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 Management 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, 2013
i

TABLE OF CONTENTS

Acronyms....................................................................................................................................... vi

Executive Summary..................................................................................................................... viii

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 Administration ......................................................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 Processes .........................................................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............................................63
ii

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 Sensitivity to DRZ Permeability............................................................................70

4.1.1 Conceptualization of DRZ Healing ...........................................................70

4.1.2 Parameters Modified for the DRZ Adjacent to the ROM Salt...................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

Appendix B: Review of WIPP Performance Assessment Computerized Database (PAPDB) 

Migration............................................................................................................................92
iii

LIST OF TABLES

Table 2-1. Panel Closure and DRZ Properties for the CCA and PAVT Analyses........................10

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

Option D PCS .............................................................................................................11

Table 2-3. Panel Closure 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. Intrusion 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 Releases, 

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. Stepwise 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 Replicate 1 .....................................................................67

Table 3-17. Stepwise Ranked Regression Results for Mean Culebra Releases, 

PCS-2012 Replicates 2 and 3 .....................................................................................68

Table 3-18. Summary 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 Normalized Release Statistics for the DRZ Test and PCS-2012 PA..................75
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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 Salt 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 (WAS_SATB), 

Scenario S1-BF...........................................................................................................44

Figure 3-9. Overall Means of Waste Panel Gas Generation (GASMOL_W), 

Scenario S1-BF...........................................................................................................45

Figure 3-10. Overall 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 Means 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...........................................................................................................52
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Figure 3-21. Overall Means of Waste Panel Gas Generation (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 Contributing 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 SPALLINGS 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.........................................................................................80
vi

ACRONYMS

CBFO Carlsbad Field Office

CCA Compliance Certification Application

CCDF Complementary Cumulative Distribution 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 Disturbed 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

mPa Mega Pascal 

MSHA Mine Safety and Health Administration

Pa Pascal 

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 Performance 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 Act
vii

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 Regression 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 Pilot Plant

WPC WIPP Panel Closure
viii

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 Pilot 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 assessment, 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 ROMPCS 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 model input 

parameters and ranges used in the 2009 Performance Assessment Baseline Calculation (PABC09) PA, with the exception of parameters that represent the panel closure system. Thus, the PCS2012 PA results may be compared to those obtained in the PABC-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 which 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 time period (0–100 years), the salt begins to consolidate from an 

unconsolidated material 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 

parameter 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 surrounding 

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 DOE’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 has successfully identified the principal features, events, and 

processes (FEPs) 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 the 
ix

values adopted for ROM salt density, porosity and permeability in the PCS-2012 PA and 

considers them to capture the range of uncertainty. Based on the information presented by DOE 

and the subsequent sensitivity analyses performed by the Agency, EPA is satisfied 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 demonstrate 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 WIPP.
1

1.0 INTRODUCTION

1.1 BACKGROUND

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 Parts 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 1996a). 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) thick Salado Mass Concrete (SMC) plug by a 9.1 meter (30 feet) open drift.

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

to modify the PCS from the Option D design to a design composed primarily of run of mine 

(ROM) salt. The PA submitted with that 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 current PA 

baseline established by the 2009 Performance Assessment Baseline Calculation (PABC-2009).

Following a series of technical exchanges between EPA and DOE, DOE submitted the PCS-2012 

PA. The major differences in the calculations from the PABC-2009 were (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 Agency’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 discusses 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 describes 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 ROMPCS design, the 

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

of the PCS-2012 PA. 

Section 4.0 discusses two separate analyses performed 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.

The Agency found that with a lowered PCS DRZ permeability, calculated average releases still 

remained below regulatory limits.

1.2 RELEVANT REGULATORY REQUIREMENTS

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. 3-33) 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 phase. (DOE 

1996a, CARD 14, 45–46)

While panel closures are not considered engineered barriers per 40 CFR Part 191 because the 

presence of panel closures has potential to influence the behavior of the repository, and therefore 

long-term compliance, detailed information on the panel closure design was included in the CCA 

to satisfy 40 CFR Part194. Section 194.14, Content of Compliance Certification Application, 

requires:

(a) A current description of the natural and engineered features that may affect 

the performance 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, engineered 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 panel was 

filled, and a concrete monolith installed afterwards. The concrete block wall could be a thinner 
3

“construction-isolation wall,” or a thicker “explosion-isolation wall” to protect against a 

postulated hydrogen or methane explosion. Likewise, the monolith 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 to “[provide] flexibility to satisfy the design limit 

for the flow of volatile organic compounds (VOCs) out of the panel” (Section 3.2.1).

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

at the time of closure based 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 longterm 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 certification 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 Salado 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 

implement the Option D panel closure system at WIPP, with Salado mass 

concrete replacing fresh water concrete (Federal Register, Vol. 63, No.95, 

p. 27355). (Emphasis 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 submitted 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, and as described in Appendix SEAL 

of Docket A-93-02, Item II-G-1) instead of fresh water concrete.

EPA’s certification decision explicitly discusses the possibility of DOE proposing a reengineered panel closure design, with the following language:

Nothing in this condition precludes DOE from reassessing the engineering of the 

panel seals at any time. Should DOE determine at any time that improvements in 
4

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, Vol. 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 from 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 operations,” 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 through 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 phase 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 public [See Guidance for the Implementation of 

EPA’s Standards for management and 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. EPA 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 system be installed 

prior to closing of the disposal system. EPA believes DOE’s proposed 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 

PCR (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 

restrictions found at 40 CFR Part 268, and instead sought a no-migration 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 promulgated 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, 104th Congress) obviating the need for a no-migration variance. The panel 

closure design remains a condition 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 Administrative 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 Mine Safety and Health Administration

Section 11 of the WIPP LWA (P.L. 102-579) establishes 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 mining 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 

stoppings” 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 closure 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 are 

described, which led to four design options 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 increased 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 activities 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 intake 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 (Dimensions in feet)
8

2.2 PRE-CCA PROPOSED DESIGNS

Various materials have 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-consolidated 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 MONOLITH 

DOE’s 1996 Compliance Certification Application (CCA) presented four optional PCS designs, 

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 1996a). These 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 closure (DOE 1996a). 

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 lives 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 candidate for this type of closure” (DOE 1996a).

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 

Rulemaking 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 performance 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. In those analyses the 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 closure and the DRZ were fixed for the entire 10,000-year 

regulatory time period.
9

From DOE 1996a

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

Certification Application
10

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 Porosity 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 2011b; and Hansen et al. 2002.

2.4 DOE POST-OPTION D DESIGN ALTERNATIVES

After the CCA was approved by the Agency, DOE conducted studies of installation 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 crushed 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 rigid 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 permeability 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 2011b). 
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 Permeability (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: 0.0548

Mean: 0.0211

 From DOE 2011b; Camphouse et al. 2012b.

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 from 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 (Marcinowski 

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 indicated 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 (Saeb 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) the 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 period differ from those 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 rigid 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 value 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 second 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 performance 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 comparison was labeled the 

Panel Closure System Impact Assessment (PCSIA) PA. The PCSIA PA results indicated that 

even though the permeability of the two closure systems differed by four orders of magnitude, 

repository performance was nearly identical (Hansen 2002a; Hansen 2002b). 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 Recertification 

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 included 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 were 

compared to the 2004 PABC.

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 change 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 permeability was slightly lower than that assumed in 

the 2002 PCSIA PA. Similar to the PCSIA PA, creep closure compressed the ROM salt
13

properties to asymptotic values estimated around 200 years after repository closure. Two discrete 

time periods were adopted in both PAs to capture the change in ROM salt properties. The first 

modeled time period 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-direction) 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-direction) 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 CHANGE 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 future 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 2011a; 

DOE 2011b). 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 PCS redesign, the Agency requested 

that DOE prepare a PA that only addressed the proposed PCS redesign. As a result, DOE 

presented an overview of their proposed approach, called the PCS-2012 PA, to the Agency at a 

Technical Exchange Meeting in April 2012 (Air 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 

represents the proposed ROM salt PCS. All other parameters, conditions, material properties, 

waste inventory, and modeling 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 meters (100 feet) of 

ROM salt placed within the panel access drift with a steel bulkhead positioned at each end. 

Alternatively, 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 meters (100 feet) of ROM salt against the explosion wall, and a standard steel ventilation 

bulkhead on the outer end (Camphouse et al. 2012a). 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 steel bulkheads are assumed to last and do not 

enter into the performance assessment modeling (Camphouse 2012c). The proposed modified 

designs 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

From DOE 2013

Figure 2-3. Typical ROM Salt Panel Closure for Drifts without Explosion Walls

From DOE 2013

Figure 2-4. Typical ROM Salt Panel Closure for Drifts with Explosion Walls
16

3.0 2012 DOE PLANNED CHANGE 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. 

Section 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 proposed 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 SALT DESIGN

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

monoliths. SMC is 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 2011b). DOE has, with the 

approval 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 simpler, easier and 

faster to construct, is less disruptive to facility operations, increases the use of natural materials, 

and is less expensive than the current Option D concrete monolith design, while also meeting the 

same operational performance requirements as the Option D concrete monolith (DOE 2011b).

3.2 ROM SALT DESIGN MATERIALS 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 closed 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 performance (Camphouse 2012c). 

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 that 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 delivered 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 degree 
17

practicable to minimize the gap between the salt and roof or sidewalls 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 with the repository environment.

DOE believes that the long-term properties of ROM salt are also advantageous during the postclosure period due to its creep characteristics when under compressive stress. DOE’s JAS3D 

numerical modeling indicates that following emplacement, an air gap will 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 inches 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 WIPP 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 

increases 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 healing process is 

expected to be essentially complete within approximately 200 years (see discussion in 

Section 3.4.2.1). Therefore, 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 PROCESSES 

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. The 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 probability 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 FEPs 
18

that are considered potentially relevant to WIPP performance. 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, 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

The FEP review for the PCS-2012 PA was conducted by Sandia National Laboratories (SNL) 

and is documented in Kirkes (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 waste 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 review for the PC3R PA (Camphouse et al. 2012a) 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, Performing 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 

impacted 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 stressinduced 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 baseline. 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 1UP The repository layout 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 baseline.

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 Disturbed 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 differ 

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.

1UP: 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 regulated 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 alternative 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 additional 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 that 
20

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 material 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 invalidate, 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 Concrete Hydration, W111 Panel 

Closure Chemical Composition, and W112 Radiological 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 omitted 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 WIPP 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 stress 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 captured 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 baseline and PA framework are suitable to 

account for the changes 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 

implementation 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’s 
21

analyses of the FEPs in Table 3-1 that were screened in are summarized 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 screening 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, potentially 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 expected 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 potentially 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 generation 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 meetings, 

documented in the Agency’s 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, DOE adequately demonstrated that 

significant gas pressure buildup is rare during early time when the porosity and permeability of 

the ROM 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 progresses, 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 confinement 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 concern, SNL presented the results 

of a numerical model of panel closure consolidation in the April 2012 technical exchange 
22

meeting, documented in the Agency’s AIR Docket A-98-49, Item II-B2-81, DOE Response to 

Technical Question 1.3e, that adequately demonstrated 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 of 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 ASSESSMENT 

The PCS-2012 PA was built upon DOE’s earlier representations 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 concerns 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 

aforementioned 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 2011b). This appendix describes

the generation mechanisms of hydrogen and methane, the two flammable gases of most concern 

in filled waste panels. The appendix also describes hydrogen and methane monitoring results in 

the waste rooms of Panels 3 and 4, and presents a worst-case analysis of hydrogen buildup in 

Panel 3. DOE concluded that explosion walls are not necessary because measured 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 assumed 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, the Agency agrees that the bulkheads will corrode in the postclosure environment, be crushed during drift creep closure, and have no impact on post-closure 

repository performance.

3.4.1.3 ROM Salt Emplacement 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. Scenario 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). Uncertainty 

regarding the ROM salt emplacement method and initial density increases the uncertainty in 

estimating its permeability during the first 100 years. The Agency believes this was adequately 

addressed by DOE assigning wide ranges to the ROM salt porosity and permeability 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 Attachment B of the PCR (DOE 2011b). 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 permeability of the loosely placed ROM salt of 10-11m

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 permeability 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 within 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 period (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 states.

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 extends from zero to 100 years as before; time period T2 extends

from 100 to 200 years, and time period T3 extends from 200 years to 10,000 years. DOE treated 

ROM salt porosity as an uncertain, sampled parameter and determined the permeability for time 

periods T2 and T3 using a correlation between permeability and porosity based on laboratory 

data from Hurtado et al. (1997). 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 permeability values from Hurtado’s data set that were 

measured with brine (not gas) as a permeant and sample permeability from a normal distribution 

centered on a linear least squares fit to a plot of Hurtado’s porosity vs. log permeability data. 

This approach resulted in an overall permeability range of -20.8 log (m

2

) or 1.58×10-21m

2

to 

-16.3 log (m

2

) or 5.01×10-17 m

2

during time period T2, and an overall range of -21.4 log (m

2

) or 

3.98×10-22m

2

to -16.8 log (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 is appropriate because 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 

porosity 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% and 
25

independently sampling the log permeability from a uniform distribution ranging from 10-12 to 

10-21m

2

(Camphouse et al. 2012b; as amended by Patterson 2012). Sampling for the three time 

periods is conditioned so that the ROM salt permeability monotonically decreases with time. The 

T1 ranges are selected based on the porosity and permeability 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 within the T1 range of 6.6% to 18.7% (Camphouse et al. 2012b). 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 expected 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. 2012b). The parameters describing twophase flow in the ROM salt are summarized in Table 3-3 and are further discussed in Camphouse 

(2012d). 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 additional 

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 permeability 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 modified 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.

Property 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 determined that the numerical 
26

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 request the problem with BRAGFLO was corrected and the PCS2012 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:

Pt = a k–b

where a is called the linear parameter, designated in WIPP PA as the property 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 salt 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_EXP. 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 capillary 

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 when 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 reviewed 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 material. 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 that 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 brine 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, the 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.

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, the 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 pore 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 compressibility 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 the 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 Agency 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 review of the model changes 

proposed by DOE focused on the short- and long-term porosity and permeability, and the twophase flow properties of the PCS DRZ. An overview of the Agency’s primary conclusions is 

presented in the following subsections. Additional details are presented in Appendix A: 

Parameter QA. A summary tabulation of the Agency’s modeling concerns 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 eventually 

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. 2012b). DOE chose to represent the PCS DRZ healing process in the PCS2012 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 name 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 waste 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 around 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. 2012b). Although the initial 

DRZ porosity would be expected to be larger than the final porosity due to healing effects, EPA 
29

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 log (m

2

) or 3.98 × 

10-20 m

2

to a maximum of -12.5 log (m

2

) or 3.16 × 10-13 m

2

. The mode of the distribution was

-16.0 log (m

2

) or 1 × 10-16 m

2

(Camphouse et al. 2012b). For time period T3, DOE proposed to 

use the same permeability range and distribution as for the healed DRZ around 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 log (m

2

) or 2 × 10-21m

2

to a 

maximum of -17.0 log (m

2

) or 1 × 10-17 m

2

. The mode of the distribution was -18.7496 log (m

2

) 

or 1.78 × 10-19 m

2

(Camphouse et al. 2012b). 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 permeability 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 independently sampling PCS DRZ 

permeability for the initial T1, T2 and final T3 time periods could lead to the fully healed T3 

permeability being greater than the T1 and T2 permeability because the two ranges overlap. The

ROM salt and surrounding DRZ can have relatively independent porosity and permeability until 

the ROM salt is sufficiently consolidated and begins to apply a backstress on the surrounding 

DRZ. The Agency agrees with the geomechanical processes that will ‘heal’ the DRZ as 

described by Camphouse et al. (2012b): “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).” The high end of the T3 permeability range for healed 

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 × 10-21m

2

) 

is three orders of magnitude higher than the low end of the permeability range for intact halite 

(1 × 10-24m

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-2009, DOE recognized that “the permeability of the salt 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. DOE 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 through 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).

The 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, investigated the effect on 

potential repository releases if the permeability of the DRZ were to approach that of intact halite 
30

200 years following repository closure. This evaluation and its results are described 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 fractures exist, and therefore 

applies only to the PCS DRZ during the initial time periods 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. (2012b). 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, the 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 Periods

T1 and T2

Conceptual Relevance 

to Time Period T3

RELP_MOD Relative permeability model 

number for the PCS DRZ

Modified Brooks-Corey 

model applies

Modified 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 the 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 selecting 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 PCS 

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 T2
31

Table 3-4. Two-Phase Flow Properties for the PCS DRZ

Property Name in 

PCS-2012 PA Parameter Description

Conceptual Relevance 

to Time Periods

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

Would 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 all 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 capillary pressure Model 1 for time periods 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. DOE’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-phase 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 periods T1 and T2 is 

consistent with its use in the PABC-2009 PA and is also conceptually appropriate. A zero value 

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 would 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 an insignificant effect on repository 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 zero value for 

PCT_EXP for time periods 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 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 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 an 

insignificant effect on repository performance. As with Property PCT_A, the Agency accepts the 
32

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 not 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 periods T1and T2 but not 

appropriate for time period T3. The Agency accepted 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 facilitates a comparison of results. 

The 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 value 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 

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

-5 years in both 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 WIPP 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 periods, it is 

also not appropriate for times 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 significantly affected by the zero 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 zero value was retained when the PCS DRZ was separately identified for modeling the 

Option D PCS because it was judged to have an insignificant effect on repository performance. 

The Agency accepts the use of zero values in the PCS-2012 PA because they are consistent with 
33

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 all 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 periods, it is not appropriate 

for times 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 gas saturation.

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 identified for modeling the Option D 

PCS because it was judged to have an insignificant effect on repository performance. The 

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, are 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. For 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 times 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, the 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 PABC2009 PA. This was done to facilitate a comparison between the results of the two PAs that would 

help identify the effects of the proposed modified PCS design on repository releases. Only those 

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. 2012a). The same 

set of six intrusion scenarios was also used to calculate brine 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 Formation. An 

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

but does not encounter a brine reservoir. E1 and E2 borehole intrusions are illustrated in 

Figure 3-1.
34

Table 3-5. BRAGFLO Salado Flow Modeling 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. 2012a; 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 

Model 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 in 
35

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 years); 

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 borehole 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 

Marker 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), the 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 site 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 calculating 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 between 

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 intrusion 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 borehole” 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, implementation 
36

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

result, the PCS-2012 PA calculated 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 

releases, 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 2012c

Figure 3-2. PABC-2009 BRAGFLO Grid and Material Map
38

From Camphouse 2012c

Figure 3-3. PCS-2012 BRAGFLO Grid and Material Map, Years 1 to 100
39

From Camphouse 2012c

Figure 3-4. PCS-2012 BRAGFLO Grid and Material Map, Years 100 to 200
40

From Camphouse 2012c

Figure 3-5. PCS-2012 BRAGFLO Grid and Material Map, Years 200 to 10,000 
41

From Camphouse 2012c

Figure 3-6. PCS-2012 BRAGFLO Grid and Material Map for an E1 Intrusion
42

3.5 PCS-2012 PA RESULTS AND ANALYSIS

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

DBRs and spallings releases, Culebra releases contribute 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 releases 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 that 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 waste 

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 moving 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 between the PCS-2012 PA and the PABC2009 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 uncertainty) 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 vector, 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 the 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. 2012a). Direct 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 nonwaste areas of the WIPP underground facility. 
43

As compared with the Option D PCS design, generally higher ROM salt PCS permeability in the 

first 200 years and lower permeability after 200 years resulted in less isolation of individual 

waste panels in early times and greater isolation in later times. The lower permeability 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 penetrations 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, releases 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 Model. 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. 2012a).

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 occurs 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. 2012a). At 200 years, the 

difference between the cumulative brine flows obtained in the two analyses is roughly 600 m3, 

and this difference remains 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 first 200 years.
44

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

500

1000

1500

2000

2500

3000

Time (years)

BRNWASIC (m

3

)

Scenario S1-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-7. Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC), 

Scenario S1-BF

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (years)

WAS_SATB

Scenario S1-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

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 require the presence of moisture in the form of 

brine. Increases in the availability of brine therefore increase gas 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 shown in Figure 3-10. 
45

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

x 107

Time (years)

GASMOL_W (moles)

Scenario S1-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-9. Overall Means of Waste Panel Gas Generation (GASMOL_W), Scenario S1-BF

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

7

8

9

10

x 106

Time (years)

WAS_PRES (Pa)

Scenario S1-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

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 interbeds to the accessible environment is negligible. 

Cumulative mean flow up the shafts amounts to only 6 or 7 m3in 10,000 years and is also 

negligible (Camphouse et al. 2012a).

3.5.1.2 Results for an E1 Intrusion at 350 Years (Scenario S2-BF)

An E1 intrusion 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 potentially large volumes of 
46

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-2009 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 over the next approximately 

5,000 years when it levels off at about 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 volume of brine flowing up the sealed intrusion borehole was 

also nearly identical, as shown in Figure 3-15, and amounted to nearly 9,000 m3in 10,000 years.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

7

8

9

10

x 106

Time (years)

WAS_PRES (Pa)

Scenario S2-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-11. Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S2-BF
47

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

0.5

1

1.5

2

x 104

Time (years)

BRNWASIC (m

3

)

Scenario S2-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-12. Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC), 

Scenario S2-BF

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (years)

WAS_SATB

Scenario S2-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-13. Overall Means of Waste Panel Brine Saturation (WAS_SATB), 

Scenario S2-BF
48

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

7

8

9

10

x 107

Time (years)

GASMOL_W (moles)

Scenario S2-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-14. Overall Means of Waste Panel Gas Generation (GASMOL_W), 

Scenario S2-BF

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Time (years)

BNBHUDRZ (m

3

)

Scenario S2-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

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 350 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 

equivalent to those of sand. The result is a depressurization of the intruded waste panel beginning 
49

200 years after the intrusion. This depressurization is illustrated in Figure 3-16, 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 PCS2012 PA and in the PABC-2009 PA are shown in Figure 3-17. As discussed above and shown 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 years 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 3-18. As discussed above, higher waste panel pressures can reduce brine inflow, while

lower waste panel pressures can allow increased brine inflow. The significant increase in brine 

inflow at 550 years shown in Figure 3-18 is a direct result of the significant pressure drop (due to 

the change in borehole plug permeability) at 550 years shown in Figure 3-16. As would be 

expected, waste panel brine saturation also significantly increases at 550 years, commensurate 

with the increase in brine inflow (Camphouse et al. 2012a).

The overall means of cumulative waste panel brine inflow are compared for the PCS-2012 PA 

and the PABC-2009 PA in Figure 3-19. 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 undisturbed 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-2012 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 PCS2012 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

0 100 200 300 400 500 600 700 800 900 1000

0

1

2

3

4

5

6

7

x 106

Time (years)

WAS_PRES (Pa)

PCS-2012 S1-BF Overall Mean

PCS-2012 S4-BF Overall Mean

From Camphouse et al. 2012a

Figure 3-16. Overall Means of Waste Panel Pressure (WAS_PRES), Scenarios S1-BF and 

S4-BF for Years 0 to 1,000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

x 106

Time (years)

WAS_PRES (Pa)

Scenario S4-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-17. Overall Means of Waste Panel Pressure (WAS_PRES), Scenario S4-BF
51

0 100 200 300 400 500 600 700 800 900 1000

0

500

1000

1500

2000

2500

3000

Time (years)

BRNWASIC (m

3

)

PCS-2012 S1-BF Overall Mean

PCS-2012 S4-BF Overall Mean

From Camphouse et al. 2012a

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

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1000

2000

3000

4000

5000

Time (years)

BRNWASIC (m

3

)

Scenario S4-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-19. Overall Means of Cumulative Brine Inflow to the Waste Panel (BRNWASIC), 

Scenario S4-BF
52

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (years)

WAS_SATB

Scenario S4-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-20. Overall Means of Waste Panel Brine Saturation (WAS_SATB), 

Scenario S4-BF

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1

2

3

4

5

6

7

8

x 107

Time (years)

GASMOL_W (moles)

Scenario S4-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-21. Overall Means of Waste Panel Gas Generation (GASMOL_W), 

Scenario S4-BF
53

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

50

100

150

200

250

Time (years)

BNBHUDRZ (m

3

)

Scenario S4-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

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 of 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 al. 2012a). 

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 continue 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. 2012a). 

3.5.3 Direct Brine Releases

Direct Brine Releases (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 intrusions 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 locations, 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, 1400, 3000, 5000, 10000

 From Camphouse et al. 2012a; 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 year regulatory period. 

Similarly, the mean waste panel brine saturation was higher at the time of intrusion in the PCS2012 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. 2012a). 

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 results 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. 2012a). 

Although the increase in the number of non-zero DBR volumes was relatively small (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. 2012a). Single realization results 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 al. 2012a

Figure 3-23. PCS-2012 PA and PABC-2009 Overall Mean CCDFs for 

Normalized Direct Brine Releases

3.5.4 Spallings Releases

Increases in repository pressures seen in the PCS-2012 PA BRAGFLO results have increased 

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 presented 

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 similar average nonzero spallings volumes in both the PCS2012 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 volume 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 

percentage 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 (Camphouse et al. 2012a). 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 PCS2012 PA than in the PABC-2009 PA.
56

From Camphouse et al. 2012a

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 Culebra dolomite are 

calculated in scenario S6-BF (an E2 intrusion at 1000 years followed 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 borehole 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. 2012a). The Agency agrees with SNL’s logic and 

concurs with this decision.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Time (years)

BNBHUDRZ (m

3

)

Scenario S6-BF

PCS-2012 Overall Mean

PABC-2009 Overall Mean

From Camphouse et al. 2012a

Figure 3-25. Overall Means of Brine Flow up the Borehole (BNBHUDRZ), Scenario S6-BF
57

3.5.6 Total Releases

Total normalized releases are calculated as the sum of the releases from the 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 3-26. As can be seen from the figure, cuttings 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 an 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 3-27 through 3-29. Out 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 

cavings 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 release and upper 95% confidence limit on the 

mean did not exceed the Agency’s release limits. 

From Camphouse et al. 2012a

Figure 3-26. PCS-2012 PA Primary Components Contributing to 

Total Normalized Releases
58

Modified from Camphouse et al. 2012a

Figure 3-27. PCS-2012 PA Replicate 1 Total Normalized Releases

Modified from Camphouse et al. 2012a

Figure 3-28. PCS-2012 PA Replicate 2 Total Normalized Releases
59

Modified from Camphouse et al. 2012a

Figure 3-29. PCS-2012 PA Replicate 3 Total Normalized Releases

From Camphouse et al. 2012a

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 the figure, mean total releases in the PCS2012 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 PCS2012 PA remains below the 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 seen
60

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 90th

percentile and upper 95 % confidence limit of the mean. Because the mean 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. 2012a).

From Camphouse et al. 2012a

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

90th

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. 2012a; Values in EPA units.

3.6 SNL PARAMETER SENSITIVITY ANALYSIS

SNL performs a parameter sensitivity analysis 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 parameters 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 intrusion. 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 PA 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 importance 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). The coefficient of determination (R2) 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 R2value is most strongly correlated with output variability and is 

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 R2value 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 the 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 uncertainties 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 statistical 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 coefficients 

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 R2value 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, cuttings and cavings, direct brine, and 
62

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 the 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 

contributors 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 CELLULS: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 change: 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 most 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 PCS 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:REPIPERM 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 Kirchner 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 uncertainty 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 analysis 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 insignificant 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 distribution 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 these 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 -0.02

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 regression 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 permeability 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 parameters 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 are 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:SOLVAR 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_WICK 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 Kirchner 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 repository releases. As evidenced from the 
66

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 the 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 

spallings 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 permeability reduces 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 0.27 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 2012
67

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 

sensitivity 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. Nearly 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 0.69 -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 distribution 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 variability 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 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.

Ranked regression results for mean Culebra releases in PCS-2012 Replicates 2 and 3 are

presented in Table 3-17. 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 

permeability 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, indicating 

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 CASTILER: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 3-18. Eight parameters were 

identified 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 parameters were the log 

permeability 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% or 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 Culebra 

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 permeability 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 weakly 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 expected 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 more 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 halite, 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 source 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 system 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 

of 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 ANALYSIS

The Agency conducted two sensitivity studies to further evaluate DOE’s results. One study 

investigated 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 closure material. The results of 

these sensitivity studies are described in the following section.

4.1 SENSITIVITY TO DRZ PERMEABILITY

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 during time periods T1 and T2. This section presents results of a sensitivity study 

performed 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-2012 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 PCS-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 whether 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 closure. 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 creep 

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 will not be mined and therefore its behavior 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 within 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 backstress 
71

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 compressive 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 dilatant strain on the in-situ halite that can eventually produce fractures 

which are generally parallel to the drift and perpendicular to the minimum principal stress. 

Dilatancy linearly increases during deformation and ceases when compressive stresses 

commence. Numerous laboratory and field tests, some of which are described below, indicate 

that dilatant damage in salt is reversible through crack closure and healing under compressive 

stress. Salt creep will impose compressive stresses that will close any gaps between the mine 

roof and the ROM salt. Eventually, these compressive stresses applied to the DRZ will approach 

lithostatic pressure, which is approximately 14.9 MPa. DOE expects these compressive stresses 

will heal the fractures (Hansen 2003). Studies suggest that the healing time for the DRZ will 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 pressures were between 10 and 15 MPa. The 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 fractures 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-tension 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 tests on fractured samples, at 

pressures similar to the WIPP confining pressure (~13.78 – 17.23 MPa), were run from 3 to 

163 hours and steady declines in permeability and porosity were seen as a function of pressure 

and time. When the fractured cores were placed under a pressure of 20 MPa 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 speculated 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 similar 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

Another 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

permeability 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 22oto 100o C. The 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. Fractures healed more quickly and 

effectively for those cores where fracture geometries were maintained undisturbed and 

conformed to the geometry created during fracture formation. Brodsky concluded that fracture 

closure and complete healing will occur if compressive 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 backstress 

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 crushed 

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 full reconsolidation 

of the adjoining DRZ will occur after the ROM salt panel closure is itself fully reconsolidated 

and stress equilibrium is achieved.

The healing studies described above were performed on relatively pure halite. In addition to 

halite, however, the 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 of 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 coupled with creep closure, will produce compressive 

backstress 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. The 

Agency decided to test 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 

POR2PERM 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 halite.
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

DRZ_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 believes 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 original 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 DRZ 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 medium.

To investigate whether a PCS DRZ permeability that approaches fully healed intact salt after 

200 years affects potential repository releases, the Agency 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 lowering 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 

parameters 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 PCS DRZ is treated as a fractured medium during time 

periods 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. 

The permeability of the PCS DRZ at time period T3 is set at one order of magnitude 

higher 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 (SAT_RBRN and SAT_RGAS) reflective of porous media.

3. Capillary pressure models for the PCS DRZ differs 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 

is appropriate for porous media.

4. In BRAGFLO, the initial brine saturation SAT_IBRN 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 assigned to the ROM salt were also modified to

those established by the 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 capillary threshold pressure is a function of 

permeability. Threshold pressure in the Brookes-Corey two-phase model is calculated 

with the following equation:

Pt = 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 Sensitivity 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/a
75

4.1.4 Results of DRZ Sensitivity Analysis

The mean and the 90th 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 for the DRZ Test and PCS-2012 PA

PCS-2012 PA and DRZ Test 

EPA Release Limit Probabilities -

Pooled Across All Replicates

Probability Analysis Mean Releases 

in EPA Units

90th 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 0.115 .019 10

DBR

0.1 PCS 2012 PA 0.0054 0.032 1

DRZ Test 0.0041 0.028 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 Table 4-3, changing the DRZ properties causes the 

total mean releases to slightly increase at the 0.1 probability and decrease at the 0.001 probability 

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 cavings 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

CCDF - PCS 2012 PA and DRZ Test Pooled Across Replicates 

Total Mean Releases 

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Release Probability Release > R

Mean PCS 2012 PA

Mean DRZ Test

Release Limits

Figure 4-1. PCS 2012 PA and DRZ Test, Mean Total Normalized Releases 

CCDF - PCS 2012 PA and DRZ Test - 

Spallings Releases

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Spallings Release (EPA Units) Probability Release > R

PCS 2012 PA Mean Spallings

Release Limits

DRZ Test Mean Spallings

Figure 4-2. DRZ Sensitivity Test and PCS-2012 PA Overall Mean CCDFs for 

Spallings Releases 
77

CCDF - PCS 2012 PA and DRZ Test - 

Mean Direct Brine

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Release Probability Release > R

DRZ Test Mean DBR

PCS 2012 PA

Release Limits

Figure 4-3. DRZ Sensitivity Test and PCS-2012 PA Overall Mean CCDFs for 

Normalized Direct Brine Releases

CCDF - PCS 2012 PA and DRZ Test -

Cuttings and Cavings

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Release Probability Release > R

PCS 2012 PA Mean Releases

DRZ Test Mean Releases

Release Limits

Figure 4-4. DRZ Sensitivity Test and PCS-2012 PA Overall Mean CCDFs for 

Normalized Cuttings and Cavings Releases
78

The Agency investigated the BRAGFLO output variables to understand why the spallings 

releases increased, and found that their increase was primarily due to an increase in waste room 

pressures. Additionally, experiment and operations room pressures in the DRZ test cases were 

lower than those observed in the PCS 2012 runs. Collectively, these pressure differences suggest 

that gas generated in the waste rooms tends to stay in the waste region. Gas is impeded from

migrating up-dip into the operations or experimental rooms that would otherwise provide 

additional gas storage volume and would reduce waste room pressure. These 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 

repository releases would continue to remain below the EPA regulatory limits over a wide range 

of DRZ permeability, 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 time 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 attributed to the 

increase in the spallings release pathway. However, the derived mean CCDF, 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. (2012b), DOE 

identifies the relative permeability and capillary pressure models intended to represent the flow 

of brine and gas within the 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 past 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 the PCS-2012 BRAGFLO report (Camphouse 

2012c) 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 consistency with the surrounding DRZ and salt properties of the original shaft seal 
79

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 the 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 initial 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 brine saturation (SAT_IBRN) was 

independently sampled from a range (0.041–0.16) that extended below some sampled values of 

the residual brine saturation. Numerical problems with the code were encountered for vectors 

where the initial brine saturation was sampled at a lower value than the sampled residual brine 

saturation. Sandia 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 

performance, 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 saturated with brine. 

BRAGFLO converged for all replicates without loosening the convergence criteria. Results are 

shown in Figures 4-5 through 4-7.

SPALLINGS

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Release (EPA Units) Probability Release > R

CP Test

PCS 2012 PA Mean Releases

Release Limits

Figure 4-5. DRZ Sensitivity Test and PCS-2012 PA Overall Mean CCDFs for 

Normalized SPALLINGS Releases 
80

DIRECT BRINE

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Release (EPA Units) Probability Release > R

CP Test

PCS 2012 PA Mean Releases

Release Limits

Figure 4-6. DRZ Sensitivity Test and PCS-2012 PA Overall Mean CCDFs for 

Normalized Direct Brine Releases

Mean Total Releases

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

R = Release (EPA Units) Probability Release > R

CP Test

PCS 2012 PA Mean Releases

Release Limits

Figure 4-7. DRZ Sensitivity Test and PCS-2012 PA Overall Mean CCDFs for 

Total Normalized Releases
81

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 in 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 spallings 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 Replicate 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 predicted 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 ASSURANCE 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 Agency’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 PAPDB (Version 2.00) on the MySQL Server 

on Solaris. The results of EPA’s more detailed review are provided in Appendix B.

5.1 PARAMETER CHANGES

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 ROM 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 distribution, 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 errors 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 the 

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 

many 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 attributable 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 WIPP 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 not 
83

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, documenting, 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, during 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 believes 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 PCS-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 DATABASE MIGRATION

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 and 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 represents 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 directly 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 are 
84

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 2012).

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 Panel Closure System (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 effects of the proposed ROMPCS 

design change on WIPP repository performance by maintaining the structure of the calculations 

as similar 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 capture the range of 

uncertainty.

The Agency reviewed DOE’s FEP analysis associated with the 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 ROMPC and accepts the values DOE adopted for the PCS DRZ to maintain

reasonable consistency with PABC-2009. The DRZ surrounding the PCS plays a significant role 

in the effective permeability of the 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 indicated 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 performance by changing the gas pressures and 

brine saturations within the waste panels. Slightly higher repository gas pressures and higher 

brine saturations, resulting in slightly higher spallings and DBR releases. However, 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 controls 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 for the ROMPCS, and has 

been adequately implemented. The 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

7.0 REFERENCES

Air Docket A-98-49; Item II-B2-80, DOE 4/17/12 Response to EPA 12/22/11 Letter.

Air Docket A-98-49; Item II-B2-81, Thompson, B. Effect of Gas Generation on Consolidation, 

DOE/EPA Technical Exchange Meeting, February 14, 2012, Dallas TX.

Brodsky, N.S., 1990. Crack Closure and Healing Studies in WIPP (Waste Isolation Pilot Plant) 

Salt Using Compressional Wave Velocity and Attenuation Measurements: Test Methods and 

Results. SAND90-7076. Sandia National Laboratories, Albuquerque, NM SAND90-7076.

Camphouse 2012c. Analysis Package for Salado Flow Modeling Done in the AP-161 (PCS2012) Performance Assessment, Revision 0. Sandia National Laboratories, Waste Isolation Pilot 

Plant, Carlsbad, NM. September. ERMS 558204.

Camphouse, R.C., 2012d. An overview of the BRAGFLO two-phase flow parameters used to 

model the run-of-mine salt panel closures implemented in the PCS-2012 PA. Memo to Paul 

Shoemaker dated June 7, 2012. Sandia National Laboratories, Carlsbad, NM. ERMS 557653.

Camphouse, R.C., D.C. Kicker, T.B. Kirchner, J.J. Long, B. Malama, and T.R. Zeitler, 2012a. 

Summary Report and Run Control for the 2012 WIPP Panel Closure System Performance 

Assessment. Sandia National Laboratories, Carlsbad, NM. October 31. ERMS 558365.

Camphouse, R., M. Gross, C. Herrick, D. Kicker, and B. Thompson, 2012b. Recommendations 

and Justifications of Parameter Values for the Run-of-Mine Salt Panel Closure System Design 

Modeled in the PCS-2012 PA. Sandia National Laboratories. Carlsbad, NM, May 3. ERMS 

557396.

Clayton, D.C., R.C. Camphouse, J.W. Garner, A.E. Ismail, T.B. Kirchner, K.L. Kuhlman, and 

M.B. Nemer, 2010. Summary Report of the CRA-2009 Performance Assessment Baseline 

Calculation, Revision 1. Sandia National Laboratories. Carlsbad, NM, ERMS 553039.

Costin, L.S. and W.R. Wawersik, 1980. Crack Healing Of Fractures in Rock Salt. SAND80-

0392. Sandia National Laboratories, Albuquerque, NM (cited in SAND2003-3407).

DOE (U.S. Department of Energy) 1986. Waste Isolation Pilot Plant Design Validation Final 

Report, DOE-WIPP-86-010. Prepared for the U.S. Department of Energy, Carlsbad Field Office, 

Carlsbad, New Mexico, by Bechtel National Inc., San Francisco, California. October.

DOE (US. Department of Energy) 1995. Conceptual Design for Operational Phase Panel 

Closure Systems. DOE-WIPP-95-2057, US. Department of Energy, Carlsbad Area Office, 

Carlsbad, New Mexico.

DOE (U.S. Department of Energy) 1996a. Title 40 CFR Part 191 Compliance Certification 

Application for the Waste Isolation Pilot Plant, DOE/CAO-1996-2184, U.S. Department of

Energy, Carlsbad Area Office, Carlsbad, New Mexico.
88

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APPENDIX A: QUALITY ASSURANCE REVIEW OF THE 

PCS-2012 PA PARAMETER DATABASE
92

APPENDIX B: REVIEW OF WIPP PERFORMANCE ASSESSMENT 

COMPUTERIZED DATABASE (PAPDB) MIGRATION