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Technical Review: Site Stability for the 2020 Saltstone Disposal Facility Performance Assessment - Ga
	ML23017A114
Person / Time
Site:	PROJ0734
Issue date:	04/18/2023
From:	George Alexander, Hans Arlt, Douglas Mandeville, Christianne Ridge; NRC/NMSS/DDUWP/RTAB
To:	;
	Alexander G, Ridge A
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ML23012A239, ML23090A081	List: ML23017A114;
References
	eConcurrence 20230331-60019
	Download: ML23017A114 (22)
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Enclosure Technical Review: Site Stability at the U.S. Department of Energy Savannah River Site Saltstone Disposal Facility Date:

April 18, 2023 Reviewers:

George Alexander, Risk Analyst, U.S. Nuclear Regulatory Commission Christianne Ridge, Sr. Risk Analyst, U.S. Nuclear Regulatory Commission Doug Mandeville, Sr., Project Manager, U.S. Nuclear Regulatory Commission Hans Arlt, Sr. Risk Analyst, U.S. Nuclear Regulatory Commission

1. Purpose and Scope The purpose of this U.S. Nuclear Regulatory Commission (NRC) Technical Review Report (TRR) is to document the NRC staff review of the U.S. Department of Energy (DOE) assessment of site stability related to the Savannah River Site (SRS) Saltstone Disposal Facility (SDF) through the review of the 2020 SDF Performance Assessment (NRCs Agencywide Documents Access and Management System Accession No. ML20190A056) and related documents. Under the Ronald W. Reagan National Defense Authorization Act for Fiscal Year (FY) 2005 (NDAA), the NRC staff performed this review to support a future decision about whether the DOE has demonstrated that radioactive waste disposal at the SDF complies with the performance objective (PO) entitled Stability of the Disposal Site after Closure in Title 10 of the Code of Federal Regulations (10 CFR) Part 61, Subpart C (i.e., 10 CFR §61.44). The PO in

§61.44, states that:

The disposal facility must be sited, designed, used, operated, and closed to achieve long-term stability of the disposal site and to eliminate to the extent practicable the need for ongoing active maintenance of the disposal site following closure so that only surveillance, monitoring, or minor custodial care are required.

As described in the NRC 2013 Monitoring Plan for the SRS SDF (ML13100A113), in addition to being directly applicable to maintaining compliance with the 61.44 PO, site stability is important in limiting the deep infiltration1 through the SDF, which is important to SDF performance because of the importance of the hydraulic isolation of saltstone. Accordingly, the stability of the SDF is also important to both the 61.41 and 61.42 performance objectives. Therefore, this TRR includes discussion of site stability with respect to the performance of the disposal structures, waste form, and the closure cap in the SDF. Relevant discussions of the effect of site stability on these barriers were also discussed in TRRs entitled Near-Field Flow and Transport (ML23017A086), Composite Barrier Layers and Drainage Layers (ML23017A089), and Cover Percolation and Adjacent Erosion (ML23017A083), and Dose and Exposure Pathways Model (ML23017A113).

This NRC TRR is related to Monitoring Area (MA) 2 (Infiltration and Erosion Control) and MA 9 (Site Stability). In the 2013 Monitoring Plan, the NRC staff identified MA 9 as being related to the PO in §61.44. Based on the analyses in this TRR, the NRC staff has identified both MA 9 and MA 2 as related to the PO in §61.44. Within those two MAs, this TRR addresses Monitoring 1 For consistency with the NRC TRR entitled Cover Percolation and Adjacent Erosion (ML23017A083),

this TRR uses the term deep infiltration to refer to the rate of water flow from the closure cap flowing towards the disposal structures.

Factor (MF) 2.02 (Erosion Control of the SDF Engineered Surface Cover and Adjacent Area),

MF 9.01 (Settlement Due to Increased Overburden), and MF 9.02 (Settlement Due to Dissolution of Calcareous Sediment).

2. Background

In 2012, the NRC staff issued a revised SDF Technical Evaluation Report (TER)

(ML121170309) (referred to as the 2012 TER in this TRR). In the 2012 TER, the NRC staff concluded the following with respect to §61.44, Stability of the Disposal Site after Closure:

Based on the uncertainty in certain risks associated with this performance objective, aspects of site stability will be included in NRC's revised monitoring plan. However, based on the results of the NRCs review, the NRC staff has reasonable assurance that DOEs disposal actions at the SDF meet this performance objective.

The NRC staff concludes the saltstone waste form will provide a monolithic structure, minimize void space, and prevent collapse and differential settlement that could occur due to consolidation of the waste.

NRC staff evaluated the dynamic settlement that would result from an earthquake with a 10,000-year return period and found it was unlikely to cause significant disruption to the SDF.

NRC staff determined that floods are unlikely to disrupt the SDF because the 10,000-year flood level for the Upper Three Runs basin near the SDF is significantly below the lowest planned elevation of a disposal structure at the SDF.

Much of SRS, including the SDF, is underlain by calcareous sediment in the Santee formation resulting in the presence of soft zones. Historically, some of these zones have consolidated, resulting in depressions on the land surface. The DOE concluded that consolidation of a soft zone would have minimal effects on the stability of a disposal structure at the SDF.

Sinks identified elsewhere at SRS are comparable in size to a four-pack of Future Disposal Cells (FDCs). If a sink developed under a four-pack, local deep infiltration could increase and disposal structures could fracture. However, because the history of sink development at the SRS is unclear and the potential development of soft zones under the SDF also is uncertain, the probability a sink would develop at the SDF within 10,000 years of closure is uncertain.

Recent studies predict greater static settlement in the SDF than addressed by the DOE in the PA.

In general, the NRC staff found more uncertainty in the potential effects of static settlement due to loading of the subsurface layers and settlement due to calcareous zones present or potentially developing under the SDF than that resulting from the potential effects of earthquakes, floods, and erosion at the SDF.

In the 2018 TRR entitled Hydraulic performance and Erosion Control of the Planned Saltstone Disposal Facility Closure Cap and Adjacent Area (ML18002A545), the NRC staff reviewed information related to MA 2 (Infiltration and Erosion Control), which is relevant to compliance with the 61.44 PO. Conclusions from that TRR related to erosion control included:

A preliminary evaluation of rock sources should be conducted to provide confidence that an acceptable rock source is available.

Pore-pressure build-up in the overlying closure cap layers could affect cover stability, vegetation, hydraulic performance of cover materials, and erosion. If an analysis containing more detailed simulations determined that the buildup of hydraulic head is realistic, an explicit evaluation of the physical stability of cover materials under this condition would be needed.

An evaluation of the cumulative effects from precipitation events over long time periods with respect to gully formation was needed to support predictions of long-term performance of the topsoil and vegetative layers. In addition, the stability of a degraded vegetative cover should be evaluated because the Bahia grass, bamboo, or pine forest could be degraded by fire or extended drought, which could adversely affect the ability of the vegetative and topsoil layers to resist erosion.

NUREG/CR-7028, Volume 1 (ML12005A110) and other studies have shown that compacted soil materials used in cover materials at the sites studied did not retain as built properties over periods of regulatory interest, which means that additional justification may be needed to support values representing continual compaction.

Subflow from the closure cap drainage layer flowing through the side slope riprap could significantly increase the water content of the backfill beneath the side slope and lead to slumping. Because saturated conditions at the surface would increase concerns of geotechnical stability, an understanding of moisture content for each layer over time is needed. If saturated conditions exist, an evaluation of the physical stability of the cover materials also would be needed.

Compared to an erosion barrier filled with sandy soil, an erosion barrier filled with controlled low strength material (CLSM) would increase the chance of lateral flow occurring on top of the erosion barrier thereby potentially decreasing the stability of the closure cap. Concerns would include the stability of the outer edges of the upper backfill layer as the backfill material may be carried out by the lateral flow into the side slopes. In addition, higher moisture contents or saturation levels may deter growth of the loblolly pine and promote vegetation more acclimated to moister soils.

The NRC staff also identified concerns related to the U.S. Department of Agriculture (USDA) Revised Universal Soil Loss Equation (RUSLE).

Much of SRS, including the SDF, is underlain by calcareous sediment that has resulted in the presence of under-consolidated soft zones in the Santee formation (WSRC-TR-99-4083 (ML13079A139)). In conjunction with those soft zones, layers of hardened sediment are commonly observed. The DOE has characterized those layers as bridges or arches in a honeycomb-like structure that acts to redistribute stresses. Historically, some of the soft zones have consolidated, resulting in depressions on the land surface. Those depressions, or sinks, at SRS typically are 3 to 5 meters (m) [10 to 15 feet (ft)] deep and, in F-Area, up to 300 m [1,000 ft] long (WSRC-TR-2007-00283 (ML101600572)). A stereoscopic examination of aerial photography of the SRS illustrated the potential presence of several basins and sinks located in and near the SDF (US Army Corps of Engineers, 1952; Figure A8). However, a subsequent investigation in 1986 stated that no evidence of ground subsidence was observed in the vicinity of where SDF is now located (87814-PT1).

Under MA 9 (Site Stability) in the 2013 SDF Monitoring Plan, the NRC staff developed two monitoring factors. Under MF 9.01 (Settlement Due to Increased Overburden), the NRC staff discussed monitoring settlement due to increased overburden because of the potential for increased settlement to increase deep infiltration into the site. Under MF 9.02 (Settlement Due to Dissolution of Calcareous Sediment), the NRC staff discussed monitoring the DOEs development of additional information related to the dissolution of calcareous sediment. That information included: (1) support for the DOE conclusion that the potential for sink formation is limited; and (2) information to determine whether reasonably projected future dissolution of calcareous sediment was significant to site stability.

3. Site Stability in the 2020 SDF PA 3.1. Overview The following information is adapted from the 2012 TER and updated with revised information from the 2020 SDF PA.

The DOE SRS is located on the Atlantic Coastal Plain within the Aiken Plateau. This region has a relatively low seismic activity (WSRC-IM-2004-00008 (ML101600352)). The largest known earthquake in the vicinity of the site occurred in Charleston, South Carolina in 1886 with a magnitude of 7.3 on the Richter Scale (USGS, 2011). In addition, the SDF is sited on a well-drained topographic high. The 100,000-year flood level for the Upper Three Runs basin near the SDF is approximately 49 m (160 ft) above mean sea-level (MSL) which is significantly below the lowest planned elevation of a disposal structure at the SDF, which is 79 m (260 ft) above mean MSL. The projected flood levels were also below the lowest elevation of the planned lower closure cap foundation layer, at the bottom of the side slope, at approximately 78 m (257 ft) above MSL.

The use of grout to stabilize salt waste at the SDF is designed to provide a monolithic structure, minimize void space, and prevent collapse. Following the closure of all of the SDF disposal structures, the DOE anticipates that two closure caps will be constructed over the SDF disposal structures, as shown in Figure 3.2-29 of the 2020 SDF PA. The DOE also plans to begin a 100-year period of institutional controls after completing the closure cap. During that period, active maintenance of the SDF will include repairs of any significant erosion or closure cap defects and to prevent pine forest succession. No active SDF maintenance is assumed to be conducted beyond the institutional control period. The closure cap will be designed to provide a minimum of 3 m (10 ft) of clean material above the SDF disposal structures for the protection of inadvertent intruders. The DOE SRS End State Vision includes ownership and control of the entire site by the federal government in perpetuity and prohibits residential use of the site.

3.2. Erosion of the Closure Cap In the DOE document SRR-CWDA-2021-00036 (ML21160A063), the DOE provided updated information regarding erosion of the closure cap and land in the vicinity of the SRS Z-Area.

Information regarding specific erosion mechanisms are provided in the following sections.

3.2.1. Gully Erosion The DOE evaluated the potential for gullying based on the flow velocity during a probable maximum precipitation (PMP) event. If the actual flow velocity, Va, is less than the permissible velocity, Vp, over a given surface with a defined slope, then the slope is considered stable with respect to gullying. The DOE provided details regarding the calculations of the actual flow velocity in Section 6.2.1 of SRR-CWDA-2021-00036. As shown in Table 3.5-9, the DOE recommended a 15-minute PMP of 6.9 cm (2.7 in) and 8.4 cm (3.3 in) for return periods of 1,000 years and 10,000 years, respectively. The DOE analyses shown in Tables 7.1-1 through 7.1-4 indicated that gully erosion of the SDF Closure Cap Top Surfaces and Side Slopes will not occur using either the 1,000 -year or 10,000-year PMP return period.

3.2.2. Sheet and Rill Erosion The DOE calculated the potential for sheet and rill erosion based on the USDA RUSLE. The RUSLE approach estimates average annual soil loos due to erosion based on climatic erosivity, soil erodibility, slope, slope length, cover management, and support practices. The DOE recommended values were provided in Table 6.3-3 of SRR-CWDA-2021-00036. In Section 7.2, the DOE estimated the depth of erosion of the SDF side slopes could range from 41 centimeters (cm) (16 inches (in)) to 180 cm (70 in) within 1,000 years and up to depths of almost 3 m (10 ft) within 10,000 years. The DOE provided two lines of support for the estimated sheet and rill erosion. First, the DOE stated that they conservatively selected many of the assumed parameter values for greater defensibility. Second, the DOE expected that the erosion barrier of the closure cap, which will be present at depths of 91 cm (36 in) to 122 cm (48 in), will provide additional resistance to erosion. However, the erosion barrier was not taken into account in the RUSLE evaluation; therefore, the DOE expects the result of the RUSLE to be conservative.

Despite the two lines of evidence that the DOE provided to support the conservatism of the analysis, the DOE noted that the analysis identified topics where additional studies may be needed to reduce uncertainties or where further design improvements may be needed before establishing the final SDF closure cap design.

3.2.3. Slope Stability For the SDF, the DOE relied on a slope stability analysis that was performed for the SRS Tank Farms (K-CLC-G-00111 (ML21196A050)), based on similarities in the material layers, side slope gradient, side slope features, and similar environmental conditions (e.g., rainfall, subsurface soil properties, seismic stresses). The authors considered a series of cases in response to various seismic stresses with static and seismic loading conditions. A peak horizontal ground acceleration (PGA) of 0.20 g (meters per square second) was assumed, in accordance with an American Society of Civil Engineers standard. The analysis also included cases that considered soil parameters for saturated soils. That is because of the likelihood that pore pressures would not have adequate time to dissipate, resulting in an undrained loading condition. For each case, the authors calculated factors of safety by dividing the forces resisting movement by the forces driving movement.

For the SRS F-Tank Farm, all of the cases met the minimum factor of safety for both the static and seismic loading condition, including the case with saturated soil parameters. For the SRS H-Tank Farm, most of the cases met the minimum factor of safety with the exception of Case 9a. Case 9a included a PGA of 0.20 g with saturated soil conditions. In response to the Case 9a results, the authors revised the PGA with less conservative values of 0.16 g and 0.10 g for Cases 9b and 9c, respectively. The results from those less conservative analysis met the minimum factor of safety.

Lastly, the authors of K-CLC-G-00111 stated that the final design for the cover system had not been completed and that the stability of the interface between the geosynthetics and engineered soil layers will require evaluation during the final design of the cover system.

3.3. Settlement In the 2020 SDF PA, the DOE discussed that settlement could occur due to either static or dynamic loads. Prior to the construction of each SDF disposal structure, the DOE conducted geotechnical investigations to evaluate settlement due to static loading (i.e., presence of disposal structures, saltstone grout, closure cap) and the dynamic settlement due to potential liquefaction from earthquakes and the settlement of soft zones. As part of the update to the 2009 SDF PA, the DOE calculated settlement projections for Saltstone Disposal Structure (SDS) 6 and SDS 7 in the 2020 SDF PA, as shown in Table 1 below.

Table 1. Projected Average Static and Dynamic Settlement Values for SDS 6 and SDS 7 (adapted from K-ESR-Z-00005 and K-ESR-Z-00008)

SDS 6 SDS 7 (inches) (a)

(inches) (a)

Average Static Settlement Heave During Excavation 1

0.4 Operations Complete 4

2 Closure Cap Complete 7

3.5 30 Years After Closure 8

5 Average Dynamic Settlement Liquefaction 0.5 0.75 Soft Zone Settlement 0.5 0.5 (a) To convert inches to centimeters, multiply by 2.54 cm/inch.

3.3.1. Static-Loading Induced Settlement In the 2020 SDF PA, the DOE discussed that uniform static settlement of the SDF disposal structures was likely to occur due to the loading from the closure cap (see Table 1 above). In addition to the uniform static settlement, the DOE estimated differential static settlement for SDS 6 to be 8.2 cm (3.2 in) across a distance of 34 m (110 ft) (K-ESR-Z-00005 (ML16106A258)). For SDS 7, the DOE estimated differential static settlement of 1.3 cm (0.5 in) across a distance of 58.2 m (188 ft) (K-ESR-Z-00008 (ML20206L078)).

In contrast with the results of the DOE documents K-ESR-Z-00005 and K-ESR-Z-00008, in the 2020 SDF PA, the DOE concluded that settlement would be expected to occur uniformly over the entire cap area and differential settlement would be negligible, assuming the subsurface conditions are relatively uniform. To address potential impacts from differential settlement on the closure cap, the DOE concluded in Section 5.8.2.4 of the 2020 SDF PA that a soil-only closure cap case provides a bounding approximation.

To address potential impacts from static and dynamic settlement on the SDF disposal structures and saltstone grout, the DOE evaluated a fast flow paths sensitivity case in Section 5.8.8.2 of the 2020 SDF PA. That sensitivity analysis modeled the effects of 5-cm (2-in) cracks spaced at approximately 1 m (3 ft) intervals in the 375-ft disposal structures. The DOE used two sub-cases to model fast flow paths that proceeded either partially or entirely through the SDF disposal structure roofs, the saltstone wasteform, the disposal structure floors, and the mud mats. The cases did not alter the modeled properties of the engineered barriers above the SDF disposal structure roofs. The dose projections from this sensitivity case were approximately 15 percent

(%) and 20% greater than the Compliance Case for the partially-and fully-penetrating fast pathway cases, respectively, at 10,000 years after SDF closure.

3.3.2. Seismic-Induced Liquefaction and Settlement For the 2020 SDF PA, the DOE evaluated seismic-induced liquefaction and settlement for SDS 6 and SDS 7, as shown above in Table 1. In Section 3.1.4.4 of the 2020 SDF PA, the DOE discussed the effects of seismic events on the SDF. The DOE evaluated seismic-induced degradation mechanisms in the design of the closure cap, which is discussed in Sections 3.2.3 and 4.2.3 in this TRR. As discussed above, the DOE conducted a fast flow paths sensitivity case to evaluate the impacts of static and dynamic settlement on the disposal structures and saltstone grout.

3.3.3. Soft Zones In Section 3.1.4.5 of the 2020 SDF PA, the DOE discussed their evaluation of soft zones in the SRS Z-Area. The DOE conducted geotechnical subsurface investigations for SDS 6 and SDS 7, as described in DOE documents K-ESR-Z-00005 and K-ESR-Z-00008, respectively. For SDS 6, soft zones were identified in three cone penetrometer tests (CPTs). Two CPTs had a total soft zone thickness of 0.46 m (1.5 ft) and the third CPT had a thickness of 1.8 m (5.8 ft). Using consolidation theory, the DOE calculated surface settlements for wide areas underlain by soft zones.

The maximum soft zone-induced settlements for SDS 6 and SDS 7 were calculated to be 1.3 cm (0.5 in) and the maximum differential settlement was also 1.3 cm (0.5 in) (K-ESR-Z-00005 and, K-ESR-Z-00008). In addition to the geotechnical analyses for SDS 6 and SDS 7, the DOE discussed in Section 4.6.4 of the 2020 SDF PA that the conclusions from the H-Tank Farm PA were relevant to the SDF based on the proximity of Z-Area to H-Area. In the H-Tank Farm review, DOE concluded that:

Soft zones beneath SRS are not cavernous voids, but are small, isolated, poorly connected, threedimensional features filled with loose, finegrained, watersaturated sediment.

Despite their underconsolidated nature, soft zones have survived for a very long time and remain structurally competent in the presence of overburden stresses.

Soft zones appear not to be a critical influence on either ground water flow or contaminant transport.

3.3.4. Settlement due to Waste Bags in SDS 4 In Section 5.8.7.3 of the 2020 SDF PA, the DOE evaluated an alternative waste disposal and closure configuration for waste bags placed into Cells C and I in SDS 4. The waste bags consist of grout and pieces of plastic and are in a pile 4 to 4.6 m (13 to 15 ft) tall and approximately 3.7 m (12 ft) in radius. The Compliance Case in the 2020 SDF PA assumed that the weight of the clean cap grout will compress these waste bags to minimize void space and prevent collapse or settlement. Accordingly, the Compliance Case assumed that the waste bags will not impact SDF performance. However, DOE did conduct an engineering evaluation to determine if the waste bags could impact SDF performance to risk inform the PA. That engineering evaluation concluded that compression of the waste bags could result in settlement of the closure cap in the range of 7.6 to 38 cm (3 to 15 in), which could direct water into Cells C and I. The DOE then evaluated the dose to a member of the public at the 100-m SDF boundary due to increased deep infiltration into Cells C and I. The other SDS 4 cells were modeled using the Pessimistic Case assumptions. The DOE concluded that although the projected peak dose is higher in this sensitivity analysis than the Compliance Case (i.e., almost 0.04 millisievert (mSv)/yr [4 millirem

{mrem/yr}] compared to 0.012 mSv/yr [1.2 mrem/yr]), it is below the 0.25 mSv/yr (25 mrem/yr) performance objective for an offsite member of the public. In addition, the DOE stated that the assumed deep infiltration in this sensitivity case was extremely high and bounding (i.e., 269 mm/yr [10.6 in/yr]) and that potential impacts would likely be minimal with peak doses intermediate between this case and the Compliance Case.

4. NRC Evaluation 4.1. Overview The NRC staff reviewed the DOE site stability analyses and conclusions included with the 2020 SDF PA and related documents in support of the NRC staffs assessment of compliance with the 61.44 PO. The NRC staffs site stability review included assessment of static and dynamic settlement, including the presence of waste bags in SDS 4, floods, gully erosion, sheet and rill erosion, and slope stability.

As discussed in the 2012 TER, the NRC staff agrees that stabilizing salt waste with grout and completely filling the SDF disposal structures will provide a monolithic structure, minimize void space, and prevent collapse and differential settlement that could occur due to waste consolidation. However, in Section 5.8.7.3 of the 2020 SDF PA, the DOE discussed an alternate waste disposal and closure configuration regarding piles of several hundred plastic bags containing contaminated grout chips in Cells C and I of SDS 4. Additional discussion regarding that analysis is provided in Section 4.3 below.

In the 2012 TER, the NRC staff determined that flooding is unlikely to disrupt the SDF, because the 100,000-year flood level for the Upper Three Runs basin near the SDF is below the lowest planned elevation of a disposal structure at the SDF. The NRC staff did not find any basis for changing that assessment in the 2020 SDF PA or supporting documents. Therefore, the NRC staff determined the previous staff determination that flooding is unlikely to disrupt the SDF remains valid.

Additional discussion regarding erosion and settlement are provided in Sections 4.2 and 4.3, below.

4.2. Erosion of the Closure Cap The NRC staff did not find the DOE analysis of erosion of the closure cap allowed the NRC staff to make a determination about site stability because there are risk-significant aspects of the design and implementation plan that the DOE has indicated are not complete. In addition, the NRC staff identified aspects of the DOE erosion analyses that use unsupported parameters or do not appear to account for saturated conditions in the closure cap and other materials above the disposal structures due to enhanced deep infiltration reduction by the closure cap. This TRR addresses those issues in Sections 4.2.1 through 4.2.6 below.

In the 2020 SDF PA, the DOE revised the closure cap conceptual model with a significant reduction in the projected deep infiltration. The reduced deep infiltration results in the closure cap being saturated with hydraulic head building up in the closure cap layers on top of the geomembrane and increasing runoff (SRR-CWDA-2021-00072 (ML21148A005)). The potential for saturation of the layers above the upper HDPE geomembrane significantly impacts the risk of erosion and slope stability. The effects of saturation on erosion and slope stability calculations for the closure cap top surface and side slopes are discussed in the following subsections.

The closure cap consists of a series of layers that work in conjunction with each other and the performance of which affects the other layers. For example, the low permeability of the High-Density Polyethylene (HDPE)/Geosynthetic Clay Liner (GCL) (HDPE/GCL) composite layer that limits deeper infiltration results in the buildup of hydraulic head in the upper sand drainage layer up to the topsoil. That creates saturated conditions that could affect plant speciation on top of the cover and result in overland flow, both of which can impact erosion rates. The buildup of head also affects slope stability and could impact the performance of the sand drainage layer.

Based on the risk significance and uncertainty in the performance of the closure cap discussed below, the NRC staff needs additional confidence in: (1) Gully Erosion, (2) Sheet and Rill Erosion, (3) Slope Stability, and (4) Sand Entrainment. NRC staff notes that one way to significantly reduce uncertainty and increase confidence in model projections would be to construct and monitor a test cover. The DOE has built test covers at the Hanford site and uranium mill tailing sites for those same reasons.

In the Fourth Set of Request for Additional Information (RAI) Questions to the DOE, the NRC staff requested additional information in RAI Comment CM&FSU-3 about potentially risk-significant discrete Features, Events, and Processes (FEPs) (i.e., closure caps erosion barrier, the tan clay confining zone, erosion types) that were not included the DOE FEPs list for the 2020 SDF PA and, therefore, were not part of the scenario and conceptual model development.

In response to that RAI Comment (SRR-CWDA-2022-00016 (ML22118A297)), the DOE indicated that the 2020 SDF PA represented the effects of erosion were represented with the soil-only sensitivity analysis, which represents the closure cap as if it did not have an erosion barrier or any other engineered features. In Section 4.2.5 below, the NRC staff discusses the DOEs soil-only sensitivity analysis. The DOE also discussed that the impacts of potential erosion have been provided in other documents (see DOE documents SRR-CWDA-2021-00036, SRR-CWDA-2021-00040, and SRR-CWDA-2021-00066). The NRC staffs review of that information is provided below.

4.2.1. Gully Erosion The DOE evaluation for potential gully erosion was provided in SRR-CWDA-2021-00036. The DOE determined that gully erosion would not occur on the Closure Cap top surfaces or side slopes. However, the NRC staff determined that the DOE analysis of gully erosion did not allow the NRC staff to make a determination about site stability for three reasons below.

First, the DOE erosion calculations were based, in part, on the PMP. In the 2020 SDF PA, the DOE revised the PMP from the 2009 SDF PA values. For example, the 15-minute PMP was 24.6 cm (9.7 in) in the 2009 SDF PA versus 6.9 cm (2.7 in) in the 2020 SDF PA. That is a significant change and inconsistent with industry standards and the NRC guidance in NUREG/KM-0015 (ML21245A418). For example, the Vogtle Nuclear Power Plant, near the SRS, assumed a 15-min PMP of 24.9 cm (9.8 in). Additional information is provided in NUREG/KM-0015. The NRC staff recalculation of the actual flow velocity with a PMP consistent with the NRC guidance and the 2009 SDF PA2 indicates that gully erosion could occur on the SDF Closure Cap top surfaces and side slopes.

Second, the DOE calculation of the PMP did not factor in climate change. In response to NRC Comment IEC-1 (SRMC-CWDA-2022-00016 (ML22118A297)), the DOE discussed that the PMP estimates already account for uncertainty and the potential influences of climate change.

However, recent studies, which are discussed in Chapter 10 of NUREG/KM-0015, provide evidence that the magnitude of extreme storms has increased with increasing air temperatures and air moisture holding capacities. Accordingly, the NRC staff concluded in NUREG/KM-0015 that future site-specific PMP studies should account for the effects of climate change.

Third, there are several parameters in the erosion formula in Equation 6-3 of the DOE document SRR-CWDA-2021-00072 that are related to the runoff coefficient that could be affected by saturated conditions, including soil infiltration, vegetative cover, and surface storage. In response to the NRC RAI Comment IEC-1 (SRR-CWDA-2021-00072), the DOE projected hydraulic head buildup on top of the geomembrane layer with saturated overlying soils and estimated the maximum depth of water for overland surface flow to be 6.5 cm (2.6 in). The NRC staff expects that hydraulic head buildup could affect the likelihood of gully erosion of the closure cap. Therefore:

SIST Gullying of the Closure Cap The NRC staff recommends opening a medium-priority monitoring factor for the development of the closure cap design to verify that gullying will not adversely affect SDF performance under a new monitoring factor related to closure cap erosion in MA 9 (Site Stability). The NRC staffs review of information related to gully erosion will include:

the assumed PMP, the effects of climate change, and the effects of saturated conditions on gully erosion calculations.

4.2.2. Sheet and Rill Erosion The DOE used the USDA RUSLE approach to calculate average annual soil loss. The NRC staff determined that this approach is acceptable because it is consistent with best practices for projecting average annual soil loss. However, the NRC staff is concerned that the DOE RUSLE analysis could underpredict erosion because the 2020 SDF PA assumed significantly less deep 2 For a calculated time of concentration of approximately 6 minutes from the DOE analysis, the NRC staff used a 6-min PMP of 11.4 cm (4.5 in) based on a 15 min PMP of 24.6 cm (9.7 in) in their analysis.

infiltration than the NRC staff expects could occur. Consequently, the DOE analysis predicted significant head buildup (i.e., saturation) above the HDPE/GCL composite barrier (SRR-CWDA-2021-00072). The occurrence of saturated conditions could affect several of the RUSLE parameters, including soil erodibility and cover management (e.g., plant growth). In the 2020 SDF PA, the DOE indicated that the closure cap design and implementation plans have not yet been finalized. Therefore:

SIST Sheet and Rill Erosion of the Closure Cap The NRC staff recommends opening a medium-priority monitoring factor for the development of the closure cap design to verify that soil loss will not adversely affect SDF performance under a new monitoring factor related to closure cap erosion in MA 9 (Site Stability).

4.2.3. Slope Stability The NRC staff did not find that the DOE slope stability analysis allowed the NRC staff to make a determination about site stability because the DOE analysis used an unsupported value of the PGA and did not address the effects of cover saturation.

In the 2020 SDF PA, the DOE relied on a slope stability analysis from the SRS Tank Farms (K-CLC-G-00111). The authors considered a series of cases with a PGA up to 0.20 g and included soil parameters for saturated soils in several cases. For the H-Tank Farm, the case with a PGA of 0.20 g and saturated soil parameters, the slope stability analysis did not meet the minimum factor of safety (i.e., the slope was projected to be unstable). The DOE concluded that the slope stability analysis could meet the minimum factor of safety by assuming less conservative parameters (e.g., lower PGA values). However, the authors also noted that the final design for the covers system had not been completed and the stability of the interface between the geosynthetics and engineered soils layers will require evaluation. The NRC staff is concerned that a case assuming a horizontal PGA of 0.20 g and saturated soil conditions did not meet the slope stability requirements for several reasons:

A PGA of 0.20 g is consistent with previously assumed values for the SDF (SRR-CWDA-2009-00017 (ML101590008)).

The DOE did not evaluate the stability of the interface between the geosynthetics and engineered soils layers.

The DOE document K-CLC-G-00111 analyzed saturated soil conditions for F-Tank Farm and H-Tank Farm because of the potential that pore pressures will not be able to dissipate during cyclic loading. However, the DOE projections for the preliminary design (SRR-CWDA-2021-00072) showed that saturated conditions are also likely to exist in the SDF closure cap with buildup of hydraulic head on top of the geosynthetic membrane even without cyclic loading.

The DOE used Equation 4.4-3 in the 2020 SDF PA in conjunction with Equation 4.4-4 to calculate leakage through the composite liner system. However, the 2020 SDF PA did not describe what the depth (or head) of water is anticipated to be present on the geomembrane. In response to the NRC RAI Comment IEC-1 (SRR-CWDA-2021-00072), the DOE estimated that the theoretical maximum head would extend up from the HDPE geomembrane to the surface, which would range from 7.4 m (24.2 ft) before erosion to 6.5 m (21.2 ft) after erosion. The NRC staff is concerned that projections of site stability for the 2009 SDF PA are not applicable to the 2020 SDF PA because of large differences in the assumed deep infiltration through the cover, in addition to changes in the assumed PGA. Specifically, regarding changes in the assumed deep infiltration through the cover, changes from the 2009 SDF PA to the 2020 SDF PA (i.e., less projected deep infiltration in the 2020 SDF PA) could result in more head buildup on the geomembrane than what was estimated in the 2009 SDF PA and the change in slope from 1%

to 3% would increase the potential for localized slope instability.

In addition, the NRC staff previously identified concerns about the potential for slumping of the closure cap (i.e., mass wasting of a coherent, saturated mass of loosely consolidated materials or a rock layer moving down slope) (ML18002A545). The 3%-sloped upper backfill/erosion barrier or the 4%-sloped ULDL/HDPE layer are potential interfaces of concern. In the January 2018 TRR entitled Hydraulic Performance and Erosion Control of the Planned Saltstone Disposal Facility Closure Cap and Adjacent Area (ML18002A545), the NRC staff also stated:

subflow from the closure cap drainage layer flowing through the side slope riprap could significantly increase the water content of the backfill beneath the side slope and lead to slumping (ADAMS Accession No. ML121170309). Because saturated conditions at the surface would increase concerns of geotechnical stability, an understanding of moisture content for each layer over time is needed.

If saturated conditions exist, an evaluation of the physical stability of the cover materials also would be needed.

In the 2020 SDF PA, the DOE indicated that the closure cap design and implementation plans have not yet been finalized. Because of the preliminary design and the reasons provided in this section:

SIST Slope Stability of the SDF Closure Cap The NRC staff recommends opening a high-priority monitoring factor for the development of a realistic slope stability analysis of the SDF closure cap to provide confidence that a relatively impermeable closure cap in a humid environment under saturated conditions can remain stable for the 10,000-year Performance Period under a new monitoring factor in MA 9 (Site Stability). The NRC staffs review of information related to slope stability, will include: the assumed PGA, the stability of the interface between the geosynthetics and engineered soils layers, and the effects of saturated conditions on slope stability.

4.2.4. Flow through the ULDL The NRC staff did not find the DOE analysis of the sand drainage layers allowed the NRC staff to make a determination about future SDF performance because the DOE analysis did not address the effects of enhanced flow through the drainage layers due to closure cap saturation.

Due to the DOE assumed reduction in leakage through the composite barrier, the DOE projected infiltrated water to be retained on the geomembrane and ultimately drained to the side slopes via the upper lateral sand drainage layer. In the NRC RAI Comment IEC-2 (ML21133A293), the NRC staff noted that the large quantities of water the DOE projected to be conveyed through this layer could result in greater-than-anticipated erosion of the side slopes.

In addition, significant flow through the upper lateral sand drainage layer (ULDL) could impact the performance of the drainage layer. The NRC staff expects that, if the flows through the sand drainage layer are significant enough to entrain, mobilize, and potentially remove sand particles over time, then the long-term performance of the sand drainage layer could become compromised (e.g., settlement of overlying sediment into voids in the sand drainage layer could result in a decrease in hydraulic conductivity).

In response to the NRC RAI Question IEC-2 (see DOE document SRR-CWDA-2021-00072),

the DOE discussed that the projected flow rates through the ULDL would not impact the erosion barrier without channeling and focused flow, which the DOE does not anticipate. In the DOE analysis, the projected flow rates were five orders of magnitude below the maximum permissible velocity. The NRC staff notes that the DOE analysis did not appear to account for hydraulic head in this evaluation and there is potential for channeling and focused flow. It is not clear to the NRC staff if hydraulic head and channeling result in flow rates in excess of the maximum permissible velocity. Also, the DOE did not discuss whether the flows through the ULDL could mobilize the sand in the ULDL. Because of the DOE assumed performance of the ULDL:

SIST Flow through the ULDL The NRC staff recommends opening a high-priority monitoring factor for information related to potential degradation of the erosion barrier and loss of sand from the ULDL due to flow and entrainment as part of a new monitoring factor in MA 9 (Site Stability).

The information the NRC staff will monitor will include the effects of saturation and hydraulic head on flow rates in the sand drainage layer.

The NRC staff also notes that the 400 millimeter/year (mm/yr) (16 in/yr) and 650 mm/yr (26 in/yr) are the DOE estimates for percolation reaching the upper lateral sand drainage layer and not annual rainfall, as the DOE indicated in Table 4.4-3 in the 2020 SDF PA. In other words, the conservatism described by the DOE in their response for evapotranspiration and runoff in SRR-CWDA-2021-00072 have already been taken into account in the values above.

4.2.5. Degradation of the Erosion Barrier The NRC staff did not find the DOE analysis of the erosion barrier allowed the NRC staff to make a determination about site stability because the DOE indicated that the DOE had not yet made the design choice for a material to fill voids in the erosion barrier, which impacts barrier erodibility. In addition, the NRC staff determined it could not base a determination on the results of the DOE non-mechanistic sensitivity analyses that the DOE provided to bound the effects of erosion barrier degradation because the potentially optimistic assumptions regarding the lower lateral drainage layer, HDPE, and GCL.

In response to RAI Question CM&FSU-3 (SRMC-CWDA-2022-00016), the DOE discussed that the soil-only sensitivity case provides insights into the performance of a closure cap that does not include the erosion barrier. Because the soil-only sensitivity case still includes potentially optimistic performance from the lower lateral sand drainage layer (e.g., silting in of the lower lateral drainage layer or sand entrainment if the overlying layers are eroded) and the HDPE/GCL composite layer on top of the disposal structure roofs (e.g., HDPE degradation at welds and edges, GCL degradation due HDPE defects), the soil-only case does not provide insight into the potential impacts of degradation of the erosion barrier.

In RAI Comment IEC-4 (ML21133A293), the NRC staff discussed that the erosion barrier is a risk-significant feature of the closure cap, which is intended to: (1) prevent erosion of the middle backfill and deeper layers of the cover; (2) prevent animal intrusion into the lower layers; and (3) limit the rate of water flow into the middle backfill. However, in the DOE 2020 SDF PA, the DOE did not describe what material will be used to fill the voids in the erosion barrier. Instead, in Section 3.2.6.8, the DOE listed that as an open issue related to the SDF closure cap concept, which is to be addressed as the design concept matures in the future. In response to RAI Comment IEC-4 (SRR-CWDA-2021-00072), the DOE stated that although the fill material remains an open design issue, the current plan is to use coarse-grained sand to fill the voids.

Based on the risk-significance of the erosion barrier, the NRC staff determined that preliminary technical bases are needed to support the assumed performance.

In Appendix F in the DOE document WSRC-STI-2008-00244, Rev. 0 (ML20206L305), the DOE described the hydraulic properties for two different types of material being considered to fill the voids between the stones: (1) sandy soil and (2) controlled low strength material (CLSM). For sandy soil, the NRC staff determined that the DOE had not demonstrated how root growth could impact barrier performance. For example, due to the depth that tap roots of the loblolly pine and longleaf pine are able to grow, those roots could grow through sandy or backfill-type soil if used in an erosion barrier and potentially impact the placement of the stones as these roots grow.

Eventually, sufficient generations of tree roots may disrupt the erosion barrier and severely degrade performance. Additional information regarding taproots was provided in the NRC TRR entitled Composite Barrier Layers and Lateral Drainage Layers (ML23017A089).

Compared to an erosion barrier filled with sandy soil, an erosion barrier filled with CLSM would increase the chance of lateral flow occurring on top of the erosion barrier due to the lower hydraulic conductivity of the CLSM; thereby, potentially decreasing the stability of the closure cap. The NRC staff expects that the volume of lateral flow that would exit at the upper reaches of the side slope above the erosion barrier could result in an increase in the current modeled flow rate on the side slope. The NRC staff also expects that lateral flow into the side slopes could undermine the stability of the edges of the upper backfill layer because the backfill material may be carried out by the lateral flow. Although Section IEC-7 in the DOE document SRR-CWDA-2011-00044, Rev. 1 (ML113320303) related to the 2009 SDF PA described that the DOE did not expect slope stability to be an issue even with a buildup of head, that section provided no reference or calculations to support that DOE position.

Lastly, in Section IEC-7 of the DOE document SRR-CWDA-2011-00044, Rev. 1, related to the 2009 SDF PA, the DOE also described that none of the nominal saturations listed pose a problem to plant health in terms of root drowning; however, that position also had no references or supporting calculations. Therefore, the NRC staff requested additional information in IEC-4 (ML21133A296) to determine whether higher saturation levels may adversely affect the vegetation assumed in the 2020 SDF PA.

In response to IEC-4 (SRR-CWDA-2021-00072), the DOE discussed that the fill material remains an open design issue, but that the current plan is to use coarse grained sand to fill the voids in the erosion barrier. The DOE discussed that they expect the sand-filled erosion barrier to limit shallow infiltration with water being retained in the topsoil and upper backfill layers.

Accordingly, the DOE expects plant roots, including loblolly pine, to proliferate in the topsoil and upper backfill rather than into the erosion barrier. If roots do grow into the sandy material between the erosion barrier rock, the DOE expects this to occur on a limited, localized basis.

The DOE further discussed that maximum projected erosion on the top surface of the closure cap of 71.9 cm (28.3 in) within 10,000 years is less than the 91.4 cm (36.0 in) of material on top of the closure cap. In addition, if or when the erosion barrier does become exposed, the DOE expects it to behave similarly to the riprap on the side slopes of the closure cap (i.e., the DOE expects the barrier will not erode).

The NRC staff recognizes that the closure cap design has not been finalized. However, in the 2020 SDF PA, the closure cap is the key barrier to radionuclide release. Accordingly, the NRC staff needs to understand the anticipated conditions to determine if there is reasonable assurance that a safe design can be constructed to accommodate the expected flow rates.

SIST Degradation of the Erosion Barrier The NRC staff recommends opening a medium-priority monitoring factor for information related to the degradation of the erosion barrier under a new monitoring factor in MA 9 (Site Stability). The information the NRC staff will monitor will include the material the DOE choses to fill voids in the erosion barrier and the effects on closure cap saturation and root growth.

4.3. Settlement 4.3.1. Static-Loading Induced Settlement The NRC staff determined that the DOE analysis of settlement did not provide a sufficient basis for making a determination about site stability because the sensitivity analyses the DOE relied on to bound the effects of static-loading induced settlement were insufficiently representative or were inconclusive.

In the 2020 SDF PA, the DOE concluded that settlement would be expected to occur uniformly over the entire cap area and differential settlement would be negligible, assuming the subsurface conditions are relatively uniform. The NRC staff determined that the DOE did not provide sufficient bases to support that position for two reasons. First, the NRC staff expects that settlement would not be uniform because the evolution and dissolution of soils around the disposal structures could vary from that of the SDF disposal structures. Second, soft zones in the subsurface are inherently not uniform. As such, settlement resulting from consolidation of these zones could result in differential settlement in overlying layers. As an example, a DOE analysis indicated that differential static settlement could be 8.2 cm (3.2 in) across a distance of 34 m (110 ft) (K-ESR-Z-00005). In addition, the NRC staff determined that the DOE did not demonstrate how much settlement key layers in the closure cap and on top of the disposal structures (e.g., lateral sand drainage layers and HDPE/GCL composite layers) can withstand before performance degrades.

Although the DOE provided sensitivity analyses to provide additional information regarding the impacts of settlement of the SDF, the NRC staff determined that the analysis did not provide a sufficient basis for determining that settlement would not adversely affect site stability. To evaluate the potential impacts from static-loading induced settlement on the closure cap, the DOE conducted a soil-only closure cap sensitivity case as a bounding approximation. Because of potentially unrealistic assumptions regarding the performance of the lower lateral sand drainage layer and the HDPE/GCL composite barrier on the disposal structure roofs in the soil-only case, the NRC staff determined that this sensitivity analysis does not provide information regarding the potential impacts from static-loading induced settlement on the closure cap. The DOE probabilistic analysis that was provided in response to the NRC Request for Supplemental Information (RSI) RSI-1 (ML20254A003) provides insight into the potential impacts of settlement on the closure cap and overall performance of the SDF (SRR-CWDA-2021-00066 (ML21217A083)). However, the results from that analysis showed that doses can exceed the performance objectives.

To evaluate the potential impacts from static-loading induced settlement on the SDF disposal structures and saltstone grout, the DOE conducted a fast flow paths sensitivity case. Because this sensitivity case assumes potentially unrealistic assumptions regarding deep infiltration through the closure cap and the lower lateral sand drainage layer and the HDPE/GCL composite barrier on the disposal structure roofs, the NRC staff determined that this sensitivity case does not provide information regarding the potential impacts from static-loading induced settlement on the closure cap. As above, the DOE probabilistic analysis in response to RSI-1 provides insight into the potential impacts of settlement on the disposal structures and saltstone grout (SRR-CWDA-2021-00066 (ML21217A083)). However, the results from that analysis showed that doses can exceed the performance objectives. Therefore:

SIST Static-Loading Induced Settlement The NRC staff recommends opening a medium-priority monitoring factor for information related to impacts of settlement on the closure cap, saltstone, and disposal structure concrete as one part of a new monitoring factor related to closure cap erosion in MA 9 (Site Stability).

4.3.2. Seismic-Induced Liquefaction and Settlement To address potential impacts of seismic-induced liquefaction and settlement, the DOE relied on the same sensitivity cases as discussed above in Section 4.3.1. Accordingly, the NRC staff determinations regarding dynamic or seismic-induced liquefaction and settlement are the same as with static-loading induced settlement. The 2013 NRC SDF Monitoring Plan addressed settlement due to increased overburden under MF 9.01 (Settlement Due to Increased Overburden). Therefore:

SIST Settlement Due to Increased Overburden and Seismic Loading The NRC staff recommends updating the text of MF 9.01 to include settlement due to seismic loading - MF 9.01 Settlement Due to Increased Overburden and Seismic Loading under MA 9 (Site Stability).

4.3.3. Soft Zones The NRC staff determined that the DOE analysis of soft zones did not allow the NRC staff to make a determination about the effect of soft zone consolidation on site stability because the DOE did not provide a sufficient basis for assuming saturation of the soft zones would make consolidation negligible for 10,000 years after SDF closure.

In the DOE documents K-ESR-Z-00005 and K-ESR-Z-00008, the DOE evaluated settlement due to collapse of soft zones that were observed to be up to 1.8 m (5.8 ft) thick for SDS 6 and SDS 7, respectively. The DOE projected the maximum settlement from these evaluations to be 1.3 cm (0.5 in) and the maximum differential settlement was also 1.3 cm (0.5 in). The thickness of those soft zones can vary significantly across the SDF. In the 2012 TER, the NRC staff discussed a soft zone beneath SDS 2A and SDS 2B that was approximately 4.3 m (14 ft) thick.

As discussed in the 2012 TER, the NRC staff stated that the DOE analyses of potential settlement due to consolidation of soft zones did not account for the potential removal of subsurface material that has resulted in subsidence observed at SRS. In the DOE document SRNL-TR-2012-00160 (ML13080A339), the DOE discussed that the formation of sinks occurred when the calcareous zones were in the unsaturated zone and dissolution occurred due to deep infiltration of acidic meteoric water. The DOE hypothesized that now that those calcareous zones were in the saturated zone, the dissolution is slow to non-existent. However, in the 2012 TER, the NRC staff discussed that the dissolution of those calcareous materials is ongoing based on the high pH and high carbonate/bicarbonate ion concentrations, as documented in the DOE document WSRC-RP-92-450 for wells completed in the Santee formation. The NRC staff acknowledged that although dissolution of calcareous sediment in the saturated zone is likely to be slow, the DOE had not demonstrated that dissolution is insignificant with respect to site stability over the course of the 10,000-year performance period. In addition, the NRC staff determined that the DOE did not provide a basis for how much dissolution of calcareous sediment would be required for consolidation of soft zones or how much settlement the closure cap can withstand without compromising the performance of key layers, such as the HDPE/GCL composite barriers and the lateral sand drainage layers.

In response to the NRC RAI Question CM&FSU-4 (SRMC-CWDA-2022-00016), the DOE stated that the FEPs Screening Team would have likely screened out subsidence had they been aware of information in the DOE document SRNL-TR-2012-00160, which discussed that karst conditions are most likely located towards the Southeastern part of the SRS. NRC staff agrees that the prevalence of karst conditions and calcareous sediment become more prevalent to the southeast; however, that does not preclude the presence of soft zones and potential impacts from these features in the SRS Z-Area. Subsurface investigations described above continue to find evidence of calcareous sediment, soft zones, and carbonate dissolution (K-ESR-Z-00005 andK-ESR-Z-00008). The 2013 NRC SDF Monitoring Plan already includes a monitoring factor about the potential impact of soft zones on site stability for the SDF. Based on the potential for soft zone consolidation to affect the disposal structures, saltstone grout, and the closure cap, the NRC staff will continue to monitor information related to the potential impacts from soft zones under MF 9.02 (Settlement Due to Dissolution of Calcareous Sediment).

4.3.4. Settlement due to Waste Bags in SDS 4 The NRC staff did not find the DOE analysis of settlement due to waste bags in SDS 4 allowed the NRC staff to make a determination about site stability because the sensitivity analysis the DOE used to address waste bag consolidation appears to underestimate water flow into cells affected by waste bag consolidation and saltstone degradation in those cells.

In the Compliance Case in the 2020 SDF PA, the DOE assumed that the weight of the clean cap grout will compress the waste bags in Cells C and I of SDS 4 and will minimize void space and prevent collapse or settlement. Accordingly, the Compliance Case assumed that the waste bags will not impact SDF performance. However, a DOE engineering evaluation concluded that settlement of the closure cap could occur if the bags are not compressed by the grout (K-CLC-Z-00028). To evaluate the dose impacts due to the potential consolidation of the waste bags after grouting and site closure, the DOE evaluated an alternative waste disposal and closure configuration with an increased deep infiltration rate of 26.9 cm/yr (10.6 in/yr) for Cells C and I and pessimistic values for the remaining cells in SDS4.

The dose results from the sensitivity analysis in Figure 5.8-75 of the 2020 SDF PA showed only a modest dose increase. The NRC staff found the result to be unexpected because of the importance of deep infiltration on contaminant release and the magnitude of increase in deep infiltration in this sensitivity case. The increase in deep infiltration for this case was more than four orders of magnitude more deep infiltration through the closure cap than the DOE modeled in the Compliance Case. To better understand the dose-limiting barrier(s), the NRC staff requested a revised sensitivity analysis in RAI Question SS-2 with the conceptual model where the overlying layers are impacted by the settlement due to consolidation of the waste bags (ML21062A214).

In response to RAI Question SS-2 (SRR-CWDA-2021-00047, Rev.1 (ML21201A247)), the DOE provided a new sensitivity analysis with a revised conceptual model where the overlying HDPE/GCL composite barrier on the roof, the roof, and the clean cap grout are damaged, in addition to the previously-assumed increase in deep infiltration. The projected doses from that sensitivity case, as shown in Figure SS-2.9 of SRR-CWDA-2021-00047, were slightly higher than the original sensitivity analysis in the 2020 SDF PA due to the assumed-damaged materials above the saltstone grout in Cells C and I. The increase in dose was limited because of the assumed performance of the saltstone grout in this analysis, which resulted in the bulk of the deep infiltration water being diverted around the saltstone.

The NRC staff remain concerned about the presence of the waste bags due to the potential for increased deep infiltration into disposal cells and the potential for greater-than-assumed degradation of saltstone grout. The DOE analysis in SRR-CWDA-2021-00047, Rev.1 showed the importance of saltstone as a secondary barrier. If saltstone grout degradation is greater than assumed in the 2020 SDF PA, then doses could be significantly greater than projected in the DOE sensitivity analysis (SRR-CWDA-2021-00047, Rev.1). In addition, the DOE analysis included a deep infiltration rate of 26.9 cm/yr (10.6 in/yr), however, it appears that deep infiltration values could also be greater than assumed in this analysis. Specifically, in Table 2 of the DOE document WSRC-STI-2008-00244, the DOE provided a series of water balance studies that were conducted around the SRS. The median annual precipitation value of eight studies was 121 cm/yr (47.8 in/yr). If settlement/collapse occurs due to the consolidation of the waste bags, then the water balance components of runoff and evapotranspiration would be reduced and the deep infiltration could significantly exceed the values assumed in that sensitivity analysis. In addition, settlement/collapse could also result in funneling of water into this zone, which could further increase the deep infiltration rate into this zone.

In response to RAI Question SS-2, the DOE discussed that the timing of degradation at 0 years after closure is conservative. The NRC staff expects that the timing is conservative with respect to the timing of the peak dose (i.e., projecting an earlier peak dose) however, the NRC staff expects the assumption is not likely to be conservative with respect to the magnitude of the peak dose. For example, if settlement occurs at 500 years, then the NRC staff expects that some saltstone degradation would have occurred and that a sudden influx of water could result in a pulse release.

Also, in response to RAI Question SS-2, the DOE discussed that Cells C and I were approximately half full of saltstone grout in July of 2021. The DOEs conceptual model for the Compliance Case is that the waste bags will be compressed by the weight of the grout, thereby minimizing void space. Because of the risk significance of deep infiltration, the potential for settlement of the waste bags to result in increased deep infiltration, and NRC staffs concern with the assumed saltstone degradation, SIST Settlement due to Waste Bags in SDS 4 The NRC staff recommends opening a medium-priority monitoring factor for information related to the settlement due to waste bags in SDS 4 under a new monitoring factor in MA 9 (Site Stability). The information the staff will monitor will include verification of the amount of grout pumped into Cells C and I relative to the volume of those cells and any dose analyses if void space remains.

5. Teleconference or Meeting There were no teleconferences or meetings with the DOE related to this TRR.

6. Follow-up Actions Except for NRC staff recommendations to revise the 2013 NRC SDF Monitoring Plan, there are no specific Follow-up Actions related to this TRR.

7. Conclusions The closure cap consists of a series of layers that work in conjunction with each other and the performance of which affects the other layers. For example, the low permeability of the HDPE/GCL composite layer that limits deeper infiltration results in the buildup of hydraulic head in the upper sand drainage layer up to the topsoil. That creates saturated conditions that could affect plant speciation on top of the cover and result in overland flow, both of which can impact erosion rates. The buildup of head also affects slope stability and could impact the performance of the sand drainage layer. Based on the risk significance and uncertainty in the performance of the closure cap discussed in the following sections, the NRC staff needs additional confidence in: (1) Gully Erosion, (2) Sheet and Rill Erosion, (3) Slope Stability, and (4) Sand Entrainment.

The NRC staff notes that one way to significantly reduce uncertainty and increase confidence in model projections would be to construct and monitor a test cover. The DOE has built test covers at the Hanford site and uranium mill tailing sites for those reasons.

The NRC staff determined that the information provided in the 2020 SDF PA and supporting documents did not provide a sufficient basis for the NRC staff to assess the stability of the SDF because the DOE indicated that the design and implementation plan for risk-significant features of the SDF is not complete. The NRC staff identified the following specific information that the NRC will monitor to assess the stability of the SDF as the DOE finalizes its closure cap design and implementation plan: