5 th Response Submittal to the EPA Enclosure 1

1 EPA Comment 2

- C

- 3 Data Supporting Water Balance Assumptions. The CRA

- 2014 PA calculations include the effects of MgO hydration, microbial degradation of CPR and iron sulfide formation on repository water balance. For PA, it is assumed that hydrogen sulfide created by CPR degradation preferentially reacts with iron hydroxide versus metallic iron (CRA

- 2014 Appendix MASS, page MASS

- 57). These hydrogen sulfide reactions are:

Fe(OH) 2 (s) + H 2 S(g) → FeS(s) + H 2 O(l) (1)

Fe(s) + H 2 S(g) → FeS(s) + H 2 (g) (2)

The assumption that hydrogen sulfide preferentially reacts with iron hydroxide increases brine production and decreases gas production compared to the assumption that all or some of the hydrogen sulfide reacts with meta llic iron.

It is also assumed for PA that carbon dioxide preferentially reacts with brucite instead of unreacted MgO. The carbonation reactions are:

1.25 Mg(OH) 2 (s) + CO 2 (g,aq) → 0.25 Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O(s) (3)

1.25 MgO(s) + CO 2 (g,aq) + 1.25 H 2 O(l) → 0.25 Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O(s) (4)

The assumption that carbon dioxide preferentially reacts with brucite increases brine production in the repository.

Please provide supporting data for these water

- balance assumptions and evaluate the potential magnitude of t he effects of these assumptions on the water balance.

DOE Response

The water balance assumptions were based on experimental studies and were made to capture the transient behavior of the gas and water generation/consumption events

. Magnesium oxide hydrati on, iron corrosion and microbial degradation of cellulosics, plastics and rubbers ( CPR ) all depend on the amount of water present. CPR degradation via sulfate reduction generates hydrogen sulfide and carbon dioxide. Anoxic iron corrosion generates hydrogen gas and iron hydroxide, while consuming water. Magnesium oxide hydration consumes water and produces brucite. The pro ducts of these reactions will be generated where the water is present in the repository.

The overall reactions for the sulfidation of the iron species used in the performance assessment calculations are hydrogen sulfide reacting with iron hydroxide (reacti on 1) and metallic iron (reaction 2) to form iron sulfide.

Fe(OH) 2 + H 2 S → FeS + 2 H 2 O (1)

Fe + H 2 S → FeS + H 2 (2)

The iron hydroxide in reaction 1 is produced by the anoxic corrosion of iron (reaction 3).

Fe + 2 H 2 O → Fe(OH) 2 + H 2 (3)5 th Response Submittal to the EPA Enclosure 1

2 The reaction rates used in performance assessment for reactions 1 and 2 depend on the sampled value of the CPR degradation rate because the hydrogen sulfide comes from CPR degradation . The reaction rate used in performance assessment for reaction 3 depends on the sample d value of the iron corrosion reaction rate. Differences in these reaction rates affect the amount of gas and water that are present. The reaction scheme for the iron species used in the performance assessment calculation is best represented by

rea ctions 3 and 1, rather than reaction 2, which captures the transient behavior of the gas and water generation/consumption.

The above representation is supported by experimental studies. In the reports Hydrogen Generation by Metal Corrosion in Simulated Wa ste Isolation Pilot Plant Environments: Progress Report for the Period November 1989 through December 1992 (Telander and Westerman 1993)
, and Hydrogen Generation by Metal Corrosion in Simulated Waste Isolation Pilot Plant Environments (Telander and Westerm an 1997)
, the authors experimentally demonstrated that anoxic corrosion of low carbon steel without the presence of hydrogen sulfide produced iron hydroxide (reaction 3)

. They also experimentally demonstrated that low carbon steel corrosion in the presence of hydrogen sulfide quickly passivated (order of days), with the specimen suspended over the brine (reaction 2) passivating quicker and corroding less than the specimen i mmersed in the brine (reaction 1) . The specimen immersed in the brine corroded more due to the competition of the iron corrosion reaction (reaction 3) and the sulfidation reaction (reaction 1) , indicating that reaction 1 would dominate in the long term co mpared with reaction 2. Since 1) experimental studies support the above representation
, and 2 ) it allows the performance assessment calculations to capture the transient behavior of the gas and water generation/consumption , it was assumed that reaction 1 would dominate reaction 2.

The overall reactions of carbon dioxide produced from CPR degradation with brucite (reaction 4) and magnesium oxide (reaction 5) as used in performance assessment calculations are:

1.25 Mg(OH) 2 + CO 2 → 0.25 Mg 5 (CO 3 ) 4 (OH) 2 ∙ 4H 2 O (4)

MgO + CO 2 → MgCO 3 (5)

The b rucite in reaction 4 is an intermediate species produced by the hydration of magnesium oxide (reaction 6)
, and the hydromagnesite (5424) in reaction 4 is a metastable species that converts to magnesite (reaction 7).

MgO + H 2 O → Mg(OH) 2 (6)

0.25 Mg 5 (CO 3 ) 4 (OH) 2 ∙ 4H 2 O → MgCO 3 + 0.25 Mg(OH) 2 + H 2 O (7)

The reaction rates used in performance assessment for reactions 4 and 5 depend on the sampled value of the CPR degradation rate because the carbon dioxide comes from CPR degradatio n . The reaction rates used in performance assessment for reactions 6 and 7 depend on the sample d value of the magnesium oxide hydration and hydromagnesite conversion reaction rates. Differences in these reaction rates affect the amount of gas and water that are present

. The reaction scheme used in the performance assessment calculations follow s the reaction path of reaction s 6, 4 and 7 instead of reaction 5 to capture the transient behavior of the gas and water generation/consumption.

The above represen tation is supported by experimental studies. I n Experimental I nvestigations of the R eaction P ath in the MgO

– CO 2 – H 2 O S ystem in S olutions with Various I onic S trengths, and T heir A pplications to N uclear W aste I solation (Xiong and Lord 2008), it was experimentally demonstrated that magnesium oxide first hydrates as brucite ( r eaction 6), then the brucite carbonate s as hydromagnesite (5424) ( r eaction 4)

. The conversion rates from brucite to hydromagnesite (5424) are fast under the5 th Response Submittal to the EPA Enclosure 1

3 WIPP

- relevant condit ions with regard to the repository performance assessment time scale

. For instance, the conversion rates for reaction 4 in terms of fraction of conversion for ERDA

- 6 and GWB brines are 5.37 × 10

– 6 hour

– 1 (4.7% per year) and 2.29 × 10

– 6 hour

– 1 (2.0% per year) at room temperature and atmospheric partial pressure of carbon dioxide (Xiong and Lord 2003). Furthermore, in Thermokinetic Analysis of the MgO Surface Carbonation Process in the Presence of Water Vapor ( Torres

- Rodriguez and Pfeiffer 2011) , it is reported that magnesium oxide does not absorb carbon dioxide in dry conditions (reaction 5) at lower temperatures (<70 o C). Since 1) experimental studies support the above representation, and 2 ) it allows the performance assessment calculations to capture the transient behavior of the gas and water generation/consumption, it was assumed that reaction 4 would dominate reaction 5.

It is worth mentioning that magnesite is the predicted thermodynamically stable phase under WIPP conditions, whereas hydromagnesite (5424) is the metastable phase (Xiong and Lord 2008) . However, the conversion from hydromagnesite (5424) to magnesite is slow, as described in Appendix MgO

- 2009
, Magnesium Oxide as an Engineered Barrier (U

. S

. DOE 2009). During the PA perio d of 10,000 years, hydromagnesite (5424) could convert to magnesite (U

. S

. DOE 2009).

One evaluation of the potential magnitude of the water balance assumptions is given in Verification of the Waste Isolation Pilot Plant Chemistry Conceptual Models (SCA 200 8). In their analysis, S

. C ohen and A ssociates determine d the potential amounts of brine volume change per panel for various water balance assumptions, summarized here in Figure 1. The values in the right column of Figure 1 indicate the volume of brine i n a panel that could be produced or consumed for various assumptions. T he potential volumes of brine in a panel that could be produced or consumed are on the same order as the maximum volume of brine in a panel

. To evaluate the potential magnitude of the water balance assumptions on performance assessment, the refinement to the repository water balance implementation was included in the CRA

- 2014 and the results were analyzed. The performance assessment calculations conducted for the CRA

- 2014 included two sub sets that are relevant to the EPA’s comment , CRA14

- BV and CRA14

- 0 (Zeitler 2013). The only parameter changes between the CRA14

- BV and CRA14

- 0 calculations were the update to the steel (iron) corrosion rate and the refinement to the repository water bal ance implementation ( implemented simultaneously ) . In the CRA14

- BV calculations, the repository water balance implementation included iron corrosion (reaction 3) as a gas producing and water consuming reaction
, and CPR degradation as a gas producing and wat er neutral reaction . This essentially assumed that the amount of water generated from CPR degradation was equal to the amount of water consumed by magnesium oxide hydration (reaction 6)
, and magnesium oxide reacted directly with carbon dioxide (reaction 5 ). Furthermore, in the CRA14

- BV, the hydrogen sulfide was assumed to react directly with the iron (reaction 2)

. The CRA14

- 0 calculations included CPR degradation as a gas and water producing reaction and reactions 1

- 7 as written above
, with reaction 1 do minating reaction 2 and reaction 4 dominating reaction 5. The reduction of the iron corrosion rate for the CRA14

- 0 calculation should have a similar effect on the water balance as the refined repository water balance implementation by reducing the amount of gas produced and increasing the amount of water available.

A comparison of the results between the CRA14

- BV and CRA14

- 0 is shown in Figure 2. Comparing the curves for the CRA14

- BV and CRA14

- 0 shows that the potential magnitude of the impact of the water balance assumptions on the normalized releases from the repository is small. The small effect on normalized releases of the water balance assumptions is likely because direct brine releases are dominated by scenarios involving intrusions into a bri ne pocket, which increases repository pressure and floods the repository and hence, dominates the amount of water in the repository and overwhelms the gas and water balance.5 th Response Submittal to the EPA Enclosure 1

4 Table 5

- 1. GWB Brine Volumes Associated with Repository Chemical Processes and Mo deled Cumulative Brine Flow for the Undistu r bed PABC Scenario

Process Brine Volume Per Panel Maximum Brine Consumed by MgO Hydration and Carbonation Magne site 0 m 3 Brucite or hydromagnesite 3,175 m 3 Maximum Brine Produced by Cellulose Degradation Magnesite 988 m 3 Hydromagnesite 649 m 3 Maximum Brine Produced by Plastics Degradation Magnesite 1,614 m 3 Hydromagnesite 1,291 m 3 Maximum Brine Produced by Rubber Degradation Magnesite 170 m 3 Hydromagnesite 116 m 3 Maximum Total Brine Production by CPR Degradation (Cellulose + Plastics + Rubber Magnesite 2,772 m 3 Hydromagnesite 2,056 m 3
Castile Brine (Clayton 2006) Maximum c alculated in PABC 13,267 m 3 Salado Brine (Clayton 2006) Maximum c alculated in PABC 7, 763 m 3 Salado or Castile Brine (Clayton 2006) Minimum necessary for direct brine release 1,045 m 3

Brine Consumed by Anoxic Corrosion Fe(OH) 2 4,060 m 3 0.25 FeS + 0.75 Fe(OH) 2 2,030 m 3



### Figure 1. Table 5

- 1 from SCA (2008).

Figure 2. Simplified version of figure 3

- 46 from Zeitler (2013).5 th Response Submittal to the EPA Enclosure 1

5 References:

S. Cohen and Associates ( SCA )

. 2008. Verification of the Waste Isolation Pilot Plant Chemistry Conceptual Models

. September 19 , 2008. Final Report prepared for the U.S. Environmental Protection Agency Office of Radiation and Indoor Air, Washington, D.C.

Telander , M.R.
, and Westerman , R.E . 1993. Hydrogen Generation by Metal Corrosion in Simulated Waste Isolation Pilot Plant Environments: Progress Report for the Period November 1989 through December 1992

. SAND92

- 7347

. Sandia Nationa l Laboratories
, Albuquerque
, NM

.
Telander , M.R.
, and Westerman , R.E

. 1997 . Hydrogen Generation by Metal Corrosion in Simulated Waste Isolation Pilot Plant Environments. SAND96

- 2538

. Sandia National Laboratories, Albuquerque
, NM

.

Torres

- Rodríguez
, D

. A

. , and Pfeiffer
, H . 2011.Thermokinetic A nalysis of the MgO S urface C arbonation P rocess in the P resence of W ater V apor. Thermochim ica Acta 516: 74

– 78

. *

U

. S

. DOE. 2009. Title 40 CFR Part 191 Subparts B and C Compliance Recertification Application for the Waste Isolation Pilot Plant Appendix MgO

- 2009 Magnesium Oxide as an Engineered Barrier. United States Department of Energy, Waste Isolation Pilot Plant, Carlsbad Field Office, Carls bad, NM.

Xiong, Y.

- L.
, and Lord, A.C.S. 2008. Experimental investigations of the reaction path in the MgO

– CO 2 – H 2 O system in solutions with various ionic strengths, and their applications to nuclear waste isolation. Applied Geochemistry, 23:1634

– 1659.

Xio ng, Y.

- L.
, and Snider, A.C. 2003. 4.3 Carbonation Rates of the Magnesium Oxide Hydration Product Brucite in Various Solutions, Sandia National Laboratories Technical Baseline Reports, p. 4.3

- 1

– 4.3

- 11, WBS 1.3.5.3, Compliance Monitoring; WBS 1.3.5.4, Repository Investigat ions, Milestone RI 03

- 210, January 31, 2003. ERMS526049

. Sandia National Laboratories, Carlsbad, NM.

Zeitler, T.R. 2013. Analysis Package for CCDFGF: 2014 Compliance Recertification Application Performance Assessment (CRA

- 2014 PA). ERMS 560074. Sandia National Laboratories, Carlsbad, NM

.

* Copyrighted reference not provided in Enclosure 2.5 th Response Submittal to the EPA Enclosure 1

6 EPA Comment 2

- C

- 5 Cumulative Effects of Water Balance Assumptions on PA. The result of several water balance assumptions is to increase brine production from waste reactions in the repository. These assumptions inclu de that hydrogen sulfide will preferentially react with iron hydroxide instead of metallic iron (Comment 2

- C

- 3); that carbon dioxide will preferentially react with brucite instead of MgO (Comment 2

- C

- 3); and that the minimum rate of hydromagnesite conversi on to magnesite is greater than zero (Comment 2

- C

- 4). Please provide a description of the cumulative effects of these assumptions on the water balance calculations for the CRA

- 2014 PA.
DOE Response

The effect of assuming hydrogen sulfide will preferentially react with iron hydroxide instead of metallic iron on the performance assessment calculations is minimal (see response to comment 2

- C

- 3). The effect of assuming carbon dioxide will preferentially react with brucite instead of magnesium oxide on the performance assessment calculations is minimal (see response to comment 2

- C

- 3). Performance assessment calculations show a minimal effect with the assumption that the minimum rate of hydromagnesite con version to magnesite is greater than zero (see response to comment 2

- C

- 4). Each of these assumptions results in an increase in the amount of brine at earlier times, but the cumulative effect of these three assumptions has a minor effect on performance ass essment calculations (see response to comment 2

- C

- 3)

.5 th Response Submittal to the EPA Enclosure 1

7 EPA Comment 2

- 32

- S1 FEP H21 Drilling Fluid Flow. Screening argument considers only boreholes intersecting the waste region. Please supplement the argument with a discussion of boreholes that intersect the non

- waste regions and the consequence to PA calculations. Provide references and specific informat ion as to whether boreholes penetrating non

- waste regions could result in the transport of radionuclides between the waste and non

- waste regions, to overlying units, or to the surface. Provide information, either directly or by reference, as to how deep bo reholes penetrating the non

- waste and waste regions of the repository are accounted for in the PA.

2

- 32

- S2 FEP H22 Drilling Fluid Loss. The screening argument considers flow into the repository from boreholes that intercept pressurized fluid in underlying formations but only for boreholes intersecting the waste region. In the current BRAGLO model gas and brine readily flow between the waste and non

- waste regions. A discussion and analysis of boreholes that could intersect the non

- waste regions and their im

pact on the PA needs to be provided.

2

- 32

- S3 FEP H23 Blowouts. Screening argument considers only boreholes intersecting the waste region and also pressurized Castile brine. In the current BRAGLO model gas and brine readily flow between the waste and non

- w aste regions. Please supplement the argument with a discussion and analysis of boreholes that could intersect the non

- waste regions on the PA.

DOE Response

Boreholes that intercept the non

- waste regions (e.g., the operations and experimental areas ) of the repository have been screened out of PA calculations since the CCA (DOE 1996). Additionally, early screening efforts demonstrated that drilling into a non

- repository area of the controlled area ( independently, within both the Salado and Culebra F ormations) resulted in low consequences to the performance of the repository (Wallace 1996 and Economy 1996). No known recent events have occurred or new information become available that would weaken or render these determinations invalid. In fact, a recent analy sis by Zeitler (2015), based on BRAGFLO results from the CRA

- 2014, strengthens the determination of drilling into non

- repository areas being of low consequence. This analysis included the impacts of a run

- of

- mine (ROM) salt panel closure system (PCS).

The current BRAGFLO model does allow for brine and gas flow between the waste and non

- waste regions, as has been the case since the CCA. A full discussion and analysis of boreholes intersecting non

waste regions is contained in the Supplemental Peer Review R eport (Wilson et al. 1996), in which the Conceptual Models Peer Review Panel (CMPRP) evaluated the conceptual model for the exploration borehole and raised questions regarding the justification for excluding inadvertent intrusions into the operations and e xperimental area s of the repository from PA calculations. DOE’s response to this issue was partially based on the fact that even though intrusions into these areas of the repository could occur, since waste will not be disposed in these areas, releases fr om cuttings, cavings, and spallings could (by their definitions) not occur since waste will not be disposed in these areas

. R adionuclides present in the repository could potentially be transported in brine into the operations and experimental areas and

co ntaminated brine could be brought to the surface during drilling. However, because the operations and experimental area s of the repository are separated from the waste regions by panel closures that will greatly reduce brine flow, insignificant quantities of brine flows across this boundary.

To respond to the CMPRP, the DOE provided intermediate BRAGFLO results that demonstrated that the panel c losures effectively limited brine movement. In their Supplemental Peer Review Report (Wilson et al. 1996), the CMPRP stated:5 th Response Submittal to the EPA Enclosure 1

8 “...with very few exceptions, the maximum brine flow through a panel seal was found in modeling results to be about 12,000 m 3 follow ing an E1 intrusion, and the expected flow would be about 3,000 m 3 . Under undisturbed conditions, the cumulative brine inflow into the operations and experimental areas from all sources ranges from near zero to about 10,000 m 3 (see Figures 3

- 4 and 3

- 5). This flow would, on the average, be expected to result in approximately a twofold dilution of brine flowing in from the waste area following an E1 intrusion. Under disturbed conditions with intrusion boreholes penetrating the operations and experimental areas, the inflow of uncontaminated brine and the dilution of contaminated brine would be even greater.

In the event that the operations and experimental areas are penetrated by intrusion boreholes, approximately 12,000 m 3 of brine could conceivably flow f rom these areas into the waste area. Performance assessment results indicate that total cumulative brine flow into the waste area is typically about 40,000 m 3 following an E1 intrusion at 1,000 years and about 30,000 m 3 following an E2 intrusion at 1,000 years. Given that about six borehole intrusions are expected to occur during the regulatory time frame, the total volume of brine potentially available to flow into the waste area could exceed 100,000

m 3 . Although the actual volume of brine inflow will de pend on the interrelationships among time of intrusion, repository creep closure, gas generation, repository pressure, and other factors, the modeling results indicate that sufficient brine is potentially available from other sources that an incremental su pply of as much as 12,000 m 3 would have no consequential effect on performance assessment results. However, if subsequent model modifications result in significantly smaller fluid volumes in the repository, the significance of this issue should be reevalua ted.”

Thus, the CMPRP determined that the DOE had satisfactorily demonstrated that representing exploration boreholes and their effects in the experimental and operations areas of the repository was not necessary, and the representation of repository fluid flow and the exploration borehole was adequately incorporated into PA models.

To update the BRAGFLO results that were used by the CMPRP to make its decision, and verify that it remains appropriate to exclude boreholes in the operations and experimental ar eas from PA calculations, BRAGFLO results from replicate 1 of the CRA

- 2014 PA calculations are examined. The maximum brine flow through a panel closure is now about 8,000 m 3 following an E1 intrusion, which occurred for brine flow across the panel closure separating the waste panel and southern rest of repository (SRoR) ( Figure 1 ). (BRAGFLO scenarios S2 and S3 consider E1 intrusions.) The expected northward flow across any panel closure is about 1,000 m 3 (for brine flow across the panel closure separating t he waste panel and SRoR). 1 For undisturbed conditions (S1), the cumulative brine inflow into the operations and experimental areas from all sources ranges from 700 to 35,000 ( Figure 2 ). This flow would, on average, result in about a seventeen

- fold dilution of brine flowing in from the waste area following an E1 intrusion (comparing (35,000 + 700)m 3 /2 = 17,850m 3 to 1,000m 3 )

.

1 See Appendix PA

- 2014, Figure PA

- 12 for a representation of the BRAGFLO grid and the discrete grid sections (including the SRoR).5 th Response Submittal to the EPA Enclosure 1

9 

### Figure 1 . Cumulative brine volume passing northward from the waste panel to the SRoR through the panel closure for the 100 vectors of replicate 1, scenario 2 (E1 intrusion at 350 years) (from Zeitler 2015)

Figure 2 . Cumulative brine flow into the experimental and operations areas for the 100 vectors of replicate 1, scenario 1 ( undisturbed repository) (from Zeitler 2015)5 th Response Submittal to the EPA Enclosure 1

10 The CRA

- 2014 PA results indicate that a total cumulative brine flow into the waste area (including the waste panel, northern rest of repository, and SRoR) is on average about 36,000 m 3 following an E1 intrusion at 1,000 years (S3) and about 20,500 m 3 following an E2 intrusion at 1,000 years (S5). Given that about 7.5 borehole intrusions are expected to occur during the regulatory period, the total volume of brine potentially available to flow into the waste area could exceed 200,000 m 3 . Thus, with an incremental supply of only 8,000 m 3 , the same conclusion as the CMPRP can reached: [the additional brine volume] “would have no consequential effect on performance assessment results.” Compared to the previous analys is, the maximum brine flow across a panel closure has decreased (12,000 to 8,000 m 3 ) and the total volume of brine potentially available to flow into the waste area has stayed the same ( 210,000 to 209,500 m 3 ). Thus, the relative amount of “incremental supp ly” of brine has decreased from 12,000/ 210,000 =0. 0571 to 8,000/ 209,500 =0. 038 , indicating approximately a 33 percent decrease in the relative amount of brine available in the non

- waste regions of the repository. Therefore, boreholes that intersect the non

- w aste regions of the repository still do not need to be accounted for in PA calculations.

The following text for FEPs H21, H22, and H23 incorporate references to previous screening work (Wallace 1996 and Economy 1996 ) , as well as the new analysis using CRA

- 2014 describe d above. Changes to the text for FEPs H21, H22, and H23 are shown in blue font below.



### SCR

- 5.2.1.1 FEP Number: H21 FEP Title: Drilling Fluid Flow

SCR

- 5.2.1.1.1 Screening Decision: SO

- C (HCN) DP (Future)

Drilling Fluid Flow associated with historical, current, near

- future, and future boreholes that do not intersect the waste disposal region has been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system. The possibility of a future deep borehole penetrating a waste panel, such that drilling

- induced flow results in transport of radionuclides to the land surface or to overlying hydraulically conductive units, is accounted for in PA calculations. The possibility of a deep bore hole penetrating both the waste disposal region and a Castile brine reservoir is accounted for in PA calculations.

SCR

- 5.2.1.1.2 Summary of New Information

No new information that affects the screening of this FEP has been identified since the CRA

- 2009 .Th e screening argument for this FEP has been updated to reference new confirmatory screening analyses (Zeitler 2015) that support the determination by the Conceptual Models Peer Review Panel (Wilson et al. 1996) that boreholes into the experimental and opera tional areas do not need to be considered in PA calculations.

SCR

- 5.2.1.1.3 Screening Argument

Borehole circulation fluid could be lost to thief zones encountered during drilling, or fluid could flow from pressurized zones through the borehole to the land surface (blowout) or to a thief zone. Such drilling

- related EPs could influence groundwater flow and, potentially, radionuclide transport in the affected units. Future drilling within the controlled area could result in direct releases of radionuclides to the land surface or transport of radionuclides between hydraulically conductive units.5 th Response Submittal to the EPA Enclosure 1

11 Movement of brine from a pressurized zone through a borehole into potential thief zones such as the Salado interbeds or the Culebra could result in geochemical change s and altered radionuclide migration rates in these units.

SCR

- 5.2.1.1.3.1 Historical, Current, and Near

- Future Human EPs

Drilling fluid flow is a short

- term event that can result in the flow of pressurized fluid from one geologic stratum to another. How ever, long

- term flow through abandoned boreholes would have a greater hydrological impact in the Culebra than a short

- term event such as drilling

- induced flow outside the controlled area. Wallace (1996a) analyzed the potential effects of flow through aband oned boreholes in the future within the controlled area, and concluded that interconnections between the Culebra and deep units could be eliminated from PA calculations on the basis of low consequence. Thus, the HCN of d rilling fluid flow associated with boreholes outside the controlled area has been screened out on the basis of low consequence to the performance of the disposal system.

As discussed in FEPs H25 through H36 (Section s SCR

- 5.2.1.5, SCR

- 5.2.1.6, SCR

5.2.1.7, SCR

- 5.2.1.8, SCR

- 5.2.1.9, SCR

- 5.2.1 .10, SCR

- 5.2.1.11, SCR

- 5.2.1.12, and SCR

- 5.2.1.1), drilling associated with water resources exploration, groundwater exploitation, potash exploration, oil and gas exploration, oil and gas exploitation, enhanced oil and gas recovery, and drilling to explore other resources has taken place or is currently taking place outside the controlled area in the Delaware Basin. These drilling activities are expected to continue in the vicinity of the WIPP in the near future.

SCR

- 5.2.1.1.3.2 Future Human EPs

For the f uture, drill holes may intersect the waste disposal region and their effects could be more profound. Thus, the possibility of a future borehole penetrating a waste panel, so that d rilling fluid flow and, potentially, b lowout results in transport of radion uclides to the land surface or to overlying hydraulically conductive units, is accounted for in PA calculations. Drilling events through the experimental and operational regions of the repository are not accounted for in PA calculations as it was determin ed that they could be excluded due to low consequence (Wilson et al. 1996). This conclusion of low consequence was recently reconfirmed by Zeitler (2015 ). This new analysis uses the latest PA results and updates the low consequence brine flow justificatio n analyses discussed in Wilson et al. (1996), where it was determined that insignificant amounts of brine flow from the repository to the non

- waste areas do not have a consequence on PA results. Additionally, drilling into a contaminated zone outside the waste area but within the controlled area was screened out due to low consequence (Economy 1996).

The units intersected by the borehole may provide sources for fluid flow (brine, oil, or gas) t o the waste panel during drilling. In the vicinity of the WIPP, the Castile that underlies the Salado contains isolated volumes of brine at fluid pressures greater than hydrostatic. A future borehole that penetrates a Castile brine reservoir could provid e a connection for brine flow from the reservoir to the waste panel, thus increasing fluid pressure and brine volume in the waste panel. The possibility of a deep borehole penetrating both a waste panel and a brine reservoir is accounted for in PA calcula tions.

Penetration of an underpressurized unit underlying the Salado could result in flow and radionuclide transport from the waste panel to the underlying unit during drilling, although drillers would minimize such fluid loss to a thief zone through the i njection of5 th Response Submittal to the EPA Enclosure 1

12 materials to reduce permeability or through the use of casing and cementing. Also, the permeabilities of formations underlying the Salado are less than the permeability of the Culebra (Wallace 1996a). Thus, the consequences associated with ra dionuclide transport to an underpressurized unit below the waste panels during drilling will be less significant, in terms of disposal system performance, than the consequences associated with radionuclide transport to the land surface or to the Culebra du ring drilling. Through this comparison, drilling events that result in penetration of underpressurized units below the
waste

- disposal region have been eliminated from PA calculations on the basis of beneficial consequence to the performance of the disposa l system.



### SCR

- 5.2.1.2 FEP Number: H22 FEP Title: Drilling Fluid Loss

SCR

- 5.2.1.2.1 Screening Decision: SO

- C (HCN) DP (Future)

Drilling Fluid Loss associated with HCN and future boreholes that do not intersect the waste disposal region has been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system. The possibility of a future Drilling Fluid L oss i nto waste panels is accounted for in PA calculations.

SCR

- 5.2.1.2.2 Summary of New Information

No new information that affects the screening of this FEP has been identified since the CRA

- 2009. The screening argument for this FEP has been updated to refere nce new confirmatory screening analyses (Zeitler 2015) that support the determination by the Conceptual Models Peer Review Panel (Wilson et al. 1996) that boreholes into the experimental and operational areas do not need to be considered in PA calculations

.

SCR

- 5.2.1.2.3 Screening Argument

Drilling fluid loss is a short

- term event that can result in the flow of pressurized fluid from one geologic stratum to another. Large fluid losses would lead a driller to inject materials to reduce permeability, or it would lead to the borehole being cased and cemented to l imit the loss of drilling fluid. Assuming such operations are successful, drilling fluid l oss in the near future outside the controlled area will not significantly affect the hydrology of the disposal system. Thus, drilling fluid l oss associated with histo rical, current, and near

- future boreholes has been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system.

In evaluating the potential consequences of drilling fluid l oss to a waste panel in the future, tw o types of drilling events need to be considered

– those that intercept pressurized fluid in underlying formations such as the Castile (defined in the CCA, Chapter 6.0, Section 6.3.2.2 as E1 events), and those that do not (E2 events). A possible hydrologi cal effect would be to make a greater volume of brine available for gas generation processes and thereby increase gas volumes at particular times in the future. For either type of drilling event, on the basis of current drilling practices, the driller is assumed to pass through the repository rapidly. Relatively small amounts of drilling fluid loss might not be noticed and might not give rise to concern. Larger fluid losses would5 th Response Submittal to the EPA Enclosure 1

13 lead to the driller injecting materials to reduce permeability, or to the b orehole being cased and cemented, to limit the loss of drilling fluid.

For boreholes that intersect pressurized brine reservoirs, the volume of fluid available to flow up a borehole will be significantly greater than the volume of any drilling fluid that

c ould be lost. This greater volume of brine is accounted for in PA calculations, and is allowed to enter the disposal room (see the CCA, Chapter 6.0, Section 6.4.7). Thus, the effects of drilling fluid l oss will be small by comparison to the potential flo w of brine from pressurized brine reservoirs. Therefore, the effects of drilling fluid loss for E1 drilling events have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system.

The consequences of dri lling fluid l oss into waste panels in the future are accounted for in PA calculations for E2 events.

SCR

- 5.2.1.2.3.1 Historical, Current, and Near

- Future Human EPs

Drilling fluid flow will not affect hydraulic conditions in the disposal system significant ly unless there is substantial drilling fluid l oss to a thief zone, such as the Culebra. Typically, zones into which significant borehole circulation fluid is lost are isolated through injection of materials to reduce permeability or through casing and ce menting programs. Assuming such operations are successful, drilling fluid loss in the near future outside the controlled area will not affect the hydrology of the disposal system significantly and will be of no consequence.

SCR

- 5.2.1.2.3.2 Future Human E Ps

The consequences of drilling within the controlled area in the future will primarily depend on the location of the borehole. Potentially, future deep drilling could penetrate the waste disposal region. Hydraulic and geochemical conditions in the waste panel could be affected as a result of drilling fluid l oss to the panel. Drilling fluid loss due to drilling through the experimental and operational regions of the repository are not accounted for in PA calculations as it was determined that such drillh oles could be excluded due to low consequence (Wilson et al. 1996). This conclusion of low consequence was recently reconfirmed by Zeitler (2015). This new analysis uses the latest PA results and updates the low consequence brine flow justification analyses discussed in Wilson et al. (1996) where it was determined that insignificant amounts of brine flows from the repository to the non

- waste areas do not have a consequence on PA results.

Penetration of an underpressurized unit underlying the Salado c ould result in flow and radionuclide transport from the waste panel to the underlying unit during drilling, although drillers would minimize such fluid loss to a thief zone through the injection of materials to reduce permeability or through the use of cas ing and cementing. Also, the permeabilities of formations underlying the Salado are less than the permeability of the Culebra (Wallace 1996a). Thus, the consequences associated with radionuclide transport to an underpressurized unit below the waste panel s during drilling will be less significant, in terms of disposal system performance, than the consequences associated with radionuclide transport to the land surface or to the Culebra during drilling. Through this comparison, drilling events that result i n penetration of underpressurized units below the5 th Response Submittal to the EPA Enclosure 1

14 waste

- disposal region have been eliminated from PA calculations on the basis of beneficial consequence to the performance of the disposal system.

For boreholes that do not intersect pressurized brine reserv oirs (but do penetrate the
waste

- disposal region), the treatment of the disposal room implicitly accounts for the potential for greater gas generation resulting from drilling fluid loss. Thus, the hydrological effects of drilling fluid l oss for E2 drillin g events are accounted for in PA calculations within the conceptual model of the disposal room for drilling intrusions.



### SCR

- 5.2.1.3 FEP Number: H23 FEP Title: Blowouts

SCR

- 5.2.1.3.1 Screening Decision: SO

- C (HCN) DP (Future)

Blowouts associated with HCN and future boreholes that do not intersect the waste disposal region have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system. The possibility of a future deep borehole penetrating a waste pan el such that drilling

- induced flow results in transport of radionuclides to the land surface or to overlying hydraulically conductive units is accounted for in PA calculations. The possibility of a deep borehole penetrating both the waste disposal region and a Castile brine reservoir is accounted for in PA calculations.

SCR

- 5.2.1.3.2 Summary of New Information

Blowouts are implemented in PA through the parameter GLOBAL:PBRINE, which represents the probability of an inadvertent intrusion borehole encounter ing pressurized brine beneath the repository. This parameter has been updated based on new data and analysis as reported in Kirchner et al. (2012). This parameter update does not change the screening argument or decision; H23 Blowouts continue to be clas sified as DP for the future timeframe. The screening argument for this FEP has also been updated to reference new confirmatory screening analyses (Zeitler 2015) that support the determination by the Conceptual Models Peer Review Panel (Wilson et al. 1996) that boreholes (and any associated blowouts) into the experimental and operational areas do not need to be considered in PA calculations. This new analysis uses the latest PA results and updates the low consequence brine flow justification analyses discus sed in Wilson et al. (1996)
, where it was determined that insignificant amounts of brine flows from the repository to the non

- waste areas do not have a consequence on PA results.

SCR

- 5.2.1.3.3 Screening Argument

Blowouts are short

- term events that can res ult in the flow of pressurized fluid from one geologic stratum to another. For the near future, a blowout may occur in the vicinity of the WIPP but is not likely to affect the disposal system because of the distance from the well to the waste panels, assu ming that AICs are in place which restrict borehole installation to outside the WIPP boundary. Blowouts associated with HCN and future boreholes that do not intersect the waste disposal region have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system. For the future, the drill holes may intersect the waste disposal region and these effects could be more profound. Thus, blowouts that occur in the waste area of the repository are5 th Response Submittal to the EPA Enclosure 1

15 included in the assessment of future activities
, and their consequences are accounted for in PA calculations. Blowouts that occur in the non

- waste regions of the repository in the future are not included in PA calculations as both Wilson et al. (1996) and Zeitler (2015) have demonstrated that these events are inconsequential to repository performance.

Fluid could flow from pressurized zones through the borehole to the land surface ( blowout ) or to a thief zone. Such drilling

- related EPs could influence groundwater flow

an d, potentially, radionuclide transport in the affected units. Movement of brine from a pressurized zone through a borehole into potential thief zones such as the Salado interbeds or the Culebra could result in geochemical changes and altered radionuclide migration rates in these units.

SCR

- 5.2.1.3.3.1 Historical, Current, and Near

- Future Human EPs

Drilling associated with water r esources exploration, groundwater exploitation, potash exploration, oil and gas exploration, oil and gas exploitation, enhanced oil and gas recovery, and drilling to explore other resources has taken place or is currently taking place outside the controlled area in the Delaware Basin. These drilling activities are expected to continue in the vicinity of the WIPP in the near future

. Naturally occurring brine and gas pockets have been encountered during drilling in the Delaware Basin. Brine pockets have been intersected in the Castile (as discussed in the CCA, Chapter 2.0, Section 2.2.1.2). Gas blowouts have occurred during drilli ng in the Salado. Usually, such events result in brief interruptions in drilling while the intersected fluid pocket is allowed to depressurize through flow to the surface (for a period lasting from a few hours to a few days). Drilling then restarts with an increased drilling mud weight. Under these conditions, blowouts in the near future will cause isolated hydraulic disturbances, but will not affect the hydrology of the disposal system significantly.

Potentially, the most significant disturbance to the disposal system could occur if an uncontrolled blowout during drilling resulted in substantial flow through the borehole from a pressurized zone to a thief zone. For example, if a borehole penetrates a brine reservoir in the Castile, brine could flow thro ugh the borehole to the Culebra over the long term and, as a result, could affect hydraulic conditions in the Culebra. The potential effects of such an event can be compared to the effects of long

- term fluid flow from deep overpressurized units to the Cul ebra through abandoned boreholes. Wallace (1996a) analyzed the potential effects of flow through abandoned boreholes in the future within the controlled area and concluded that interconnections between the Culebra and deep units could be eliminated from PA calculations on the basis of low consequence. Long

term flow through abandoned boreholes would have a greater hydrological impact in the Culebra than short

- term, drilling

- induced flow outside the controlled area. Thus, the effects of fluid flow during d rilling in the near future have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system.

In summary, blowouts associated with historical, current, and near

- future boreholes have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system.5 th Response Submittal to the EPA Enclosure 1

16 SCR

- 5.2.1.3.3.2 Future Human EPs

— Boreholes that Intersect the Waste Disposal Region

The consequences of drilling within the controlled area in the future will depend primarily on the location of the borehole. Potentially, future deep drilling could penetrate the waste disposal region. If the borehole intersects the waste in the disposal rooms, radionuclides could be transported as a result of d rilling fluid flow , e.g ., releases to the accessible environment may occur as material entrained in the circulating drilling fluid is brought to the surface. Also, during drilling, contaminated brine may flow up the borehole and reach the surface, depending on fluid pressure wi thin the waste disposal panels; blowout conditions could prevail if the waste panel were sufficiently pressurized at the time of intrusion.

SCR

- 5.2.1.3.3.3 Hydraulic Effects of Drilling

- Induced Flow

The possibility of a future borehole penetrating a waste panel, so that d rilling fluid flow and, potentially, blowout result in transport of radionuclides to the land surface or to overlying hydraulically conductive units, is accounted for in PA calculations. The hydraulic effects of drilling through the exper imental and operational regions of the repository are not accounted for in PA calculations as it was determined that they could be excluded due to low consequence (Wilson et al. 1996). This determination of low consequence was recently reconfirmed by Zeit ler (2015 ). This new analysis uses the latest PA results and updates the low consequence brine flow justification analyses discussed in Wilson et al. (1996)
, where it was determined that insignificant amounts of brine flows from the repository to the non

- waste areas do not have a consequence on PA results.

The units intersected by the borehole may provide sources for fluid flow (brine, oil, or gas) to the waste panel during drilling. In the vicinity of the WIPP, the Castile that underlies the Salado conta ins isolated volumes of brine at fluid pressures greater than hydrostatic. A future borehole that penetrates a Castile brine reservoir could provide a connection for brine flow from the reservoir to the waste panel, thus increasing fluid pressure and brin e volume in the waste panel. The possibility of a deep borehole penetrating both a waste panel and a brine reservoir is accounted for in PA calculations.

Future boreholes could affect the hydraulic conditions in the disposal system. Intersection of pocke ts of pressurized gas and brine would likely result in short

- term, isolated hydraulic disturbances, and will not affect the hydrology of the disposal system significantly. Potentially the most significant hydraulic disturbance to the disposal system could occur if an uncontrolled blowout during drilling resulted in substantial flow through the borehole from a pressurized zone to a thief zone. For example, if a borehole penetrates a brine reservoir in the Castile, brine could flow through the borehole to t he Culebra and, as a result, could affect hydraulic conditions in the Culebra. The potential effects of such an event can be compared to the effects of long

- term fluid flow from deep overpressurized units to the Culebra through abandoned boreholes. Wallac e (1996a) analyzed the potential effects of such interconnections in the future within the controlled area (but that do not intersect waste), and concluded that flow through abandoned boreholes between the Culebra and deep units could be eliminated from PA calculations on the basis of low consequence.

These changes have been added to Enclosure 4, CRA

- 2014 Errata Tracking

.5 th Response Submittal to the EPA Enclosure 1

17 References:

Economy, K. 1996. “Drilling Into a Salado Zone of Contamination Within the Controlled Area; Drilling Into a Non

- Salado Zone of Contamination Within the Controlled Area.” Summary Memo of Record for S

- 9 and NS

- 6 FEPs Screening. August 21, 1996. ERMS 416368. Sandia National Laboratories
, Albuquerque, NM.
Wallace, M. 1996. “Leakage from Abandoned Boreholes.” Summary Memorandum of Record for NS

7b, SWCF

- A 1.1.6.3:PA:QA:TSK:NS

- 7b. ERMS 240819. Sandia National Laboratories, Albuquerque, NM.
Wilson, C., D. Porter, J. Gibbons, E. Oswald, G. Sjoblom, and F. Caporuscio. 1996. Conceptual Models Supplementary Peer Review Report (Dec ember). ERMS 243153. Carlsbad Area Office , Carlsbad, NM

. U.S. Department of Energy (DOE). 1996. Title 40 CFR Part 191 Compliance Certification Application for the Waste Isolation Pilot Plant (October). 21 vols. DOE/CAO

- 1996

- 2184. Carlsbad Field Office, Carlsbad, NM.

Zeitler, T. 2015. “Evaluation of Brine Flow Through Panel Closures using CRA

- 2014 BRAGFLO Results.” Summary Memo to Records. June 2015. ERMS Pending. Sandia National Laboratories
, Carlsbad, NM.5 th Response Submittal to the EPA Enclosure 1

18 EPA Comment 2

- 32

- S6 FEP W1 Disposal Geometry. In the screening argument please provide evidence that the modeled excavated volume is the expected mined volume of the underground workings at the time of closure.

DOE Response

Consistent with the DOE’s response to EPA comment 2

- 32

- G1, this type of information is best left in the supporting documentation, rather than within the screening document. With regard to the actual mined volume modeled in PA, please see DOE’s response to EPA Comment 1

- 23

- 7 (Franco 2015).

Referen ce:
Franco, J.R. 2015. Letter to J. Edwards (Subject: Response to Environmental Protection Agency Letter Dated December 17, 2014 Regarding the 2014 Compliance Recertification Application). March 18, 2015. U.S. Department of Energy, Carlsbad Field Offic e, Carlsbad, NM.5 th Response Submittal to the EPA Enclosure 1

19 EPA Comment 2

- 32

- S11 FEP W20 Salt Creep. Please supplement the screening argument with a discussion of salt creep and consolidation to the ROM salt in the ROMPCS, and healing of the adjacent DRZ. Such a discussion can be found in Camphou se et al. (2012, Section 2.0. ERMS 557396). The screening argument for this FEP states that “Salt creep in the Salado is accounted for in PA calculations (the CCA, Chapter 6.0, Section 6.4.3.1).” The cited CCA section discusses these FEPs only in the conte xt of the waste region. In addition, this is the only FEP that addresses DRZ healing, which is expected to vary spatially.

2

- 32

- S12 FEP W21 Changes in the Stress. Screening argument was combined with that for W20 Salt Creep; please see comments for FEP W2 0. Additionally, please supplement the screening argument with discussions of 1) the coupling between consolidation of the ROM salt in the ROMPCS and healing of the adjacent DRZ (DRZ healing cannot occur until the ROM salt is consolidated and applies a bac k stress sufficient to compress and heal the DRZ); and 2) lateral extrusion of the ROM salt when under compressive stress from drift creep closure.

DOE Response

The screening argument and decision for these FEPs have not changed, however, the following poi nters will be placed in Appendix SCR

- 2014 to more precisely define where the new ROM salt panel closures are described and how they are implemented within PA. Changes are shown in blue font below.



### SCR

- 6.3.1.2 FEP Numbers: W20 and W21 FEP Titles: Salt Creep (W20) Change in the Stress Field (W21)

SCR

- 6.3.1.2.1 Screening Decision: UP

Salt Creep in the Salado and any resultant Changes in the Stress Field are accounted for in PA calculations.

SCR

- 6.3.1.2.2 Summary of New In formation

Salt creep and changes in stress will affect the consolidation of the ROM salt PCS over time. Modifications to relevant parameters are described in Camphouse (2013a). Implementation of the new ROM salt PCS is described in Appendix PA

- 2014, Sect ion PA

- 4.2.8
, and Appendix MASS

- 2014, Section MASS

- 4.1.3. These changes are downstream of the FEPs screening process, and will not change the screening decision; these FEPs will remain classified UP.

SCR

- 6.3.1.2.3 Screening Argument

Salt creep will lead to changes in the stress field, compaction of the waste and containers, and consolidation of the long

- term components of the sealing system. It will also tend to close fractures in the DRZ, leading to reductions in porosity and permeability, increases in pore fluid pressure, and reductions in fluid flow rates in the repository. Salt creep in the Salado is accounted for in PA calculations (the CCA, Chapter 6.0, Section 6.4.3.1). The long

- term repository seal system relies on the consolidation of the crush ed

- salt seal material and healing of the DRZ around the shaft seals and in and around the panel closures to achieve a low permeability under stresses induced by salt creep. Shaft seal5 th Response Submittal to the EPA Enclosure 1

20 and panel closure performance is discussed further in Section SCR

- 6.3.5 .1 (FEPs W36, W37, W113, and W114).

This change has been added to Enclosure 4, CRA

- 2014 Errata Tracking

.5 th Response Submittal to the EPA Enclosure 1

21 EPA Comment 2

- 32

- S13 FEP W25 Disruption Due to Gas Effects. Please supplement the screening argument with a discussion of the potential for high waste panel gas pressures to delay the consolidation of the ROM salt, thereby maintaining a higher permeability in the PCS for a longer period of time.

DOE Response

The current parameters used in the implementation of the ROMPCS are described in Appendix PA, Section PA

- 4.2.8 and its supporting references. Gas pressurization effects were not considered in the consolidation of the ROMPCS because physical (creep) closure an d complete consolidation are expected to occur before significant gas pressures can develop. Figure 6

- 1 of Camphouse (2013) indicates gas pressures will be below lithostatic pressure for 1000 years or more, supporting this assertion. Because gas pressure s are not expected to affect the ROMPCS final permeability, this FEP argument has not been changed.

Reference:

Camphouse, C. 2013. Analysis Package for Salado Flow Modeling Done in the 2014 Compliance Recertification Application Performance Assessment (CR A

- 2014 PA). Revision 0. April 30, 2013. ERMS 559980. Sandia National Laboratories
, Carlsbad, N M

.5 th Response Submittal to the EPA Enclosure 1

22 EPA Comment 2

- 32

- S14 FEP W27 Gas Explosions. Please update the screening argument to reflect the LANL inventory with nitrates and added organic matter th at resulted in an exothermic reaction.

DOE Response

The Accident Investigation Board report (DOE 2015) and the Technical Assistance Team report (SRNL 2015) specifically say that there was no evidence of an explosion. Both reports cite the steep angle of

r epose for the MgO as good evidence that there was no explosion. Both conclude that the radiological release was the result of an incompatible mixture that heated up, pressurizing the drum to the point of venting and igniting nearby flammable emplacement m aterials. Therefore, the DOE believes that this type of information is best described in FEP W72, Exothermic Reactions (see following DOE response to EPA Comment 2

- 32

- S21 ).

References :
DOE (U.S. Department of Energy). 2015. Accident Investigation Report. Phase 2. Radiological Release Event at the Waste Isolation Pilot Plant, February 14, 2014. April 2015. U.S. Department o f Energy, Office of Environmental Management.

SRNL (Savannah River National Laboratory). 2015. Waste Isolation Pilot Plant Technical Assessment Team Report. March 17, 2015. Revision 0. SRNL

- RP

- 2014

- 01198. Savannah River National Laboratory
, Aiken, SC.5 th Response Submittal to the EPA Enclosure 1

23 EPA Comment 2

- 32

- S21 FEP W72 Exothermic Reactions. Please supplement the screening argument with a discussion of the impact of exothermic reactions in the waste panels.

DOE Response

It is clear that the event of February 14, 2014
, has no effect on compliance with 40 CFR 191 Subparts B and C (or the CRA

- 2014) , primarily because it occurred during the operational timeframe and not the post

- closure disposal period. Additionally, this accide nt was the result of operational and administrative deficiencies and should not be considered a normally anticipated event. Procedural changes and safeguards will be implemented to assure that such incompatibilities will not occur within waste drums in th e future. For th ese reason s , the screening argument has not been changed to assume that such events will occur in the future. The screening argument will, however, be changed to describe the incident and the processes that are understood to have occurred . This event and resulting administrative changes should be considered as operational period activities and have no bearing upon the long

- term conditions within the repository.

With regard to this specific event, while nitrate is not depleted by anoxic co nditions, it does react with cellulose and will likely be reacting during the operational phase of the repository until the nitrate salts are depleted. However, rather than resulting in a thermal runaway, the more likely scenario of nitrate depletion , whi ch has been observed in sibling containers being stored and monitored at LANL, is by smooth oxidation that does not give rise to a thermal runaway. These reactions reduce the nitrate salts
, with a concomitant oxidation of the cellulose, to produce carbon a nd nitrogen oxides and other products of combustion. Although not seen in the gas phase, metal oxides are also produced. The heat of reaction on these monitored drums has not been detected by the external therm o couples because it is dissipated as fast as i t is formed. However, the gaseous products are easily detected by gas chromatograp h y , so it is clear that the reaction is ongoing but the rate is slow and therefore the impact is small, resulting in no loss of containment. While the oxidation by nitrate does produce heat, this heat load is already bounded by the CPR being converted to gas (the calorie release from the nitrate oxidation of cellulose is very similar to the calorie release from the air oxidation of cellulose). It is expected that the exoth ermic heat load from these reactions will be fairly small compared to the heat capacity of the quantity of salt in the repository. The impact of exothermic reactions from the presence of waste with nitrate salts is expected to be insignificant and is not included in PA.

The following text from Appendix SCR

- 2014 present s an updated screening argument for FEP W72 Exothermic Reactions has been provided to reflect the February 2014 event. Changes are shown in blue font.5 th Response Submittal to the EPA Enclosure 1

24 

### SCR

- 6.3.4.1 FEP Numbers: W29, W30, W31, W72, and W73 FEP Titles: Thermal Effects on Material Properties (W29) Thermally

- Induced Stress Changes (W30) Differing Thermal Expansion of Repository Components (W31) Exothermic Reactions (W72) Concrete Hydration (W73)

SCR

- 6.3.4.1.1 Screening Decision: SO

- C

The effects of Thermally

- Induced Stress
, Differing Thermal Expansion of Repository Components , and Thermal Effects on Material Properties in the repository have been eliminated from PA calculations on the basis of low consequence to performance of the disposal system.

The thermal effects of Exothermic Reactions , including Concrete Hydration , have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system.

SCR

- 6.3.4.1.2 Summary of New Information

This FEP has been updated to include the most recent inventory information as presented in Van Soest (2012). Thermal calculations have been update d with the updated quantities of reactants and provided below. Additionally, planned Salt Disposal Investigations (SDI) experiments as detailed in Patterson (2011) or the Salt Defense Disposal Investigations (SDDI) (Franco 2012) will place heaters in newl y excavated tunnels in the northern experimental region of the WIPP. Mining has been completed, but heater tests have not yet commenced. An evaluation conducted by Kuhlman (2011) for the SDI planned change notice (PCN) shows that any thermal pulse from these experiments will be very minimal, on the order of 0.02 °C or less. Therefore, the screening argument and decision for this FEP is unaffected by the conduct of these experiments.

On February 14, 2014, an exothermic reaction involving the mixture of the organic materials (Swheat Scoop ® absorbent and/or neutralizer) and nitrate salts occurred inside a waste drum emplaced in Room 7 of Panel 7. This exothermic reaction resulted in pressurization of the drum, failure of the drum locking ring, and displa cement of the drum lid. The visual evidence associated with the identified breach was consistent with an exothermic reaction within the drum. This reaction led to pressure buildup of combustible gases within the drum. The drum lid extruded beyond the lid retention ring, deflected the lid, and resulted in rapid release of the materials from the drum. The combustible gases and solids ignited
, which then spread to other combustible materials within the waste array, i.e., fiberboard and polyethylene slip s heets, reinforcement plates, stretch wrap, cardboard stiffeners and polypropylene super sack fabric. The cause of this event has been identified by the Accident Investigation Board (DOE 2015) as being a specific deficiency that, “if corrected, would preve nt recurrence of the same accident.”5 th Response Submittal to the EPA Enclosure 1

25 SCR

- 6.3.4.1.3 Screening Argument

Thermally induced stress could result in pathways for groundwater flow in the DRZ, in the anhydrite layers and MBs, and through seals, or it could enhance existing pathways. Conversely, elevated temperatures will accelerate the rate of salt creep and mitigate fracture development. Thermal expansion could also result in uplift of the rock and ground surface overlying the repository, and thermal buoyancy forces could lift the waste upward in the salt rock.

The distributions of thermal stress and strain changes depend on the induced temperature field and the differing thermal expansion of components of the repository, which depends on the components’ elastic properties. Thermal effects on material properties (such as permeability and porosity) could potentially affect the behavior of the repository.

Exothermic reactions expected in the WIPP repository include MgO hydration, MgO carbonation, aluminum (Al) corrosion, and cement hydration 1 (Bennett et al. 1996). Wang (1996) has shown that the temperature rise by an individual reaction is proportional to , where V is the maximum rate of brine inflow into a waste panel for a reaction limited by brine inflow (or a specified maximum reaction rate for a reaction limited by its own kinetics) and M is the quantity of the reactant. MgO hydration, cement hydration, and Al corrosion are assumed to be limited by brine inflow because they all consume water and have high reaction rates. The amounts of reactants are tabulated in Table SCR

- 3.

Table SCR

- 3

. Changes in Inventory Quantities from the CCA to the CRA

- 2014

SCR

- 1.1.1 Inventory CCA CRA

- 2004 CRA

- 2009 CRA

- 2014 MgO (tons) 85,600 a 72,760 (because of the elimination of mini

sacks) a 59,385 e 51,430 h

Cellulosics (tons) 5,940 b 8,120 c 8,907 f 5,127 i
Plastics (tons) 3,740 b 8,120 c 10,180 f 10,487 i
Rubber (tons) 1,100 b 1,960 c 1,885 f 1,379 i Aluminum alloys (tons) 1,980 b 1,960 c 2,030 f 504 i
Cement (tons) 8,540 b 9,971 d 13,888 g 11872 j

a U.S. DOE (2000a)
b U.S. DOE (1996b). Only CH

- TRU wastes are considered. Total volume of CH

- TRU wastes is 1.1 × 10 5 m 3 . This is not scaled to WIPP disposal volume.

c Appendix DATA

- 2004, Attachment F. Only CH

- TRU wastes are considered. Total volume of CH

- TRU waste is 1.4 × 1 0 5 m 3 . This is not scaled to WIPP disposal volume.
d This estimate is derived from data in Leigh (2003) and includes both reacted and unreacted cement (1.2 × 10 7 kg × 1.4 × 10 5 /168485/1000 kg/ton = 9971 tons

cement).

e This estimate is derived by assuming that Panel 1 has an MgO excess factor of 1.95, three panel equivalents have a 1.67 excess factor, and the remaining 6

panel equivalents have a 1.2 excess factor, resulting in a 1.416 projected excess factor for a full repository. The projecte d excess fac tor is then multiplied by the equivalent cellulose value of 28,098 × (40.3/27) (the MgO molar ratio).
f This value is derived using material densities reported in Leigh et al. (2005b), and total CH

- TRU waste volume (1.45 × 10 5 m 3 reported in Leigh et al. (2005a)).

g This value is derived from data in Leigh (2003) and Leigh et al. (2005a). ((1.2 × 10 7 kg) × 39/29 × (1.45 × 10 5 )/168485/1000 kg/ton = 13,888 tons cement).

h This estimate is derived by assuming that Panel 1 has an MgO excess factor of 1.95, three panel equivalents have a 1.67 excess fac tor, and the remaining 6 panel

equivalents have a 1.2 excess factor, resulting in a 1.416 projected excess factor for a full repos itory. The projected excess factor is then multiplied by the equivalent cellulose value of 24,334 × (40.3/27) (the MgO molar ratio).

i This value is derived from Van Soest (2012) and contains CH, RH, packaging, and emplacement materials. j This value is derived from Van Soest (2012) and contains reacted and unreacted cements for both CH and RH wastes.

1 Only materials in the waste are included in these calculations. Other materials such as concrete in shaft seals have not been included because their effects occur prior to final repository closure or shortly thereafter and have no impact on repository performance. 

### VM5 th Response Submittal to the EPA Enclosure 1

26 Similarly, MgO carbonation, which consumes CO 2 , is limited by CO 2 generation from microbial degradation. Given a biodegradation rate constant, the total CO 2 generated per y ea r is proportional to the total quantity of biodegradable materials in the repository. For the LANL drums
, the mass of added desiccant is a small f raction of the total mass of CPR in the repository, and will not have a significant impact on long

- term performance. Using the computational methods in Wang and Brush (1996a and 1996b), the inventory of biodegradable materials has been changed from 23,8 84 (8,120 + 1.7 × 8,120 + 1,960) tons for the CRA

- 2004 2 to 28,098 (8,907 + 1.7 × 10,180 + 1,885) tons of equivalent cellulosics for the CRA

- 2009. 1 For the CRA

- 2014, this value changes to 24,334 (5,127 + 1.7 × 10,487 + 1,379) tons of equivalent cellulosics. This decrease in biodegradable materials corresponds to a proportional decrease in CO 2 generation, all other factors (such as brine saturation) being equal. For MgO carbonation and microbial degradation, t he calculated temperature rises have been updated for the changes in both microbial gas generation and waste inventory and are presented in Table SCR

- 4. Temperature rises ( o C) by exothermic reactions are revised as follows

. CCA conditions following a dril ling event show that Al corrosion could, at most, result in a short

- lived (two y ea rs) temperature increase of about 6 °C (10.8 °F) above ambient room temperature (about 27 °C (80 °F)) (Bennett et al. 1996). A temperature rise of 6 °C (10.8 °F) represented the maximum that could occur as a result of any combination of exothermic reactions occurring simultaneously. Revised maximum temperature rises by exothermic reactions for CRA

- 2014 are still less than 12 oC (22 °F) (as shown in Table SCR

- 4). Such small t emperature changes cannot affect material properties. Thus, thermal effects on material properties in the repository have been eliminated from PA calculations on the basis of low consequence to the performance of the disposal system.

Table SCR

- 4

. CCA and CRA Exothermic Temperature Rises

Reactant CCA a CRA

- 2004 a CRA

- 2009 a CRA

- 2014 a MgO hydration < 4.5 < 4.7 < 4.2 < 3.9 MgO carbonation < 0.6 < 0.7 < 0.6 < 0.6 Microbial degradation < 0.8 < 1.4 < 1.5 < 1.4
Aluminum corrosion < 6.0 < 6.8 < 6.9 < 3.4
Cement hydration < 2.0 < 2.5 < 3.0 < 2.7

a All values are in degrees Celsius.

All potential sources of heat and elevated temperature have been evaluated and found not to produce high enough temperature changes to affect the repository’s performance. Sources of heat within the repository include radioactive decay and exothermic chem ical reactions such as backfill hydration and metal corrosion. The rates of these exothermic reactions are limited by the availability of brine in the repository. In general, the various sources of heat are not great enough to jeopardize the performance of the disposal system.
These changes have been added to Enclosure 4, CRA

- 2014 Errata Tracking

.

2 The 1.7 molar conversion rate for plastic is based on ana lyses presented in Wang and Brush (1996a and 1996 b).5 th Response Submittal to the EPA Enclosure 1

27 EPA Comment 2

- 32

- S22 FEP W73 Concrete Hydration. Please supplement the screening argument with a discussion of the impact on the PA based on a reduced concret e inventory due to DOE now using Option D concrete monoliths in the panel closure system. Update the analysis to include where explosion walls are or will be installed.

DOE Response

The concrete evaluated within the screening argument reflects only that co ncrete intrinsic to WIPP waste, and does not consider concrete hydration due to panel closures or other engineered repository elements. Concrete hydration as it relates to this FEP is solely due to cement (or concrete) within the waste (see for example Ta ble SCR

- 3 in Appendix SCR

- 2014). The reason that concrete hydration from panel closures has not been included in this analysis is because any related thermal rise is considered a short

- term phenomenon that will only exist during the operational period, or at the very earliest part of the post

closure disposal period. Now that concrete will not be used in the panel closure system, concrete hydration from engineered elements of the repository is even less important. The same can be said for any explosion w alls currently emplaced or that will be emplaced in the future. These elements, which consist of concrete blocks with mortar joints, have no effect on the long

- term performance of the repository. What small amount of cement mortar used will have long bee n reacted during the operational period and will not have any thermal effect during the disposal period.5 th Response Submittal to the EPA Enclosure 1

28 EPA Comment 2

- 32

- S24 FEP W111 Panel Closure Chemical Composition. Please update the screening argument to include the chemical composition of the steel bulkheads that are part of the panel closure design.

DOE Response

Steel (iron) bulkheads have been used in each panel for ventilation control and are not considered par t of the engineered panel closure system. Their presence is primarily to control ventilation during construction of the ROMPCS. In cases where a block explosion wall has already been emplaced as part of the partial closure process for some panels, a stee l bulkhead will not be used, as the block wall will serve as the ventilation barrier. In cases where the block walls exist, ROM salt will be placed directly against the wall. In cases where the steel bulkhead is used, the ROM salt will have a safe stand

- off distance so as not to collapse or compromise the steel bulkhead. In either case, these engineered features are not considered long

- term functional elements of the ROMPCS, and are not factored into PA parameters used to represent the closures.

Steel (i ron) bulkheads are used throughout the mine for ventilation controls, and may or may not be removed prior to repository closure. Any additional iron mass associated with steel bulkheads used in construction of the ROMPCS is inconsequential to repository p erformance because modeling predictions show un

- degraded iron at the end of the 10,000

- year performance period (see DOE response to comment 1

- 23

- 5). That is, considering only the iron

- based alloys in the waste and waste containers results in a significant portion of the iron inventory un

- degraded. Therefore, slightly increasing the iron inventory due to additional steel bulkheads or other facility elements will result in even more un

- degraded iron at the end of the performance period. Because of this, an d the fact that the steel bulkhead is not considered a PCS element, this FEP remains unchanged.5 th Response Submittal to the EPA Enclosure 1

29 EPA Comment 2

- 32

- S26 FEP W115 Chemical Degradation of Panel Closures. The screening decision for this FEP was changed from UP (screened in) to SO

- P (screened out

– low probability). Please supplement the screening argument with a discussion of the chemical degradation of the steel bulkheads, which are part of the ROM salt panel closure system. Please also provide technical justification for the changed screenin g decision in light of the presence of the bulkheads.

DOE Response

As explained in the response to 2

- 32

- S24, the steel bulkhead is not considered an engineered element of the ROMPCS and is only important to control ventilation during construction of the closure. Degradation of the steel bulkhead will proceed throughout t he performance period, as it will for all steel in the repository. This has no effect on the long

- term properties of the ROMPCS and is not included in PA. However, degradation of steel within the repository is represented in PA and is a desirable process that serves to maintain a reducing chemical environment
, which in turn supports lower actinide solubilities.5 th Response Submittal to the EPA Enclosure 1

30 EPA Comment 3

- C

- 11. Appendix SOTERM Typographical Errors/Errata. Section SOTERM

- 3.6.1.3, Page SOTERM

- 67, line 3: Section SOTERM

- 3.5.1.1 and Sec tion SOTERM

- 3.5.1.2 should be Section SOTERM

3.6.1.1 and Section SOTERM

- 3.6.1.2

Appendix SOTERM, Section SOTERM

- 6.0 References: The correct date for the following reference is 201 0: Reed, D.T., J.

- F. Lucchini, M. Borkowski, and M.K. Richmann. 2009 2010. Reduction of Higher Valent Plutonium by Iron under Waste Isolation Pilot Plant (WIPP)

- Relevant Conditions: Data Summary and Recommendations. LCO

- ACP

- 09, L ANL \ ACRSP Report. Los Alamos, NM: Los Alamos National Laboratory.
DOE Response
These are typo gr aphical error s in Appendix SOTERM

- 2014

. The cross

- references in Section 3.6.1.3 should be SOTERM

- 3.6.1.1 and SOTERM

- 3.6.1.2
, as the EPA has noted. This sentence in Appendix SOTERM

- 2014 is changed to read:

The overall issue of a thermodynamic driver for higher

- valent Pu oxides, although it has received much recent attention in the literature, is not yet resolved, but has a relatively insignificant impact on the WIPP regardless of the mechanisms at work. A prolonged unsaturated phase in the WIPP could lea d to the formation of some PuO 2+x , but this will be quickly overwhelmed in an aqueous environment and the higher

- valent Pu will be reduced to Pu(III/IV) species, as described in Section SOTERM

- 3.6.1.1 and Section SOTERM

- 3.6.1.2. Both DBR and transport

- rele ase scenarios assume brine inundation and, correspondingly, the rapid introduction of reducing conditions.

Although the plutonium internal report is labeled as a 2009 report, it was final ized in 2010

– so DOE agree s with the EPA that 2010 is the correct da te of reference for the report. The reference for this report is changed to the following:

“ Reed, D.T., J.

- F. Lucchini, M. Borkowski, and M.K. Richmann. 2010. Reduction of Higher Valent Plutonium by Iron under Waste Isolation Pilot Plant (WIPP)

- Relevan t Conditions: Data Summary and Recommendations. LCO

- ACP

- 09, LANL \ ACRSP Report. Los Alamos National Laboratory , Los Alamos, NM

. ”

A copy of th is reference is provided in Enclosure 2.

Also, “Reed et al. 2009” is replaced with “Reed et al

. 2010” in the following locations:

SOTERM vi Figure SOTERM

- 15 caption SOTERM 18 Line 8 SOTERM 36 Line 27 SOTERM 62 Line 14 SOTERM 65 Line 4 (Figure 15 caption)

These typographical error s ha ve been added to Enclosure 4, CRA

- 2014 Errata Tracking

.