ML21062A217

Enclosure - Request for Additional Information Questions

ML21062A217
Person / Time
Site:	PROJ0734
Issue date:	03/01/2021
From:	Stephen Koenick Division of Decommissioning, Uranium Recovery and Waste Programs
To:	Folk J US Dept of Energy, Savannah River Operations Office
Harry Felsher, 301-415-6559
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ML21062A214	List: ML21040A492 ML21062A217
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Enclosure First Set of Request for Additional Information Questions For the U.S. Nuclear Regulatory Commission Technical Review Reports Regarding the 2020 Savannah River Site Saltstone Disposal Facility Performance Assessment INTRODUCTION:

The U.S. Nuclear Regulatory Commission (NRC) staff identified the following Request for Additional Information (RAI) Questions while drafting Technical Review Reports (TRRs) regarding the U.S. Department of Energy (DOE) 2020 Savannah River Site (SRS) Saltstone Disposal Facility (SDF) Performance Assessment (PA).

Each NRC RAI Question falls within one of the following technical topics: (1) Performance Assessment Methods; (2) Saltstone Performance; (3) Infiltration and Erosion Control; (4) Disposal Structure Performance; (5) Far-Field Transport; (6) Inadvertent Human Intruder; (7) Biosphere; (8) Inventory; (9) Site Stability; (10) Selection of Features, Events, and Processes; (11) Conceptual Models and Future Scenario Uncertainty; (12) Near-Field Flow; and (13) Radionuclide Release. Not all technical topics will be in each set of RAI Questions.

Each NRC RAI Question will be identified by its technical topic and the number of the RAI Question in that technical topic. Each RAI Question contains what the NRC staff needs, a Basis, and a Path Forward. The Path Forward provided by the NRC staff is one possible approach to a resolution of the RAI Question. The NRC staff understands that there may be more than one approach to adequately address the technical issue raised in each RAI Question.

Adequate DOE responses to some RAI Questions may depend on the nature of the resolution of other RAI Questions.

This first set of NRC RAI Questions for the TRRs is related to the following four technical topics:

Biosphere, Inventory, Inadvertent Human Intruder, and Site Stability. In addition, the NRC staff has some Clarifying Comments (CCs) about the 2020 SDF PA.

RAI Questions for the Technical Topic of Biosphere (BIO):

BIO-1 The NRC staff needs a technical basis for restricting modeled radionuclide deposition to the leafy fraction of plants to assess projected dose from the plant ingestion pathway.

Basis Equation 4.4-139 in the 2020 SDF PA limits projected deposition of radionuclides onto plants to the leafy portion of the plants, which is modeled as 22 percent (%) of plant mass consumed. It is not clear to the NRC staff why deposition on non-leafy edible plant parts that are exposed to deposition (e.g., fruits, grains, non-leafy vegetables grown above ground) would not also contribute to dose. Intermediate results from the DOE GoldSim models for the Compliance Case (e.g., for Saltstone Disposal Structure (SDS) 9 and SDS 6) for the 2020 SDF PA indicate that the modeled activity of iodine-129 (I-129) deposited onto leaves exceeds the total root uptake of I-129. The same models show technetium-99 (Tc-99) deposition is equal to approximately half of the total root uptake of Tc-99. Therefore, increases in modeled radionuclide deposition on plants is likely to increase modeled dose from the plant ingestion

2 pathway, which is a significant contributor to the projected peak dose for a member of the public1 who uses well water within 10,000 years of site closure.

Path Forward Provide a technical basis for excluding edible non-leafy portions of plants exposed to deposition from the calculation of dose from the plant ingestion pathway. Alternatively, provide revised dose calculations that account for deposition on both leafy and non-leafy edible plant parts that could be exposed to radionuclide deposition.

BIO-2 The NRC staff needs additional information about the development of the soil-to-plant factors used in the 2020 SDF PA to assess projected dose from the plant ingestion pathway.

Basis Section 4.4.8.3.4 of the 2020 SDF PA states that soil-to-plant transfer coefficients were developed based on the wet-weight of plants. That statement is consistent with the implementation in the GoldSim dose model for the 2020 SDF PA, which uses the soil-to-plant transfer factors with wet-weight plant consumption factors to calculate radionuclide intake from the plant ingestion pathway. However, the DOE document that the 2020 SDF PA cites as the reference for the soil-to-plant transfer factors (SRR-CWDA-2013-00058, Rev. 2) does not state whether the listed soil-to-plant transfer factors are on a wet-or dry-weight basis. Instead, that document indicates that when wet-weight values were provided in the source documents, dry-to-wet ratios were applied, which implies that the factors were converted from a wet-weight to a dry-weight basis. The NRC staff was unable to reproduce the soil-to-plant transfer factors listed in SRR-CWDA-2013-00058, Rev. 2 based on the cited reference hierarchy for the soil-to-plant transfer factors, the assumed crop yield percentages in Table 9.2-2 of SRR-CWDA-2013-00058, Rev. 2, and the referenced dry-to-wet ratios in the DOE document PNNL-13421, Rev. 0.

Combining the dry-to-wet ratios for different plant parts from the DOE document PNNL-13421, Rev. 0 with the fractional yields used in the 2020 SDF PA results in an overall dry-to-wet ratio of 0.3. Because the DOE projects that plant ingestion is 19% of the projected dose for a member of the public at 10,000 years after closure in the Compliance Case and this ratio has a linear effect on the projected dose, the dry-to-wet conversion factor to the soil-to-plant transfer factors could impact the overall uncertainty of the projected dose. Therefore, the NRC staff needs to understand the development of the soil-to-plant transfer factors used in the 2020 SDF PA to evaluate the DOE dose projections.

Path Forward Provide illustrative calculations of the soil-to-plant transfer factors for Tc and Iodine listed in Table 4.4-113 of the 2020 SDF PA. The calculations should identify the specific literature sources used (i.e., which documents in the hierarchy provided in SRR-CWDA-2013-00058, Rev. 2 were used) and show any weighting by plant parts or conversions between a wet-and dry-weight basis.

BIO-3 1 The NRC staff will use the DOE dose analyses for a member of the public to evaluate the projected dose to an average member of the critical group. As defined in 10 CFR Part 20, the critical group means the group of individuals reasonably expected to receive the greatest exposure to residual radioactivity for any applicable set of circumstances.

3 The NRC staff needs additional information about the effect of uncertainty in certain transfer factors to evaluate dose projections for both a member of the public and inadvertent human intruder (IHI).

Basis The 2020 SDF PA does not represent the uncertainty in transfer factors because the DOE Fiscal Year (FY) 2014 Special Analysis Document for the SDF did not demonstrate that those factors contributed significantly to uncertainty in dose projections (SRR-CWDA-2013-00058, Rev. 2). However, the relative significance of uncertainty in individual parameters on the uncertainty in dose projections can change when a model changes. For example, the soil-to-plant and feed-to-meat transfer factors were both identified as contributing significantly to the uncertainty in the Tc-99 dose in Sector B in the DOE FY 2013 Special Analysis Document for the SDF. Similarly, the water-to-fish bioconcentration factor was identified as significantly contributing to uncertainty to dose in several sectors in the FY 2013 Special Analysis Document.

Although none of those transfer factors were identified among the top eight contributors to uncertainty in the projected dose for any sector in the FY 2014 Special Analysis Document, their identification in the FY 2013 Special Analysis Document demonstrates that model changes can affect which parameters have the most significant effect on dose. Model changes between the FY 2014 Special Analysis Document and the 2020 SDF PA could have a similar effect.

Furthermore, several parameters that were identified as important to performance in the FY 2014 Special Analysis and the 2020 SDF PA (e.g., infiltration rate, Tc solubility) were modeled differently in the 2020 SDF PA than they were in the FY 2014 Special Analysis Document, increasing the chance that the relative importance of uncertainty in parameters would change. Therefore, parameters that were not identified as one of the top eight contributors to uncertainty in dose projections in the FY 2014 Special Analysis Document could be worth including in the uncertainty analysis in the 2020 SDF PA based on consideration of the uncertainty in the parameter values, the dominant radionuclides, and the major exposure pathways.

As stated in the description of Monitoring Factor 10.08 in the NRC Monitoring Plan for the SDF, Rev. 1, transfer factors typically have significant uncertainty. No additional information related to transfer factors was introduced between the FY 2013 Special Analysis Document, when several transfer factors were identified as significantly affecting the uncertainty in dose, and the 2020 SDF PA. Therefore, it appears that transfer factors related to the dominant radionuclides and main dose pathways could significantly affect uncertainty in the dose projections in the 2020 SDF PA.

The DOE identified water ingestion, plant ingestion, and fish ingestion as the main contributors to dose to the member of the public in the Compliance Case in the 2020 SDF PA. Although calculation of the dose from water ingestion does not involve an environmental transfer factor, calculation of the projected dose from plant ingestion and fish ingestion do involve environmental transfer factors. Therefore, the DOE should evaluate the effect of the uncertainty in transfer factors related to the plant and fish ingestion pathways in the 2020 SDF PA model for Tc-99 and I-129 for the member of the public. Similarly, the DOE identified water ingestion and plant ingestion as major dose pathways for the IHI in the 2020 SDF PA. Therefore, the DOE should evaluate the effect of the uncertainty in transfer factors related to the plant ingestion pathway for Tc-99 and I-129 for the projected dose to the IHI.

Path Forward Provide an analysis of the effect of uncertainty in the soil-to-plant transfer factor for Tc-99 and

4 I-129 on the uncertainty in the projected dose for the member of the public and IHI. Provide an analysis of the effect of uncertainty in the water-to-fish transfer factors for Tc-99 and I-129 on the projected dose for the member of the public.

RAI Questions for the Technical Topic of Inventory (INV):

INV-1 The NRC staff needs additional information about the development of the chemical inventories of Iodine reported in Tables 3.3-8, 3.3-9, and 3.3-10 of the 2020 SDF PA and how they relate to the radiological inventories of I-129 reported in Tables 3.3-5, 3.3-6, and 3.3-7 of the 2020 SDF PA.

Basis The chemical inventories of Iodine reported in Tables 3.3-8, 3.3-9, and 3.3-10 of the 2020 SDF PA are too small to account for the radiological inventories of I-129 reported in Tables 3.3-5, 3.3-6, and 3.3-7 of the 2020 SDF PA. Both the chemical inventories of Iodine and the radiological inventories of I-129 reported in the 2020 SDF PA are shown below in Table INV-1.

Using a specific activity of 6.5x109 Becquerels per kilogram (Bq/kg) (0.18 Curies per kg [Ci/kg])

(10 CFR Part 71, Appendix A), the NRC staff also calculated the mass of Iodine corresponding to the reported radiological inventory of I-129. Based on the values in Table INV-1, it appears that the reported chemical inventories of Iodine are insufficient to account for the radiological inventories of I-129 even if there are no other Iodine isotopes present (i.e., if other isotopes were present the chemical inventories would need to be even greater than shown in the last column of Table INV-1).

Table INV-1: Radiological and Chemical Inventories for the SDF Reported in the 2020 SDF PA Compared to the Chemical Inventory Calculated by the NRC Staff Based on the Reported Radiological Inventories Radiological Inventory Reported in the 2020 SDF PA (Ci)

Chemical Inventory Reported in the 2020 SDF PA (kg)

Chemical Inventory Calculated by the NRC Staff Based on Reported Radiological Inventory, Assuming I-129 is the Only Iodine Isotope Present (kg)

Realistic 15.7 3.58 87.3 MPAD 16.6 3.68 92.4 Pessimistic 24.2 3.71 135 The NRC staff uses radiological inventory of Iodine to assess the projected dose to a member of the public and an IHI. In addition, the NRC staff uses chemical inventory of Iodine to assess the potential effect of stable Iodine on I-129 uptake and dose.

Path Forward Provide the technical basis and any calculations supporting the chemical inventories of Iodine in Tables 3.3-8, 3.3-9, and 3.3-10 of the 2020 SDF PA. Explain the apparent inconsistency

5 between the reported chemical and radiological inventories and provide updated values, if necessary. The projected chemical inventories of Iodine should address contributions of other Iodine isotopes, including stable Iodine, based on available information.

INV-2 The NRC staff needs a justification for the DOE assumption that transfers made after 2015 did not significantly affect tank farm concentrations measured after June 2015. In addition, the NRC staff needs information about the uncertainty that the DOE assumption would contribute to the projected SDF inventory of I-129 at closure.

Basis Section 4.3 of the DOE document SRR-CWDA-2015-00077, Rev. 2 states that Any concentration from June of 2015 or newer was assumed to still be valid, regardless of any transfer activity occurring since that time. However, the DOE did not provide a justification for that assumption. Almost half of the samples (i.e., 20 of the 43) listed in Table 4-3 of that DOE document were taken in or after June 2015. Therefore, the projected inventory of the SDF could be affected by the DOE assumption that those concentrations were valid.

The sensitivity analysis documented in Section 5.8.5.3 of the 2020 SDF PA shows that changes to the I-129 inventory have an approximately linear effect on the projected dose to a member of the public from I-129. Because I-129 is one of the two radionuclides that dominate the projected dose for a member of the public from the SDF, the NRC staff needs information about the uncertainty attributable to the DOE assumption that measured tank farm concentrations taken in or after June 2015 are valid to understand the uncertainty in the projected dose to a member of the public.

Path Forward Provide a justification for the DOE assumption that transfers made after 2015 did not significantly affect tank farm concentrations measured after June 2015. Provide an estimate of the uncertainty that the DOE assumption would contribute to the projected SDF inventory of I-129 at closure.

RAI Questions for the Technical Topic of Inadvertent Human Intruder (IHI):

IHI-1 The NRC staff needs information about the difference between the deterministic projected doses to a chronically exposed IHI as reported in Sections 6.4.1 through 6.4.3 of the 2020 SDF PA and the corresponding deterministic doses projected by the NRC staff with the GoldSim models for the Compliance Case for SDS 9.

Basis The NRC staff was unable to replicate the deterministic dose projections for the IHI chronic exposure scenario as reported in Sections 6.4.1 through 6.4.3 of the 2020 SDF PA. The NRC staff used the Compliance Case model for SDS 9 in the deterministic mode in the configuration provided by the DOE, except for changing the value of the element IntruderInventorySwitch.

In that configuration, the GoldSim model is used only as a dose calculator with inputs from the SDF Aquifer Transport Model implemented with the PORFLOW code. The model results and the corresponding dose projections reported in the 2020 SDF PA are provided in Table IHI-1 below. As stated in the notes in the GoldSim model, the values for the inventory switch element are 0 (no drilling source term), 1 (soil-source term), or 2 (disposal structure-source term).

6 Table IHI-1: Comparison of the Deterministic Projections of the Peak Doses to an IHI in Different Chronic Exposure Scenarios Reported in the 2020 SDF PA with Doses the NRC staff Generated Using the DOE Compliance Case Model for SDS 9 Assumed Time of Intrusion (years after closure)

Reported in the 2020 SDF PA (mrem/yr)

Generated by the NRC Staff Using the DOE SDF GoldSim Dose Calculator with PORFLOW inputs (mrem/yr)

No Drilling Source 100 1.9 0

Soil-Source 100 2.2 86 SDS Source 1,371*

170 1,093

• The value for the degradation of the SDS 9 roof was taken from Table 4.4-45 of the 2020 SDF PA based on the statement in Section 6.4.3 of the 2020 SDF PA that intrusion into a disposal structure was assumed to occur at the conservative estimate of the time of the disposal structure roof degradation.

Path Forward Provide any additional information related to the deterministic model configuration used to calculate the chronic IHI dose projections reported in Sections 6.4.1 through 6.4.3 of the 2020 SDF PA. Alternatively, provide revised deterministic projections of the peak doses to the IHI in the chronic soil drilling and disposal structure drilling exposure scenarios.

IHI-2 The NRC staff needs additional information about the radionuclide contributions to the projected chronic dose to an IHI in the deterministic soil drilling scenario.

Basis Section 6.4.1 of the 2020 SDF PA, which reports deterministic model results for the IHI, states that the peak of the Chronic IHI dose from the soilbased drill cuttings is predominantly from Tc99 and I129. However, the NRC staff could not replicate that result for the IHI with the GoldSim models for the Compliance Case provided by the DOE run in deterministic mode. The NRC staff used the Compliance Case model for SDS 9 in the deterministic mode in the configuration provided by the DOE, except for changing the value of the element IntruderInventorySwitch to 1 (i.e., soil drilling source). In that configuration, the GoldSim model is used only as a dose calculator with PORFLOW inputs. In the NRC staffs model runs, the Dose_IHI_rads element in the IHI_1_m_Boundary container showed the peak projected dose to occur 100 years after SDF closure. The main contributors to that peak were Strontium-90 (Sr-90) (77%), Cesium-137 (Cs-137) (14%), and Tc-99 (5.8%).

Path Forward Provide any additional information related to the model configuration used to calculate the deterministic chronic IHI dose projections reported in Section 6.4.1 of the 2020 SDF PA.

Alternatively, provide a revised projection of the main radionuclide contributors to the deterministic peak projected dose to the IHI in the chronic soil drilling exposure scenario.

7 IHI-3 The NRC staff needs additional information about the calculation of the inventory for the IHI soil-source drilling scenario to assess the projected dose to the IHI.

Basis Section 6.2.1.1 of the 2020 SDF PA states that the inventory for the soil drill cuttings scenarios (both acute and chronic) is based on groundwater concentrations calculated by the Aquifer Transport model with certain adjustments applied. Section 6.2.1.1 continues, These assumed ground water concentrations for each radionuclide were then converted into a soil drill cutting inventory based on the total volume of the drill cutting material. However, additional information is needed to understand how the DOE performed this conversion from the aqueous concentrations to the inventory in the soil. For example, it was not stated whether the soil Kd values in Table 4.3-4 of the 2020 SDF PA were used to convert aqueous concentrations to concentrations sorbed to the soil. If they were not, the inventory of radionuclides with a Kd value greater than 0.625 milliliters per gram (mL/g) (i.e., the reciprocal of a soil density of 1.6 g/mL) could be underestimated because those radionuclides would have more activity per mL of soil than they would per mL of water. Although conservatisms were applied to the water concentrations used (e.g., using the greatest radionuclide concentrations from any location at any time) those conservatisms would not necessarily compensate for the use of aqueous rather than soil concentrations for sorptive radionuclides.

For example, the radionuclide with the greatest projected dose to the chronic IHI in the soil drilling scenario as projected by the DOE GoldSim model run in deterministic mode by the NRC staff (see RAI Question IHI-2) is Sr-90. Table 4.3-4 of the 2020 SDF PA shows Sr-90 Kd values ranging from 5 mL/g for vadose zone or sandy soils to 50 mL/g in leachate-impacted clayey soils. Similarly, the greatest projected dose for the acute IHI in the soil drilling case, as projected by the GoldSim model, is from Cs-137. Table 4.3-4 of the 2020 SDF PA shows a Cs-137 Kd ranging from 10 mL/g in vadose zone or sandy soils to 50 mL/g in backfill or clayey soils. For either of those radionuclides (i.e., Sr-90 or Cs-137), the Kd values would imply that significantly more of each radionuclide would be present on the soil column than in an equal volume of water once equilibrium was reached.

Path Forward Describe how sorption to soils was accounted for in the soil drill cuttings scenario for the IHI or why it was not necessary to account for sorption to soils. Alternatively, provide a revised dose projection for the acute and chronic IHI in the soil drilling exposure scenario based on a revised inventory that accounts for sorption to soil.

IHI-4 The NRC staff needs a technical basis for the most likely garden size and the range of garden sizes used in the probabilistic analysis for the IHI in the chronic exposure scenario.

Basis Sections 6.4.1 and 6.4.3 of the 2020 SDF PA state that in both the soil drilling scenario and disposal structure drilling scenario, the main dose pathway to a chronic IHI is the ingestion of contaminated plants grown in a garden onsite. Garden size is reported to be a key parameter influencing the projected dose to an IHI in Section 6.6.1.3 of the 2020 SDF PA. That analysis varied garden sizes between 100 square meters (m2) (1,080 square feet (ft2)) and 1,000 m2 (10,800 ft2) with a most likely value of 100 m2 (1,080 ft2). However, no technical basis was provided for that range.

8 The SDF GoldSim model calculates the fraction of produce that is locally-grown from the crop yields and garden size to ensure that the garden size is consistent with the modeled consumption of local produce. The most likely garden size in the probabilistic model, 100 m2 (1,080 ft2), corresponds to a fraction of local produce of 0.266, which is very similar to the mean value of the fraction of local produce consumed for households who farm, (0.275) as seen in Table 13-68 of the U.S. Environmental Protection Agency (EPA) 2011 Exposure Factors Handbook. Given that the 100 m2 (1,080 ft2) garden area corresponds to a central tendency of local produce and that no other basis was presented for the range of garden sizes included in the uncertainty analysis, it is not clear to the NRC staff why the 100 m2 (1,080 ft2) garden size is used as the lower bound, rather than the central tendency, of the garden sizes used in the uncertainty analysis.

Although the EPA 2011 Exposures Factors Handbook does not provide a distribution for the fraction of produce consumed that is locally-produced, it does provide related information. For example, Table 13-10 in the EPA 2011 Exposures Factors Handbook shows that the 25th percentile value for the mass of locally-produced vegetables for households who garden (all regions) is 41% of the median value. A similar reduction in the fraction of locally-grown produce consumed would correspond to a proportional reduction in garden size in the SDF model because the relationship between the garden size and the fraction of produce that is grown locally is modeled as linear. In an independent analysis conducted with the DOE GoldSim model for the Compliance Case for SDS 9 run in deterministic mode with a soil drilling source, the NRC staff determined that changing the garden size from 100 m2 (1,080 ft2) to 41 m2 (441 ft2) increased the projected dose to the chronic IHI by 55%.

Path Forward Provide a technical basis for the range of garden sizes used in the probabilistic analysis for the chronic IHI dose, including an explanation of how the range of garden sizes tested accounts for the expected variability in the fraction of produce consumed that is locally-produced.

Alternatively, provide a technical basis for a revised probability distribution for garden size and a revised dose projection for the IHI in the chronic exposure scenario based on that revised garden size distribution.

IHI-5 The NRC staff needs information about the impact of an IHI well on infiltration and radionuclide release.

Basis Section 4.6.9 of the 2020 SDF PA states that the impact of an IHI well drilled near or into a disposal structure was not considered because the soil-only closure cap sensitivity analysis would show the effect. However, the soil-only closure cap sensitivity analysis is not a good indicator of the effect of an IHI well on infiltration and radionuclide release because the soil-only closure cap sensitivity analysis includes performance from an undisturbed lower lateral drainage layer (LLDL) and high-density polyethylene (HDPE)/geosynthetic clay liner (GCL), which would be punctured by an IHI well. Furthermore, a well that intersected a disposal structure would also create a pathway through the disposal structure and puncture the HDPE between the mudmats.

The uncertainty analysis in Section 6.6.3.1 of the 2020 SDF PA identifies infiltration as a key parameter affecting the projected dose for an IHI. Therefore, processes that are expected to affect infiltration, such as penetrating the closure cap, LLDL, and HDPE/GCL layer under the LLDL are expected to affect the projected dose significantly. Disruption of the HDPE between

9 the mudmats also could increase flow from the disposal structures, increasing radionuclide release and thereby increasing the projected dose.

Path Forward Provide revised analyses for the projected IHI dose in the chronic soil-source term and disposal structure-source term drilling cases (i.e., Sections 6.4.1 and 6.4.3 of the 2020 SDF PA) that consider the effects of the IHI well on infiltration and radionuclide release.

RAI Questions for the Technical Topic of Site Stability (SS):

SS-1 The NRC staff needs additional information about how the surface settlement from the 1.5 m (5 ft)-wide soft zones was superimposed to represent a 46 m (150 ft)-wide soft zone.

Basis In the DOE document K-CLC-Z-00026, Rev. 0, the DOE discussed that the surface settlement due to soft zones was computed by superimposing the settlement troughs from multiple 1.5 m (5 ft)-wide soft zones to represent soft zones ranging from 7.6 m (25 ft) to 46 m (150 ft).

Figure 4 of that document showed that the superimposition of additional segments increases the total surface settlement up to a width of 38 m (125 ft). However, each additional 1.5 m (5 ft)-wide segment appears to result in progressively less surface settlement. This result is counterintuitive to the NRC staff. As the width of the soft zone increases, there is expected to be a decrease in the relative amount of overlying material to fill in the underlying consolidated zone. For example, an infinitely long soft zone would not have any adjacent material in the direction of the soft zone to collapse into the underlying consolidated zone.

If the superimposition of soft zone segments results in progressively more settlement, then there could be more surface settlement than assumed in the 2020 SDF PA. This additional settlement could impact the performance of key barriers (e.g., HDPE/GCL, drainage layers) and result in increased infiltration and contaminant release.

Path Forward Provide additional information regarding the details of the calculation of settlement using the superimposition of the individual 1.5 m (5 ft)-wide soft zones.

SS-2 The NRC staff needs additional information about the risk significance of settlement due to compression of the waste bags in SDS 4.

Basis In Section 5.8.7.3 of the 2020 SDF PA, the DOE discussed surface settlement due to compression of waste bags in Cells C and I of SDS 4. As documented in the DOE document K-CLC-Z-00028, Rev. 0, the maximum surface settlement of the closure cap could reasonably vary - from 7.6 cm (3 inches) to 38 cm (15 inches), based on the range of assumed compressibility for the waste bags. The DOE then evaluated the impact of potential settlement by considering an alternative conceptual model with an infiltration rate of 26.9 cm/yr (10.6 inches/year) for Cells C and I. This infiltration rate was based on a fully degraded closure cap from the DOE document WSRC-STI-2008-00244, Rev. 0.

10 The modeled dose results from this sensitivity case are shown in Figure 5.8-75 of the 2020 SDF PA. However, the dose results with the increased infiltration for Cells C and I are not intuitive to the NRC staff. Table 4 of SRR-CWDA-2018-00062 shows the assumed inventories for Tc-99 and I-129 for SDS 4. Relative to the other cells in SDS 4, Cells C and I have a reduced inventory of Tc-99; but, an increased inventory of I-129. The NRC staff would expect to see a more significant dose impact due to settlement and increased infiltration into Cells C and I. This result is unexpected because of the importance of infiltration on contaminant release and the magnitude of increase in infiltration in this sensitivity case, which was more than four orders of magnitude more infiltration through the cover. It is not clear if the assumed increase in infiltration through the cover is still being diverted by underlying layers (e.g., HDPE/GCL, LLDL, SDS 4 roof, clean cap grout). If this sensitivity case contains significant diversion of the infiltration, then that would also be unexpected for NRC staff. A conceptual model of surface settlement due to consolidation of underlying plastic bags would appear to be capable of disrupting the overlying hydraulic barriers such as the HDPE/GCL, LLDL, SDS 4 roof, clean cap grout. Accordingly, the reasons for the projected dose impacts due to increased infiltration of water into Cells C and I and the interaction of that water with the radionuclide inventory in those cells is not clear to the NRC staff.

Path Forward Provide the PORFLOW Vadose Zone Flow Model files and the Vadose Zone Transport Model files for Sr-90, Tc-99, I-129 and Cs-137 for the sensitivity case described in Section 5.8.7.3. of the 2020 SDF PA. If the model results from this sensitivity case indicate that the majority of the water that is assumed to infiltrate through the closure cap is being diverted by other layers (e.g., HDPE/GCL, LLDL, SDS 4 roof, clean cap grout), then provide a technical basis for why the DOE expects these layers to divert water in light of the assumed settlement. Alternatively, provide an analysis for this sensitivity case with the conceptual model where all the overlying layers are impacted by the settlement due to consolidation of the waste bags.

Clarifying Comments (CC) about the 2020 SDF PA from the NRC Staff:

CC-1 Paladium-107 (Pd-107) is included in the inventories in Tables 3.3-5 through 3.3-7 of the 2020 SDF PA; however, it was not included in the SDF GoldSim model. Please explain why Pd-107 was not included in the GoldSim model so that the NRC staff can document the DOE screening process.

CC-2 Table 10.3-1 of the 2020 SDF PA provides a value for the parameter Flocal,FISH and labels the parameter the Fraction of households that fish. However, the recommended value of Flocal,FISH in Table 10.3-1 (i.e., 0.325) corresponds to the 2011 Exposure Factors Handbook fraction of locally-caught fish consumed for households that fish, which is consistent with how the value is used in the GoldSim model. Please verify the DOE description of the Flocal,FISH parameter.

CC-3 Section 6.2.1.1 of the 2020 SDF PA states that groundwater concentrations used in the soil drilling scenario for the IHI were multiplied by a factor of eight to ensure greater defensibility if a well were slightly closer than 1 m from a disposal structure. Please provide the reasoning used in the development of that factor so that the NRC staff can assess the degree of conservatism it introduced.

11 CC-4 Section 7.1.7 of the 2020 SDF PA states The only modeling case that showed IHI doses that exceeded performance objectives relied on unrealistic assumptions and were presented for informational purposes only. The NRC staff could not locate IHI dose projections that exceeded the performance objectives in the 2020 SDF PA. Please direct the NRC staff to the modeling cases being referred to by the DOE.

CC-5 Settlement data from grouted disposal structures provides information about the stability of SRS Z-Area. Please provide the most recent reports on settlement data for the 46 m (150 ft) diameter disposal structures that have been grouted.

CC-6 In the DOE document SRNL-TR-2012-00160, Rev. 0 the DOE discussed that a multi-year soft zone investigation by the Georgia Institute of Technology was underway. Please provide any additional information related to soft zones that was developed since 2012, including any additional insights on the subsurface conditions that can lead to the formation and collapse of soft zones.

References:

U.S. Department of Energy (DOE), PNNL-13421, Rev. 0, A Compendium of Transfer Factors for Agricultural and Animal Products, June 2003. ML101600004

___, WSRC-STI-2008-00244, Rev. 0, Saltstone Disposal Facility Closure Cap Concept and Infiltration Estimates, May, 2008. ML101600430.

___, K-CLC-Z-00026, Rev. 0, Calculation Sheet: Soft Zone Induced Settlements for Saltstone Disposal Unit 6, April 18, 2012. ML20206L064

___, SRNL-TR-2012-00160, Rev. 0, A Review of Subsurface Soft Zones at Savannah River Site with Emphasis on H-Area Tank Farm, July 2012. ML13080A339

___, SRR-CWDA-2013-00062, Rev. 2, Fiscal Year 2013 Special Analysis for the Saltstone Disposal Facility at the Savannah River Site, October 2013. ML14002A069

___, SRR-CWDA-2014-00006, Rev. 2, Fiscal Year 2014 Special Analysis for the Saltstone Disposal Facility at the Savannah River Site, September 2014. ML15097A366

___, K-CLC-Z-00028, Rev. 0, Calculation Sheet: Evaluation on the Stability of Saltstone Disposal Facility Closure Cap System over Saltstone Disposal Unit 4 with Cells C and I Containing Stacked Waste Bags, January 30, 2015. ML20206L066

___, SRR-CWDA-2015-00077, Rev. 2, Evaluation of I-129 Concentration Data to Improve Liquid Waste Inventory Projections, February 2018. ML18170A269

___, SRR-CWDA-2018-00062, Rev. 0, Memorandum: Saltstone Disposal Unit (SDU) 1 Cells A

- C and SDU 4 Cells A - L Inventories in Support of Saltstone Disposal Facility Performance Assessment Modeling, October 10, 2018. ML20206L247

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___, SRR-CWDA-2013-00058, Rev. 2, Dose Calculation Methodology for Liquid Waste Performance Assessments at the Savannah River Site, January 2019. ML20206L207

___, 2020 Savannah River Site Saltstone Disposal Facility Performance Assessment, July 6, 2020. Package: ML20190A055 U.S. Environmental Protection Agency (EPA), EPA-600/R-090/052F, Exposure Factors Handbook - 2011 Edition, September 2011. ML20206K992 U.S. Nuclear Regulatory Commission (NRC), NDAA-Waste Incidental to Reprocessing Monitoring Plan for the Savannah River Site Saltstone Disposal Facility, Rev. 1, September 2013. ML13100A113

___, Acknowledgement Letter for the 2020 Savannah River Site Saltstone Disposal Facility Performance Assessment, July 10, 2020. ML20148M201

___, Preliminary Review Letter for the 2020 Savannah River Site Saltstone Disposal Facility Performance Assessment, October 5, 2020. ML20254A003