-6642, the most detailed 

risk study of materials to date, have to be used to calculate the risk for comparison with QHG 2, 

a clear understanding of their basis has to be arrived at to use them in this particular context. 

Until such understanding is established, an interim approach is taken, as discussed below. 

A few initial approaches have been developed for use in this report, mainly to serve as a basis 

for further discussion and elucidation: 

(1) One approach (Approach 1) is to assume that the per-facility public risk reported in 

Table 5.1 represents the risk to just one person who is inadvertently exposed as a result 

of mishandling a lost source from a facility. The industry-wide risk is assumed to 

represent the aggregate risk over all facilities. The industry-wide population at risk is 

then obtained from the per facility population at risk by looking at the data on the total 

number of devices, and estimates of the average number of devices per facility. 

(2) Another approach (Approach 2) is to evaluate the public population at risk on a perfacility basis alone. This approach assumes that the facility is located in a small town of 

approximately 10,000 population, and the risk arises from the lost or stolen source 

getting into the waste stream. In this approach, the entire population of the town is 

considered to be potentially at risk of exposure. Conceptually, this approach to 

identifying the population at risk represents a very different situation compared to the 

stationary facilities, such as fuel fabrication plants. Since the materials devices only 

have a small radioactive inventory, actual radiation risk can only be to the person (or 

members of his/her family) who inadvertently picks up a lost device. (This is unlike the 

plume exposure from accidents at a large facility that can affect many offsite people.) 

However, since the whole town has potential access to a lost source, in one sense, the 

entire population can be considered to be at risk. 

(3) A third approach (Approach 3) is to evaluate the public population at risk on an industry 

wide basis only by using the annual loss rate for these devices nationwide 

(approximately six devices per year) and assuming that the loss occurs in different small 

towns, each with an average population given in Approach 2. This approach is similar to 

Approach 2 except that the numerator in the average individual-risk calculation is based 

on the industry-wide estimate in Table 5.1. 

The results of these calculations of the average individual risk of latent cancer, using each of the 

above approaches, are shown in Table 5.3. 
 5-14 

Table 5.3. Average Public Individual Risk of Latent Cancer Based on 

Different Approaches for Estimating the Population at Risk

Average Individual Risk of Latent Cancer (per year) 

(QHG 2 = 2x10-6/year)

Approach 1 Approach 2 Approach 3

Device/Facility

Per-Facility 

Basis

Industry-Wide 

Basis

Per-Facility Basis 

Only

Industry-Wide Basis 

Only

Gas Chromatographs Total: 

120,000 number/facility = 10 

1.3X10-9 5X10-10 1.3X10-13 1x10-10

Static Eliminators Total: 

75,000 number/facility = 5 

8.5X10-11 4X10-9 8.5X10-15 8x10-10

Fixed gauges (gamma) 

Total: 40,000 number/facility 

= 2 

1.2X10-8 7.2X10-8 1.2X10-12 1.5x10-8

Fixed gauges (beta) 

Total: 100,000 

number/facility = 5 

2.9X10-10 7.8X10-10 2.9x10-14 1.6x10-10

The results for the individual risk of latent cancer calculated, using the different approaches for 

the population at risk shown in Table 5.3, are broadly similar for Approaches 1 and 3. The 

results using Approach 2 are much smaller because the per-facility risk estimate from 

NUREG/CR-6642 is now being distributed over a much larger population. All the computed risk 

values, however, fall below the QHG 2 risk guideline value of 2x10-6 /yr. 

NUREG/CR-6642 also contains estimates for worker risk both on a per-facility and an industrywide basis, for the systems/devices analyzed in the case study. For gas chromatographs, static 

eliminators, and fixed gauges (beta), no accident sequences (down to a frequency of 

5x10-7/yr) were identified that could lead to an early fatality. However, for fixed gauges 

(gamma), there is an identified accident sequence consisting of a device damaged by a 

crushing event that causes failure of the shielding and encapsulation. This accident sequence 

can lead to a dose of almost 600 rem (WB) that is considerably in excess of the threshold for 

early fatality. The frequency of this sequence is estimated at 1.8x10-6 /yr. The population at risk 

is assumed to be, on average, two workers per facility. This is the population at risk for acute 

fatality. Hence, the average individual risk of acute fatality is estimated to be 9.0x10-7 /yr. This 

can be compared with the proposed worker early-fatality risk guideline of 1x10-6 /yr. 

Data on accidental doses that could lead to a risk of severe injury to workers were not available. 

However, data on smaller accident doses that could lead to a risk of latent cancer are provided 

in NUREG/CR-6642, on both a per-facility and an industry-wide basis. These are shown in 

Table 5.4. 
 5-15 

Table 5.4. Worker Accident Risks from Byproduct Materials Systems in the Case Study 

(Based on data from NUREG/CR-6642)

Material System Worker Risk (mrem/year)

Per-Facility Industry-wide

Dominant Accident 

Cause

Gas Chromatographs 2.8X10-3 1.2X10+2 Leaking or damaged 

source 

Fixed gauges (gamma) 3.1X10-2 4.5X10+4 Leaking source or shielding 

failure from fire 

Fixed gauges (beta) 4.5X10-4 2.3X10+1 Leaking source or shielding 

failure from fire 

Static eliminators (assumed to 

be about 10% of all small 

sealed sources) 

1.2X10-5 1.9X10+2 Leaking or damaged 

source 

To compute the average individual risk of latent cancer for the workers, and to compare it with 

the proposed latent cancer QHG, an estimate of the worker population at significant risk is 

needed. Although this is likely to differ among industries, it is assumed, for purposes of 

discussion, that the average number of workers at risk per facility is two and the average 

number at risk on an industry-wide basis is 10,000. Using these assumptions, the average 

individual risk of latent cancer calculated using the worker risk coefficient of 4x10-4 latent 

cancers per rem is shown in Table 5.5. 

Although all the average individual risks of latent cancer for workers are considerably below the 

risk guideline of 1x10-5 /yr, the differences between the risks calculated on a per-facility and 

industry-wide basis, especially for static eliminators and fixed gauges (gamma), need to be 

understood. These are likely to be caused by the differences between the way in which the 

estimates in NUREG/CR-6642 were generated and the assumptions used in estimating the 

number of workers at risk for these materials and devices. 

Table 5.5 Average Individual Risk of Latent Cancer for Workers

Average Individual Risk of Latent Cancer (per year) 

QHG 5 = 1X10-5/yr

Material System/Device

Per-facility basis Industry-wide basis

Gas Chromatographs 5.6X10-10 4.8X10-9

Static Eliminators 2.4X10-12 7.6X10-9

Fixed gauges (gamma) 6.2X10-9 1.8X10-6

Fixed gauges (beta) 9.0X10-11 9.2X10-10
 5-16 

5.5.2 Estimation of Risk for Fuel Cycle Facilities

(Reserved for future use) 

(Reserved for Table 5.6) 

5.5.3 Summary

The factors that affect the population at risk for various health outcomes at the very different 

kinds of facilities and activities regulated by NMSS/FSME have been discussed in Section 5.5. 

From the standpoint of risk estimation, the facilities and activities are divided into three 

categories: (1) stationary facilities, such as the fuel cycle facilities, with comparatively large 

inventories of radioactive and hazardous chemical materials; (2) activities involving storage and 

transport of radioactive materials, including reactor spent fuel; and (3) industrial and medical 

facilities and activities using radioactive materials. Various issues that affect the nature and 

kind of risks presented by these facilities, to both the offsite public and the onsite workers, were 

discussed and elaborated previously. 

Section 5.5 presents risk data and an estimation of the QHGs for selected facilities and 

activities. In Section 5.5.1, available risk data from NUREG/CR-6642 are presented for three 

specific materials systems: gas chromatographs, static eliminators, and fixed gauges (beta and 

gamma), which were studied earlier in the case studies presented in the Integration Report 

(Ref. 5.2). These data are then combined with a number of assumptions regarding the 

population at risk from these devices to calculate the risk measures, average individual risk of 

early fatality, and average individual risk of latent cancer for both the public and the workers. A 

number of issues emerge from this preliminary exercise: 

(1) For the three materials systems examined, the only early fatality risk identified is to 

workers from accidents involving fixed (gamma) gauges that use americium-241 (Am241); the other systems/devices have too low an inventory of radioactive material to 

pose an early-fatality risk. 

(2) NUREG/CR-6642 is a rich source of data, but the information is not completely available 

in the printed reports; more interaction between the authors and users would be useful in 

developing a firmer base for selecting among the various approaches identified for 

estimating the population at risk in specific activities. 

(3) Data corresponding to the severe-injury outcome were not available either for workers or 

the public. 

(4) The estimation of latent cancer risks for both public and the workers shows that the risks 

fall below the corresponding risk guidelines QHG 2 and QHG 5, in some cases, by 

several orders of magnitude. These calculations reinforce the conclusions that were 

reached in the case studies reported in the Integration Report. 
 5-17 

5.6 Draft Risk Guidelines - Other Approaches for Consideration

The discussion on the draft risk guidelines thus far has focused on the base approach. Per 

recommendations from Ref. 5.9, other alternative approaches are being evaluated. Meanwhile, 

other policy and implementation issues (see Appendix I) are also being considered. 

5.7 International Activities on Safety Goals and Risk Guidelines

The results of a survey of information and activities relevant to the development of risk 

guidelines in the materials and waste arenas were reported in Ref. 5.10. This reference 

contains the results of a search of the international literature on safety-goal development. While 

there is a particular focus on information from Japan and the U.K., a broad search across many 

other countries has also been performed. Discussions were held with various individuals who 

were knowledgeable about the objectives of this program. Documents were gathered and 

reviewed from readily available sources (libraries and the web). Cognizant safety experts in 

several countries were contacted to obtain relevant current information on the subject area. 

These sources do not have the same approach to regulation or radioactive material as does 

NRC. However, they are useful in obtaining comparative insights for safety guideline 

development. 

This literature review determined that much material exists, in several countries and 

organizations, that is useful to constructing risk guidelines in the materials and waste arenas. 

It should be noted that this review was not exhaustive. As additional information becomes 

available, it should be considered in the development of the risk guidelines, as appropriate. 

The work in the U.K. is reasonably mature and many relevant ideas exist in that body of work. 

The work in the field of radiation protection carried out by such standards-setting organizations 

as the International Commission on Radiation Protection (ICRP), the National Council on 

Radiological Protection and Measurements, and the National Academy of Sciences form an 

important foundation for risk-guideline development, and due consideration should be given to 

that material. Other countries, such as Japan, have taken some steps toward safety-goal 

development for power reactors, but less so for materials and waste. There is some helpful 

material on the key issues relevant to this program (e.g., worker/public, accident/normal, 

individual/societal risk). However, the specific application of this information to these issues and 

the tailoring of the ideas to the NMSS/FSME-specific regulatory environment is not directly 

evident. 

ICRP has developed a conceptual framework (Ref. 5.11) for protection from potential exposure. 

This document extends the ICRP philosophy of radiation protection (Ref. 5.12) to the accident 

regime by considering nonroutine exposures and their likelihoods. ICRP distinguishes between 

normal exposure and potential exposure by regarding the former as those exposures that are 

reasonably expected to occur (planned plus unintended high-probability, low-consequence 

events). ICRP regards potential exposures as uncertain events that fall outside the general 

boundaries considered for normal exposure. For the risk to individual health from potential 

exposures, ICRP “...recommends that limits of risk be the same order of magnitude as the 

health risk implied by the dose limits for normal exposures.” This concept and the range of 

probabilities vs. health effects to the individual are broadly consistent with the approach and 
 5-18 

recommendations of this report. For the convenience of the reader, the range of (annual) 

probabilities, suggested by ICRP, for four health impacts on the individual, are shown in Table 

5.7. 

Table 5.7. ICRP 64 Framework for Potential Exposures

Health Impact Annual Probability Range

Treated as Normal Exposures 1x10-1 to 1x10-2

Stochastic Effects, but above Dose Limits 1x10-2 to 1x10-5

Some Radiation Effects are Deterministic 1x10-5 to 1x10-6

Death is Likely to Result < 1x10-6

5.8 References

5.1 U.S. Nuclear Regulatory Commission, “Safety Goals for the Operation of Nuclear Power 

Plants; Policy Statement,” Federal Register, Vol. 51, p. 30028 (51 FR 30028), August 4, 

1986. 

5.2 U.S. Nuclear Regulatory Commission, “Risk-Informing the Materials and Waste Arenas: 

Integration of Case Studies and Related Risk Assessments,” ADAMS Accession 

Number ML 022130067, February, 2002. 

5.3 U.S. Nuclear Regulatory Commission, “Staff Requirements - SECY-99-100 - Framework 

for Risk-Informed Regulation in the Office of Nuclear Material Safety and Safeguards,” 

June 28, 1999. 

5.4 United Kingdom Health and Safety Executive, “Safety Assessment Principles for Nuclear 

Plants,” ISBN 0 11 882043 5, 1992, http://www.hse.gov.uk/nsd/saps.htm.

5.5 WISQARS, "Web-based Injury Statistics and Query Reporting System," Centers for 

Disease Control and Prevention, National Center for Injury Prevention and Control, 

http://webapp.cdc.gov/ncipc/wisqars.

5.6 National Cancer Institute, "2001 Cancer Progress Report," National Institutes of Health, 

U.S. Department of Health and Human Services, Washington, DC, 2002. 

5.7 United States Department of Labor, Bureau of Labor Statistics, http://www.bls.gov.

5.8 U.S. Nuclear Regulatory Commission, “Risk Analysis and Evaluation of Regulatory 

Options for Nuclear Byproduct Material Systems,” NUREG/CR-6642, February 2000. 

5.9 Advisory Committee on Nuclear Waste, Letter to Commission, dated June 9, 2004. 
 5-19 

5.10 Letter report, “International Activities on Safety Goal Development,“ Brookhaven 

National Laboratory, June 6, 2002. 

5.11 International Commission on Radiological Protection, “Protection from Potential 

Exposure: A Conceptual Framework,” ICRP Publication 64, 1993. 

5.12 International Commission on Radiological Protection, “Recommendations of the 

International Commission on Radiological Protection,” ICRP Publication 60, 1991. 
 6-1 

6 RISK COMMUNICATION 

6.1 Introduction 

To be effective in communicating risk-informed decisions, it is important to consider early in the 

process who needs to be informed and involved, as well as who will be impacted, and to build in 

communication steps that encourage discussion and clarification throughout the process. 

A communication plan should be developed as one of the early activities. Potential internal 

stakeholders (e.g., line and senior management, other divisions or offices, etc.) and external 

stakeholders (other government agencies, industry, affected state and local governments, 

affected citizens, etc.) should be identified if they may use or be impacted by the risk. This 

enables analysts and decision-makers to be prepared for communication activities during and at 

the end of this risk-informed process. Placing emphasis on communication during the process 

will help identify topics that require clarification by the staff and focus attention on ensuring that 

all participants share an understanding of the subject, objective, terms, and assumptions at 

hand; this will encourage discussion and prevent misunderstandings among team members and 

therefore enable everyone to stay on track. This is especially important when working with 

multidisciplinary teams that include both risk analysts and analysts from other (e.g., engineering 

and licensing) disciplines. The NRC’s Risk Communication Guidelines (Refs. 6.1 and 6.2) 

emphasize the importance of explicitly addressing communication challenges early in a process. 

This process follows that guidance and other internal communication guidelines. 

In addition, the task of communicating and implementing the decision is where the risk 

information becomes translated into effective action. The effectiveness of risk information will 

be limited or non-existent if there is no effective implementation or in cases where stakeholders 

disagree with the way in which risk information was used. Also, alternatives will typically have 

risk tradeoffs (e.g., public vs. remedial worker exposures, or trading off larger but low probability 

hazards for smaller but relatively certain hazards). Therefore, it is important to document and 

communicate the decision. 

6.2 Document the Decision 

Once the decision has been made, the staff should document the decision. The analysis report 

should contain appropriate supplemental material. Documentation of the decision should 

include not only what the decision was, but also the following: 

• Insights obtained from the decision-maker 

• How various factors were considered in reaching the final decision 

• Factors not considered in the technical analysis of the issue 

• Any contingencies or need for subsequent decision points 

Decision-makers typically need information presented in summary format for rapid assessment 

and ease of understanding the impacts and complexity of an issue. However, enough 

background information should be documented to introduce the issue and the decision that is to 

be made. The decision that is required should be stated clearly and concisely. 
 6-2 

Each of the options developed should be presented individually and concisely. The preferred 

option should be presented first. The driving factors for accepting or rejecting the option must 

be presented. Typically these factors address satisfaction or non-satisfaction of the principles of 

risk-informed decisionmaking that are relevant to the issue, plus a discussion of how the degree 

of uncertainty (or certainty) of supporting information affects application of the principles to the 

option. Other relevant criteria must also be addressed, if any. The communication document 

must provide sufficient information to document the logical basis for accepting and rejecting 

options. 

The logic for accepting the recommended option and not accepting other options must be 

summarized, drawing from the individual option discussions. Where more than one option is 

acceptable, the basis for preferring the recommended option must be provided. 

Where there are technical issues particularly important for the decision-makers to recognize in 

order to make a properly informed decision, those issues must be briefly described in the 

communication document. Decision-makers need narrative descriptions that provide qualitative 

insight into causes, uncertainties, assumptions, sensitivities and affected outcomes for a given 

situation. Less information is needed regarding the details of numerical results, statistical 

methods, and analyses. Other relevant information, such as generic implications, stakeholder 

concerns, or known or anticipated impacts of a decision on other regulations, should be 

documented. A list of staff contacts for each relevant issue or input to the document should be 

provided at the end of the document. 

6.3 Communicate the Decision 

The decision and related information should be communicated to the various stakeholders in 

accordance with the Communication Plan developed (Ref. 6.3). The Communication Plan 

should be examined at this point to ensure that it addresses the right audiences and that the key 

messages reflect the process and its outcomes. Audiences may include internal and external 

stakeholders. Key messages may include the information conveyed in the decision 

communication, as well as background information relevant to specific audiences. 

6.4 References 

6.1 U.S. Nuclear Regulatory Commission, “Effective Risk Communication—Guideline for 

Internal Risk Communication,” NUREG/BR-0318, December 2004. 

6.2 U.S. Nuclear Regulatory Commission, “Effective Risk Communication—The Nuclear 

Regulatory Commission’s Guideline for External Risk Communication,” NUREG/BR-0308, 

January 2004. 

6.3 Communication Plan Guidance at: http://www.internal.nrc.gov/communications/plans/guidance.
APPENDIX A 

GLOSSARY 
 A-1 

APPENDIX A 

GLOSSARY 

accidental exposure — occupational or public exposure to radiation, chemicals, or other 

hazardous materials in excess of regulatory limits due to failure or degradation of a system, 

process, structure, component, barrier, or procedure designed to prevent exposure in excess of 

regulatory requirements. 

ALARA — an acronym for "As Low As Is Reasonably Achievable," meaning that every 

reasonable effort should be made to maintain exposures to ionizing radiation as far below the 

dose limits as 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 improvements in relation to benefits to the public health 

and safety, and other societal and socioeconomic considerations, and in relation to the 

utilization of nuclear energy and licensed materials in the public interest (see 10 CFR Part 

20.1003). 

backfit — as per 10 CFR Part 50.109(a)(1), “the modification of or addition to systems, 

structures, components, or design of a facility; or the design approval or manufacturing license 

for a facility; or the procedure or organization required to design, construct or operate a facility; 

any of which may result from a new or amended provision in the Commission rules or imposition 

of a regulatory staff position interpreting the Commission rules that is either new or different 

from a previously applicable staff position...” A generic backfit is one that applies to multiple 

facilities. A plant-specific backfit applies to a single facility and is subject to the requirements of 

NRC Manual Chapter 0514 and the NRC Guidelines. 

Chemical exposure — occupational or public exposure to a licensed material or hazardous 

chemicals that are used or stored at a NMSS/FSME-regulated facility. 

Complementary cumulative distribution function — estimates of the probability that a given 

consequence will be exceeded. 

Consequences — effects of normal and accidental exposures to radiological, chemical, or other 

hazards resulting from the operation of NMSS/FSME-regulated facilities, activities, and 

materials. 

Critical group — a group of individuals reasonably expected to receive the greatest exposure to 

residual radioactivity, chemical exposure, or other hazards for any applicable set of 

circumstances from the operation of NMSS/FSME-regulated facilities, activities, and materials. 

Defense in depth — a design and operational philosophy with regard to NMSS/FSME-regulated 

facilities that calls for multiple layers of protection to prevent and mitigate accidents. It includes 

the use of controls, multiple physical barriers to prevent release of radiation, redundant and 

diverse key safety functions, and emergency response measures. 
 A-2 

Deterministic radiation exposure — health effects, the severity of which varies with the dose, 

and for which a threshold is believed to exist. Radiation-induced cataract formation is an 

example of a deterministic effect (also called a nonstochastic effect) (see 10 CFR Part 20.1003). 

Event likelihood (highly unlikely, unlikely, likely) — qualitative or quantitative statement of the 

frequency of occurrence of events at an NMSS/FSME-regulated facility over the licensing 

period. Classification of occurrence as highly unlikely, unlikely, and likely is defined on an 

individual facility basis. (See 10 CFR Part 70.61) 

Inadvertent exposure — unplanned or unexpected occupational or public exposure to radiation, 

chemicals, or other hazardous materials in excess of regulatory limits due to failure or 

degradation of a system, process, structure, component, barrier, or procedure designed to 

provide exposure in excess of regulatory requirements. 

Licensing basis — set of NRC requirements applicable to a specific plant and a licensee’s 

written commitments for ensuring compliance with and operation within applicable NRC 

requirements and plant-specific design basis (including all modifications and additions to such 

commitments over the life of the license). (See 10 CFR 54.3) 

Minimal cut set — smallest set of primary events, inhibit conditions, or undeveloped fault events 

(or combination thereof) which must all occur in order for the top event to occur. 

Nuclear material and waste activity — licensed activity or activities being conducted by specific 

NMSS/FSME -regulated material or waste licensees. 

Population at risk — population at, and surrounding, NMSS/FSME -regulated facilities that is at 

risk of injury and death “attributable” to radiological causes, or subject to the “greatest” or most 

“significant” risk depending on the risk metric involved. 

Potential exposures — potential occupational or public exposure to radiological, chemical, or 

other risks from the operation of NMSS/FSME -regulated facilities, activities, and materials. 

Probabilistic risk assessment — an analytic tool that computes the probability of health, 

environmental, and economic consequences of events resulting from equipment or human 

failure. A Level 3 PRA assesses the offsite health effect risk. 

Property damage—damage incurred to private or licensee-owned property resulting from an 

accident at an NMSS/FSME-regulated facility or activity. 

Quantitative health guidelines—risk metric values that are “not a significant addition to other 

risks.” The QHGs form a set of minimal risk reference levels for each of the risk metrics used in 

NMSS/FSME and are applied across all NMSS/FSME-regulated activities. 

Quantitative health objectives—quantified restatement of the qualitative risk guidelines that 

define specific objectives necessary to ensure public and occupational health from the operation 

of NMSS/FSME-regulated facilities, activities, and materials. 

Red Oil- substance of varying composition formed when an organic solution, typically tri-butyl 

phosphate (TBP) and its diluent, comes in contact with concentrated nitric acid at a temperature 
 A-3 

above 120 °C. The solvent TBP is often used to extract uranium or plutonium from acid 

solutions. Even though red oil is relatively stable below 130 °C, it can decompose explosively 

when its temperature is raised above 130 °C. 

Regulatory limit—specific limitations applicable to actions or effects of NMSS/FSME-regulated 

activities per regulations contained in Chapter 10 Parts 20, 40, 50, or 72 or other applicable 

parts of the Code of Federal Regulations, either by direct citation or reference. 

Risk (unacceptable, tolerable, insignificant)—quantitative or qualitative assessment of the 

probability of an accident or event occurring resulting in impacts to public or occupational health 

or environmental impacts. Insignificant risks result in small impacts on public or worker health 

or the environment, tolerable risks reflect an increasing probability of occurrence that may 

require additional oversight or assessment, and unacceptable risks reflect a high probability of 

occurrence that requires action to rectify the situation. 

Risk guidelines—qualitative and quantitative statements designed to guide regulatory 

requirements in a consistent manner to ensure protection of the public, workers, and the 

environment from the operation of NMSS/FSME-regulated facilities, activities, and materials. 

Risk guidelines are not substitutes for NRC regulations or the “defense in depth” principle. 

Risk-informed approach— represents a philosophy whereby risk insights are considered 

together with other factors to establish requirements that better focus licensee and regulatory 

attention on design and operational issues commensurate with their importance to public health 

and safety. A “risk-informed” approach enhances the deterministic approach by: (a) allowing 

explicit consideration of a broader set of potential challenges to safety, (b) providing a logical 

means for prioritizing these challenges based on risk significance, operating experience, 

and/or engineering judgment, (c) facilitating consideration of a broader set of resources to 

defend against these challenges, (d) explicitly identifying and quantifying sources of uncertainty 

in the analysis (although such analyses do not necessarily reflect all important sources of 

uncertainty), and (e) leading to better decisionmaking by providing a means to test the 

sensitivity of the results to key assumptions. Where appropriate, a risk-informed regulatory 

approach can also be used to reduce unnecessary conservatism in purely deterministic 

approaches, or can be used to identify areas with insufficient conservatism in deterministic 

analyses and provide the bases for additional requirements or regulatory actions. “Riskinformed” approaches lie between the “risk-based” and purely deterministic approaches. The 

details of the regulatory issue under consideration will determine where the risk-informed 

decision falls within the spectrum. 

Risk metric—a more quantitative assessment of risk. 

Stochastic radiation exposure—effects that occur by chance, generally occurring without a 

threshold level of dose, whose probability is proportional to the dose and whose severity is 

independent of the dose. In the context of radiation protection, the main stochastic effects are 

cancer and genetic effects. 

Subsidiary guidelines—restatement of the quantitative health guidelines to be applicable to 

specific NMSS/FSME-regulated facilities, uses, or materials. 
 A-4 

Subsidiary objectives—restatement of the quantified health objectives to be applicable to 

specific NMSS/FSME-regulated facilities, activities, or materials. 

Unplanned exposure—see Inadvertent Exposure. 

Value-impact analysis—an estimate of the benefits (values) and costs (impacts) associated with 

an action or decision. These should be evaluated in monetary terms when feasible, using 

qualitative terms where conversion to monetary equivalents is not possible. A value-impact 

analysis is a substantial part of a regulatory analysis. It may also be described as a benefit-cost 

analysis.
APPENDIX B 

VALUE-IMPACT ANALYSIS
 B-1 

APPENDIX B 

VALUE-IMPACT ANALYSIS

As illustrated in Figure 4.1, one outcome of the RIDM application process may be to perform a 

cost-benefit analysis (value-impact analysis) for new requirements. Detailed instructions on 

performing this analysis are provided in NUREG/BR-0184, “Regulatory Analysis Guidelines of 

the U. S. Nuclear Regulatory Commission.” This appendix will provide a generic template for 

performing the analysis, including a summary of the main attributes. It must be noted that, 

given the breadth of NMSS/FSME-regulated facilities and activities, no one generic template 

can be developed. Rather, each analysis must be tailored to suit the particular application being 

reviewed. 

The performance of a value-impact analysis may often require the use of an existing risk 

assessment. If such an assessment is not available, as is the case in many NMSS/FSMElicensed applications, it must be performed. This assessment can be performed with varying 

degrees of formality and rigor. It may be a relatively simple, application-specific analysis, or a 

more detailed facility-risk assessment. The latter may require the assistance of a risk expert. 

Attempts should be made to include an uncertainty/sensitivity analysis. Such an analysis 

should include a review of all relevant factors available (assumptions, data, models) that can 

affect the risk. The number of factors that may affect the risk may be large, and not all may be 

treated using the same level of detail. However, an effort to identify them is important. When 

assembled, the factors can be screened to identify the subset to be included in the analysis. 

This screening process should consider the contribution of each to the risk of the change being 

analyzed, including the degree of uncertainty associated with each. The reviewers’ judgment 

and experience will be a factor in the screening process. In addition to exposure data, cost 

measures are also included. Typically, this information (labor rates, interest rates, replacementequipment costs, etc.) is readily available. 

The attributes that may be affected at an NMSS/FSME-licensed facility are shown in Table B-1. 

Most of these attributes can be analyzed quantitatively in terms of monetary impact, or through 

radiation exposure, which can be converted to dollars. Others may not be readily quantifiable 

and may be treated in a more qualitative manner. Efforts should be made, however, to quantify 

the attributes, if possible. Not every attribute will be pertinent to all value-impact analyses. The 

data (site-specific or generic) and assumptions used for evaluating each parameter should be 

documented by the analyst. In the evaluation of each attribute, the analyst should consider 

uncertainty and should determine the best estimate, as well as minimum and maximum 

estimates. A summary of the results should be tabulated (Table B-2). 

A brief description of each attribute is provided below. 

Public Health (Accident): This attribute measures expected changes in radiation exposures to 

the public because of changes in accident frequency or consequences associated with the 

proposed action. The appropriate distance from the facility to be analyzed needs to be 

established on a case-by-case basis. Typically, the effect of the proposed action will be to 

decrease public exposure (given in person-rems). This decrease multiplied by the monetary 

conversion factor ($/person-rem) will be given a positive monetary value. It is possible that the 
 B-2 

proposed action could increase public exposure from potential accidents. In this case, the 

increase would assume a negative sign. 

Public Health (Routine): This attribute accounts for changes in radiation exposures to the public 

during normal facility operations (non-accident). Analysis of this attribute will involve actual 

estimates since accident probabilities are not involved. The product of the decrease in public 

exposure multiplied by the monetary conversion factor would be taken as positive. An increase 

in public exposure (multiplied by the monetary conversion factor) would be taken as negative. 

Occupational Health (Accident): This attribute measures the health effects (both immediate and 

long-term) associated with facility workers as a result of changes in accident frequency or 

accident mitigation. A decrease in worker radiological exposure is taken as a positive, and an 

increase in worker exposure is considered to be negative. 

Occupational Health (Routine): This attribute accounts for radiological exposure to workers 

during normal facility operations. For many types of proposed actions, there may be an 

increase in worker exposure. This may be a one-time effect (e.g., installation or modification of 

equipment in a radiological area), or it may be an ongoing effect (e.g., routine surveillance or 

maintenance of contaminated equipment). Some actions may involve a one-time increase with 

an offsetting reduction of future exposure. This attribute represents an actual estimate of health 

effects; accident probabilities are not relevant. A decrease in worker radiological exposure is 

taken as a positive, and an increase in worker exposure is considered to be negative. 

Offsite Property: This attribute measures the expected total monetary effects on offsite property 

resulting from the proposed action. Changes to offsite property can take various forms, both 

direct (e.g., land, food, water) and indirect (e.g., tourism). This attribute is typically the product 

of the change in accident frequency and the property consequences resulting from the 

occurrence of an accident (e.g., interdiction cost measures such as decontamination, cleanup, 

and evacuation). A reduction in offsite property damage is taken as a positive, an increase is 

considered to be negative. 

Onsite Property: This attribute measures the expected monetary effect on onsite property, 

including decontamination and replacement costs from the proposed action. This attribute is 

typically the product of the change in accident frequency and the onsite property consequences 

if an accident were to occur. A reduction in expected onsite property damage is taken as a 

positive, an increase is considered to be negative. Care should be taken in estimating monetary 

savings associated with this attribute, since values may be difficult to estimate accurately, and 

the estimated values may significantly outweigh other values and impacts associated with an 

alternative. 

Industry Implementation: This attribute accounts for the projected net economic effect on the 

affected licensees to install or implement mandated costs. Costs should include procedural and 

administrative activities, equipment, labor, and materials. Additional costs above the status quo 

are considered negative; cost savings would be considered positive. This attribute reflects 

actual estimated costs; accident probabilities are not involved. This attribute is measured very 

differently from those associated with accident-related health effects and onsite and offsite 

property. 
 B-3 

Industry Operation: This attribute measures the projected net economic effect from routine and 

recurring activities required by the proposed action. Additional costs above the status quo are 

taken to be negative; cost savings are taken to be positive. Costs falling into this category 

generally occur over long periods of time (facility lifetime) and are sensitive to the discount 

factor used. Additional costs above the status quo are considered negative; cost savings would 

be considered positive. This attribute reflects actual estimated costs; accident probabilities are 

not involved. 

NRC Implementation: This attribute measures the projected net economic effect, on NRC, to 

implement the proposed action. Costs already incurred, including all predecisional activities 

performed by NRC are viewed as sunk costs and are not to be included. Additional costs above 

the status quo are taken to be negative; cost savings are positive. NRC may seek 

compensation (license fees) from the licensees to provide needed services; any compensation 

received should not be subtracted from the cost to NRC, because NRC is the entity consuming 

real resources (labor and capital) to meet its responsibilities. Any fees provided by the licensee 

should be viewed as transfer payments and, as such, are not real costs, from a societal 

perspective. 

NRC Operation: This attribute measures the projected net economic effect on NRC after the 

proposed action is implemented. Additional inspection, evaluation, or enforcement activities are 

examples of these costs. Additional costs above the status quo are taken to be negative; cost 

savings are taken to be positive. As with industry operating costs, NRC operating costs 

generally occur over long periods of time and are sensitive to the assumed discount factor. If 

NRC seeks compensation from the licensee for its services, the compensation should not be 

subtracted. 

Other Government: This attribute measures the net economic effect of the proposed action on 

the Federal government (other than NRC) and State and local governments. Additional costs 

above the status quo are taken to be negative; cost savings are taken as positive. This attribute 

will be affected less than many other attributes, but can be relevant in certain types of actions 

(e.g., changes to offsite emergency planning, provision of offsite services, and new 

requirements affecting Agreement States). The government entities may seek compensation 

from the licensee to provide needed services; any compensation received should not be 

subtracted from the cost to the government units. 

General Public: This attribute accounts for direct, out-of-pocket costs paid by members of the 

public as a result of implementation or operation of the proposed action. Examples could 

include items such as increased cleaning costs because of dust and construction-related 

pollutants, property value losses, or other inconveniences. Increases in costs from the status 

quo are taken to be negative; decreases in costs from the status quo are taken as positive. This 

attribute is not related to offsite property losses from accidents. The general public attribute 

measures real costs that will be paid due to the implementation of the proposed action. These 

costs exclude taxes, as they are transfer payments with no real resource commitment from a 

societal perspective. Any costs that are reimbursed by the applicant or licensee should be 

accounted for here and not duplicated under industry costs. 

Improvements in Knowledge: This attribute accounts for the potential value of new information, 

especially from the assessments of the safety of licensee activities. Some NRC actions have as 

a goal the improvement in the state of knowledge for factors, such as accident probabilities or 
 B-4 

consequences, with an ultimate objective of facilitating safety enhancement or reduction in 

uncertainty. Quantitative measurement of improvements in knowledge depends largely on the 

type of action being investigated. The value of assessments directed at a fairly narrow issue 

(e.g., reducing the failure rate of a component) may be quantifiable in terms of safety or 

monetary equivalent. If this is so, these values and impacts should be treated by the other 

attributes and not included here. If potential values from the assessments are difficult to 

identify, or not quantified elsewhere, they should be evaluated with this attribute. 

Regulatory Efficiency: This attribute measures regulatory and compliance improvements 

resulting from the proposed action. These may include changes in industry reporting 

requirements and NRC’s inspection and review efforts. Achieving consistency with international 

standards groups may also improve regulatory efficiency for both NRC and the industry groups. 

This attribute is qualitative in nature. However, in some instances, changes in regulatory 

efficiency may be quantifiable, in which case the improvements should be accounted for under 

other attributes (NRC implementation or industry operation). Regulatory efficiency actions that 

are not quantifiable should be addressed under this attribute. 

In addition to these attributes, other considerations, including safeguards and security, antitrust, 

and environmental considerations, may need to be evaluated. For safeguards and security, it 

must be determined if the existing level of safeguards is adequate and what effect the proposed 

action has on the level of security. Environmental considerations are typically handled under 

the National Environmental Policy Act requirements. If not, they must be evaluated but handled 

separately from the value-impact analysis. 
 B-5 

Table B-1. Value-Impact Analysis Generic Template

 Attribute Units 

Affected 

(yes/no) 

Change in 

Accident 

Frequency 

Accident 

Frequency 

(per year) 

Expected 

Dose 

(rem) 

Facility 

Life Cycle 

(years) 

Co

($2

Public Health (Accident) $ Public Health (Routine) $ Occupational Health (Accident) $ Occupational Health (Routine) $ Offsite Property $ Onsite Property $ Industry Implementation $ NRC Implementation $ NRC Operation $ Other Government $ General Public $ Improvement in Knowledge Qualitative Regulatory Efficiency Qualitative Antitrust Considerations Qualitative Safeguards and Security Considerations 

 $ 

Environmental Considerations $ Other Considerations $/Qualitative 
 B-6 

Table B-2. Summary of Value-Impact Results Table Template 

Present Value Estimates (

Quantitative Attribute 

Low Best 

Accident 

Public 

Routine 

Accident 

Health 

Occupational 

Routine 

Offsite 

Property 

Onsite 

Implementation 

Industry 

Operation 

Implementation 

NRC 

Operation 

Other Government 

General Public 

NET VALUE (Sum) 
APPENDIX C 

EXAMPLES OF THE APPLICATION 

OF SCREENING CONSIDERATIONS 
 C-1 

APPENDIX C 

EXAMPLES OF THE APPLICATION 

OF SCREENING CONSIDERATIONS 

C.1 Introduction

Three examples are provided below to illustrate how the screening considerations may be 

applied in the materials use and waste arenas. The examples are chosen from three broad 

areas of regulatory activity. The first example is the in situ leach (ISL) approach for uranium 

recovery facilities, which represents a broad program area for regulatory consideration. The 

second example is the License Termination Rule (LTR) for site decommissioning, which 

represents a change to specific regulation and guidance. The third example is the shipment of 

the Trojan Reactor Vessel Package, which represents a specific licensing action. 

The application of the screening considerations is not only useful in making a decision as to 

whether to apply RIDM to a particular activity, but also, how to do so (i.e., what information is 

needed, how it should be organized, and what kinds of issues can, or need to be addressed by 

applying the RIDM approach). 

Note that the screening considerations have been reduced from seven to four as described in 

Chapter 2. The information provided in this Appendix reports on case studies that used the 

original seven screening considerations. 

C.2 In situ Leach Uranium Recovery Facilities 

C.2.1 Background

In the United States, uranium recovery is carried out by conventional uranium mills and by ISL 

facilities. 10 CFR Part 40 applies broadly to all facilities receiving title to, or receiving, 

possessing, using, transferring, or delivering, source and byproduct materials, and it has been 

used for uranium recovery licensing. Criteria for the operation of conventional uranium mills and 

for the disposition of their tailings and waste are given in 10 CFR Part 40, Appendix A. ISLs 

developed subsequent to the issuance of Appendix A, and thus certain aspects of the ISL 

operation, are not covered by this regulation. This includes extraction of uranium from ore beds, 

certain chemical processes leading up to the conversion of uranium to yellowcake, and the 

integrity and restoration of the groundwater. 

In the absence of specific regulatory criteria for ISLs, the NRC imposes specific “license 

conditions” on ISL licensees, to protect public health and safety and the environment. It has 

been recognized that this is not an optimum regulatory framework for ISLs and thus a riskinformed approach has been explored. Accordingly, the NMSS/FSME staff directed its 

contractor to perform a risk assessment that would address the specific aspects of an ISL, as it 

relates to health, safety, and the environment. The following illustrates how the screening 

criteria could have been applied to this broad program area. The discussion below relies 

heavily on the study performed for the Fuel Cycle Licensing Branch by the CNWRA; namely, 

NUREG/CR-6733 (Ref. C.1). 
 C-2 

C.2.2 Application of Screening Considerations

(1) Could a risk-informed regulatory approach help address the Commission’s 

strategic objective in the NRC’s Strategic Plan ? 

Although ISLs are new, relative to the conventional uranium mills, it is generally accepted that a 

comparable level of safety is being achieved. Thus, because this relatively new activity has not 

been tested by the regulations to the same degree as the conventional mills, risk information 

could prove to be useful in assessing the level of safety being achieved by ISLs. Secondly, 

because the framework for the regulation of ISLs is not as complete as for conventional mills, 

risk information could help to provide insights and guidance for improving the regulatory 

framework. Thus, risk information would be helpful in maintaining the level of safety and in 

clarifying the regulatory approach to ensuring that safety is maintained. 

It is evident that aspects of the regulatory framework for ISLs can be streamlined through use of 

risk information. The current approach, using license conditions, is subject to rejection or 

modification through legal challenges and adds substantial uncertainty. Further, there is no 

consistent approach to licensing ISLs with a clear focus on the relevant safety issues. Ensuring 

consistency of requirements across the broad class of ISL licensees is difficult due to the 

widespread use of license conditions that may differ from site to site. 

Because the license condition approach is subject to rejection or modification through legal 

challenge, it could lead to undue economic and operational impacts for the licensee. It would be 

important to have a justifiable basis for any changes to the regulations or policy so that 

regulatory burdens are in proportion to the hazards posed. There is a clear need to focus 

licensing and inspection policies on the areas that have the maximum impact on safety, while 

still maintaining overall safety. 

Furthermore, the license condition approach does not promote consistent decisions. Each case 

is handled on its own merits, and these specific decisions would not necessarily promote a clear 

picture to the public of the level of safety being achieved by the ISL industry. The case-by-case 

process is not transparent and thus does not enhance confidence. Risk information, especially 

a risk assessment performed across the broad program area of ISL regulation, would help to 

communicate important safety information on an integrated and even-handed basis. 

Note: NUREG/CR-6733 provides a program-area-wide perspective for the risks from ISL 

operations and groundwater recovery. As a follow-on, NMSS/FSME is using the risk 

information and insights gained from this study to promulgate appropriate guidance to the staff 

for the regulation of ISLs. This study was performed on a generic basis for ISLs and thus 

provided many of the overarching features of the risk profile for this class of facilities. This can 

lead to regulatory consistency and effectiveness as the insights from this study are incorporated 

in the licensing framework. NUREG/CR-6733 also provides insights as to the relative risk at an 

ISL, including risks posed by both radiological and chemical hazards.
 C-3 

(2) Do data and/or analytical models of sufficient quality exist or could they be 

reasonably developed to support applying a risk-informed approach to a 

regulatory activity?

Before the completion of NUREG/CR-6733, there was limited information, in an integrated 

format, on ISL risks. However, much information was available on a fragmentary basis. For 

example, inspection reports, environmental impact statements, the NMED database (Ref. C.2), 

health physics surveys of uranium recovery facilities, guides to chemical hazards, in situ mining 

studies, and industrial hygiene reports were available to the authors of NUREG/CR-6733. No 

doubt, it took some effort to acquire this information and even more effort to synthesize it 

appropriately to produce the risk assessment. 

Note: The authors of NUREG/CR-6733 remarked that they did not follow a conventional 

approach to performing their risk assessment because “...as the analysis evolved, the authors 

recognized that for screening purposes, the approach should be tailored to the nature of the 

specific materials, activities, and regulatory requirements associated with the uranium ISL 

facilities.” Essentially, they customized their study to the information that was available to them 

and obtained useful information, on a realistic basis, regarding the risks posed by hazards at 

ISL facilities. 

(3) Can startup and implementation of a risk-informed approach be realized at a 

reasonable cost to NRC, an applicant, or licensee, and/or the public, and provide a 

net benefit? 

To the extent that risk information is recognized to be realistic, it should provide information of 

value. For ISL facilities, an integrated perspective for risks and safety of the operations that are 

particular to those facilities was lacking. As discussed above, much of the information in this 

area already existed in fragmentary form. Thus, elaborate and expensive testing and research 

programs were not required. The main cost to NRC was in synthesizing this information into a 

risk assessment. This cost was reasonable, given the fact that an entire class of facilities would 

benefit. 

Note: This type of reasoning probably factored into the decision to carry out the NUREG/CR6733 study. 

(4) Would any other existing factor(s) limit the utility of implementing a risk-informed 

approach? 

In this case, the greatest limitation is probably the ability of the uranium recovery industry to 

financially participate in new risk-informed activities. Clearly, this is a financially stressed 

industry, as is evidenced by its unwillingness to embrace the proposed 10 CFR Part 41 

development purely for financial reasons. Nevertheless, it is cost-effective for NRC to pursue a 

risk-informed approach because of the above-mentioned considerations. 

Note: Although NMSS/FSME followed up on the risk information developed in NUREG/CR-6733 

by developing more generic guidance for licensing and regulatory reviews pertaining to ISLs, a 

new rule specifically for ISL was not pursued because of the resources required. 
 C-4 

C.3 License Termination Rule

C.3.1 Background

Before the issuance of the LTR in 1997 (Ref. C.3), the regulations associated with 

decommissioning facilities licensed by NRC were derived from the Atomic Energy Act of 1954 

and its amendments and pertained to facilities licensed under 10 CFR Parts 30, 40, 50, 60, 61, 

70, and 72. In particular, the approach to decommissioning power reactors, as exemplified by 

Regulatory Guide 1.86, "Termination of Operating Licenses for Nuclear Reactors," June 1974 

(Ref. C.4), was a deterministic approach. The criterion for unrestricted release was acceptable 

surface-contamination levels for a variety of radionuclides including isotopes of uranium, 

thorium, transuranics, Strontium-90, and various other beta-gamma emitters. Limits of the 

average, maximum, and removable levels of these radionuclides, in amounts of disintegrations 

per minute per area (dpm/100 cm2

) were prescribed, and licensees had to demonstrate that all 

contamination at the site was within these limits. As the complexities of specific site evaluations 

began to grow, the need for a more effective and efficient approach to decommissioning was 

given increased consideration. The anticipation of a growing workload in the decommissioning 

area also contributed to the need for a revision to the regulations. 

The LTR approach is based on dose criteria. An acceptable dose level of 25 mrem/yr total 

effective dose equivalent (TEDE) to the average member of the critical group is defined in the 

regulations as the criterion for unrestricted release of a reactor site that is undergoing 

decommissioning. Inasmuch as radiation dose translates directly into a health risk, albeit a 

small additional risk of latent cancer at these low dose levels, the current regulation can be 

thought of as incorporating elements of a risk-informed approach in the decision process related 

to site release. The following discussion demonstrates how application of the screening 

considerations would have led to this evolution had they been applied before the development 

of the LTR. 

C.3.2 Application of Screening Considerations

(1) Could a risk-informed regulatory approach help address one or more of the goals 

in the Commission’s Strategic Plan ? 

Before the LTR, there was a need to ensure that there would be a clear and consistent 

regulatory basis for determining the extent to which lands and structures must be remediated to 

ensure safety before decommissioning a site can be considered complete and the license 

terminated. Here there is a clear need to have a level regulatory playing field across the 

spectrum of sites and facilities to be regulated. 

It was evident that the Commission desired a more efficient and consistent approach to 

licensing actions related to the numerous and frequently complex anticipated site-remediation 

activities. Thus, criteria based on dose would lead to a streamlining of the regulatory process. 

For some sites, an elaborate and detailed assessment of the residual doses might be 

warranted. However, for other sites, the residual contamination may be very low. It would, 

therefore, be useful to have an approach that gives the licensee the option to use a less 

resource-intensive screening method to demonstrate that an acceptable level of safety had 

been achieved. 
 C-5 

 

It is helpful to support a decision or relate information on a situation by providing as broad and 

clear a perspective as possible. Understanding the risks and their trade-offs can only enhance 

the perspective. It is also important to communicate the benefits as well as the risks of a given 

situation or decision. 

Note: The LTR provides a level playing field. The residual-dose criterion is a performancebased approach that defines adequate safety and does not rely on an evaluation of the 

complexities of a facility to achieve compliance. In the case of the Trojan application, the NRC 

review was straightforward. The staff verified that the licensee performed the appropriate 

analysis to support the goal of unrestricted release. However, Trojan is a site with low levels of 

residual contamination from licensed operation and the same may not be the case at other sites 

to be reviewed in the future. A licensee or applicant may want to examine the criteria 

associated with restricted and unrestricted release and evaluate the risks associated with 

alternatives on a site-specific basis, to determine the most effective approach, consistent with 

protecting health, safety, and the environment. In the case of Trojan, the licensee made a 

business-risk decision by opting to use the screening approach in its License Termination Plan, 

because the site would likely meet the criteria for unrestricted site release. The LTR approach 

was developed a few years after the Trojan decommissioning process started. The 

transparency of the LTR approach, which is a risk-informed approach, seemed to contribute to a 

stabilization of the interactions with local stakeholders. 

(2) Do data and/or analytical models of sufficient quality exist or could they be 

reasonably developed to support applying a risk-informed approach to a 

regulatory activity? 

Physical models for dose assessment already exist, and they can express parameter 

uncertainties (referred to in this literature as probabilities). As Ref. C.5 indicates, PA is the 

overall methodology of choice for performing a decommissioning risk assessment. 

Note: Central to the risk-informed approach is the concept of uncertainty. Expressing and 

understanding uncertainties allows the analyst and decision-maker to assess the risks for the 

activity in question. There are models for working with uncertainties that have been useful in 

other contexts, and they should also be useful for assessing the risk related to 

decommissioning. 

(3) Can startup and implementation of a risk-informed approach be realized at a 

reasonable cost to NRC, an applicant, or licensee, and/or the public, and provide a 

net benefit? 

Several models and tools are available and are being refined in the area of dose assessment. 

There is strong interest in this area, for decommissioning, including international efforts. Much 

of the investment has been made already. It is now a matter of synthesizing this information 

into dose (risk) assessments. 

Note: Additional refinements of the risk-informed approach may occur for site 

decommissioning. However, it is not necessary to totally revamp the overall process at once. 

An evolutionary process is more appropriate and likely to be warranted. 
 C-6 

(4) Would any other existing factor(s) limit the utility of implementing a risk-informed 

approach? 

There are no factors that precluded the development of the LTR. On the contrary, it appeared 

to be an idea whose time had come (on the path to better regulation). 

C.4 Trojan Reactor Vessel Package

C.4.1 Background

The Trojan Nuclear Plant in Rainier, Oregon, was permanently shut down in 1993 after 

approximately 17 years of service. As part of the decommissioning process, Portland General 

Electric (PGE) removed offsite and disposed of the large components (four steam generators 

and pressurizer) in 1995. Because of the size (1000 tons) and radiological content (2 million 

curies) of the reactor vessel, several options were considered: 

(1) Ship the reactor vessel, with its internals intact, by barge, up the Columbia River, and 

transport it over land to the US Ecology site in Richland, Washington; 

(2) Store the reactor vessel at the original site of the reactor; 

(3) Dispose of the reactor vessel in one piece, with only those internals left inside that were 

Class C radioactive waste or less; and 

(4) Dispose of the reactor vessel and internals separately. 

The application submitted to NRC by the licensee, PGE, proposed to transport the vessel by 

Option 1 and a detailed plan was developed to implement it. The plan included removal of the 

reactor vessel from its containment, preparation of the vessel with shielding and impact limiters, 

a special-purpose transporter and barge, and transport routing and timing that reduced specific 

risks. The transport process was successful. The following discussion demonstrates how 

application of the screening considerations would have led to the conclusion that a RIDM 

approach should have been applied to the Trojan Reactor Vessel Package (TRVP) activity. 

C.4.2 Application of Screening Considerations

(1) Could a risk-informed regulatory approach help address one or more of the goals 

in the Commission’s Strategic Plan ? 

Option 1 could not meet the regulation in 10 CFR Part 71 that requires transport packages to be 

capable of surviving a 1 meter (3-foot) drop and transport packages to be tested by a 0.3 meter 

(one-foot) drop onto a flat, unyielding surface. The safety question was: Was an alternative 

transport equivalent in safety to a transport conforming to the specific regulatory requirement? 

PGE devised a special mode of package protection and transport by barge. It submitted a 

probabilistic safety study for this special transportation by barge, and a probabilistic safety 

evaluation for the overland shipment. A risk analysis for external events was also included. 

PGE concluded that it could conduct the transport in a manner that was safer than transport 

conforming to the prescriptive requirements. The NRC staff reviewed the risk assessments and 

agreed that the risk was low enough. 
 C-7 

This review could be protracted by such considerations as determining the consequences of 

ramming or sinking, or whether hypothetical one meter (3.3 feet) drops could occur. Resolving 

such technical issues might have been inefficient and ineffective in reaching a timely resolution. 

A risk-informed approach to evaluating alternatives would be worthy of exploration. 

Currently, a proposal for the revision of 10 CFR Part 71 has been submitted. One of the issues 

identified in the revision is whether NRC should propose Part 71 amendments to provide a 

standard for reviewing large-object packages, such as the Trojan Reactor Vessel, versus the 

current exemption process. As stated in SECY-01-0035, “...The proposed action would result in 

enhanced regulatory efficiency by standardizing the requirements to provide greater regulatory 

certainty and clarity, and would ensure consistent treatment among licensees requesting 

authorization for special packages” (Ref. C.6). The current regulations, under Part 71, require 

an exemption for large-object packages and require Commission approval. The licensee would 

view the regulatory process as a challenge. In addition, the alternatives to shipment of the 

vessel, intact, would appear to be substantially more costly, as well as cause higher 

occupational doses to the workers. 

In conclusion, the risk assessment of the alternatives, barge transport versus truck transport, led 

to an outcome that not only used a risk-informed approach but considerably reduced the 

regulatory burden on the licensee and the ratepayers in the area. 

A risk-informed approach would provide a clear and understandable technical basis for 

demonstrating that the alternative mode of transport was equally safe. NRC and the licensee 

could conduct public meetings that would address safety and regulatory concerns involved with 

the shipment, in which the risk information would show that a wide spectrum of accidents had 

been considered and accommodated by the design. 

In performing the risk assessment, PGE self-identified an elevated risk and made a design 

change to lower the risk. Risk information benefited communications: the stakeholders in the 

area readily understood the notion of the relative risk of “one barge vs. 44 trucks.” Regulations 

often prescribe just one way to provide and demonstrate safety, but risk analysis can open the 

path to many more options, with equivalent or better safety. 

(2) Do data and/or analytical models of sufficient quality exist or could they be 

reasonably developed to support applying a risk-informed approach to a 

regulatory activity? 

The identification of potential accidents would use standard techniques. Risk methods can 

readily be applied to the problem posed and the options considered. Statistical data on the 

frequency of various maritime and inland waterway accidents are available from government 

sources. 

(3) Can startup and implementation of a risk-informed approach be realized at a 

reasonable cost to NRC, an applicant or licensee, and/or the public, and provide a 

net benefit? 
 C-8 

There could be a large potential for savings in this case. These reductions in burden on the 

applicant and NRC staff would offset the added cost of the risk assessment and associated 

activities. Potential net benefits are identified below by stakeholder: 

Licensee: The shipment of the entire reactor, with its internals, would provide a benefit over the 

alternative recommendations of indefinite onsite storage and/or segmentation of the reactor 

vessel package. With the latter, portions of the internals with the highest activation levels would 

no longer qualify for disposal under 10 CFR Part 61 regulations. It would increase occupational 

radiation exposure by at least 100% (i.e., 134 to 154 person-rem vs. 67 person-rem). It would 

result in approximately 44 individual cask shipments to the US Ecology facility. It would 

increase radiation exposure to the transportation workers by at least 1000% (i.e., 1.06 to 1.19 

person-rem vs. 0.09 person-rem). By shipping the reactor vessel as a whole, with internals, the 

decommissioning schedule would be shortened. 

Public: The alternative of segmenting the reactor vessel into 44 individual cask shipments by 

truck would increase the public exposure to radiation by at least 2300% (i.e., from 0.48 to 0.56 

person-rem vs. 0.02 person-rem), although the levels still would have been low. From the 

modal safety perspective, 44 truck shipments would have had greater risk of a transportation 

accident than the single barge shipment. 

NRC: NRC staff benefits from a relatively clear safety argument for the exemption. NRC also 

recognized, as a result of this activity, that the regulatory framework may be altered to improve 

efficiency and effectiveness. 

Note: The TRVP transport study brought the benefit-cost of using a risk-informed approach into 

sharp focus. The risk assessment showed that the alternative action proposed by the licensee, 

and approved ultimately by NRC, resulted in a significant cost savings. However, this was an 

unusual outcome, as both risk and cost were decreased by implementing the proposed 

alternative. It is more usual that a certain amount of risk is decreased at an increased cost, or 

vice versa. 

(4) Would any other factor(s) limit the utility of implementing a risk-informed 

approach? 

There are no precluding factors in this case. Close stakeholder participation can help to assure 

that there is public acceptance for a course of action. 

Note: The exemption was granted and the shipment was made in accordance with all applicable 

regulatory rules and regulations. According to the response at the public meetings and local 

newspaper articles, there was no adverse stakeholder opposition to the risk-informed approach 

taken. 
 C-9 

C.5 References

C.1 U.S. Nuclear Regulatory Commission, “A Baseline Risk-Informed Performance-Based 

Approach for In situ Leach Uranium Extraction Licensees,” NUREG/CR-6733, June 

2001. 

C.2 U.S. Nuclear Regulatory Commission, “The Nuclear Material Events Database,” 

http://nmed.inel.gov. 

C.3 U.S. Nuclear Regulatory Commission, “Radiological Criteria for License Termination 

[Final Rule),” Federal Register, 62(139), 39058-39092. 

C.4 U.S. Nuclear Regulatory Commission, "Termination of Operating Licenses for Nuclear 

Reactors," Regulatory Guide 1.86, June 1974. 

C.5 Eisenberg, N. A., et. al., “Development of a Performance Assessment Capability in 

Waste Management Programs of the USNRC,” Risk Analysis, Vol. 19, pp. 847-875, 

1999. 

C.6 U.S. Nuclear Regulatory Commission, “Proposed Rule for Revising 10 CFR Part 71 for 

Compatibility with IAEA Transportation Safety Standards (TS-R-1), and for Making Other 

NRC-Initiated Changes,” SECY-01-0035, March 2, 2001.
APPENDIX D 

SUPPLEMENTAL INFORMATION 

ON PERFORMING RISK ASSESSMENTS 

FOR NMSS/FSME-REGULATED FACILITIES AND APPLICATIONS
 D-1 

APPENDIX D 

SUPPLEMENTAL INFORMATION 

ON PERFORMING RISK ASSESSMENTS 

FOR NMSS/FSME-REGULATED FACILITIES AND APPLICATIONS

Chapter 3 provided an overview of the four main risk assessment methods that can be used to 

assess the risk associated with NMSS/FSME -licensed facilities and applications. This 

Appendix describes some of the common features inherent to these methods.

D.1 The Project Team

One of the most important factors in ensuring a successful risk analysis is the knowledge and 

experience of the team. Although the exact makeup of the team will be a function of the 

assessment method chosen and the facility or process to be analyzed, there are some general 

guidelines that should be followed. Regardless of the assessment method, facility and process 

experts are needed, since they have the practical understanding of the process (or facility) and 

how it operates and the problems that may be expected. Other required subject matter 

specialties may include: human factors, radiation protection, physical and chemical phenomena, 

environmental protection, and statistics. The latter category is essential in developing reliability 

estimates and understanding the quantitative results from PRAs. 

D.2 Facility/Process Hazard Identification and Screening

This early stage of the risk assessment is a scoping task that consists of defining the objective 

and outputs needed, then identifying those hazards and types of processes that must be 

assessed in order to produce that output. Hazards are those materials, objects, or 

characteristics of the facility that can do harm. By processes is meant those types of chronic 

conditions or episodic events by which hazards can result in harm. For example, for the hazard 

of radioactive materials, there are processes of chronic intake or episodic releases. In this 

scoping task, there are competing goals of technical adequacy and efficiency. Technical 

adequacy is the need to identify a set of hazards and processes with sufficient completeness so 

that the resultant output of the assessment can meet its purposes. Efficiency involves 

screening out some of these hazards and processes, while still obtaining a technically adequate 

result. For example, if the output of the assessment is to be a risk value that is to be compared 

to a criterion value, hazards and processes whose total contribution to that risk value can be 

shown to be negligible relative to the criterion may be screened out. This type of screening 

inherently involves joint consideration of both consequences and frequency. By using simple 

bounding assessments, certain hazards and processes can be screened out. 

The Preclosure Risk Assessment Methodology (PRAM) (Ref. D.1) was used as a foundation for 

the discussion on hazard identification and screening for NMSS/FSME -regulated facilities. This 

methodology described one way to identify and analyze accident scenarios that contributed to 

the radiological risk associated with the preclosure operation of an HLW geologic repository. 

However, the approach is sufficiently generic that it can be extended to the broad spectrum of 

materials and waste applications. 
 D-2 

Once all the credible initiating events have been identified, a screening process is used to 

evaluate the magnitude and nature of the most severe challenges posed by these events. This 

determination can be made by first assessing the frequency for each event. For NMSS/FSMEregulated facilities, an actual frequency cutoff level should be developed. If used, only events 

occurring more frequently than this cutoff will need to be considered further. For events 

occurring with a higher frequency, a boundary hazard assessment is performed. The specific 

hazards, in terms of radiation dose or chemical consequences to the population at significant 

risk needs to be identified and assessed for each application. Certain applications, such as 

medical equipment or gauges, may not need a chemical consequences analysis. 

If the risks result in no public, occupational, or environmental releases that exceed risk 

guidelines, a conclusion can be reached without the expenditure of significant additional 

resources on the analysis. If, however, the risks are significant, additional evaluation is needed 

as to the actions, if any, required. 

The first step, as discussed in the previous section, is to develop a detailed and structured 

description of the facility or process. This should include information on processes, operation, 

and any engineered safety features designed to mitigate or prevent off-normal occurrences. As 

discussed in previous sections, all hazards, including nonradiological, should be considered. 

Chemical accidents, although not regulated by NRC, may lead to radiological consequences or 

the inability of the operator to perform his/her duties. Based on this information, all credible 

initiating events for the facility or process are identified. The events considered may come from 

several sources, including an expert panel or a review of available failure and reliability 

databases (e.g., NMED). The latter would highlight historical initiating events for similar facility 

groups. The exact method employed to identify the initiating events, as discussed in the 

previous section, would be dependent on the facility group being analyzed. Generally, for 

NMSS/FSME licensees, the primary concern would be events that would pose a radiological 

risk to the workers, the public, or the environment. Based on this, an initiating event would be 

defined as an event that threatens the integrity of the isolation of the sources of radiation. An 

initiating event is a deviation from the normal processes, which must be controlled to prevent a 

direct challenge to the barriers designed to isolate the radioactive materials. An initiator may or 

may not lead to an off-normal event, depending on the response of the various safety and 

protection systems available. In addition to initiating events, a radiological risk evaluation must 

be performed, accounting for occupational, public, and environmental risks. It is also important 

to consider external events as initiators. Events such as seismic occurrences, floods, and high 

winds may be initiators, or at the very least, may affect the magnitude of the off-normal events. 

The consequence assessment can take many forms, to determine the exact effect on the facility 

or process. As discussed in the previous section, qualitative and quantitative tools are 

available. Regardless of the method chosen, there are several key outputs that should result 

from such an analysis. Once the consequences are identified and analyzed, including specific 

failures or degradations of any safety systems, the facility can determine what steps must be 

taken to decrease the probability of occurrence. For many of the NMSS/FSME -regulated 

facilities, errors by facility personnel are expected to be the leading cause. In these instances, 

corrective steps may take the form of increased training requirements, revised procedures or 

processes, or facility modifications. In other groups, such as the long-term storage of 

radiological waste (both LLW and HLW), environmental consequences may be anticipated 

because of barrier degradation over a long period of time. For these facilities, improved or 

additional barriers and monitoring systems may be identified. 
 D-3 

If risks can be shown to be bounded by characteristic values, it would suggest that a higher 

annual frequency cutoff might be acceptable. However, care should be taken to ensure that a 

regulatory threshold is not violated. 

D.3 Delineation of Accident Sequences

This section describes the tasks associated with the delineation of an accident sequence (also 

referred to as a scenario). These tasks include: (1) identification of accident-initiating events 

and their screening; (2) construction of potential accident/event sequences that could result in 

unacceptable consequences; (3) identification and description of controls (i.e., structures, 

systems, equipment, and components, as well as procedures for normal and off-normal 

operating conditions) that are relied on to limit or prevent potential accidents or mitigate their 

consequences; and (4) identification of measures taken to ensure the availability and reliability 

of identified safety systems. Recognizing the different uses of nuclear material and methods of 

disposal, and the varying characteristics of the hazards associated with each activity, the extent 

to which the above tasks are performed depends on the particular hazard targeted in the risk 

assessment. This section attempts to provide some general guidance for the four categories of 

the NMSS/FSME -regulated uses of nuclear material and nuclear waste disposal, as grouped in 

SECY-99-100. 

D.3.1 Identification of Initiating Events 

The first step in developing a risk model is to define a set of initiating events (IEs). An IE is 

defined as a physical event or a human error whose occurrence is followed by a sequence of 

events that may result in either a successful state or a failed state of systems and operator 

actions leading to an undesirable or unacceptable consequence. The IE identification requires 

a systematic search across the complete spectrum of activities (both internal and external to the 

sequence being analyzed for risk) associated with the process for possible energized sources 

that could lead to unacceptable consequences. For NRC-licensed nuclear material and waste 

disposal, the unacceptable consequences involve the exposure of workers or members of the 

public to excessive levels of radiation and hazardous concentrations of certain chemicals, 

and/or to an adverse impact on the environment. The purpose of identifying IEs is to answer 

the question “What can go wrong?” in the operation of a facility or a process. The IEs can be 

categorized as process upsets, management system failures, human errors, and external 

events (e.g., high winds, floods, and earthquakes). 

There is a body of published literature available that provides techniques to identify IEs for the 

four categories of the NMSS/FSME -regulated uses of nuclear material and waste disposal. For 

Group 1, which includes HLW and LLW disposal, decommissioning, and mill tailings 

reclamation, Ref. D.2 provides the technical basis for the PA technique. Detailed guidance for 

accident scenario analysis is also provided in the paper. To complement the PA, the PRAM 

document (Ref. D.1) can be a useful tool in providing guidance for selecting and screening the 

IEs. However, the total risk of low frequency/high consequence events should be considered 

before screening them out from further analysis. 
 D-4 

For Group 2, the regulated activities involve the use of engineered casks to isolate nuclear 

material under various normal and off-normal conditions. For spent fuel transportation, 

NUREG/CR-6672 (Ref. D.3) estimates the risks associated with spent fuel shipment via either 

rail or truck. Risks were estimated for both incident-free transport and transportation accidents. 

The latter was further divided into severe accidents that result in the release of radioactive 

materials from the cask to the environment and less severe accidents that cause the cask 

shielding to be degraded but result in no release of radioactive material. The initiating events 

for transportation accident scenarios were mainly associated with collision accidents that could 

result in the failure of the containment seals and lead to the release of radioactive material. 

These accident scenarios were identified by reviewing past studies of transportation accidents, 

in particular the Modal Study (Ref. D.4). Other available databases, such as Department of 

Agriculture (Ref. D.5) and Geographic Information System (Ref. D.6) data were used to quantify 

the frequency of accident occurrence. 

A pilot PRA (Ref. D.7) was recently performed by RES to quantify the risk associated with dry 

cask storage at a specific reactor site. The three aspects of the dry cask storage operation 

examined included handling, transfer, and storage. Event trees were established and IE 

frequencies were quantified. This pilot PRA provides a road map for performing a dry cask 

PRA, including identification of initiating events. Several external events were also examined, 

such as aircraft crashes, tornados, flooding, and earthquakes. 

Group 3 of the NMSS/FSME -regulated activities encompasses primarily the nuclear fuel cycle 

facilities and their radiological and chemical hazards. The chemical hazards include hazardous 

releases resulting from the processing of licensed nuclear material or the potential for chemical 

hazards to adversely affect radiological safety. One such example is UF6 vaporization in the 

fuel fabrication process. For this group of facilities, with multiple sources of hazards, NRC 

requires an ISA analysis for systematic identification of risks and their quantification. NUREG1513 (Ref. D.8) provides guidance for performing an ISA; it also provides an example of ISA 

analysis for a fuel-conversion process. The AIChE (Ref. D.9) is another useful document for 

information on ISA analysis. Table 1.3 of this reference provides a list of possible IEs, 

propagating events, risk-reduction factors (controls), and incident outcomes. 

Group 4 of the NMSS/FSME -regulated activities involves the use of either sealed or unsealed 

byproduct material in industrial and medical applications, such as irradiators and radiography. 

Since many Group 4 applications involve devices that do not require sophisticated hardware 

systems and components to operate, their risk assessments do not warrant the type of PRA 

analysis that is most appropriate for understanding a sophisticated process involving complete 

system configurations. The radiological risk associated with these activities can be assessed 

using the BHA methods described in NUREG/CR-6642 (Ref. D.10). The BHA assesses both 

the source hazard and the barriers that confine it. The barriers or controls are considered either 

as physical hardware or as administrative controls, such as procedures, instructions, and 

enforcement measures. The BHA identifies deviations from normal operations that could lead 

to worker or public radiation exposures, along with possible causes, effects, and preventive and 

mitigative barriers/controls. The question “What can go wrong?” is still valid for identifying the 

initiating events, such as breach of confinement from a spill, rupture, or mechanical damage; 

loss of physical control (e.g., a source in an unintended location outside the barrier); and failure 

of administrative controls. 
 D-5 

In addition to the discussions above for the four categories, a recent IAEA report (Ref. D.11) can 

also be used for the selection and grouping of initiating events. Appendix III of the IAEA report 

provides a list of typical initiating events for nonreactor nuclear facilities. 

D.3.2 Construction of Potential Accident Event Sequences

Regardless what method is chosen for a risk assessment, the next step after IE identification is 

to construct potential accident sequences. An accident sequence is defined as a specific 

unplanned sequence of events that could result in an undesirable consequence. Therefore, an 

important product of the risk assessment is a description of all accident sequences identified 

and recorded during the analysis process. To identify potential accident sequences, either an 

inductive or deductive approach can be applied, each having its own advantages. As noted in 

Ref. D.8, the inductive (bottom-up) approach attempts to identify possible accident sequences 

by examining deviations from normal operating conditions (e.g., event trees), whereas the 

deductive approach identifies a top event (usually a severe consequence), and attempts to 

explain the various ways that the top event can occur (fault tree). Generally, the inductive 

approaches are useful in identifying a broad range of potential accidents, whereas the deductive 

approaches provide a deeper understanding of the mechanism by which a particular accident 

might occur. 

The PRA methodology includes logic models, data, physical models, and health and economicloss models. The logic model identifies those events that are undesirable in the risk arena. 

Two types of logic trees, an event tree and a fault tree, are used in logic models. Both trees 

have their foundation in Boolean algebra. The event-tree approach is an inductive method that 

moves forward in time to delineate events generally through a two-level, logic-type tree: yes/no, 

fail/success. The fault-tree approach starts with an undesired event (e.g., environmental 

release) and moves backward in time to determine what led to the event. The PRA method 

combines both approaches to allow a simultaneous movement forward and backward. The 

principal advantage of this is that it reduces the number of event combinations that would be 

present if only one type of approach were used. 

Figures D.1 and D.2 provide simplified examples of an event tree and a fault tree, respectively. 

As shown in Figure D.1, the event tree begins with an initiating event with two separate systems 

designed to prevent the IE from occurring. Given that the IE occurs, were Systems A and B 

found in a success state or failure state, and what probabilities are assigned to these branches? 

In actual situations, this model would be much larger and more involved. If there were N 

systems, there would be 2N possible states at the end. When only System A or B is sufficient to 

perform the overall safety function for the IE, then an event where A is successful, but B fails, 

will be a safe event. Similarly, the complementary situation (A fails and B is successful) will not 

be hazardous. As shown in the figure, three events present no hazard, but the failure of both 

systems will lead to a hypothetical environmental release. The fault-tree models are used when 

System A comprises subsystems. Going backwards in time using the “and” and “or” gates in 

the basic logic models, and using Boolean algebra, if both A1 and A2 fail, the overall system 

fails. Again, actual fault trees are larger and more complex. The eventual fault trees are put 

together and finally quantified through probabilistic expressions. 
 D-6 

The development of potential accident sequences requires both a clear understanding of the 

systems and operating procedures for the subject facility or process, and, as a prerequisite, a 

team of qualified analysts with training to perform risk analyses. Depending on the facility or 

process being analyzed, accident sequences could be a full-blown event tree if a PRA or ISA 

were required (for a process operated with many systems and components), or a simple event 

tree for a sealed device, using a BHA. 

Regardless of the modeling devices chosen, the goal of this task is to develop a logical 

representation linking the IEs to the corresponding possible end states, with potential 

consequences. This is accomplished by constructing an event-sequence model that represents 

the possible combination of safety function and operator responses after the occurrence of each 

of the initiator groups. An event-sequence model, therefore, describes the sequence of events 

that, after an IE, leads either to a successful state or to a failed state of systems and operator 

actions intended to mitigate the consequences. 

System failures are logical combinations of simpler events (e.g., component failures). It may be 

convenient to represent the system response as multiple discrete states, rather than simply 

binary (success or failure). Also, it may be convenient to include, along with the response of 

systems and facility operators, phenomenological questions, or "events," in the event-sequence 

model. An example of the latter might be asking whether an explosive amount of hydrogen 

could build up in an enclosed space during a certain critical time. The probabilistic answer to 

such a question might be a function of the uncertainties in the hydrogen generation or release 

process and depend on the outcome of a previously queried element of the scenario. 

As discussed in Chapter 3 and this Appendix, the reference documents for the four groups of 

the NMSS/FSME -regulated activities can also be used to guide the development of the 

potential accident sequences. Further, the methods that are popular in the chemical industry, 

such as HAZOP (Refs. D.12–D.14) may also be employed. As noted in Ref. D.15, tools that 

can be used include: (1) cause-consequence diagrams; (2) event-sequence diagrams (ESDs); 

(3) state-space diagrams and Markov analysis; (4) block diagrams; (5) GO charts; and (6) 

general mathematical simulations of physical systems (e.g., Monte Carlo).
 D-7 

Figure D.1 A Simplified Event Tree 

SYSTEM-A-FAILS

TRAIN-A1

0.0E+0

A11

0.0E+0

A12

TRAIN-A2

0.0E+0

A21

0.0E+0

A22

system A Fails

(/A)

system A Fails

(/A)

/A

Component A11 Component A12 Component A21 Component A22

 SIMPLIFIED-FAULT-TREE - Fig 3.4 2004/08/26 Page 8

Figure D.2 A Simplified Fault Tree 

Success

Failure 

A B = OK

A /B = OK

/A B = OK

/A /B = Environmental 

Release

Initiating Event System A System B
 D-8 

D.3.3 Identification and Description of Safety Measures, Controls, and 

Barriers

Once the IEs and accident sequences are established, the safety measures and controls should 

be identified and described and their reliability should be assessed. The safety measures and 

safety features are systems or procedures that need to function correctly to prevent or mitigate 

the occurrence of the accidents that are postulated for each IE. The depth of analysis of safety 

measures and controls varies, depending on the process and the associated hazard to be 

addressed. For instance, a medical device may require safety measures pertaining to access 

control, confinement, and shielding of the source, which can be characterized mainly in terms of 

procedural controls, rather than in terms of hardware, whereas in a fuel fabrication facility, 

safety measures may consist of mechanical and electrical systems, emergency power systems, 

etc., which involve both hardware and procedural safety measures. 

Factors to consider in describing the safety measures include whether they have the 

appropriate sensitivity to detect the need for action after specific IEs, and whether they have a 

sufficiently rapid response time so that the fault condition arising from the initiator can be 

corrected before the accident occurs. 

It should also be established that the safety measures are capable of functioning correctly under 

the conditions created by the scenario. This consideration includes the conditions brought 

about directly as a consequence of the initiator, as well as operational limitations that are a 

result of additional potential degraded conditions (such as from the independent loss of 

environmental control, leading to high ambient temperature and humidity). 

Clearly, any safety systems that would fail because of the initiator should be identified, and any 

safety systems so affected should not be claimed in the safety assessment. Likewise, limiting 

conditions for safety measure effectiveness that might be encountered subsequent to an initiator 

should be described. 

In choosing the safety measures needed to protect against the occurrence of a particular event 

sequence, both the number and the effectiveness of such measures should be addressed. 

Similarly, for administrative controls, training measures and audit/inspection measures should 

be tailored to ensure the specific reliability needed for each control. For example, if the facility is 

relying on a single individual on duty at a particular time to take action (i.e., close a valve or turn 

a switch) to avoid a major accident, that person should receive periodic training and his or her 

performance should be monitored. Another example could be a medical application involving 

the use of a device with a sealed source. If the device needs to be confined within a set 

boundary (e.g., operation room), a control measure should be in place to effectively prevent the 

practitioner from taking the device outside the set boundary. 

The relevant information should be obtained from the design specifications for the safety 

measures, and, for existing facilities, from testing carried out during commissioning and/or as 

part of a regular examination, inspection, maintenance, and testing program. These bases 

should be documented in the risk assessment. 

In some cases, it may be necessary to consider specific development and testing programs (for 

non-standard instrumentation, for example). If expert judgment and knowledge are used, it is 

important to document the bases for the judgment and knowledge. 
 D-9 

D.3.4 External Events

The events that are initiated external to the process or facility under consideration are generally 

grouped in the category usually referred to as external events. The typical external events that 

could require analysis include seismic events, fires, and high wind. Floods and other external 

IEs involving accidents related to transportation and events at nearby facilities should also be 

addressed. 

The extent to which the external IEs should be addressed varies, depending on the nature and 

characteristics of the materials being processed, and systems and components required to 

mitigate hazards associated with the operation of a particular facility/process. Unlike the NPP, 

which has a concentrated source of radiation hazard, the NMSS/FSME -regulated activities 

encompass a wide variety of sources of hazards that employ diverse types of safety measures 

and barriers. Therefore, for NMSS/FSME facilities, the external events should be addressed on 

a case-by-case basis for accident prevention and consequence mitigation. For example, 

seismic events should be addressed for facilities that rely on hardware for production operations 

and safety measures and controls, such as fuel cycle facilities; however, seismic events may 

not have much impact on the operation of certain medical applications with sealed sources. On 

the other hand, waste repositories rely on manmade and natural barriers for safety. For these 

facilities, the natural phenomena hazards, such as seismic and weather-related events (high 

winds and floods), can degrade the repository barriers and should be considered and evaluated 

for decisions involving both site selection and waste emplacement. 

Screening for credible external events for a particular NMSS/FSME -regulated facility or process 

is an important step in determining what external events should be included in a risk model. For 

those events with very low-occurrence frequencies, a simplified consequence analysis may aid 

the decision to screen a particular event in or out. Those events being screened in would enter 

into a risk model for further analysis, whereas the events being screened out because of low 

consequences would not require further treatment. In either case, the basis for the decision 

should be documented. 

Once an event is screened in, potential accident sequences initiated by that event need to be 

identified and quantified. The process of sequence identification is much more complicated for 

external events, especially for seismic events and fires. The risk analysis methodology for 

seismic events is much more involved, because of the nature of earthquakes and the groundshaking effects across the board on the structures, systems, and components (SSCs) 

associated with a facility or a process. A typical analysis for the seismic risk includes a seismic 

hazard analysis, an initial SSC screen, a seismic-initiated accident sequence analysis, a 

component fragility analysis, a sequence quantification, and a consequence analysis. Many 

guidance materials (Refs. D.16–D.20) for a seismic PRA have been published over the years, 

though mostly directed at NPP facilities. However, the basic elements of these guidance 

documents should be equally applicable to a seismic-risk analysis for an NMSS/FSME -

regulated facility. The task-by-task procedure for a risk analysis of fires includes a fire-hazard 

analysis, a fire-propagation analysis, a facility and systems analysis, and a release frequency 

analysis. Two methodologies, FIVE (Ref. D.21) and FPRAIG (Ref. D.22), were developed by 

the Electric Power Research Institute (EPRI) in response to Supplement 4 to GL 88-20 

(Ref. D.23), which called for all commercial NPPs in the U.S. to perform an individual plant 

examination of external events (IPEEE). FIVE and FPRAIG were the tools provided to NPP 
 D-10 

licensees to perform a systematic examination of fires risk. A similar approach can also apply to 

NMSS/FSME -regulated facilities. 

D.4 Facility or Process Systems Analysis

This section discusses the aspects of the risk assessment that require formal consideration 

and/or detailed analysis in the system evaluation of a facility or process. These include system 

and process dependencies and a human performance evaluation. 

D.4.1 System and Process Dependencies and Interactions

As noted in Ref. D.11, dependent failures can be dominant contributors to the frequency of 

undesirable end-states and they should be taken into account in the analysis, regardless of the 

modeling approach selected. In cases where event trees are not used for event-sequence 

modeling, attention must be paid to the proper handling of the dependencies that appear in the 

fault trees and to ensuring that they are identified and their logic interrelations are modeled 

correctly. 

The different types of dependencies that can occur include the following: 

• Functional and physical dependencies; 

• Human interaction dependencies; and 

• Component failure dependencies. 

Functional dependencies between safety systems, measures, or components can arise when 

the function of one system or group of components depends on the function of another system 

or component. These can arise from a number of causes, including the following: 

• Shared component; 

• Common actuation systems; 

• Common isolation requirements; and 

• Common support systems (e.g., power, cooling, indication and control, and ventilation). 

Functional and physical dependencies share many commonalities. Functional dependencies 

include physical interaction between systems, measures, or components that can occur when 

the loss of function of a system or component causes a physical change in the environment of 

another system or component (e.g. where loss of trace heating on a piece of pipe allows it to 

freeze in cold weather). 

Physical dependencies can arise in two ways. First, an IE can cause the failure of a safety 

system, measure, or component, as well as the failure of some of the safety systems or 

components required to provide protection. Secondly, an internal hazard (such as a fire or a 

flood) or an external hazard (such as extreme environmental conditions or a seismic event) can 

cause an initiating event and failure of some of the safety systems, measures, or components 

required to provide protection. 
 D-11 

It is important to analyze the interaction between the physical process progression and the 

performance of the required system or components. To correctly incorporate the effects of 

physical processes on the accident sequences, the operability of the required systems must be 

assessed; i.e., the effect of accidental environmental conditions on the engineered safety 

features and their support systems must be analyzed in detail. 

One example of how physical processes may influence the progression of events is the loss of 

the heating, ventilation, and air conditioning (HVAC) system. Increasing temperature and 

humidity may affect the functioning of mechanical or electrical equipment, the ability of 

operators to take appropriate action, and the quality of information provided to the operators. 

The equipment within the facility for enabling the operator to perform his tasks is a strong 

influencing factor. This is particularly true of computer systems. As mentioned, a modern 

facility may contain more or less sophisticated computer-based operator support systems that 

monitor the facility performance and display information to the human operator via a manmachine interface (graphic display). The functionality of such systems is determined by the 

design goals specified during their development. 

The ability and appropriateness of such systems to correctly inform the human operator of the 

different facility states need to be investigated and included in the facility-modeling process. For 

example, an IE may incapacitate some monitoring functions (the IE may change the 

temperature of the environment in which a sensor is situated, causing it to become unreliable). 

As a result, the ability of the operating staff to correctly interpret the information displayed is 

reduced and this should be reflected in the facility model. 

Human interaction dependencies arise when the operators make errors during the repair, 

maintenance, testing, or calibration tasks that lead to the unavailability or failure of safety 

systems, measures, or components after an IE. Human interaction dependencies can also 

arise during the “post-accident” phase, when manual actions require the operator to interact with 

multiple components. 

Component failure dependencies cover those failures of identical components that are 

otherwise not analyzed. Such failures may be caused by errors in design, manufacturing, 

installation, calibration, or operational deficiencies and are treated quantitatively by commoncause failure methods or other dependence-quantification approaches. Common-cause failure 

probabilities are usually quantified by using the alpha-factor approach, the beta-factor approach, 

the Multiple Greek Letter approach, or the binomial failure-rate model to assess the probabilities 

of common-cause failures on similar (redundant) components.

D.4.2 Human Performance Analysis

This section highlights the human performance that relates to the IEs and subsequent system 

responses. Human actions include all those identified, during the course of risk-model 

development as having potential impacts on the accident sequence structure and results of the 

model quantification. Human performance analysis usually considers only errors of omission, 

although some recent developments have been published providing guidance on how errors of 

commission can be evaluated and modeled. 
 D-12 

As discussed in Ref. D.11, the evaluation of human performance depends on the complexity 

and the degree of automation of the facility or process. Since humans must perform many 

functions and tasks, accurate evaluation of human performance is crucial for the assessment of 

risks and their consequences. The depth of human performance analysis is driven by the scope 

and objectives of a particular risk assessment and influences the selection of analysis 

methodology. This chapter divides human-performance analysis into two broad categories: 

qualitative or quantitative. 

D.4.2.1 Qualitative Human-Reliability Analysis

A qualitative human-reliability analysis (HRA) is necessary to identify those possible operator 

actions that, if not properly performed, will have an adverse impact on the development of the 

accident. 

HRA generally involves the evaluation of tasks within a procedure or sequence, while taking into 

account such factors as the complexity of the task, the conditions under which it is performed, 

and the mental and physical characteristics and limitations of the operator. 

HRA task analysis includes: task decomposition, hierarchical task analysis, time line analysis, 

task simulation, and ergonomics checklists. Each technique has particular applications, 

limitations, advantages, and disadvantages. The safety assessor must decide, possibly in 

consultation with a human-reliability specialist, which techniques should be applied and to what 

extent. 

D.4.2.2 Quantitative HRA

Where the depth of analysis is such that potential human errors are represented in safetyassessment accident sequences (e.g., fault trees or event trees), quantification of human-error 

probabilities through quantitative HRA is required. 

As discussed in Ref. D.11, probability values may be assigned to human errors, using one or 

more of the following information sources or techniques. In all cases, the individual conditions 

of the human error under consideration, such as the performance-shaping factors, must be 

taken into account and the value adjusted as required. Suitably qualified and experienced 

human-reliability specialists should be consulted if necessary. 

Existing Safety Assessments

Where the safety assessor is revising a safety assessment and is able to confirm that there 

have been no significant changes to the facility’s operating procedures and conditions, it may be 

possible to use the previous safety assessment as a basis for the current one. 
 D-13 

Human-Error Databases

It may be possible to select a human-error value by comparing the postulated human errors to 

ones that have been compiled as part of a database, and for which values have previously been 

assigned. Extreme care must be taken when using such a database. Justification must be 

provided that the context and factors influencing human performance are sufficiently similar for 

the sequence under consideration, when compared to the bases for the actions found in the 

database. 

Derivation of Human-Error Probabilities, using Quantitative HRA Methods

If relevant human-error databases are not available, human-error probabilities can be calculated 

using various methods published in the literature. There appears to be a consensus on the 

usefulness and applicability of certain techniques for evaluating human performance, such as 

those discussed in Ref. D.24, an IAEA report on HRA (Ref. D.25,), and NUREG/CR-2300, “PRA 

Procedures Guide - A Guide to the Performance of Probabilistic Risk Assessments for Nuclear 

Power Plants” (Ref. D.16), which provides guidance on PRA, including HRA. Examples of 

specific techniques include: the THERP method (Ref. D.26), SLIM-MAUD (Ref. D.27), 

ATHEANA (Refs. D.28, D.29), and HEART (Ref. D.30). 

As indicated in Ref. D.11, the development of methodologies to represent and understand 

human performance is continuing on several fronts. In the selection and application of a 

specific methodology, four guidelines should be taken into consideration: 

• The assessment applied to each action evaluated should be consistent. 

• All actions should be evaluated within the context of specific event sequences. 

• The evaluation should have as a goal that the qualitative ranking of all actions be correct. 

• The quantitative evaluation of the actions should be traceable. 

Software-Based Quantification Tools

There are some software tools that will calculate a value for a human error. The program 

requires the input of parameters that could influence the human-error probability, such as the 

complexity of the operation, the availability of instructions, the degree of training, and the time 

available to carry out the operation. 
 D-14 

D.4.2.3 Human-Interaction Dependencies

As discussed in (Ref. D.11), dependencies between different operators or between different 

tasks performed by the same operator can significantly affect the overall level of reliability. The 

human performance analysis should therefore be carefully performed to identify any 

dependencies between different operators, between different errors committed by the same 

operator, and even between hardware failures and operator actions (e.g., if operators routinely 

‘work to’ mechanical trips/cut-outs rather than use them as safeguards against operator error). 

Where dependencies exist, their effects need to be evaluated and quantified. In some cases, 

this is already accounted for in the individual human-error data, and in other cases, 

dependencies are accounted for by the effects of performance-shaping factors. Specialist 

advice should be sought where dependencies are identified that are not accounted for in the 

human error data. Where specific dependencies cannot be identified, human-performance 

limiting values are used to limit the claims for human reliability for multiple operator actions. 

Human-error evaluations implying error likelihoods that are less than 1E-4 require particular 

justification, whereas evaluations implying error likelihoods that are less than 1E-6 are 

extremely suspect and most likely should not be used. 

D.5 Quantification of Accident Sequences

To quantify the frequencies of the accident sequences, failure rates are assigned to the systems 

and components involved in the sequences, and frequencies are assigned to each IE. 

Combining the appropriate system successes and failures with each class of IEs yields a logical 

representation of each accident sequence. The following sections describe the approaches to 

accident sequence quantification, general procedures for accident-sequence quantification, and 

discussion on the computer tools available to assist in the quantification of accident sequences. 

D.5.1 Approaches to Accident-Sequence Quantification

As discussed in NUREG/CR-2300, “PRA Procedures Guide - A Guide to the Performance of 

Probabilistic Risk Assessments for Nuclear Power Plants” (Ref. D.16), two distinct approaches 

have been used in NPPs to quantify the frequency of accident sequences. The fault-tree linking 

approach consists of combining system and component failures that are not necessarily 

independent; the event tree with boundary conditions considers combining event-tree tops that 

are all independent. The first approach automatically takes into account intersystem 

dependencies within a sequence; the second method involves two steps: 1) the quantification of 

each independent top event and 2) the multiplication of the probabilities of those top events to 

get a sequence frequency. The choice of a quantification method depends on the method used 

to create event/fault-tree models. There is literature available on system quantification 

techniques, such as the Fault Tree Handbook (Ref. D.31), on techniques to derive reliability 

parameters (Ref. D.32), and on importance measures (Refs. D.33–-D.35). 
 D-15 

D.5.1.1 Fault-Tree Linking

In fault-tree linking, the accident sequence is represented by a fault tree whose top event is 

connected to “and” gates, with inputs representing the top gates of the system fault trees for 

each system employed in the accident sequence. Since fault trees are the logic models for 

combining faults (primary events) within a system or sequence, a set of Boolean expressions 

can be established that can be further reduced to minimal cut sets, using Boolean algebra. 

Dependencies among systems or components are thus automatically accounted for in the 

Boolean reduction process. With this process, the quantification takes place on the overall 

sequence cut sets, as opposed to the individual systems or components. Furthermore, if 

multiple sequences are initiated by the same IE, these sequence fault trees can be combined 

with an “or” gate, to form a single logic model. Since IEs are assumed to be mutually exclusive, 

the estimated frequencies for sequences with different IEs can then be summed to produce an 

estimated frequency of the damage state for the facility or process. 

The fault-tree linking technique uses “and” gates to model the top events of the event tree, and 

systems and components and their interdependencies for ensuring success or failure of each 

top event are logically modeled in the fault tree. The resultant linked fault trees could be large 

and complex. In these situations, computer codes may be used in spite of the many cut sets 

that can be generated for quantification. 

D.5.1.2 Event Tree with Boundary Conditions

In this approach, system dependencies are explicitly modeled in the event tree. Each system in 

an event tree is quantified for every set of boundary conditions that has a unique effect on the 

system-failure probability. The set of boundary conditions consists of a set of components and 

systems, including dependencies that affect the failure state of the system being quantified. 

Therefore, the quantification involves the calculation of conditional probabilities, since specific 

component and system states are assumed. Events are then combined within the event tree by 

multiplication, to obtain estimated frequencies or approximate frequency distributions for each 

sequence. The frequencies for sequences initiated from the same initiator are summed up to 

obtain the frequency for the group. 

Because the conditional probabilities are calculated for each top event to explicitly account for 

the dependency effects, the resultant fault trees for the event-tree top events are thus simpler 

and independent. In some cases, analyses can be performed by hand without resorting to 

computer-assisted fault-tree reductions. 

D.5.2 General Procedure for Sequence Quantification 

Accident sequence analysis begins with the sequence identification, usually followed by 

grouping similar sequences into damage categories or bins. The damage bins are the groups of 

sequences that contribute to the same radiological and chemical consequences. The accident 

sequences that make an insignificant contribution to the total frequency of the bin may be 

screened out. Once the accident sequences to be quantified have been screened, the 

quantification can be performed using either fault-tree linking or an event tree with boundary 

conditions. Both approaches, if rigorously applied, should result in equivalent solutions. 

However, since in practice, both methods employ approximations and assumptions, the final 

results for any given solution may vary if the assumptions used are not carefully examined. 
 D-16 

For such facilities as fuel enrichment or fabrication, where extensive SSCs are employed in the 

facility processes, fault-tree linking provides a structure that automatically incorporates 

interdependencies among the SSCs, can be used to identify common-cause failures and 

perform the common-cause analysis, and identifies the dominant contributors to the failure 

frequencies, as well as their rankings. However, implementation of this approach requires 

considerable effort, usually involving computer programs. For situations where few sequences 

are identified as associated with the hazard being assessed and where less hardware is 

involved, (such as use of byproduct materials in certain medical applications), the fault-tree 

linking would not be a good choice for risk assessment. In these applications, the event trees 

with boundary conditions can provide a clear and quick insight into the risk associated with a 

particular sequence. Detailed task-by-task guidance can be found in NUREG/CR-2300, “PRA 

Procedures Guide - A Guide to the Performance of Probabilistic Risk Assessments for Nuclear 

Power Plants,” Section 6.3 (Ref. D.16). 

D.5.3 Available Tools for Sequence Quantification 

A number of computer codes are currently available for the quantification of event-tree and faulttree-logic models. Each code is written with particular objectives toward aiding or improving the 

solution of complex models. However, it is difficult to reach a consensus on any given code, 

because many different factors may affect the selection of a particular code, such as availability 

of computer facilities, staff expertise, and the specific objectives of the analysis. Detailed 

descriptions of various codes can be found in Refs. D.11, D.16, D.36, D.37, and D.38. 

D.6 Consequence Analysis 

Radiological consequences from direct radiation exposure, inadvertent criticality, and 

radionuclide release are the impacts on a receptor of the radioactive material (or ionizing 

radiation) released during an accident, or present in the air, land, or water in and/or near a 

previously contaminated area. In the case of nonreactor nuclear material facilities, receptors 

would generally consist of the offsite public, facility workers, and co-located workers. The 

identification of the population at significant risk is critical in evaluating the risk and is discussed 

in further detail in Chapter 5 of this document. Note that the consequence analysis is intended 

to be all inclusive, encompassing transportation of nuclear material, storage packages, and 

casks, as well as fixed facilities. The radiological impacts are usually evaluated in terms of the 

following risk metrics: a radiation dose, either an effective dose equivalent, or an 

organ/pathway specific dose, and an acute or chronic health effect (e.g., early fatality/injury and 

latent cancer fatality/injury). Some facilities such as uranium-milling operations and enrichment 

facilities, also use hazardous chemicals. Chemical consequences are estimated in terms of the 

values of the air concentrations at the location of the potential receptors that are translated into 

various limits, such as immediately dangerous to life and health (IDLH) values, short-term 

exposure limit (STEL) values, and emergency response procedure guideline (ERPG) values. 
 D-17 

D.6.1 Consequences of Accidents

The estimation of all consequences for accidents should be recognized (e.g., direct radiation, 

inadvertent criticality, and radionuclide release). Such an estimate may include the following 

steps: 

• Determination of a source term; 

• Transport of the release inside and outside the facility (several codes take into 

consideration the probabilistic simulation of weather conditions for outdoor airborne 

transport); 

• Calculation of the dose rate and exposure time at potential receptor locations inside the 

facility; 

• Calculation of doses and health effects at offsite locations in terms of specified distances 

and locations from the release point; and 

• Estimation of complementary cumulative distribution functions (CCDFs) or exceedance 

functions for doses and/or health effects. 

D.6.1.1 Source Term

The source term is the amount of radiological material released in the accident, the rate of its 

release, the timing of the release in relation to the initiation of the accident (or the recognition of 

accident occurrence by the facility personnel), the duration of the release, and the energy of the 

release. Depending on where the release occurs, various source terms that take into account 

building ventilation systems and decontamination factors can be defined (e.g., release to the 

compartment where the accident occurs, release to the facility building, and release to the 

environment). 

Guidance on source terms, including releases, release rates, and related parameters from 

potential accidents at nonreactor nuclear facilities (including facilities regulated by NRC and the 

Department of Energy (DOE)) is contained in NUREG-1320 (Ref. D.39) and DOE-HDBK-94 

(Ref. D.40). 

It is important to note that certain accidents, criticality accidents in particular, involve an endstate that leads to a direct beam of ionizing radiation, such as neutrons and photons, that can 

directly affect a potential receptor, usually a facility worker, if shielding is missing or inadequate. 

The analysis of criticality accidents involves estimating the number of fissions in the initial and 

secondary pulses and the total time and the release fractions of any materials released, such as 

gases, halogens, and solids. Guidance on methods and codes to calculate the radiation dose 

from various sources is contained in NRC Regulatory Guides. Shielding codes, such as DORT, 

MCNP, and SCALE, needed to carry out the calculations, are available from the Radiation 

Safety Information Computational Center at the Oak Ridge National Laboratory. 

D.6.1.2 Transport of the Release
 D-18 

A release is transported from its original location (e.g., a room) to other areas inside the facility 

building and ultimately to the environment. Transport mechanisms include airborne, 

groundwater, surface water, and others. The release transport takes into account various 

barrier and release paths. Transport also involves estimating potential depletion mechanisms 

inside the building, such as gravitational settling, impaction, thermophoresis, electrophoresis, 

filtration, tortuosity of the release path. Transport outside the building depends on site-specific 

weather conditions, such as wind speed and direction, building wake factors, surface roughness 

of the area over which the release is transported, atmospheric stability factors A - F (if a 

Gaussian transport model is used to calculate dispersion), and height of the inversion layer. 

Knowledge of topographical features may be important in some cases where terrain effects 

have a significant impact on dispersion. 

The source term to the building is used to estimate the facility worker dose, depending on 

receptor location. The source term to the environment is used to calculate the offsite (public) 

dose or the doses to co-located workers in physically separate buildings at the facility site. 

Guidance on in-facility transport models is contained in DOE reports (Refs. D.41, D.42, D.43). 

Guidance on ex-facility transport models is contained in several NRC and DOE sources. 

Allowable release rates and concentrations for various pathways are contained in Federal, 

State, and local regulations. Guidance on meeting these release levels is also available (e.g., 

regulatory guides). Both probabilistic models (that take into account weather variability over a 

year or more and perform a Latin Hypercube or a Monte Carlo Sampling to estimate a CCDF of 

dose vs. distance) and deterministic models are used, depending on the objective of the 

calculation. The NRC probabilistic consequence assessment code MACCS (Ref. D.44) has 

been extensively used to calculate the offsite consequences of accidents at NPPs and DOE 

nonreactor nuclear facilities. Another analytical tool is the RADTRAN code developed by 

Sandia National Laboratory. RADTRAN combines user-determined demographic, routing, 

transportation, packaging, and materials data with meteorological data (partly user-determined) 

and health physics data to calculate expected radiological consequences of incident-free 

radioactive materials transportation and associated accident risks (Ref. D.45). 

D.6.2 Consequences at Decontaminated/Decommissioned Sites 

Different methods can be used to estimate consequences when a certain level of contamination 

is continuously present in the environment. In this case, the objective is to estimate the 

potential exposure of persons who are resident on, or in the vicinity of, the site. The exposure 

arises from contaminated soils, groundwater, and, potentially, decommissioned buildings. 

Guidance on probabilistic models for estimating doses, such as RESRAD and D&D, is available 

in NUREG-1757, "Consolidated NMSS Decommissioning Guidance--Characterization, Survey, 

and Determination of Radiologic Criteria" (Ref. D.46). 

D.6.3 Chemical Consequences
 D-19 

Chemical consequences of potential accidents at certain types of NRC-regulated facilities, such 

as enrichment plants, need to be estimated for both workers and the offsite public. A number of 

models for performing these calculations have been used within the chemical industry (AIChE) 

and also at DOE and NRC sites. Many chemical codes include an estimate of the source term 

and the transport of the release in one package to calculate an air (or a water body) 

concentration at a specified point assumed to be the receptor location. AIChE publications, 

Environmental Protection Agency (EPA) documents, and DOE reports contain guidance on the 

use of particular codes in specific contexts (e.g., heavy-gas dispersion). 

D.6.4 Offsite Health Effects

To estimate offsite health effects, all pathways, including inhalation, cloud shine, ground shine, 

ingestion, and resuspension inhalation, have to be considered. Consequence codes model the 

exposure based on site-specific population distribution data, farmland, and water bodies at the 

site, and the counter-measures specified as part of the emergency response. Ingestion doses 

have to take into account the transport of radionuclides through the food chain and the foods 

grown or making up the average diet in a particular region. NRC’s MACCS code (and the 

COSYMA code used in Europe) have detailed individual modules for incorporating these factors 

in the dose calculation. 

D.6.5 Onsite Health Effects

Onsite exposure and health effects have to consider the dispersion of released material through 

the facility and the location of workers, as well as an estimate of the exposure time. The latter 

will depend on several factors, such as the time taken to evacuate the facility, the recognition of 

the accident by personnel, the activation of alarms, and the training of the workforce in 

emergency response procedures. 
 D-20 

D.7 Data Analysis

Essential elements of a risk analysis are the collection of data and its assessment with respect 

to its quality and inherent uncertainties. In contrast to the nuclear power plant arena, where the 

radiation source is concentrated in the core and the safety systems are designed in the 

standardized fashion, the NMSS/FSME-regulated activities encompass much broader and 

diverse industrial and medical applications of nuclear material and waste disposal, where 

multiple sources of not only radiation hazards but also a variety of chemical hazards, exist. The 

safety systems and components that are intended to prevent accidents and mitigate their 

consequences for NMSS/FSME-regulated activities also have diverse features. Therefore, data 

collection for NMSS/FSME-regulated activities may be much more complex than for NPPs. 

However, key methods that were developed from NPP experiences can also be applied to 

NMSS/FSME-regulated activities for data analysis. One of the general rules applicable to most 

categories is that data used to characterize or quantify key parameters should be unbiased and 

estimated as realistically as possible. The use of conservative estimates of the key parameters 

could lead to unrealistic, overly conservative risk assessments. The elements of data required 

for risk assessments include: (1) data for sequence frequency estimation; (2) data for 

assessment of component/system reliability (i.e., fragilities); common-cause failures and 

initiating event frequency; (3) data required for human performance analysis; and (4) data 

required for consequence assessments, including source term characterization and the effect of 

its releases on the public and workers, as well as the environment. 

Certainly, plant-specific information should always be investigated thoroughly to collect the data 

required for a risk analysis. Depending on the situation associated with a particular activity, 

data that are useful for risk assessment may be scarce. The analysts should be aware of, and 

encouraged to use, generic databases or other information, but when using such generic 

information or data for a particular application (i.e., a system), the underlying activities on which 

the database or information is based should be comparable to those of the system being 

analyzed. 

For initiating event identification, the NMED database (Ref. D.47) may provide numerous realevent records that may be used to derive initiating events. NMED, maintained by the Idaho 

National Engineering and Environmental Laboratory, is the nuclear material events database 

that contains records of incidents involving radioactive material licensed under NRC regulations 

or compatible Agreement States. Reportable events include medical misapplications, loss of 

materials, malevolent acts (intentional violations), and release of materials. For component 

failure rate data, the Savannah River database (Ref. D.48) may be consulted. For facilities such 

as fuel fabrication and enrichment plants, where seismic events may be of concern, the 

Earthquake Experience and Test Data (Refs. D.49, D.50) may be used for component fragilities. 

Also as noted in Ref. D.11, examples of analyses are available for treating recovery events 

within nonreactor risk assessments (Refs. D.51–D.53). Ref. D.11 also provides other 

references for assessing human-error probabilities and consequences, as well as the data 

references for release effects on the public. 
 D-21 

D.8 Sensitivity, Uncertainty, and Importance Analyses

The reason for adapting the probabilistic approach versus deterministic means for 

characterizing or quantifying a process or a phenomenon is the probabilistic nature of the 

universe and the associated uncertainties caused by natural phenomena and a limited humanknowledge base. Therefore, as important parts of a risk assessment, sensitivity analysis, 

uncertainty analysis, and importance analysis should be performed in conjunction with a 

baseline risk assessment to gain confidence in, and understanding of, the results. This section 

discusses these three key analyses and provides some guidance on how they may be 

performed. 

D.8.1 Sensitivity Analysis 

The purpose of a sensitivity analysis is twofold: (1) to determine the sensitivity of the calculated 

risk measures, in terms of either individual doses or cumulative releases of radiation or 

chemicals, to possible dependencies among system or component failures, and among human 

errors; and (2) to address those modeling assumptions suspected of having a potentially 

significant impact on the results. These assumptions are generally in areas where information 

is lacking and heavy reliance must be placed on the analyst’s judgment. Sensitivity analysis 

can then be accomplished by substituting alternative assumptions for conservatism and 

evaluating their individual impacts on the results. If significant sensitivities are exhibited with 

respect to failures of certain dependencies, the risk analysis should describe what conditions, 

precautions, and actions are in place to help ensure against them; if the risk results are 

sensitive to data, then the data should be closely scrutinized to ensure the accuracy and 

adequacy of their underlying representations. 

Sensitivity analyses should be performed in three areas: potential component-failure 

dependencies, potential human performance dependencies, and major assumptions or data 

employed. For component failure dependencies, the sensitivity analysis is carried out by 

searching for sensitivity to dependence (hardware coupling), on a system-by-system basis, so 

that each system can be examined in more depth than would be the case if entire accident 

sequence cutsets were searched. Whenever the effect of a given dependence failure is 

substantial, the corresponding impact on the risk results should be assessed, along with a 

discussion of measures that can be taken to reduce the potential dependence failure or its 

contribution to the calculated risk. 

The human-error-dependence sensitivity analysis should be performed in a manner similar to 

the component-dependence sensitivity analysis. The suspected dependence cutsets are 

identified as those containing multiple human errors of any type. Rather than being 

requantified, these suspected dependence cutsets should be analyzed, and defenses, 

management controls, or conditions should be established to eliminate the dependencies 

between human errors in the suspected cutsets. 

The sensitivity analysis of assumptions or data employed in the risk assessment can be 

performed by first searching for suspected conservative assumptions or data that may have 

important impacts on the risk. Then, alternative assumptions or a different set of data are used 

and their impact on the risk results is assessed. For example, most seismic PRAs performed 

for the NRC’s IPEEE program used the hazard curves developed by EPRI for the mean CDF 

estimate, and it was then supplemented with the sensitivity analysis using Lawrence Livermore 
 D-22 

National Laboratory’s hazard curves (Ref. D.54). After comparing the analyses with the two 

sets of hazard curves, the majority of the seismic PRAs concluded that the seismic CDF is not 

sensitive to which hazard curve is chosen, even though one curve may be substantially higher 

than the other. This is because, in a conventional seismic PRA, the CDF is calculated as the 

numerical convolution of the conditional-damage-state frequency and the slope of the respective 

hazard curve. Therefore, as long as the shapes of the hazard curves are similar, the calculated 

CDF should be similar, regardless of the magnitudes of the curves. 

D.8.2 Uncertainty Analysis

Two types of uncertainties should be examined: aleatory uncertainty and epistemic uncertainty. 

Aleatory uncertainty is associated with physical phenomena that cannot be reduced by model or 

data improvements. For instance, the uncertainty associated with the thickness of a fuel cask is 

aleatory because of the allowable tolerance in the manufacturing process. Epistemic 

uncertainty represents the uncertainty associated with the human understanding of a particular 

process or physical phenomenon. As the knowledge base increases, our understanding of the 

physical phenomena is also enhanced, thereby reducing the uncertainty associated with the 

data and the model. Epistemic uncertainty is reducible as the modeling aspects are refined. In 

practice, the probability of a sequence may be regarded as representing an aleatory uncertainty 

and the probability of parameters and models as representing epistemic uncertainties. 

However, such distinctions are not absolute. For example, in calculating the consequences of 

disruptive geologic events (fault movement, igneous activity), the timing of such an event may 

be chosen as a model parameter but clearly it represents an aleatory uncertainty (Ref. D.35). 

Sources of uncertainties should be carefully considered to ensure that all uncertainties 

associated with initiating events, sequence modeling, source-term characterization, and data 

employed are included, properly characterized, and propagated through the risk model. Some 

uncertainty is associated with the manner in which the analyst applies the methods and how 

skillfully or accurately he or she is able to represent the process or system with the adopted 

modeling method; these uncertainties can be addressed by training the analyst and by using 

consistent procedures, proper guidance, and review. Modeling uncertainties can also be 

reduced by making models as realistic as possible, with compensating assumptions and 

modeling constraints. 

Another source of uncertainty is associated with human performance, especially for 

NMSS/FSME-regulated activities, such as medical and industrial uses of nuclear material. 

Many accidents involving radiation exposure have been caused by human errors. The 

uncertainties associated with human errors should be adequately addressed in any risk model. 

As a minimum, the following sources of uncertainties associated with human performance 

should be considered (Ref. D.16): 

• The dearth of data available on human performance; 

• The inexactness of models of human performance that purport to describe how people 

act in various situations and conditions; 

• The identification of all relevant performance-shaping factors and their interactions and 

effects; 

• The skill and knowledge of the human-reliability analyst; and 
 D-23 

• The variability in the performance of a given individual and among the performances of 

different individuals. 

The amount of acceptable uncertainty will vary from application to application. For example, a 

larger degree of uncertainty may be more acceptable for very low-risk applications, where there 

would be no effect on risk. Instances where the uncertainty has a profound effect on the overall 

risk need to be examined much more carefully. 

D.8.3 Importance Analysis

The purpose of the importance analysis is to identify the important IEs, accident sequences, 

system failures, and component failures, as well as human errors that are the primary 

contributors to the underlying risk. The importance measures are usually calculated in a 

hierarchical fashion, to allow tracing from the important sequence to the system failures to the 

importance component failures or human errors contributing to the system failure. For a 

process that involves both active and passive systems, importance measures may require 

consideration of components that may function either in a binary state or in a continuous 

fashion, as discussed extensively in Ref. D.35. 

Importance measures that have been frequently applied to reactor PRAs include Birnbaum, 

Fussell-Vesely, Risk Reduction Worth, and Risk Achievement Worth (Refs. D.32, D.33, D.55, 

D.56). These importance measures calculate either the fractional contribution of the sequence 

containing the primary event of interest to the total calculated risk or the degradation ratio, which 

is computed by the conditional-failure probability of a process or system, given the assumed 

failure of a primary event of interest, divided by the unconditional failure probability of the 

process or system. However, these importance measures almost always apply to processes or 

systems that can be modeled, consisting of binary-state components (i.e., either "success" or 

"failure"). Given the wide variety of NMSS/FSME-regulated activities, it is insufficient to 

consider only systems whose functionality is modeled as success or failure and whose 

components are subsystems that are similarly modeled in one of two binary states. Although 

these tools can be used for certain NMSS/FSME activities, such as fuel fabrication facilities, 

other NMSS/FSME activities employ not only systems that function in a binary state, but also 

systems that function in a continuous manner, such as HLW repositories and LLW disposal 

facilities. A repository relies on both engineered and natural components to prevent the release 

of radioactivity from the nuclear waste to the environment. Systems relied on to perform such 

functions act in a passive manner and perform on a continuous scale. Therefore, the 

determination of a failure state or failure rate for such components is generally meaningless. 

Techniques have been developed (Ref. D.35) to measure the importance of a component 

whose functionality is continuous. These measures were based on risk, in comparison to 

traditional importance measures, which are based on frequency of failure and are intended to be 

more suitable to systems comprising components with behaviors mostly easily and naturally 

represented as continuous rather than binary. It appears that these importance measures can 

be used to evaluate systems comprising both continuous- and binary-behavior components. 

For reactor PRAs, the importance measures are calculated for more than twenty dominant 

contributors, to ensure a good understanding of the effect of failures of dominant contributing 

components on the CDF. For NMSS/FSME-regulated activities, the extent to which the 

importance measures should be used depends on the underlying risk associated with the 
 D-24 

particular activity and the number and complexity of the systems and components involved in 

the particular process or facility. In any event, the effort expended for importance calculations 

should be commensurate with providing a good understanding of the key systems and 

components and their contribution to the total calculated risk, so that risk management for the 

evaluated process or facility can be planned accordingly. 

D.9 Documentation of Results

When the analysis is completed, the results must be documented and presented to reviewers 

and assessors for evaluation and decisionmaking. The documentation should contain the 

following broad elements: 

• Statement of purpose of the analysis (i.e., the regulatory issues being addressed); 

• Methodology used to addressed the problem on a technical level; 

• Major assumptions being employed in the assessment (e.g., scope and depth of analysis); 

• Data being used in the analysis; 

• Presentation of results, including an assessment of uncertainties; and 

• Conclusions of the analysis relative to the stated purpose of the study. 

Specific deliverables should be tailored to the purpose of the study and the obligations of the 

performers. Various procedural guides (Refs. D.11, D.8, D.16, D.37, and D.57) contain specific 

suggestions on how to document the particular results of studies. 

D.10 Peer Review and Quality of Risk Assessment 

Evaluation of the quality of a risk assessment via a process of peer review is an integral 

component of the risk assessment. Compliance with the intent of guidelines published by NRC 

or the industry (Refs. D.8, D.16, D.37, D.57) on specific types of analyses may be helpful in 

streamlining the peer-review process, even though many of the standards and assumptions 

may not be directly applicable to NMSS/FSME-regulated activities. Currently, a standard for 

evaluating the quality of a PRA is being developed only for operating power reactors. The 

importance of PRA quality to the RIDM approach has been recognized by the NRC, and an 

acceptable and pragmatic means of achieving the requisite PRA quality for regulatory purposes 

has been proposed (Ref. D.58). 

The submittal should document the particular review process used to validate the results, the 

qualifications of the reviewers, the review findings, and the resolution of contentious issues. 

Industry risk-comparison studies can be helpful in ensuring that the scope of the risk 

assessment is appropriate to the risk issues analyzed and that the level of detail and the overall 

quality of the analysis are adequate. This can entail a comparison of risk assessments between 

similar facilities and activities. Based on the peer review, the licensee or applicant should justify 

the technical adequacy of the scope and quality of the risk assessment. This justification and 

the peer review results should be provided in sufficient detail to allow the NRC staff to perform 

an independent assessment of the quality of the risk assessment and verification of the results 

and conclusions offered. Such documentation may also be beneficial in documenting intent in 

case of adjudication. 
 D-25 

D.11 References 

D.1 Youngblood, R. W., et aI., Brookhaven National Laboratory, "Preclosure Risk 

Assessment Methodology: PRAM Procedures Guide," PRAM-04/06/90-RY-234, April 

1990. 

D.2 Eisenberg, N. A., et aI., "Development of a Performance Assessment Capability in the 

Waste Management Programs of the U.S. Nuclear Regulatory Commission," Risk 

Analysis, Vol.19, No.5, 1999. 

D.3 U.S. Nuclear Regulatory Commission, "Re-examination of Spent Fuel Shipment Risk 

Estimates," NUREG/CR-6672, Vols.1 and 2, March 2000. 

D.4 U.S. Nuclear Regulatory Commission, "Shipping Container Response to Severe 

Highway and Railway Accident Conditions," NUREG/CR-4829, February 1987. 

D.5 Department of Agriculture (U.S.), State Soil Graphics (STATSGO) Database. 

D.6 ARC/INFO Coverages and Arc View Project, K. C. Bayer Digital Map of the U.S., 

Geologic Data Systems, Inc., Denver Colorado. 

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Cask Storage System at a Nuclear Power Plant,” NUREG-1864, September 2006. 

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NUREG-1513, May 2001. 

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Procedures, Second Edition with Worked Examples, Center for Chemical Process 

Safety, AIChE: New York, 1992. 

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Options for Nuclear Byproduct Material Systems," NUREG/CR-6642, February 2000. 

D.11 IAEA Technical Document, "Procedures For Conducting Probabilistic Safety 

Assessment (PSA) For Non-Reactor Nuclear Facilities," IAEA-TECDOC (draft), 2001. 

D.12 Loss Prevention in the Process Industries, Second Edition, Lees, F.P., ed., ButterworthHeinemann, Oxford, United Kingdom, 1996. 

D.13 Lawley, H. G., "Operability Studies and Hazard Analysis," Loss Prevention, Vol. 8, No. 

105, 1974. 

D.14 Chemical Industries Association, "A Guide to Hazard and Operability Studies," London, 

England, 1977. 

D.15 U.S. Nuclear Regulatory Commission, "A Baseline Risk-Informed Performance-Based 

Approach For In situ Leach Uranium Extraction Licensees," NUREG/CR-6733, June 

2001. 
 D-26 

D.16 U.S. Nuclear Regulatory Commission, "PRA Procedures Guide--A Guide to the 

Performance of Probabilistic Risk Assessments for Nuclear Power Plants," NUREG/CR2300, January 1983. 

D.17 Kennedy, R. P., et a!., "Probabilistic Seismic Safety Study of an Existing Nuclear Power 

Plant," Nuclear Engineering and Design, Vol. 59, No.2, August 1980. 

D.18 Kennedy, R. P., et a!., "Seismic Fragilities for Nuclear Power Risk Studies," Nuclear 

Engineering and Design, Vol. 79, No. 1, May 1984. 

D.19 EPRI, "Methodology for Developing Seismic Fragilities," TR-103959, June 1994. 

D.20 Kennedy, R. P., "Overview of Methods for Seismic PRA and Margin Analysis Including 

Recent Innovations," NEAlCSNI/R, Japan, 1999. 

D.21 EPRI, "Fire-Induced Vulnerability Evaluation (FIVE)," TR-100370, April 1992. 

D.22 EPRI, "Fire PRA Implementation Guide," TR-105928, December 1995. 

D.23 U.S. Nuclear Regulatory Commission, "Individual Plant Examination of External Events 

(IPEEE) for Severe Accident Vulnerabilities 10CFR 50.54," Generic Letter 88-20, 

Supplement 5, September 8, 1995. 

D.24 Kirwan, B., A Guide to Practical Human Reliability Assessment, published by Taylor & 

Francis, ISBN 0-7484-0052-4, 1994. 

D.25 IAEA, "Human Reliability Analysis in Probability Safety Assessments for Nuclear Power 

Plants," Safety Series No. 50-P-10, Vienna, 1995. 

D.26 U.S. Nuclear Regulatory Commission, "Handbook of Human Reliability Analysis," 

NUREG/CR-1278, August 1978. 

D.27 U.S. Nuclear Regulatory Commission, "SLIM-MAUD: An Approach to Assessing Human 

Error Probabilities using Structured Expert Judgment," NUREG/CR-3518, July 1984. 

D.28 U.S. Nuclear Regulatory Commission, "A Technique for Human Error Analysis 

(ATHEANA)," NUREG/CR-6350, May 1996. 

D.29 U.S. Nuclear Regulatory Commission, "Technical Basis and Implementation Guidelines 

for a Technique for Human Error Analysis (ATHEANA)," NUREG-1624, September 

1999. 

D.30 Williams, J. C., "A Data-based Method for Assessing and Reducing Human Error to 

Improve Operational Performance," Proceedings of the IEEE 4th Conference on Human 

Factors in Power Plants, Monterey, California, USA, June 1988. 

D.31 U.S. Nuclear Regulatory Commission, "Fault Tree Handbook," NUREG-0492, January 

1981. 
 D-27 

D.32 Henley, S. J., et aI., Reliability Engineering and Risk Assessment, Prentice-Hall, Inc., 

Englewood Cliffs, NJ, 1981. 

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Winston, Inc., New York, "1975. 

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1977. 

D.35 Eisenberg, N. A., et aI., "Importance Measures for Nuclear Waste Repositories," 

Reliability Engineering & System Safety, Vol. 70, 217-239, 2000. 

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INEEL, Idaho Falls, Idaho, 83415. 

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NUREG/CR-2815, August 1985. 

D.38 IAEA, "Procedures for Conducting Probabilistic Safety Assessments of Nuclear Power 

Plants (Level 1)," Safety Series No. 5Q-P-4, Vienna, 1992. 

D.39 U.S. Nuclear Regulatory Commission, "Nuclear Fuel Cycle Facility Accident Analysis 

Handbook," NUREG-1320, May 1988. 

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Fractions for Nonreactor Nuclear Facilities," DOE-HDBK-301Q-94, December 1994. 

D.41 Spore, J. W., et aI., "In-Facility Transport Code Review," unpublished report prepared by 

Los Alamos National Laboratory In-Facility Transport Working Group, July 1996. 

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December 1982. 

D.43 Browitt, D., et aI., "Users Guide for the KBERT 1.0. Code," Sandia National 

Laboratories, SAND95-1324, July 1995. 

D.44 U.S. Nuclear Regulatory Commission, "Code Manual for MACCS2," NUREG/CR-6613, 

May 1998. 

D.45 Neuhauser, K.S., and Kanipe, F.L., “RADTRAN 5 User Guide,” SAND2000-2354, Sandia 

National Laboratories, July 7, 2003. 

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Guidance--Characterization, Survey, and Determination of Radiologic Criteria ," 

NUREG-1757 Volume 2, 2003. 

D.47 Idaho National Engineering and Environmental Laboratory, "Nuclear material Event 

Database (NMED)," http://nmed.inel.aov/. 
 D-28 

D.48 Dexter, A. H., et aI., "Component Failure Rate Data with Potential Applicability to 

Nuclear Fuel Reprocessing Plant," Du Pont, Savannah River Lab., Rpt. DP-1633, 1982. 

D.49 Kennedy, R. P., et aI., "Use of Seismic Earthquake Experience and Test Data to Show 

Seismic Ruggedness of Certain Classes of Equipment in Nuclear Power Plants," 

SSRAP, April 16, 1990. 

D.50 U.S. Nuclear Regulatory Commission, "Seismic Fragility of Nuclear Power Plant 

Components," NUREG/CR-4659, June 1991. 

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Frequency in Reprocessing Facility —Hydrogen Explosion in Plutonium Solution 

Vessel," Proceedings of 2000 Annual Meeting of the AES of Japan, 2000. 

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Allowing for Tolerable System Downtime," Proceedings of RECOD 87, Paris, 1987. 

D.53 Institute of Nuclear Safety/Nuclear Power Engineering Corporation, "Annual Report 

About Development of Nuclear Facility Safety Analysis Code in Heisei 8," INS/S96-15, 

1997. 

D.54 U.S. Nuclear Regulatory Commission, "Perspectives Gained from the Individual Plant 

Examination of External Events (lPEEE) Program," NUREG-1742, October 2001. 

D.55 Cheok, M. C., et aI., "Use of Importance Measures in Risk-Informed Regulatory 

Applications," Reliability Engineering and System Safety, Vol. 60 pp. 213-226, 1998. 

D.56 Sherry, R., "Importance Measures for Risk-Informed Regulatory Applications: 

Information Content and Uncertainties," High Level Radioactive Waste Management: 

Proceedings of the Seventh Annual lnternational Conference, ANS, 1996. 

D.57 Nuclear Energy Institute, "Industry Guidance Document on Preparation of an ISA 

Summary," November 2000. 

D.58 U.S. Nuclear Regulatory Commission, "Stabilizing the PRA Quality Expectations and 

Requirements," Commission Action Memoranda COMNJD-03-002, September 8, 2003. 
APPENDIX E 

SUMMARY OF NMSS/FSME RISK ISSUES 

AND STATUS OF RISK INFORMATION 
 E-1 

APPENDIX E 

SUMMARY OF NMSS/FSME RISK ISSUES 

AND STATUS OF RISK INFORMATION 

E.1 Introduction

This appendix contains work carried out in Fiscal Year 2002. Some of the findings may no 

longer be relevant. The information is provided so that NMSS/FSME Divisions can benefit from 

the work that has been done. 

The scope of NMSS/FSME-regulated areas and applications is broad; it includes ensuring 

public health and safety through licensing, inspection, and environmental reviews for all NRC 

activities (except operating power and all nonpower reactors). NMSS/FSME is also responsible 

for the safeguards technical review of all licensing activities, including the import and export of 

special nuclear material (excluding reactors). 

It has grouped its regulatory responsibilities into four main program areas: (1) fuel cycle safety 

and safeguards; (2) industrial and medical nuclear safety; (3) spent nuclear fuel storage and 

radioactive materials transportation; and (4) radioactive waste management. Each area has 

unique risk areas that have benefited (or could benefit) from risk insights and the application of 

risk guidelines. 

E.2 NMSS/FSME Risk Issues

NMSS/FSME continually uses risk information in providing regulatory oversight. Risk insights 

have been employed in each of the main program areas, and future risk issues have been 

identified that could benefit from this application. Each main program area has, to some extent, 

applied the principles of risk-information. Table E-1 summarizes the potential future 

applications identified by the Risk Task Group for each area. 

 Table E-1. Potential NMSS/FSME Program Areas for Applying a Risk-Informed Approach 

Program Area Potential Application 

Fuel Cycle Safety and 

Safeguards - 

Conversion, 

Enrichment, and Fuel 

Fabrication Facilities 

1. Develop a regulatory guide for review of exemption requests that 

may result in increased risk. 

2. Prepare guidance for performing risk-informed inspections and 

identifying the risk significance of inspection findings. 

3. Prepare guidance for risk-informed ISA reviews. 
 E-2 

Program Area Potential Application 

Industrial and Medical 

Nuclear Safety - Use 

of byproduct, source, 

or special nuclear 

material 

1. Develop a vulnerability assessment for materials licensees. 

2. Reevaluate the sealed source and device review process to consider 

radiological risks for devices. 

3. Reevaluate licensing and device review process for radiography 

licensees to reflect insights from radiological risk studies. 

4. Revise license renewal process to reflect risk and past performance. 

Radioactive Materials 

Transportation 

1. Review relative safety and risks associated with compliance with 

current regulations applicable to handling, storage, transportation, and 

disposal of spent commercial fuel. 

2. Review and update risk estimates for transportation of radioactive 

material (other than spent nuclear fuel). 

3. Conduct risk-informed review of licensing and certification process 

for transport of non-spent fuel radioactive materials and independent 

spent fuel storage installations ISFSIs). 

4. Develop a risk-informed approach to the inspection process for 

transportation and spent fuel storage. 

Radioactive Waste 

Management 

1. Assess relative safety and risks associated with regulatory 

compliance for handling, storage, transportation, and disposal of spent 

nuclear fuel. 

2. Develop a generic method for performing risk analyses for dry cask 

storage facilities. 

3. Develop Total System Performance Assessment Code sensitivity and 

uncertainty analysis to support risk-informed licensing and inspection. 

4. Incorporate human reliability insights into preclosure safety analysis. 

5. Develop risk-informed licensing and inspections programs for HLW 

programs. 
 E-3 

E.3 Status of Risk Information

Sources of risk information are available to facilitate the application of a risk-informed approach 

to NMSS/FSME-regulated activities. For each major NMSS/FSME program area, a list of 

representative sources of risk-related information is provided. The risk-related references 

provided are not intended to be all-inclusive. They are, rather, representative references that 

will provide a foundation for the application of risk information and risk guidelines. The use of 

these data is not intended to replace actual site-operating data; they are intended to supplement 

the information and to serve as a basis for those facilities and applications that do not have such 

data. In addition, other regulatory agencies (i.e., DOE, Environmental Protection Agency (EPA), 

OSHA) have published reports pertaining to risk that may be applicable to specific aspects of 

these program areas. 

One potential source of applicable data for all NMSS/FSME program areas is NMED, 

administered by the Division of Industrial and Medical Nuclear Safety, NMSS. It contains data 

from materials, fuel cycle, and nonpower reactor licensees on events, such as personnel 

radiation overexposures, medical misadministrations, loss of radioactive material, and potential 

criticality events.

E.3.1 Fuel Cycle Safety and Safeguards - Conversion, Enrichment, and Fuel 

Fabrication Facilities

The following references contain examples of risk information applicable to these facilities: 

• NUREG/BR-0184: “Regulatory Evaluation Technical Evaluation Handbook” (Appendix D 

provides risk information related to public health (accident and routine), occupational 

health (accident and routine) and onsite and offsite property damage for in situ leach 

mining, milling, conversion, enrichment, fuel fabrication, MOX fuel fabrication and fuel 

reprocessing facilities). 

• NUREG-0706: “Generic Environmental Impact Statement (GElS) for Uranium Milling” 

• NUREG-1531: “Final Environmental Impact Statement for the Atlas Site” 

• NUREG-1569: “Standard Review Plan for In situ Leach Uranium Extraction License 

Applications” 

• NUREG-1620: “Standard Review Plan for the Review of a Reclamation Plan for Mill 

Tailings Sites Under Title II of the Uranium Mill Tailings Radiation Control Act” 

• NUREG-0713: “Occupational Radiation Exposures at Commercial Nuclear Power Plant 

Reactors and Other Facilities” 

• NUREG/CR-6733: “A Baseline Risk-Informed, Performance-Based Approach For In situ 

Leach Uranium Extraction Licensees”

In addition to these representative NRC-published references, DOE has published data 

pertaining to risks associated with enrichment plants and handling and storage issues 

associated with UF6 containers. 
 E-4 

E.3.2 Industrial and Medical Nuclear Safety -- Use of Byproduct, Source, or 

Special Nuclear Material

The following references contain examples of risk information applicable to these facilities: 

• NUREG/BR-0184, “Regulatory Evaluation Technical Evaluation Handbook” [Appendix D 

includes risk information pertaining to non-fuel cycle facilities (i.e., research, testing, 

experimental, diagnostic and therapeutic facilities; measurement, calibration, and 

irradiation facilities; manufacturing and distribution facilities; and service organizations)) 

• NUREG/CR-6642, “Risk Analysis and Evaluations of Regulatory Options for Nuclear 

Byproduct Material Systems.” 

• NUREG-1717 “Radiological Assessment of Exemptions for Source and Byproduct 

Materials.” 

• NUREG-0713, “Occupational Radiation Exposures at Commercial Nuclear Power Plant 

Reactors and Other Facilities.” 

• NUREG/CR-5392, “Discrete Event Simulation as a Risk Analysis Tool for Remote 

Afterloading Brachytherapy.” 

• NUREG/CR-6088, “Summary of 1991-1992 Misadministration Events.” 

• NUREG/CR-5145, “Cellular Investigations of 3M Series 900 Static Eliminators.” 

• NUREG-1631, “Source Disconnects Resulting from Radiography Drive Cable Failures.” 

• NUREG/BR-0024, “Working Safely in Gamma Radiography.” 

E.3.3 Radioactive Materials Transportation

The following references contain examples of risk information applicable to this activity: 

• NUREG-0170, “Final Environmental Statement on the Transportation of Radioactive 

Material by Air and Other Means.” 

• NUREG/CR-4829, “Shipping Container Response to Highway and Railroad Accident 

Conditions.” 

• NUREG/CR-6672, “Reexamination of Spent Fuel Shipment Risk Estimates.” 

• NUREG-0713, “Occupational Radiation Exposures at Commercial Nuclear Power Plant 

Reactors and Other Facilities.” 

• NUREG/BR-0184, “Regulatory Evaluation Technical Evaluation Handbook” (Appendix D 

includes risk information pertaining to transportation accidents) 

• Generic Letter 96-07,” Interim Guidance on Transportation of Steam Generators.” 

In addition to these representative NRC-published references, DOE has published data 

pertaining to risks associated with enrichment plants and the transportation of UF6 containers. 
 E-5 

Additional risk-informed data may be obtained from NRC licensing documents related to specific 

facilities. Although facility-specific, this information may be useful in other, similar applications. 

The following references are typical of the information available: 

• Trojan Reactor Vessel SER and Certificate of Compliance 

• West Valley TN-BRP and TN-REG Spent Fuel Transportation Casks SER and 

Certificate of Compliance 

E.3.4 Radioactive Waste Management – Hight Level Waste, Low Level 

Waste, Spent Fuel Storage 

The following references contain examples of risk information applicable to these facilities: 

• NUREG/BR-0184, “Regulatory Evaluation Technical Evaluation Handbook” (Appendix D 

includes risk information on spent fuel storage, HLW storage, TRU storage, geological 

waste disposal, and shallow land disposal) 

• NUREG-0713, “Occupational Radiation Exposures at Commercial Nuclear Power Plant 

Reactors and Other Facilities” 

• NUREG-1757,”Consolidated NMSS Decommissioning Guidance” 

• NUREG-1573, “A Performance Assessment Methodology for Low-Level Radioactive 

Waste Disposal Facilities” 

Additional data may be obtained from NRC licensing documents related to specific facilities. 

Although facility-specific, such information may be useful in other, similar applications. The 

following references are typical of the information available: 

• Exemption to 10 CFR 72.102F9(f)(1) “Seismic Design Requirements for TMI-2 ISFSI” 

• Exemption from 10 CFR 72.212 and 72.214 for the VSC-24 Dry Storage Cask and ISFSI 

• PFS, FEIS and SER (contains information on seismic issues, airplane crash hazards, 

and confinement integrity).

E.4 Need for Additional Risk Information

Significant resources exist for use in applying a risk-informed approach and implementing risk 

guidelines for NMSS/FSME facilities. These data, when supplemented with other applicationspecific information, should provide a solid foundation to assist in documenting and reviewing 

risk-informed submittals and risk guideline applications. 

The usefulness of risk-related data increases as new data are obtained and published. 

Probabilistic risk assessments, ISAs, performance assessments, and barrier/hazard analyses 

will serve to supplement the existing data and result in increasingly accurate results. 

Evidence in the NMSS/FSME materials arena, as well as other nuclear applications, has shown 

that human reliability plays a significant role in operational safety. Additional data are needed to 

more accurately model this variable. In addition, some of the data sources are dated (e.g., data 

from GElS for the Atlas Mill dates to 1980). Although such historical data are valuable, they 
 E-6 

need to be updated with more recent operating and inspection data to reduce inherent 

uncertainties. 

Revised data is also needed in the area of spent fuel behavior during transportation. Use of 

higher enrichment and higher burn up fuels may need additional characterization studies. 
APPENDIX F 

(RESERVED) 
APPENDIX G 

EXAMPLE OF THE USE OF RISK GUIDELINES 
 G-1 

APPENDIX G 

EXAMPLE OF THE USE OF RISK GUIDELINES 

G.1 Introduction

The following example is provided to illustrate how quantitative risk guidelines could be useful in 

NMSS/FSME activities. This specific example describes how a petition for rulemaking might be 

handled in a risk-informed manner and addresses the population at risk, implications for internal 

and external stakeholders, the options for numerical guidelines, and possible decision 

outcomes. 

G.2 Example: Worker Acute Fatality from an Irradiator Accident

The American National Standards Institute submitted a petition for a rulemaking to remove a 

current regulatory requirement. The petition was for a rule change to remove the requirement 

for panoramic industrial sterilization irradiators to have a qualified operator physically present 

onsite, or at the facility, at all times during operations. These irradiators use gamma sources 

that often total between (2 million) and (5 million) curies for pool-type irradiators. Entry of a 

worker into the shielded irradiation room during operation would result in a fatal exposure. Such 

entry is prevented by a number of safety features and practices, one of which is the presence of 

the trained operator onsite. The request was to have the qualified operator on call but not 

present onsite at all times. In the case of system malfunctions, the operator would be called to 

take appropriate action. This is estimated to increase the risk to a non-operator worker who 

loads products on the continuously operating conveyor leading into the irradiator, since he might 

attempt to remedy a malfunction in a way that would expose him to a lethal dose. Personnel 

entry without the qualified operator’s knowledge is difficult, and there are automatic protective 

features. However, unauthorized access is possible for certain designs. Such accidents have, 

in fact, occurred on several occasions overseas, although not at NRC-regulated facilities. 

The use of a risk-informed process to evaluate this proposed rule change would have positive 

implications for both internal and external stakeholders. For internal stakeholders, use of a 

RIDM process would provide NRC staff with a consistent, systematic, and defensible way to 

make risk-management decisions. For external stakeholders, the use of a risk-informed 

process would provide an objective way to assess a change that would reduce the licensees’ 

burden, as a tradeoff against a small increase in the risk of an accidental acute fatality to 

workers. The question is whether the increase in risk to workers would be acceptable. 

To quantify the change in risk that would occur from a possible rule change, staff conducted a 

risk assessment. Staff assessed the risk under the current regulatory framework, as well as the 

change in risk from the proposed rule change. Staff estimated the risk of worker acute fatality 

per facility to be 5x10-8 /yr, under the current regulatory framework. If the operator were not 

always required to be present, but only on call, as the petition requested, staff estimated the risk 

as 4x10-6 /yr. 

Using the draft risk guidelines (QHG4 is the base approach) indicated 4X10-6 /yr is no longer 

less than the 1X10-6 /yr limit of the negligible category. Therefore, the estimated individual risk 

with the removal of the current requirement is not considered negligible. However, with a factor 
 G-2 

of 4 greater than QHG 4, the risk is unlikely to be in the unacceptable region. The decisionmaker would need to determine whether it is cost-beneficial to grant the exemption. Such a 

discussion also needs to be balanced by the consideration of security needs, assurance of 

defense in depth, and maintenance of adequate safety margin.
APPENDIX H 

(RESERVED) 
APPENDIX I 

KEY ISSUES ASSOCIATED WITH THE PROPOSED APPROACH 

AND IMPLEMENTATION OF GUIDANCE 

FOR RISK-INFORMED DECISIONMAKING
 I-1 

APPENDIX I 

KEY ISSUES ASSOCIATED WITH THE PROPOSED APPROACH 

AND IMPLEMENTATION OF GUIDANCE 

FOR RISK-INFORMED DECISIONMAKING

As NMSS/FSME plans to apply the proposed RIDM process and the proposed draft risk 

guidelines to selected applications in the coming years, certain issues with potential policy 

implications as well as process implementation become evident. These require further 

evaluation in order to facilitate a smooth implementation of this proposed process. These 

issues are organized into two groups: those with potential policy implications and those related 

to implementation. These issues will be evaluated during the application phase of the proposed 

systematic RIDM process for NMSS/FSME-regulated activities. 

ISSUES WITH POTENTIAL POLICY IMPLICATIONS

(1) Application of the accident risk guidelines for the public to workers 

Workers take some voluntary risks in their jobs and are compensated for it. Workers also 

receive training on radiation protection. For these reasons, the staff has taken the approach 

that the draft accident risk guidelines for workers can be higher than those for the public. 

However, since the accident risk guidelines correspond to a level of risk considered to be 

negligible additional risk, where further reductions may not be warranted, perhaps this level 

should be the same for workers as for the public. The appropriateness, practicality, and 

regulatory coherence of such an approach will be evaluated further. 

(2) Application of uniform or activity-specific accident risk guidelines for workers 

The draft accident risk guidelines for workers are intended for uniform application across all 

NMSS/FSME-regulated activities. However, it may be more reasonable to adjust these accident 

guidelines, depending upon the risk in comparable industries, while at the same time retaining 

the principle of keeping the risk a small fraction of the accident risk in the comparable industries. 

Also, the risk from routine operations may be a consideration in setting accident risk guidelines. 

These issues are to be evaluated further. 

(3) Application of the accident risk guidelines on an industry-wide or site-specific 

 basis 

The calculation of risk for regulated activities, where only small numbers of people are exposed 

to the risk (such as is the case for many NMSS/FSME-regulated activities), is very sensitive to 

how this population is identified and modeled. The more people in the population at risk, the 

less sensitive the calculations are to specific individuals in the model. Accordingly, application 

of the risk guidelines across an entire industry could result in more realistic average risk 

estimates but could limit their application to site-specific issues. The advantages and 

disadvantages of each approach will be assessed during the application phase. 
 I-2 

(4) Extent of application of the accident risk guidelines to NMSS/FSME-regulated 

activities (i.e., is there sufficient guidance available for those regulated activities 

(e.g., high-level waste and decommissioning) that already rely on quantitative 

criteria to address accident risk) 

Certain NMSS/FSME-regulated activities already have quantitative acceptance criteria that 

include the consideration of accident risk. It may not be appropriate to apply additional accident 

risk guidelines to these activities. Accordingly, an evaluation will be made during the application 

period to determine whether or not certain regulated activities should be excluded. 

(5) Consideration of the accident risk from chemical exposure in the scope of the risk 

guidelines 

NRC is responsible for regulating the chemical hazards associated with certain NMSS/FSMEregulated activities (e.g., uranium enrichment). However, risk assessments have not 

traditionally included chemical risk. The need for, and practicality of, including such a risk in the 

scope of risk-informed NMSS/FSME-regulated activities will be assessed during the application 

period. 

ISSUES WITH IMPLEMENTATION

(6) Determining the approach for the preliminary draft accident risk guidelines that is 

best suited for each NMSS/FSME-regulated activity 

In response to an Advisory Committee on Nuclear Waste and Materials (ACNW&M) 

recommendation, the latent cancer QHG values, which are in units of fatalities per year, can be 

converted into equivalent doses per year by using the ICRP conversion factors of 5x10-4

fatalities/rem for the public, and 4x10-4 fatalities/rem for workers. 

(7) The need for subsidiary accident risk guidelines for some materials or waste 

activities 

The development of reactor safety goals led to establishing two subsidiary objectives; namely, 

CDF (preventive measure) and LERF (mitigative measure). Subsidiary objectives may be 

useful because they are easier to use and could provide guidance on measures such as 

prevention versus mitigation. As it gains more experience from applying a risk-informed 

approach to NMSS/FSME-regulated activities, the staff may determine the need for developing 

subsidiary objectives for some activities. 

(8) Determining the appropriate population to be considered in estimating the risks 

and evaluating against the risk guidelines 

The staff chose the term “average member of a population at significant risk” for estimating the 

individual risk associated with the draft risk guidelines. This is analogous to the concept of an 

average member of a critical group or “reasonable maximally exposed individual,” as 

recommended by ACNW&M. However, the population at risk varies by regulated activity and is 

a key factor in the assessment. The appropriate population will need to be defined for each 

type of activity regulated by NMSS/FSME. 
 I-3 

(9) Application of defense in depth in a risk-informed decisionmaking approach 

As discussed in the agency’s Strategic Plan, defense in depth is an element of NRC’s safety 

philosophy that employs successive compensatory measures to prevent accidents or lessen 

their effects. Defense in depth ensures that key safety functions are not dependent upon a 

single element of design, construction, maintenance, or operation. For example, defense in 

depth can provide for multiple lines of defense, where necessary, to address uncertainties. 

Preliminary high-level guidance on the application of this philosophy to NMSS/FSME-regulated 

activities has been included in the draft guidelines and will be tested and further refined in the 

application period. 

(10) Development and application of standards for nonreactor risk assessment quality 

and documentation 

The quality of a risk assessment is a key factor for ensuring sound risk-informed 

decisionmaking. There is an ongoing effort in RES to develop standards for PRA quality and 

documentation for reactor applications. The staff will consider the question of what constitutes 

quality in nonreactor risk assessments (such as ensuring realistic conservatism) after acquiring 

additional experience with specific risk-informed NMSS/FSME applications, and will determine 

the need for a standard or standards in this area. 
APPENDIX J 

APPLYING THE RIDM METHODOLOGY 

TO FUEL CYCLE FACILITIES 
 J-1 

APPENDIX J 

APPLYING THE RIDM METHODOLOGY 

TO FUEL CYCLE FACILITIES 

J.1 Review Red Oil Accidents in the Mixed-Oxide Fuel Fabrication 

Facility 

The NRC is in the process of reviewing a license application to construct and operate a facility 

to manufacture mixed-oxide (MOX) fuel at the U.S. Department of Energy’s (DOE’s) Savannah 

River site in South Carolina (Refs. J.1, J.2, J.3 and J.4). The responsibility for ensuring that the 

facility is designed, constructed, and operated safely resides with the facility operator Duke 

COGEMA Stone & Webster (DCS). The NRC's role is to provide sufficient oversight and 

regulation to ensure that public health and safety, the common defense and security, and the 

environment remain protected. 

The NRC is undertaking a review of the processes proposed to be employed at the MOX facility 

to ensure that the facility is operated within the envelope of the applicable regulations governing 

safety contained in the Code of Federal Regulations, Title 10 Part 70. Specifically, the NRC 

staff has been using RIDM methodology to assess the risk of red oil excursions in the MOX 

facility. The analysis performed by the NRC was based on limited design information available 

in the MOX fuel fabrication facility (FFF) Construction Authorization Request (CAR) (Ref. J.1). 

Red oil is defined as a substance of varying composition formed when an organic solution, 

typically tri-butyl phosphate (TBP) and its diluent, comes in contact with concentrated nitric acid 

at a temperature above 120 °C. The solvent TBP is often used to extract uranium or plutonium 

from acid solutions. Even though red oil is relatively stable below 130 °C, it can decompose 

explosively when its temperature is raised above 130 °C. It is important to analyze the risk from 

a red oil accident at the MOX FFF because several red oil accidents (Ref. J.5) have occurred in 

the United States and overseas (for example, at the Hanford Site in 1953, at the Savannah 

River Site in 1953 and 1975, and at the Tomsk-7 site at Seversk, Russia). 

Since the analysis of red oil accident in the MOX FFF was based on limited design information 

available in the CAR, the analysis included a preliminary risk assessment of certain systems 

vulnerable to the red oil hazard. The results of this red oil excursion analysis were very helpful 

to the NRC review staff in focusing attention on items important to safety. Considerably more 

detail on the design of safety controls is now available in the MOX FFF operating license 

application. NRC would like to have the red oil analysis updated based on information provided 

in the operating license application. 

J.2 Prioritize Processes and Event Sequences 

The MOX FFF applicant has performed an Integrated Safety Analysis (ISA), which attempts to 

identify all event sequences that could result in high or intermediate consequences, as defined 

in 10 CFR Part 70.61. The MOX FFF application contains roughly 80 processes, and each 

process could have a dozen or more event sequences. NRC needs to define a methodology 

and establish criteria to prioritize the processes and event sequences since there are too many 

sequences for a detailed risk assessment. The criteria should derive from two primary factors; 
 J-2 

1) the likelihood that there is a design deficiency having the particular event sequence and 2) 

the effect on safety if the deficiency exists. For the first factor, some experience has been 

compiled in NMSS/Division of Fuel Cycle Safety and Safeguards (FCSS) Interim Staff Guidance 

(ISG) documents. For the second factor, consideration should be given for how a deficiency 

could affect subsidiary factors such as individual risk, defense in depth, safety margins and 

collective risk. An effort is under way to use simplified methods to assess only the higher risk 

sequences. Examples of simplified methods are available in NMSS/FCSS ISG-01 and NUREG1718, “Standard Review Plan for the Review of Application for a Mixed Oxide (MOX) Fuel 

Fabrication Facility.” The processes and event sequences would be prioritized to focus on 

criticality safety and chemical hazards. 

J.3 Evaluate Process Hazards Analysis in Integrated Safety 

Assessment 

The first step in an Integrated Safety Analysis (ISA) under 10 CFR Part 70 Subpart H is a 

Process Hazard Analysis (PHA). The PHA uses systematic techniques to attempt to identify all 

sequences of potential events in the facility that could lead to high or intermediate 

consequences ( > 5 rem to the public or > 25 rem to a worker). Since the completion of the 

baseline ISAs by existing fuel cycle facilities in October 2004, events have occurred at these 

facilities which could have led to high consequences and which had not been identified in the 

PHA/ISA. These unidentified events are referred to by staff as “unanalyzed events”, since they 

were not identified or analyzed in the ISA. 

Since events have occurred at fuel cycle facilities that had not been identified in the PHA, NRC 

would like to evaluate the effectiveness of the Process Hazard Analysis (PHA) in the ISA and 

document some insights from previous PHA work. An effort is under way to evaluate the 

effectiveness of PHA as performed by fuel cycle facility licensees and identify possible 

deficiencies in licensee PHA processes. 

In order to produce lessons learned and general insights that can be applied in future ISA 

reviews, the staff would seek answers to the following questions: 

(1) Why were these events not identified? 

(2) Is there a weakness in the methods used by the particular licensees where the events 

occurred? 

(3) Are there other contributing causes? 

(4) How great is the concern that other unanalyzed event sequences exist? 

(5) What could be done that might improve this situation? 

J.4 References 

J.1 AMixed Oxide Fuel Fabrication Facility Construction Authorization Request,” Duke 

Cogema Stone & Webster, in transmittal letters to the U.S. Nuclear Regulatory 

Commission, October 31, 2002 through February 9, 2005. 
 J-3 

J.2 Draft Letter Report, ”Risk-Informed Review of the MOX Facility Licensing Application 

with Focus on Potential Hazards in the MOX Facility,” Brookhaven National Laboratory, 

August 2006. 

J.3 Draft Letter Report, “Risk Assessment of Red Oil Excursions in the MOX Facility,” 

Brookhaven National Laboratory, November 2006. 

J.4 Letter Report, “Risk-Informed Review of the MOX Facility Licensing Application,” 

Brookhaven National Laboratory, February 2007. 

J.5 U.S. Defense Nuclear Facilities Safety Board, “Control of Red Oil Explosions in Defense 

Nuclear Facilities,” DNFSB/TECH-33, November 2003. 
APPENDIX K 

ISSUE CHARACTERIZATION 
 K-1 

APPENDIX K 

ISSUE CHARACTERIZATION 

The purpose of this guidance (Refs. K.1, K.2) is to characterize the issue, in terms of the 

physical impact on the facility and the potential impact on safe operation, well enough to make 

an initial assessment of those organizations that must be involved and of the information 

needed to perform the later steps in the process (e.g., determine which options make sense, 

decide which technical approaches are appropriate). 

The following items should be included in a detailed description of issue: 

• Describe the issue. 

• Identify how the issue was identified. 

• Identify the structures, systems, and components (SSCs) or operational characteristics 

affected. 

• Describe the nature of the effect on the identified SSCs or operational characteristics. 

• Identify the safety function(s) affected. 

• Identify the plant conditions/ operating environments under which the issue would affect 

the safety of the plant. 

• Identify the regulations (or other requirements/ commitments such as design basis, 

licensing basis, generic letters) that may be challenged by this issue. 

• Assess the expected duration of the issue (when applicable)? 

• How long has the issue or condition been present? 

• Does it appear that this is/was a temporary condition? 

• Is the effect static, or is it likely to worsen with time? 

• Identify potential internal (e.g., line and senior management, other divisions or offices, 

etc.) who may use or be impacted by the risk. 

• Identify external stakeholders (other government agencies, industry, affected state and 

local governments, affected citizens, etc.) who may use or be impacted by the risk. 

Additional guidance on Issue Characterization may be obtained from NUREG-BR-0184, 

“Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory Commission.” (Ref. K.3). 
 K-2 

References 

K.1 U.S. Nuclear Regulatory Commission, “An Approach for Using Probabilistic Risk 

Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing 

Basis,” Regulatory Guide 1.174, Revision 1, November 2002. 

K.2 U.S. Nuclear Regulatory Commission, LIC-504, Revision 2, “Integrated Risk-Informed 

Decisionmaking Process for Emergent Issues,” February 12, 2007. 

K.3 U.S. Nuclear Regulatory Commission, NUREG/BR-0184, “Regulatory Evaluation 

Technical Evaluation Handbook,” January 1997. 
APPENDIX L 

DEFINING ALTERNATIVE ACTIONS 
 L-1 

APPENDIX L 

DEFINING ALTERNATIVE ACTIONS 

The purpose of this activity is to define the decisionmaking environment, define the alternative 

actions, describe the decision criteria for acceptance/rejection, describe the factors that 

determine the analysis approach, and generate a preliminary list of alternative actions/options. 

To do this, staff should complete the following tasks: 

• Define the alternative actions for resolution. These could include, for example, shutting 

the facility down immediately, shutting it down within a specified time period, continuing 

operation with the implementation of compensatory actions (for example, continuing 

operation with increased monitoring), or issuance of an immediately effective order to 

place or maintain the facility in a safe condition, or delaying the decision until more 

information is available. 

• Describe how the alternative action addresses the regulatory issue. 

• Describe what tools, analysis, resources, and costs are needed to implement the 

alternative action/option. 

• Describe the guidelines or criteria for acceptability or rejection of each alternative action. 

Staff should focus considerations on maintaining defense in depth and safety margins, 

and minimizing the risk impact. 

• Describe the factors that determine the approach to the analysis of the regulatory issue 

and the selection of actions. A variety of factors, technical and non-technical, can 

influence this decision. For example, resource limitations, particularly those related to 

the time available to make a decision, can be an important factor in determining the 

available options. 

NUREG-BR-0184 (Ref. L1) provides additional guidance on how to define alternative actions. 

References 

L.1 U.S. Nuclear Regulatory Commission, NUREG/BR-0184, “Regulatory Evaluation 

Technical Evaluation Handbook,” January 1997. 
APPENDIX M 

INITIAL RISK ASSESSMENT 
 M-1 

APPENDIX M 

INITIAL RISK ASSESSMENT 

After the regulatory issue is characterized, staff should make an initial risk-informed evaluation. 

This activity will not only determine and document the significance of the issue, but also provide 

a base case to facilitate the assessment of various options developed later. Also, much of the 

information gathered and analytical work performed at this point will be directly applicable to the 

risk-informed evaluation of each option: 

• Is the regulatory issue risk-significant enough that an immediate facility shutdown should 

be considered? 

• Is defense in depth significantly degraded? (e.g., multiple barriers are moderately to 

significantly degraded; functional redundancy or diversity is significantly compromised; 

vulnerability to single failures is significantly increased, etc.). 

• Is there a significant loss of safety margin? 

• Is the issue of low risk? If “yes” then document the finding of low risk. 

• Are there other NRC processes for dealing with this issue? If “yes,” initiate the 

applicable processes. 

• Are there generic implications associated with this issue, either already identified or 

identified as a potential generic issue? Generic implications may trigger other agency 

processes. 

• Which NRC organizations may be affected by or concerned about this issue? 
APPENDIX N 

ASSESSING THE IMPACT OF THE ISSUE 

ON DEFENSE IN DEPTH 
 N-1 

APPENDIX N 

ASSESSING THE IMPACT OF THE ISSUE 

ON DEFENSE IN DEPTH 

It is generally assumed that if the current regulations are met, there is adequate defense in 

depth at least before the condition/issue arose. However, the analysts should assess the effect 

of the condition/issue on defense in depth (Refs. N.1, N.2, N.3). The analyst should first 

consider which of the high level aspects of defense in depth is affected by the issue (for 

example, barrier integrity, emergency preparedness). The analyst should also assess the 

effectiveness of compensatory measures (for example, operator actions) to compensate for the 

degradation of defense in depth. It is important to note that for an event sequence whose 

outcome is a given level of consequences and a given level of uncertainty in the risk, there 

should be a minimum level associated with defense in depth or safety margins. Thus, a 

relaxation of a safety requirement that reduces below the minimum level for defense in depth or 

safety margins should be rejected. 

The following questions are needed to help the analyst to assess the effects of the 

condition/issue on defense in depth: 

 

Assessment of impact of issue on barrier integrity 

Assess impact on barrier integrity (i.e., degradation of the effectiveness of barriers). 

• Does the issue significantly change the failure probability of any individual barrier? 

• Is the degradation mechanism understood and information (e.g., test or operational data) 

available regarding the degradation-time relationship for short-term and long-term 

solutions? 

• Is the independence of barriers degraded? If so, which barriers? 

• Does the issue introduce new or additional failure dependencies among barriers that 

significantly increase the likelihood of failure compared to the existing conditions? 

• Does the issue result in a significant increase in the existing challenges to the integrity of 

the barriers? 

Assessment of potential for impact on multiple layers of defense in depth 

• Are the remaining elements of defense in depth intact? What are they and what is the 

reason for assuming they are intact? 

The intent of this question is to ascertain that the independence of the different layers of 

defense in depth is not compromised. 

Assessment of whether the risk model can address the defense in depth element 

• Can the impact of the degradation be quantified and evaluated through the risk model? 
 N-2 

Assessment of effectiveness of option in maintaining defense in depth 

The analyst needs to consider how a given option changes the defense in depth assessment. 

The analyst should use the defense in depth guidance above when considering the various 

alternative actions: 

• Does the option propose actions that can compensate for the degradation of defense in 

depth? 

• Discuss the proposed actions. Explain how and to what degree the action(s) can be 

successful (what level of confidence can be associated with this compensatory 

measure). 

• Does the option identify a programmatic activity that is proposed as a compensatory 

measure for the identified issue? For example, reliance on operator actions as monitors 

of plant conditions. 

• Describe how the option addresses degradation in defense in depth. 

• Identify sources of uncertainty with respect to (1) the assessment of the impact of the 

degradation of defense in depth, and (2) either the compensatory measures or 

monitoring approach. 

• List assumptions made to address the uncertainties and how they support the option. 

Assess the confidence level in the option. 

• Document why the methods used in the analyses above are considered adequate to 

support the conclusions. 

References 

N.1 J.N. Sorensen, G.E. Apostolakis, T.S. Kress, and D.A. Powers, “On the Role of Defense 

 in Depth in Risk-Informed Regulation,” presented at PSA ‘99. 

N.2 U.S. Nuclear Regulatory Commission, “Risk-Informed and Performance-Based 

Regulation”, White Paper, 1999. 

N.3 U.S. Nuclear Regulatory Commission, Letter from B.J. Garrick and D.A Powers to R.A 

Meserve, "Use of Defense in Depth In Risk-Informing NMSS Activities," May 25, 2000. 
APPENDIX O 

ASSESSING THE IMPACT OF THE ISSUE 

AND ALTERNATIVE ACTIONS ON SAFETY MARGINS 
 O-1 

APPENDIX O 

ASSESSING THE IMPACT OF THE ISSUE 

AND ALTERNATIVE ACTIONS ON SAFETY MARGINS 

Margins can be (1) conformance to codes and standards or their alternatives approved for use 

by the NRC, (2) design margins, (3) safety margins or safety limits, or (4) safety analysis 

acceptance criteria in the licensing basis. It is important to note that for an event sequence 

whose outcome is a given level of consequences and a given level of uncertainty in the risk, 

there should be a minimum level associated with defense in depth or safety margins. Thus, a 

relaxation of a safety requirement that reduces below the minimum level for defense in depth or 

safety margins should be rejected. 

The following questions are needed to help the analyst to assess the effects of the 

condition/issue on safety margins: 

Assessment of issue on safety margin 

• Define “safety margins” relevant to this application. 

• What is the basis for the original safety margin and how much conservatism was built in? 

• Where was safety margin lost or degraded? 

• Identify which aspect of safety margin is compromised and describe the impact. Provide 

an assessment of the actual extent of loss of safety margin, if measurable or otherwise 

determinable. 

• Document an assessment of the significance of the loss of safety margin, including 

assumptions made as to the degree of loss, and the expected consequences of 

functional failure of the affected elements. Document the uncertainty associated with an 

evaluation of the available margin. 

The results of this assessment can be used to guide the uncertainty evaluation for the risk 

analysis and will provide useful information to help in the overall decisionmaking process. In 

evaluating safety margins, we can also look at the risk profile of the plant. It would be important 

for the decision-maker to know if the issue creates or exacerbates a situation where risk is 

dominated by a few elements (structure, systems and components (SSCs) or operator actions) 

or a few accident sequences. 

• Can the loss of safety margin be quantified in such a way as to provide input to a PRA 

evaluation? 
 O-2 

Assessment of alternative actions on safety margin 

The analyst needs to consider how an alternative action changes the safety margins 

assessment. The analyst should use the following questions to assess the impact of the 

alternative action on safety margin. 

• Describe how the option addresses such degradation in safety margin. 

• Are there measures that can be taken to compensate for the potential impact of a loss of 

margin? 

• Describe any compensatory measures and how they address this degradation. 

Compensatory measures may be proposed by the licensee or by the staff to identify 

potential options to address the issue. For example, compensatory actions could take 

the form of monitoring the rate of degradation. Proof of viability would likely require an 

assessment of the significance of the initial degradation. 

• Identify sources of uncertainty with respect to (1) the assessment of the impact of the 

degradation of safety margin (the base case), and (2) either the compensatory measures 

or monitoring approach. 

• List assumptions made to address the uncertainties and how they support the alternative 

action. Assess the confidence level in the alternative action. 
APPENDIX P 

REACHING CONSENSUS ON A RECOMMENDATION 
 P-1 

APPENDIX P 

REACHING CONSENSUS ON A RECOMMENDATION 

This appendix is intended to assist the risk management team in reaching a consensus. In that 

step, the technical staff involved with the analysis will summarize the results for each decision 

option. The principal analysts in each discipline (e.g., PRA, engineering, licensing, etc.) will 

participate in the integration process with other technical staff through a facilitated discussion. 

The goal of this step is to come to agreement on a recommendation to take forward to decisionmakers. This group, the integration team, will work to achieve consensus through the following 

process: 

• Summarize the results of individual assessments and present to the group. 

• Led by a team leader chosen for his or her facilitation skills by the lead organization 

branch chief, discuss the results and evaluations of individual assessments. (The team 

leader must facilitate the sharing of information and deliberation.) It is critical during this 

discussion to raise issues, ask questions, and raise and address concerns about risk 

information, data sources, and other related subjects. The group discussion may identify 

the need for additional analyses, reframing of the issue, the involvement of additional 

staff, or other issues. 

• With the help of the team leader, decide upon a decision option to recommend to 

decision-makers. As part of these deliberations, the following questions should be 

considered for each input to the decision: 

3⁄4 Do the results of the assessment support the option? 

3⁄4 Is the appropriate regulatory principle met? 

3⁄4 What is the basis for this conclusion? 

3⁄4 Are the analysis tools and supporting data technically adequate? 

9 If the tools are not adequate, describe the inadequacy and potential 

impacts on the results. 

9 If the supporting data are inadequate, describe the data limitations and 

potential impacts on the results. 

3⁄4 Is the conclusion clear and unambiguous? 

3⁄4 What are the uncertainties that affect this assessment? 

3⁄4 What confidence do we have in the results/conclusion? 

3⁄4 What is the significance of the results? 

3⁄4 In the case that an individual assessment does not support the option, do the 

results of the other assessments have more weight? 

3⁄4 How important is this specific assessment to this particular option? 
 P-2 

• Attempt to reach consensus on the driving factors or considerations. Consensus is 

defined as agreement among the integration group members on which decision option is 

considered “preferred” for taking forward to decision-makers. Given the potentially 

subjective nature of the risk-informed process, the team may not be able to reach a 

consensus. If the integration team cannot achieve consensus, options include the 

following: 

3⁄4 The dissenters can agree to let the group move on, comparable to abstaining in 

voting. This minority opinion will be communicated to the decision-maker along 

with the recommended decision option(s). 

3⁄4 Iterate back through the process and conduct additional analysis. 

3⁄4 If the disagreement cannot be reconciled by additional information or analysis, 

seek management guidance to review the analysis and make a decision based 

on available analyses. 

• Document the results of the assessment as described above.