DOCKET: EPA-HQ-OAR-2014-0609

                                     FINAL

                 TECHNICAL SUPPORT DOCUMENT FOR SECTION 194.24
EVALUATION OF THE COMPLIANCE RECERTIFICATION ACTINIDE SOURCE TERM, GAS GENERATION, BACKFILL EFFICACY, WATER BALANCE AND CULEBRA DOLOMITE DISTRIBUTION COEFFICIENT VALUES

                    U. S. Environmental Protection Agency
                      Office of Radiation and Indoor Air
                 Center for Waste Management and Regulations
                         1301 Constitution Ave., N.W.
                             Washington, DC 20004

                                   June 2017
Table of Contents

Acronym and Abbreviation List	viii
Preface		xii
Executive Summary and Report Outline	xiii
1.0	Introduction	1
2.0	Geologic Setting, Salado Formation Mineralogy, and Brine Chemistry	3
2.1	Geologic Setting	3
2.2	Salado Mineralogy	3
3.0	Gas Generation	5
3.1	Salado Formation and Castile Formation Brine Chemistry	5
3.2	Iron Corrosion	6
3.2.1	Corrosion Reaction Products and Stoichiometry	7
3.2.2	Corrosion Rates	8
3.2.3	Passivation of Iron Surfaces by H2S	11
3.2.4	Steel Surface Area	12
3.2.5	PA Implementation	19
3.2.6	Effects of Anoxic Corrosion Gas Generation Rates on Repository Releases	21
3.3	Corrosion of Aluminum, Lead and Other Metals	21
3.4	Microbial Gas Generation	24
3.4.1	CCA PA, PAVT, and CRA-2004 PA Microbial Gas Generation	25
3.4.2	Microbial Gas Generation Evaluation during AMWTF Review	27
3.4.3	PABC04 Microbial Gas Generation	27
3.4.4	Microbial Gas Generation Review for MgO Planned Change Request	32
3.4.5	Microbial Gas Generation for the CRA-2009 PA and PABC09	32
3.4.6	Microbial Gas Generation for the CRA-2014 PA	34
3.5	Effects of Chemically Incompatible Materials on Gas Generation for the CRA-2014 PA	35
3.6	Radiolysis	37
3.7	Conclusions Regarding WIPP Gas Generation	38
4.0	Backfill Efficacy	40
4.1	CCA PA Conceptual Model and Implementation	40
4.1.1	MgO Reactions in the Repository	40
4.1.2	Placement of MgO Backfill	42
4.2	Reviews of MgO Efficacy and Implementation Changes Prior to the CRA-2004 PA	43
4.3	CRA-2004 PA and PABC04 Review of MgO Backfill Efficacy	43
4.3.1	Characterization of Premier MgO	43
4.3.2	MgO Hydration and Carbonation Experiments	44
4.3.3	MgO Excess Factor Calculations	45
4.4	Planned Change Request to Reduce MgO Excess Factor	45
4.5	CRA-2009 MgO Backfill Efficacy Review	49
4.6	Planned Change Notification Regarding Alternative Placement of MgO Supersacks	51
4.7	CRA-2014 MgO Backfill Efficacy Review	52
4.7.1	MgO Placement	52
4.7.2	MgO Characteristics and Reactivity Testing	52
4.7.3	MgO Hydration Reactions	53
4.7.4	MgO Carbonation Reactions	53
4.8	Backfill Efficacy Conclusions	53
5.0	Actinide Oxidation States	55
5.1	Actinide Oxidation State Information Developed for the CCA PAVT and PABC04	55
5.2	Actinide Oxidation State Information Developed for the PABC09	56
5.3	Actinide Oxidation State Information Developed for the CRA-2014 PA	57
5.3.1	Uranium and Neptunium Oxidation States	57
5.3.2	Plutonium Oxidation States	57
5.3.3	Incorrect Citations	71
5.4	Actinide Oxidation States Summary and Conclusions	71
6.0	Dissolved Actinide Source Term	73
6.1	CCA and PAVT Dissolved Actinide Source Term	73
6.1.1	FMT Code and Database	74
6.1.2	Oxidation-State Analogy	74
6.1.3	Uranium(VI) Solubility	74
6.1.4	CCA PA and PAVT Actinide Solubility Uncertainties	75
6.1.5	Effects of Organic Ligands on Actinide Solubilities	75
6.1.6	Actinide Solubility Calculations for the CCA PAVT	76
6.2	CRA-2004 PA Dissolved Actinide Source Term	78
6.2.1	FMT Code and Database	78
6.2.2	Salado Brine Formulation	78
6.2.3	Carbon Dioxide Buffering in the Absence of Significant Microbial Activity	78
6.2.4	Uranium(VI) Solubility	79
6.2.5	Actinide Solubility Uncertainties Used in PA	80
6.2.6	Temperature Effects on Actinide Solubilities	81
6.2.7	Effects of Organic Ligands on Actinide Solubilities	82
6.3	PABC04 Dissolved Actinide Source Term	82
6.3.1	FMT Code and Database	83
6.3.2	Carbon Dioxide Buffering Reactions	83
6.3.3	Uranium(VI) Solubility	83
6.3.4	Actinide Solubility Uncertainties Used in PA	83
6.3.5	Organic Ligand Concentrations	84
6.3.6	Results of the PABC04 Actinide Solubility Calculations	85
6.4	CRA-2009 PA Dissolved Actinide Source Term	86
6.4.1	Waste Inventory	86
6.4.2	Minimum Brine Volume	87
6.4.3	Repository Chemical Conditions	87
6.4.4	Actinide Aqueous Speciation and Solubility Data	91
6.5	PABC09 Dissolved Actinide Source Term	100
6.5.1	FMT Code and Database	100
6.5.2	Chemical Conditions Assumptions	101
6.5.3	Organic Ligand Concentrations	101
6.5.4	Results of Actinide Solubility Calculations	101
6.5.5	Actinide Solubility Uncertainties	102
6.6	CRA-2014 PA Dissolved Actinide Source Term	106
6.6.1	Waste Inventory	106
6.6.2	Brine Chemistry	108
6.6.3	Brine Volumes and Organic Ligand Concentrations	109
6.6.4	EQ3/6 Code and Database	109
6.6.5	Results of Actinide Solubility Calculations	125
6.6.6	Actinide Solubility Uncertainties	127
6.7	CRA14_SEN4 PA Dissolved Actinide Source Term	130
6.7.1	Revised Selection Criteria for Am(III) and Th(IV) Actinide Solubility Uncertainty Distributions	130
6.7.2	Th(IV) Uncertainty Data Selection	132
6.7.3	Am(III) Uncertainty Data Selection	134
6.7.4	CRA14_SEN4 Th(IV) Solubility Uncertainty Results	135
6.7.5	CRA14_SEN4 Am(III) Solubility Uncertainty Results	136
6.8	Actinide Source Term Summary and conclusions	137
7.0	Colloidal Actinide Source Term	139
7.1	Mineral Fragment Colloids	139
7.2	Intrinsic Colloids	142
7.3	Humic Colloids	144
7.4	Microbial Colloids	145
7.5	Colloids Summary and Conclusions	148
8.0	Total Mobilized Actinide Calculations for PA	150
8.1	CRA-2014 PA Total Mobilized Actinide Results	150
8.2	CRA14_SEN4 PA Total Mobilized Actinide Results	152
8.3	Sensitivity of Total Mobilized Actinide Concentrations to Parameter Changes	152
9.0	Actinides Included in Performance Assessment Calculations	158
10.0	Culebra Dolomite Distribution Coefficients	160
10.1	Distribution Coefficients Used in the CCA PA and PAVT	160
10.2	Distribution Coefficients Used in the CRA-2004 PA, PABC04 and CRA-2009 PA	161
10.3	Distribution Coefficients used in the PABC09 and CRA-2014 PA	162
10.4	Culebra Dolomite Distribution Coefficients Conclusions	164
11.0	Effects of Heterogeneous Waste Loading	165
12.0	Effects of Chemical Processes on Repository Water Balance	168
12.1	Anoxic Corrosion and Sulfidation of Iron-Based Metals	168
12.2	Lead Corrosion	169
12.3	Microbial Degradation of CPR	170
12.4	Backfill Reactions	170
12.4.1	MgO Hydration Rate	170
12.4.2	MgO and Brucite Carbonation Rates	171
12.4.3	Hydromagnesite Conversion Rate	172
12.5	Effects of Water Balance Assumptions on PA	174
13.0	Sensitivity Study	176
13.1	Effects of CRA14_SEN4 PA Parameter Changes on Releases	176
13.2	Possible Effects of Additional Chemistry Parameter Changes	177
13.3	Sensitivity Calculations Conclusions	178
14.0	Summary and Conclusions	179
15.0	References	187

List of Tables
Table 3-1.	Brine A, GWB and ERDA-6 Compositions	6
Table 3-3.	CRA-2014 PA Inventories of Iron-Based Metals and Alloys in Waste and Steel Packaging Materials	7
Table 3-4.	WIPP PA Inundated Steel Corrosion Rates (STEEL:CORRMCO2) and Humid Steel Corrosion Rates (STEEL:HUMCORR)	9
Table 3-5.	WIPP Container Surface Areas, Tare Weights and Surface Area to Tare Weight Ratios	15
Table 3-6.	Container plus Iron Waste Surface Areas Calculated per 55-Gallon Drum Equivalent Using Different Surrogate Assumptions	16
Table 3-7.	Surface Area per Drum Equivalent, Weighted by Waste Container Volume and Number	17
Table 3-8.	Steel Surface Area per Unit Volume (DS) from the CRA-2014 PA and Calculated Using Different Waste Surface Area to Mass Surrogate Assumptions for Idealized and Reported Waste Emplacement	19
Table 3-9.	CRA-2014 PA Inventories of Aluminum, Lead and Other Metals and Alloys in Waste and Packaging Materials	22
Table 3-10.	Possible Effects of Shielded Containers on WIPP Lead Inventory	24
Table 3-11.	Inundated Microbial Gas Generation Rates (WAS_AREA:GRATMICI) Used in PA	26
Table 3-12.	Humid Microbial Gas Generation Rates (WAS_AREA:GRATMICH) Used in PA	26
Table 5-1. 	CRA-2014 Baseline Actinide Solubilities (Brush and Domski 2013a) in mol/L (M). Actinide concentrations in brine are calculated by equilibrating a representative brine (GWB or ERDA-6) with WIPP-relevant solid phases (such as minerals), organic ligands, and actinides. Solubilities are calculated for different brine volumes released from the repository, ranging from the minimum brine volume needed for a release to 5-times that volume (discussed in Section 6.6.3).	66
Table 5-2. 	Higher Oxidation State Assumption Plutonium Concentration Ranges Using CRA-2014 Baseline Concentrations	69
Table 6-1.	Organic Ligand Concentrations Calculated for the CCA PAVT, PABC04, PABC09 and CRA-2014 PA Using Brine Volumes and Organic Ligand Inventories That Were Updated for Each PA	76
Table 6-2.	Actinide Solubility Calculations for the CCA PAVT, PABC04, PABC09 and CRA-2014 PA (1x brine volume)	77
Table 6-3.	High Ionic Strength Uranium Solubility Data Evaluated by EPA (2006c)	79
Table 6-4.	Cumulative Density Function Ranges Established by the PABC04 Actinide Solubility Uncertainty Analysis	84
Table 6-5.	Reported Ligand Inventories for the PABC04, PABC09 and CRA-2014 PA	86
Table 6-6.	Mass Balance Calculations Comparing Moles of Portlandite in Cement to Moles of Magnesium Available from Brine and Polyhalite, Minimum Brine Volume of 17,400 m[3]	89
Table 6-7.	PABC09 Organic Ligand Concentrations and Concentrations Used in the Sensitivity Calculations by Brush and Xiong (2004) and Brush et al. (2008)	90
Table 6-8.	Comparison of the Th(IV) and Am(III) Solubility Uncertainty Distribution Statistics for the PABC04, PABC09, CRA-2014 PA and CRA14_SEN4 PA	103
Table 6-9.	Comparison of Th(IV) Difference Values Calculated for the PABC09 and for Individual Investigations	103
Table 6-10.	Am(III) PABC09 Uncertainty Distribution and Range of Difference Values Calculated for Excluded Data	104
Table 6-11.	CRA-2014 PA Inventory Mass Balance Calculations Comparing Moles of Portlandite in Cement to Moles of Magnesium Available from Brine and Polyhalite Using Minimum Brine Volume of 17,400 m[3]	107
Table 6-12.	CRA-2014 PA Curium and Neptunium Inventory Decayed through 2033	108
Table 6-13.	Borate Speciation in WIPP Brines Calculated with EQ3/6	114
Table 6-14.	CaEDTA[2-] and MgEDTA[2-] Formation Constants	117
Table 6-15.	Th(IV)-carbonate Experimental Conditions	122
Table 6-16.	Log Ks[0] Values for Am(OH)3(c), Am(OH)3(am) and Nd(OH)3(am)	123
Table 6-17.	CRA-2014 PA Actinide Solubility Modeling Results Using 1x and 5x Brine Volumes[a] (Brush and Domski 2013b).	126
Table 6-18.	Th(IV) Uncertainty Distribution Data Evaluation	133
Table 6-19.	Am(III) Uncertainty Distribution Data Evaluation	134
Table 6-20.	Comparison of Th(IV) Difference Value Statistics Calculated for the CRA14_SEN4 PA and Individual Investigations	136
Table 6-21.	Comparison of Am(III) Difference Value Statistics Calculated for the CRA14_SEN4 PA and Individual Investigations	137
Table 7-1.	Actinide Concentrations Associated with Mineral Fragment and Intrinsic Colloids	140
Table 7-2.	Proportionality Constants and Maximum Concentrations for Humic  Colloids Used in All WIPP PAs	140
Table 7-3.	Proportionality Constants and Maximum Concentrations for Microbial Colloids	141
Table 8-1.	CRA-2014 PA Mean Dissolved and Colloidal Actinides	154
Table 8-2.	PABC09 Mean Dissolved and Colloidal Actinides	155
Table 8-3.	CRA14_SEN4 PA Mean Dissolved and Colloidal Actinides	156
Table 8-4.	Mean Total Mobilized Concentrations in Castile Brine for the PABC09, CRA-2014 PA and CRA14_SEN4 PA	157
Table 10-1.	Comparison of Matrix Kd Values for the CCA PA and PAVT With Matrix Kd Values for the CRA-2004 PA, PABC04, CRA-2009 PA and PABC09	161
Table 12-1.	Hydromagnesite Conversion Rates (WAS_AREA:HYMAGCON)	173

List of Figures

Figure 3-1.	Humid Steel Corrosion Rate Data (Roselle 2013)	11
Figure 5-1.	Plutonium Eh-pH Diagram Showing Plutonium Aqueous Speciation (Simpkins 2011) 	59
Figure 5-2.	Reactions Controlling the Solubility of Plutonium in WIPP Brines	67
Figure 5-3.	Effects of Redox Conditions on Baseline Plutonium Concentrations for WIPP PA, Illustrated Using CRA-2014 Am(III) and Th(IV) Baseline Solubilities (GWB, 1x brine volume). WIPP Repository Brine Aqueous Plutonium Concentrations Are Controlled by Pu(OH)3(s) Solubility Under the Most Reducing WIPP Conditions and by Reductive Dissolution of PuO2(am) Under the Less Reducing WIPP Conditions	68
Figure 6-1. 	Solubility of ThO2::xH2O(am) in 0.1 M to 4 M NaCl with a Total Carbonate Concentration of 0.02 M	94
Figure 6-2.	U(VI) Solubility Measured in Carbonate-Free Brine as a Function of pcH	97
Figure 6-3.	Hydromagnesite Solubilities	112

Acronym and Abbreviation List

Ad	Surface area of steel associated with a waste disposal drum
AMWTF	Advanced Mixed Waste Treatment Facility
Brine A	Simulated Salado brine formulation
CAPMIC	Maximum Actinide Concentration Associated with Microbes
CCA	Compliance Certification Application
CCDF	Complementary cumulative distribution function
CDF	Cumulative distribution function
CEMRC	Carlsbad Environmental Monitoring and Research Center
CFR	Code of Federal Regulations
CH	Contact handled
Ci	Curie
CPR	Cellulosics, plastics, and rubber
CRA	Compliance Recertification Application
CRA-2004	First Compliance Recertification Application, March 2004
CRA-2009	Second Compliance Recertification Application, March 2009
CRA-2014	Third Compliance Recertification Application, March 2014
CRA14_SEN4 PA	Sensitivity study of CRA-2014 PA
D	Difference value, log10(measured solubility)  -  log10(predicted solubility)
DATA0.FM1	EQ3/6 thermodynamic database used for the CRA-2014 PA
DATA0.FM2	Revised EQ3/6 thermodynamic database
DBR	Direct brine release
DL	Direct load
DOE	U.S. Department of Energy
DRSPALL	Spallings released to accessible environment from a future drilling event
DRZ	Disturbed rock zone
DS	Steel surface area per unit disposal volume
EDTA	Ethylenediaminetetraacetic acid
EEF	Effective Excess Factor
EF	Excess Factor
EEG	Environmental Evaluation Group
Eh	Oxidation/reduction potential
EPA	U.S. Environmental Protection Agency
EQ3/6	Geochemical modeling code used for CRA-2014 PA
ERDA-6	Simulated Castile brine formulation
EXAFS	Extended x-ray absorption fine structure
FMT	Fracture Matrix Transport geochemical modeling code
FEP	Feature, event and process
FMT_021120.CHEMDAT	FMT Thermodynamic database used for the CRA-2004 PA
FMT_050405.CHEMDAT	FMT Thermodynamic database used for the PABC04 and PABC09
FMT_970407.CHEMDAT	FMT Thermodynamic database used for the PAVT
GWB	Generic Weep Brine, simulated Salado brine formulation
Kd	Distribution coefficient
kPa	kilopascal
Ksp[0]	Solubility product at zero ionic strength
LANL	Los Alamos National Laboratory
LIBD	Laser-induced breakdown detection
LOI	loss on ignition
m	meter
M	molar, moles/liter
MB	Marker bed
meq	milliequivalent
MH2 		Molecular weight of H2 [kg H2 per mole H2, 2.02  10[-3], parameter REFCON:MW_H2].
MPa	megapascal
u[0]/RT 	Standard chemical potential (dimensionless)
nd	Number of waste drums that can be close-packed into a single room in the repository
OP	Overpack
Pa	Pascal
PA	Performance assessment
PABC04	CRA-2004 Performance Assessment Baseline Calculations
PABC09	CRA-2009 Performance Assessment Baseline Calculations
PAVT	Performance Assessment Verification Testing
PCR	Planned change request
ρFe 	Themolar density of steel (1.41 x 10[5] moles/m[3][)]
pH	Negative log of the hydrogen ion activity
pcH	Negative log of the hydrogen ion concentration, molar units
pmH	Negative log of the hydrogen ion concentration, molal units
PVC	Polyvinylchloride
qr gc 			Rate of gas production per unit volume of waste due to anoxic corrosion of iron-based metals (kg/m[3]/sec)
Rci 	Corrosion rate under inundated conditions, STEEL:CORRMCO2 (m/s)
Rch 	Corrosion rate under humid conditions, STEEL:HUMCORR (m/s)
REFCON:ASDRUM	A performance assessment parameter name used to represent the surface area of a waste drum
REFCON:DRROOM	A performance assessment parameter name used to represent the number of drums emplaced in a waste panel
REFCON:MW_H2	A performance assessment parameter name used to represent the molecular weight of hydrogen gas
REFCON:VROOM	A performance assessment parameter name used to represent the volume of a waste drum

RH	Remote handled
RSI	Institute for Regulatory Science
Sb,eff 	Effective brine saturation due to capillary action in the waste materials
Sg* 	Equals 1- Sb,eff if Sb,eff > 0, or equals 0 if Sb,eff = 0
Sm	Measured solubility
SNL	Sandia National Laboratories
Sp	Solubility predicted by FMT or EQ3/6 calculations
STEEL:CORMCO2	Represents inundated steel corrosion rate (m/s)
STEEL:HUMCORR	Represents the humid steel corrosion rate (m/s)
STEEL:STOIFX	Represent the stoichiometric coefficients for gas generation from steel corrosion (m/m)
STTP	Actinide Source-Term Waste Test Program
TRLFS	Time-resolved laser fluorescence spectroscopy
TRU	Transuranic waste
VR	Initial volume of a single room in the repository
WAS_AREA:BIOGENFC	A performance assessment parameter name used to represent the gas generation from microbes in the waste panels
WAS_AREA:GRATMICI	A performance assessment parameter name used to represent biodegradation of cellulose plastics and rubber under anaerobic and brine inundated conditions

WAS_AREA:GRATMICH	A performance assessment parameter name used to represent biodegradation of cellulose plastics and rubber under anaerobic and humid conditions
WAS_AREA:PROBDEG	A performance assessment property used to estimate the probability of biodegradation of organic materials in a waste panel
WDS	WIPP Waste Data System
WIPP	Waste Isolation Pilot Plant
XAFS	X-ray absorption fine structure
XANES	X-ray absorption near-edge spectroscopy
XC (H2│Fe) 		Stoichiometric coefficient for gas generation due to the corrosion of steel [1 mole H2 per mole Fe, parameter (Kicker and Herrick 2013)]
XRD	X-ray diffraction

   Preface

The U.S. Department of Energy (DOE) is required to submit a Compliance Recertification Application (CRA) to the U.S. Environmental Protection Agency (EPA) for the Waste Isolation Pilot Plant (WIPP) facility every five years including an updated assessment of future WIPP performance. During EPA's review of DOE's CRA-2014 performance assessment (PA), events associated with the February 2014 repository fire and radionuclide release have closed portions of the underground facility. This temporary closure has created a situation where certain parts of the underground facility could not be accessed for ground control. Panel 9 may be abandoned along with plans to modify the design change used for panel closures in panels 3, 4, 5 and 6.

Because the CRA PAs are predictions of post-closure repository performance and the EPA knows there will be modifications to the current repository design, modifying the CRA-2014 PA at this time to incorporate alternative parameter values would not add more reality to predictions of repository post-closure performance. Consequently, the EPA adopted the CRA-2014 PA as originally submitted by DOE as the baseline, rather than have DOE conduct a revised PA baseline calculation (PABC). In lieu of requesting a PABC-2014, the EPA requested that DOE conduct a set of sensitivity studies to address some of the significant technical concerns arising from the EPA's CRA-2014 review. The inputs to these sensitivity studies broadly address many of the EPA's technical concerns that could potentially impact long-term repository performance. The Agency has reviewed the results of these studies and determined that there exists an adequate level of confidence -- that is, a reasonable expectation -- that the repository will continue to comply with EPA regulations. These results are described and evaluated in this report, but are addressed in more detail in a companion Technical Support Document, "Review of EPA Sensitivity Studies of DOE CRA-2014 WIPP Compliance Recertification Performance Assessment."

Additionally, the EPA recommends further work that can be conducted to evaluate many of the technical concerns identified in the EPA's review of the CRA-2014 PA, as well as incorporate future repository design changes. The EPA will work with DOE to determine the best path forward for resolution of EPA's concerns, which could include additional data reviews, independent technical reviews, and possibly additional sensitivity analyses to reach a consensus for the next CRA. It is anticipated that the results of these efforts will be incorporated into the CRA-2019 PA or otherwise be made available during EPA's review of the CRA-2019 PA.

Executive Summary and Report Outline

The Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico is an underground facility designed for the permanent disposal of defense-related transuranic (TRU) waste. The U.S. Department of Energy (DOE) operates the WIPP repository under the regulatory oversight of the U.S. Environmental Protection Agency (EPA). DOE submitted the Compliance Certification Application (CCA) to the EPA in 1996. After extensive review of the CCA and supplemental information provided by DOE, including additional performance assessment (PA) calculations referred to as Performance Assessment Verification Testing (PAVT), EPA certified that DOE had met the regulatory requirements and WIPP began accepting waste in March 1999.
DOE is required to submit a Compliance Recertification Application (CRA) every 5 years after the initial receipt of waste at WIPP, and the first CRA (CRA-2004) was submitted to EPA in March 2004 (DOE 2004). EPA reviewed the CRA-2004 and supplemental information provided by DOE, including PA calculations referred to as the CRA-2004 Performance Assessment Baseline Calculations (PABC04), and certified that DOE continued to meet WIPP regulatory requirements (EPA 2006a). DOE submitted the second CRA (CRA-2009) to EPA in March 2009 (DOE 2009). The CRA-2009 PA included the actinide solubility calculations incorporated in the PABC04. The EPA performed a completeness review of the CRA-2009 and instructed DOE to perform a new PA to capture changes since the 2004 recertification (Cotsworth 2009a). This PA, the CRA-2009 Performance Assessment Baseline Calculations (PABC09), was performed by DOE and reviewed by EPA. Based on their review of the CRA-2009 and PABC09, EPA certified that DOE continued to meet WIPP regulatory requirements (EPA 2010b). At the time of the recertification decision, however, EPA (2010b) noted a number of uncertainties related to WIPP chemistry and also identified issues related to WIPP repository chemical processes for additional investigation prior to the CRA-2014.
DOE submitted the third CRA (CRA-2014) to EPA in March 2014 (DOE 2014b). This Technical Support Document summarizes EPA's review of chemistry-related issues important to PA, including actinide source term modeling, gas generation, backfill efficacy, water balance calculations and actinide transport in the Culebra Dolomite above the repository for the WIPP CRA-2014.
Section 1.0 provides a summary of WIPP certification and recertification activities since the CCA. It also summarizes the regulatory requirements and important waste chemical characteristics and repository chemical processes considered during certification and recertification. WIPP waste characteristics and components that have been identified as potentially affecting the chemistry of the system and repository releases include actinide solubilities and oxidation state distributions; formation of colloidal suspensions containing radionuclides; production of gas from steel corrosion and waste degradation; ferrous metals inventory; cellulosics, plastics and rubber (CPR) inventory; chelating agents (organic ligands) and oxyanions; and the activity of each isotope of the radionuclides present in the waste [40 CFR 194.24(b), EPA 1997b].
A summary of the geologic setting, mineralogy of the Salado Formation surrounding the repository and WIPP brine chemistry is provided in Section 2.0. The WIPP repository is excavated within the Salado Formation, a relatively thick bedded salt, composed mainly of layers of impure halite [NaCl], with interbedded layers of anhydrite [CaSO4(s)], polyhalite [K2MgCa2(SO4)4::2H2O(s)] and mudstone. The Salado Formation is underlain by the Castile Formation. The Castile Formation is mainly composed of thick beds of high-purity halite, alternating with thick beds of interlaminated carbonate and anhydrite. Pressurized brines are associated with the deformed areas of this formation. The Salado Formation is overlain by the Rustler Formation, of which the Culebra Dolomite is the most significant to WIPP PA. The Culebra Dolomite, which is mostly composed of dolomite [CaMg(CO3)2(s)] with small amounts of anhydrite or gypsum [CaSO4::2H2O(s)], is the most transmissive continuously saturated unit above the WIPP facility. It therefore provides the most direct pathway from the repository to the accessible environment, apart from releases directly to the surface.
PA calculations have used representations of both Salado and Castile brines. Small quantities of intergranular and intragranular Salado brines are associated with the salt at the repository horizon. These brines are highly concentrated (ionic strengths up to 8 M), with a composition of mostly sodium, magnesium, potassium, chloride and sulfate, and with smaller amounts of calcium, carbonate and borate. The brine formulation used to represent Salado brine in WIPP laboratory experiments and PA calculations is denoted as Generic Weep Brine (GWB). Since the CRA-2004 PA calculations to the present CRA-2014 PA, it is the only brine formulation used to represent that of the Salado (Snider 2003b). Castile brine may enter the repository if a borehole penetrates a waste panel and also penetrates an area of pressurized brine in the underlying Castile Formation. The brine composition used to represent that of the Castile is withdrawn from the ERDA-6 well and used to represent all Castile brines in WIPP related laboratory experiments as well as WIPP PA calculations. Brine taken from the ERDA-6 borehole has lower magnesium and chloride concentrations and a lower ionic strength than GWB -- the Salado brine analog.

Section 3.0 provides a review and discussion of the Gas Generation conceptual model and its implementation for WIPP PA. The Gas Generation conceptual model includes the assumptions that significant gas generation will occur in the repository through corrosion of ferrous metals and through microbial degradation of CPR. The model also assumes that essentially all carbon dioxide [CO2] will be removed from the gas phase by reaction with the magnesium oxide (MgO) backfill.
The PA uses two anoxic steel corrosion rates depending on brine saturation: humid and inundated. These rates cannot occur unless brine is present and are proportional to the estimated surface area of iron-based waste and steel waste containers. Anoxic corrosion of iron-based metals in the waste and waste containers produces hydrogen and the reaction products are two solids, Fe(OH)2::xH2O(s) and FeS(s). New project-specific data was used in the CRA-2014 PA to modify and lower the inundated corrosion rates from those used in prior PAs. Upon EPA's review of the updated experimental data and DOE revisions to the 2014-CRA PA, the EPA concluded that DOE's corrosion rate updates as most likely being underestimated. Consequently, the EPA recommends that the inundated and humid corrosion rates used in the CRA-2014 PA be reassessed by DOE for the CRA-2019 PA such that corrosion rates are aligned with current theoretical and experimental data. EPA's evaluation of the steel surface area calculations demonstrated that the steel surface area per unit volume used in the CRA-2014 PA is a reasonable value for WIPP waste that has been emplaced through December 31, 2014. EPA recommends that DOE continue to update the steel surface area per unit volume using future inventories and waste emplacement data to demonstrate that the value used in PA continues to be representative for the CRA-2019.
DOE assumed that all H2S(g) generated by CPR degradation would instantaneously react with Fe(s) and Fe(OH)2(s). This assumption resulted in lower gas pressures and higher brine volumes. EPA's assessment of the available experimental data concludes this assumption is inconsistent with that data and indicates H2S could passivate iron surfaces. Therefore, EPA concludes it would not be appropriate for DOE to assume H2S(g) would instantaneously react with Fe(s) and Fe(OH)2(s) in the CRA-2019 PA.
For the PABC04 and all later PA calculations, significant microbial degradation of cellulosics was assumed for 100% of the PA realizations, with significant microbial degradation of plastics and rubber assumed for 25% of the PA realizations. For the PABC04 and all later PA calculations, an initial gas pressure was used to account for rapid initial microbial gas generation rates, with lower rates used to account for slower long-term microbial gas generation through CPR degradation. Based on the available data that include WIPP-specific long-term degradation experiments, the current approach for modeling microbial gas generation rates is appropriate for PA.
Based on information available at the time of the CCA, lead and aluminum corrosion are assumed to be insignificant to PA because of the small amount of gas production that is likely to result relative to anoxic corrosion of iron-based metals. Evaluation of the CRA-2014 PA aluminum inventory shows that aluminum corrosion is unlikely to have significant effects on gas generation. The CRA-2014 PA lead inventory is also relatively small, indicating that lead corrosion is unlikely to affect gas generation rates. However, WIPP has emplaced some remote-handled (RH) shielded containers that contain significant amounts of lead and may use more shielded containers in the future. The CRA-2014 PA inventory does not include the large future increase in lead inventory that may occur as shielded containers are placed in the repository. Bounding calculations of the effects of shielded containers on lead inventory using information from Crawford and Taggart (2007) indicate that the amounts of lead associated with the shielded containers might significantly increase gas generation rates. EPA believes bounding calculations for the CRA-2019 PA would be appropriate to assess the possible effects of additional lead from shielded containers on gas generation rates and repository performance.
The Gas Generation Conceptual model includes the assumption that radiolysis of water in the waste and brine, and irradiation of plastics and rubber in the waste will not significantly affect the amounts of gas generated. However, lower microbial and anoxic corrosion gas generation rates were used in the CRA-2014 PA compared to those used in the CCA PA or PAVT, consequently, it is no longer certain that radiolytic gas generation will be relatively insignificant.  Based on these modifications to the microbial and anoxic contributions to gas generation, gas generation from radiolysis may now be more significant than assumed in the past. Therefore, the EPA believes that it would be appropriate to include an assessment of the potential importance of radiolytic gas generation in the 2019 PA.
Section 4.0 describes the MgO backfill included in the WIPP repository to control repository chemical conditions and minimize actinide solubilities in the postclosure repository. EPA determined that the MgO backfill met the requirements for an engineered barrier to control CO2 fugacities and buffer brine. MgO reacts with water in the repository to create brucite [Mg(OH)2(s)], and brucite dissolution prevents low brine pH values that could increase actinide solubilities. Brucite also reacts with CO2 produced by microbial degradation of CPR to form hydromagnesite [Mg5(CO3)4(OH)2::4H2O(s)], controlling CO2 fugacities at low levels that minimize actinide solubilities in repository brines. Hydromagnesite may convert over long time periods to magnesite [MgCO3(s)], which is the more stable phase and which will control CO2 fugacities at even lower levels.

Evaluation of the available data indicates that the MgO backfill will control CO2 fugacities under inundated conditions and maintain chemical conditions in WIPP brines that limit actinide solubilities. The current emplacement plan ensures adequate amounts of MgO will be present in the repository, because the quantities of CPR and MgO are monitored in each room of the repository during emplacement and additional MgO is emplaced if necessary. Because of uncertainties in the rate at which hydromagnesite will convert to magnesite in the repository environment, it is assumed for PA that the brucite-hydromagnesite reaction will buffer CO2 fugacity at levels consistent with relatively low actinide solubilities under inundated conditions. Current data suggests the backfill material will adequately control chemical conditions in the repository.

Section 5.0 provides a review of the expected actinide oxidation states that will dissolve in brines that enter the repository. The actinide oxidation states assumed for PA have remained unchanged since the CCA. Am(III), Cm(III) and Th(IV) are assumed to be the only stable oxidation states for these radionuclides. However, U, Np, and Pu may be present in different oxidation states. It is assumed for PA that U may exist in either the +IV or +VI oxidation state and Np may exist in either the +IV or +V oxidation states. These assumptions remain valid, based on the available data. U(IV) and Np(IV) are likely to be the most stable oxidation states under the reducing conditions in the WIPP repository. However, U(VI) may persist because of the effects of complexation on uranium reduction. The assumption that Np may be present in the +V oxidation state in the WIPP repository brines is conservative, because of the greater solubility of Np(V) solid phases.
Available data continue to support the assumption that Pu will be present in the +III and +IV oxidation states, rather than the +V and +VI states. DOE has assumed that Pu concentrations in the PA realizations with reducing conditions is controlled by equilibrium between Pu(III) solid and Pu(III) aqueous species. For the PA realizations with more oxidizing conditions, DOE has assumed the Pu solubility is controlled by equilibrium between Pu(IV) solid and Pu(IV) aqueous species. However, EPA's evaluation of recent experimental data has indicated that reductive dissolution of Pu(IV) solid to form Pu(III) aqueous species may be more realistic for the more oxidizing realizations, which would result in a higher range of Pu concentrations in the more oxidizing realizations. DOE disagrees with EPA's assessment of likely Pu oxidation states. Based on EPA's evaluation of the available experimental data, the Agency is recommending that DOE conduct an independent technical review to establish the appropriate Pu oxidation states. Additionally, the EPA would like to see the recommendations from this review be implemented as soon as possible, and ideally implemented in the CRA-2019 PA.

Section 6.0 addresses the calculation of Am, Cm, Np, Pu, Th, and U solubilities used in PA. Representative Salado or Castile brines are equilibrated with Salado minerals and MgO hydration and carbonation products. The CRA-2014 PA solubilities of Am(III), Th(IV) and Np(V) are calculated for WIPP repository conditions using the EQ3/6 code and database, replacing the FMT code and database used for previous WIPP PAs. The Dissolved Actinide Source Term conceptual model includes the assumption that actinides in the same oxidation state will exhibit the same solubilities and aqueous speciation. Accordingly, the solubilities of Cm(III) and Pu(III) are assumed equal to the solubility calculated for Am(III) and the solubilities of U(IV), Np(IV) and Pu(IV) are assumed equal to the calculated Th(IV) concentration. Calculated Np(V) solubilities are used only for neptunium, because it is the only actinide predicted to be present in WIPP brines in the +V oxidation state. Because DOE has not developed an aqueous speciation and solubility model for U(VI), an upper-limit concentration of 10[-3] M is assumed for PA.

The organic ligands acetate, citrate, ethylenediaminetetraacetic acid (EDTA) and oxalate could affect actinide solubilities in the repository because these ligands are water soluble and present in significant quantities in the WIPP inventory. The effects of organic ligands on actinide solubilities were not included in the CCA PAVT actinide solubility calculations because it was assumed that other constituents, including transition metals in the waste and magnesium from the MgO backfill, would compete with the actinides for binding sites on the organic ligands. The effects of organic ligands on actinide solubilities have been included in subsequent PAs, showing that EDTA and other ligands may affect +III actinide concentrations.

EPA's review of DOE's thermodynamic database indicates actinide solubilities and aqueous speciation in the WIPP thermodynamic database have not been updated since 2005.  The Agency had informed the DOE that a thorough review and update of the thermodynamic database of available and relevant actinide solubility and speciation experimental data be considered in the 2014-CRA. At the time, the EPA recommended DOE evaluate data presented by Altmaier et al. (2005, 2006) to be considered in a revision of thorium-carbonate speciation. Additionally, EPA recommended the results of the Neck et al. (2009) Nd(OH)3(s) solubility studies be evaluated and considered in a database update as they relate to the solubility of Am(OH)3(s) (EPA 2010b). During EPA's evaluation of DOE's 2014-CRA thermodynamic database it was discovered no revisions related to the actinide solubility and aqueous speciation had been incorporated as recommend during our PABC09 review. This omission was communicated to DOE as part of the Agency's CRA-2014 completeness review. In response, DOE revised the WIPP thermodynamic database that included changes to the solubility of hydromagnesite; changes to borate, EDTA and citrate chemistry; addition of the MgSO4(aq) dissociation constant; and addition of lead aqueous speciation and solubility data.

EPA's preliminary review of the DOE's revised database indicates that DOE's modified hydromagnesite solubility is not well-supported.  The Agency recommends this solubility be revised back to its previous value. Additionally, DOE did not revise the aqueous speciation and solubility data for Am(III) and Th(IV) based on data that have become available since the PABC04. EPA expects DOE to revise the Th(IV)-carbonate aqueous species and the Am(OH)3(s) solubility data in the WIPP database that is aligned and consistent with updated and relevant experimental data prior to the CRA-2019 PA. EPA also expects DOE to continue updating the database for CRA-2019 such that it is aligned and consistent with recent information. If data is to be excluded from any updates, EPA expects a defensible argument for that exclusion.

The calculated solubilities for the +III, +IV, and +V actinides are constant values, with associated uncertainties to bound the upper and lower solubility values. They are developed by comparing the measured solubility of actinides in peer-reviewed studies to their modeled solubility. The studies used for this uncertainty distribution are chosen using a predetermined set of selection criteria. For the CRA-2014 PA DOE evaluated the +III and +IV actinide solubility uncertainty distributions using revised data selection criteria and sampled these revised distributions in order to represent the uncertainty in modeling. EPA found DOE's use of the selection criteria for this study to be inconsistent and amended the data criteria for a sensitivity analysis PA (CRA14_SEN4 PA). The revised criteria included studies previously omitted by DOE as well as some expanded criteria such as the inclusion of studies that emphasize the presence of borate and carbonate.
The mean and median of the +IV actinide solubility uncertainty distribution decreased between the PABC04 and PABC09, then increased for the CRA-2014 PA. For the CRA14_SEN4 PA, the amended uncertainty distribution resulted in a decrease of the mean and median +IV actinide solubility when compared to the CRA-2014 PA values, with many close to zero. These results indicate DOE predicted higher +IV actinide solubilities for the CRA-2014 PA given the data used in the EPA directed analysis.
The mean and median values of the +III actinide solubility uncertainty distribution decreased significantly between the PABC09 and CRA-2014 PA. Using the EPA-amended data selection criteria, the mean and median +III actinide solubility distribution for the CRA14_SEN4 PA increased compared to the CRA-2014 PA values. The combined effect of the modified +IV and +III uncertainty distributions was to increase overall releases because more emphasis was placed on the more soluble +III actinides. EPA recommends that DOE use the amended data selection criteria included in the SEN4 PA as the starting point for calculations of the +III and +IV actinide solubility uncertainty distributions for the CRA-2019 PA.
The WIPP colloidal actinide source term is summarized in Section 7.0. Four types of colloids can form in WIPP repository brines, including microbial, humic, intrinsic and mineral fragment colloids. Actinide concentrations associated with microbial and humic colloids are based on proportionality constants with maximum values. The concentrations of actinide intrinsic colloids and mineral fragment colloids used in PA are constant values based on the results of experimental investigations. No new data have been identified for actinide-humic colloids, so the humic colloid parameters remain unchanged for the CRA-2014 PA.
Experimental data have been reported indicating that mineral fragment colloids may form from MgO hydration and carbonation products and mobilize Th(IV) in MgCl2 brines.  EPA's assessment of DOE's mineral-fragment colloid parameter used in PA calculations indicate this parameter is not aligned with the empirical data. The EPA concludes that it would be appropriate for DOE to evaluate the potential formation of mineral-fragment colloids from MgO hydration and carbonation under WIPP-relevant conditions as soon as possible, and ideally for use in the CRA-2019 PA.
In the absence of evidence of the formation of actinide intrinsic colloids for radionuclides other than plutonium, previous PAs have set the concentrations of Th, U, Np, and Am intrinsic colloids to zero. More recent Th(IV) solubility investigations, however, identified the existence of intrinsic Th(IV) colloids in dilute solution and in brines. DOE performed experiments to evaluate the formation of intrinsic colloids using a narrow pcH range and used data from these experiments to increase the intrinsic colloid parameters used in the CRA-2014 PA. Review of the data used to develop these parameters indicates that the Th(IV) intrinsic colloid parameter for the CRA-2019 PA would be more appropriately based on data collected over a wider pcH range than the range used by DOE, and would increase Th(IV) intrinsic colloid concentrations. EPA's assessment indicates DOE's concentration range for intrinsic colloid is not aligned with intrinsic colloid formation in predicted pcH conditions in the repository. Therefore, EPA concludes that it would be appropriate for DOE to develop a Pu(III) intrinsic colloid parameter range, aligned with this data, that includes and an upper bounding value.
DOE conducted a series of experiments to determine revised parameters for actinides associated with microbial colloids. DOE reduced the maximum concentrations for the microbially associated actinides based on a revised conceptual model assumption. In previous PAs it was assumed that the maximum for the microbially associated actinide concentration is the concentration at which no growth was observed, but DOE changed this upper limit for the CRA-2014 PA to the concentration calculated from an assumed maximum cell concentration multiplied by the moles of actinides per cell. Experiments also used short reaction times, the result of which is increased uncertainty because important actinide-cell interactions (e.g., bioaccumulation and biomineralization) have been ignored and could result in a colloid underestimation. These approaches deviate from the conceptual model and as a result, the Agency does not accept this change. Until DOE can provide a more acceptable change to this parameter, EPA will accept the maximum concentrations used in previous CRAs for the CRA-2019 PA. The Agency also finds that further uncertainty was introduced in the experimental procedures due to the improper use of filters, resulting in added intrinsic colloids to solutions and a non-representative calculation of microbial colloid formation. In future experiments, the Agency expects DOE to provide the appropriate controls to demonstrate intrinsic colloids have been removed from their post-test solutions. Because of the uncertainty introduced by the effects of intrinsic colloids, it would also be appropriate for DOE to compare the CCA PAVT proportionality constants to the constants calculated from the recent investigations and use the higher values for the CRA-2019 PA.
Section 8.0 provides a comparison of the mean calculated total mobilized actinides for the PABC09, CRA-2014 PA and the CRA14_SEN4 PA. Evaluation of the total mobilized actinide concentrations calculated for these PAs demonstrates that mineral fragment colloids and intrinsic colloids make only minor contributions to mean total mobilized concentrations. Mean total Am(III) and Pu(III) mobilized concentrations for the CRA-2014 PA were significantly lower than their respective concentrations for the PABC09 and CRA14_SEN4 PA because of the negative mean +III actinide solubility uncertainty parameter and decreases in microbially associated colloids used for the CRA-2014 PA. In addition, mean and median total mobilized concentrations in all cases were higher for Pu(IV) than for Pu(III), despite the higher baseline Pu(III) solubility. These results show the importance of the microbial colloid parameters in PA.

Section 9.0 summarizes the actinide elements and isotopes included in WIPP PA calculations. Not all radionuclides in the WIPP inventory are included in PA calculations because of computational and time constraints. Consequently, the number of radionuclides included in PA has been reduced according to an algorithm initially developed for the CCA. The results of PA calculations indicate that radionuclide releases are dominated by Pu and Am. These assumptions have not changed since the CRA-2009.

Distribution coefficient (Kd) values used to calculate radionuclide transport through the Culebra are described in Section 10.0. These distribution coefficients are based on the results of WIPP-specific sorption studies carried out on Culebra rock samples and pure dolomite. The PABC09 Kd distributions were also used for the CRA-2014 PA and continue to be appropriate.
Section 11.0 examines the likely effects of assuming homogeneous waste emplacement for WIPP PA on chemical processes in the repository. This assumption is potentially significant because of heterogeneous waste loading that has occurred in the WIPP repository. Furthermore, the empirical and theoretical data of excavated openings in salt indicate each waste panel will be isolated from one-another for the majority of the regulatory period due to the creep-closure process. Therefore, it is highly unlikely that brine in contact with waste in one panel of the repository will contact brine in another panel. Because chemical equilibrium is assumed between brine and actinide solubility-controlling solids, heterogeneous distribution of the radionuclide inventory between the panels will not affect the calculated actinide solubilities. DOE has adequately accounted for the effects of possible heterogeneous distribution of organic ligands between panels in the actinide solubility calculations. Panels filled with waste following the approval of shielded RH containers may have significantly higher lead contents from the container shielding.

Section 12.0 addresses the effects of chemical processes on the repository water balance. Anoxic corrosion was the only chemical process included in calculations of the quantity of brine in the repository for the CCA PAVT, PABC04 and PABC09. The revised CRA-2014 PA water balance added the effects of iron sulfidation, microbial degradation of CPR, MgO hydration, MgO and brucite carbonation and the hydromagnesite conversion reaction. DOE has updated corrosion rates without considering steel passivation. MgO hydration rates included in the water balance were based on data from Premier MgO, which is present in the repository in smaller quantities than Martin Marietta MgO. Premier MgO hydrates more slowly than Martin Marietta MgO. EPA's assessment of DOE's MgO hydration rates used in the 2014-CRA PA indicate this parameter is not aligned with the experimental data for Martin Marietta MgO hydration, the predominate supplier of MgO. Therefore, EPA recommends that DOE develop hydration rates using Martin Marietta MgO and use these rates in combination with Premier MgO hydration rates in the CRA-2019 PA. A `composite' hydration rate will likely decrease brine volumes in the repository. Updates to the CRA-2014 PA water balance are a significant improvement over the water balance calculations used for previous WIPP PAs. The available information indicates that the effects of the CRA-2014 PA water balance issues identified by EPA are likely to be reasonably minor, but DOE should address these issues for the CRA-2019 PA water balance.

Section 13.0 summarizes the results of an additional PA (CRA14_SEN4) that incorporated some changes to the CRA-2014 PA and assesses the effects of these changes on repository releases. Chemistry parameter changes included revised distributions for the +III and +IV actinide solubility uncertainties based on criteria amended by EPA and changes to the stoichiometric parameters for reaction of H2S with Fe(s) and Fe(OH)2(s) to reflect surface passivation. Total releases calculated for the CRA14_SEN4 PA were minimally changed at high probabilities relative to total releases for the CRA-2014 PA. However, direct brine releases (DBRs) were noticeably increased in the CRA14_SEN4 PA compared to the CRA-2014 PA because of higher total mobilized Pu(III) and Am(III) concentrations resulting from the increased +III actinide solubility in the revised uncertainty distribution, an increased probability of encountering pressured brine during a drilling intrusion and increased repository pressure because of the removal of the iron sulfidation reactions from the repository water balance.
Section 14.0 summarizes EPA's conclusions regarding chemistry issues related to the CRA-2014. Review of the CRA-2014 and supporting information, including the results of the CRA-2014 PA and CRA14_SEN4 PA, demonstrates that because of their importance to releases from the repository, issues related to the plutonium oxidation states in WIPP repository brines and solids and DOE's proposed changes to the microbial colloid conceptual model are particularly important to resolve as soon as possible. In addition, EPA recommends that DOE take the following actions for performance assessment, ideally for inclusion in the CRA-2019:
       Use the revised, higher humid and inundated anoxic corrosion rate ranges identified by EPA that account for the effects of CO2 and high pressure
       Confirm that the steel surface area per unit disposal volume is representative of WIPP waste using updated inventory data, the sheet metal surrogate and 55-gallon drum surrogate assumptions and updated waste emplacement data
       Develop a reasonable upper-bound estimate for the lead inventory and lead surface area per unit disposal volume that accounts for the potential future inventory of lead associated with shielded containers; it would be appropriate for DOE to use available project-specific lead corrosion data to assess the effects of lead corrosion on gas generation and brine consumption in the WIPP repository
       Use the available radiolytic H2(g) generation data from monitoring of sealed WIPP panels to evaluate whether radiolytic gas generation rate is significant relative to the rates of gas generation by microbial degradation and anoxic corrosion
       Remove the assumption that all H2S(g) generated by CPR degradation will instantaneously react with Fe(s) and Fe(OH)2(s) from the gas generation rate calculations by revising the relevant stoichiometric coefficients
       Because of their importance to releases from the repository, perform an independent technical review of likely plutonium oxidation states in solids and aqueous species in the repository
       Use the solubility of hydromagnesite established by Robie and Hemingway (1973) in the EQ3/6 database
       Review the results of investigations published after the CRA-2014 cutoff date related to complexation of +III actinides by borate species and precipitation of borate solid phases, and if appropriate, revise the EQ3/6 thermodynamic database to incorporate the data
       Use the data provided by Altmaier et al. (2005, 2006) to develop revised aqueous speciation and Pitzer parameters for the Th(IV)-carbonate aqueous species and incorporate the results in the EQ3/6 database
       Use the results of the Neck et al. (2009) Nd(OH)3(s) solubility study to update the solubility of Am(OH)3(s) in the EQ3/6 database
       Provide supporting information for the revised data in the EQ3/6 database DATA0.FM2 to demonstrate agreement with acceptance criteria; EPA would like to see this supporting information sufficiently in advance of the CRA-2019 to enable EPA review and acceptance before its use to calculate dissolved actinide concentrations for the CRA-2019 PA
       Modify the criteria to select solubility data for the +III and +IV actinide solubility uncertainty distributions
       Experimentally evaluate formation of mineral fragment colloids from MgO hydration and carbonation products and their effects on total mobilized actinides
       Revise the Th(IV) intrinsic colloid parameter to be consistent with upper bounding values for samples collected within the pcH range of 8.5  -  10.5
       Provide documentation for the data used to determine the Pu(III) and Am(III) intrinsic colloid parameters and develop a Pu(III) intrinsic colloid parameter that is an upper bounding value
       Incorporate the effects of bioaccumulation and biomineralization in addition to biosorption in the development of revised microbial colloid parameters; until the effects of bioaccumulation and biomineralization have been included in the revised parameters, the Agency will only accept the higher proportionality constants (either from the CCA PA or CRA-2014 PA) for the CRA-2019 PA
       Use the higher CAPMIC parameters developed for the CCA based on the conceptual model assumption that these parameters represent the concentrations at which no growth of microbial colloids occurs
       Use 2 nm filtration to remove intrinsic colloids from post-test solutions in future microbial colloid experiments to establish the dissolved concentration for calculating proportionality constants for microbial colloids
       Develop MgO hydration rates that adequately account for the more rapid hydration rates of Martin Marietta MgO relative to Premier MgO for use in the CRA-2019 PA water balance
       Use the lower rate range established by EPA for the hydromagnesite conversion rate for the CRA-2019 PA water balance

Introduction

The Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico is an underground facility designed for the permanent disposal of transuranic (TRU) defense-related waste. The U.S. Department of Energy (DOE) operates the WIPP repository under the regulatory oversight of the U.S. Environmental Protection Agency (EPA). DOE submitted the Compliance Certification Application (CCA) to the EPA in 1996. After extensive review of the CCA and supplemental information provided by DOE, including an additional performance assessment (PA) calculation referred to as the Performance Assessment Verification Testing (PAVT), EPA certified that DOE had met the regulatory requirements and WIPP began accepting waste in March 1999.
Waste characteristics and waste components must be analyzed and the results must be included in any certification application [40 CFR 194.24(b)]. Those waste characteristics and components that could affect releases must be incorporated into PA [40 CFR 194.32(a)]. All additional data collected since the most recent CCA or Compliance Recertification Application (CRA) must be included in the CRA [40 CFR 194.15(a)], including any geochemical information, analyses, or results of laboratory experiments that are relevant to the performance of the repository. WIPP waste characteristics and components that have been identified as potentially affecting the chemistry of the system and releases include actinide solubilities and oxidation state distributions; formation of colloidal suspensions containing radionuclides; production of gas from the waste; ferrous metals inventory; cellulosics, plastics, and rubber (CPR) inventory; chelating agents (organic ligands) and oxyanions; and the activity of each isotope of the radionuclides present in the waste [40 CFR 194.24(b), EPA 1997b].
DOE is required to submit a CRA every 5 years after the initial acceptance of waste at WIPP and the first CRA (CRA-2004) was submitted to EPA in March 2004 (DOE 2004). EPA reviewed the CRA-2004 and supplemental information provided by DOE, including PA calculations referred to as the CRA-2004 Performance Assessment Baseline Calculations (PABC04), and certified that DOE continued to meet WIPP regulatory requirements (EPA 2006a).
DOE submitted the second CRA (CRA-2009) to EPA in March 2009 (DOE 2009). The CRA-2009 PA included the actinide solubility and colloidal actinide source term calculations from the PABC04. EPA performed a completeness review of the CRA-2009 and instructed DOE to perform a new PA to capture changes since the 2004 recertification (Cotsworth 2009a). DOE performed this revised PA, the CRA-2009 Performance Assessment Baseline Calculations (PABC09). Based on their review of the CRA-2009 and PABC09, EPA certified that DOE continued to meet WIPP regulatory requirements (EPA 2010a). At the time of the recertification decision, EPA (2010b) noted some uncertainties related to WIPP chemistry and also identified issues related to WIPP repository chemical processes for additional investigation by DOE prior to the CRA-2014.
DOE submitted the third CRA (CRA-2014) in March 2014 (DOE 2014b) that included the CRA-2014 PA. Because of uncertainties associated with some of the  parameters used in the PA, EPA requested a sensitivity study of the CRA-2014 PA (CRA14_SEN4 PA), which included changes to the probability of encountering a Castile brine reservoir, changes to the uncertainty distributions for the +III and +IV actinide solubilities, a change to the shear strength of degraded waste, changes to the stoichiometric parameters for the iron sulfidation reactions, use of a corrected version of the DRSPALL code for calculating spallings releases, and use of a corrected panel closure length.
This Technical Support Document summarizes EPA's review of chemistry-related issues important to PA, including actinide source term modeling, gas generation, backfill efficacy and actinide transport in the Culebra Dolomite above the repository for the WIPP CRA-2014 and CRA14_SEN4. The principal portions of CRA-2014 (DOE 2014b) addressed during this review are Appendix SOTERM and its supporting references, portions of §194.44 and Appendix MgO and their supporting references, portions of Appendix PA and its supporting references and portions of Appendix MASS and its supporting references. Information has also been obtained from DOE's responses to EPA's requests for additional information and from the results of reviews of chemistry-related issues carried out by EPA since the CCA.
Chemistry-related topics addressed in this report include Salado Formation mineralogy surrounding the repository and the chemistry of Salado and Castile brines (Section 2.0); gas generation caused by microbial degradation of CPR and anoxic corrosion of metals in the repository (Section 3.0); the ability of the magnesium oxide (MgO) backfill to control chemical conditions (Section 4.0); actinide oxidation states assumed in source term modeling (Section 5.0); dissolved actinide source term modeling (Section 6.0); colloidal actinide source term modeling (Section 7.0); PA calculations of total mobilized actinides (Section 8.0); an assessment of the actinides included in PA (Section 9.0); distribution coefficients (Kds) used in PA to model the release of actinides by transport through the Culebra member of the Rustler Formation (Section 10.0); the effects of heterogeneous waste loading (Section 11.0); the effects of chemical processes on repository water balance (Section 12.0); and the sensitivity of repository releases to parameter changes made in the CRA14_SEN4 (Section 13.0).
The results of this review indicate that DOE has modified its assessment of repository chemical processes as additional information has become available since the CCA PAVT, PABC04 and PABC09. EPA has identified additional modifications to incorporate in DOE's assessment of repository chemical processes for the CRA-2019 PA.
Geologic Setting, Salado Formation Mineralogy, and Brine Chemistry

WIPP repository chemical processes are influenced by the chemistry of the waste and by interaction of the waste with brines that may contact the waste and with the minerals in the Salado Formation that surrounds the repository. As a result, the geologic setting, Salado Formation mineralogy and the chemistry of brines that could contact the waste can affect actinide releases from the repository and subsequent transport. The Salado mineralogy and the chemistry of brines that may contact WIPP waste are described in the CRA-2014 (DOE 2014b, Appendix SOTERM, Section 2.0).
Geologic Setting

The WIPP repository is excavated within the Salado Formation. The low permeability of the Salado Formation surrounding the WIPP repository provides a significant hydrologic barrier between the repository and other, more transmissive water-bearing strata. A more permeable, transitory region forms within the Salado around the repository in response to the stress field caused by repository excavation; this region is called the Disturbed Rock Zone (DRZ). The DRZ is relatively thin compared to the thickness of the Salado Formation surrounding the repository (DOE 2014b, Appendix PA, Section 2.2.1).
The Salado Formation is underlain by the Castile Formation. The Castile Formation is mainly composed of thick beds of high-purity halite, alternating with thick beds of interlaminated carbonate and anhydrite [CaSO4(s)] (DOE 2014b, Appendix SOTERM, Section 2.1). The Castile Formation has been significantly deformed in some areas near the WIPP repository and pressurized brines are associated with the deformed areas.
The Salado Formation is overlain by the Rustler Formation (DOE 2014b, Appendix SOTERM, Section 2.1). This formation is composed of evaporite layers called, from bottom to top, the Los Medaños Member, the Culebra Dolomite Member, the Tamarisk Member, the Magenta Dolomite Member, and the Forty-Niner Member. The Culebra Dolomite is mostly composed of dolomite [CaMg(CO3)2(s)], with small amounts of anhydrite or gypsum [CaSO4::2H2O(s)] (Lambert 1992). The Culebra Dolomite is the most transmissive continuously saturated unit above the WIPP facility. It therefore provides the most direct pathway from the repository to the accessible environment, apart from releases directly to the surface (DOE 2004, Section 2.0). As a consequence, this aquifer has been studied more extensively than other portions of the Rustler Formation (Lambert 1992; DOE 2004, Section 2.0). The Magenta Dolomite has lower transmissivity than the Culebra Dolomite, and is unfractured at the WIPP site (DOE 2004, Section 2.0).
Salado Mineralogy

In the area around WIPP, the Salado Formation is a thick bedded salt, composed mainly of layers of impure halite [NaCl], with interbedded layers of anhydrite, polyhalite [K2MgCa2(SO4)4::2H2O(s)] and mudstone (Lambert 1992). These sulfate-bearing interbeds are called Marker Beds (MB). Within the Salado Formation, these beds are labeled using numbers that range from MB 100 near the top of the formation to MB 144 near the bottom (DOE 2004, Section 2.0).
Stein (1985) provided mineralogical analyses of drill core from the Salado Formation above and below the repository horizon. Based on x-ray diffraction (XRD) data, Stein (1985) identified quartz [SiO2(s)], magnesite [MgCO3(s)], anhydrite, gypsum, halite, polyhalite, bassanite [CaSO4::0.5H2O(s)], alkali feldspar [(NaK)AlSi3O8(s)], and clay minerals in the Salado Formation. Brush (1990) used these data to calculate the average mineralogical composition of the Salado to use for geochemical modeling of the effects of different backfill additives. Because this XRD evaluation of the mineralogical composition was only semiquantitative, Brush (1990) assumed that halite constituted 93.2 wt% of the Salado Formation, with the remaining fraction composed of equal parts magnesite, anhydrite, gypsum and polyhalite (1.7 wt% each). Brush (1990) did not include clay minerals, feldspar and quartz in the assumed mineralogy because Pitzer data for aqueous silica species were not available at that time for use in geochemical modeling. Bassanite was not included in the assumed mineralogy, because it was identified in only one sample.
GAS GENERATION

The Gas Generation conceptual model includes the assumption that significant gas generation will occur in the repository from corrosion of ferrous metals and from microbial degradation of CPR. The Gas Generation conceptual model also includes the assumptions that essentially all carbon dioxide [CO2] will be removed from the gas phase by reaction with the MgO backfill and that radiolysis of water in the waste and brine and radiation of plastics and rubber in the waste will not have significant effects on the amounts of gas generated (Wilson et al. 1996a and 1996b). The model assumes that corrosion of other metals in the WIPP inventory, such as aluminum and lead, will not contribute significantly to gas generation rates because of low inventories.
The Gas Generation conceptual model implemented in WIPP PA is called the average stoichiometry model, to differentiate it from alternative gas generation conceptual models considered for the CCA (Wilson et al. 1996a). The current Gas Generation conceptual model is described in detail by SC&A (2008b, Section 2.0 and Appendix A.1).
Salado Formation and Castile Formation Brine Chemistry

Small quantities of intergranular and intragranular Salado brines are associated with the salt at the repository horizon. These brines are highly concentrated (ionic strengths up to 8 molar), with a composition of mostly sodium, magnesium, potassium, chloride and sulfate, and with smaller amounts of calcium, carbonate and borate. Two formulations of these Salado brines have been used in WIPP laboratory experiments and PA calculations -- Brine A and Generic Weep Brine (GWB) (Snider 2003b). In the CCA PAVT, the Brine A formulation was used to simulate intergranular Salado brines. However, the GWB Salado brine formulation has been used in the CRA-2004 PA and all subsequent WIPP PA calculations. A detailed discussion of GWB brine and a comparison of this brine to Brine A were provided by Snider (2003b).
GWB was developed based on chemical analyses of intergranular fluids that seep into the WIPP excavations from the DRZ (Snider 2003b). DOE developed the GWB formulation because of the potential effects of magnesium concentrations in the brine on the modeled and experimental chemical behavior of the MgO backfill; the concentration of magnesium in GWB is lower than in Brine A (Table 2-1). Snider (2003b) verified the calculations supporting the formulation of GWB and compared the concentrations of major cations and anions in several WIPP brines. The comparison between GWB and Brine A indicated that GWB has higher boron, sodium, bromide, chloride and sulfate concentrations, and lower calcium, potassium and magnesium than Brine A. Brine A has a relatively low reported inorganic carbon concentration, whereas none was reported for GWB (Table 2-1). The ionic strength of GWB is approximately 6.5 compared to approximately 5.9 for Brine A.
EPA (2006c) reviewed DOE's proposed use of the GWB brine formulation to model actinide solubilities for the CRA-2004 PA and concluded that the GWB formulation was adequately documented. EPA concurred with DOE that the GWB brine formulation more closely matches the composition of Salado intergranular brines than Brine A, and that use of the GWB formulation in place of Brine A for the CRA-2004 PA and future actinide solubility calculations is appropriate. Only minor differences were observed in actinide solubility calculations carried out with GWB and Brine A.
Table 3-1.	Brine A, GWB and ERDA-6 Compositions
                                   Component
                                    Brine A
                                      GWB
                                    ERDA-6
Boron
                                     0.020
                                     0.155
                                     0.063
Calcium
                                     0.020
                                     0.014
                                     0.012
Potassium
                                     0.770
                                     0.458
                                     0.097
Magnesium
                                     1.44
                                     1.00
                                     0.019
Sodium
                                     1.83
                                     3.48
                                     4.87
Bromide
                                     0.010
                                     0.026
                                     0.011
Chloride
                                     5.35
                                     5.51
                                      4.8
Sulfate
                                     0.040
                                     0.175
                                     0.170
Total Inorganic Carbon
                                     0.010
                                 Not reported
                                     0.016
pH
                                      6.5
                                 Not reported
                                     6.17
Specific Gravity (kg/L)
                                      1.2
                                      1.2
                                     1.216
Total Dissolved Solids (mg/L)
                                    306,000
                                 Not reported
                                    330,000
  All units are moles/L solution, unless otherwise specified.
  Source: Brush and Xiong (2003a)

Castile brine may enter the repository if a borehole that penetrates the repository also penetrates an area of pressurized brine in the underlying Castile Formation. Borehole ERDA-6 encountered pressurized brine in the Castile Formation. The composition of this ERDA-6 brine has been used to represent the likely composition of Castile brine in laboratory experiments and all WIPP PA calculations (Table 2-1).
Iron Corrosion

Relatively large quantities of reduced iron will be present in the repository in the form of steel waste containers and as iron-based alloys in the waste. The most recent inventory estimates of the masses of iron-based waste material and packaging materials in contact-handled (CH) and remote-handled (RH) waste are provided by Van Soest (2012, Table 5-5). This estimate is based on the annual inventory report as of December 31, 2011 (DOE 2012) and includes emplaced, stored and projected WIPP waste, scaled to a full repository. These estimates indicate that over 49,000 metric tons of iron-based materials will be present in the repository at closure (Table 3-1). CH and RH waste constitutes 25% of the iron-based metals and alloys inventory, and packaging materials make up 75% of this inventory (Table 3-1).

Table 3-3.	CRA-2014 PA Inventories of Iron-Based Metals and Alloys in Waste and Steel Packaging Materials
                                   Parameter
                                   Mass (kg)
                                   Mass (%)
CH Waste Iron-Based Metal and Alloys
                                  10,900,000
                                      22
CH Steel Packaging Materials
                                  30,000,000
                                      61
RH Waste Iron-Based Metal and Alloys
                                   1,350,000
                                       3
RH Steel Packaging Materials
                                   6,860,000
                                      14
Repository Total
                                  49,110,000
                                      100
      Source: Van Soest (2012)
The Gas Generation conceptual model includes assumptions related to iron corrosion (Wilson et al. 1996a and 1996b, SC&A 2008b):
       The small amounts of oxygen trapped in the repository immediately after closure will be consumed by oxic corrosion or aerobic microbial degradation, and these processes will not generate a significant amount of gas
 Anoxic corrosion can occur in the repository as soon as the shafts are sealed
 Anoxic corrosion of steel in the repository cannot occur unless brine is present and in contact with the steel, and the corrosion rate is assumed to be a function of both brine saturation and steel surface area
 Carbon dioxide generated by microbial degradation of CPR will not passivate steel, because CO2 will be consumed by reaction with the MgO backfill
 Hydrogen sulfide (H2S) might passivate steel by forming FeS(s) on the steel surfaces if H2S is produced in sufficient quantities; however, reducing chemical conditions would continue to be maintained by the FeS(s).
 Fe will be represented by Fe(s) and/or Fe(OH)2
Corrosion Reaction Products and Stoichiometry

Anoxic corrosion of steel in the waste and waste containers produces hydrogen (H2). The chemical reactions that represent the stoichiometry of anoxic corrosion of steel in the WIPP repository for CRA-2014 are:

Fe(s) + (x+2) H2O <--> Fe(OH)2::xH2O(s) + H2
                                                                            (1)

Fe(s) + H2S <--> FeS(s) + H2
                                                                            (2)

In the CRA-2004 (DOE 2004, Appendix PA, Attachment SOTERM-2.2.3) and CRA-2009 (DOE 2009, Appendix SOTERM-2.3.4), DOE listed additional iron corrosion reactions based on WIPP-specific experiments (Telander and Westerman 1993 and 1997):

3 Fe(s) + 4 H2O <--> Fe3O4(s) + 4 H2
                                                                            (3)

Fe(s) + H2O + CO2 <--> FeCO3(s) + H2
                                                                            (4)

Reactions (3) and (4) indicate that iron corrosion could produce magnetite [Fe3O4(s)] or siderite [FeCO3(s)]. The possible occurrence of reaction (3) is important, because reaction (3) produces 1.33 moles of gas per mole of iron, whereas reactions (1) and (2) produce only one mole of gas per mole of iron. EPA previously considered whether inclusion of reactions (3) and (4) in the Gas Generation conceptual model is appropriate (SC&A 2008b). The available experimental data indicate that magnetite formation will not occur at the low temperatures expected in the WIPP repository. Siderite formation is not expected to be a significant process in the repository, because of the presence of the MgO backfill, which will control CO2 fugacities at low levels. Consequently, reactions (1) and (2) describe the expected anoxic corrosion reactions in the WIPP repository (SC&A 2008b).
DOE (2014b, Appendix SOTERM, Section 2.3.4) stated that based on new iron corrosion experiments, Roselle (2013) had identified green rust [idealized formula Fe(III)2Fe(II)4(OH)12CO3·2H2O] as the "most likely corrosion product in experiments with low atmospheric CO2 concentrations (<350 ppm)." The corrosion reaction to form green rust was given as:

6 Fe(s) + CO2 + 15 H2O --> Fe(III)2Fe(II)4(OH)12CO3·2H2O(s) + 7 H2
(5)

However, EPA noted that Roselle (2013) did not identify green rust as an iron corrosion product and that green rust formation could affect the moles of gas produced per mole of iron consumed (Edwards 2015a, Completeness Comment 4-C-6). In response, DOE reviewed the available data regarding likely iron corrosion products (Shrader 2016), including WIPP-specific data from Telander and Westerman (1993, 1997), Roselle (2009, 2010, 2011a, 2011b, 2013) and Nemer et al. (2011) as well as data related to corrosion of iron archaeological artifacts in chloride-rich environments (Neff et al. 2005, Réguer et al. 2007, Bellot-Gurlet et al. 2009, Rémazeilles et al. 2009, Saheb et al. 2009). The available data indicate that the most likely corrosion product is Fe(OH)2::xH2O(s), as observed by Telander and Westerman (1993), or possibly Fe2(OH)3Cl(cr). Consequently, EPA believes the molar ratio of hydrogen gas produced per mole of iron should remain equal to 1 for anoxic corrosion of iron in the WIPP environment.
Corrosion Rates

Two parameters are used in PA to represent iron corrosion rates, STEEL:CORRMCO2, representing the inundated steel corrosion rate (m/s) and STEEL:HUMCORR, representing the humid steel corrosion rate (m/s). The same values and distributions for these parameters were used for the CCA PAVT, PABC04 and PABC09. In these PAs, the humid corrosion rate for steel was assumed equal to 0.0 m/s (Wang and Brush 1996).
For the CCA PA, DOE estimated the inundated corrosion rate would range from 0.0 to 1.59 x 10[-14] m/s, with a uniform distribution over this range (Wang and Brush 1996). However, EPA requested that DOE double the upper bound of the inundated corrosion rate because of the rate increase observed at pressures from 70 to 127 atm by Telander and Westerman (1997), which are comparable to WIPP repository gas pressures (Camphouse 2013a). MFG (2000) confirmed these corrosion rates and established iron corrosion would also maintain repository reducing conditions. Consequently, the sampled range for the inundated steel corrosion rate was from 0 to 3.17 x 10[-14] m/s, with a uniform distribution for the CCA PAVT, PABC04 and PABC09 (Fox 2008).
DOE (2014b, Appendix PA) established a revised range and distribution for the inundated steel corrosion rate for the CRA-2014 PA (Table 3-2) based on WIPP-specific experiments (Roselle 2009, 2010, 2011a, 2011b, 2013). The humid corrosion rate remained unchanged for the CRA-2014 PA at zero, based on a recommendation by Roselle (2013).
Table 3-4.	WIPP PA Inundated Steel Corrosion Rates (STEEL:CORRMCO2) and Humid Steel Corrosion Rates (STEEL:HUMCORR)

                                 Distribution
                                    Maximum
                                     Mean
                                    Minimum
Inundated Steel Corrosion Rates (m/s)
CCA PA
                                    Uniform
                                1.59 x 10[-14]
                               7.95 x 10[-15]
                                     0.0
CCA PAVT, PABC04, PABC09
                                    Uniform
                               3.17 x 10[-14]
                               1.585 x 10[-14]
                                     0.0
CRA-2014
                                    Student
                               1.84 x 10[-14]
                               6.06 x 10[-15]
                               3.29 x 10[-16]
Revised (Zeitler and Hansen 2015a)
                                      CDF
                               3.96 x 10[-14]
                               6.76 x 10[-15]
                                     0.0
EPA-Revised (increased pressure)
                                      CDF
                               7.92 x 10[-14]
                                1.35 x 10[-14]
                                     0.0
Humid Steel Corrosion Rates (m/s)
CCA PA, CCA PAVT, PABC04, PABC09, CRA 2014 PA
                                      --
                                     0.0
                                     0.0
                                     0.0
Revised (Zeitler and Hansen 2015b)
                                      CDF
                               1.03 x 10[-15]
                               2.71 x 10[-16]
                                      0.0

EPA requested that DOE provide copies of the spreadsheets containing the iron and lead corrosion rate data (Edwards 2015b, Comment 2-C-8) and received the data from DOE (Franco 2015a).
EPA requested justification for the revised inundated steel corrosion rate because Roselle (2013) used only data obtained at a CO2 atmospheric concentration of 0 ppm to determine the revised rates (Edwards 2015a, Comment 4-C-2). EPA observed that CO2 atmospheric concentrations in equilibrium with repository brines, while low, are nonzero and corrosion experiments conducted at higher CO2 appeared to have higher maximum corrosion rates. EPA stated that the minimum sampled range should remain at zero because of the possibility of passivation of the steel surface by H2S reported by Telander and Westerman (1993, 1997). EPA also requested that DOE provide justification for the distribution selected for the inundated steel corrosion rate parameter. DOE responded with a revised assessment of the corrosion rate data (Shrader 2016, Zeitler and Hansen 2015a).
DOE (Zeitler and Hansen 2015a) acknowledged that it was appropriate to include corrosion experiments conducted with nonzero CO2 concentrations because CRA-2014 calculations (Brush and Domski 2013b) predicted a gas-phase CO2 concentration of 3.14 ppm in equilibrium with WIPP brines. Zeitler and Hansen (2015a) recalculated the inundated steel corrosion rate by including data collected at CO2 concentrations of 0 and 350 ppm and 1 atm total pressure (Table 3-2). The data set included 64 samples tested at 0 ppm CO2 and 64 samples tested at 350 ppm CO2. Four of the 350 ppm CO2 samples were excluded because of negative measured reaction rates, and an additional data point was added to represent the lower-bound zero reaction rate, resulting in a final data set of 125 observations. Zeitler and Hansen (2015a) accumulated these rates into a cumulative distribution function (CDF) that could be used in PA because it represents the range of variability in the inundated corrosion rate data. This data analysis and distribution is reasonable, but does not account for the previously noted effects of higher repository pressure on the inundated corrosion rate. Consequently, EPA recommends the inundated corrosion rates measured by Roselle (2013) and accumulated into the CDF for PA be increased by a factor of two, as previously directed by EPA for the CCA PAVT (Table 3-2).
Humid steel corrosion rates previously used in PA have been zero based on assessments of data available from Telander and Westerman (1993, 1997). EPA observed that the Roselle (2013) data appeared to be non-zero and requested that DOE assess the humid steel corrosion rate using data obtained with gas-phase CO2 concentrations of 0 and 350 ppm, which bracket anticipated repository conditions (Edwards 2015a, Comment 4-C-3). DOE responded with a revised assessment of the humid corrosion rate data (Shrader 2016, Zeitler and Hansen 2015b).
DOE (Zeitler and Hansen 2015b) considered 16 data points from Roselle (2013), eight each for samples tested at 0 ppm and 350 ppm CO2. After constructing separate CDFs for 0 ppm and 350 ppm CO2 data, Zeitler and Hansen (2015b) developed a common set of percentiles by linearly interpolating between the CDFs using a CO2 concentration of 3.14 ppm, consistent with predicted WIPP repository conditions from Brush and Domski (2013b). The resulting CDF ranges from 0 m/s to 1.03 x 10[-15] m/s with a mean value of 2.71 x 10[-16] m/s (Zeitler and Hansen 2015b). Because of the limited number of data points, any selected distribution would be debatable. The upper bound for the humid corrosion rate is reasonable, because only a single data point obtained at 350 ppm CO2 significantly exceeds this rate and a lower bound of zero is appropriate considering the possibility of surface passivation by H2S and low to slightly negative rates observed in some of the experiments (Figure 3-1). The mean value also appears adequately descriptive of the data. Because this CDF is reasonable, we believe the CRA-2019 PA will be strengthend if DOE used the distribution developed by Zeitler and Hansen (2015b) for the humid steel corrosion rate. A nonzero humid corrosion rate represents a revision to the Gas Generation conceptual model, but this change is adequately supported by new experimental data.

Figure 3-1.	Humid Steel Corrosion Rate Data (Roselle 2013)

Passivation of Iron Surfaces by H2S
EPA requested that DOE provide the basis for the PA assumption that all H2S produced by microbial degradation of cellulose will instantaneously react to produce iron sulfide (Edwards 2015a, Comment 4-C-5), noting that WIPP-specific corrosion experiments had indicated H2S passivated steel (Telander and Westerman 1997). In their response, DOE cited Wang and Brush (1996), who opined that steel passivation is unlikely based on the length of time necessary for repository H2S concentrations to increase to the levels used in the WIPP-specific experiments (Shrader 2015). In addition, Wang and Brush (1996) stated that the lack of total steel passivation in the WIPP repository is consistent with other studies, citing Ikeda et al. (1983) and Schmitt (1983). Wang and Brush (1996) noted that although total passivation of iron corrosion by H2S has not been observed, varying degrees of corrosion inhibition (i.e., a significant decrease in corrosion rates) has been observed in the presence of H2S.
DOE did not provide the Ikeda et al. (1983) and Schmitt (1983) articles, but their titles indicate that they address the effects of CO2 on steel corrosion. The effects of CO2 on steel corrosion rates are minimally significant to the WIPP repository environment because of the low CO2 concentrations that will be maintained by the MgO backfill. Although complete passivation of steel in the repository by H2S is uncertain, DOE's response includes the observation that corrosion may be inhibited by H2S. Consequently, DOE has not adequately justified the assumption of instantaneous reaction of H2S with iron and iron hydroxide in the repository.
The iron metal sulfidation and ferrous hydroxide sulfidation reactions are reaction (2) and:

Fe(OH)2(s) + H2S(g) --> FeS(s) + 2 H2O(l)
(6)

Reaction (2) has no net effect on the amount of gas or brine in the repository. However, reaction (6) reduces the amount of gas and increases the brine volume.
The available data (e.g., Nešić et al. 2008) indicate that H2S(g) in the repository will initially rapidly react with iron and form a thin layer of FeS(s) on iron surfaces. This time period is consistent with DOE's assumption of instantaneous reaction of H2S. However, after this initial period of rapid reaction, reaction rates decline significantly as iron sulfidation rates are controlled by diffusion through this adherent inner FeS(s) layer and a porous outer layer of FeS(s) reaction product. The assumption of instantaneous iron sulfidation reactions is inconsistent with this later period of slower reaction. Because H2S presumably preferentially reacts with iron hydroxide versus metallic iron (DOE 2014b, CRA-2014 Appendix MASS, page MASS-57), overestimation of the iron sulfidation rate would result in an underestimation of the gas pressure and overestimation of the brine volume in the repository (reaction 6). The effects of underestimating the repository gas pressure appear likely to be non-conservative, because implementation of revised (lower) iron corrosion rates and water balance refinements in the CRA-2014 PA decreased the number of vectors with direct brine releases, which affects overall mean releases (Zeitler 2013, Figures 3-26 and 3-46; DOE 2014b, CRA-2014 Appendix PA, page PA-241). Because the assumption of instantaneous iron sulfidation may have non-conservative effects on repository releases, DOE should assume no reaction of H2S with iron in the repository will occur, consistent with the WIPP-specific data from the experiments reported by Telander and Westerman (1997). EPA communicated with DOE regarding the validity of the iron sulfidation assumptions (Economy 2016). DOE agreed that there are insufficient data to support the assumption of instantaneous iron sulfidation reactions and stated that the stoichiometric coefficients for equations PA.89 and PA.90 (DOE 2014b, Appendix PA) should be set equal to zero, removing these reactions from PA (Patterson 2016b).
The stoichiometric coefficients for reactions (2) and (6) were set equal to zero for the CRA14_SEN4 PA (Zeitler and Day 2016). The effects of this change and other changes made for the CRA14_SEN4 PA are discussed in Section 13.0.
Steel Surface Area

The gas generation rate caused by anoxic corrosion of iron-based metal in the repository is proportional to the steel surface area per unit volume, DS (DOE 2014b Appendix PA Equation PA.67). The value of DS is calculated from (Appendix PA equation PA.75):

DS= AdndVR
(7)

Where:
      Ad is the surface area of steel associated with a waste disposal drum (parameter REFCON:ASDRUM, m[2]/drum)

      VR is the initial volume of a single room in the repository (parameter REFCON:VROOM, m[3]), and

      nd is the ideal number of waste drums that can be close-packed into a single room (parameter REFCON:DRROOM).

Based on information developed prior to the CCA PA, the surface area of steel associated with a 55-gallon waste disposal drum (Ad) is approximately 6 m[2]/drum (Brush 1990). This estimated surface area included both the surface area of the steel drum and the iron-based waste contents. A generic room size was assumed to be 33 ft by 300 ft by 13 ft (VR, 3,640 m[3]) allowing for 6,800 (nd) 55-gallon drums. These parameters result in DS = 11.2 m[2]/m[3], which was used in the CRA-2014 PA and all previous WIPP PA calculations. EPA requested that DOE evaluate the potential effects of waste inventory and emplacement data collected since the CCA PAVT on the calculation of DS (Edwards 2015a, Comment 4-C-4).
Technical Exchanges Over CRA-2014 Steel Surface Area

DOE responded with an evaluation of the PA parameters associated with the steel surface area (Shrader 2015, Day 2015). The results of these calculations show that the current value of DS adequately represents current waste inventory, container types and waste emplacement in the repository. Because the waste inventory, relative numbers of different containers and emplacement practices may evolve over time, we expect future WIPP PAs to include recalculations of appropriate DS values. A detail of this calculation is discussed below.
DOE (Day 2015) identified the different waste containers in the repository based on a WIPP Waste Data System (WDS) query as of December 31, 2014 refining surface area calculations and iron inventory. The inner plus outer surface areas for all authorized waste containers were calculated from the container dimensions (DOE 2014b, Appendix DATA, Attachment B). For authorized payload container types that were overpacked, the overpacked containers were not included in the average iron-based metal waste weight, but were instead used to directly calculate the steel surface areas based on their known geometry (Day 2015). For the 100-gallon drums containing compressed Advanced Mixed Waste Treatment Facility (AMWTF) waste, the mass of iron in the compressed drums was included with the iron-based waste in the surface area calculations (Patterson 2016b).
Because only the masses and not surface areas of iron-based waste are known from the inventory (Table 3-1), Day (2015) developed surrogate assumptions relating the surface area of the iron-based waste to its mass: a spherical assumption and a sheet metal assumption. For the spherical surrogate assumption, the surface area of the iron-based waste was assumed to be equivalent to the surface area of the same mass of 1-cm diameter steel spheres. For the sheet metal surrogate assumption, the surface area to mass ratio of the waste was assumed to be equivalent to the surface area to tare weight ratio of its container. Day (2015) did not provide evidence demonstrating that these assumptions are representative of WIPP waste or that these two surrogate assumptions are bounding of the possible range of surface area to mass ratios for WIPP waste.
In response to an EPA request for additional information (Economy 2016), DOE indicated that the spherical surrogate assumption is justified because 1 cm is a reasonable lower limit for identifying metal waste during radiographic examination of the waste drums (Patterson 2016b). Because the assumed spherical shape provides a minimum surface area to volume ratio, the surface area associated with the inventory of iron waste (6.5 x 10[5] m[2]) is 27% of the total iron-based waste and container surface area (2.38 x 10[6] m[2], Patterson 2016b).
Recalculation of Steel Surface Area per Unit Volume

The sheet metal surrogate assumption used by Day (2015) does not provide a consistent upper bound for the range of the iron-based waste surface area to mass ratio because its results differ from the spherical surrogate assumption results by varying amounts, depending on container type (Day 2015, Table 2). For example, the sheet-metal surrogate provides an estimated iron-based waste surface area to mass ratio that is approximately twice the surface area to mass ratio for the spherical surrogate for 55-gallon drums. However, the estimated iron-based waste surface area values are approximately equal for the direct-load standard waste boxes.
The tare weights and surface areas reported by Day (2015, Table 1 and Appendix A) were used to calculate the surface area to tare weight ratios of the WIPP waste containers (Table 3-3). The ratios in Table 3-3 range over more than an order of magnitude, from 0.01 m[2]/kg for the shielded container overpack of one drum up to 0.30 m[2]/kg for the 85 gallon overpack of one drum. DOE justified the sheet metal surrogate assumption by stating that larger pieces of iron-based waste with lower surface area to mass ratios would be placed in more robust containers with lower surface-area to tare weight ratios. The sheet metal surrogate assumption results in an iron-based waste surface area that is 35% of the total iron-based waste plus container surface area (Patterson 2016b).
Most of the iron-based waste in the AMWTF containers (100-gallon drums) likely has a surface area to mass ratio consistent with 55-gallon drums, because the AMWTF drums are expected to contain an average of four compacted 55-gallon drums (Hansen et al. 2003b). Based on the tare weight for 55-gallon drums (28.6 kg), the mass of four 55-gallon drums represents 67% of the average iron-based waste weight in these containers (170.59 kg, Table 3-4). The two surrogate assumptions used by Day (2015) likely underestimate the steel surface areas in the AMWTF iron-based waste because the surface area to mass ratio for the AMWTF waste using the spherical assumption is 0.076 m[2]/kg and the ratio used for the sheet metal surrogate assumption is 0.12 m[2]/kg, both of which are lower than the 0.16 m[2]/kg ratio for a 55-gallon drum.
The potential effect on DS of a higher surface area to mass ratio for iron-based waste was assessed by assuming all iron-based waste had a surface area to mass ratio consistent with the ratio for a 55-gallon drum (55-gallon drum surrogate). This assumption is a reasonable approach for evaluating the possible maximum effects of the relatively large amount of iron-based metal waste in the AMWTF drums that is composed of compacted 55-gallon drums.

Table 3-5.	WIPP Container Surface Areas, Tare Weights and Surface Area to Tare Weight Ratios
Container Type
                               Tare weight (kg)
            Surface area of outer and overpacked containers (m[2])
                 Container surface area/tare weight (m[2]/kg)
55-gallon drum (DL)[1]
                                     28.6
                                     4.58
                                     0.16
12-in. standard pipe overpack (DL)
                                     110.3
                                     6.34
                                     0.06
S100 pipe overpack (DL)
                                     70.5
                                     6.34
                                     0.09
S300 pipe overpack (DL)
                                     109.8
                                     6.34
                                     0.06
85-gallon drum (OP[2] one drum)
                                      37
                                     10.92
                                     0.30
100-gallon drum (DL of compacted pucks)[3]
                                     56.7
                                     6.62
                                     0.12
Standard waste box (DL)
                                     290.3
                                     21.93
                                     0.08
Standard waste box (OP four drums)
                                     290.3
                                     40.23
                                     0.14
Standard large box 2 (DL)
                                   1,224.49
                                     52.58
                                     0.04
Ten drum overpack (DL)
                                    770.98
                                     31.42
                                     0.04
Ten drum overpack (OP ten drums)
                                    770.98
                                     77.19
                                     0.10
Shielded container (OP one drum)
                                    782.77
                                     8.98
                                     0.01
RH FL waste canister (DL)
                                    799.09
                                     14.06
                                     0.02
RH RL waste canister (DL)
                                    517.01
                                     14.06
                                     0.03
RH RL waste canister (OP 3 drums)
                                    517.01
                                     27.79
                                     0.05
Shield Plug
                                     n.r.
                                     10.18
                                     n.c.
[1]	DL = direct load
[2]	OP = overpack
[3]	AMWTF waste

The results of the steel surface area per drum equivalent calculations using the three surrogate assumptions are summarized in Table 3-4. RH waste that is not placed on the disposal room floor was not included in these calculations. The results show that there is a wide variation in the calculated steel surface area per drum equivalent depending on the container type and the average iron-based metal waste per container. Because a single Ad value is used for the entire repository, a representative steel surface area per drum equivalent must be calculated from an average that is weighted for the volumes occupied by each container type. Seven-packs of 55-gallon drums can be stacked in three layers within a disposal room. Consequently, there are 21 containers (7 drums stacked in three layers) present in each stack of 55-gallon drums. Day (2015, Appendix A) calculated for the other container types the number of containers that would occupy the same space as a stack of 55-gallon drums and the equivalent number of stacks that would be occupied by the number of each container (Table 3-5). The contribution of each waste container type to the steel surface area per drum equivalent was calculated by multiplying the surface area per drum equivalent (Table 3-4) by the number of equivalent stacks and dividing by the total number of stack equivalents. The results of this calculation for the three surrogate waste assumptions are provided in Table 3-5.

Table 3-6.	Container plus Iron Waste Surface Areas Calculated per 55-Gallon Drum Equivalent Using Different Surrogate Assumptions
Container Type
                             Number of containers
            Average iron-based metal waste per container type (kg)
                             Containers per stack
         Spherical surrogate, surface area/drum equivalent (m[2]/drum)
       Sheet metal surrogate, surface area/ drum equivalent (m[2]/drum)
      55-gallon drum surrogate, surface area/drum equivalent (m[2]/drum)
55-gallon drum (DL)
                                    96,494
                                     6.41
                                      21
                                     5.07
                                     5.61
                                     5.61
12-in. standard pipe overpack (DL)
                                    25,626
                                       2
                                      21
                                     6.49
                                     6.45
                                     6.66
S100 pipe overpack (DL)
                                      814
                                       0
                                      21
                                     6.34
                                     6.34
                                     6.34
S300 pipe overpack (DL)
                                      45
                                       0
                                      21
                                     6.34
                                     6.34
                                     6.34
85-gallon drum (OP one drum)
                                       5
                                     27.76
                                      12
                                     7.45
                                     10.92
                                     8.78
100-gallon drum (DL of compacted pucks)
                                    34,255
                                    170.59
                                       9
                                     8.41
                                     11.37
                                     14.54
Standard waste box (DL)
                                     6,807
                                    234.36
                                       3
                                     5.69
                                     5.66
                                     8.49
Standard waste box (OP four drums)
                                     6,054
                                     17.9
                                       3
                                     5.94
                                     6.10
                                     6.16
Standard large box 2 (DL)
                                      228
                                    492.13
                                     0.678
                                     2.91
                                     2.38
                                     4.24
Ten drum overpack (DL)
                                      24
                                    928.91
                                      1.5
                                     7.30
                                     4.95
                                     12.87
Ten drum overpack (OP ten drums)
                                     6,023
                                     22.87
                                      1.5
                                     5.64
                                     5.68
                                     5.78
Shielded container (OP one drum)
                                       9
                                     16.4
                                       9
                                     4.38
                                     3.93
                                     4.97

Table 3-7.	Surface Area per Drum Equivalent, Weighted by Waste Container Volume and Number
Container Type
                             Number of containers
                             Containers per stack
                          Number of equivalent stacks
Spherical surrogate assumption, contribution to surface area per drum equivalent, (m[2]/drum)
Sheet metal surrogate assumption, contribution to surface area per drum equivalent (m[2]/drum)
55-gallon drum surrogate assumption, contribution to surface area per drum equivalent (m[2]/drum)
55-gallon drum (DL)
                                    96,494
                                      21
                                     4,595
                                     1.27
                                     1.41
                                     1.41
12-in. standard pipe overpack (DL)
                                    25,626
                                      21
                                     1,220
                                     0.43
                                     0.43
                                     0.44
S100 pipe overpack (DL)
                                      814
                                      21
                                      39
                                     0.01
                                     0.01
                                     0.01
S300 pipe overpack (DL)
                                      45
                                      21
                                       2
                                  7 x 10[-4]
                                  7 x 10[-4]
                                  7 x 10[-4]
85-gallon drum (OP one drum)
                                       5
                                      12
                                       0
                                  2 x 10[-4]
                                  2 x 10[-4]
                                  2 x 10[-4]
100-gallon drum (DL of compacted pucks)
                                    34,255
                                       9
                                     3,806
                                     1.75
                                     2.36
                                     3.02
Standard waste box (DL)
                                     6,807
                                       3
                                     2,269
                                     0.70
                                     0.70
                                     1.05
Standard waste box (OP four drums)
                                     6,054
                                       3
                                     2,018
                                     0.65
                                     0.67
                                     0.68
Standard large box 2 (DL)
                                      228
                                     0.678
                                      336
                                     0.05
                                     0.04
                                     0.08
Ten drum overpack (DL)
                                      24
                                      1.5
                                      16
                                     0.01
                                     4E-03
                                     0.01
Ten drum overpack (OP ten drums)
                                     6,023
                                      1.5
                                     4,015
                                     1.24
                                     1.24
                                     1.27
Shielded container (OP one drum)
                                       9
                                       9
                                       1
                                  2 x 10[-4]
                                  2 x 10[-4]
                                  3 x 10[-4]
Total
                                    18,318
                                     6.12
                                     6.88
                                     7.97

The largest contributions to the average weighted surface area per drum come from the 100-gallon drum containers with AMWTF waste, the 55-gallon drums and ten-drum overpacks (OP ten drums) because these container types comprise the largest numbers of equivalent stacks (Table 3-5). Significant contributions to the average weighted surface area per drum also come from the standard waste boxes (DL and OP four drums) and the 12-in. standard pipe overpacks, which also represent relatively large numbers of equivalent stacks. The total surface area per drum ranges from 6.12 m[2]/drum for the spherical surrogate up to 7.97 m[2]/drum for the 55-gallon drum surrogate. The most significant increase in the surface area per drum when using the 55-gallon drum surrogate instead of the spherical surrogate assumption comes from the 100-gallon drums containing compressed AMWTF waste. Because a large fraction (~67%) of the iron-based metal waste inside these 100-gallon drums will be compressed 55-gallon drums, this result shows that the spherical surrogate assumption likely underestimates a significant contribution to the steel surface area in WIPP waste. The representativeness of the different surrogate assumptions cannot be assessed given the available data, because there is no information regarding the physical dimension of the iron-based waste (Patterson 2016b). However, the spherical surrogate appears to provide an unrealistic and non-conservative result, so the most reasonable value for Ad likely falls within the range from the sheet metal surrogate value (6.88 m[2]/drum) through the 55-gallon drum surrogate value (7.97 m[2]/drum). Based on a panel volume of 46,097.6 m[3] (Schreiber 1991) and seven rooms per panel, Day (2015) calculated an idealized waste room volume of 6,585 m[3]. Day (2015) used the CH-TRU waste emplacement information in the CRA-2014 (DOE 2014b, Appendix DATA Attachment B, Figure DATA-B-11) to establish that there are an idealized 10,908 55-gallon drum equivalent container positions per room. Both the room volume and number of drums per room increased relative to the values used in the CRA-2014 PA, but as noted by Day (2015), the revised nd/VR value (10,908 drums/room / 6,585 m[3]/room = 1.66 drums/m[3]) is smaller than the nd/VR value used in the CRA-2014 PA (6,804 drums/room / 3,644 m[3]/room = 1.87 drums/m[3]), which would reduce the steel surface area per unit volume (DS). Using the idealized emplacement number of drums, both the sheet metal surrogate and 55-gallon drum surrogate assumptions result in average steel surface area to unit volume values greater than the value used in the CRA-2014 PA (Table 3-6).
Day (2015) reported that the waste emplaced through December 31, 2014 occupies a total of 40 rooms. Based on the total number of stacks (Table 3-5) and 21 55-gallon drums per stack, the emplaced waste represents a total of 384,678 drum equivalents, yielding 9,616 drums/room. This number of drums per room represents a fill factor of 88%. If the steel surface area is recalculated using the more realistic number of drums per room, the sheet metal surrogate and 55-gallon drum surrogate assumptions result in average steel surface area to unit volume values of 10.0 m[2]/m[3] and 11.6 m[2]/m[3], which range from slightly below to slightly above the 11.2 m[2]/m[3] value used in the CRA-2014 PA (Table 3-6). These results indicate that the current value of DS is a reasonable approximation of the steel surface area for WIPP waste that has been emplaced through December 31, 2014.

Table 3-8.	Steel Surface Area per Unit Volume (DS) from the CRA-2014 PA and Calculated Using Different Waste Surface Area to Mass Surrogate Assumptions for Idealized and Reported Waste Emplacement
Calculation
                     Surface Area Per Drum (m[2]/drum, Ad)
                         Number of Drums per Room (nd)
                     Disposal Room Volume (m[3]/room, VR)
              Steel Surface Area per Unit Volume (Ds, m[2]/m[3])
Idealized Waste Emplacement
CRA-2014 PA
                                       6
                                     6,804
                                     3,644
                                     11.2
Spherical surrogate
                                     6.12
                                    10,908
                                     6,585
                                     10.1
Sheet metal surrogate
                                     6.88
                                    10,908
                                     6,585
                                     11.4
55-gallon drum surrogate
                                     7.97
                                    10,908
                                     6,585
                                     13.2
Reported Waste Emplacement
Spherical surrogate
                                     6.12
                                     9,616
                                     6,585
                                      8.9
Sheet metal surrogate
                                     6.88
                                     9,616
                                     6,585
                                     10.0
55-gallon drum surrogate
                                     7.97
                                     9,616
                                     6,585
                                     11.6

These calculations of average DS depend on both the proportions of different wastes and waste containers and the fill factor achieved during waste emplacement. If the proportions of different wastes in the repository change over time, e.g., a higher proportion of AMWTF waste is emplaced, or if higher fill factors are achieved in the future, the average DS value could be significantly affected. Accordingly, future WIPP PAs should include recalculation of the appropriate DS values. Because of the potential effects of DS on brine releases (DOE 2014b, Appendix PA), EPA also expects DOE to assess the panel-by-panel variation of DS to determine whether the average DS adequately represents the possible range of this parameter calculated for the different panels.
The current value of DS (11.2 m[2]/m[3]) combined with the reported waste emplacement nd and VR values (9,616 drums/room and 6,585 m[3], respectively) results in an Ad value of 7.67 m[2]. Because the parameters REFCON:DRROOM, REFCON:VROOM and REFCON:ASDRUM are used only to calculate DS, revising these parameter values to be more consistent with available data will have no practical effect on PA. EPA asked in an August 2016 teleconference with DOE whether REFCON:VROOM is used in any other PA calculations, because equations in the CRA-2014 indicate that VR is used to calculation Dc, the estimated cellulosics and rubber density, and Dm, the MgO mass concentration (DOE 2014b, Appendix PA, Equations PA.76 and PA.93). DOE responded that REFCON:DRROOM, REFCON:VROOM and REFCON:ASDRUM are used only to calculate DS, and the cited equations contained typographical errors (Patterson 2016b).

PA Implementation

In the CCA PAVT, CRA-2004 PABC and CRA-2009 PA, the inundated corrosion rate was a sampled parameter (STEEL:CORRMCO2) uniformly distributed from zero to 3.17 x 10[-14] m/sec (Table 3-2). For the CRA-2014 PA, DOE (2014b) used a revised range and distribution (Table 3-2) based on the experimental results of Roselle (2013). Zeitler and Hansen (2015a) revised this range and distribution to include corrosion data obtained at low CO2 concentrations, and an additional revision to this range and distribution should be used for the CRA-2019 PA to account for the effects of increased pressure on the inundated corrosion rate (EPA-revised rate, Table 3-2).
Gas generation by anoxic corrosion of steel has been implemented for all WIPP PA calculations by assuming the humid corrosion rate parameter (STEEL:HUMCORR) is equal to zero. However, Zeitler and Hansen (2015b) developed a CDF for the humid corrosion rate parameter that ranges from 0 m/s to 1.03 x 10[-15] m/s with a mean value of 2.71 x 10[-16] m/s. EPA recommends DOE incorporate this parameter range to represent the humid steel corrosion rate in the CRA-2019 PA calculations.
The equation used to calculate the anoxic steel corrosion gas generation rate is (DOE 2014b, CRA-2014 Appendix PA-4.2.5):

qr gc = (Rci Sb,eff + Rch Sg*) Ds ρFe XC (H2│Fe) MH2
                                                                            (8)

Where:
      qr gc is the rate of gas production per unit volume of waste due to anoxic corrosion of iron-based metals (kg/m[3]/sec)
      Rci is the corrosion rate under inundated conditions, STEEL:CORRMCO2 (m/s)
      Sb,eff is the effective brine saturation due to capillary action in the waste materials
      Rch is the corrosion rate under humid conditions, STEEL:HUMCORR (m/s)
      Sg* equals 1- Sb,eff if Sb,eff > 0, or
          equals 0 if Sb,eff = 0
      Ds is the steel surface area per unit volume in the repository (11.2 m[2]/m[3]), calculated from the parameters REFCON:ASDRUM (6 m[2]/drum), REFCON:DRROOM (6,800 drums/room), and REFCON:VROOM (3,640 m[3]/room) (Kicker and Herrick 2013)
      ρFe is themolar density of steel (1.41 x 10[5] moles/m[3], Telander and Westerman 1993)
      XC (H2│Fe) is the stoichiometric coefficient for gas generation due to the corrosion of steel [1 mole H2 per mole Fe, parameter STEEL:STOIFX (Kicker and Herrick 2013)]
      MH2 is the molecular weight of H2 [kg H2 per mole H2, 2.02  10[-3], parameter REFCON:MW_H2 (Kicker and Herrick 2013)].
For each realization, the sampled anoxic inundated and humid corrosion rates are used to calculate anoxic corrosion gas generation per unit volume of waste as a function of the effective brine saturation.
In Appendix PA Section 4.2.5 (page PA-71), the gas and brine production rates in Equation PA.65 and PA.66 are described in terms of unit waste volume. However, the individual components of these gas and brine production rates are calculated per unit disposal volume rather than per unit waste volume (Equations PA.67 through PA.74). EPA will expect DOE to resolve this apparent inconsistency.
Effects of Anoxic Corrosion Gas Generation Rates on Repository Releases

DOE (Triay 2002) requested EPA approval for the disposal of compressed waste from the AMWTF. During review of the AMWTF planned change request (PCR), EPA questioned whether higher iron surface areas expected in compressed waste would affect modeled releases from the repository because of increased anoxic corrosion and repository pressures (TEA 2004). DOE assessed the sensitivity of the PA results to changes in the assumed iron surface area and found that increased iron surface area and anoxic corrosion resulted in decreased overall gas production, with increased gas pressures during the first 2,500 years, followed by decreased gas pressures during later time periods (Stein and Zielinski 2004). Decreased overall gas production was predicted because higher initial gas pressures forced brine from the repository and decreased microbial gas production. DOE concluded that increased iron surface areas would not have significant effects on total repository releases, and EPA concurred with this conclusion (TEA 2004).
The inundated steel corrosion rates used in the CRA-2014 PA were lower than the rates used in the CRA-2009 PABC (Table 3-2). The lower corrosion rates resulted in reduced spallings releases and fewer vectors with the conditions necessary for DBR in the CRA-2014 PA than the CRA-2009 PABC, leading to lower mean total releases (DOE 2014b, Appendix PA page PA-241). Further evaluation of the inundated and humid corrosion rate data shows that higher rates should have been used in the CRA-2014 PA (Zeitler and Hansen 2015a, 2015b). The revised inundated corrosion rate adjusted for increased pressure has a similar mean to the CRA-2009 PABC mean inundated corrosion rate (Table 3-2). The revised humid corrosion rate (Zeitler and Hansen 2015b) is also higher than the CRA-2009 PABC humid corrosion rate. The higher mean inundated and humid corrosion rates would be expected to lead to higher mean total releases. However, because these higher rates have not been incorporated in a new PA, the magnitude of the effects of these increased rates on repository releases cannot be quantified. The effects of the nonzero humid corrosion rate are particularly difficult to assess, even qualitatively, because previous WIPP PAs have not included humid anoxic corrosion.
Corrosion of Aluminum, Lead and Other Metals

DOE (2014b, Appendix SOTERM-2.3.5) addressed the potential effects of lead corrosion on repository chemical conditions, but did not address the effects on gas generation of the corrosion of metals other than iron in the WIPP inventory, such as lead and aluminum. Peer review of the Gas Generation conceptual model determined that the quantities of aluminum in the repository will be too small to cause significant gas generation (Wilson et al. 1996a, 1996b, 1997a, 1997b). Whether this assumption continues to be true can be assessed by comparing the relative masses of iron and aluminum in the CRA-2014 PA inventory (Table 3-7). The aluminum inventory is less than 1% of the iron-based metals inventory, so aluminum corrosion is unlikely to significantly affect gas generation rates.
Table 3-9.	CRA-2014 PA Inventories of Aluminum, Lead and Other Metals and Alloys in Waste and Packaging Materials
                          Waste or Packaging Material
                                   Mass (kg)
CH Waste Aluminum-Based Metal and Alloys
                                    437,000
RH Waste Aluminum-Based Metal and Alloys
                                    20,100
CH Waste Other Metals and Alloys
                                    762,000
RH Waste Other Metals and Alloys
                                    471,000
CH Lead Packaging Materials
                                      --
RH Lead Packaging Materials
                                     8,280
Total Waste Other Metals and Alloys and Lead Packaging Materials
                                   1,241,280
Total Waste Aluminum-Based Metals and Alloys, Waste Other Metals and Alloys and Lead Packaging Materials
                                   1,698,380
Total Iron-Based Waste and Packaging Materials
                                  49,110,000
      Source: Van Soest 2012

Anoxic corrosion of lead can be expected to consume brine and generate H2, based its reaction stoichiometry:

Pb(s) + H2O <--> PbO(s) + H2
                                                                            (9)

The WIPP lead inventory from packaging materials is reported, but the lead inventory associated with CH and RH waste is only included as "other metals and alloys" (Van Soest 2012). The total inventory of lead will be less than the sum of the "other metals and alloys" plus the lead packaging materials (Table 3-7). The 1,241,280 kg upper limit on lead inventory reported for the CRA-2014 is less than 3% of the iron inventory, so if the data in Table 3-7 adequately represent the final lead inventory at closure, lead corrosion would not be expected to significantly affect gas generation rates.
DOE submitted a PCR to allow use of shielded containers for disposal of RH waste (Moody 2007) and EPA approved this request (Edwards 2011, 2013). Because lead is used in the container shielding, disposal of RH waste in these containers will increase the amounts of lead in the repository. EPA requested information from DOE regarding how lead shielding in RH shielded containers is included in the PA (Edwards 2014, Comment 1-24-1). DOE responded that only one shielded container had been placed in the repository as of the CRA-2014 PA inventory cut-off date of December 31, 2011 (Franco 2015b). Its contribution to the volume, radionuclide inventory, waste inventory and packaging inventory was included under RH waste. DOE stated that the mass of lead contributed by this single container was 431 kg out of the total lead packaging inventory of 8,280 kg reported by Van Soest (2012).
EPA requested information from DOE regarding the volume of RH waste that has been and will be placed in the shielded containers on the waste panel floors (Edwards 2014, Comment 1-24-2). DOE responded that since the CRA-2014 PA inventory cut-off date of December 31, 2011, nine shielded containers have been emplaced in WIPP (Franco 2015b). DOE further stated that "As of 12/31/2013, no other lead-shielded containers had been identified by the sites to be shipped to WIPP."