## Review of DOE’s Planned Change Request to Modify the WIPP Panel Closure System

Contract Number EP

- D

- 10

- 042 Work Assignment No. 4

- 02

Prepared for:

U.S. Environmental Protection Agency

Office of Radiation and Indoor Air Center for Waste Managemen t and Regulations

1310 L Street N.W. Washington, DC 20005

Jonathan P. Walsh Work Assignment Manager

Prepared by:

S. Cohen & Associates 1608 Spring Hill Road Suite 400 Vienna, Virginia 22182

November 20, 2013i 

### TABLE O F CONTENTS

Acronyms

................................ ................................ ................................ ................................ ....... vi

Executive Summary

................................ ................................ ................................ ..................... v iii

1.0 Introduction

................................ ................................ ................................ .......................... 1

1.1 Background

................................ ................................ ................................ .............. 1

1.2 Relevant Regulatory Requirements

................................ ................................ ......... 2

1.2.1 EPA

– 40 CFR 194

................................ ................................ ...................... 2

1.2.2 40 CFR 191 Subpart A

................................ ................................ ................. 4

1.2.3 40 CFR 264, 268 (RCRA)

................................ ................................ ........... 5

1.2.4 Mine Safety and Health A dministration

................................ ...................... 5

2.0 Brief History of PCS Design

................................ ................................ ................................ 6

2.1 Background

................................ ................................ ................................ .............. 6

2.2 Pre

- CCA Proposed Designs

................................ ................................ ..................... 8

2.3 Final Design

– Option D Concrete Monolith

................................ ........................... 8

2.4 DOE Post

- Option D Design Alternatives

................................ .............................. 10

2.4.1 2002 Proposed PCS Design Modifications

................................ ................ 11

2.4.2 2006 Proposed PCS Design Modifications

................................ ................ 12

2.5 2011 PC3R Planned Change Requests

................................ ................................ .. 13

3.0 2012 DOE Planned Change Request for PCS Design

................................ ....................... 16

3. 1 Rationale for ROM Salt Design

................................ ................................ ............. 16

3.2 ROM Salt Design Materials and Components

................................ ....................... 16

3.3 Analysis of Features, Events, and Process es

................................ ......................... 17

3.3.1 SNL’s Identification of Potentially Relevant FEPs

................................ ... 18

3.3.2 EPA Review of SNL’s FEP Assessment

................................ ................... 20

3.3.3 Adequacy of SNL’s FEP Review

................................ .............................. 22

3.4 Development of PCS

- 2012 Performance Assessment

................................ ........... 22

3.4.1 ROM Salt PCS Design

................................ ................................ ............... 22

3.4.2 ROM Salt Modeling

................................ ................................ ................... 23

3.4.3 PCS DRZ Modeling

................................ ................................ ................... 28

3.4.4 Development of the PCS

- 2012 PA Numerical Model

............................... 33

3.5 PCS

- 2012 PA Results and Analysis

................................ ................................ ...... 42

3.5.1 Results Compared with PABC 2009 PA

................................ ................... 43

3.5.2 Cuttings and Cavings Releases

................................ ................................ .. 53

3.5.3 Direct Brine Releases

................................ ................................ ................. 53

3.5.4 Spallings Releases

................................ ................................ ...................... 55

3.5.5 Culebra Releases

................................ ................................ ........................ 56

3.5.6 Total Releases

................................ ................................ ............................ 57

3.6 SNL Parameter Sensitivity Analysis

................................ ................................ ...... 60

3.6.1 Parameter Regression Results

................................ ................................ .... 61

3.6.2 Sensitivity of Cuttings and Cavings Releases

................................ ............ 63ii 3.6.3 Sensitivity of Direct Brine Releases

................................ .......................... 64

3.6.4 Sensitivity of Spallings Releases

................................ ............................... 65

3.6.5 Sensitivity of Culebra Releases

................................ ................................ . 67

3.6.6 Summary of Sensitivity Results

................................ ................................ . 68

3.7 EPA Assessment of DOE’s Analysis

................................ ................................ ..... 69

4.0 EPA Sensitivity Analysis

................................ ................................ ................................ ... 70

4.1 Sensitivi ty to DRZ Permeability

................................ ................................ ............ 70

4.1.1 Conceptualization of DRZ Healing

................................ ........................... 70

4.1.2 Parameters Modified for the DRZ Adjacent to the ROM Sa lt

................... 72

4.1.3 Modified Parameter Values for Sensitivity Study

................................ ..... 73

4.1.4 Results of DRZ Sensitivity Analysis

................................ ......................... 75

4.2 Sensitivity to Capillary Pressure in the ROM Salt

................................ ................. 78

5.0 EPA Quality Assurance Review

................................ ................................ ........................ 82

5.1 Parameter Changes

................................ ................................ ................................ . 82

5.2 Database Migration

................................ ................................ ................................ 83

6.0 Summary and Conclusions

................................ ................................ ................................ 85

7.0 References

................................ ................................ ................................ .......................... 87

Appendix A: Quality Assurance Review of the PCS

- 2012 PA Parameter Database

.................... 91

Appen dix B: Review of WIPP Performance Assessment Computerized Database (PAPDB) Migration

................................ ................................ ................................ ............................ 92iii 

### LIST OF TABLES

Table 2

- 1. Panel Closure and DRZ Properties for the CCA and PA VT Analyses

........................ 10

Table 2

- 2. Panel Closure and DRZ Properties for Explicit Representation of the

Option D PCS

................................ ................................ ................................ ............. 11

Table 2

- 3. Panel C losure and DRZ Properties for the 2006 PCS PA

................................ ............ 13

Table 3

- 1. FEPs Identified by SNL

- Related to the Panel Closure Modification

........................... 19

Table 3

- 2. DCCA FEPs Excluded from the CCA that Relate to Panel Closures

.......................... 19

Table 3

- 3. Two

- Phase Flow Properties for the ROM Salt

................................ ............................. 25

Table 3

- 4. Two

- Phase Flow Properties for the PCS DRZ

................................ ............................. 30

Table 3

- 5. BRAGFLO Salado Flow Modeling Scenarios

................................ ............................. 34

Table 3

- 6. Int rusion Scenarios Used in Calculating Direct Brine Releases

................................ .. 54

Table 3

- 7. Total Normalized Release Statistics for the PCS

- 2012 and PABC

- 2009 PAs

............. 60

Table 3

- 8. Stepwise Ranked Regression Results for Mean Total Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

................................ ................................ ..... 62

Table 3

- 9. Stepwise Ranked Regression Results for Mean Total Rel eases,

PCS

- 2012 Replicates 2 and 3

................................ ................................ ..................... 63

Table 3

- 10. Stepwise Ranked Regression Results for Mean Cuttings and Cavings Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

................................ ................................ ..... 64

Table 3

- 11. Stepwise Ranked Regression Results for Mean Cuttings and Cavings Releases,

PCS

- 2012 Replicates 2 and 3

................................ ................................ ..................... 64

Table 3

- 12. Stepwise Ranked Regression Results for Mean Direct Brine Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

................................ ................................ ..... 65

Table 3

- 13. Stepwise Ranked Regression Results for Mean Direct Brine Releases,

PCS

- 2012 Replicates 2 and 3

................................ ................................ ..................... 65

Table 3

- 14. Stepwise Ranked Regression Results for Mean Spallings Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

................................ ................................ ..... 66

Table 3

- 15. Stepwi se Ranked Regression Results for Mean Spallings Releases,

PCS

- 2012 Replicates 2 and 3

................................ ................................ ..................... 66

Table 3

- 16. Stepwise Ranked Regression Results for Mean Culebra Releases,

PCS

- 2012 and PABC

- 2009 Replic ate 1

................................ ................................ ..... 67

Table 3

- 17. Stepwise Ranked Regression Results for Mean Culebra Releases,

PCS

- 2012 Replicates 2 and 3

................................ ................................ ..................... 68

Table 3

- 18. Su mmary of Contributions to Variability for Parameters Related to the

Proposed PCS Design Change

................................ ................................ .................... 68

Table 4

- 1. Log Permeability Ranges for ROM Salt and the Adjacent DRZ from 200 to

10,000 Years, and for Intact Salado Halite

................................ ................................ . 73

Table 4

- 2. Parameter Values Modified for the DRZ Sensitivity Study

................................ ......... 74

Table 4

- 3. Total Normaliz ed Release Statistics for the DRZ Test and PCS

- 2012 PA

.................. 75iv 

### LIST OF FIGURES

Figure 2

- 1. Plan and Cross Section Views of a Typical Waste Panel (Dimensions in feet)

........... 7

Figure 2

- 2. Four Optional PCS Designs Presented in the 1996 WIPP Compliance

Certification Application

................................ ................................ .............................. 9

Figure 2

- 3. Typical ROM S alt Panel Closure for Drifts without Explosion Walls

....................... 15

Figure 2

- 4. Typical ROM Salt Panel Closure for Drifts with Explosion Walls

............................ 15

Figure 3

- 1. Illustration of E1 and E2 Borehole Intrusions

................................ ............................ 34

Figure 3

- 2. PABC

- 2009 BRAGFLO Grid and Material Map

................................ ....................... 37

Figure 3

- 3. PCS

- 2012 BRAGFLO Grid and Material Map, Years 1 to 100

................................ . 38

Figure 3

- 4. PCS

- 2012 BRAGFLO Grid and Material Map, Years 100 to 200

............................. 39

Figure 3

- 5. PCS

- 2012 BRAGFLO Grid and Material Map, Years 200 to 10,000

........................ 40

Figure 3

- 6. PCS

- 2012 BRAGFLO Grid and Material Map for an E1 Intrusion

........................... 41

Figure 3

- 7. Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC),

Scenario S1

- BF

................................ ................................ ................................ ........... 44

Figure 3

- 8. Overall Means of Waste Panel Brine Saturation (WA S_SATB),

Scenario S1

- BF

................................ ................................ ................................ ........... 44

Figure 3

- 9. Overall Means of Waste Panel Gas Generation (GASMOL_W),

Scenario S1

- BF

................................ ................................ ................................ ........... 45

Figure 3

- 10. Ov erall Means of Waste Panel Pressure (WAS_PRES), Scenario S1

- BF

................ 45

Figure 3

- 11. Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S2

- BF

................ 46

Figure 3

- 12. Overall Means of Cumulative Brine Inflow to the Waste Panel

(BRNWASIC), Scenario S2

- BF

................................ ................................ ................. 47

Figure 3

- 13. Overall Means of Waste Panel Brine Saturation (WAS_SATB),

Scenario S2

- BF

................................ ................................ ................................ ........... 47

Figure 3

- 14. Overall Means of Waste Panel Gas Generation (GASMOL_W),

Scenario S2

- BF

................................ ................................ ................................ ........... 48

Figure 3

- 15. Overall M eans of Brine Flow up the Borehole (BNBHUDRZ), Scenario S2

-

BF

................................ ................................ ................................ ............................... 48

Figure 3

- 16. Overall Means of Waste Panel Pressure (WAS_PRES), Scenarios S1

- BF and

S4

- BF for Years 0 to 1,000

................................ ................................ ......................... 50

Figure 3

- 17. Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S4

- BF

................ 50

Figure 3

- 18. Overall Means of Cumulative Brine Inflow to the Waste Panel

(BRNWASIC), Scenarios S1

- BF and S4

- BF for Years 0 to 1,000

............................ 51

Figure 3

- 19. Overall Means of Cumulative Brine Inflow to the Waste Panel

(BRNWASIC), Scenario S4

- BF

................................ ................................ ................. 51

Figure 3

- 20. Overall Means of Waste Panel Brine Saturation (WAS_SATB),

Scenario S4

- BF

................................ ................................ ................................ ........... 52v

Figure 3

- 21. Overall Means of Waste Panel Gas Generati on (GASMOL_W),

Scenario S4

- BF

................................ ................................ ................................ ........... 52

Figure 3

- 22. Overall Means of Brine Flow up the Borehole (BNBHUDRZ),

Scenario S4

- BF

................................ ................................ ................................ ........... 53

Figure 3

- 23. PCS

- 2012 PA and PABC

- 2009 Overall Mean CCDFs for

Normalized Direct Brine Releases

................................ ................................ .............. 55

Figure 3

- 24. PCS

- 2012 PA and PABC

- 2009 Overall Mean CCDFs for Normalized

Spallings Releases

................................ ................................ ................................ ...... 56

Figure 3

- 25. Overall Means of Brine Flow up the Borehole (BNBHUDRZ),

Scenario S6

- BF

................................ ................................ ................................ ........... 56

Figure 3

- 26. PCS

- 2012 PA Primary Components Cont ributing to

Total Normalized Releases

................................ ................................ ......................... 57

Figure 3

- 27. PCS

- 2012 PA Replicate 1 Total Normalized Releases

................................ ............. 58

Figure 3

- 28. PCS

- 2012 PA Replicate 2 Total Normalized Releases

................................ ............. 58

Figure 3

- 29. PCS

- 2012 PA Replicate 3 Total Normalized Releases

................................ ............. 59

Figure 3

- 30. PCS

- 2012 PA Confidence Limits on Overall Mean for

Total Normalized Releases

................................ ................................ ......................... 59

Figure 3

- 31. PCS

- 2012 PA and PABC

- 2009 Overall Mean CCDFs for

Total Normalized Releases

................................ ................................ ......................... 60

Figure 4

- 1. PCS 2012 PA and DRZ Test, Mean Total Normalized Releases

............................... 76

Figure 4

- 2. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Spallings Releases

................................ ................................ ................................ ...... 76

Figure 4

- 3. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized Direct Brine Releases

................................ ................................ .............. 77

Figure 4

- 4. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized Cuttings and Cavings Releases

................................ ............................... 77

Figure 4

- 5. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized S PALLINGS Releases

................................ ................................ ............ 79

Figure 4

- 6. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized Direct Brine Releases

................................ ................................ .............. 80

Figure 4

- 7. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Total Normalized Releases

................................ ................................ ......................... 80vi 

### ACRONYMS

CBFO Carlsbad Field Office

CCA Compliance Certification Application

CCDF Complementary Cumulative Distributi on Function

CFR Code of Federal Regulations

CRA Compliance Recertification Application

DBRs direct brine releases

DCCA Draft Compliance Certification Application

DOE U.S. Department of Energy

DP screened in for disturbed performance scenarios

DRZ Di sturbed Rock Zone

EPA U.S. Environmental Protection Agency

ERMS Electronic Record Management System

FEPs Features, Events, and Processes

LWA Land Withdrawal Act

m 2 square meter

m 3 cubic meter

m Pa M ega Pascal

MSHA Mine Safety and Health Administrat ion

Pa P ascal

PA Performance Assessment

PABC Performance Assessment Baseline Calculations

PAPDB Performance Assessment Parameter Database

PAVT Performance Assessment Verification Test

PC3R Panel Closure Redesign and Repository Reconfiguration Performan ce Assessment

PCR Planned Change Request

PCS Panel Closure System

PCSIA Panel Closure System Impact Assessment

PMR Permit Modification Request

QA Quality Assurance

R 2 Coefficient of Determination

RCRA Resource Conservation and Recovery Actvii

ROM Run

- of

- Mine

SMC Salado Mass Concrete

SNL Sandia National Laboratories

SO

- C screened out based on no, low, or beneficial consequence

SO

- P screened out based on a very low probability of occurrence

SO

- R screened out based on a regulatory directive

SRRC Standard Rank Reg ression Coefficient

TBM Technical Baseline Migration

TRU Transuranic

TSD Technical Support Document

UP screened in for undisturbed performance scenarios

VMPa Milli Pascal

VOC Volatile Organic Compound

VVP Verification Validation Plan

WIPP Waste Isolation P ilot Plant

WPC WIPP Panel Closureviii 

### EXECUTIVE SUMMARY

In September 2011

, the Department of Energy ( DOE ) submitted a Planned Change Request (PCR) to the Environmental Protection Agency ( EPA ) to modify the Panel Closure System (PCS) at the Waste Isolation P ilot Plant (WIPP). DOE proposed to replace the Option D design

, which is required by Condition 1 of 40 CFR Part 194 , Appendix A, with a new design called the

Run

- of

- Mine Salt Panel Closure System (ROMPC or ROMPCS)

. DOE later submitted a performance assessm ent, the PCS

- 2012 PA, to demonstrate that with the redesigned panel closure, WIPP remains compliant with the disposal regulations at 40 CFR Part 191. This Technical Support Document discusses the Agency’s review and approval of DOE’s PCS

- 2012

PA.

A panel is a group of mined rooms in the WIPP underground, connected by two drifts. Following completion of waste disposal activities in each panel, DOE intends to seal these drifts with engineered structures called panel closures. DOE’s previous Option D design included a concrete block explosion wall and a poured concrete monolith. The ROM PCS design consists of two steel bulkheads placed at each end of 30.5 meters ( 100 feet ) of mined salt.

The approach used to develop the PCS

- 2012 PA was to maintain the same m odel input parameters and ranges used in the 2009 P erformance A ssessment Baseline Calculation (PABC

09) PA, with the exception of parameters that represent the panel closure system

. Thus, the PCS

2012 PA results may be compared to those obtained in the PAB C

- 2009 PA as a means to quantify changes in the PA results solely based on the panel closure design changes.

The Option D concrete monolith is a rigid mass with constant physical properties, whereas the ROMPC is emplaced as a relatively loose material wh ich is compressed into a solid mass due to creep closure of the repository. The PCS

- 2012 uses three time periods to simulate the creep closure process . During the first tim e period (0

– 100 years)

, the salt begins to consolidate from a n unconsolidated materi al to a rigid mass. During the second time period (100

– 200 years)

, the salt

continues to consolidate and is in the process of healing . The third time period (200

– 10,000 years) represents a state of consolidation that approaches intact halite. A range of

pa rameter values is used for each of the three specified time periods to capture uncertainties in the ROM salt properties as the panel closure system consolidates.

Since the panel closure is a system that includes both the closure itself and the surroundin g disturbed rock zone (DRZ), the Agency closely examined the effects that the DRZ permeability has on repository performance during time period T3. The DOE assumes that the DRZ surrounding the panel closures will also heal with time.

EPA concurred with D OE’s proposal to maintain the properties of the panel closure DRZ used to
model the Option D PCS, but EPA also investigated the impact on PA results if the PCS DRZ

was to heal even further and approach the permeability of intact salt.

The Agency concludes that DOE ha s successfully identified the principal features, events, and processes ( FEP s) necessary to achieve the intent of the PCS

- 2012 PA , and agrees with DOE’s

use of three time periods to represent progressive salt consolidation

. The Agency accepts t heix

values adopted for ROM salt density, porosity and permeability in the PCS

- 2012 PA and

considers the m to capture the range of uncertainty

. Based on the information presented by DOE and the subsequent sensitivity analyses performed by the Agency, EPA is s atisfied that the

revised PCS

- 2012 PA provides an adequate performance measure for the implementation of the ROMPCS at WIPP. The Agency concludes that the results of DOE’s PCS

- 2012 PA and the additional analyses conducted by the Agency adequately demonstr ate that the containment requirements of 40 CFR Part 191.13 continue to be met with ROM salt as the primary material in the PCS. The Agency, therefore, accepts DOE’s proposal to replace the mandatory Option D PCS design with the proposed ROMPCS design at W IPP.1 

### 1.0 INTRODUCTION

1.1 B ACKGROUND

The Waste Isolation Pilot Plant (WIPP), located in southeastern New Mexico, has been developed by the Department of Energy (DOE) for the deep disposal of transuranic (TRU)

waste. The Environmental Protection Agency ( EPA ) regulates containment of TRU waste in

accordance with EPA’s radioactive waste disposal standards at Code of Federal Regulations (CFR), Title 40 Part s 191 and 194 . Condition 1 of 40 CFR Part 194 (EPA 1998) specifies that the Panel Closure System (PCS) to be used in the WIPP repository is the one designated as “Option D” in the Compliance Certification Application (CCA) (DOE 1996 a )

. The Option D design

consists of a 3 .7 meter ( 12 feet ) thick concrete block explosion wall separated from a 7.9 meter ( 26 feet ) t hick Sa lado Mass Concrete (SMC) plug by a 9.1 meter (30 feet) open drift

.

DOE submitted a PCR to the EPA in September 2011 (DOE 2011 b ) requesting Agency approval

to modify the PCS from the Option D design to a design composed primar ily of run of mine (RO M) salt. The PA submitted with th at PCR is called the PC3R PA

. It included the ROM salt

PCS design as well as proposed alterations to the repository configuration

. The Agency requested DOE to perform another PA where only the PCS was modified from the curr ent PA baseline established by the 2009 Performance Assessment Baseline Calculation ( PABC

- 2009)

.

F ollowing a series of technical exchanges between EPA and DOE, DOE submitted the PCS

- 2012

PA . T he major differences in the calculations from the PABC

- 2009 wer e (1) the parameterization of the panel closure based on salt instead of concrete, ( 2) the time dependence

on the panel closure properties, and ( 3) the reduction in the DRZ permeability at 200 years.

This Technical Support Document (TSD) discusses the Ag ency’s review and assessment of

DOE’s PCS

- 2012 PA supporting the request to modify the PCS design. The organization of this

TSD is as follows:

Section 1.2 describes the original motivation for Condition 1, which requires the Option D panel closure, and di scusses the regulatory requirements that impact the design of the panel closure.

Section 2 .0 provides a brief history of the evolution of the PCS design and its representation in WIPP performance assessment prior to the current PCR and PA. Section 2 .1 des cribes where the panel closures are to be installed in the waste panel access drifts

.

Section 3 .0 discusses in detail the rationale used by DOE to adopt the ROM PCS design

, the materials and components used in the design , and parameterization

, implementati on , and results

of the PCS

- 2012 PA.

Section 4 .0 discusses two separate analys e s p erformed by the Agency and compares their results

with the PCS

- 2012 PA

. Section 4.1 assesses whether lowering the long

- term permeability of the

healed PCS DRZ to values that approach those of intact halite affect repository containment requirements. Section 4. 2 discusses an analysis to assess whether the modeling results are

sensitive to capillary pressure in the ROM salt

.2

Section 5 .0 describes the Agency’s quality assurance reviews of DOE’s database migration and of the parameter dataset used in the PCS 2012 PA. Detailed information on these reviews is included in Appendices A and B.

Section 6 .0 presents EPA’s overall summary and conclusions of DOE’s PCS 2012 PA results

. Th e Agency found that with a lowered PCS DRZ permeability, calculated average releases still remained below regulatory limits.

1.2 R ELEVANT REGULATORY REQUIREME NTS

1.2.1 EPA

– 40 CFR PART 194

DOE’s original Compliance Certification Application (CCA) included panel closures in the design of the repository

. As noted in EPA’s documentation,

[CCA] Chapter 3.3.2 (p. 3

- 27) indicates that the original intention of the panel closure system was to support Resource Conservation and Recovery Act (RCRA) closure of the waste disposal panels and to prevent the release of potentially unacceptable levels of volatile organic compounds from the filled waste disposal panels during waste management operations in the remainder of the repository (up to 35 years). The CCA stated (p. 3

- 3 3) that although the design of the panel closure system was based on its need to protect human health and the environment during the operations period, the use of these systems will also influence fluid connections between panels during the post

- closure ph ase. ( DOE 1996a, CARD 14, 45

– 46 )

While panel closures are not considered engineered barriers per 40 CFR Part 191 b ecause the presence of panel closures has potential to influence the behavior of the repository, and therefore long

- term compliance, detaile d information on the panel closure design was included in the CCA to satisfy 40 CFR Part 194

. Section 194.14, Content of Compliance Certification Application, requires:

(a ) A current description of the natural and engineered features that may affect the pe rformance of the disposal system. The description of the disposal system shall include, at a minimum, the following information:

... ( b)(1) “Information on materials of construction including, but not limited to: Geologic media, structural materials, engin eered barriers, general arrangement, and approximate dimensions.”

Options for closing filled waste panels were described in Detailed Design report for an Operational Phase Panel Closure System , DOE/WIPP

- 96

- 2150 (DOE 1996b) , included in the CCA as Appendix PCS. The contents of Appendix PCS are discussed in Section 3 .0 of this document. Appendix PCS proposed a composite system including a concrete block isolation wall

that would be installed between the waste face and the main drift immediately after the pan el was filled, and a concrete monolith installed afterwards. The concrete block wall could be a thinner3

“construction

- isolation wall,” or a thicker “explosion

- isolation wall” to protect against a postulated hydrogen or methane explosion. Likewise, the mono lith could be of simple rectangular construction, filling the open drift, or the disturbed rock zone (DRZ) could be excavated to remove fractures and an enlarged monolith installed to prevent further fracturing. The design options, according to DOE, were t o “[provide] flexibility to satisfy the design limit

for the flow of volatile organic compounds ( VOCs ) out of the panel” (Sec tion 3.2.1)

. Appendix PCS proposed that DOE would determine which panel closure configuration to install

at the time of closure bas ed on the age of the drift being closed, the degree of fracturing, and the nature of the waste in the panel, but did not offer any specific criteria for making such a determination. Chapter 3.3.2 of the CCA (discussed on pages 114

– 115 of EPA’s TSD for CARD 14) stated that the panel closures would maintain their structural integrity into the period of long

term compliance, that the permeability of the panel closure and surrounding DRZ will control long

- term fluid flow into or out of the waste panel, and that degradation of the freshwater concrete barrier component may occur by interaction with brine. The CCA did not include any analysis of the impact of the panel closure design on long

- term system performance. Without this information, the Agency felt it was necessary for panel closures emplaced in the repository to exhibit the physical properties that had been used to represent them in the compliant performance assessment. As stated in the Federal Register notice for certification:

Condition 1 of the certifi cation relates to the panel closure system... The EPA based its certification decision on DOE’s use of the most robust design (referred to in the CCA as “Option D ) .” The Agency found the Option D design to be adequate, but also determined that the use of Sal ado mass concrete

– using brine rather than fresh water

– would produce concrete seal permeabilities in the repository more consistent with the values used in DOE’s performance

assessment

. Therefore, Condition 1 of EPA’s certification requires DOE to imple ment the Option D panel closure system at WIPP, with Salado mass

concrete replacing fresh water concrete ( Federal Register , Vol. 63

, No.95,

p. 27355 ). ( E mphasis added

. )

Thus, as part of EPA’s certification decision, the following text was added to 40 CFR 194:

Appendix A, Condition 1: §194.14.14(b), Disposal system design, panel closure
system. The Department shall implement the panel seal design designated as Option D in Docket A

- 93

- 02, Item II

- G

- 1 (October 29, 1996, Compliance Certification Application s ubmitted to the Agency). The Option D design shall be implemented as described in Appendix PCS of Docket A

- 93

- 02, Item II

- G

- 1, with the exception that the Department shall use Salado Mass concrete (consistent with that proposed for the shaft seal system, a nd as described in Appendix SEAL of Docket A

- 93

- 02, Item II

- G

- 1) i nstead of fresh water concrete.

EPA’s certification decision explicitly discusses the possibility of DOE proposing a re

engineered panel closure design, with the following language:

Nothin g in this condition precludes DOE from reassessing the engineering of the panel seals at any time

. Should DOE determine at any time that improvements in4

materials of construction techniques warrant changes to the panel seal design

, DOE must inform EPA

. If EPA concurs, and determines that such changes constitute a significant departure from the design on which certification is based, the Agency is authorized under § 194.65 to initiate a rulemaking to appropriately modify the certification ( Federal Register , V ol. 63 No.95, p. 27362 )

.

DOE proposed and withdrew requests to modify the panel closure design in 2001, 2002, and 2006, including a request to defer entirely to NMED on the design of the panel closure. EPA declined to do so, and a letter from EPA to DOE f rom 2007 clearly articulates Agency’s position on the regulation of panel closures. While acknowledging that panel closures are “primarily operational period features that are designed to control volatile organic compound emissions during disposal operatio ns,” and that “EPA’s Compliance Criteria (40 CFR 194) do not require a panel closure system for WIPP , panel closures are part of the disposal system and their characteristics must be properly incorporated into performance assessment [under 40 CFR 194.14].” The letter also states that “EPA has determined that a change in the panel closure design is a significant departure from the most recent compliance application. Therefore, under §194.65, EPA is required to address the change in panel closure design throu gh the Agency’s rulemaking process” ( Reyes 2007 )

.

1.2.2 40 CFR Part 191 Subpart A

EPA’s regulatory role at WIPP does not solely pertain to long

- term performance. 40 CFR 191 Subpart A [ 40 CFR 191.03 (b) ] sets dose limits to the public during the operational pha se of the repository.

Subpart A does not prescribe how WIPP should meet the operational dose limits . EPA’s approach has been to ensure that DOE has a reliable monitoring system in place to measure any releases and calculate any potential dose to the publ ic [See Guidance for the Implementation of EPA’s Standards for management a n d Storage of Transuranic Waste (40 CFR Part 191, Subpart A) and the Waste Isolation Pilot Plant, EPA 402

- R

- 97

- 001]. EPA annually inspects DOE’s monitoring system, as well as their operational safety and emergency response procedures.

Although the detonation or deflagration due to a buildup of explosive gases in a waste panel could theoretically result in a release, the CCA indicated that this is an extremely remote possibility

. EP A accepted this, and has never considered the panel closure to be necessary for compliance with Subpart A. EPA’s 2007 letter approved delaying installation of the final panel closures, stating that “EPA regulations do not require that any panel closure sys tem be installed prior to closing of the disposal system. EPA believes DOE’s propose d gas monitoring in the panels would provide useful information for long

- term performance” ( Reyes 2007 ).

The results of monitoring VOCs and explosive gases in Panels 3 and 4 are included in DOE’s

PC R (DOE 2011b). There is no indication that explosive conditions will be encountered in the underground

. The Agency concludes that the proposed changes to the panel closure system do not impact DOE’s ability to comply with 40 CFR Part 191 Subpart A.5

1.2.3 40 CFR Parts 264, 268 (RCRA)

WIPP is subject to the Resource Conservation and Recovery Act (RCRA). As stated in Appendix PCS, the panel closure system was originally intended “to support a Resource Conservation and Recovery Act (RCRA) Part B permit application and a no

- migration variance petition ” [ DOE/WIPP

- 96

- 2150 (DOE 1996b ) ]

. At the time, DOE recognized that it would be exceedingly difficult to chemically treat mixed hazardous and transuranic waste to meet the land disposal

restrict ions found at 40 CFR Part 268, and instead sought a no

- migra tion variance petition under § 268.6 ( DOE 1996a )

.

In 1996, Congress amended the WIPP Land Withdrawal Act (LWA) Section 9(a)(1) to exempt transuranic waste at WIPP from “treatment standards promulg ated pursuant to section 3004(m)” and “the land disposal prohibitions in section 3004(d), (e), (f), and (g) of the Solid Waste Disposal Act,” ( 1992, 104 th Congress ) obviating the need for a no

- migration variance. The panel

closure design remains a conditio n of WIPP’s RCRA Part B permit to reduce the exposure of workers and the public to VOCs.

The State of New Mexico has adopted EPA’s hazardous waste regulations (40 CFR Parts 264, 268, and 270) in title 20, Chapter 4, Part 1 of the New Mexico Administrativ e Code C 4.1 (Federal Register 60, No. 79 20240

, April 25, 1995). A RCRA permit modification to allow the redesigned panel closure will be considered separately by NMED.

1.2.4 M ine S afety and H ealth A dministration

Section 11 of the WIPP LWA (P.L. 102

- 579) esta blishes that the Mine Safety and Health Administration (MSHA) as the regulator of mine safety at WIPP

. According to the CCA, WIPP is Category IV mine under 30 CFR 57 Subpart T, meaning that non

- combustible ore is extracted, and any methane liberated by min ing is not capable of creating an explosive environment ( DOE

1996a ). In Category IV mines, Subpart T regulations require testing for methane at least once per shift (§57.22226), but do not specify requirements for the constructions of “barriers and stoppin gs” as they do for other mine categories. In its design documents, however, DOE has adopted the use of the term “substantial barrier.” MSHA defines “substantial construction” as

“ Construction of such strength, material, and workmanship that the object will withstand air blasts, blasting shock, ground movement, pressure differentials, wear, and usage which may be expected to occur in the mining environment” (30 CFR § 57.22002)

. DOE has stated that the “substantial barriers” referenced in the proposed panel cl osure design documents will be steel

bulkheads.6 

### 2.0 BRIEF HISTORY OF PCS DESIGN

Section 2 .0 provides a summary of the evolution of the PCS design and its representation in WIPP performance assessment. Alternative designs considered by DOE prior to the CCA a re described, which led to four design option s presented in the CCA. One of those designs, identified as Option D, was approved by EPA and NMED as suitable for licensing. Difficulties in implementing the Option D design were subsequently identified by DOE, and alternative PCS designs involving the use of ROM salt were proposed by DOE. This process culminated in the proposed ROMPCS design reviewed in this TSD. The Agency has followed the PCS design process, and both the Agency and DOE have gained an increase d understanding of how the PCS impacts repository performance.

2.1 BACKGROUND

The rooms of each WIPP waste panel are connected to the main repository by two drifts

, the air intake drift and the air exhaust drift. Following completion of waste disposal activi ties in a panel, the panel is closed by sealing these two drifts. Plan and cross section views of a typical waste panel are shown in Figure 2

- 1. The air intake and exhaust drifts are shown in Cross

Sections C

- C and D

- D. As shown in the figure, the air inta ke drift is slightly larger than the air

exhaust drift.7 From DOE 2013

Figure 2

- 1 . Plan and Cross Section Views of a Typical Waste Panel ( D imensions in feet)8

2.2 PRE

- CCA PROPOSED DESIGNS

Various materials h ave been considered for the WIPP panel closures since repository design studies began. A 1986 design considered concrete, salt blocks, and bentonite fill ( DOE 1986). A 1990 design considered two concrete plugs keyed into the drift walls with pre

- consolidat ed salt (tamped salt with salt bricks) between them and crushed salt filling the drift on the outer sides (Nowak et al. 1990). A 1995 design consisted of a concrete block wall on the waste end, a ‘simple crushed salt’ fill extending 1.5 times the diameter of the drift, a construction bulkhead,

and a 2

- times entry diameter length of concrete with grouting of the DRZ (DOE 1995).

2.3 FINAL DESIGN

– OPTION D CONCRETE MO NOLITH

DOE’s 1996 Compliance Certification Application (CCA) presented four optional PCS desi gns, shown in Figure 2

- 2

. The designs consisted of concrete monoliths with and without removal of the surrounding DRZ, combined with thick explosion isolation walls or thin construction isolation walls constructed from concrete cinderblocks (DOE 1996 a ). Th ese options included performing contact grouting around the concrete monolith as needed.

DOE stated that one of the four options would be selected for each filled waste panel based on ground conditions and timing of installation relative to repository cl osure (DOE 1996 a ). Although specific criteria for making the selection were not provided, DOE expected that the most robust panel closures would be needed for those waste panels that would be filled first because they would have the longest functional live s during repository operations. DOE concluded that “...Option D may be appropriate for panels whose entries require significant removal of the DRZ and whose closure will precede final facility closure by more than 20 years. Panel 1 is the most likely candida te for this type of closure” (DOE 1996 a ).

As described in Section 1

. 2. 1 above , the Agency concluded that certification of compliance could not be granted when a potentially important feature affecting post

- closure repository performance was not specified but left to future decisions based on incomplete criteria. The Agency therefore mandated the selection of PCS Option D and the use of Salado Mass Concrete (SMC) rather than fresh water concrete for the monolith as Condition 1 of the Final Certification

Rul emaking for 40 CFR Part 194 (EPA 1998). This decision was based on the conclusion that Option D represented the most robust of the alternative designs provided by DOE and that SMC, made with simulated Salado brine, would provide a long

- term seal performanc e that was more consistent with the properties of the generic panel closure represented in DOE’s CCA PA and PAVT ( EPA 1998). The properties of the panel closure and surrounding DRZ used in the CCA PA and PAVT are presented in Table 2

- 1. I n those analyses t he DRZ around the panel closure was given the same properties as the DRZ around the waste panel and did not account for any possible healing that might occur. The panel closure design was not explicitly represented and the properties of both the panel clos ure and the DRZ were fixed for the entire 10,000

- year regulatory time period.9 From DOE 1996 a

Figure 2

- 2

. Four Optional PCS Designs Presented in the 1996 WIPP Compliance Certification Application10

Table 2

- 1 . Panel Closure and DRZ Properties for the CCA and PAVT Analyses

Property Parameter Type Parameter Value Panel Closure (CCA and PAVT) Permeability (X, Y, and Z

– directions) Constant 1 × 10 - 15 m 2

Effective P orosity Constant 0.075 DRZ (CCA) Permeability (X, Y, and Z

– directions) Constant 1 × 10 - 15 m 2
Effective Porosity Cumulative Min: 0.0029

Max: 0.0329

Mean: 0.0157 DRZ (PAVT) Permeability (X, Y, and Z

- directions Loguniform Min: 3.98 × 10 - 20 m 2

Max: 3.16 × 10 - 13 m 2
Effective Porosity Cumulative Min: 0.0029

Max: 0.0329

Mean: 0.0157 From Vugrin and Wagner 2006; DOE 2011 b ; and Hansen et al. 2002

.

2.4 DOE POST

- OPTION D DESIGN ALTE RNATIVES

After the CCA was approved by the Agency, DOE conducted studies of i nstallation of the

concrete monolith. In 2001

, DOE proposed several modifications, including replacing SMC with generic, salt

- based concrete, replacing fresh water grouting with salt

- based grouting, using local carbonate river rock aggregate in lieu of cru shed quartz, and an option to allow steel forms to be left in place or removed (Triay 2001). These modifications were accompanied by a PCR from

DOE that was later withdrawn.

Representation of the Option D PCS in WIPP PA also evolved during this time. The Option D concrete monolith design was explicitly incorporated into the 2002 Technical Baseline Migration PA (Stein 2002). This representation was carried forward to the CRA

- 2004 and CRA

- 2009 PAs. The representation of Option D included the impacts of a rig id structure on the surrounding

DRZ. The properties of the panel closure and surrounding DRZ used for explicitly representing the Option D PCS are presented in Table 2

- 2 . The Option D panel closure permeability used in PA is a composite effective permeabil ity of the concrete monolith, the open drift, and the explosion wall. The permeability of the concrete monolith alone was assigned a constant value of

1.78 × 10 - 19 m 2 , equal to the mode of the distribution for SMC ( DOE 2011 b ).11

Table 2

- 2 . Panel Closure and DRZ Properties for Explicit Representation of the Option D PCS

Property Parameter Type Parameter Value

Panel Closure Composite Permeability (along the drift) Constant 9.01 × 10 - 19 m 2
Composite Permeabilit y (perpendicular to the drift) Constant 1.93 × 10 - 13 m 2
Effective Porosity Constant 0.15

DRZ

Permeability (X, Y, and Z

– directions) Triangular Min: 2 × 10 - 21 m 2

Max: 1 × 10 - 17 m 2

Mode: 1.78 × 10 - 19 m 2
Effective Porosity Cumulative Min: 0.0039

Max: 0.05 48

Mean: 0.0211

From DOE 2011 b ; Camphouse et al. 2012 b

.

The Agency received a PCR in 2002 proposing a PCS design consisting of an explosion wall and a closure of ROM salt. This design change was accompanied by a request for EPA to remove

Condition 1 fr om the WIPP certification and to cede approval of the final PCS design to the New Mexico Environment Department (Triay 2002). The Agency informed DOE that the PCR would not be considered until after the 2004 Compliance Recertification Authorization (Marcin owski 2002). DOE resubmitted essentially the same 2002 PCS design in a new PCR in November of 2006, but eliminated the requests for removal of Condition 1 and ceding of approval (Moody 2006). This PCR was accompanied by a new performance assessment that in dicated the repository would remain in compliance with the proposed PCS design modification (Vugrin and Dunagan 2006). DOE’s proposed 2002 and 2006 PCS design modifications are described in the following paragraphs.

2.4.1 2002 Proposed PCS Design Modifications

DOE’s proposed 2002 design modifications consisted of a 30

- foot concrete block explosion wall constructed on the waste disposal side of the closure drift and 30.5 meters ( 100 feet ) of ROM crushed salt placed against the outer side of the explosion wall (S aeb and Case 2002). This alternate design was called the WIPP Panel Closure (WPC) system. The rationale to adopt the WPC design instead of the Option D design was primarily based three considerations: ( 1) the WPC was simpler to engineer and install, ( 2) th e MSHA specifications for mine safety during the operational phase could be met, and ( 3) long

- term repository performance would not be negatively impacted over the regulatory time frame.

The characteristics of ROM salt behavior during the post

- closure pe riod differ from th ose of the concrete monolith in the Option D design. Because the ROM salt is easily compressed as the waste panel access drifts creep close, its density is increased and its permeability is reduced. In contrast, the properties of the rig id Option D cement monolith are expected to be relatively stable for 10,000 years. Consequently, assessments of the gradual decrease in ROM salt permeability with time were considered early on

. The concrete block explosion walls were only intended to

last during the 35 year operational period and these walls were therefore not taken into consideration in long

- term PA modeling.12

Hansen and Thompson (2002) estimated that consolidation of the ROM salt due to creep closure would reduce its density

, approaching intact halite by 200 years. Since density and permeability

are related for halite , once the ROM salt has reached a final density, its permeability would

decrease no further . They estimated that the permeability of healed ROM salt would range

between 10 - 15 and 10 - 19 m 2 during the 200 to 10,000 year time frame. Their estimates were based on observed salt creep measurements and laboratory experiments

. By comparison, the effective permeability of the Option D PCS design along the drift was given a constant val ue of 9.01 × 10 - 19 m 2 for the entire regulatory time frame and took into account all three components of the Option D design

- a 12 foot thick concrete block explosion wall, a 30 foot open area, and a 26 foot long concrete monolith (Hansen et al. 2002).

Two discrete time periods were modeled to capture the change in ROM salt properties during the regulatory time frame. The first time period, 0 to 200 years, was meant to capture the flow properties of a loose and less compact, lower density ROM salt. The s econd time period, 200 to 10,000 years, was meant to capture a range of uncertain end

- point values when it was assumed the ROM salt would be compressed to its fully consolidated state.

SNL evaluated the impact of the proposed ROM salt PCS on repository p erformance by comparing the results of two PAs, one using the lower end

- point PCS permeability of

10 - 19 m 2 representing the Option D design, and the other using a higher end

- point PCS permeability of 10 - 15 m 2 representing the ROM salt design. This compari son was labeled the Panel Closure System Impact Assessment (PCSIA) PA

. The PCSIA PA results indicated that even though the permeabilit y of the two closure systems differed by four orders of magnitude, repository performance was nearly identical (Hansen 200 2 a ; Hansen 2002 b ). However, due to scheduling and regulatory time constraints the WPC design was not formally presented to EPA. The Option D design therefore continued to be used in the 2004 Compliance Rece rtification Application (CRA).

2.4.2 2006 Proposed PCS Design Modifications

In 2006

, DOE submitted another PCR to adopt a panel closure design that was the immediate predecessor to the 2012 design considered in this TSD. The 2006 design was similar to the ROM WPC design considered in the 2002 PCSIA PA and inc luded an explosion wall and 30.5 meters ( 100 feet ) of ROM salt (Vugrin and Dunagan 2006). The rationale to change from the Option D concrete monolith design to ROM salt was the same as in 2002 ; the ROM salt panel closure was cheaper and easier to install, and would provide the same safety during the operational period as the Option D design

. A 2006 PA was conducted using ROM salt as the PCS material with a few

modifications from the 2002 PA. The 2006 PA is called the PCS PA and the results wer e compared to the 2004 PA BC

.

The 2006 PCS PA was run using the same BRAGFLO grid as the 2004 CRA PA and PABC but modified to make the PCS permeability and porosity representative of ROM salt

. By 2006 there was a better understanding of how the properties of ROM salt ch ange due to consolidation. This resulted in modifying the estimated range of ROM permeability and porosity from that adopted

in 2002

. In the 2006 PCS PA

, the end

- point permeabilit y was slightly lower than th at assumed in

the 2002 PCSIA PA

. Similar to the P CSIA PA, creep closure compressed the R OM salt13

properties to asymptotic value s estimated around 200 years after repository closure . T wo discrete time periods were adopted in both PAs to capture the change in ROM salt properties . The first modeled time peri od was 0 to 200 years and the second time period was 200 to 10,000 years (Vugrin and Dunagan 2006). Table 2

- 3 provides the porosity, and permeability values that were assigned to each time period.

Table 2

- 3

. Panel Closure and DRZ Properties for the 2006 PCS PA

Time Period Property Parameter Type Parameter Value

Panel Closure

0

- 200 Years Permeability (X

- direction) Constant 8 × 10 - 15 m 2

Permeability (Y

- direction) Constant 8 × 10 - 12 m 2
Permeability (Z

- dir ection) Constant 8 × 10 - 12 m 2
Effective Porosity Constant 0.27

200

- 10,000 Years Permeability (X

- direction) Triangular Min: 1.6 × 10 - 23 m 2

Max: 4.0 × 10 - 23 m 2

Mode: 6.3 × 10 - 21 m 2
Permeability (Y

- direction) Constant 5 × 10 - 14 m 2
Permeability (Z

- direc tion) Constant 5 × 10 - 14 m 2
Effective Porosity Constant 0.05

DRZ

0

- 200 Years Permeability (X, Y, and Z

- directions) Loguniform Min: 4.0 × 10 - 20 m 2

Max: 3.2 × 10 - 13 m 2

Effective Porosity Cumulative Min: 0.0039

Max: 0.0548

Mean: 0.0211

200

- 10,000 Years Permeability (X, Y, and Z

- directions) Triangular Min: 2 × 10 - 21 m 2

Max: 1 × 10 - 17 m 2

Mode: 1.78 × 10 - 19 m 2
Effective Porosity Cumulative Min: 0.0039

Max: 0.0548

Mean: 0.0211 From Vugrin and Dunagan 2006; Camphouse et al. 2012b

.

The 2006 PCS PA results showed that repository performance using the ROM salt PCS remained within the regulatory limits (Vugrin and Dunagan 2006)

. However, the 2006 PCR was withdrawn due to scheduling and time constraints, and the Option D PCS was again used in the

PABC

- 2009 PA for the 2009 CRA.

2.5 2011 PC3R PLANNED CH ANGE REQUESTS

The PC3R PA was prepared by SNL to evaluate the combined impacts on long

- term WIPP repository performance of two PCRs submitted to EPA by DOE in 2011. These PCRs consisted of a proposal to relocate futu re waste Panels 9 and 10 to the south end of the repository and a proposal to replace the Option D PCS design with an ROM salt design (DOE 2011 a ; DOE 2011 b ). The impacts of these changes were determined by comparing repository performance calculated in the PC3R PA, which included the two proposed design changes, against the baseline PABC

- 2009, which included the current repository layout and the current Option D PCS design (Clayton et al. 2010).14

In order to isolate and assess the impacts of the proposed PC S redesign, the Agency requested that DOE prepare a PA that only addressed the proposed PCS redesign. As a res u lt, DOE presented an overview of their proposed approach , called the PCS

- 2012 PA, to the Agency at a Technical Exchange Meeting in April 2012 ( Ai r Docket A

- 98

- 49, Item II

- B2

- 81)

. The PCS

- 2012

PA is identical to the PABC

- 2009 PA with the exception that the modeled panel closure system
represent s the proposed ROM salt PCS. All other parameters, conditions, material properties, waste inventory, and mo deling scenarios are the same as those used in the PABC

- 2009 PA.

DOE’s current PCS design PCS

- 2012 PA consists of a minimum of 30.5 meter s (100 feet) of ROM salt placed within the panel access drift with a steel bulkhead positioned at each end.

Alternati vely, for waste Panels 1, 2, and 5 where explosion walls have already been constructed, the proposed PCS design consists of the explosion wall on the waste panel end, emplacement of

30.5 meter s ( 100 feet ) of ROM salt against the explosion wall, and a stand ard steel ventilation bulkhead on the outer end (Camphouse et al. 2012 a ). No concrete monoliths have yet been constructed and the proposed design does not include additional explosion walls. For long

- term performance, neither the explosion walls nor the st eel bulkheads are assumed to last and do not enter into the performance assessment modeling (Camphouse 2012c)

. The proposed modified

des igns are illustrated in Figures 2

- 3 and 2

- 4

. The Agency’s review of the PCS

- 2012 PA is

described in Section 3 .0

.15 Fr om DOE 2013

Figure 2

- 3 . Typical ROM Salt Panel Closure for D rifts without Explosion Walls

From DOE 2013

Figure 2

- 4 . Typical ROM Salt Panel Closure for Dr ifts with Explosion Walls16



### 3.0 2012 DOE PLANNED CHA NGE REQUEST FOR PCS DESIGN

Section 3 .0 addresses DOE’s analysis of its Planned Change Request (PCR) for modifying the PCS from the approved Option D design to the ROM salt design described in Section 2

. Sectio n 3 .0 describes DOE’s rationale for the proposed design change, the materials and components of the proposed design, the development of DOE’s PCS

- 2012 PA, the results of that PA, and the Agency’s evaluation of those results.

DOE documented its request to change the PCS design in a PCR submitted to EPA in September 2011 (DOE 2011b). The Agency’s evaluation of DOE’s proposed modeling approach of the

ROM salt for PCS

- 2012 PA is presented in Sections 3.4.2.1 and 3.4.2.2. The Agency’s evaluation of DOE’s propos ed modeling of the PCS DRZ is presented in Section 3.4.3. Additional details are presented in Appendix A: Parameter QA.

3.1 RATIONALE FOR ROM SA LT DESIGN

The most challenging aspect of implementing the Option D design is constructing the SMC

monoliths. SMC i s a salt saturated concrete developed for use in the WIPP Shaft Seals. During a series of test pours, DOE determined that SMC as formulated would not be able to meet the specification requirements for the Option D monoliths (DOE 2011 b ). DOE has, with the

a pproval of EPA and NMED, emplaced temporary closures in various filled panels while gathering information for a revised final closure design. That effort led to the currently proposed ROM salt PCS design. DOE has described its ROM salt design as being simp ler, easier and faster to construct, is less disruptive to facility operations, increas es the use of natural materials,

and is less expensive than the current Option D concrete monolith design, while also meeting the same operational performance requiremen ts as the Option D concrete monolith (DOE 2011 b ).

3.2 ROM SALT DESIGN MATE RIALS AND COMPONENTS

As described in Section 2 .0 , the ROM salt PCS design consists of two steel bulkheads (or one steel bulkhead and one concrete block explosion wall, if an explosion wall was previously constructed in the panel), and a minimum of 30.5 meters ( 100 feet ) of ROM salt between the two

bulkheads. Each bulkhead would consist of a steel frame covered with sheet metal. The bulkheads are designed to limit air flow through close d waste panels during WIPP’s operational period and have no post

- closure design function. After repository closure, DOE concludes that the bulkheads would corrode and be crushed by halite creep, and would therefore have a negligible effect on repository pe rformance (Camphouse 2012 c )

.

The waste disposal panels and associated access and ventilation drifts in the WIPP underground facility are excavated using a drum

- type continuous miner that shreds the intact halite into

centimeter size or smaller chunks tha t can be scooped and hauled like gravel. The ROM designation means that the salt used for the PCS is mined from the WIPP underground facility without grading or processing. The ROM salt material obtained from WIPP mining operations is planned to be deliver ed by haul truck or load haul dump units to the panel closure area and emplaced as

- is, in a loose state or possibly compacted in lifts. Water may also be added to facilitate compaction. The final closure material would be pushed into place to the degree17

pr acticable to minimize the gap between the salt and roof or side walls of the drift (Zimmerly

et al. 2012). In addition to its ease of handling, the ROM salt is a noncombustible, natural material that is chemically compatible wi th the repository environment.

DOE believes that the long

- term properties of ROM salt are also advantageous during the post

closure period due to its creep characteristics when under compressive stress. DOE’s JAS3D numerical modeling indicate s that following emplacement, an air gap wi ll form between the drift roof and top of the ROM salt because the initial rate of compaction of the ROM salt from its own weight exceeds the rate of drift creep closure. Two years after salt emplacement, the air gap is predicted to reach a maximum of 19 i nches and then begin to close due to inward creep of the drift roof. Modeling results indicate that the air gap closure would be complete after approximately 23 years. In addition to the roof collapse and gradual closing of the air gap, the permeability of the ROM salt will also decrease as the salt consolidates under its own weight (DOE 2013). After the air gap closes, air flow resistance would be predominately due to the consolidated ROM salt and surrounding disturbed rock zone (DRZ) rather than the steel bulkheads (DOE 2013). Although DOE’s modeling estimate of 23 years for air gap closure is approximate, the air gaps are expected to have closed in all panel closures by 100 years after repository closure when the period of institutional control of the WIP P site is assumed to end.

After the air gap closes, the ROM salt will be compressed by creeping halite in the drift roof, and to a lesser extent in the drift walls and floor. Although the period of time for this process is uncertain, after about 100 years the compression of the ROM salt is expected to significantly reduce its porosity and permeability. This reduction in porosity of the ROM salt creates a backpressure which would result in greater resistance to further compaction

. As the backpressure

increa ses the fractures and fissures in the surrounding DRZ would begin to close. This process has been called “healing” the DRZ because it eventually returns the DRZ to a state that approaches the very low porosity and permeability of intact halite. This healin g process is expected to be essentially complete within approximately 200 years (see discussion in

Section 3 .4.2.1). T herefore, in addition to its pre

- closure attributes of low cost and ease of handling, DOE concludes that the ROM salt and surrounding DRZ of the ROM salt PCS design

would also have the post

- closure attribute of a very low permeability for most of the 10,000

- year regulatory period.

3.3 ANALYSIS OF FEATURES , EVENTS, AND PROCES SES

DOE demonstrated the effects of the proposed ROM salt PCS design on long

- term repository performance by conducting a PA called the PCS

- 2012 PA. The first step in preparing a new PA is to identify and evaluate the features, events, and processes (called FEPs) associated with the proposed design changes that could affect repository performance. The focus of the FEP review is to identify new FEPs or changes to existing FEPs that may need to be included in the new PA as a result of changes that have occurred in repository design or conceptual models since the previous PA. Th e identified FEPs are screened for inclusion or exclusion in the new PA. FEPs may be screened in if they are potentially significant to repository performance, and such FEPs may already be included in the PA models. FEPs may be screened out if their probab ility of occurrence is low; if they have no, low, or beneficial consequences on repository performance; or if they are excluded on a regulatory basis. This process produces a comprehensive list of FEPs18

that are considered potentially relevant to WIPP perfo rmance. This list is called the FEPs

baseline.

The following screening classifications are used in WIPP FEP analyses:

DP Screened in for disturbed performance scenarios

UP Screened in for undisturbed performance scenarios

SO

- C Screened out based on no, l ow, or beneficial consequence

SO

- P Screened out based on a very low probability of occurrence

SO

- R Screened out based on a regulatory directive

The FEP review for the PCS

- 2012 PA was conducted by Sandia National Laboratories (SNL)

and is documented in Kir kes (2011). For the PCS

- 2012 PA, the baseline for comparison is the

PABC

- 2009 PA, which has been reviewed and accepted by EPA. The principal changes in the

PCS

- 2012 PA that potentially affect long

- term repository performance are a revised design for

the wa ste panel closures consisting of ROM crushed salt (a new repository feature), the modeling

of time

- dependent consolidation of the ROM salt panel closures due to halite creep (a new process), and the modeling of time

- dependent consolidation of the disturbed rock zones (DRZ) around the redesigned panel closures (a revised modeling approach)

.

3.3.1 SNL’s Identification of Potentially Relevant FEPs

SNL’s summary report for the PCS

- 2012 PA uses the FEP review described in Kirkes (2011),

which documents the FEP rev iew for the PC3R PA (Camphouse et al. 2012 a ) and which SNL concluded was applicable for the PCS

- 2012 PA since the PCS design had not changed. SNL’s reviews of potential impacts on the current FEP baseline were guided by SNL Specific Procedure

SP 9

- 4, Perfo rming FEPs Baseline Impact Assessments for Planned and Unplanned Changes (Kirkes 2009). The Agency focused its review on those FEPs related to the ROM salt PCS.

SNL’s screening review began by identifying all FEPs in the current baseline that may be impac ted by the PC3R planned changes. The FEPs identified through this process and related to the proposed ROM salt PCS are presented in Table 3

- 1. These FEPs include the geometric effects of the proposed panel closure redesign, the physical characteristics of the new panel closure material, consolidation of the ROM salt PCS and the surrounding DRZ, and stress

induced salt creep rates.

SNL’s next step was to identify any FEPs associated with the PCS design changes that are not

included in the current FEP baseli ne. This step was accomplished by reviewing the more extensive FEP list prepared in 1995 for the Draft Compliance Certification Application (DCCA).

Twelve additional FEPs relevant to the role of the PCS as a seal were identified and are listed in

Table 3

- 2 (Kirkes 2011).19

Table 3

- 1

. FEPs Identified by SNL

- Related to the Panel Closure Modification

Related FEPs Screening

Classifications Relationship to Proposed Change

W1 Disposal Geometry 1 UP The repository la yout must be adequately represented in the BRAGFLO grid

W109 Panel Closure Geometry UP Proposed PCS geometry differs from the

current PA baseline.

W110 Panel Closure Physical Properties UP Proposed PCS material properties differ

from the current PA basel ine.

W111 Panel Closure Chemical Composition 2 SO

- C (Beneficial) Proposed PCS chemical composition

differs from current PA baseline.

W112 Radiological effects on Panel Closures SO

- C Proposed PCS material properties differ

from current PA baseline.

W18 Di sturbed Rock Zone (DRZ) UP Proposed repository layout may affect the behavior of the surrounding DRZ

W20 Salt Creep UP Proposed repository layout may alter current salt creep expectations

W73 Concrete Hydration SO

- C Proposed PCS material properties diffe r from current PA baseline.

W113 Consolidation of Panel Closures UP Proposed PCS material properties differ

from current PA baseline.

W114 Mechanical Degradation of Panel Closures UP Proposed PCS material properties differ

from current PA baseline.

W115 Chemical Degradation of Panel Closures UP Proposed PCS chemical properties differ

from current PA baseline.

1 UP: Screened in, undisturbed performance scenario

2 SO

- C: Screened out, low or beneficial consequence

Source: Kirkes 2011, Table 1

Table 3

- 2

. DCCA FEPs Excluded from the CCA that Relate to Panel Closures

DCCA FEP Name Reason for Exclusion from the CCA Boundary Conditions

– Seals Considered a “modeling issue,” not a FEP

Correlation

– Seals Considered a “modeling issue,” not a FEP

Groundwater flow: initial conditions

- Seals Considered a “modeling issue,” not a FEP Time dependence

– Seals Considered a “modeling issue,” not a FEP Advection/dispersion: hazardous

constituents

– Seals Issues not regulat ed by 40 CFR 191

Diffusion: hazardous constituents

– Seals Issues not regulated by 40 CFR 191 Dissolution, speciation, sorption, precipitation:

hazardous constituents

– Seals Issues not regulated by 40 CFR 191

Buffer additives

– Seals Applies to alterna tive repository design Design modifications

– Seals Applies to alternative repository design Faulty seal emplacement

– Seals Applies to alternative repository design Inadequate seal or compaction: voidage

– Seals Applies to alternative repository design

Seal material deficiencies Applies to alternative repository design

Source: Kirkes 2011

SNL’s FEP review next evaluated if any FEPs listed in Table 3

- 2 should be included in the present baseline FEPs list. SNL determined that none of these additi onal FEPs warranted inclusion. Exclusion of four DCCA FEPs, because they are modeling issues, still holds. SNL concluded that the boundary conditions, correlations, initial conditions, and time dependence are necessary elements of models and should not be part of the FEP list. SNL also determined that20

the three “hazardous constituent” FEPs should continue to be excluded, because they are not part of the constituents regulated by 40 CFR Part 191.

One FEP listed in Table 3

- 2, Design modification

– Seals, was determined to be better handled by parameter and model changes, and should not be considered as part of the baseline FEP lists (Kirkes 2011). Four final seal

- related FEPs

— buffer additives, faulty seal emplacement, inadequate seal or compaction, and materi al deficiencies

— are all related to negligence or poor construction practices. SNL concluded that these are generally not considered FEPs, because they are more closely related to WIPP site operational practices and should be excluded from the

baseline FEP list.

SNL’s final FEP review step, as required by SP 9

- 4, Section 2.4.4, was to review the screening arguments and decisions associated with the FEPs listed in Table 3

- 1 and determine whether they remain valid. If the proposed PCS design changes did not i nvalidate, change, or render incomplete the existing screening arguments or decisions for the identified FEPs, then the current PA framework was considered suitable to account for the changes.

SNL determined that three FEPs listed in Table 3

- 1; W73 Concre te Hydration, W111 Panel Closure Chemical Composition, and W112 Radiolo gical Effects on Panel Closures, would continue to be screened out because of expected low consequence, as was done in the CRA

- 2009 FEPs review. Significant quantities of concrete are o mitted from the panel closure redesign; therefore, less thermal rise was expected (W73). Including radionuclide sorption and sequestration in the ROM salt would have a beneficial effect on performance, and so the process

is screened out (W111). Because of the low levels of radioactivity in WIPP waste, SNL expected that no radiological effects will be seen on the panel closure material (W112). SNL determined that these three FEPs can continue to be screened out, due to their continued low consequences

to WIP P performance.

The remaining FEPs listed in Table 3

- 1 have previously been screened in and are therefore currently incorporated in WIPP PA. SNL made the following determinations regarding these FEPs. FEPs associated with salt creep and changes in the stre ss field will remain unaffected

because of the PCS design changes and any effect is already included in the WIPP models. Panel

closure

- related FEPs continue to be included in the WIPP PA, and changes due to the proposed panel closure modification are captu red by modeling implementation changes, such as grid alterations and parameter changes (Kirkes 2011 ).

Kirkes (2011) concludes that no screening decision conflicts or impacts were identified as a result of SNL’s review, and therefore the current FEP baseli ne and PA framework are suitable to account for the change s in the PC3R (and, by extension, the PCS

- 2012) impact assessment.

3.3.2 EPA Review of SNL’s FEP Assessment

The Agency agrees that for screened

- in FEPs, the details of conceptual and numerical implement ation and parameterization can be considered modeling issues, and can be documented and justified in analysis plans and reports. The Agency’s reviews of DOE’s approach to parameterizing and modeling the screened

- in FEPs are presented elsewhere in this TSD. SNL’s21

analyses of the FEPs in Table 3

- 1 that were screened in are summ arized in Kirkes (2011 ). The Agency accepts the reasons presented by Kirkes for screening in those FEPs.

Screened

- out FEPs generally require more comprehensive justification as part of a FEP review

than screened

- in FEPs. This is because the treatment of screened

- in FEPs can be documented and justified in subsequent analysis plans and analysis reports, while screened

- out FEPs are not considered further. Depending on the issue of concern, adequate justification for screening out FEPs can range from reasoned arguments to detailed technical analyses. Three FEPs listed in

Table 3

- 1 that are potentially relevant to the proposed PCS design change were screened out, all on the basis of past scre ening conclusions and reasoned arguments of continuing low or beneficial consequence (Kirkes 2011). The Agency accepts the reasons given by SNL for screening out these FEPs.

The Agency also agrees that the FEPs derived from the DCCA and listed in Table 3

- 2 do not need to be added to the current FEP baseline, in part for the reasons given by SNL (Kirkes 2011) and in part because some of those FEPs are redundant to FEPs already in the baseline.

During its review, the Agency identified two additional, potenti ally relevant FEPs that are aspects of FEPs that are in the current FEP baseline. These additional FEPs are described below. The Agency concludes that although the two FEPs are potentially relevant but were not considered in SNL’s FEP review, neither is ex pected to have a significant impact on repository performance and their omission does not affect compliance.

3 .3.2. 1 Impact of Repository Gas Pressure on Reconsolidation of ROM Salt PCS

FEPs W25 Disruption Due to Gas Effects and W26 Pressurization are p otentially relevant to the proposed PCS design change, because high gas pressures could delay the consolidation of the ROM salt, thereby maintaining a higher permeability in the PCS for a longer period of time. Hansen and Thompson (2002) note that if gas g eneration occurs and gas penetrates the ROM salt, pore pressures may build up, leading to a slowing or stopping of consolidation. However, in response to this Agency concern, in the April and February 2012 technical exchange meeting s, documented in the Age ncy’s A ir Docket A

- 98

- 49, Item II

- B2

- 81, DOE Response to Technical Question 1.3c, and Air Docket A

- 98

- 49, Item II

- B2

- 80, D OE adequately demonstrated that significant gas pressure buildup is rare during early time when the porosity and permeability of

the R OM salt would be relatively high, and would therefore have little impact on the average releases that are of regulatory concern.

3 .3.2. 2 Impact of Lateral Halite Movement at the Unconstrained Ends of ROM Salt PCS during Creep Closure

As creep closure p rogresses, the ends of the emplaced ROM salt will be unconstrained and extruded laterally. This movement and lack of constraint is likely to result in a higher porosity and permeability at the ends of the ROM salt fill than in the center where greater conf inement is provided. This would limit the effectiveness of the end regions as panel closures and is an aspect

of FEPs W20 Salt Creep and W114 Mechanical Degradation of Panel Closures that was not considered in SNL’s FEP review. In response to this Agency c oncern, SNL presented the results

of a numerical model of panel closure consolidation in the April 2012 technical exchange22

meeting , documented in the Agency’s AIR Docket A

- 98

- 49, Item II

- B2

- 81, DOE Response to Technical Question 1.3e, that adequately demon strated that such end effects would have little impact on PCS performance.

3.3.3 Adequacy of SNL’s FEP Review

The Agency concludes that SNL’s FEP baseline review successfully identified the principal modeling changes that were necessary to achieve the purpose o f the PCS

- 2012 PA. Although the additional FEPs identified by the Agency are potentially relevant

, DOE has provided additional

information to screen out these FEPs

.

3.4 DEVELOPMENT OF PCS

- 2012 PERFORMANCE ASS ESSMENT

The PCS

- 2012 PA was built upon DOE’s ear lier representation s of a ROM salt PCS described

above in Section 2 .0. The Agency’s panel ROM salt design and modeling concerns were discussed in Technical Exchange Meetings between EPA and DOE, and in subsequent correspondence ( Air Docket A

- 98

- 49; Item II

- B2

- 81, DOE Response to Technical Question 1.3c,

and Air Docket A

- 98

- 49; Item II

- B2

- 80)

. The first of these meetings was held in Dallas, Texas, in February 2012, and the second was held in Albuquerque, New Mexico, in April 2012. The Agency’s preliminary co ncerns related to the ROM salt design and the modeling approach proposed by DOE for the PCS

- 2012 PA are summarized in the following subsections.

3.4.1 ROM Salt PCS Design

The basic components of the currently proposed design were introduced as part of the

afor ementioned PC3R PA. The Agency’s review of the ROM salt PCS design in that PA raised initial concerns that were also relevant to DOE’s final design as modeled in the PCS

- 2012 PA

.

3 .4.1.1 Elimination of Explosion Walls from PCS Design

Justification for eliminating explosion walls from the ROM salt PCS design was provided by DOE in the PCR submitted to EPA in September 2011 (DOE 2011 b ). This appendix describes the generation mechanisms of hydrogen and methane, the two flammable gases of most concern

in fi lled waste panels. The appendix also describe s hydrogen and methane monitoring results in the waste rooms of Panels 3 and 4, and present s a worst

- case analysis of hydrogen buildup in Panel 3. DOE concluded that explosion walls a re not necessary because mea sured levels of hydrogen and methane were well below the lower explosive limit.

3 .4.1.2 Elimination of Steel Bulkheads and Explosion Walls from Post

- closure Modeling

For long

- term performance, neither the explosion walls nor the steel bulkheads are assu med to last and do not enter into the performance assessment modeling (Camphouse 2012c)

. DOE explains that as a result of the loading caused by inward creep of the surrounding halite explosion walls will not be significant structures after the initial 100 year time period due to the brittle, non

- plastic behavior of concrete (Camphouse 2012c). Similarly, the PCS steel bulkheads are only designed to control the pressures of ventilation air and would not be sufficiently strong to impede drift creep closure.23

In view of the brittle nature of the concrete blocks, the high stresses that will eventually be imposed by creep closure, and the presence of explosion walls in only three of the 10 WIPP waste panels, the Agency agrees that explosion walls do not need to be explicitly modeled in the

PCS

- 2012 PA. Furthermore, t he Agency agrees that the bulkheads will corrode in the post

closure environment, be crushed during drift creep closure, and have no impact on post

- closure repository performance.

3 .4.1.3 ROM Salt Empla cement Method

The method used by DOE for emplacing the ROM salt can affect its initial porosity and permeability

. Mechanical compaction and higher moisture content reduce the initial porosity and permeability of the salt.

At the time the PCS

- 2012 PA was performed, the method of emplacing the ROM salt was uncertain. DOE had initiated tests of three different emplacement scenarios that were as yet incomplete. Scenario 1 consisted of 1% (by weight) of water added to the ROM salt and compaction of the salt. S cenario 2 consisted of compaction of the ROM salt with no additional water. Scenario 3 consisted of the ROM salt simply being placed and pushed up tight against the

roof and walls without compacting or adding water (Zimmerly et al. 2012). U ncertainty

regar ding the ROM salt emplacement method and initial density increase s the uncertainty in estimating its permeability during the first 100 years. The Agency believes t his was adequately addressed by DOE assigning wide ranges to the ROM salt porosity and permea bility values in

the PCS

- 2012 PA.

3.4.2 ROM Salt Modeling

3 .4.2.1 Porosity and Permeability of the ROM Salt during Consolidation

Initial information on the porosity and permeability of the ROM salt during the consolidation process was provided by DOE in Atta chment B of the PCR (DOE 2011 b ). This attachment consisted of a 2002 memorandum describing changes in porosity and permeability with time of the ROM salt during drift creep closure (Hansen and Thompson 2002). Hansen and Thompson estimated an initial permea bility of the loosely placed ROM salt of 10 - 11 m 2 and a porosity of about 33%. Following emplacement, salt consolidation and later drift closure from halite creep would compress the ROM salt and reduce both its permeability and porosity. Based on the data of Hurtado et al. (1997), Hansen and Thompson expected the ROM salt permeability to be 10 - 15

m 2 or lower when its porosity reached 10%, and about 10 - 19 m 2 when the porosity reached 5%. As consolidation proceeds, the material will attain sufficient density so that its response would assume the constitutive response of intact salt (Hansen and Thompson 2002 ). Hansen and Thompson do not expect significant back pressure from the consolidating salt until the porosity drops to about 10% and permeabilit y lower than 10 - 15 m 2 is achieved (Hansen and Thompson 2002). Based on the measured, unimpeded closure rates of WIPP drifts and considering the delaying effects of back pressure buildup, Hansen and Thompson believed that porosity on the order of 10% may be achieved wi thin 40 to 60 years following ROM salt emplacement (Hansen and Thompson 2002).24

DOE initially proposed to model the consolidation of the ROM salt in the PCS

- 2012 PA using two time periods: a time period ( T1 ) extending from 0 to 100 years, and a time perio d ( T2 ) extending from 100 to 10,000 years. To better model the consolidation process, the Agency recommended that DOE model three time periods instead of two and use ranges of permeability values that more accurately reflect intermediate consolidation stat es.

The expected conditions during the three time periods are as follows:

T1: ROM salt healing, but producing no backstress. DRZ static or expanding. T2: ROM salt placing backstress on DRZ. Both are in process of healing. T3: Forces balanced and healing generally complete

.

DOE responded to the Agency’s recommendations by incorporating three time periods in the

PCS

- 2012 PA: time period T1 extend s from zero to 100 years as before; time period T2 extend s from 100 to 200 years, and time period T3 extend s f rom 200 years to 10,000 years. DOE treated ROM salt porosity as an uncertain, sampled parameter and determine d the permeability for time periods T2 and T3 using a correlation between permeability and porosity based on laboratory data from Hurtado et al. (1 997). DOE proposed to sample the new T2 porosity from 2.5% to 7.5% using a uniform distribution. This range retained the same average porosity of 5% that DOE believes could be achieved by 100 years. DOE proposed to sample the T3 porosity from

0.1% to 5.19% , again using a uniform distribution

. Although this range and distribution are the same as used for intact halite in WIPP PA, the T3 porosity value was independently sampled in

the PCS

- 2012 PA. Assigning the same range and distribution assumes that the ROM salt PCS will fully compact and have properties consistent with those of intact halite during time period

T3.

Because there is also uncertainty in the permeability that is correlated with a given sampled porosity, DOE proposed to use only the permeabilit y values from Hurtado’s data set that were measured with brine (not gas) as a p erm eant and sample permeabilit y from a normal distribution centered on a linear least squares fit to a plot of Hurtado’s porosity vs. log permeability data. This approach result ed in an overall permeability range of

- 20.8 lo g ( m 2 ) or 1.58×10 - 21 m 2 to

- 16.3 lo g ( m 2 ) or 5.01×10 - 17 m 2 during time period T2, and an overall range of

- 21.4 lo g ( m 2 ) or

3.98×10 - 22 m 2 to

- 16.8 lo g ( m 2 ) or 1.58×10 - 17 m 2 during time period T3.

The Agency agreed that incorporating three time periods better represents the ROM salt reconsolidation process. The Agency also agreed that using only Hurtado’s brine data set for correlating permeability to porosity during time periods T2 and T3 i s appropriate becau se the data represented the porosity range expected after 100 years based on DOE’s proposed ROM salt emplacement methods

. Correlating those data with the porosity ranges was also deemed appropriate for the halite by the Agency because brine is expected to accumulate in the repository in later time and, as previously mentioned, the permeability of consolidating ROM salt is reduced by the presence of moisture.

In recognition of the uncertainty in the rate of salt consolidation and the correlation between

por osity and permeability , the DOE proposed to change the porosity and permeability of the ROM salt during time period T1 from a constant to values sampled over a range. DOE proposed sampling the porosity from a uniform distribution over the range of 6.6% to 18.7% and25

independently sampling the log permeability from a uniform distribution ranging from 10 - 12 to
10 - 21 m 2 (Camphouse et al. 2012 b ; as amended by Patterson 2012). Sampling for the three time periods i s conditioned s o that the ROM salt permeability mo notonically decreas es with time. The T1 ranges a re selected based on the porosit y and permeabili ty of the ROM salt expected by DOE at 50 years after emplacement, which is the midpoint of time period T1. The porosity range uses JAS3D modeling predictions at 50 years for initial emplacement porosities ranging from 20% to 35%, depending on whether the ROM salt was compacted upon emplacement. The permeability range is based in part on laboratory measurements of the permeability of crushed salt with porosities w ithin the T1 range of 6.6% to 18.7% (Camphouse et al. 2012 b ). A more detailed discussion of the porosity and permeability values for the ROM salt is presented in Appendix A

.

3 .4.2.2 ROM Salt Two

- Phase Flow Properties

Two

- phase brine and gas flow is expec ted to occur within the WIPP repository during the regulatory time frame and is simulated for all materials in WIPP PA except the anhydrites using

a modified Brooks

- Corey model (Camphouse et al. 2012 b ). The parameters describing two

phase flow in the ROM s alt are summarized in Table 3

- 3 and are further discussed in Camphouse (2012 d ). EPA conducted a sensitivity study of the two

- phase flow properties and determined that they did not appreciably affect performance assessment results. The results of this addit ional study are presented in Section 4.0 of this TSD.

Table 3

- 3 . Two

- Phase Flow Properties for the ROM Salt

Property Name in PCS

- 2012 PA Parameter Description

RELP_MOD Relative permeability model number for the ROM salt
CAP_MOD Capillary pressure model number for the ROM salt
PCT_A Threshold capillary pressure linear parameter for the ROM salt
PCT_EXP Threshold capillary pressure exponential parameter for the ROM salt

KPT Flag for selecting the permeabi lity used in calculating threshold capillary

pressure for the ROM salt

PC

- MAX Maximum allowable capillary pressure for the ROM salt
PO_MIN Minimum allowable brine pressure for the ROM salt
PORE_DIS Brooks

- Corey pore distribution parameter for the ROM salt
SAT_IBRN Initial brine saturation of the ROM salt at the beginning of time period T1
SAT_RBRN Residual brine saturation of the ROM salt
SAT_RGAS Residual gas saturation of the ROM salt

Property RELP_MOD. Relative permeability Model 4, a modi fied Brooks

- Corey model, was used for the ROM salt. The Agency believes that the modified Brooks

- Corey model adequately

describes two

- phase flow in the ROM salt and is appropriate and consistent with the use of this

model in the baseline PABC

- 2009.

Proper ty CAP_MOD. Capillary pressure Models 1 and 2 both relate to the Brooks

- Corey relative permeability model but Model 1 is typically used by SNL when the capillary pressure function is turned off, and Model 2 is typically used when that function is turned on . Although the database approved by the Agency for the PCS

- 2012 PA specified use of capillary pressure Model 2, Model 1 was used instead because it avoided numerical problems with the BRAGFLO code when using Model 2. In response to an Agency request, SNL d etermined that the numerical26

problems resulted from assigning an initial brine saturation that was less than the residual brine saturation, as more fully described below. Because capillary pressure would actually exist in the ROM salt, at the Agency’s requ est the problem with BRAGFLO was corrected and the PCS

2012 PA was rerun by the Agency using Model 2.

Property PCT_A. As applied to the ROM salt, threshold capillary pressure is the minimum pressure that repository gas must attain before it can overcome capillary pressure and create a continuous flowpath for gas in a partially saturated medium. Threshold capillary pressure for the

Brooks

- Corey model is calculated as :

P t = a k

– b

where a is called the linear parameter, designated in WIPP PA as the proper ty PCT_A, k is the permeability of the material in the X

- direction, and b is the exponential parameter, designated in WIPP PA as the property PCT_EXP. Positive values for PCT_A activate the capillary pressure model, while zero values deactivate it. The PCS

- 2012 PA database approved by the Agency included a value of 0.56 for PCT_A because, as described above, capillary pressure would actually exist in the ROM salt and the Agency wanted to determine its effect on releases. However, SNL set PCT_A for the ROM s alt to zero in the PCS

- 2012 PA to deactivate the capillary pressure model and avoid numerical problems with BRAGFLO. As described above, the problem with BRAGFLO was corrected and the PCS

- 2012 PA was rerun using a value of 0.56 for PCT_A.

Property PCT_EX P. PCT_EXP is the exponential parameter in the aforementioned threshold capillary pressure equation. SNL typically sets PCT_EXP to zero when PCT_A is set to zero. A

value of

- 0.346 was accepted by the Agency for this property as part of an activated capill ary pressure model in the PCS

- 2012 PA. SNL subsequently reset the value of PCT_EXP to zero because of numerical problems with BRAGFLO. As described above, the problem with

BRAGFLO was corrected and the PCS

- 2012 PA was rerun using a value of

- 0.346 for PCT_ EXP.

Property KPT. KPT is a flag for using either constant or varying permeability in the threshold pressure equation presented above. When KPT is set to 0.0, the permeability is not varied as a function of time but changes when the material changes. The Agency accepted a value of 0.0 for this parameter because the threshold capillary pressure would be appropriately recalculated for the new ROM salt PCS materials during T1, T2, and T3.

Property PC_MAX. PC_MAX is the maximum capillary pressure allowed whe n using capillary pressure Model 2. The Agency notes that capillary pressure can become very large at low brine saturations, and it is appropriate to limit the maximum value of this parameter to avoid numerical problems with BRAGFLO. The Agency has reviewe d the value of this parameter and found it to be reasonable and appropriate for use in the PCS

- 2012 PA.

Property PO_MIN. PO_MIN is the minimum allowable capillary pressure. It is only used in capillary pressure Model 3, which was not used in PCS

- 2012 or in PABC

- 2009. The value of this parameter, therefore, has no impact on the PA results.27

Property POR_DIS. POR_DIS is a pore distribution index that is required for the Brooks

- Corey

two

- phase flow model used in WIPP PA. The Agency has reviewed the assigned value for this parameter and found it to be reasonable given the range of values for pore size distributions for ‘consolidated rock’ reported in Hurtado et al. ( 1997 )

.

Property SAT_IBRN. As applied here, SAT_IBRN is the initial brine saturation of the ROM salt at the beginning of time period T1. A value lower than the residual brine saturation (described below) was used in the PCS

- 2012 PA because of the drying that was expected to occur in ROM salt that would be stockpiled prior to use as a panel closure m aterial. Although the Agency considers the drying process to be realistic, the residual brine saturation is defined as the lowest possible brine saturation in the Brooks

- Corey model, which does not consider drying effects. It was therefore not surprising t hat assigning even lower brine saturation as an initial condition caused numerical problems. As described above, the numerical problems were

resolved when the PCS

- 2012 PA was rerun using an artificially higher value for SAT_IBRN. The Agency accepts the use of a higher value because the moisture content of the ROM salt is expected to increase upon repository closure when mine ventilation ceases.

Property SAT_RBRN. As applied here, SAT_RBRN is the residual brine saturation of the

ROM salt. The residual brin e saturation is the saturation at which brine flow ceases because a continuous flow path for brine through the medium no longer exists. Ignoring evaporation and chemical effects, the residual saturation is therefore the lowest brine saturation that can be attained in an initially saturated medium. In previous PAs, t he range of the distribution for this parameter went to zero, which the Agency considers to be unrealistic for a porous medium.
However, the end members of a distribution are rarely sampled and a llowing this range to extend to zero was not expected to significantly impact repository performance. The Agency agreed to
retain this distribution for the PCS

- 2012 PA to simplify comparison of results.

Property SAT_RGAS. As applied here, SAT_RGAS is the residual gas saturation of the ROM salt. Parallel with residual brine saturation, the residual gas saturation is the saturation at which gas flow ceases because a continuous flow path for gas through the medium no longer exists. In previous PAs, t he range of the distribution for this parameter went to zero, which the Agency considers to be unrealistic for a porous medium. However, the end members of a distribution are rarely sampled and allowing this range to extend to zero was not expected to significantly impact repository performance

. The Agency agreed to retain this distribution for the PCS

- 2012 PA to simplify comparison of results.

3 .4.2.3 Bulk Compressibility of the ROM Salt during Consolidation

Bulk compressibility is used in BRAGFLO to calculate p ore volume compressibility, which is part of the hydraulic diffusivity term used in calculating transient fluid flow in a compressible porous medium. Pore volume compressibility is equal to the bulk rock compressibility divided by the porosity. Bulk rock c ompressibility is also used to determine resistance to crushing (also called the back pressure) provided by a material under a confining stress. However, bulk compressibility is not used in this manner for the panel closures in the PCS

- 2012 PA because the process of creep closure around the waste panel seals is not explicitly modeled.28

The same constant value of 8.0 × 10 - 11 Pa - 1 was proposed by DOE for this parameter for all three time periods in the PCS

- 2012 PA. The proposed value was originally used for t he compacted,

crushed salt materials in the WIPP shaft seal

. Although the Agency considers its value to be a reasonable estimate for compacted salt, the initial compressibility of the salt will be higher if it is emplaced without compaction. However, the A gency believes that the proposed value of this parameter is acceptable for all three time periods because the ROM salt may be compacted upon emplacement.

3.4.3 PCS DRZ Modeling

DOE proposed to model the PCS DRZ using characteristics of the modeling approaches that had previously been used for the Option D design and for the DRZ around the waste panels. This approach helped to facilitate comparison with the results of the baseline PABC

- 2009 PA and was approved by the Agency for the PCS

- 2012 PA. The Agency’s revi ew of the model changes proposed by DOE focused on the short

- and long

- term porosity and permeability, and the two

phase flow properties of the PCS DRZ. An overview of the Agency’s primary conclusions is presented in the following subsections . Additional d etails are presented in Appendix A : Parameter QA. A summary tabulation of the Agency’s modeling c oncerns is presented in

Section A .4.

3 .4.3.1 Porosity and Permeability of the PCS DRZ during Healing

The PCS DRZ is the disturbed rock zone that forms around the ROM salt fill material as a result of stress relief within the halite due to drift excavation. The stress gradients cause open fracturing

near the drift walls and dilation from micro

- fracturing of the halite farther from the drift walls.

As described above in Section 3 .2, healing of the PCS DRZ occurs as a result of drift creep closure and begins when the ROM salt begins to resist further compaction and applies a backpressure that starts to close fractures in the surrounding DRZ. This process eventuall y returns the DRZ to a state that approaches the very low porosity and permeability of intact halite

.

DOE expects healing of the PCS DRZ to be essentially complete within approximately 200 years (Camphouse et al. 2012 b ). DOE chose to represent the PCS DRZ healing process in the PCS

2012 PA by combining time periods T1 and T2, such that one set of parameters (given the material name DRZ_1) represents the disturbed PCS DRZ during time periods T1 and T2, while a second set of parameters (given the material na me DRZ_PCS) represents the nearly healed PCS DRZ during time period T3. The material DRZ_1 has the same material properties as the DRZ around a waste panel. Therefore, for the first 200 years the properties of the PCS DRZ are

identical to those of the wast e panel DRZ.

DOE proposed not to vary the porosity of the PCS DRZ with time in the PCS

- 2012 PA, and also proposed to use the same sampled porosity value as for the DRZ around the waste panels. This

same approach was also used for the porosity of the DRZ a round the Option D monolith in the

PABC

- 2009 PA. The DRZ porosity was sampled from a cumulative distribution over the range of 0.39% to 5.48%, with a median value of 1.29% (Camphouse et al. 2012 b ). Although the initial DRZ porosity would be expected to be larger than the final porosity due to healing effects, EPA29

accepts the range and distribution for the PCS DRZ porosity used in PCS

- 2012 , since they were also used in PABC

- 2009 and therefore facilitate a comparison of results.

For time periods T1 and T2, DOE proposed to use the same sampled permeability value for the PCS DRZ as for the DRZ around the waste panels. The log permeability was therefore proposed to be sampled from a uniform distribution ranging from a minimum of

- 19.4 lo g ( m 2 ) or 3.98 × 10 - 20 m 2 to a maximum of

- 12.5 lo g ( m 2 ) or 3.16 × 10 - 13 m 2 . The mode of the distribution was

- 16.0 lo g ( m 2 ) or 1 × 10 - 16 m 2 (Camphouse et al. 2012 b ). For time period T3, DOE proposed to use the same permeability range and distribution as for the healed DRZ aroun d an Option D concrete monolith. DOE therefore proposed to independently sample the log permeability from a triangular distribution ranging from a minimum of

- 20.699 lo g ( m 2 ) or 2 × 10 - 21 m 2 to a
maximum of

- 17.0 lo g ( m 2 ) or 1 × 10 - 17 m 2 . The mode of the d istribution was

- 18.7496 lo g ( m 2 )

or 1.78 × 10 - 19 m 2 (Camphouse et al. 2012 b ). The rationale for this distribution was that it would be slightly higher than the permeability range for intact halite (10 - 24 to 10 - 21 m 2 ) and it would be the same as the permea bility range for the Option D concrete monolith, such that flow between the DRZ and the monolith would be equally probable and the concrete monolith seal ‘system’ would act as a single unit (Stein 2002).

The Agency was concerned that DOE’s approach of ind ependently sampling PCS DRZ permeabilit y for the initial T1

, T2 and final T3 time periods could lead to the fully healed T3 permeab ility being greater than the T1 and T2 permeability because the two ranges overlap. The ROM salt and surrounding DRZ can have relatively independent porosit y and permeabilit y until

the ROM salt is sufficiently consolidated and begins to apply a backstress on the surrounding DRZ. The Agency agrees with the geo mechanical processes that will ‘heal’ the DRZ as described by Camphouse et al. (2012 b ) : “As ROM salt reaches higher fractional densities during consolidation, backstress will be imposed on the surrounding rock mass leading to eventual healing of the disturbed rock zone (DRZ).” T he high end of the T3 permeability range for hea led DRZ (1 × 10 - 17 m 2 ) , however, is nearly the same as the mode of the T1 distribution for the ROM salt (1 × 10 - 16 m 2 ) and the low end of the T3 permeability range for healed DRZ (2.14 × 10 - 21 m 2 ) is three orders of magnitude higher than the low end of the permeability range for intact halite (1 × 10 - 24 m 2 ). The Agency ’s interpretation of the data is that the permeability of a healed DRZ

could be closer to the permeability of intact halite.

In the PABC

- 20 09, DOE recognized that “the permeability of the sal t immediately above and below the rigid concrete monolith component of Option D will approach the intrinsic permeability of the undisturbed Salado halite,” but assigned the permeability values to the DRZ_PCS identical to those for the material CONC_PCS

. DO E acknowledged that this parameterization was based “not directly on experimental data (although the general range of the distribution agrees with experimental observations of healed salt” but instead “ensures that any fluid flow is equally probable throug h or around the Option D panel closures, and represents the range of uncertainty that exists in the performance of the panel closure system” (DOE 2009).

T he Agency agreed with the PCS DRZ values used in PCS

- 2012 to maintain consistency with

PABC

- 2009 and simplify a comparison of results

. EPA , however, investigate d the effect on potential repository releases if the permeability of the DRZ were to approach that of intact halite30

200 years following repository closure

. This evaluation and its r esults are descr ibed in

Section 4 .0.

3 .4.3.2 PCS DRZ Two

- Phase Flow Properties

The two

- phase flow characteristics of the PCS DRZ are expected to be different from those of the ROM salt because of the effects of open fractures in the DRZ that would not be present in the ROM salt. The open fractures would allow gas to enter the PCS DRZ at very low capillary pressures that could drop to essentially zero if the fracture apertures are wide enough to not be bridged by capillary action. This difference occurs only when open fra ctures exist, and therefore applies only to the PCS DRZ during the initial time period s T1 and T2. As the fractures close and the DRZ is healed, capillary effects will be present. The properties describing two

- phase flow in the PCS DRZ and their conceptual application to the three time periods are summarized

in Table 3

- 4. The two

- phase flow parameters used for the PCS DRZ in the PCS

- 2012 PA are summarized in Camphouse et al. (2012 b ). The basic functions of these properties are the same as

described for the ROM salt in Table 3

- 4. As described below and in Section 4.0, t he Agency

evaluated an alternative set of two

- phase flow parameter values from those used in the PCS

- 2012

PA as part of its analysis correlating DRZ permeability with the permeability of intact halite. The results of that analysis showed that the repository continued to meet the containment requirements of 40 CFR Part 191.13 under these alternative assumptions.

Table 3

- 4 . Two

- Phase Flow Properties for the PCS DRZ

Property Name in

PCS

- 2012 PA Parameter Description Conceptual Relevance

to Time Period s T1 and T2 Conceptual Relevance

to Time Period T3

RELP_MOD Relative permeability model

number for the PCS DRZ Modified Brooks

- Corey model applies Modif ied Brooks

- Corey model applies

CAP_MOD Capillary pressure model number

for the PCS DRZ Capillary pressure could go to zero, consistent

with a fractured medium Capillary pressure

would not be zero

PCT_A Threshold capillary pressure linear

parameter for th e PCS DRZ Could be zero,

consistent with a zero capillary pressure Would not be zero

PCT_EXP Threshold capillary pressure

exponential parameter for the PCS

DRZ Could be zero,

consistent with a zero capillary pressure Would not be zero

KPT Flag for sele cting the permeability

used in calculating threshold capillary pressure for the PCS DRZ A zero value applies A zero value applies

PC

- MAX Maximum allowable capillary

pressure for the PCS DRZ Would not be used when capillary pressure is zero Would be used and

would not be zero

PO_MIN Minimum allowable brine pressure

for the PCS DRZ Would not be used Would not be used

PORE_DIS Brooks

- Corey pore distribution

parameter for the PCS DRZ Would be used Would be used

SAT_IBRN Initial brine saturation of the PC S

DRZ at time

- 5 years Would be set to a value
>SAT_RBRN at

time

- 5 years Would be calculated

based on saturation at end of time period

T1 and T231 Table 3

- 4 . Two

- Phase Flow Properties for the PCS DRZ

Property Name in

PCS

- 2012 PA Parameter Description Conceptual Relevance

to Time Period s T1 and T2 Conceptual Relevance

to Time Period T3

SAT_RBRN Residual brine saturation of the PCS

DRZ Could go to zero,

consistent with a

fractured medium Wou ld not be zero

SAT_RGAS Residual gas saturation of the PCS

DRZ Could go to zero,

consistent with a

fractured medium Would not be zero

Property RELP_MOD. Relative permeability Model 4, a modified Brooks

- Corey model, was used by DOE for the PCS DRZ in al l three time periods. The Agency believes that the modified

Brooks

- Corey model adequately describes two

- phase flow in the PCS DRZ and is appropriate

and consistent with the use of this model in the baseline PABC

- 2009.

Property CAP_MOD. DOE’s use of capill ary pressure Model 1 for time period s T1 and T2 is

consistent with its use in the PABC

- 2009 PA and is also theoretically appropriate. Model 1 is used when the capillary pressure is set to zero, which is appropriate for a fractured PCS DRZ in early times. D OE’s use of capillary pressure Model 1 for time period T3 is also consistent with

its use in the PABC

- 2009 PA . A zero value would not be appropriate if all fractures were closed in a healed DRZ. The Agency accepts the use of Model 1 in both early and late time periods in

the PCS

- 2012 PA because its continued use facilitates comparison of results. The Agency has performed a sensitivity study that evaluates the impacts of DOE’s parameter values for the PCS DRZ on repository performance, including the two

- phas e flow parameters. The results of this study are presented in Section 4 .0.

Property PCT_A. DOE’s assignment of a zero value to PCT_A for time period s T1 and T2 is

consistent with its use in the PABC

- 2009 PA and is also conceptually appropriate. A zero va lue for this property deactivates the capillary pressure model, which is appropriate for a fractured PCS DRZ in early times. DOE’s assignment of a zero value to PCT_A for time period T3 is also

consistent with its use in the PABC

- 2009 PA . A zero value woul d not be appropriate if all

fractures were closed in a healed DRZ. The Agency accepted use of a constant zero value for PCT_A in the PABC

- 2009 PA to resolve numerical problems in BRAGFLO and because its use was judged to have a n in significant effect on rep ository performance. The Agency accepts the use of zero values for PCT_A in both early and late time periods in the PCS

- 2012 PA because its continued use facilitates comparison of results.

Property PCT_EXP. As with Property PCT_A, DOE’s assignment of a z ero value for PCT_EXP for time period s T1 and T2 is consistent with its use in the PABC

- 2009 PA and is also conceptually appropriate because capillary pressures can drop to zero in open fractures. DOE’s assignment of a zero value for PCT_EXP for time perio d T3 is also consistent with its use in the

PABC

- 2009 PA . A zero value would not be appropriate if all fractures were closed in a healed

DRZ

. The Agency accepted use of a constant zero value for PCT_EXP in the PABC

- 2009 PA to resolve numerical problems in BRAGFLO and because its use was judged to have a n in significant effect on repository performance. As with Property PCT_A, the Agency accepts the32

use of zero values for PCT_EXP in both early and late time periods in the PCS

- 2012 PA because

its continued use facilitates comparison of results.

Property KPT. DOE’s assignment of a zero value for KPT is accepted by the Agency because the threshold capillary pressure would be appropriately recalculated for the healed PCS DRZ during time period T3 because it is a new material

.

Property PC_MAX. DOE has adopted the same value of PC_MAX for the PCS DRZ as for the ROM salt. The Agency has reviewed the value of this parameter and found it to be reasonable and appropriate for use in the PCS

- 2012 PA. This parameter is no t used in capillary pressure

Model 1 and the results were therefore insensitive to its value . As described above, the Agency considers Model 1 to be appropriate for the PCS DRZ in time period s T1 and T2 but not appropriate for time period T3. T he Agency acc ept ed the use of Model 1 for the entire regulatory

time frame in the baseline PABC

- 2009 PA and also accepts its use in the PCS

- 2012 PA because

it is consistent with its use in the baseline PABC

- 2009 PA and facilitate s a comparison of results.

T he effect of DOE’s selection of two

- phase flow parameters is included in an EPA sensitivity study presented in Section 4 .0.

Property PO_MIN. As described above, PO_MIN is only used in capillary pressure Model 3,

which was not used in PCS

- 2012 or in PABC

- 2009. The val ue of this parameter, therefore, has no impact on the PA results.

Property POR_DIS. POR_DIS is a pore distribution index that is required for the Brooks

- Corey

two

- phase flow model used in WIPP PA. The Agency has reviewed the assigned value for this

parame ter and found it to be reasonable given the range of values for pore size distributions for ‘consolidated rock’ reported in Hurtado et al. ( 1997 )

.

Property SAT_IBRN. The initial brine saturation of the PCS DRZ was set to 1.0 at time period

- 5 years in bot h the PABC

- 2009 and the PCS

- 2012 PAs. It is not a new property for the DRZ and is included here for completeness. Initial conditions are set for many parameters in W IPP PA at

5 years before repository closure so that the models can calculate what the value would be at

time zero.

Property SAT_RBRN. SAT_RBRN was assigned a constant value of zero for the PCS DRZ for

all three time periods. Although as described above, the Agency considers a value of zero to be

reasonable for a fractured medium and therefore reasonable for the T1 and T2 time period s, it is

also not appropriate for time s within period T3 when the open fractures have closed

. At later times, however, most of the DRZ porosity will be filled with brine and therefore brine flow will not be significa ntly affected by the z ero residual brine saturation.

Use of a zero value for this parameter is also appropriate for the DRZ around a waste panel where gas pressure fluctuations are expected to disrupt the DRZ and retard fracture closure. This

constant zer o value was retained when the PCS DRZ was separately identified for modeling the Option D PCS because it was judged to have a n in significant effect on repository performance.

T he Agency accepts the use of zero values in the PCS

- 2012 PA because they are con sistent with33

their use in the baseline PABC

- 2009 PA , are unlikely to significantly affect the results, and facilitate a comparison of results.

Property SAT_RGAS. As with SAT_RBRN, SAT_RGAS was assigned the same constant

value of zero for the PCS DRZ for a ll three time periods. The Agency’s response is also the same. Although as described above, the Agency considers a value of zero to be reasonable for a

fractured medium and therefore reasonable for the T1 and T2 time period s , it is not appropriate

for time s within period T3 when the open fractures have closed. At later times, however, gas generation will have dropped off significantly and most of the porosity within the DRZ will be

filled with brine thereby reducing the sensitivity to residual ga s saturatio n.

Use of a zero value for this parameter is appropriate for the DRZ around a waste panel where gas pressure fluctuations are expected to disrupt the DRZ and retard fracture closure. This constant zero value was retained when the PCS DRZ was separately id entified for modeling the Option D PCS because it was judged to have a n in significant effect on repository performance

. T he Agency accepts the use of zero values in the PCS

- 2012 PA because they are consistent with their

use in the baseline PABC

- 2009 PA , ar e unlikely to significantly affect the results, and facilitate a comparison of results.

3 .4.3.3 Bulk Compressibility of the PCS DRZ during Healing

In BRAFLO, the porosity is calculated as a function of the bulk compressibility and prevailing pressure. Fo r the PCS

- 2012 PA, DOE proposed a constant value of 7.41 × 10 - 10 Pa - 1 for bulk compressibility for all three time periods

. Although the compressibility of the PCS DRZ should be higher during early times when the fractures are open and lower during later ti mes as the

fractures close the DRZ porosity is sampled from a distribution that would, in effect, cover a range of expected bulk compressibilities

. DOE’s also used the proposed value in the PABC

- 2009

PA. Therefore, t he Agency accepted DOE’s constant value for the PCS

- 2012 PA

.

3.4.4 Development of the PCS

- 2012 PA Numerical Model

The PCS

- 2012 PA was developed by SNL to be as similar as possible to the baseline PABC

2009 PA. This was done to facilitate a comparison between the results of the two PAs that would

hel p identify the effects of the proposed modified PCS design on repository releases. Only t hose aspects of the PABC

- 2009 calculations that were potentially impacted by the panel closure redesign were updated for the PCS

- 2012 PA

. For example, the PCS

- 2012 PA used the same waste inventory information, drilling rate and plugging pattern parameters, and radionuclide solubility parameters as were used in the PABC

- 2009 PA (Camphouse et al. 2012 a ). The same

set of six intrusion scenarios was also used to calculate b rine and gas flow. These scenarios are

summarized in Table 3

- 5 . An E1 intrusion assumes the borehole passes through a waste

- filled panel and into a pressurized brine reservoir beneath the repository in the Castile F ormation. An

E2 intrusion assumes that th e borehole passes through the repository and the Castile Formation

but does not encounter a brine reservoir. E1 and E2 borehole intru sions are illustrated in Figure 3

- 1

.34 Table 3

- 5 . BRAGFLO Salado Flow Modelin g Scenarios

Scenario Description

S1

- BF Undisturbed Repository

S2

- BF E1 intrusion at 350 years

S3

- BF E1 intrusion at 1,000 years

S4

- BF E2 intrusion at 350 years

S5

- BF E2 intrusion at 1,000 years

S6

- BF E2 intrusion at 1,000 years and E1 intrusion at 2, 000 years From Camphouse et al. 2012 a ; BF = BRAGFLO

Figure 3

- 1 . Illustration of E1 and E2 Borehole Intrusions

Most computational changes for the PCS

- 2012 PA were made to the BRAGFLO Salado Flow

Mod el to accommodate the revised geometry, time periods, and material properties associated with the modified PCS design. The revised geometry was accommodated by changes to the

baseline PABC

- 2009 BRAGFLO grid. The baseline grid with an E1 intrusion is shown in35

Figure 3

- 2. This two

- dimensional grid includes the Option D concrete monolith, which was to be “keyed in” to the DRZ and extend vertically from Anhydrite AB above to anhydrite Marker Bed 139 below, cutting off the lower and part of the upper DRZ around the monolith. The grid dimensions on the figure are in meters.

The revised grid for the PCS

- 2012 PA is illustrated in four figures with the same cell dimensions but different material properties: Figure 3

- 3 shows the grid for time period T1 ( 0 to 100 year s); Figure 3

- 4 shows the grid for time period T2 (100 to 200 years), Figure 3

- 5 shows the grid for time period T3 (200 years to 10,000 years), and Figure 3

- 6 shows the T3 grid with activated

intrusion boreho le material properties.

As shown in Figure 3

- 3, during time period T1 the ROM salt is given the material name PCS_T1 and the surrounding PCS DRZ is given the same properties as the DRZ around the waste panel and around the rest of the WIPP underground facility. Segments of Anhydrite AB and anhydrite

Mar ker Bed 139 remain in place within the PCS DRZ for two reasons. First, SNL does not expect the marker beds to be as significantly disrupted around a panel closure as they would be around a waste panel. Additionally, and as explained by Camphouse (2012c), t he presence of the marker beds within the PCS DRZ help the two

- dimensional model capture the impacts of brine and gas that could bypass the panel closure by flowing through the interbed layers out of plane of

the model . Institutional control of the WIPP si te is assumed to remain in force during time period T1 and no borehole intrusions of the waste are expected.

The material properties during time period T2, shown in Figure 3

- 4, are the same as during T1 except for the ROM salt, which has the new material name PCS_T2 and new material properties. Institutional control is expected to end after 100 years and borehole intrusions could occur during T2. Although the first E1 or E2 borehole intrusion is modeled as occurring at 350 years, for purposes of calculatin g direct brine releases (DBRs) or spallings releases the first borehole intrusion (which could be either an E1 or E2) is modeled as occurring at 100 years after repository closure. Repository conditions at intermediate times are linearly interpolated betwe en the specific modeling dates.

As shown in Figure 3

- 5, during time period T3 the material properties change for both the ROM salt and the PCS DRZ. The ROM salt has the new material name PCS_T3 and new material properties. The surrounding DRZ has the new material name DRZ_PCS and new material properties that are different from those around a waste panel. Although borehole intrusions could occur during T3, they are not shown in the figure.

Figure 3

- 6 shows the BRAGFLO grid with T3 properties and an intrusi on borehole penetrating the representative waste panel. The figure shows an E1 intrusion where the borehole penetrates a Castile brine reservoir beneath the repository. If an E2 intrusion is modeled, the material properties in the zone marked “lower boreho le” are changed to represent the lower DRZ, Salado, and Castile properties unaffected by a borehole.

Results obtained in the six scenarios from BRAGFLO are used to initialize flow and material properties in subsequent codes in the PA computational suite. These include the calculation of direct brine releases and spallings releases. As discussed later in Section 3 .5.5, implementation36

of the ROM salt PCS design has a negligible impact on potential releases from the Culebra. As a result, the PCS

- 2012 PA calcu lated CCDFs of individual vectors for total normalized releases using Culebra release results calculated in the PABC

- 2009. The PCS

- 2012 PA calculated CCDFs of individual vectors for total normalized releases, cuttings and cavings releases, spallings

releas es, and DBRs. Although the use of the ROM salt PCS design had no impact on cuttings and cavings releases, new cuttings and cavings release volumes were calculated in the PCS

- 2012 PA as a by

- product of the spallings calculation. Mean CCDFs for each release pathway were calculated by replicate and across all replicates. The 95% confidence limit on the mean across all replicates was also calculated.37 From Camphouse 2012 c

Figure 3

- 2 . PABC

- 2009 BRAGFLO Grid and Material Map38 From Camphouse 2012 c

Figure 3

- 3 . PCS

- 2012 BRAGFLO Grid and Material Map, Years 1 to 10039 From Camphouse 2012 c

Figure 3

- 4 . PCS

- 2012 BRAGFLO Grid and Material Map, Years 100 to 20040 From Camphouse 2012 c

Figure 3

- 5 . PCS

- 2012 BRAGFLO Grid and Material Map, Years 2 00 to 10,00041 From Camphouse 2012 c

Figure 3

- 6 . PCS

- 2012 BRAGFLO Grid and Material Map for an E1 Intrusion42

3.5 PCS

- 2012 PA RESULTS AND ANALYSIS

The principal contributions to tot al repository releases are from cuttings and cavings releases,

DBRs and spallings releases, Culebra releases contribut e little to the total . All of these types of releases result from the assumed inadvertent penetration of repository waste by an oil or gas exploratory borehole. Releases by lateral transport through anhydrite interbeds or through vertical transport via sealed repository shafts do not involve intrusion boreholes but are negligible compared with those involving intrusion boreholes.

Cuttings releases consist of solid repository waste cut by a drill bit and carried to the ground surface by the circulating drilling mud. Cavings consist of solid repository waste eroded from the borehole wall by circulating drilling mud. Cuttings and cavings relea ses are calculated together. They are the largest contributors to total releases at high probabilities because they occur with every borehole intrusion. Direct Brine Releases ( DBRs ) are releases of radionuclides dissolved in pressurized repository brine th at flows up an intrusion borehole to the surface. DBRs occur only under specific conditions of repository pressure and brine saturation, and are the greatest contributors to releases at lower probabilities. Spallings releases consist of solid repository wa ste entrained in pressurized gas that escapes into an intrusion borehole. Releases of dissolved radionuclides can also occur up an abandoned intrusion borehole and into the overlying Culebra dolomite horizon where they can be transported laterally by movin g groundwater to the accessible environment. Spallings and Culebra releases are small compared with cuttings, cavings, and DBRs. DBR, spallings, and Culebra releases are driven by a buildup of repository gas pressure from waste degradation. Comparisons bet ween the PCS

- 2012 PA and the PABC

2009 PA of total releases and releases from each of the contributing mechanisms are presented in the following subsections.

The PA methodology accommodates both uncertainty in the occurrence of future events (aleatory unc ertainty) and uncertainty in the true value of a material property (epistemic uncertainty). Aleatory uncertainty is accounted for in PA by the generation of random sequences of future events. Epistemic uncertainty is accounted for by sampling the values of uncertain parameters from assigned distributions. One set of sampled parameter values is called a vector.

In the PCS

- 2012 PA, the PA models were executed for three replicates of 100 vectors each, constituting 300 sets of sampled parameter values. Each vec tor, containing a different set of sampled parameter values, is executed 10,000 times for 10,000 possible sequences of future events. The normalized repository releases for each of these 10,000 sequences of future events are ranked and plotted, as required by EPA, in the form of a complementary cumulative distribution function (CCDF) that provides the probability of the release being greater than a particular value. PA results are presented as a distribution of 100 CCDFs of normalized releases

for each of t he three replicates (EPA 1996).

The primary effect of the proposed panel closure design modifications on repository releases was determined by SNL to result from the changes in pressure and brine saturation in the waste panels (Camphouse et al. 2012 a ). Di rect brine and spallings releases are particularly sensitive to those parameters and those parameters in turn are sensitive to the permeability of the panel closures and surrounding DRZ which control fluid flow between the waste panels and the non

waste ar eas of the WIPP underground facility.43

As compared with the Option D PCS design, generally higher ROM salt PCS permeabilit y in the first 200 years and lower permeabilit y after 200 years resulted in less isolation of individual waste panels in early times a nd greater isolation in later times. The lower permeabilit y tended to trap early

- time brine inflows into the waste panels resulting in generally higher brine saturations and gas pressures in the undisturbed repository after 200 years when borehole penetrat ions more commonly occur. The generally increased brine saturations and gas pressures encountered by borehole penetrations of the repository resulted in increases in overall mean DBR and spallings

releases. Those increases in turn resulted in increases in the overall mean total repository releases which, however, remained within regulatory limits. Cuttings and cavings are important contributors to total releases but are not affected by waste panel pressure and brine saturation. As previously mentioned, rele ases through the Culebra remained essentially unchanged from those calculated using the Option D design.

3.5.1 Results Compared with PABC 2009 PA

The important parameters of waste panel pressure and brine saturation are calculated by the

BRAGFLO Salado Flow Mo del. Illustrations of results are plotted on the following pages using BRAGFLO output variables.

During BRAGFLO calculations, aleatory uncertainty is addressed by defining a set of six scenarios for which brine and gas flow are calculated for each of the 300 generated vectors. The

six scenarios used in the PCS

- 2012 PA are described in Section 3 .4.4 (see Table 3

- 5 ) and are unchanged from those used for the PABC

- 2009. Results are discussed in terms of overall means, which are obtained by forming the average of the 300 realizations calculated for a given quantity and scenario. Undisturbed results were presented by SNL for Scenario S1

- BF. Intruded results were presented for Scenarios S2

- BF and S4

- BF, as these are representative of the intrusion types

considered in Scenarios S3

- BF and S5

- BF, with the only differences being the timing of drilling

intrusions. Results from scenario S6

- BF were also briefly discussed (Camphouse et al. 2012 a ).

3 .5.1.1 Results for an Undisturbed Repository (Scenario S1

- BF)

During the first 200 years after emplacement the permeability of the ROM salt panel closure material and surrounding DRZ are higher than the permeability of the Option D concrete monolith and its surrounding DRZ. As a result, greater flows of both brine and gas occur between the waste panels and the non

- waste parts of the WIPP underground facility. Figure 3

- 7 compares the overall mean of cumulative brine flow into the waste panel between the PCS

- 2012

and the PABC

- 2009 PAs for a repository with no borehole intrusions. The greater brine flow in

the PCS

- 2012 PA is clearly evident in the figure. Most of this increase occ urs in the first

200 years when the permeability of the ROM salt and DRZ are relatively high. After 200 years the ROM salt and DRZ are compressed by creep closure and the ROM salt assumes its long

- term properties with the DRZ healed above and below it (Camphouse et al. 2012 a )

. At 200 years, the difference between the cumulative brine flows obtained in the two analyses is roughly 600 m 3

, and this difference r emains fairly constant for the remainder of the regulatory period. The increase of brine flow into the waste panel results in a corresponding increase in the waste panel

brine saturation

, as shown in Figure 3

- 8 . Again, most of the increase occurs in the fi rst 200 years.44 From Camphouse et al. 2012 a Figure 3

- 7 . Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC),

Scenario S1

- BF

From Camphouse et al. 2012 a Figure 3

- 8 . Overall Means of Waste Panel Brine Saturation (WAS_SATB), Scenario S1

- BF

Gas is expected to be generated in the waste panels due to the corrosion of metals, primarily iron, and to microbial activity. Both of these processes req uire the presence of moisture in the form of brine. Increases in the availability of brine therefore increase ga s generation, as seen in Figure 3

- 9 . The increase in gas generation translates to a long

- term increase in the waste panel mean pressure, as show n in Figure 3

- 10

.45 From Camphouse et al. 2012 a Figure 3

- 9 . Overall Means of Waste Panel Gas Generation (GASMOL_W), Scenario S1

- BF

From Camphouse et al. 2012 a Figure 3

- 10 . Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S1

- BF

The greater brine saturations and gas pressures in the waste panels would tend to increase DBRs, spallings, and possibly also Culebra releases, but only if a waste panel is penetrated by an intrusion borehole. As previously mentioned, in the undisturbed repository scenario the only release pathways are through sealed shafts or laterally through anhydrite interbeds. Based on past PA results, lateral flow through anhydrite int erbeds to the accessible environment is negligible

. Cumulative mean flow up the shafts amounts to only 6 or 7 m 3 in 10,000 years and is also negligible (Camphouse et al. 2012 a ).

3 .5.1. 2 Results for an E1 Intrusion at 350 Years (Scenario S2

- BF)

An E1 intru sion is the result of a borehole that penetrates a waste panel and also penetrates a brine reservoir in the underlying Castile Formation. Such boreholes create a pathway for waste to reach the ground surface or the Culebra dolomite, and also release potent ially large volumes of46

pressurized Castile brine into the waste panel, increasing waste panel pressure and brine saturation, and impacting other waste panel conditions.

The overall means of waste panel pressure obtained in the PCS

- 2012 PA and the PABC

- 200 9 PA

for scenario S2

- BF are plotted in Figure 3

- 11 . As seen in that figure, the mean waste panel pressure increases rapidly to nearly 10 MPa when the brine reservoir is hit at 350 years, drops slightly and then climbs again reaching a slightly higher peak at about 1,000 years. This second pressure peak is due to increased waste degradation rates resulting from increased saturation with Castile brine. The excess pressure subsequently dissipates ov er the next approximately 5,000 years when it levels off at ab out 8.5 MPa. As seen in the figure, the second pressure peak is slightly higher in the PCS

- 2012 PA than in the PABC

- 2009 PA. This higher second peak is due to a generally lower long

- term permeability of the consolidated ROM salt than the Option D

concrete monolith that does not allow the excess pressure in the waste panel to dissipate as quickly.

The generally higher waste panel pressure following the E1 intrusion slightly retards the inflow of Salado brine to the waste panel, as shown in Figure 3

- 12 . The reduction of brine inflow to the waste panel translates to a reduction in waste panel brine saturation, as shown in Figure 3

- 13 , but the cumulative mean gas generation was nearly identical between the two PAs, as shown in Figure 3

- 14 . The cumulative mean v olume of brine flowing up the sealed intrusion borehole was also nearly identical, as shown in Figure 3

- 15 , and amounted to nearly 9,000 m 3 in 10,000 years.

From Camphouse et al. 2012 a Figure 3

- 11 . Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S2

- BF47 From Camphouse et al. 2012 a Figure 3

- 12 . Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC),

Scenario S2

- BF

From Camphouse et al. 2012 a Figure 3

- 13 . Overall Means of Waste Panel Brine Saturation (WAS_SATB),

Scenario S2

- BF48 From Camphouse et al. 2012 a Figure 3

- 14 . Overall Means of Wa ste Panel Gas Generation (GASMOL_W),

Scenario S2

- BF

From Camphouse et al. 2012 a Figure 3

- 15 . Overall Means of Brine Flow up the Borehole (BNBHUDRZ), Scenario S2

- BF

3 .5.1. 3 Results for an E2 Intrusion at 3 50 Years (Scenario S4

- BF)

An E2 intrusion is the result of a borehole that penetrates a waste panel but does not penetrate a brine reservoir in the underlying Castile Formation. An E2 intrusion typically has little effect on waste panel conditions at the time of intrusion because the borehole is assumed to be immediately plugged with low permeability concrete near the ground surface. The concrete plugs are modeled as failing 200 years after the intrusion, and the entire borehole is given properties equival ent to those of sand. The result is a depressurization of the intruded waste panel beginning49

200 years after the intrusion. This depressurization is illustrated in Figure 3

- 1 6 , where the effects on mean waste panel pressure of an E2 intrusion at 350 years are compared with mean waste panel pressure if no intrusion had occurred.

The overall means of waste panel pressure obtained for an E2 intrusion at 350 years in the PCS

2012 PA and in the PABC

- 2009 PA are shown in Figure 3

- 1 7 . As discussed above and show n in Figure 3

- 10 for the undisturbed scenario, at the time of an E2 borehole intrusion the mean waste panel pressure obtained in the PCS

- 2012 PA is slightly greater than in the PABC

- 2009 PA. Consequently, the mean waste panel pressure is also greater 200 y ears after the intrusion when the borehole plugs fail. This pressure difference is augmented by an increase in gas generation rates (discussed below) and continues into later times. The result is a slightly higher mean pressure following an E2 intrusion in the PCS

- 2012 PA when compared to the PABC

- 2009 PA.

The impact of the E2 intrusion on cumulative waste panel brine inflow can be clearly seen in Figure 3

- 1 8 . As discussed above, higher waste panel pressures can reduce brine inflow, while lower waste panel pressures can allow increase d brine inflow. The significant increase in brine inflow at 550 years shown in Figure 3

- 1 8 is a direct result of the significant pressure drop (due to the change in borehole plug permeability) at 550 years shown in Figure 3

- 1 6

. As would be expected, waste panel brine saturation also significantly increases at 550 years, commensurate with the increase in brine inflow (Camphouse et al. 2012 a ).

The overall means of cumulative waste panel brine inflow are compared for the PCS

- 2012 PA

and the PABC

- 2009 PA in Figure 3

- 1 9 . As is evident in that figure, the mean waste panel cumulative brine inflow is greater in the PCS

- 2012 PA. This increase is due to the increased waste panel brine inflow that occurs in the first 200 years for undistur bed conditions, shown in Figure 3

- 7 . After the borehole plugs fail at 550 years, a similar increase in brine inflow occurs in both PAs corresponding to similar decreases in waste panel pressure. As would be expected, the

increased brine inflow in the PCS

- 2 012 PA is accompanied by an increase in brine saturation, as shown in Figure 3

- 20

.

As in the undisturbed case, the increase in mean waste panel brine saturation results in an increase in mean waste panel gas generation. Comparative gas generation results for the PCS

2012 PA and the PABC

- 2009 PA are shown in Figure 3

- 21 . The increase in gas generation rates and the accompanying increase in waste panel pressure results in an increase in brine flow up the sealed borehole, as shown in Figure 3

- 22

.50 From Cam phouse et al. 2012 a Figure 3

- 16 . Overall Means of Waste Panel Pressure (WAS_PRES), Scenarios S1

- BF and

S4

- BF for Years 0 to 1,000

From Camphouse et al. 2012 a Figure 3

- 17 . Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S4

- BF51 From Camphouse et al. 2012 a Figure 3

- 18 . Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC),

Scenarios S1

- BF and S4

- BF for Years 0 to 1,000

From Camphouse et al. 2012 a Figure 3

- 19 . Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC),

Scenario S4

- BF52 From Camphouse et al. 2012 a Figure 3

- 20 . Overall Means of Waste Panel Brine Saturation (WAS_SATB),

Scenario S4

- BF

From Camphouse et al. 2012 a Figure 3

- 21 . Overall Means of Waste Panel Gas Generat ion (GASMOL_W),

Scenario S4

- BF53 From Camphouse et al. 2012 a Figure 3

- 22 . Overall Means of Brine Flow up the Borehole (BNBHUDRZ), Scenario S4

- BF

3 .5.1. 4 Results for an E2 Intrusion at 1000 Years Followed by an E1 Intrusion at 2000 Years (Scenario S6

- BF)

This BRAGFLO scenario models an E2 intrusion occurring at 1000 years, followed by an E1 intrusion into the same panel at 2000 years. The effects of these two intrusions are qualitatively

similar to those o f individual E1 and E2 intrusions discussed in Sections 3 .5.1.2 and 3 .5.1.3 above. However, brine flows up the intrusion borehole calculated in this scenario are used in PA code PANEL to determine the radionuclide source term to the Culebra (Camphouse et a l. 2012 a ).

Releases to the Culebra are discussed in Section 3 .5.5 below.

3.5.2 Cuttings and Cavings Releases

Cuttings and cavings releases and direct brine releases (DBRs) were the two primary sources contributing to total releases in the PABC

- 2009 PA, and co ntinue to be so in the PCS

- 2012 PA. Cuttings and cavings releases are not impacted by the change in panel closure design, and so remain unchanged from those calculated in the PABC

- 2009 PA (Camphouse et al. 2012 a ).

3.5.3 Direct Brine Releases

Direct Brine Rele ases ( DBRs ) are calculated using the BRAGFLO DBR version of the BRAGFLO code in WIPP PA. These two uses of BRAGFLO require different computational grids. Results obtained from the BRAGFLO Salado Flow Model brine and gas flow calculations are mapped to the DBR grid to provide initial conditions for borehole intrusions in the DBR calculations. The mapped results include gas pressure, brine saturation, and other repository conditions. The BRAGFLO DBR Model is then used to calculate brine flow up the intruding borehole as a function of the gas pressure, brine saturation, and other properties of the intersected waste panel at the time of intrusion.54

Five intrusion scenarios are considered in the DBR calculations, and are listed in Table 3

- 6 . The types of intrusi ons considered in the DBR calculation and the times at which they occur are shown in the table. Repository conditions corresponding to other times are interpolated from the conditions calculated at the times shown in the table. The scenarios, intrusion loc ations, and timings used for the PCS

- 2012 PA are the same as those used for the PABC

- 2009 PA.

Table 3

- 6 . Intrusion Scenarios Used in Calculating Direct Brine Releases

Scenario First Intrusion Time

(year and type) Intrusion Times in Subsequent Years (year)

S1

- DBR None 100, 350, 1000, 3000, 5000, 10000

S2

- DBR 350, E1 550, 750, 2000, 4000, 10000

S3

- DBR 1000, E1 1200, 1400, 3000, 5000, 10000

S4

- DBR 350, E2 550, 750, 2000, 4000, 10000

S5

- DBR 1000, E2 1200, 14 00, 3000, 5000, 10000 From Camphouse et al. 2012 a ; DBR = Direct Brine Release

Maximum DBR volumes consistently increased from PABC

- 2009 to the PCS

- 2012 PA. The largest increases were observed in Scenarios S4

- DBR and S5

- DBR, which are associated with

E2 intrusions. As previously discussed, E2 intrusion scenarios in the PCS

- 2012 PA yielded waste panel pressures at the time of intrusion that were higher, on average, than in the PABC

- 2009 PA and the pressures remained higher for the duration of the 10,000 y ear regulatory period. Similarly, the mean waste panel brine saturation was higher at the time of intrusion in the PCS

2012 PA, resulting in higher long

- term waste panel brine saturations. Because DBR volumes are strongly dependent on waste panel pressure and brine saturation at the time of intrusion, increases in these two quantities led to the increased maximum DBR volumes observed in the two E2 intrusion scenarios. The higher pressures and brine saturations also led to a higher overall

number of non

- zero direct brine releases (Camphouse et al. 2012 a ).

A more moderate increase in maximum DBR volumes was observed for E1 intrusions. SNL concludes that this is because the generally lower long

- term permeability of the ROM salt as compared to Option D PCS res ults in increased waste panel gas pressures which tend to inhibit brine flows from an intruded Castile brine reservoir into the waste panel. Because a combination of increased pressure and increased brine saturation are needed for a direct brine release to the ground surface, the reduced brine saturation resulted in only a relatively slight 8% to 10% increase in the maximum DBR volumes for E1 intrusions (Camphouse et al. 2012 a ).

Although the increase in the number of non

- zero DBR volumes was relatively sm all (about 6%) and the increase in average release volume was also small (about 10%), the maximum release volume more than doubled for E2 intrusions, resulting in a noticeable increase in overall mean DBRs (Camphouse et al. 2012 a ). Single realization resul ts such as this occur due to rare combinations of pressure and saturation that are highly favorable to generating extreme events. A comparison of overall mean DBRs between the PCS

- 2012 PA and the PABC

- 2009 PA is shown in Figure 3

- 23

.55 From Camphouse et a l. 2012 a Figure 3

- 23 . PCS

- 2012 PA and PABC

- 2009 Overall Mean CCDFs for

No rmalized Direct Brine Releases

3.5.4 Spallings Releases

Increases in repository pressures seen in the PCS

- 2012 PA BRAGFLO results have incr eased spallings releases. The volume of solid waste material released to the ground surface as spallings is calculated at four values of repository pressure (10, 12, 14, and 14.8 MPa). The code CUTTINGS_S is then used to linearly interpolate between these DRSPALL volumes to determine releases at the pressures calculated by BRAGFLO.

DRSPALL volumes used in the PABC

- 2009 were also used in the PCS

- 2012 PA. The intrusion scenarios and times used in the calculation of spallings volumes are the same as those pre sented

in Table 3

- 6 for the calculation of DBRs, and intermediate results are interpolated in the same

manner as for DBRs.

The PCS

- 2012 PA results show increases in the maximum spallings volumes across all three replicates. Replicate 1 showed very simila r average nonzero spallings volumes in both the PCS

2012 and the PABC

- 2009 PA calculations. However, in Replicates 2 and 3 the average nonzero spallings volumes were higher for the PCS

- 2012 PA calculations compared to PABC

- 2009, with the most significant v olume increases occurring in Scenarios S2

- DBR, S3

- DBR, and S5

- DBR. Considering the total from all scenarios across all three replicates, the general trend shows a slightly higher average nonzero spallings volume, a larger maximum volume, and a larger

perce ntage of vectors with spallings (Kicker 2012). The increases in spallings volumes and in the number of vectors that result in nonzero spallings volume translate to an increase in spallings releases, because both analyses use the same waste inventory (Camph ouse et al. 2012 a ). A comparison of overall mean spallings releases between the PCS

- 2012 and the PABC

- 2009 PAs is shown in Figure 3

- 24 . As can be seen in the figure, releases are noticeably larger in the PCS

2012 PA than in the PABC

- 2009 PA.56 From Campho use et al. 2012 a Figure 3

- 24 . PCS

- 2012 PA and PABC

- 2009 Overall Mean CCDFs for Normalized Spallings

Releases

3.5.5 Culebra Releases

As discussed in Section 3 .5.1.3, brine flows up the intrusion borehole to the Cu lebra dolomite are

calculated in scenario S6

- BF ( an E2 intrusion at 1000 years foll owed by an E1 intrusion at

2000 years). These results are used in PA code PANEL to determine the radionuclide source term to the Culebra. Mean brine flows up the intrusion b orehole determined in the PCS

- 2012 and

PABC

- 2009 PAs are shown in Figure 3

- 25 . As shown in that figure, the mean flows are essentially the same. Based on this result and because the radionuclide source terms and Culebra flow conditions were unchanged, SNL concluded that releases from the Culebra would be essentially the same in both PAs

. SNL therefore used the results obtained from the PABC

- 2009

PA in the PCS

- 2012 PA (Camphouse et al. 2012 a ). The Agency agrees with SNL’s logic and

concurs with this decision

.

From Camphouse et al. 2012 a Figure 3

- 25 . Overall Means of Brine Flow up the Borehole (BNBHUDRZ), Scenario S6

- BF57

3.5.6 Total Releases

Total normalized releases are calculated as the sum of the releases from t he individual pathways.

Overall mean releases for the PCS

- 2012 PA from the four primary contributing pathways

— cuttings and cavings, DBRs, spallings, and Culebra pathways

— are shown along with total releases in Figure 3

- 2 6 . As can be seen from the figure, cu ttings and cavings releases dominate at higher probabilities and DBRs dominate at lower probabilities. Releases from spallings and Culebra pathways are approximately an order of magnitude lower than cuttings and cavings at high probabilities and are also a n order of magnitude lower than DBRs at low probabilities. Spallings and Culebra releases therefore have only a small effect on total releases.

Total normalized releases for the PCS

- 2012 PA are shown by replicate and vector in Figures 3

- 2 7 through 3

- 2 9 . O ut of the 300 vectors in the PA, one vector in Replicate 2 exceeded the regulatory limit. The exceedance was at a high probability and was due to unusually high cuttings and cavings releases in that vector; however, as previously mentioned, cuttings and

ca vings releases in the PCS

- 2012 PA were unchanged from those in the PABC

- 2009 PA and this

exceedance was unrelated to the proposed PCS design modification. The EPA limits apply not to single vectors but to the mean releases and to a demonstration that there is at least a 95 % level of statistical confidence that the mean release meets the Agency’ s containment requirements (EPA 1996). The overall mean and 95% confidence limits are shown for the PCS

- 2012 PA in Figure 3

- 30 . As shown in the figure, the mean relea se and upper 95% confidence limit on the mean did not exceed the Agency’s release limits

.

From Camphouse et al. 2012 a Figure 3

- 26 . PCS

- 2012 PA Primary Components Contributing to

Total Normalized Releases58 Modified f

rom Camphouse et al. 2012 a Figure 3

- 27 . PCS

- 2012 PA Replicate 1 Total Normalized Releases

Modified f

rom Camphouse et al. 2012 a Figure 3

- 28 . PCS

- 2012 PA Replicate 2 Total Normalized Releases59 Modified f

rom Camphouse et al. 2012 a Figure 3

- 29 . PCS

- 2012 PA Replicate 3 Total Normalized Releases

From Camphouse et al. 2012 a Figure 3

- 30 . PCS

- 2012 PA Confidence Limits on Overall Mean for

Total Normalized Releases

Comparisons of overall mean total releases between the PCS

- 2012 and the PABC

- 2009 PAs are shown in Figure 3

- 31 and Table 3

- 7 . As can be seen in t he figure, mean total releases in the PCS

2012 PA did not noticeably increase at a probability of 0.1 but did increase at a probability of 0.001. This increase is primarily due to increases in DBRs, but the overall mean for the PCS

2012 PA remains below th e Agency’s regulatory limits at both points of compliance. Table 3

- 7 provides total release statistics at probabilities of 0.1 and 0.001 for the two PAs, and also provides the Agency’s regulatory limits at those probabilities for comparison. As can be seen60

from the table, total releases were higher in the PCS

- 2012 PA than in the baseline PABC

- 2009 PA but remained below the regulatory limits for the mean total release as well as the 90 th
percentile and upper 95 % confidence limit of the mean. Because the mea n total repository releases continued to remain below their regulatory limits in the PCS

- 2012 PA, DOE concluded

that if the Option D panel closure is replaced with the ROM salt PCS design , the WIPP repository would remain in compliance with the containment requirements of 40 CFR Part 191 (Camphouse et al. 2012 a ).

From Camphouse et al. 2012 a

Figure 3

- 31 . PCS

- 2012 PA and PABC

- 2009 Overall Mean CCDFs for

Total Normalized Releases

Table 3

- 7 . Total Normalized Release Statistics for the PCS

- 2012 and PABC

- 2009 PAs

Probability Analysis Mean

Total

Release 90 th

Percentile Lower 95%

Confidence

Limit Upper 95%

Confidence

Limit EPA Release

Limit

0.1 PCS

- 2012 PA 0.10 0.17 0 .10 0.10 1

PABC

- 2009 PA 0.09 0.16 0.09 0.10 1

0.001 PCS

- 2012 PA 1.51 1.00 0.33 2.81 10

PABC

- 2009 PA 1.10 1.00 0.37 1.77 10 From Camphouse et al. 2012 a ; Values in EPA units.

3.6 SNL P ARAMETER SENSITIVITY ANALYSIS

SNL performs a parameter sensitivity an alysis following completion of each PA to identify the physical properties with uncertain values that most strongly influence the variability in PA results. The sensitivity analysis for the PCS

- 2012 PA was documented by Kirchner (2012) and is

the source of the information presented in this section. Physical properties that are potentially important to repository performance and have uncertain values, such as the permeability of the ROM salt 100 years after repository closure, are sampled in WIPP PA. Such pa rameters are assumed to have specific values but those values are uncertain because of a lack of knowledge.61

Additional uncertainty arises because of a lack of knowledge about the occurrence of future events, such as the timing and location of a drilling in trusion. This type of uncertainty is not addressed in this sensitivity analysis. This sensitivity analysis attempts to resolve the question of

which sampled physical properties contribute the most to the variability (uncertainty) observed in

the PCS

- 2012 P A results.

The code STEPWISE Version 2.21 was used by SNL to determine the relative importance of the sampled parameters in the PCS

- 2012 PA. Sampled parameter values and corresponding release data calculated by the PA codes are input into STEPWISE, which then relates the releases with the parameter values through stepwise linear multivariate regression analyses

. The results for the

PCS

- 2012 PA are compared with those from the baseline PABC

- 2009 PA to determine whether the PCS design changed the relative im portance of the sampled parameters.

One of the assumptions of the statistical model used in STEPWISE is that the dependent (output) variable (the repository release) shows a linear response to the independent (input) variables (the sampled parameters). Th e coefficient of determination (R 2 ) serves as a measure of the degree to which a given parameter or set of parameters contribute to output variability. The single parameter with the highest R 2 value is most strongly correlated with output variability and i s therefore the greatest contributor to that variability. This parameter is followed in ranked order by other parameters that, when included, cumulatively improve the correlation and therefore sequentially contribute to the variability. The cumulative R 2 v alue represents the proportion of total variation explained by the fitted regression using the listed variables, starting with the greatest contributor to the variance. SNL concludes that the results are usually well suited for ranking the importance of th e contributions of the input variables to the response of the output variable ( Kirchner 2012)

.

Most of the regression models produced by STEPWISE do not include all of the variable parameters, even after ranking the data. This indicates that the uncertain ties in many of the parameters have statistically insignificant effects on the releases. This is not to say that these

non

- significant parameters have no influence on the releases. Their exclusion from the tabulated results reflects the inability of this s tatistical technique to rank their importance with an acceptable degree of confidence. SNL notes that correlations that explain only a few percent of the variation can occur due to random sampling and may be spurious ( Kirchner 2012 )

.

For an input variable to be retained, its Standard Rank Regression Coefficient (SRRC) must be statistically distinguishable from zero. The regression coefficient is a measure of the linear contribution of an input to the prediction of the output variable. Negative regression c oefficients indicate an inverse relationship between the input and output variables, where a decrease in an input variable results in an increase in an output variable. Both the cumulative R 2 value and the SRRC value are presented in the tabulated results of SNL’s sensitivity analysis and are reproduced in this Agency review.

3.6.1 Parameter Regression Results

SNL’s sensitivity study results for mean total releases in PCS

- 2012 and PABC

- 2009 Replicate 1 are presented in Table 3

- 8 . As described in Section 3 .5, cu ttings and cavings, direct brine, and62

spallings releases account for the majority of the total releases estimated in both the PCS

- 2012

and the PABC

- 2009 PAs. Also in both PAs, the variable parameter BOREHOLE:TAUFAIL was the greatest single contributor to t he uncertainty in mean total releases. This parameter is the shear strength of degraded waste and is used in calculating cavings releases. There is little consistency in the ranking of the other variable parameters identified as less significant

contributo rs to the variability in total releases. These other parameters contribute less than an additional 14 % to that variability.

Table 3

- 8 . Stepwise Ranked Regression Results for Mean Total Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

Step PCS

- 2012 Replicate 1 PABC

- 2009 Replicate 1

Variable R 2 SRRC Variable R 2 SRRC

1 BOREHOLE:TAUFAIL 0.69

- 0.83 BOREHOLE:TAUFAIL 0.76

- 0.88

2 CASTILER:PRESSURE 0.71 0.16 SOLMOD3:SOLVAR 0.79 0.17

3 DRZ_1:PRMX_LOG 0.74 0.17 CELLUL S:FBETA 0.81

- 0.14

4 SOLMOD3:SOLVAR 0.76 0.14 CASTILER:PRESSURE 0.83 0.12

5 S_HALITE:POROSITY 0.78 0.13 GLOBAL:PBRINE 0.85 0.13

6 PCS_T1:SAT_IBRN 0.79

- 0.12 SHFTU:SAT_RGAS 0.85

- 0.10

7 GLOBAL:TRANSIDX 0.80 0.12 GLOBAL:TRANSIDX 0.86 0.09

8 CULEBRA:MINP _FAC 0.81 0.09 BOREHOLE:DOMEGA 0.87 0.08

9 S_MB139:RELP_MOD 0.82

- 0.10

10 S_MB139:SAT_RBRN 0.83

- 0.09

From Kirchner 2012

Only two parameters identified in the analysis for PCS

- 2012 Replicate 1 are related to the proposed PCS design chang e: DRZ_1:PRMX_LOG, the log permeability of the PCS DRZ during time periods T1 and T2, and PCS_T1:SAT_IBRN, the initial brine saturation of the ROM salt. Parameter DRZ_1:PRMX_LOG contributed about 3% to the variability in total releases and

is the third mos t significant parameter in the list. The initial brine saturation contributed only about 1%, which is not considered significant and may be spurious.

Ranked regression results for mean total releases in PCS

- 2012 Replicates 2 and 3 are presented

in Table 3

- 9 . Again, the parameter BOREHOLE:TAUFAIL was the greatest single contributor to the uncertainty in total releases for both replicates. Two parameters identified in the analysis of Replicate 2 are related to the proposed PCS design change: PCS_T1:POROSITY, the ROM salt porosity during T1 ; and PCS_T1:PORE_DIS, the ROM salt pore distribution parameter during T1. Only one parameter identified in the analysis of Replicate 3 is related to the proposed PCS design change: DRZ_1:PRMX_LOG, the permeability of the PC S DRZ during time periods T1 and T2. Each of these parameters contributed only about 1% to the variability in total mean releases. The Agency notes that parameter DRZ_1:PRMX_LOG was also identified in the analysis for Replicate 1.63 Table 3

- 9 . Stepwise Ranked Regression Results for Mean Total Releases,

PCS

- 2012 Replicates 2 and 3

Step PCS

- 2012 Replicate 2 PCS

- 2012 Replicate 3

Variable R 2 SRRC Variable R 2 SRRC

1 BOREHOLE:TAUFAIL 0.71

- 0.84 BOREHOLE:TAUFAIL 0. 74

- 0.85

2 CASTILER:PRESSURE 0.75 0.22 S_HALITE:POROSITY 0.77 0.18

3 CULEBRA:MINP_FAC 0.77

- 0.14 CASTILER:PRESSURE 0.80 0.15

4 S_HALITE:POROSITY 0.79 0.12 SOLMOD3:SOLVAR 0.81 0.12

5 SOLMOD3:SOLVAR 0.80 0.12 BOREHOLE:DOMEGA 0.82 0.12

6 SPALLMOD:REPIPER M 0.81

- 0.10 GLOBAL:PBRINE 0.83 0.10

7 PCS_T1:POROSITY 0.82

- 0.10 GLOBAL:PBRINE 0.84 0.09

8 PCS_T1:PORE_DIS 0.83

- 0.09 DRZ_1:PRMX_LOG 0.85 0.09

9 CASTILER:PRMX_LOG 0.84 0.09 GLOBAL:TRANSIDX 0.86 0.08

10 STEEL:CORRMCO2 0.86

- 0.08

From Kirc hner 2012

3.6.2 Sensitivity of Cuttings and Cavings Releases

Ranked regression results for mean cuttings and cavings releases in PCS

- 2012 and PABC

- 2009 Replicate 1 are presented in Table 3

- 10 . Cuttings and cavings are the greatest source of releases at higher probabilities because they are removed to the ground surface in every borehole intrusion. The parameter BOREHOLE:TAUFAIL remained the greatest single contributor to the uncertainty in cuttings and cavings releases and accounted for nearly all the uncertain ty in those releases. As is evident in the table, the results of the regression analysis for PCS

- 2012 and

PABC

- 2009 Replicate 1 are identical. The only parameter related to the proposed PCS design change that was identified in the PCS

- 2012 Replicate 1 anal ysis was DRZ_PCS:PRMX_LOG, the permeability of the DRZ surrounding the ROM salt backfill during time period T3. This parameter accounted for less than 1% of the uncertainty in cuttings and cavings releases and is considered by SNL to have had an insignific ant effect (Kirchner 2012). This same parameter was

identified in the PABC

- 2009 Replicate 1 analysis but it represented the DRZ surrounding the Option D concrete monolith in that PA. As discussed in Section 3 .4.3, the same permeability range and distributi on was used for this parameter in both PAs.

Ranked regression results for mean cuttings and cavings releases in PCS

- 2012 Replicates 2 and 3 are presented in Table 3

- 11 . The only parameter related to the proposed PCS design change that

was identified in th ese analyses was PCS_T2:POR2PERM, the distribution used to calculate permeability from sampled porosity values during time period T2. This parameter only appeared in the analysis of Replicate 2. It accounted for less than 1% of the uncertainty in cuttings and cavings releases and is considered by SNL to have had an insignificant effect. No parameters related to the proposed PCS design change were identified in Replicate 3. The parameter SHFTL_T1:PRMX_LOG in Replicate 2 applies to the lower shaft and not to the revised PCS design.64

Table 3

- 10 . Stepwise Ranked Regression Results for Mean Cuttings and Cavings Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

Step PCS

- 2012 Replicate 1 PABC

- 2009 Replicate 1

Variable R 2 SRRC Variable R 2 SRRC

1 BOREHOLE:TAUFAIL 0.98

- 0.99 BOREHOLE:TAUFAIL 0.98

- 0.99

2 BOREHOLE:DOMEGA 1.00 0.11 BOREHOLE:DOMEGA 1.00 0.11

3 SHFTL_T1:PRMX_LOG 1.00 0.02 SHFTL_T1:PRMX_LOG 1.00 0.02

4 (Composite):OXSTAT 1.00

- 0.02 (Composite):OXSTAT 1.00

- 0.0 2

5 CULEBRA:HMBLKLT 1.00 0.02 CULEBRA:HMBLKLT 1.00 0.02

6 DRZ_PCS:PRMX_LOG 1.00 0.01 DRZ_PCS:PRMX_LOG 1.00 0.01

From Kirchner 2012

Table 3

- 11 . Stepwise Ranked Regression Results for Mean Cuttings and Cavings Releases,

PCS

- 2012 Replicates 2 and 3

Step PCS

- 2012 Replicate 2 PCS

- 2012 Replicate 3

Variable R 2 SRRC Variable R 2 SRRC

1 BOREHOLE:TAUFAIL 0.98

- 0.99 BOREHOLE:TAUFAIL 0.98

- 0.99

2 BOREHOLE:DOMEGA 0.99 0.11 BOREHOLE:DOMEGA 0.99 0.12

3 SHFTL _T1:PRMX_LOG 0.99 0.02 (Composite):OXSTAT 0.99 0.02

4 PCS_T2:POR2PERM 0.99 0.02 SOLMOD3:SOLVAR 1.00

- 0.02

5 STEEL:CORRMCO2 1.00 0.02

6 GLOBAL:TRANSIDX 1.00 0.01

From Kirchner 2012

3.6.3 Sensitivity of Direct Brine Releases

Ranked regressio n results for mean direct brine releases (DBRs) in PCS

- 2012 and PABC

- 2009 Replicate 1 are presented in Table 3

- 12 . DBRs are the greatest source of releases at lower probabilities because they occur only under specific, relatively rare combinations of waste panel pressure and brine saturation. As evidenced from the sensitivity study results, variability in DBRs is influenced by many parameters and no single parameter dominates. The only parameter related to the proposed PCS design change that was identified in the PCS

- 2012 Replicate 1 analysis was DRZ_1:PRMX_LOG, the log permeability of the DRZ surrounding the ROM salt during time periods T1 and T2. This parameter accounted for about 2% of the variability in

DBRs and was expected because a higher PCS DRZ perm eability in early time allows more brine to flow into the waste panels. The resulting increase in brine saturation increases the

likelihood of a DBR

.

Ranked regression results for mean DBRs in PCS

- 2012 Replicates 2 and 3 are presented in

Table 3

- 13 . Two p arameters related to the proposed PCS design change were identified in both replicates: DRZ_PCS:PRMX_LOG, the log permeability of the PCS DRZ during time period T3, and DRZ_1:PRMX_LOG, the log permeability of the PCS DRZ during time periods T1 and T2.

The first of these, DRZ_PCS:PRMX_LOG, accounted for 5% of the variability in DBRs in each replicate. The second, DRZ_1:PRMX_LOG, accounted for 2% of the variability in DBRs in each replicate. A third parameter, PCS_T3:POROSITY, the porosity of the ROM salt in time period T3, was only identified in Replicate 3 and accounted for about 1% of the variability in DBRs. Although the contribution of PCS_T3:POROSITY on the variability in DBRs is not significant,65

the contributions of DRZ_PCS:PRMX_LOG and DRZ_1:PRMX_LOG a re potentially significant because of their higher magnitudes and/or frequencies of occurrence. Both had positive regression coefficients, indicating that increases in PCS DRZ permeability are correlated with increasing DBRs.

Table 3

- 12 . Stepwise Ranked Regression Results for Mean Direct Brine Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

Step PCS

- 2012 Replicate 1 PABC

- 2009 Replicate 1

Variable R 2 SRRC Variable R 2 SRRC

1 CASTILER:PRESSURE 0.15 0.40 SOLMOD3:S OLVAR 0.19 0.42

2 SOLMOD3:SOLVAR 0.28 0.36 CASTILER:PRESSURE 0.35 0.40

3 STEEL:CORRMCO2 0.40

- 0.33 GLOBAL:PBRINE 0.45 0.33

4 GLOBAL:PBRINE 0.47 0.26 STEEL:CORRMCO2 0.55

- 0.30

5 S_HALITE:POROSITY 0.50 0.17 S_MB139:RELP_MOD 0.58

- 0.18

6 S_MB139:RELP_MOD 0.53

- 0.19 WAS_AREA:BIOGENFC 0.60 0.15

7 WAS_AREA:SAT_WICK 0.56

- 0.15 S_HALITE:POROSITY 0.63 0.15

8 DRZ_1:PRMX_LOG 0.58 0.15 CONC_PCS:SAT_RGAS 0.65

- 0.14

9 SOLMOD4:SOLVAR 0.60 0.16 S_MB139:PRMX_LOG 0.67 0.14

10 SHFTU:SAT_RBRN 0.62 0.14 WAS_AREA:SAT_WI CK 0.69

- 0.14

11 BOREHOLE:DOMEGA 0.64

- 0.14

12 WAS_AREA:BIOGENFC 0.66 0.13

13 WAS_AREA:SAT_RBRN 0.67

- 0.13

From Kirchn er 2012

Table 3

- 13 . Stepwise Ranked Regression Results for Mean Direct Brine Releases,

PCS

- 2012 Replicates 2 and 3

Step PCS

- 2012 Replicate 2 PCS

- 2012 Replicate 3

Variable R 2 SRRC Variable R 2 SRRC

1 SOLMOD3:SOLVAR 0.16 0.43 CASTILER:PRESSURE 0.20 0.41

2 CASTILER:PRESSURE 0.31 0.31 SOLMOD3:SOLVAR 0.38 0.42

3 STEEL:CORRMCO2 0.44

- 0.38 STEEL:CORRMCO2 0.48

- 0.32

4 GLOBAL:PBRINE 0.52 0.29 GLOBAL:PBRINE 0.56 0.28

5 WAS_AREA:SAT_WICK 0.57

- 0.22 CULEBRA:MINP_FAC 0.60 0.22

6 DRZ_PCS:PRMX_LOG 0.62 0.22 DRZ_PCS:PRMX_LOG 0.65 0.20

7 WAS_AREA:SAT_RBRN 0.65

- 0.18 WAS_AREA:SAT_RBRN 0 .68

- 0.18

8 S_MB139:PRMX_LOG 0.67 0.17 DRZ_1:PRMX_LOG 0.70 0.17

9 WAS_AREA:PROBDEG 0.70

- 0.17 CASTILER:COMP_RCK 0.72

- 0.16

10 DRZ_1:PRMX_LOG 0.72 0.14 SPALLMOD:REPIPOR 0.74 0.13

11 SHFTU:PRMX_LOG 0.74 0.16 WAS_AREA:SAT_WICK 0.75

- 0.12

12 BH_SAND:PRMX_ LOG 0.76

- 0.14 PCS_T3:POROSITY 0.76 0.11

13 S_HALITE:POROSITY 0.78 0.14 SPALLMOD:TENSLSTR 0.78 0.11

14 CULEBRA:APOROS 0.80 0.12 GLOBAL:CLIMTIDX 0.79 0.10

15 SPALLMOD:REPIPOR 0.81 0.11 S_HALITE:POROSITY 0.80 0.10

From Kirchner 2012

3.6.4 Sensitivity of Spallings Releases

Ranked regression results for mean spallings releases in PCS

- 2012 and PABC

- 2009 Replicate 1 are presented in Table 3

- 14 . Spallings releases are smaller than DBRs and cuttings and cavings releases, and have only a small effect on total r epository releases. As evidenced from the66

sensitivity study results, variability in spallings releases is influenced by many parameters and no single parameter dominates. The only parameter related to the proposed PCS design change that

was identified in t he PCS

- 2012 Replicate 1 analysis was DRZ_PCS:PRMX_LOG, the log permeability of the DRZ surrounding the ROM salt during time period T3. This parameter accounted for about 4% of the variability in spallings releases in that replicate. This same
parameter was identified in the PABC

- 2009 Replicate 1 analysis but it represented the DRZ surrounding the Option D concrete monolith in that PA. As discussed in Section 3 .4.3, the same permeability range and distribution was used for this parameter in both PAs.

Table 3

- 14 . Stepwise Ranked Regression Results for Mean Spallings Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

Step PCS

- 2012 Replicate 1 PABC

- 2009 Replicate 1

Variable R 2 SRRC Variable R 2 SRRC

1 S_HALITE:POROSITY 0.13 0.37 S_HALITE:POROSITY 0.14 0.34

2 SPALLMOD:PARTDIAM 0.21

- 0.29 SPALLMOD:PARTDIAM 0.22

- 0.28

3 DRZ_PCS:PRMX_LOG 0.25

- 0.21 CULEBRA:DPOROS 0.26

- 0.23

4 SPALLMOD:REPIPOR 0.29

- 0.20 CASTILER:PRESSURE 0.30 0.19

5 BH_SAND:PRMX_LOG 0.32

- 0.18 SHFTL_T2: PRMX_LOG 0.33

- 0.18

6 WAS_AREA:PROBDEG 0.36 0.17

7 DRZ_PCS:PRMX_LOG 0.39

- 0.18

8 S_MB139:PRMX_LOG 0.42 0.16

From Kirchner 2012

Ranked regression results for mean spallings releases in PCS

- 2012 Replicates 2 and 3 are presented in Table 3

- 15 . The parameter DRZ_PCS:PRMX_LOG, the log permeability of the PCS DRZ in time period T3, is the only parameter related to the proposed PCS design change and was identified in both replicates. This parameter accounted for 3% of the variability in spallin gs releases in Replicate 2 and 7% of the variability in Replicate 3. The regression coefficients for these parameters were negative, indicating that an increase in the value of DRZ_PCS:PRMX_LOG results in a decrease in spallings releases. This relationship was expected because higher PCS DRZ permeabilit y reduce s the gas pressures and brine saturations that drive spallings releases.

Table 3

- 15 . Stepwise Ranked Regression Results for Mean Spallings Releases,

PCS

- 2012 Replicates 2 and 3

Step PCS

- 2012 Replicate 2 PCS

- 2012 Replicate 3

Variable R 2 SRRC Variable R 2 SRRC

1 S_HALITE:POROSITY 0.15 0.37 S_HALITE:POROSITY 0.17 0.41

2 BH_SAND:PRMX_LOG 0.22

- 0.28 SPALLMOD:PARTDIAM 0.27

- 0.28

3 CASTILER:PRESSURE 0.29 0.2 7 DRZ_PCS:PRMX_LOG 0.34

- 0.26

4 SPALLMOD:PARTDIAM 0.34

- 0.23 WAS_AREA:SAT_RGAS 0.37

- 0.18

5 WAS_AREA:GRATMICI 0.37 0.18 SPALLMOD:TENSLSTR 0.40

- 0.18

6 SHFTL_T1:PRMX_LOG 0.40 0.16 STEEL:CORRMCO2 0.43 0.16

7 DRZ_PCS:PRMX_LOG 0.43

- 0.16 BH_SAND:PRMX_LOG 0 .46

- 0.16

From Kirchner 201267

3.6.5 Sensitivity of Culebra Releases

Ranked regression results for mean Culebra releases in PCS

- 2012 and PABC

- 2009 Replicate 1 are presented in Table 3

- 16 . Culebra releases are smallest of all releases evaluated in SNL’s

sen sitivity study except at very low probabilities and contribute little to total repository releases (Kirchner 2012). As with DBR and spallings releases, variability in Culebra releases is influenced by many parameters and no single parameter dominates. Near ly all parameters influencing the variability of Culebra releases in the PABC

- 2009 PA also influenced variability

in the PCS

- 2012 PA.

Table 3

- 16 . Stepwise Ranked Regression Results for Mean Culebra Releases,

PCS

- 2012 and PABC

- 2009 Replicate 1

Step PCS

- 2012 Replicate 1 PABC

- 2009 Replicate 1

Variable R 2 SRRC Variable R 2 SRRC

1 BH_SAND:PRMX_LOG 0.46 0.63 BH_SAND:PRMX_LOG 0.46 0.63

2 (Composite):MKD_U 0.58

- 0.32 (Composite):MKD_U 0.58

- 0.36

3 PCS_T2:POR2PERM 0.62

- 0.20 DRZ_PCS:PRMX_LOG 0.61 0.17

4 DRZ_PCS:PRMX_LOG 0.65 0.17 SOLMOD4:SOLVAR 0.63

- 0.17

5 CULEBRA:HMBLKLT 0.67 0.15 CULEBRA:HMBLKLT 0.65 0.16

6 SOLMOD4:SOLVAR 0.70

- 0.16 CULEBRA:APOROS 0.67

- 0.14

7 CULEBRA:APOROS 0.71

- 0.13 (Composite):MKD_PU 0.6 9

- 0.14

8 STEEL:CORRMCO2 0.73

- 0.12 STEEL:CORRMCO2 0.71

- 0.13

9 GLOBAL:TRANSIDX 0.74 0.11

From Kirchner 2012

Two parameters related to the proposed PCS design change were identified in the PCS

- 2012 Replicate 1 analysis: PCS_T2:POR2PERM, the dis tribution used to calculate permeability from sampled porosity values during time period T2, and DRZ_PCS:PRMX_LOG, the log permeability of the DRZ surrounding the ROM salt during time period T3. The parameter PCS_T2:POR2PERM accounted for about 4% of the v ariability in Culebra releases, and the parameter DRZ_PCS:PRMX_LOG accounted for about 3% of the variability. The parameter DRZ_PCS:PRMX_LOG also accounted for about 3% of the variability in the PABC

- 2009 Replicate 1 analysis but it represented the DRZ sur rounding the Option D concrete monolith in

that PA. As discussed in Section 3 .4.3, the same permeability range and distribution was used for this parameter in both PAs.

Ranked regression results for mean Culebra releases in PCS

- 2012 Replicates 2 and 3 are presented in Table 3

- 1 7 . Three parameters related to the proposed PCS design change were

identified in the PCS

- 2012 Replicate 3 analysis: PCS_T1:PORE_DIS, the pore distribution parameter for the ROM salt during time period T1, DRZ_PCS:PRMX_LOG, the log

pe rmeability of the DRZ surrounding the ROM salt during time period T3, and PCS_T1:PRMX_LOG, the log permeability of the ROM salt during time period T1. Parameters PCS_T1:PORE_DIS and DRZ_PCS:PRMX_LOG each accounted for about 2% of the variability

in Culebra releases, and PCS_T1:PRMX_LOG accounted for about 1% of the variability.

No parameters related to the proposed PCS design change were identified in the PCS

- 2012 Replicate 2 analysis. The regression coefficients for these parameters were positive, indicati ng that increases in their values results in increases in Culebra releases.68

Table 3

- 17 . Stepwise Ranked Regression Results for Mean Culebra Releases,

PCS

- 2012 Replicates 2 and 3

Step PCS

- 2012 Replicate 2 PCS

- 2012 Replicate 3

Variable R 2 SRRC Variable R 2 SRRC

1 BH_SAND:PRMX_LOG 0.55 0.71 BH_SAND:PRMX_LOG 0.41 0.62

2 GLOBAL:OXSTAT 0.67 0.22 (Composite):MKD_U 0.55

- 0.31

3 CULEBRA:MINP_FAC 0.70

- 0.16 CULEBRA:APOROS 0.59

- 0.21

4 (Composite):MKD_U 0.73

- 0.21 WAS_AREA:SAT_WICK 0.62

- 0.15

5 CONC_PLG:PRMX_LOG 0.74

- 0.12 GLOBAL:CLIMTIDX 0.64 0.18

6 STEEL:CORRMCO2 0.75

- 0.11 STEEL:CORRMCO2 0.67

- 0.17

7 CULEBRA:HMBLKLT 0.77 0.11 CASTILER:PRESSURE 0.69 0.14

8 AM+3:MKD_AM 0.78

- 0.12 PCS_T1:PORE_DIS 0.71 0.14

9 CA STILER:PRESSURE 0.79 0.11 DRZ_PCS:PRMX_LOG 0.73 0.12

10 PHUMOX3:PHUMCIM 0.80 0.11 PCS_T1:PRMX_LOG 0.74 0.12

11 WAS_AREA:SAT_RGAS 0.81

- 0.10 S_HALITE:COMP_RCK 0.75 0.11

12 CULEBRA:APOROS 0.82

- 0.10 CASTILER:PRMX_LOG 0.76 0.11

13 S_MB139:SAT_RBRN 0.77

- 0.11

From Kirchner 2012

3.6.6 Summary of Sensitivity Results

A summary of the contributions of the parameters related to the proposed PCS design change to the variability in PCS

- 2012 PA releases is presented in Table 3

- 1 8 . Eight parameters were

identi fied in SNL’s sensitivity study as contributing to the mean values of total releases, cuttings and cavings releases, DBRs, spallings releases, or Culebra releases. Of these eight parameters, only two were identified in more than two replicates. Those param eters were the log permeabilit y of the PCS DRZ during the three time periods. The remaining six parameters only appeared sporadically and all, with the exception of PCS_T2:POR2PERM which appeared at a 4% level in Culebra Replicate 1, were at levels of 2% o r less. The Agency considers that correlations to parameters that appear only sporadically, have little direct logical relationship with the type of release, or have weak correlations of 2% or less

, are likely be spurious.

Table 3

- 18 . Summary of Contributions to Variability for Parameters Related to the Proposed PCS Design Change

Variable

Parameter Total Mean

Releases Mean Cuttings

and Cavings

Releases Mean Direct

Brine Releases Mean Spallings

Releases Mean Cu lebra

Releases

R1 R2 R3 R1 R2 R3 R1 R2 R3 R1 R2 R3 R1 R2 R3
DRZ_1:PRMX_LOG 3%

- - 1%

- - - - - - 2% 2% 2%

- - - - - - - - - - - - DRZ_PCS:PRMX_LOG

- - - - - - <1%

- - - - - - 5% 5% 4% 3% 7% 3%

- - 2%
PCS_T2:POR2PERM

- - - - - - - - <1%

- - - - - - - - - - - - - - 4%

- - - - PCS_T1:SAT_IBRN 1%

- - - - - - - - - - - - - - - - - - - - - - - - - - - - PCS_T1:POROSITY

- - 1%

- - - - - - - - - - - - - - - - - - - - - - - - - - PCS_T1:PORE_DIS

- - 1%

- - - - - - - - - - - - - - - - - - - - - - - - 2%
PCS_ T3:POROSITY

- - - - - - - - - - - - - - - - 1%

- - - - - - - - - - - - PCS_T1:PRMX_LOG

- - - - - - - - - - - - - - - - - - - - - - - - - - - - 1% Note: R1, R2, and R3 are replicates of the PCS

- 2012 PA.69

The strongest correlations to the log permeabilit y of the PCS DRZ were for DBRs. DRZ_1:PRMX_LOG was weakly correlated to mean DBRs at a 2% level in all three replicates, and DRZ_PCS:PRMX_LOG was more strongly correlated at a 5% level in Replicates 2 and 3, but not in Replicate 1. DRZ_1:PRMX_LOG was also w eakly correlated with mean total releases in Replicates 1 and 3, and DRZ_PCS:PRMX_LOG was also more strongly correlated with all three replicates of mean spallings releases and with two replicates of mean Culebra releases. These correlations would be expec ted because the permeability of the PCS DRZ largely controls brine and gas flow into and out of a waste panel. Very weak to no correlations were found between the PCS DRZ permeability and cuttings and cavings releases, which is also expected

because these releases are independent of brine and gas flow.

In summary, the Agency concludes that correlations of 1% or less are probably spurious. Correlations of 2% to 3% are weak and may also be spurious unless accompanied by a logical pattern of incidence in mor e than one realization. Higher correlations are stronger, particularly when accompanied by a logical pattern of incidence. The strongest and most frequent correlations involved the influence of DRZ_PCS:PRMX_LOG on DBRs, spallings, and Culebra

releases. The significance of the influence of parameter DRZ_PCS:PRMX_LOG on the results

of the PCS

- 2012 PA was further explored in an EPA sensitivity study of the relationship between the value of that parameter and the permeability of intact halit e, and is documented in

Section 4.1

.

3.7 EPA ASSESSMENT OF DOE’S ANALYSIS

The Agency concludes that the results of the PCS

- 2012 PA indicate that the containment requirements of 40 CFR Part 191 continue to be met with the ROM salt PCS design.

To avoid numerical problems with the BRAGFLO code, DOE adjusted several two

- phase flow parameters for the ROM salt: CAP_MOD, PCT_A, and PCT_EXP. These parameters determine the presence or absence of capillary effects and are described in Section 3 .4.2.2. At the Agency’s request, the sour ce of the numerical problems was identified and corrected by SNL, and Replicate 1 of the PCS

- 2012 PA was rerun by the Agency using more realistic two

- phase flow
parameters . The results of this sensitivity analysis are described in Section 4

. 2 of this TSD

.

The results of the PCS

- 2012 PA also clarified that the time

- dependent permeability of the panel closures has a noticeable effect on repository performance at lower probabilities by changing the gas pressures and brine saturations within the waste panels. The results also clarified that the panel closures should be viewed as a system that includes both the closure itself and the surrounding DRZ. In particular, the DRZ surrounding the ROM salt plays a significant role in the effective permeability of this sy stem because its cross

- section area for brine and gas flow is large compared to that of the ROM salt. As described in Section 3 .4.3.1, a sensitivity analysis was performed by the Agency to determine the effects on releases of correlating the permeability

o f the healed DRZ to the permeability of intact halite. The results of this sensitivity analysis are

described in Section 4 .1 of this TSD.70 

### 4.0 EPA SENSITIVITY ANAL YSIS

The Agency conducted two sensitivity studies to further evaluate DOE’s results. One study

in vestigated the impact on calculated releases if the DRZ surrounding the PCS healed to a lower permeability than the one used in the CRA

- 09 and PCS

- 2012 PAs. The other investigated the impact of activating the capillary pressure model within the panel closu re material. The results of these sensitivity studies are described in the following section.

4.1 SENSITIVITY TO DRZ P ERMEABILITY

The Agency concurs with DOE’s expectation that the backstress imposed on the DRZ by the ROM salt panel closure due to salt creep will result in PCS DRZ properties approaching those of

intact halite after approximately 200 years (T3 in the PCS

- 2012 PA) . A more fully healed DRZ

will acquire properties that resemble a consolidated material rather than the fractured material

modeled du r ing time periods T1 and T2. This section presents results of a sensitivity study performe d by the Agency to evaluate the impact on the PCS

- 2012 PA releases if a lower permeability is assigned to the PCS DRZ.

4.1.1 Conceptualization of DRZ Healing

The PCS

- 201 2 PA was designed to compare the ROM salt panel closure system with Option D panel closure

. The only parameter values modified for the PCS

- 2012 PA were those used for the panel closures, such as density, permeability, porosity and compressibility

. In the P CS

- 2012 PA

, no modifications were made to the coupling between the responses of the adjacent DRZ relative to the changing properties of the ROM salt panel closure system. The Agency believed additional calculations needed to be performed to understand whe ther coupling the properties of the healed PCS DRZ to those of the pre

- damaged halite would affect the PCS

- 2012 PA results for the following reasons.

There is a major difference between the response of the ROM salt PCS and the Option D PCS due the creep c losure. For the ROM salt PCS, the permeability and porosity will become progressively lower

, while density will increase as it consolidates and responds to creep closure

. For the Option D design, the material properties were assumed not to be impacted by c reep closure. Additionally, for the Option D PCS because the DRZ above and below was to be largely

mined out to the anhydrite layers (~1.3 meters in the lower DRZ to the Marker Bed 139, and ~2.6 meters up to Marker Bed AB in the upper DRZ), there would be minimal DRZ halite remaining. Therefore, the properties of the DRZ were modeled as a constant . The DRZ flow properties were not correlated to that of intact halite, but rather to the Option D concrete . In contrast, for the ROM salt PCS, the DRZ halite wil l not be mined and therefore its beh avior over time relative to the ROM salt needs to be considered by assigning parameter values representative of healed DRZ in the T3 time period.

Due to creep closure and consolidation, DOE expects the density, porosity and permeability of ROM salt to approach that of nearly intact halite withi n approximately 200 to 300 years. Creep closure is also expected to impose backstress from the ROM salt to the adjacent DRZ around

300 years. In the PCS

- 2012 PA

, the DRZ properties do not change as a function of backstress71

created by the ROM salt . Additionally, the permeability of the PCS DRZ was based on the 2009 PABC DRZ properties which were similar to those of the anhydrite layers. The Agency believes the response of the compres sive stresses of the ROM salt panel closure imposed on the PCS DRZ between 200 and 300 years will result in a nearly healed PCS DRZ with properties similar to that

of the intact halite. A brief overview of DRZ formation and its response to stress follows.

Due to repository drift excavation

, a DRZ is formed as lithostatic compressive stresses are reduced. This produces dil a tant strain on the in

- situ halite that can eventually produce fractures which are generally parallel to the drift and perpendicular to t he minimum principal stress. Dilatancy linearly increases during deformation and cease s when compressive stresses

commence

. Numerous laboratory and field tests , some of which are described below, indicate

that dil a tant damage in salt is reversible through crack closure and healing under compressive

stress

. S alt creep will impose compressive stresses that will close any gaps between the mine

roof and the ROM salt . Eventually, these compressive stresses a pplied to the DRZ will approach lithostatic pressure , w hich is approximately 14.9 MPa

. DOE expects the se compressive stresses will heal the fractures ( Hansen 2003 )

. Studies suggest that the healing time for the DRZ wi ll be within several hundred years after repository closure . The healed DRZ will eventually be similar

to that of in

- situ halite.

Pfeifle and Hurtado (1998) performed salt healing experiments on stress induced fractures by applying compressive pressures of 1, 5, 10 and 15 MPa to the fractures. Fractures healed within a few years when compressive p ressures were between 10 and 15 MPa

. T he authors concluded that is would take between 200 and 300 years to heal the fractures at compressive pressures below

10 MPa.

DOE’s JAS

- 3D numerical calculations predicted that the salt creep will create compressive pressure on the ROM salt and DRZ of at least 12 MPa by 200 years ( Herrick 2012 )

. Based on the

1998 healing experiments and JAS

- 3D calculations, the Agency finds it reasonable to assume

that a compressive pressure of at least 10 MPa would heal the fracture s in the DRZ within 200 to 300 years.

Experiments performed by IT Corporation (1987) support the assumption that fractures will heal at pressures similar to lithostatic. IT Corporation conducted permeability tests on salt cores with both longitudinal

- tens ion and saw tooth fractures at stepped confining pressures between

~ 3.5 MPa to 20 MPa. All tests indicated a strong correlation between permeability and confining

pressures regardless of the degree of fracturing

. Compression t ests on fractured samples

, at

pressures similar to the WIPP confining pressure (~13.78

– 17.23 MPa)

, were run from 3 to

163 hours and st eady decline s in permeability and porosity were seen as a function of pressure and time

. When the fractured cores were placed under a pressure of 20 M Pa for 8 days, the permeability decreased to within the same order of magnitude or less of the intact permeability.

Cores with tensile induced fractures had lower end

- point permeability than saw tooth fractures. The authors speculate d the tensile fracture cores healed more readily because the fractured surfaces of each half had similar paired geometries

. Because the fractures in the DRZ are similar to tensile induced fractures, the results from the IT experiments support a healed DRZ to have properties simi lar to those of intact halite at the confining pressures above ~13.78 MPa which is within the range of repository confining pressures of ~14.9 MPa.72

A nother line of evidence indicating that fracture healing occurs relatively quickly are those performed to measure regained tensile strength along a fractured zone . These experiments indicated fracture healing was due to grain boundaries bonding along a fracture face under compressive pressure . This bonding along the fracture face would presumably result in

per meability similar to the pre

- fractured value due to grain

- boundary bonding. Fracture tensile strength “healing” tests were conducted by Costin and Wawersik ( 1980 ) under confining pressures ranging from 10 to 35 MPa and temperatures between 22 o to 100 o C. T he experiments lasted between 4 to 8 days. The majority of specimens healed to within 70 %

– 80% of original strength

. Brodsky ( 1990 ) tested fracture healing and microstructure hardening of induced damaged salt cores at pressures of 5, 10 and 15 MPa

. Fracture s healed more quickly and effectively for those cores where fracture geometries were maintained undisturbed and

conform ed to the geometry created during fracture formation

. Brodsky conclude d that fracture closure and complete healing will occur if compress ive pressures persist for an extended period

of time and inferred the time for fracture closure would be within a few tens of years

.

DOE and EPA agree that the porosity and density of the ROM salt are correlated and affected by creep closure . For the ROM salt, as porosity is reduced and density increases, the backst r ess from the progressively denser ROM salt will place the DRZ under greater compressive stresses.

Hansen et al. (1993) report little to no backstress is produced during the consolidation of cr ushed

salt until fractional densities approach 0.90. Hansen and Thompson (2002) indicate that as the porosity of the ROM salt is further reduced to a value of 0.05 (a density of 0.95), healing of the

DRZ will begin to occur. If ROM salt is emplaced in the access drifts, then f ull reconsolidation of the adjoining DRZ will occur after the ROM salt panel closure is itself fully reconsolidated

and s tress equilibrium is achieved.

T he healing studies described above were performed on relatively pure halite. In addition to halite, however, t he DRZ also consists of thin layers of clay, and anhydrite, which may inhibit fracture healing due to grain boundary bonding

. Therefore, for the Agency’s sensitivity study the permeability of the PCS DRZ is set at one order o f magnitude higher than the permeability of

intact halite.

4.1.2 Parameters Modified for the DRZ Adjacent to the ROM Salt

During the 200 to 10,000 year time period (T3 in the PCS

- 2012 PA) the ROM salt panel closures will have relatively high densities, and coup led with creep closure , will produce compressive

back stress that nearly heal the adjoining DRZ. Laboratory experiments predict a nearly healed

DRZ will have a permeability more like that of intact halite than uncompacted ROM salt. T he Agency decided to tes t the impact on potential releases that a lower PCS DRZ permeability

, than

assumed in the PCS

- 2012 PA

, may have on overall releases

.

The range of sampled permeabilities for Salado halite and the adjacent DRZ _PCS during the T3 time period, and the derived ROM salt T3 permeability (derived from the sampled porosity and

POR 2 PERM parameter ) are given in Table 4

- 1. These values show the range for the DRZ_PCS permeability is more like that of the ROM salt rather than of a nearly healed h alite.73

Table 4

- 1

. Log Permeability Ranges for ROM Salt and the Adjacent DRZ from 200 to

10,000 Years, and for Intact Salado Halite

Log Permeability (log m 3 ) Mean Median Minimum Maximum

Derive ROM PCS

- - 19.36

- 19.34

- 20.94

- 17.53

DR Z_PCS

- 18.82

- 18.81

- 20.48

- 17.2
S_Halite PRMX_LOG

- 22.50

- 22.505

- 23.97

- 21.01

Given laboratory and experimental results the Agency believe s the properties of the DRZ adjacent to the ROM salt should be reflective of the following:

1. The permeability of the reconsolidated PCS DRZ should be somewhat greater than the permeability of undisturbed, intact halite because it is assumed that healing of the

fractures in the DRZ may not be complete and therefore the material may not completely return to its origin al state.

2. After 200 years the DRZ is assumed to reconsolidate and heal with few remaining

fractures

.

3. During time period T3 residual brine and gas saturations in the PCS DRZ are set to values reflective of porous media.

4. Capillary pressure in the PCS DR Z will vary during the three time periods

. Because the

PCS DRZ will initially be fractured, threshold capillary pressures can approach zero.

Therefore

, not invoking a threshold capillary pressure is appropriate for the first few hundred years

. An activated capillary pressure model , however, is appropriate when the

PCS DRZ heals and behaves as a porous medi um

.

To investigate whether a PCS DRZ permeability that approaches ful ly healed intact salt after

200 years affects potential repository releases, the Age ncy performed a sensitivity study which is

discussed in the next few subsections

.

4.1.3 Modified Parameter Values for Sensitivity Study

To investigate the sensitivity on potential releases to the DRZ permeability and two

- phase flow parameters, the input files used for the PCS

- 2013 PA were modified by lower ing the PCS DRZ permeability to a value one order of magnitude greater than that of intact halite, assigning porous media two phase flow parameters to the nearly healed DRZ

, and setting the two

- phase flow

par ameters for the ROM salt to those established by EPA and DOE for the PCS

- 2013 PA

. Additional details of EPA’s sensitivity study are provided below and in Table 4

- 2

.

1. The disturbed salt material of the P CS DRZ is treated as a fractured medium during time

pe riods T1 and T2. After 200 years

, the DRZ is assumed to reconsolidate and nearly heal

with few remaining fractures and is treated as a porous medium during time period T3.

Th e permeability of the PCS DRZ at time period T3 is set at one order of magnitude

h igher than the statistically sampled permeability of intact halite. This approach assigns the same uncertainty to the PCS DRZ as to intact halite.74

2. During the T3 time period the PCS DRZ is assigned a nonzero value for residual brine and gas saturations (SA T_RBRN and SAT_RGAS) reflective of porous media.

3. Capillary p ressure m odels for the PCS DRZ differ s during the time periods

a. Capillary pressure model (CAP_MOD) = 1 is appropriate for the PCS DRZ during time periods T1 and T2. The capillary pressure model is not activated when the capillary pressure may approach zero and is appropriate for the PCS DRZ during time period T1 when open fractures may exist, and still persist through the T2 time period.

b. Capillary pressure model (CAP_MOD) = 2 is appropriate for the DRZ_PCS during time period T3. The CAP_MOD 2 switch activates nonzero capillary pressures when values of PCT_A and PCT_EXP are nonzero. An activated capillary pressure model

i s appropriate for porous media.

4. I n BRAGFLO , the initial brine saturation SAT_IB RN must be greater than the residual

brine saturation SAT_RBRN to avoid numerical issues. Therefore

, SAT_IBRN for the DRZ_PCS was set at 0.9999 at T =

- 5 years and was calculated by the model for later time periods.

5. The two

- phase flow parameter values as signed to the ROM salt were also modified to
those established by t he Agency and DOE. The healed DRZ during the T3 time period is treated as a porous media. And, as such, it is appropriate to invoke the Brookes

- Corey two phase model in a healed DRZ where c apillary threshold pressure is a function of permeability. Threshold pressure in the Brookes

- Corey two

- phase model is calculated with the following equation :

P t = a k

– b

The same values for the linear parameter ‘a’, PCT_A, and the exponent ‘

- b’, PCT_EXP, given in the above equation that are used in the ROM Salt PCS are adopted for the healed

DRZ

. These values are discussed in detail in Section 3.4.2.2.

Table 4

- 2 . Parameter Values Modified for the DRZ Sensiti vity Study

Property T1 T2 T3 T1 + T2 T3

PCS DRZ_PCS
CAP_MOD 2 2 2 1 2

KPT 0 0 0 0 0
PC_MAX 1E9 1E9 1E9 1E9 1E9
PCT_A 0.56 0.56 0.56 0 0.56
PCT_EXP

- 0.346

- 0.346

- 0.346 0

- 0.346
SAT_IBRN Due to BRAGFLO numerical issues, SAT_IBRN must be greater than SAT_RBRN

in BRAGFLO. Therefore, SAT_IBRN for the DRZ_PCS is set at 1.0 at T =

- 5 years and is calculated by the model for later time periods.
SAT_RBRN 0.1 0.1 0.1 0 0.1
SAT_RGAS 0.05 0.05 0.05 0 0.05

POROSITY Unchanged Unchanged

PERMEABILITY Unchanged Unchanged 10*k for intact halite

POR2PERM Unchanged n/a75

4.1.4 Results of DRZ Sensitivity Analysis

T he mean and the 90 th percentile releases from the pooled results for all three replicates of the

DRZ and PCS

- 2012 sensitivity analyses are presented in Table 4

- 3. A graphical comparison of

the mean releases against the EPA release limits is shown in Figure 4

- 1

.

Table 4

- 3

. Total Normalized Release Statistics f or the DRZ Test and PCS

- 2012 PA

PCS

- 2 01 2 PA and DRZ Te st
EPA Release Limit Probabilities

- Pooled Across All Replicates

Probability Analysis Mean Releases

in EPA Units 90 th Percentile

Releases EPA Release

Limit

Total

0.1 PCS 2012 PA 0.098 0.17 1

DRZ Test 0.1005 0.165 1

0.001 PCS 2012 PA 1.512 1.0 10

DRZ Test 1.198 0.891 10

Spallings

0.1 PCS 2012 PA 0.00053 0.002 1

DRZ Test 0.0029 0.008 1

0.001 PCS

- 2012 PA 0.078 0.01 10

DRZ Test 0.11 5 .019 10

DBR

0.1 PCS 2012 PA 0.005 4 0.032 1

DRZ Test 0.0041 0. 02 8 1

0.001 PCS

- 2012 PA 1.49 0.9 10

DRZ Test 1.15 0.79 10

Cuttings and Cavings

0.1 PCS 2012 PA 0.0795 0.14 1

DRZ Test 0.0796 0.14 1

0.001 PCS

- 2012 PA 0.296 0.32 10

DRZ Test 0.295 0.32 10

As indicated by total releases presented in T able 4

- 3, changing the DRZ properties causes the

to tal mean releases to slightly increase at the 0.1 probability and decrease at the 0.001 probabil ity

level from those of the PCS

- 2012 PA

. The probability versus releases (in EPA Units) for the primary release mechanisms spallings, direct brine and cuttings and c avings are depicted in Figures 4

- 2 through 4

- 4 , respectively

. Of all the release mechanisms

, spallings releases are

affected the most by changing the DRZ properties

.76

Figure 4

- 1 . PCS 2012 PA and DRZ Te st, Mean Total Normalized Releases

Figure 4

- 2 . DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for Spallings Releases77

Figure 4

- 3 . DRZ Sensitiv ity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized Direct Brine Releases

Figure 4

- 4 . DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized Cuttings and Cavings Releases78

The Agency investigated the BRAGFLO output variables to understand why the spallings

releases increased , and found that the ir increase was primarily due to an increase in waste room

pressures

. Additionally

, experiment and operations room pressures in the DRZ test ca ses were lower than those observed in the PCS 2012 runs. Collectively

, these p ressure differences suggest

that gas generated in the waste rooms tends to stay in the waste region . Gas is impeded from migrat ing up

- dip into the operations or experimental room s that would otherwise provide

additional gas storage volume and would reduce waste room pressure . The se higher waste room pressures observed in the DRZ test cases subsequently result in greater spallings releases.

The overall mean for the DRZ Sensitivity test remains below the Agency’s regulatory limits at both compliance points. The statistical results from this study also suggest the mean total

rep ository releases would continue to remain below the EPA regulatory limits over a wide range of DRZ permeabi lity

, including those representative of a fully

- healed DRZ with properties

similar to intact halite.

In conclusion, the results indicate modifying the PCS 2012 PA DRZ flow parameter values, and invoking the capillary pressure model in the DRZ at the T3 ti me period lead to a slight increase

in the total mean releases at the 0.1 probability level and decreased releases, by approximately

20% , at the 0.001 probability level. The shift in the mean total releases is mainly att ributed to the

increase in the s pall ings release pathway. However, the derived mean CCD F , a function of all release mechanisms, indicates that releases still fall well below the EPA release limit.

4.2 SENSITIVITY TO CAPILLARY PRESSURE IN THE ROM SALT

The representation of two

- phase flow within the panel closure system was addressed during multiple technical discussions between EPA and DOE. In Camphouse et al. ( 2012 b ) , DOE identifies the relative permeability and capillary pressure models intended to represent the flow of brine and gas within th e panel closure system, including both the ROM salt , where the capillary pressure model was activated, and the adjoining DRZ (DRZ_PCS) , where the capillary pressure model was not activated . The specific parameters, most adopted from the original salt

shaft seal model ( Hurtado et al. 1997 ) are fully described above in Section 3 .4.3.2.

In an email and memorandum dated June 15, 2012, Sandia staff indicated that in the event that the BRAGFLO code failed to converge, the threshold pressure linear parameter PCT _A (originally set to 0 .56 Pa) and the threshold pressure exponential parameter PCT_EXP (set at

- 0.346, dimensionless) for the ROM salt would both be changed to 0 .0 , disabling the capillary pressure model in the ROM salt . This practice has been used in pas t PAs, and effectively allows

brine and gas to flow without needing to overcome a threshold capillary pressure.

In the PCS

- 2012 calculations, BRAGFLO failed to converge for many vectors when the capillary pressure model was implemented. Section 3.2.1 of t he PCS

- 2012 BRAGFLO report (Camphouse

2012 c ) discusses the issue:

“As a result, values of CAP_MOD = 1, PCT_A = 0.0 Pa, and PCT_EXP = 0.0 are prescribed to materials PCS_T1, PCS_T2, and PCS_T3 in the PCS

- 2012 PA in order to have panel closure

material con sistency with the surrounding DRZ and salt properties of the original shaft seal79

model, as well as to prevent numerical difficulties in BRAGFLO. These values correspond to capillary pressure modeling being disabled for these materials ...”

Upon receipt of th e BRAGFLO report, EPA questioned what the impacts of the parameter change would be. The Agency asked SNL staff to further investigate both the numerical problem, and the potential for the capillary pressure model to impact performance. During a trip by EPA staff to the SNL office in Carlsbad ( October 2, 2012 ) to review the documentation of the

PCS

- 2012 PA, SNL analysts presented their findings. The failure of the model to converge was a function of the interaction between the capillary pressure model

, the i nitial brine saturation (SAT_IBRN) , and the residual brine saturation (SAT_RBRN) . The two

- phase flow numerical

model defines a minimum brine saturation below which brine cannot flow as the residual brine saturation. At the same time, however, an initial br ine saturation (SAT_IBRN) was independently sampled from a range ( 0 .041

– 0 .16) that extend ed below some sampled values of the residual brine saturation. Numerical problems with the code were encountered for vectors

where the initial brine saturation was sam pled at a lower value than the sampled residual brine

saturation. S andia staff presented several test cases showing the interactions between the
parameters.

In order to test the potential impact of the enabled capillary pressure model on repository perfor mance, EPA elected to execute Replicate 1 of the PCS

- 2012 with the model enabled for the ROM salt, as originally intended . BRAGFLO convergence was accomplished by modifying the initial conditions so that the panel closure materials are initially fully satu rated with brine. BRAGFLO converged for all replicates without loosening the convergence criteria. Results are shown in Figures 4

- 5 through 4

- 7

.

Figure 4

- 5

. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mea n CCDFs for

Normalized SPALLINGS Releases80

Figure 4

- 6

. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Normalized Direct Brine Releases

Figure 4

- 7

. DRZ Sensitivity Test and PCS

- 2012 PA Overall Mean CCDFs for

Total Normalized Releases81

The capillary pressure model does not impact releases due to cuttings or cavings since those releases are simply calculated based upon the drill

- bit diameter , and i n the case of the PCS

- 2012 PA, releases to the Culebra were not recalculated. Applying the capillary pressure model to the

ROM salt could affect spalli ngs and DBR releases. At the 0.1 probability compliance point, mean total releases due to spallings were slightly increased, and mean total direct brine releases were slightly decreased for EPA R eplicate 1

, when capillary pressure effects were included . The contributions of different release mechanisms are the same at a probability of 0.001. The total

mean pr edicted releases are indistinguishable between the PCS

- 2012 Replicate 1 and EPA’s study.

Based upon the insensitivity to total releases, the Agency concludes that inactivating the capillary pressure model for the ROM salt to achieve model convergence does not affect the

results of the PCS

- 2012 performance assessment.82 

### 5.0 EPA QUALITY ASSURANC E REVIEW

Section 5.1 describes the Agency’s review of the parameter changes made since PABC

- 2009 to the parameters used in the PAPDB

. A detailed discussion of the Agenc y’s parameter review is presented in Appendix A

. Section 5.2 discusses SNL ’s migration of the Performance Assessment Parameter Database (PAPDB, Version 1.0) used in the WIPP compliance calculations from the Microsoft SQL Server on a Windows Server to the P APDB (Version 2.00) on the MySQL Server

on Solaris

. The results of EPA’s more detailed review are provided in Appendix B.

5.1 P ARAMETER C HANGES

SNL has made 78 parameter changes since PABC

- 2009 to the approximately 1,700 parameters

in the PAPDB

. Most of the changes involved the assignment of existing parameter names and values to newly defined materials for the three time periods used to model consolidation of both the ROM salt and the surrounding disturbed rock zone (DRZ) above and below the ROM salt.

All RO M salt materials for the third time period were new because three different time periods had not previously been used in WIPP PA. Three data errors were identified by the Agency in

the PCS

- 2012 PAPDB. These involved an erroneous value for the mean of a dis tribution, an erroneous designation of a distribution type (loguniform instead of uniform), and the transposition of the mean and median values of a distribution. However, none of these errors adversely affected PCS

- 2012 results. EPA has brought these erro rs to DOE’s attention and has received assurances that they will be corrected.

A review of primary and secondary supporting documents was conducted for all parameter changes and all documents were quickly retrieved from the WIPP Records Center. Although t he parameter values were generally correct, many of the supporting documents were not found to adequately justify those parameter values as used in the PCS

- 2012 analysis, and many had transparency weaknesses because the cited source documents were prepared for other analyses and the associated metadata described those analyses rather than the PCS

- 2012 PA. For example, the designated supporting document may have described a parameter that was used in an earlier analysis for a time period extending from zero to 200 years, but was used in the PCS

- 2012 PA for a time period extending from zero to 100 years. Transparency issues generally become problems for those trying to understand an analysis they are not already familiar with. DOE’s approach was to explain the differences in the supporting documentation, however EPA believes that the database should be consistent with the supporting documentation. The justification weaknesses were resolved in part by the Agency’s independent review of the technical basis for

ma ny of the parameter values, and in part because of the need to leave some existing parameter values unchanged to minimize differences between the PCS

- 2012 and the baseline PABC

- 2009, so that the differences in performance results could be readily attributa ble to the principal effects of changing the PCS design. However, modeling the progressive consolidation of ROM salt in a waste panel access drift and the subsequent consolidation of the surrounding DRZ involves processes and parameters that are new to WIP P PA.

A database interface evaluation was performed for all new parameters and the correct parameter values were retrieved from the PAPDB with the exception of one instance where the distribution identified for the parameter in the PAPDB was not properly sampled. However, this error did not83

affect the PCS

- 2012 PA results because the returned value was the desired value. The Agency has brought this error to DOE’s attention and has received assurances that it will be corrected. Procedures for developing, doc umenting, controlling, and changing parameters, and for entering those parameters into the PAPDB, were also reviewed. The Agency found all procedural changes to be consistent with earlier versions and found all revisions to be appropriate.

In summary, dur ing this review the Agency discovered several parameter transparency, implementation and quality assurance control issues. The number of these parameter

deficiencies was far more than have been raised by the Agency in previous PA reviews. The Agency believ es that the Department should have identified and corrected these parameter issues as part of their routine technical and quality assurance program reviews . This is particularly significant because relatively few parameter changes were required for DOE’s P CS

- 2012 analyses. As demonstrated by additional EPA analyses, however, EPA concluded shortcomings associated with the parameters did not affect the overall PCS

- 2012 PA results.

5.2 D ATABASE M IGRATION

In 2011, SNL migrated the PAPDB ( Version 1.0) used in the WIPP compliance calculations from the Microsoft SQL Server on a Windows Server to the PAPDB (Version 2.00) on the MySQL Server on Solaris. The purpose of the migration was to facilitate the access to the parameter data from the PA codes on Solaris, Linux a nd VMS. In addition, the design of the database was also modified to improve its ease of use. A detailed discussion of the Agency’s review of the (PAPDP) migration, design modifications and testing activities performed by SNL is presented in Appendix A. A summary of those activities and findings are presented below.

SNL defines a parameter as any value or distribution of values used directly or indirectly as input to a PA. The PAPDB contains the parameters used in WIPP PAs, the names of documents

related to those parameters, the names of the codes that use the parameters, and a log of the parameters that were exported to the various codes during analyses. In the PAPDP a parameter may be represented by a single value or assigned a distribution which represe nts epistemic uncertainty. The values of parameters can change through time as new information becomes available. The database contains the different versions of the parameter identified by a version number and also by the analysis where the parameter was first defined and the effective date for its use. Specific versions of a parameter can be associated with one or more analyses. The purpose of the database is to securely store the parameter data used in the PA analyses, to allow

access to that data direct ly from the applications that need those data, and to log the usage of the data. In addition, the PAPDB is designed to access the data from a user interface to facilitate querying the database to document the parameter values used in the various analyses

The primary objectives of the Agency’s review of the PAPDP migration activities were to assess the accuracy of the database migration to the new platform and to assess the transcription accuracy, documentation, and traceability of changes to the parameter values, distributions and metadata. The PAPDB parameter database and associated metadata are maintained in Carlsbad, New Mexico, by SNL. Hard copies of documentation supporting the parameters and their values
are maintained in SNL’s WIPP Records Center in Carlsbad. The supporting documents are84

tracked by SNL’s Electronic Record Management System (ERMS), through which a unique identifying number is assigned to each document.

The Agency has reviewed the PAPDP life

- cycle documentation that SNL has developed to assess whether the PAPDP migration to a new platform meet the requirements specified in § 194.22 and § 194.23. In addition to the life

- cycle documentation, the Agency reviewed several migration reports (Kirchner 201 2 ).

The PAPDB Version 2.00 was shown to meet the requirements listed in the Verification

Validation Plan ( VVP ) . Data could be successfully obtained from the database or inserted into the database from a FORTRAN application and permission to write into the database was properly controlled using group membership on Solaris.85 

### 6.0 SUMMARY AND CONCLUSIONS

The EPA has reviewed the PCS

- 2012 PA submitted by DOE in support of the Planned Change Request (PCR) to modify the P anel C losure S ystem (PCS) from the Option D design, as specified in Condition 1 of 40 CFR 194, to a design primarily consisting of run of mine (ROM)

salt. The PCS

- 2012 PA was developed by DOE to isolate the effect s of the proposed ROM PCS design change on WIPP repository performance by maintaining the structure of the calculations

as simila r as possible to that used in the current regulatory baseline PABC

- 2009. This approach facilitated a comparison between the results of the two PAs and helped isolate the effects of the proposed PCS design change on repository releases.

The Agency reviewed DOE ’ s ROM salt design and method of emplacement and its treatment of the hydrologic properties of salt during reconsolidation . The primary uncertainties are the rate of change and values of porosity and permeability of the ROM salt with time. DOE adopted a wide range of values for these parameters. The Agency accepts the values adopted for porosity and permeability in the PCS

- 2012 PA and agrees that they adequately captur e the range of uncertainty.

The Agency reviewed DOE’s FEP analysis associated with th e proposed ROM salt panel closure design and concludes that SNL’s FEP baseline review adequately indentified the principal modeling changes that were necessary to achieve the purpose of the PCS

- 2012 PA. The Agency
accepts the computational changes adopted in the PCS

- 2012 PA made to the BRAGFLO Salado Flow Model in order to accommodate the revised geometry, time periods, and material properties associated with the modified PCS design.

The Agency agrees with DOE’s conceptualization of long

- term “ healing ” of the disturbed rock zone adjacent to the ROM PC and accepts the values DOE adopted for the PCS DRZ to maintain

reasonable consistency with PABC

- 2009. T he DRZ surrounding the PCS plays a significant role in the effective permeability of th e system because its cross

- section area for brine and gas flow is large compared to that of the ROM salt. The Agency performed a subsequent study of the sensitivity of PCS

- 2012 PA results to an alternative parameterization of the healed DRZ

. The Agency’s study indicate d that lowering the permeability of the healed PCS DRZ and invoking

the capillary pressure did not result in a significant change in total predicted releases.

The results of the PCS

- 2012 PA indicate that the time

- dependent permeability of the panel

closures has a noticeable effect on repository performan ce by changing the gas pressures and

brine saturations within the waste panels. S lightly higher repository gas pressure s and higher brine saturations, resulting in slightly higher spallings and DBR releases. Howev er, overall releases were well below the EPA limits. DOE concluded the primary effect of the proposed panel closure design modifications on repository releases were the changes in pressure and brine saturation in the waste panels (Camphouse et al. 2012a). This is because direct brine, spallings, and Culebra releases are sensitive to those parameters and those parameters in turn are sensitive to the permeability of the panel closures and surrounding DRZ which control s fluid flow between the waste panels and the non

- waste areas of the WIPP underground facility

. The Agency agrees with this conclusion.86

With a review of the information presented by DOE and its own analyses, EPA is satisfied that

the revised PCS

- 2012 PA provides an adequate performance measure fo r the ROMPCS , and has

been adequately implemented. T he Agency has a reasonable expectation that the WIPP still complies with the containment requirements of 40 CFR Part 191.13 with ROM salt as the primary material in the PCS.87 

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December.91



### APPENDIX A: QUALITY ASSURANCE RE VIEW OF THE

PCS

- 2012 PA PARAMETER DATABASE92



### APPENDIX B : REVIEW OF WIPP PERFO RMANCE ASSESSMENT

COMPUTERIZED DATABASE (PAPDB) MIG RATION