ML23017A088

Technical Review: Future Scenarios and Conceptual Models for the 2020 Saltstone Disposal Facility Performance Assessment - Ha

ML23017A088
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Site:	PROJ0734
Issue date:	04/18/2023
From:	Hans Arlt NRC/NMSS/DDUWP/RTAB
To:
Arlt H, Ridge A
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ML23090A081	List: ML23017A083 ML23017A084 ML23017A085 ML23017A086 ML23017A087 ML23017A088 ML23017A089 ML23017A090 ML23017A113
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eConcurrence 20230331-60017
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Text

Technical Review: Future Scenarios and Conceptual Models for the U.S. Department of Energy 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site

Date

April 18, 2023

Reviewers

Hans Arlt, Sr. Risk Analyst, U.S. Nuclear Regulatory Commission (NRC)

Christianne Ridge, Sr. Risk Analyst, NRC

1.0 Purpose and Scope

The purpose of this NRC staff Technical Review Report (TRR) is to document the NRC staff review of the U.S. Department of Energy (DOE ) Future Scenarios and Conceptual Models for the DOE 2020 Performance Assessment (PA) for the Saltstone Disposal Facility (SDF) at the Savannah River Site (SRS). The NRC staff performed this review to support a future decision about whether the DOE has demonstrated that radioactive waste disposal activities at the SDF comply with the performance objectives of Title 10, Code of Federal Regulations (CFR) Part 61.

This TRR also supports NRC monitoring of the SDF under the NRC document Plan for Monitoring Disposal Actions Taken by the U.S. Department of Energy at the Savannah River Site Saltstone Disposal Facility in Accordance with the National Defense Authorization Act for Fiscal Year 2005, Revision 1 (available in the NRC Agencywide Documents Access and Management System under Accession No. ML13100A113) (referred to as the NRC SDF Monitoring Plan in this TRR). Specifically, this TRR supports NRC monitoring under both Monitoring Factor (MF) 10.02 (Defensibility of Conceptual Models) and M F 10.14 (Scenario Development and Defensibility), which are both under M onitoring Area 10 (Performance Assessment Model Revisions). In this TRR, the NRC staff included recommended changes to the NRC SDF Monitoring Plan in the NRC Staff Evaluation (Section 4) and summarized those changes in the Conclusions (Section 7).

The NRC staff reviewed the modeling parameters and equations the DOE used in the PA m odel for the 2020 SDF PA (SRR-CWDA -2019-00001 (ML20190A056)). The review scope included aspects of the PA Model that affect the projected dose to hypothetical members of the public at two locations: (1) 100 meters (m) (~328.1 feet (ft)) from the SDF boundary and (2) the nearest contaminated streams 1. The NRC staff will compare the projected dose to a member of the public at those locations to the 0.25 millisievert (mSv) (25 millirem [mrem]) dose limit under 10 CFR 61.41, Protection of the general population from releases of radioactivity. The review scope also includes aspects of the model that affect a member of the public that could also affect the projected dose to an individual who inadvertently intrudes into the SDF 100 years or more after site closure (referred to as an inadvertent intruder in this document). Howeve r, aspects of the PA Model that only affect an inadvertent intruder (e.g., inhalation of suspended soil while drilling a well) are addressed in the TRR on Intrusion Analysis (ML23017A085).

1 For each modeled year, the DOE chose either McQueen Branch or Upper Three Runs, depending on which had the greater radionuclide concentrations in the groundwater where groundwater seeped into the stream.

2.0 Background

Because the SDF and the SRS Tank Farms (i.e., H -Tank Farm and F-Tank Farm) are both located at t he SRS, the scenarios and conceptual m odels for those facilities share technical bases. Many of the features, events, and processes (FEPs) that could affect the SDF performance could also affect the performance of the SRS Tank Farms. For the 2020 SDF PA, the DOE provided information about FEPs, conceptual models, and future scenarios in two documents:

• SRR-CWDA-2017-00057, Rev. 0, Features, Events, and Process es for the Saltstone Disposal Facility Performance Assessment (ML18170A253)

• SRR-CWDA-2018-00006, Rev. 0, Conceptual Model Development for the Saltstone Disposal Facility Performance Assessment (ML18143B265)

The NRC draft guidance document, Guidance for Conducting Technical Analyses for 10 CFR Part 61 (NUREG -2175; ML15056A516) defines scenario development as the process of developing the scope of the analysis that will be implemented in the conceptual and numerical models. The NUREG also indicates that approaches to scenario development can typically be described as bottom-up or top-down. In a bottom-up approach, the analysis will identify, categorize, and systematically screen the FEPs that could affect the performance of the disposal facility being analyzed. In a top-down approach, the analyst uses the safety assessment and safety functions to develop scenarios. In the 2020 SDF PA, the DOE used a bottom-up approach of: (1) generating a list of FEPs, (2) systematically considering their interactions and effect on the SDF, and (3) screening the FEPs to be represented in the Conceptual Model (SCM) and future scenarios.

To identify and screen FEPs, the DOE relied on a team of subject matter experts. The NRC addressed the use of expert elicitation in the guidance document, Branch Technical Position on the Use of Expert Elicitation in the High-Level Radioactive Waste Program (NUREG -1563; ML033500190).

3.0 DOE Future Scenarios and Conceptual Model for the 2020 SDF PA

3.1 Overview

The DOE developed future scenarios and conceptual models, also known as site conceptual models, for the 2020 SDF PA based in part on the DOE identification of FEPs that could affect SDF performance. In the document SRR-CWDA-2017-00057, the DOE provided the following definitions:

• A feature is an object, structure, or condition that has a potential to affect disposal system performance

• An event is a natural or human-caused phenomenon that has a potential to affect disposal system performance and that occurs during an interval that is short relative to the period of performance

• A process is a natural or human-caused phenomenon that has a potential to affect disposal system performance and that operates during all or a si gnificant part of the period of performance.

In the document SRR-CWDA-2018-00006, the DOE indicated that it defined a scenario as a subset of important FEPs that are used to identify conditions or the future evolution of the

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disposal site. The DOE based that definition on the definition provided in the NRC draft guidance document Guidance for Conducting Technical Analyses for 10 CFR Part 61 (NUREG-2175). The term scenario is a frequently used term in technical analyses documents.

This TRR sometimes refers to the term future scenario to distinguish it from other types of scenarios such as exposure or receptor scenarios, which describe the FEPs associated with the behaviors and activities of the people who may be exposed to radiation. This TRR uses the terms scenario and future scenario interchangeably. The DOE indicated that it considered the Central Scenario to be the scenario that represents the most probable and defensible future evolution of a disposal site. The DOE referred to any futur e scenario other than the Central Scenario as an alternative scenario. In SRR-CWDA -2018-00006, the DOE indicated that an alternative scenario usually reflects scenarios that are plausible, although they are less likely than the Central Scenario. The DOE also indicated that alternative scenarios may include disruptive events. In the draft guidance document NUREG -2175, the NRC staff indicated that the Central Scenario typically would not include disruptive events because disposal sites typically are sited to avoid disruptive events.

In SRR-CWDA -2018-00006, the DOE indicated that a conceptual model is a well -defined, qualitative description of how related FEPs behave or are impacted within the bounds of a specific scenario. The DOE then stated that an alternative conceptual model is an additional and different conceptual model that introduces an alternative approach for addressing simulated conditions. In NUREG-2175, the NRC stated: Plausible conceptual models of a system are estimates of how the system may function. The DOE provided an example of an alternative conceptual model for the 2020 SDF PA in terms of saltstone degradation. Specifically, the DOE indicated that the conceptual model for the Central Scenario assumed that the main manifestation of saltstone degradation would be gradually increasing saturated hydraulic conductivity, whereas an alternative conceptual model would be that saltstone degradation primarily proceeded through the development of cracks through the wasteform.

Figure 1 in this TRR provides an overview of the process the DOE used to develop FEPs, conceptual models, and scenarios for the future evolution of the SDF. In brief, the DOE developed the Central Scenario for the SDF (i.e., the expected future evolution of the SDF) based on expert judge ment. The DOE then developed the site conceptual model to represent how the site would function in the Central Scenario. The DOE developed a list of FEPs from standard FEP lists and previous site analyses. Then, the DOE screened that list to include FEPs relevant to the SDF. The DOE reviewed the FEP list to determine which of the FEPs were included in the Central Scenario. The DOE created alternative scenarios to address FEPs that the Central Scenario did not reflect. The DOE then created alternative conceptual models to represent how the site would function in the alternative scenarios.

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Figure 1. The DOE Model Development Process for the 2020 SDF PA (F rom Figure 1.2-1 in SRR -CWDA-2018-00006)

Figure 1 also shows the scope of the system description and assessment context. In SRR-CWDA-2018-00006, the DOE based the definitions of the assessment context and system description on the definitions in the NRC draft guidance in NUREG -2175. The DOE stated that the assessment context is the collection of information that provides the basis for the performance assessment. The DOE also stated that the assessment context includes the purpose for the PA, the regulatory framework, assessment end points, waste characteristics, and assessment timeframes. In SRR -CWDA -2018-00006, the DOE states the system description describes the disposal system and the natural environment of the site. Consistent with the NRC guidance in NUREG -2175, the DOE indicated that the system description should include descriptions of the site, the natural setting, the disposal facility, the interaction of the site and disposal facility, the waste to be disposal of, and processes c ontrolling contaminant release, transport, and characteristics of potentially affected members of the public.

3.2 Features, Events, and Processes

The DOE document SRR-CWDA -2017-00057, describes the DOE processes for identifying and screening FEPs for the 2020 SDF PA. This section addresses those steps in subsections 3.2.1

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and 3.2.2.

3.2.1 Identification of Features, Events, and Processes

To identify FEPs, analysts often begin with standardized lists from different organizations and previous analyses for the disposal site (i.e., in this case, the SDF). An analyst would then compile the FEPs from those sources, remove duplicates, and use that list as input in the FEPs screening process. An alternative approach is to consider potential failure modes of a site and then to list FEPs involved in those failures. That approach is typically referred to as a top-down approach. The NRC guidance document NUREG-2175 provides additional information about the general bottom-up and top-down FEP identification processes. For the 2020 SDF PA, the DOE mostly took a bottom-up approach. The DOE complied FEPs from the following sources:

• Safety Assessment Methodologies for Near Surface Disposal Facilities, Results of a Coordinated Research Project, Volume 1 (ISBN 92 -0 -104004-0 )

• Features, Events and Processes for the Disposal of Low -Level Radioactive Waste FY 2011 Status Report (FCRD-USED-2011-000297)

• Features, Events, and Processes for the Total System Performance Assessment:

Analyses (ANL -WIS -MD -000027, Rev. 0)

• Encyclopedia of Features, Events and Processes for the Swedish SFR and Spent Fuel Repositories (SKI Report 02:35)

That compilation resulted in a list of 1,383 FEPs. The DOE then organized the FEPs to make it easier to recognize duplicate entries.

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Table 1 in this TRR shows the six groups and 33 sub-groups the DOE used to organize the FEPs. After the DOE removed duplicates and combined similar FEPs, the list included 245 FEPs. The DOE then reviewed the 2009 SDF PA and supporting documents and identified an additional 100 FEPs to be added to the list. Therefore, the list included 345 FEPs before the DOE began the FEPs screening process.

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Table 1. Categories for Features, Events, and Processes (A dapted from Table 3.0-2 in SRR-CWDA-2017-00057)

Group Sub-Group 1.1 General 1.0 Assessment Basis 1.2 Regulations and Controls 1.3 Models and Calculations 1.4 Other Assessment Factors 2.1 Human Characteristics 2.2 Land and Water Management 2.3 Future Human Activity 2.0 External Factors 2.4 Biological Factors 2.5 Geologic Features 2.6 Geologic Processes 2.7 Climate 2.8 Water Cycle 3.1 General Closure System 3.2 Pre-Closure Activities 3.3 Closure System Components 3.0 Closure System 3.4 Closure System Hydrology 3.5 Chemical Processes 3.6 Thermal Processes 3.7 Material Degradation 3.8 Other Closure System Factors 4.1 Contaminant Description 4.0 Contaminant Factors 4.2 Contaminant Properties 4.3 Concentrations 4.4 Exposure Factors 4.5 Other Contaminant Factors 5.1 Flow Factors 5.0 Flow and Transport 5.2 Flow Processes 5.3 Release and Transport 5.4 Other Flow and Transport Factors 6.1 Intrusions 6.0 Disruptive Events 6.2 Seismic Events 6.3 Igneous Events 6.4 Other Events

3.2.2 Screening of Features, Events, and Processes

After developing an initial comprehensive list of FEPs, the DOE determined which of the FEPs should be accounted for in the PA model. That step is typically referred to as screening. In general, FEPS that are screened in are accounted for in the PA model or supporting analyses and FEPs that are screened out do not need to be considered further. The DOE document SRR-CWDA-2017-00057 indicated that the DOE might not explicitly include all FEPs that are screened in in the PA model. Instead, the DOE could account for the FEPs through bounding analysis, secondary analysis, design consideration, administrative procedures, PA maintenance processes, or other methods.

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The DOE developed a team of nine subject matter experts to conduct the screening analysis for the 2020 SDF PA. Collectively, the team members had advanced education and professional experience in radioactive waste disposal, mixed chemical and radioactive waste disposal, cementitious waste forms, waste stabilization, surface chemistry, geology, hydrogeology, groundwater monitoring, groundwater flow modeling, statistical and data analyses, risk assessment, and performance assessment modeling. Although the FEPs screening team included nine members, only eight were available 2 for each Phase.

Initial Programmatic Screening

Before the DOE FEPs screening team began its process, the DOE screened in 50 FEPs and screened out three FEPs based on the assessment context. The DOE referred to that process as programmatic screening. For example, the DOE programmatically screened in FEP 1.1.01 (Assessment Purpose), FEP 1.107 (Assessment Endpoints ), and 1.1.08 (Transparency of Assessment Approach) because the DOE requires those considerations as part of PA development. The DOE also programmatically screened out three FEPs prior to the FEP screening team beginning its work:

• Prospective evaluation assessment approach (FEP 1.3.11)

• Retrievability (FEP 1.4.06)

• Site development (FEP 3.1.02)

The DOE screened out a prospective evaluation assessment approach (FEP 1.3.11) because it was a generic FEP designed to focus on individual model components, which was not appropriate for a holistic PA model. The DOE screened out retrievability (FEP 1.4.06) because it is related to ensuring waste can be retrieved after disposal, which is inconsistent with the DOE plan for the SDF. The DOE screened out site development (e.g., constructing roads, urban residential buildings, industrial buildings) because the DOE determined it would lead to speculation about future land use.

Phase I Screening

After the initial screening by DOE, the screening team used a two-phase screening process. In the first phase, each subject matter expert worked alone to evaluate each FEP based on two criteria: (1) the experts perceived frequency of the FEP or perceived probability of the FEP occurring within 10,000 years of site closure and (2) the experts perceived impact (i.e.,

consequence) of each FEP occurring. The team members assigned each FEP a score of 0, 1, or 2 for both frequency and consequence using the descriptions in

2 Dr. Simner participated in Phase I only and Dr. Garrabrants participated in Phase II only.

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Table 2 and Table 3, below. The DOE instructed each team member that, in the case of uncertainty, the team member should assign the higher score. The team members could also indicate NR, meaning they had no recommendation for the FEP. Each team member then determined a recommended action for each FEP based on the matrix in Table 4, below.

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Table 2. FEPs Screening Criteria for Perceived Frequency (Table 4.1-1 in SRR -CWDA-2017-00057)

Description Perceived Frequency Score Anticipated, Expected, or Already Will occur in 10,000 years 2 Known to Exist or Occur Unlikely or Extremely Unlikely May occur in 10,000 years 1 Beyond Extremely Unlikely Not expected to occur in 10,000 years 0

Table 3. FEPs Screening Criteria for Percieved Impact (Table 4.1-2 in SRR -CWDA-2017 -00057)

Description Perceived Impact to Member of Public Score High Significant Impact on Release or Dose 2 Moderate Moderate Impact on Release or Dose 1 Negligible No or Negligible Impact on Release or Dose 0

Table 4. FEPs Screening Criteria Matrix (Table 4.1-3 in SRR -CWDA-2017-00057)

Frequency Beyond Extremely Unlikely or Extremely Anticipated, Unlikely Unlikely Expected, or Already Known to Exist or Occur Impact 0 1 2 High 2 Considered Screened In Screened In Moderate 1 Screened Out Considered Considered Negligible 0 Screened Out Screened Out Screened Out

To conclude Phase I, the team assembled the recommended actions from each subject matter expert for each FEP. If the members unanimously decided to screen a FEP in, it was screened in at that point. Similarly, if the members unanimously decided to screen a FEP out, it was screened out at that point. In any other case, the FEP was considered and proceeded to Phase II.

The DOE document SRR -CWDA -2017-00057 did not indicate how the DOE considered NR responses when determining whether the team members recommendations were unanimous.

That is, the DOE did not specify whether a response of NR would be interpreted as being the same as or different from otherwise unanimous responses. However, the Phase I screening responses in Appendix A of SRR-CWDA-2017- 00057, indicated that any NR response resulted in the FEP being referred to Phase II screening. For example, the team members unanimously recommended to screen in the FEP for Degradation of Waste Form (Saltstone)

Matrix (FEP 3.7.05) except for one team member who gave an NR response, and the DOE referred the FEP to Phase II screening. Similarly, the team members unani mously recommended to screen out the FEP for Recrystallization of Vitrified Wastes (FEP 3.6.07) except for two team members who provided NR responses and the DOE referred the FEP to Phase II screening.

During Phase I, the DOE screened out two FEPs: FEP 6.4.10 (Changes to the Earths Tidal Processes) and FEP 6.4.11 (Changes in the Earths Magnetic Field). The DOE did not screen in any FEPs during Phase I.

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Phase II Screening

In Phase II, the team members discussed the 290 FEPs that had not been screened in or out.

The default position of the team was to screen FEPs in and the team documented a justification for any FEPs the team screened out. The DOE document SRR -CWDA -2017-00057 indicated that, during Phase II, the team considered factors in addition to the frequency and impact factors considered in Phase I. Specifically, the DOE indicated that the team considered the available knowledge and potential interactions with other FEPs. The DOE document SRR-CWDA-2017-00057 did not indicate whether the DOE considered a lack of available information to be a basis for screening a FEP in our out. However, the Phase II justifications provided in of SRR-CWDA -2017- 00057 indicated that the DOE screened out some FEPs because any attempt to address thos e FEP would be highly speculative. That DOE document stated : Excluding FEPs that are considered highly speculative is consistent with guidance found in DOE G 435.1-1, Chapter IV. In general, the DOE used that justification with FEPs that involved hypothetical future states. The DOE used that basis for the following 11 FEPs:

• Ozone layer failure (FEP 2.2.08)

• Technological developments (FEP 2.3.04)

• Retrograde developments (FEP 2.3.06)

• Species evolution (FEP 2.4.04)

• Tectonic activity and p rocesses (FEP 2.6.01)

• Orogeny (formation of m ountains) (FEP 2.6.02)

• Site development (FEP 3.1.02)

• Waste inventory (unexpected sources) (FEP 4.3.07)

• Accidents and unplanned events (FEP 6.4.1)

• Extraterrestrial events (FEP 6.4.09)

• Impacts from meteorites of space debris (6.4.09)

The next most common basis the DOE provided for screening out a FEP was that the screening team considered the FEP to be too broad and determined that it wa s redundant with other more specific FEPs included in the process. T he DOE used that basis for screening out the following nine FEPs:

• Natural/semi-natural land and water use (FEP 2.2.03)

• Pollution (soil, groundwater, air, etc.) (FEP 2.2.07)

• Biomes (FEP 2.4.01)

• Vegetation (FEP 2.4.02)

• Animal populations (2.4.03)

• Climate change (FEP 2.7.08)

• Response to climate change (FEP 2.7.09)

• Rind (chemically altered zone) forms in the near field (FEP 3.5.22)

• Hydrological response to geological changes (FEP 5.2.05)

The remaining FEPs that the DOE screened out were screened out for various site-specific reasons. For example, the DOE screened out sedimentation (FEP 2.6.06) because the SDF is higher than the surrounding area and because the team expected sedimentati on to enhance, rather than diminish SDF performance. As another example, the DOE screened out recrystallization of vitrified wastes (FEP 3.6.07) because it does not apply to the cementitious saltstone wasteform. After screening, the DOE had a list of 284 FEPs to address with the 2020 SDF PA. Table 5 shows the number of FEPs screened in and out at each stage of the process.

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Table 5. FEPs Screening Summary (A dapted from SRR-CWDA-2017- 00057, Table 4.4-1 )

Screening Phase Screened In Screened Out Prior to Phase I (programmatic decisions) 50 3 Phase I 0 2 Phase II 234 56 Total 284 61

3.3 Future Scenario Development

Typically, the next step after FEPs screening is the development of scenarios for the future evolution of the site. As discussed in Section 3.1 in this TRR, the DOE referred to the scenario it found to be the most likely as the Central Scenario and other, less likely scenarios as alternative scenarios. The DOE developed the Central Scenario through expert judgement. In the 2020 SDF PA, the DOE referred to nine modeling cases as alternative scenarios. During the development of alternative conceptual models (see Section 3.4 in this TRR) the DOE identified 163 FEPs represented by the Central Scenario. The DOE determined an additional 56 scenarios were addressed by the modeling cases it referred to as alternative scenarios (Table 6 below). This TRR addresses the disposition of the remaining 65 FEPs in Section 3.4.

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Table 6. FEPs the DOE Indicated are Addressed in Alternative Scenarios (Adapted from Tables 4.4-2 through 4.4-10 in SRR -CWDA-2018-00006)

Modeling FEPs DOE Indicated are Addressed Case Name External Closure Contaminant Flow and Disruptive Factors (a) System (a) Factors (a) Transport (a) Events (a) 3.2.04, 3.2.05, 6.2.01, 6.2.03, Early 3.2.09, 3.7.02, 5.1.03 6.2.04, 6.4.02, Releases 2.7.03 3.7.06, 3.7.08, 5.1.07 6.4.03, 6.4.04, 3.7.09, 3.7.19, 5.4.06 6.4.06, 6.4.07 3.8.07 Infiltration 2.6.05, 2.6.07, 3.7.08 5.1.03 6.2.01, 6.2.03, Variability 2.6.11, 2.6.14 5.1.07 6.2.04, 6.4.06 Fast Flow 3.2.09, 3.4.05, 5.1.03 Paths 3.7.02, 3.7.06, 5.1.14 6.2.01, 6.2.03, through 2.7.03 3.7.09, 3.7.15, 5.3.09 6.2.04, 6.4.04, Disposal 3.7.19, 3.8.02, 5.4.05 6.4.06, 6.4.07 Structures 3.8.07 Fast Flow 2.5.08 Paths 2.5.10 3.7.08 5.1.10 6.2.01 through 2.5.11 3.7.19 5.1.13 6.2.04 Ground 2.6.14 5.3.09 6.4.06 Water No Closure 2.6.07, 2.6.11, 3.2.08, 3.8.06 6.1.04 Cap 2.6.14, 2.7.03 6.1.06 3.2.04, 3.2.10, Stratified 3.3.17, 3.4.08, 4.1.06 6.4.07 Saltstone 3.5.13, 3.7.09, 3.8.02 Perched 2.5.11 3.5.21 5.2.09 6.4.02 Water Colloid 3.5.18 5.3.06 Transport 3.5.24 4.5.07 5.3.11 3.5.25 5.4.04 Inadvertent 6.1.01 Human 6.1.03 Intruder 6.1.06 (a) See Table 1 in this TRR for FEP categories and Table 9 in this TRR for FEP descriptions

The NRC staff requested additional information about the DOE development of alternative scenarios in a Request for Additional Information Question (ML22026A391). The NRC staff indicated that some of the modeling cases that the DOE referred to as alternative scenarios appeared to be more like sensitivity analyses, which can focus on single parameters or features, rather than alternative scenarios, which envision an alternative evolution of the site and represent changes to all of the systems that would be affected in that alternative scenario. In that RAI Question the NRC staff used the Early Releases, Infiltration, and Fast Flow Paths through Disposal St ructures Scenarios as examples. The NRC staff noted that the DOE indicated that those modeling cases accounted for FEPs related to subsidence and seismic activities (i.e., FEPs 6.2.01 through 6.2.04). However, in the RAI Question the NRC staff indicated that those modeling cases do not appear to represent plausible future scenarios with seismic activity or subsidence because they do not include damage to either the closure cap or to the drainage and composite layers above the disposal structure roofs. Consequently, the

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infiltration rates and the saltstone inflow rates that the DOE projected in those modeling cases remained similar to those for the Central Scenario compliance case, which is not what the NRC staff would expect in a future scenario that included seismic activity or subsidence. Table 7 in this TRR lists the FEPs for which the NRC staff requested additional information in the Fourth Set of NRC RAI Questions for the 2020 SDF PA ( ML22026A391).

Table 7. Disruptive FE Ps for Which the NRC staff Requested Additional Information (F rom text in ML22026A391)

FEP Description Number 6.2.01 Seismicity 6.2.03 Seismicity-Induced Damage or Changes to System Components 6.2.04 Effects of Subsidence 2.6.07 Erosion and Weathering 2.6.11 Mass Wasting 5.1.07 Episodic or Pulse Flow and Release

In response to the NRC RAI Question, the DOE addressed the FEPs in Table 7 above in three groups (see DOE document SRMC-CWDA-2022-00016 (ML22118A297)). The first group included the FEPs related to seismicity and subsidence (i.e., FEP 6.1.01, 6.2.03, and 6.2.04), the second group included the FEPs relatedto erosion, weathering, and mass wasting (i.e., FEP 2.6.07 and 2.6.11), and the third group addressed episodic or pulse flow and release (i.e., FEP 5.1.07).

The DOE indicated that it expected damage to the closure cap and disposal structures from seismicity and subsidence to have very low probability. In addition, the DOE indicated that the PA modeling team had reviewed additional information that had not been reviewed by the FEPs screening team and determined that the probability of subsidence was lower than the FEPs screening team had expected. For seismicity, the DOE provided two reasons: (1) ground motion would move the disposal structures with the materials around them (e.g., engineered barriers above the disposal structure roofs) so that the relative motion would be limited and (2) the DOE will inform the design of the closure caps with a slope stability analysis, and the closure cap will be designed and built to minimize damage from seismic events. For seismicity, the DOE provided a reference that indicated that calcareous zones (also called soft zones) at SRS are primarily located south of the SDF (see DOE document SRNL-TR -2012 -00160 (ML13080A339) ). The DOE stated: If the FEPs Screening Team had studied this information prior to performing the FEPs screenings, it is likely that the FEPs Screening Team could have screened out FEPs 6.2.01, 6.2.03, and 6.2.04 on the basis that their qualitative perceptions may have changed. In addition, the DOE indicated that the DOE response to the NRC Request for Supplemental Information (RSI) included a sufficient range of infiltration rates and saltstone properties that it would account for the effects of seismicity or subsidence.

For erosion, weathering, and mass wasting (FEPs 2.6.07 and 2.6.11) the DOE indicated that the FEPs had been addressed in the DOE RSI response because the RSI response included uncertainty ranges for the performance of engineered layers within the closure cap sufficient to account for the FEPs. In addition, for mass wasting (FEP 2.6.11), the DOE indicated the FEP was addressed in the retreating cap sensitivity analysis in the 2020 SDF PA. The DOE also indicated that the DOE would perform a slope stability analysis before finalizing the design of the closure cap, and that the DOE would mitigate the risk of mass wasting based on the slope stability analysis prior to finalizing the closure cap design.

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For episodic or pulse flow and release (FEP 5.1.07), the DOE provided additional information to address the FEP in the DOE document SRMC-C WDA-202 0-00016. In that document, the DOE provided projections of dose to a member of the public 100 m from the SDF boundary under different flow conditions the DOE designed to mimic episodic flow. To represent episodic flow, the DOE varied the modeled Darcy velocities in the vadose zone and aquifer thickness over 10- year modeling time steps based on the c ompliance case annual average values. For the increased flow time periods, the DOE doubled the c ompliance case Darcy velocities in the vadose zone and multiplied the Darcy v elocities in the saturated z one by a factor of 1.5. For the decreased flow time periods, the DOE set the vadose zone Darcy velocities to zero and multiplied the saturated zone Darcy velocities by 0.5. The DOE also changed the modeled thickness of the saturated zone to reflect changes in the flow. For the increased flow time periods, the DOE increased the thickness of the saturated zone by 3.7 m (12 ft). For the decreased flow time periods, the DOE decreased the thickness of the saturated zone by 3.7 m (12 ft). The results of the DOE analysis showed small vacillations around the projected dose for the compliance case. Based on that analysis, the DOE indicated that the FEP had been addressed and was not expected to have a significant effect on the projected dose.

3.4 Conceptual Model Development

The DOE described the process the DOE used to develop conceptual models for the 2020 SDF PA in SRR-CWDA-2018-00006. The first step was to develop an interaction matrix (IM). In SRR-CWDA-2018-00006, the DOE described an IM as a tool for mapping specific FEPs to various system components and interactions between system components, as a method for developing modeling scenarios. As described in the NRC guidance in draft NUREG-2175, the first step in developing an IM is to divide a system into components. For the 2020 SDF PA, the DOE divided the system into the 26 components shown in Table 8.

Figure 2 in this TRR shows the DOE IM for the Central Scenario for the 2020 SDF PA. The IM uses the 26 components in Table 8 as row and column headers. Each matrix entry represented the effect of the row component on the column component. For example, an entry in the first row and second column would be identified as IM 01.02 and would represent the effect of precipitation on the closure cap. Entries on the diagonal line reference single components because they represent the same component row and column. For example, in Figure 2 in this TRR, element IM 15.15 represents the performance of the lower mud mat. The DOE referred to entries with the same row and column header as leading diagonal elements.

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Table 8. System Components for Central Scenario (A dapted from Table 3.1-1 in the 2020 SDF PA)

Component Number Description 1 Precipitation 2 Closure Cap 3 Disposal Structure Roof 4 Waste (Decontaminated Salt Solution) 5 Saltstone Hydraulic Conductivity 6 Saltstone Reducing Capacity 7 Pore Water Chemistry 8 Gaseous Phases 9 Disposal Structure Columns 10 Disposal Structure Joints / Waterstops 11 Disposal Structure Interior Liner 12 Disposal Structure Walls and Floor 13 Upper Mud Mat 14 Liner Between Mud Mats 15 Lower Mud Mat 16 Backfill 17 Groundwater Chemistry 18 Vadose Zone 19 Saturated Zone 20 1-Meter or 100-Meter Well 21 Surface Streams 22 Soil 23 Vegetation 24 Livestock 25 Human 26 Exposure / Risk

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Figure 2. Interaction Matrix for the Central Scenario for the 2020 SDF PA (from Figure A-1 in SRR-CWDA-2017 -00057)

The DOE used the IM and the final FEPs list to identify the need for alternative conceptual models (SRR-CWDA -2018-00006). Figure 3 below provides an overview of the process that the DOE used, which the DOE referred to as the FEPs auditing process. Although the DOE document SRR-CWDA -2018-00006 indicated that alternative conceptual models were based on future scenarios, the process overview shown in Figure 3 below illustrates the development of alternative conceptual models directly from the FEPs auditing process. T he DOE began with the FEPs list and evaluated whether each FEP was represented by a leading diagonal element (LDE) or an off-diagonal element (ODE) in the IM for the Central Scenario. If the DOE could not map the FEP to an IM entry, then the DOE evaluated whether the FEP could be represented with an alternative conceptual model. If it could not, then the DOE determined whether the FEP could be represented with a sensitivity analysis. If the FEP could not be represented with either an alternative conceptual model or a sensitivity analysis, then the DOE provided an explanation for excluding the FEP from the PA.

Although Figure 3 below shows creation of an alternative conceptual model and creation of a sensitivity analysis as separate steps, the DOE did not explicitly distinguish between an alternative conceptual model and a sensitivity analysis in SRR -CWDA -2018-00006 or in the 2020 SDF PA. During an Onsite Observation Visit (OOV) with the DOE on July 9-11, 2018, the NRC staff clarified that that a sensitivity analysis was typically designed to provide information about the importance of a parameter or engineered feature to projected system performance and does not necessarily reflect a plausible site condition (e.g., no closure cap), whereas an alternative conceptual model would represent a plausible site condition (ML18219B859).

Consequently, a sensitivity analysis might represent changes to one parameter or feature in isolation, whereas an alternative conceptual model typically represents changes to multiple parameters or features that might function differently in the alternative conceptual model.

Based in the IM (Figure 2 above), the DOE determined 163 FEPs were addressed in the conceptual model for the Central Scenario. During the first step of the FEPs auditing process illustrated in Figure 3, below, the DOE eliminated 51 FEPs from inclusion in the PA Model. The DOE eliminated most of those FEPs for one of three reasons: (1) they were addressed in documentation independent of the PA Model (e.g., FEP 1.1.01, assessment purpose), (2) the DOE planned to address them outside of the PA model (e.g., FEP 1.4.03, development of expertise), or (3) the DOE determined the FEP was too broad and was represented by more specific FEPs in the PA Model.

For many of the FEPs the DOE eliminated from the PA Model during this step, the D OE provided more than one reason for exclusion from the PA model. For example, the DOE excluded FEP 1.3.03, model confidence, both because it was too broad (i.e., reason 3) and because the DOE addresses model confidence outside of the PA model with research, monitoring, and maintenance (i.e., reason 2). Of the 22 FEPs the DOE identified as being too broad during the FEP audit process, only one, F acility Factors (3.1.03), had no other reason for exclusion.

Table 9 in this TRR lists the 70 FEPs that the DOE determined should be addressed in the PA model but were not addressed in the conceptual model for the Central Scenario or excluded during FEPs audit process (i.e., 284 FEPs screened in - 163 in the Central Scenario - 51 excluded from the PA model = 70 FEPs).

Figure 3. Auditing of Features, Events, and Processes List for Conceptual Model Development (From Figure 1.2-2 in SRR -CWDA-2018-00006)

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Table 9. FEPs Not Addressed in the Conceptual Model for the Central Scenario or Excluded During FEPs Auditing (Table 4.4-1 in SRR -CWDA-2018-00006)

No. Name No. Name 2.4.05 Microbial Activity 3.7.09 Localized Degradation 2.5.08 Geologic Discontinuities and Boundary 3.7.15 Waste Form (Saltstone) and Concrete Conditions (Fractures, Faults, and Cracks) (SDU) Macroscopic Fracturing 2.5.10 Unconsolidated Soft Zones 3.7.19 Site Stability 2.5.11 Undetected Geologic Features 3.8.02 Void Space Formation 2.6.05 Mechanical Effects on Geologic Features 3.8.06 Incomplete Closure 2.6.07 Erosion and Weathering 3.8.07 Mechanical Effects at Engineered Barrier System Component Interfaces 2.6.11 Mass Wasting 4.1.06 Waste Allocation and Emplacement 2.6.14 Closure Cap Performance (Differential 4.5.05 Radiation Effects on the Waste Closure Settlement) System 2.7.03 Cold Weather Effects 4.5.06 Radiolysis Effects 2.7.06 Acid Rain 4.5.07 Radionuclide Interaction with Corrosion Products 2.8.05 Focused Infiltration 5.1.03 Focusing of Flow Along Preferred Flow Paths (Fingers, Weeps, Faults, Fractures) 3.2.04 Manufacturing and Commissioning of 5.1.07 Episodic or Pulse Flow and Release Components 3.2.05 Construction 5.1.10 Alteration and Chemical Weathering Along Flow Paths 3.2.07 Alternate Container Design & Construction 5.1.12 Film/Laminar Flow 3.2.08 Discrete Capping of Disposal Units 5.1.13 Calcareous Zone Flow 3.2.09 Repairs of Construction Defects 5.1.14 Deformation at Flow Interfaces 3.2.10 Preparation of Cementitious Materials 5.2.09 Perched Water Develops 3.3.07 Ancillary Equipment / Piping/Transfer Lines 5.3.06 Colloid Facilitated Transport 3.3.17 Variability of Field Emplaced Saltstone 5.3.09 Fast Transport Pathways 3.4.05 Saturation of Fractures 5.3.11 Solid-Mediated Migration of Contaminants 3.4.08 Dispersivity Inside SDUs 5.4.04 Organic matter impacts on sorption 3.4.10 Osmotic Pressure 5.4.05 Oxidation along Fractures 3.4.13 Condensation on Closure System Surfaces 5.4.06 Rinse Release 3.5.13 Dispersion of the Oxidation Front 6.1.01 Inadvertent Human Intrusion 3.5.18 Complexation in the Natural System 6.1.03 Drilling Activities 3.5.20 Reaction Kinetics 6.1.04 Excavating and Mining Activities 3.5.21 Pooling Water Above SDUs 6.1.06 Animal/Plant Intrusion 3.5.24 Colloid Generation 6.2.01 Seismicity 3.5.25 Chelating Agent Effects 6.2.03 Seismic-Induced Damage or Changes to System Components 3.6.01 Thermal Processes and Conditions in the 6.2.04 Effects of Subsidence Engineered System 3.6.02 Thermal Processes and Conditions in the 6.4.02 Flooding or Drainage System Failure Natural System 3.6.05 Thermo-Chemical Alteration, Near-Field 6.4.03 Releases Prior to Closure 3.7.02 Mechanical Degradation Mechanisms 6.4.04 Forest/Brush Fire 3.7.06 Concrete Shrinkage/Expansion 6.4.06 Movement of the Waste Form 3.7.08 Swelling of Backfill and Emplacement 6.4.07 Cave-In, Collapse, or Rockfall Materials

Although Figure 3 in this TRR illustrates a process to identify the need for alternative conceptual

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models though the FEPs auditing process, the DOE appeared to use the process to generate future scenarios. For example, the DOE document SRR -CWDA -2018-00006 does not list alternative conceptual models identified through the FEPs auding process, in contrast with the plan shown in Figure 3. In general, the DOE appeared to equate alternative conceptual models with future scenarios, although the DOE defined the two terms differently in SRR -CWDA -2018-00006. This TRR provides a list of the future scenarios the DOE identified through the FEPs auditing process in Section 3.4.

As discussed in Section 3.3 in this TRR, the DOE determined that 56 of the 70 FEPs in Table 9, above, were accounted for in modeling cases the DOE refer red to as alternative scenarios. The DOE stated in Table 4.5-1 in SRR-CWDA -2018- 00006 that three of the remaining 14 FEPs were addressed with sensitivity analyses based on the Central Scenario (i.e., equations and parameters were equal to those of the Central Scenario except for the specific changes listed in parenthesis). However, in the DOE Response to Clarifying Comment (CC)-4 in SRMC -CWDA-2022-00016, the DOE declared that not all of the recommendations from SRR -CWDA-2018-00006 were directly implemented, and that there were no sensitivity models associated with the three FEPs although the DOE response did provide insights into the potential impacts related to the three FEPs based on the results of other, more indirect, sensitivity cases.

• Acid rain (FEP 2.7.06) (lower pH infiltrating water)

• Alternate container design and construction (FEP 3.2.07) (different disposal structure design and locations)

• Ancillary equipment and piping or transfer lines (FEP 3.3.07) (transfer lines include residual waste)

Finally, the DOE determined that it would not address the remaining 11 FEPs in the PA Model because no further action was needed. The DOE used that basis to screen out the following FEPs:

• Microbial activity (FEP 2.4.05)

• Focused infiltration (FEP 2.8.05)

• Osmotic pressure (FEP 3.4.10)

• Condensation on closure system surfaces (FEP 3.4.13)

• Reaction kinetics (FEP 3.5.20)

• Thermal processes and conditions in the engineered system (FEP 3.6.01)

• Thermal processes and conditions in the natural system (FEP 3.6.02)

• Thermo-chemical alteration, near -field (3.6.05)

• Radiation effects on the waste closure system (4.5.05)

• Radiolysis effects 4.5.06)

• Film/Laminar flow (5.1.12)

That step was similar to a FEP being screened out during the Phase II screening process if the FEPs screening team determined that a FEP was either not probable or consequential enough (or both) to warrant inclusion in the PA model. The main difference was that the FEPs screening team members were selected based on their expertise in a broad range of applicable technical fields. In contrast, the DOE did not indicate the individual contributors responsible for excluding FEPs during the FEP auditing process. The DOE provided the basis for determining that no further action was needed for each of 11 FEPs above in Table 4.5-1 (SRR-CWDA -2018-00006).

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4 NRC Staff Evaluation

The NRC SDF Monitoring Plan does not include a monitoring area for scenarios and conceptual models. Instead, it groups monitoring factors related to scenarios and conceptual models with other monitoring factors under MA 10 ( Performance Assessment Model Revisions). MA 10 includes three monitoring factors related to scenarios and conceptual models: (1) MF 10.01 (Implementation of Conc eptual Models) related to the mathematical and computational models that the DOE uses to implement its conceptual models; (2) MF 10.02 (Defensibility of Conceptual Models) related to the support that the DOE presents for its SDF conceptual models, and (3) MF 10.14 (Scenario Development and Defensibility ) related to the DOE process for developing future scenarios and the support for those scenarios. Because of the importance of those areas to understanding model uncertainty, the NRC staff recommends creating a new Monitoring Area tilted Future Scenarios and Conceptual Models to focus on issues related to the development of plausible future scenarios and conceptual models. Therefore, the NRC staff also recommends moving the three monitoring factors discussed above to the new MA. In addition, the NRC staff also recommends changing the priority of MF 10.01 from high to medium, as discussed in NRCs Model Integration TRR (ML23017A090).

Recommendation FSCM-0 1 The NRC staff recommends opening a new monitoring area entitled Future Scenarios and Conceptual Models under the performance objectives of §61.41 and §61.42.

Recommendation FSCM-0 2 The NRC staff recommends closing the medium priority MF 10.14 and opening a new monitoring factor entitled Scenario Development and Defensibility under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Scenario Development and Defensibility after the DOE updates the performance assessment and the NRC staff determines that future scenarios are appropriate.

Recommendation FSCM-0 3 The NRC staff recommends closing the high priority MF 10.02 and opening a new monitoring factor entitled Defensibility of Conceptual Models under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of

§61.41 and §61.42. The NRC staff expects to close the new monitoring factor Defensibility of Conceptual Models after DOE updates the performance assessment and the NRC staff determines that the conceptual models are appropriate.

In addition, in the Model Integration TRR ( ML23017A090), the NRC staff recommended closing the high priority MF 10.01 and opening a new monitoring factor entitled Implementation of Conceptual Models under the new Future Scenarios and Conceptual Models monitoring area.

The Model Integration TRR (ML23017A090) also recommended changing the high priority status of the new monitoring factor to medium priority and expects to close the new monitoring factor Implementation of Conceptual Models under the performance objectives of §61.41 and

§61.42 after DOE updates the performance assessment and the NRC staff determines that the conceptual models are appropriately implemented.

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4.1 Overview

Although the general scenario development for the 2020 SDF PA can be described as a bottom -up approach, there are top-down elements involved, such as the Central Scenario having been chosen based on expert judgement to represent the most probable and defensible future evolution of Z -Area before any FEPs analysis was undertaken. Only after the DOE developed the site conceptual model to represent how the site would function in the Central Scenario did the DOE then develop a list of FEPs. The NRC staff finds that approach acceptable if a scenario development analysis is performed in a consequential and logical manner so that risk sign ificant FEPs, plausible alternative conceptual models, and plausible alternative future scenarios are included and assessed in the PA process.

In general, the NRC staff found most of the steps for developing an initial FEPs list, screening the FEPs, and developing alternative modeling scenarios to be well documented and transparent by the DOE. However, the NRC staff found that the DOE basis for screening out FEPs considered to be too broad was insufficiently documented (see Section 4.2.2 in this TRR).

The NRC staff also determined that the basis for eliminating 11 FEPs from the PA model during the FEPs auditing process on the basis that no fur ther action was needed was insufficiently documented. Although the DOE provided bases for determining no further action was needed to address each FEP, the DOE did not indicate why the judgement of the individuals who conducted the auditing process superseded the judgment of the FEPs screening team members, who were selected based on their expertise in relevant fields. In general, the NRC staff reviewed the technical issues specific to the FEPs excluded on this basis in TRRs specific to each topic area. For example, the DOE excluded reaction kinetics (FEP 3.5.20) on the basis that the DOE assumed all reactions in saltstone would come to equilibrium. The NRC staff addressed that technical issue in a TRR entitled Near Field Flow and Transport (ML23017A086).

In addition, the NRC staff determined that the DOE appeared to use the terms alternative conceptual model, sensitivity analysis, and scenario interchangeably. That lack of precision was detrimental to ensuring that the PA model accounted for all relevant FEPs because the DOE labeled certain modeling cases as alternative scenarios although they treated the FEPs in a more isolated manner, characteristic of a sensitivity analysis (see Section 4.2.2 in this TRR) and frequently excluded FEPs based on the results of such sensitivity analyses. The NRC staff found that rationale for screening out individual FEPs not to be acceptable, if the analyses were performed so that interdependencies and interrelationships of the FEP were ignored, that is, where potential impacts on or changes to other FEPs were not addressed. The DOE probabilistic models documented in SRR-CWDA-2021-00040 ( ML21160A064) and SRR-CWDA-2021-00066 (ML21217A083) have the capability to capture such interdependencies and interrelationships between FEPs and include parameter ranges for many features effected by various degradation processes, including the degradation processes associated with climate change, a potentially disruptive event. The NRC staff TRR entitled Percolation Through and Potential Erosion near the Closure Cap (ML23017A083) discussed the DOE climate change modeling efforts in more detail.

The DOE stated that it is likely that the FEPs analysis could have benefited from including additional team members because the results of the FEPs analys is were limited by the experiences and knowledge base of the FEPs Team members. For example, the DOE declared that screening out FEP 3.4.11 Water Table Variability was an oversight in the DOE Response

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to RAI Question Conceptual Model and Future Scenario Uncertainty (CM &FSU )-2 (SRR-CWDA-2022-00016) and realized that water table variability to be a relevant and potentially significant phenomenon affecting radionuclide and chemical transport from the disposal structures. The potential impact of water table variability is especially strong when associated with such potentially disruptive events as climate change with wetter and drier conditions. In the DOE Response to RAI Question CM&FSU-3, which had asked for additional information on discrete FEPs that were not included in the original list of FEPs (discussed in Section 3.4 of this TRR), the DOE stated that it was likely that the FEPs effort could have benefited from including more team members although efforts were made to assemble a team with a wide range of backgrounds.

Finally, evaluating the DOE FEPS analysis and scenario development process was complicated by features of design that have not yet been determined (e.g., from features as specific as the fill material of the erosion barrier to the overall design of the closure cap itself) and by analyses that have not yet been performed (e.g., site stability analyses). Those aspects limit the NRC staff ability to asses s FEPs analyses and model completeness by having such potentially significant processes depend on a future design and work.

4.2 Features, Events, and Processes

4.2.1 Identification of Features, Events, and Processes

The NRC staff found the DOE process for developing an initial list of FEPs to be acceptable because it was thorough and well documented. The NRC staff found the process to be thorough because it relied on multiple relevant standard FEPs list and previous analyses of waste disposal at the SRS. The NRC staff found the process to be well documented because the NRC staff was able to trace the origin of the FEPs.

4.2.2 Screening of Features, Events, and Processes

The NRC staff determined that several parts of the DOE process for screening FEPs were acceptable because they were transparent and suited for the purpose. Specifically, the NRC staff determined that the two-phase screening process was acceptable because it fostered independent assessments by the subject matter experts before they met to discuss the FEPs.

The NRC staff also found that the experts perceived frequency and probability of each FEP were suitable for the purpose of making screening decisions. In addition, the NRC staff found that the numeric scale the DOE used for frequency and probability was acceptable because it provided acceptable transparency into the screening decisions. T he NRC staff determined that the composition of the FEPs screening team was acceptable because the members of the team had professional expertise and education necessary to adequately address FEPs relevant to SDF performance. However, the DOE stated in their comment response to CM&FSU -3 in SRMC-CWDA -2022-00016 that the FEPs analysis could have benefited from including additional team members with a greater range of disciplines and knowledge in topic areas since the results of the FEPs analysis were limited by the experiences and knowledge base of the FEPs team members. In addition, the NRC staff reviewed the insights provided by the DOE in the comment response to CC-4 in SRMC -CWDA -2022-00016 into the potential impacts related to FEP 2.7.06 (acid rain), FEP 3.2.07 (alternate container design and construction), and FEP 3.3.07 (ancillary equipment and piping or transfer lines) and found them to be acceptable bases to screen out the FEPs because the rationale was clear and logical. The NRC staff also

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found the DOE decision to programmatically screen in 50 FEPs prior to the FEPs screening teams two-part process to be acceptable because screening in additional FEPs is conservative (i.e., it would tend to increase the projected dose). The NRC staff found the DOE decision to programmatically screen out three FEPs prior to the FEP screening team assessment to be acceptable because the justifications given by the DOE to screen out FEP 1.3.11 and FEP 1.4.06 were found to be sound by the NRC staff. As for FEP 3.1.02, Site development, is a FEP closely tied with human behavior and exposure scenarios. Exposure scenarios are developed based on the natural characteristics of the site and the natural features and processes of the site (i.e., developed within the Central Scenario or plausible alternative scenario). H umans will build their livelihoods and center their large and small activities around the natural environment that they live in. The DOE contemplated plausible site development by analyzing various exposure scenarios involving the construction and occupation of homes, farms, etc. on the site in the inadvertent intruder assessment. The Intrusion Analysis TRR (ML23017A085) discusses this in more detail.

However, the NRC staff did not find the DOE process for screening FEP s to be entirely acceptable. The NRC staff determined that the use of general FEPs instead of more specific FEPs related to discrete features of the SDF was unclear and caused significant discrete FEPs to be left out the FEPs analysis. For example, in an RAI Question (CM&FSU-3 in SRR -CWDA -

2022-00016), the NRC staff asked the DOE for additional information about the effects of treating the closure cap as a single component in the IM rather than addressing separate engineered features of the closure cap (e.g., drainage layers, HDPE/GCL barrier) separately. In response, the DOE indicated that the DOE did not represent some of the engineered features of the SDF with specific FEPs because the DOE did not expect them to affect SDF performance.

For one of the specific engineered features, the closure caps erosion barrier, the DOE indicated that the 2020 SDF PA represented the erosion barrier explicitly even though the FEPs analysis grouped the erosion barrier with the rest of the closure cap. In addition, the DOE indicated that the DOE did not represent some FEPs explicitly in the 2020 SDF PA because the 2020 SDF PA represented those FEPs non-mechanistically in sensitivity analyses. For the erosion barrier, the DOE also indicated that the soil -only closure cap sensitivity case demonstrated how the SDF would function without an erosion barrier. The NRC staff did not find that sensitivity analysis adequately addressed the effects of erosion, as discussed further below.

In SRMC-CWDA -2022- 00016, the DOE stated: it is acknowledged that defining and including more explicit FEPs during the FEPs process could have led to improved conceptual model development and analyses. Although it is difficult to discern the level of detail FEPs should be considered at, the NRC staff finds that the FEPs team members (i.e., subject matter experts) with their professional knowledge should be able to judge the appropriate level of detail for each FEP. For example, in CM&FSU -3 (SRMC -CWDA-2022-00016), the DOE brought up the example of the thermocouple trees installed in some of the disposal structures. The DOE determined at some point that these features, which occupy a relatively small amount of surface area, were not anticipated to have a large effect on the system performance. If such, a decision should be informed by a team of experts. However, the DOE may not have provided that level of detail to the FEPs team members so that they did not have an opportunity to provide their input.

That is, the team members may not have had risk-significant engineered features broken out into enough detail so they could weigh in on specific engineered features (e.g., drainage layers, composite barrier layers).

The NRC staff found the DOE responses (as described above) for not including significant

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discrete or specific FEPs not to be adequate because (1 ) the FEPs analysis team members were apparently not given an opportunity to evaluate the specific FEPs in question, (2 ) specific FEPs were not included in the scenario and conceptual model process that may have led to a risk significant alternative scenario or alternative conceptual model from being developed although the discrete FEP may have been explicitly included in the PA, and (3 ) sensitivity cases cannot evaluate the significance of FEPs as accurately as a plausible alternative scenario or alternative conceptual model unless the sensitivity analyses include all interdependencies and interrelationships of the FEP in question, which is especially true for one-off, one-on type of sensitivity cases.

However, for certain FEPs, sensitivity analyses may provide a sufficient basis to screen out FEPs if their interdependencies and interrelationships with other FEPs are negligible and not risk significant. This may be difficult to determine (or else the subject matter experts could have screened the FEP out), and for many FEPs, this is not the situation. For example, the comment response to CM&FSU-3 ( SRMC-CWDA -2022-00016) states that the s oil-only closure cap case provided insights relative to potential influence of a post-closure system that does not include the erosion barrier. This modeling case did not directly include erosion and mass wasting as part of the modeling, although increased infiltration rates were included to simulate the effects of erosion and mass wasting. However, that case did not include potential degradation of the lower lateral drainage layer above the roof due to the filling in of the sand by fine particle being transported downward since no upper lateral drainage layer would exist above the lower drainage layer to trap the fine particles. In addition, if the closure cap were to consist of soil only, the steep side slopes of the cover c ould quickly undergo significant erosion so that the roof and wall of the disposal structure itself may become exposed and degrade. This one example shows potentially important FEPs frequently have impacts on other FEP s either by their presence or by their absence. Therefore, the NRC staff determined that subsequent DOE FEPs analyses should be monitored to ensure that future analyses include a reasonable set of technical analyses to evaluate performance. The NRC staff sh ould monitor the identification and screening of FEPs including the adequacy of the level of detail for each FEP. The NRC staff will also monitor justifications for FEP exclusions from the PA process to ensure that the se justifications have strong bases (e.g., that inappropriate one-off type sensitivity analys es are not used as an exclusion justification).

Recommendation FSC M-0 4 The NRC staff recommends opening a new medium priority monitoring factor entitled Identification and Screening of Features, Events, and Processes under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of

§61.41 and §61.42. The NRC staff expects to close the new monitoring factor Identification and Screening of Features, Events, and Processes after the staff determines that the identification and screening of features, events, and processes (FEPs) has resulted in no risk-significant FEPs being excluded from consideration in the alternative scenario or conceptual model development process.

As discussed in the Section 4.1 of this TRR, many potentially risk significant degradation processes are assumed not to take place by the DOE because the specific features in question are part of a future design which has not been finalized, but the DOE assumes that the future designs will prevent much of the degradation from occurring so that performance will not be negatively affected. E valuating the DOEs FEPS analysis and scenario development process is complicated by this reliance on future actions and limits the NRC staffs ability to assess FEPs

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analyses and model completeness. Therefore, the NRC staff recommends monitoring those future designs with regard to the model support they provide in excluding or minimiz ing degradation processes discussed in this TRR.

Recommendation FSC M-0 5 The NRC staff recommends opening the new high priority monitoring factor Future Designs and Analyses as They Pertain to Potential Degradation Processes and Performance in the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Future Designs and Analyses as They Pertain to Potential Degradation Processes and Performance after the staff determines that DOEs future designs and analyses (e.g., slope stability analyses) can exclude or minimize the occurrence of degradation processes previously identified by the NRC staff (e.g., future closure cap design will specify a fill material for the erosion barrier to limit the flow of water towards the disposal structures but also allows sufficient water to flow through the erosion barrier to minimize saturated conditions and possible mass wasting in the layers above it).

4.3 Future Scenarios

As previously discussed, t he DOE reviewed the FEP list to determine which of the FEPs were included in the Central Scenario, then created alternative scenarios to address FEPs that the Central Scenario did not reflect, and based on Figure 1 in the TRR, created alternative conceptual models to represent how the site would function in the alternative scenarios.

However, Figure 3 in this TRR shows they developed alternative conceptual models directly from the FEPs during the FEPs auditing process. T he NRC staff determined that the DOEs alternative scenarios lacked plausibility, that is, the scenarios did not represent a plausible set of FEPs that could occur in the designated scenario. Instead, it appeared to the NRC staff that the DOE developed the alternative scenarios to evaluate the sensitivity of particular parameters or FEPs. The DOEs alternative scenarios mostly did not reflect alternative scenarios with a plausible alternative evolution of the site.

Future Scenario Development

The NRC staff determined that the DOEs top-down approach for developing the Central Scenario is acceptable because the DOE had provided strong technical bases with data and information to support the development of this scenario. However, there was an exception to the NRC staff conclusion and that was the exclusion of climate change. As discussed in Section 3.2.2 in this TRR, the FEP 2.7.08, Climate Change, was screened out in the 2020 SDF PA. It was identified as being too broad in scope with other more discrete FEPs having been identified and screened in. The NRC staff is not opposed to this approach; however, it was unclear to the NRC staff how this substitution of more detailed FEPs for the more general FEP, Climate Change, was carried out. Although rainfall was one of these more discrete FEPs, the DOE did not discuss the inclusion of other more relevant FEPs within the PA. Climate change is a general and very inclusive FEP, so that the number of FEPs related to this general FEP is large as are the number of interdependencies and interrelationships between it and other FEPs.

Simple sensitivity analyses that only increase or decrease r ainfall rates would not be equivalent to a future scenario with climate change. Based on the DOE Response to the RSI (see Section 3.6.3 in SRR-CWDA -2021-00036), climate transitions can occur within 10,000 years. Although

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the DOE excluded climate change and the response to climate change (FEP 2.7.09) in the 2020 SDF PA, so that no alternative future scenario including climate change could be developed for deterministic SDF PA, the DOEs probabilistic models documented in SRR-CWDA -2021-00040 and SRR-CWDA -2021- 00066 have the capability to capture impacts of other FEPs due to potentially disruptive events involved with climate change that can promote degradation processes. Therefore:

Part of Recommendation FSC M-0 6 The NRC staff recommends monitoring how the processes and impacts related to potential climate change are incorporated into the SDF PA. The NRC staff recommends monitoring how the processes of climate change affect PA performance results under the new monitoring factor entitled Scenario Development and Defensibility under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of §61.41 and §61.42.

The NRC staff TRR entitled Percolation Through and Potential Erosion near the Closure Cap (ML23017A083) discussed the DOE climate change modeling efforts in detail.

Alternative Future Scenarios Identified

In general, the NRC staff found the DOEs alternative scenarios lacked plausibility. That is, interdependencies and interrelationships between FEPs were not addressed and frequently not all plausible FEPs for a specific alternative scenario were included.

The NRC staff determined that the Infiltration and Soil -Only Closure Cap alternative scenarios did not adequately address erosion or mass wasting because they did not include damage to the drainage and composite layers above the disposal structure roofs, which would cause the models to underestimate inflow into the disposal structures compared to a seismic scenario.

Also, as discussed above, the NRC staff determined that the DOE soil-o nly modeling case did not represent the site evolution without a closure cap because a closure cap that did not include riprap or the erosion barrier would undergo significant erosion of the 0.5 degree or less side slopes and the upper perimeters of Saltstone Disposal Structure ( SDS) 7 through SDS 12 would be exposed. The NRC staff expects that exposure of those disposal structures would lead to accelerated degradation of the roofs and walls, which would increase the likelihood of exposing saltstone grout. The NRC staff expects that such an implausible alternative scenario, if it did occur, could preclude meeting the performance objectives for protection of an individual who inadvertently intrudes on the site and for site stability (i.e., 10 CFR 61.42 and 61.44). However, implausible future scenarios generally do not require analysis because the results are not plausible and the risk significance of individual FEPs are difficult interpret.

In the 2020 SDF PA, the DOE originally indicated that the Infiltration Case addressed erosion and mass wasting (FEPs 2.6.07 and 2.6.11). The NRC staff determined that the Infiltration modeling case also did not adequately address erosion or mass wasting because the modeling case only affected the infiltration rate and did not change the modeled performance of the drainage layer or HDPE/GCL barrier above the disposal structures, both of which could be affected by erosion or mass wasting. In the DOE document SRR-CWDA -2022-00016, the DOE indicated that the Retreating Cap Case addressed mass wasting. However, the NRC staff determined that the Retreating Cap Case also did not adequately address mass wasting because that modeling case also only changed the model i nfiltration rates over portions of the

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disposal structures and did not address plausible accelerated degradation of the engineered barriers above the disposal structures or the disposal structure roofs. The DOE also indicated that mass wasting would be limited because the DOE planned to perform a slope stability analysis prior to finalizing the closure cap design and the DOE will alter the closure cap design to ensure slope stability.

The NRC staff determined that the reliance on a future design and impl ementation of the closure cap to limit the effects of erosion, weathering, and mass wasting limited the ability of the NRC staff to assess the effects of those FEPs on SDF performance. Therefore:

Part of Recommendation FSC M-0 6 The NRC staff recommends monitoring the development of model support for the performance of the SDF due to erosion, weathering, and mass wasting. The NRC staff recommends monitoring the development of model support under the new monitoring factor entitled Scenario Development and Defensibility under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of §61.41 and §61.42.

Erosion or mass wasting [FEP 2.6.07 (Erosion and Weathering) and FEP 2.6.11 (Mass Wasting)] were discussed in detail in the NRCs Site Stability TRR (ML23017A114).

The DOE screened in the FEP 6.2.01, Seis micity and FEP 6.2.03, Seismic -Induced Damage or Changes to System Components based on a combination of probability (or likelihood) and consequence (or impact) as seen in Table 4 of this TRR. However, in SRMC -CWDA -2022-00016, the DOE indicated that it expected seismic damage of the disposal structures or saltstone grout to be improbable for three reasons: (1) the DOE designed the disposal structures to withstand the design basis seismic event for the SRS; (2) the relative motion of the disposal structures will be limited because the DOE will fill voids with clean grout, backfill ar ound the disposal structures with soil, and bury the structures below the closure cap; and (3) the DOE plans to conduct a slope stability analysis before finalizing the closure cap design and the DOE will design the closure cap to limit the effects of the design basis seismic event. Two of the three reasons listed above are dependent on future actions by the DOE. For information on this, see the proposed MF 1 4.04, Monitoring DOEs Future Designs and Analyses as They Pertain to Potential Degradation Processes and Performance, discussed above.

In addition, the DOE indicated that the DOE response to the NRC RSI included a sufficient range of infiltration rates and saltstone properties that it would account for the effects of seismicity. The DOEs probabilistic models do have the capability to capture more interdependencies and interrelationships between FEPs then the deterministic modeling and the many sensitivity analyses performed in the 2020 SDF PA; however, seismicity and its impacts (similar to climate change) are general and very inclusive FEPs, so that the number of related FEPs is large. Table 4.1 -5 in the 2020 SDF PA lists the following associations:

• liquefaction of the backfill materials and soils,

• shaking and damage to the waste form or engineered components,

• rockfalls,

• extension or creation of fractures or faults,

• damage from repeated vibration,

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• damage from physical contact between components,

• damage from rockfall,

• damage from stress resulting in dynamic or static loading, and

• damages related to movement or displ acement of components.

The results of an evaluation in the NRC staff TRR entitled Site Stability (ML23017A114) found 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 seismic-induced liquefaction and settlement were insufficiently representative or were inconclusive, so that NRC staff will monitor this issue in one of the monitoring factors under Monitoring Area 9 (Site Stability). Based on that, the NRC staff determined that excluding seismicity as FEP is premature.

Regarding FEP 6.2.02, Effects of Subsidence, the DOE indicated that the FEPs process did not use all of the available information about each FEP at the SRS (SRMC -CWDA -2022-00016). In that DOE R esponse, the DOE provided additional information about the lo cation of sink holes at the SRS and indicated that, if the FEPs screening team had considered that information during the screening process, the DOE expects the team would have screened out FEP 6.2.02. The NRC staff addressed the additional information the DOE provided about sinkholes at the SRS in a separate TRR on Site Stability ( ML23017A114). In that TRR, 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. Therefore, the NRC staff determined that excluding the effects of subsidence as a FEP is premature.

The NRC staff determined that the DOE evaluation of FEP 5.1.07, Episodic or Pulse Flow and Release was not adequate to address short-term episodes of high flow rates, such as individual storms. In the 2020 SDF PA, the DOE addressed FEP 5.1.07 with model runs that changed the flow rates over long time periods (i.e., hundreds or thousands of years). Although the DOE provided additional information in the DOE document SRMC -C WDA-2022-00016 that addressed 10-year changes in flow, the NRC staff did not find those sensitivity analyses adequately addressed FEP 5.1.07 for two reasons. First, the DOE conducted those analyses with an abstracted model that was not capable of the type of flow analysis that the NRC staff expects would be required to represent the effec ts of storm events on flow through the closure cap, engineered features above the disposal structures, disposal structures, and saltstone grout.

Second, the DOE modeled episodic flow by adjusting the modeled Darcy velocities and aquifer thickness based on the compliance case average values. The NRC staff determined that the DOE did not show that doubling an annual average Darcy velocity represented the flow that the SDF would encounter during a storm event. In the DOE document SRMC -CWDA -2022-00016, the DOE stated: The assumed episodic flow conditions exceed the degree of variability that can be expected under normal conditions. However, the DOE did not provide a basis for determining whether doubling the Darcy velocities exceeded potential episodic flows.

Furthermore, because the DOE based the increased flow rates on variations of annual averages, rather than the variability experienced during storm events, the NRC staff expected that the variation the DOE used does not adequately represent variation that could occur during storm events. Therefore, the NRC staff was unable to use the sensitivity cases the DOE provided in SRMC-CWDA -2022-00016 to assess the potential effects of episodic or pulse flow

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on SDF performance.

The DOE document SRMC-CWDA-2022-00016 indicates that the DOE does not expect episodic flow to affect disposal structure degradation because disposal structure degradation is dominated by diffusive processes (SRMC -CWDA -2022-00016). However, the NRC staff concluded that DOE determination does not have sufficient support because it does not address the potential for advective processes to become more significant to disposal structure degradation during episodic flow. Because the DOE did not model the flow that could occur during storm events, the NRC staff did not have enough information to evaluate the DOE assumption that disposal structure degradation would continue to be dominated by diffusive process during episodic flow. The DOE document SRMC -CWDA -2022- 00016 also stated :

Assuming alternating periods that 10 continuous years of no flow and 10 continuous years with double the flow is an extreme assumption. However, the 10-year duration of the modeled changes in elevation does not address the central issue of the R AI Question, which is about episodic or pulse flow, such as could occur because of a storm event. Although the duration of such events is much shorter than 10 years, the NRC staff expects the effect on the Darcy velocities in the closure cap and disposal structures could be much greater than double the compliance case annual average flow. Therefore:

Part of Recommendation FSC M-0 6 The NRC staff recommends monitoring the development of model support for the performance of the SDF in the case of episodic flow. The NRC staff recommends monitoring the development of model support for the performance of the SDF in the case of episodic flow under the new monitoring factor entitled Scenario Development and Defensibility under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of §61.41 and §61.42.

Although the DOE referred to the Stratified Saltstone, Perched Water, and Colloid Transport modeling cases as alternative scenarios, the NRC staff determined that it would be more appropriate to refer to those modeling cases as sensitivity analyses because they did not include FEPs capable of changing the evolution of the site, such as disruptiv e events. Although the NRC staff does not consider those cases to be alternative scenarios, the NRC staff determined that those modeling cases addressed many of the FEPs in Tables 4.4-7, 4.4-8, and 4.4-9 in SRR -CWDA -2018-00006 because the number of interdependencies and interrelationships between these FEPs and other FEPs are minimal so that the results of these modeling cases are more straightforward and clearer to interpret.

4.4 Conceptual Models

Conceptual Model Development

The NRC staff determined that the DOE top-down approach for developing the conceptual model for the Central Scenario i s acceptable because the outcome of a review of the SDF-related FEPs by the NRC staff arrived at a similar conclusion. However, as previously discussed in Section 4.2 of this TRR, the DOE created alternative conceptual models representing how the site would function for the DOE alternative scenarios which lacked plausibility. Similar to the DOE alternative future scenarios, the DOE conceptual models also appeared to be developed to evaluate the sensitivity of particular parameters or FEPs and frequently lacked plausibility (i.e., interdependencies and interrelationships between FEPs were not addressed and not all

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plausible FEPs for a specific alternative s cenario were included.) Most of DOEs alternative conceptual models described how related FEPs behave or are impacted within the bounds of implausible future scenarios that were not realistic.

Alternative Conceptual Models Identified by the NRC Staff

The NRC staff determined that the DOE had not evaluated an exposure or receptor scenario based on a plausible alternative conceptual model of the DOE Central Scenario involving potentially significant contaminant transport to the 100-meter boundary within the Upper Three Runs Aquifer - Upper Aquifer Zone (UTRA-UAZ) with the UTRA-UAZ being the hypothetical receptors main water source. The NRC staff found evidence for lateral flow in the UTRA-UAZ to be convincing and sufficient for the development of an alternative conceptual model within the Central Scenario. Sufficient water may be available in the UTRA -UAZ to be the main source of water for a hypothetical receptor and the NRC staff expects that the plausibility of such a receptor scenario would increase with a wetter climate.

The DOE indicated that bleed water and contaminated rainwater weeping from Cell G of SDS 4 in the 1990s (see DOE document SRR-CWDA 2016 00052 (ML16134A185)) had reached the unsaturated zone and saturated zone ( see DOE document SRR-CWDA 2016 00004 (ML16105A043)). Evidence indicated that contaminants from SDS 4 had traveled through the unsaturated zone and were then transported in the saturated zone on top of the TCCZ toward Well ZBG 2 and beyond. The results of the 2015 Z-Area groundwater characterization (SRNS -

RP-2015-00902 (ML16057A135)) study provided convincing evidence that the plume has traveled past the boundaries of the Z -Area and that horizontal transport dominates vertical transport. Because the SDS 4 plume traveled much quicker in the lateral direction compared to the vertical direction, the majority of future contaminants may also travel much further in a lateral direction than the DOE current conceptual model allows. That difference between observed and modeled water flows in the UTRA -UAZ could result in two different exposure scenarios, depending on the conceptual model being used. The plume data indicates that initial contaminants could remain longer and more concentrated in the UTRA -UAZ than in the UTRA-Lower Aquifer Zone (LAZ) and that the DOE needs to consider the possibility that a hypothetical receptor could obtain water from the more accessible UTRA -UAZ. The NRC staff TRR entitled Hydrogeology, Groundwater Monitoring, and Far -Field Model ing (ML23017A084) provided detailed information on that topic.

The Figures 5-4 and 5-6 in the DOE document SRR-CWDA -2018-00036 ( ML20206L238) presented a general conceptual model of contamination migration from SDS 4 to Well ZBG-2 that is in alignment with NRC staffs alternative conceptual model (see Figure 4 below). The general conceptual model presented shows vertical unit gradient flow in the unsaturated or vadose zone down to the saturated zone and then a predominately lateral flow in the UTRA -

UAZ with contaminants gradually flowing downward into the TCCZ and UTRA-L AZ. The NRC staff asked the DOE about the effects of an alternativ e conceptual model involving potentially significant contaminant transport within the UTRA -UAZ in the RSI Comment 8 ( ML20254A003).

The NRC staff requested information about flow in the UTRA-UAZ that was consistent with the observations from the contaminant plume from SDS 4 in the Second Set of RAI Questions for the 2020 SDF PA (ML21133A296) in RAI Far-Field (FF) -1. The DOE R esponse to RSI-8 (SRR -

CWDA-2021-00065) presented an alternative General S eparations Area (GSA) groundwater flow model that better represented hydrologic conditions near SDS 4, especially the elevations for the top and bottom of the TCCZ. In addition, the DOE presented particle tracking results for

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a refined Z-Area flow field cut from the alternative model. The DOE Responses to both RSI-8 and RAI FF-1 were not sufficient to the NRC staff because neither SRR -CWDA-2021-00065 nor the DOE Response in SRR-CWDA -2021-00072 for RAI FF-1 addressed lateral flow in the UTRA-UAZ.

Figure 4. Proposed Generalized Conceptual Model of M igration of C ontamination from SDS 4 to Well ZBG 2 (majority of contaminants move laterally within the UTRA-UAZ before flowing through the TCCZ) (F rom Figure 5-4 from SRR-CWDA-2018-00036)

The DOE Response to CM&FSU-5 in SRMC -CWDA-2022- 00016, partially addressed the NRC staffs request for additional information. The response included modeling results showing that gross alpha and nonvolatile beta were found in the water sample obtained from ZDPT 11 (SRNS -RP-2015-00902). The response also provided individual cross-sections for each modeled plume with depth for each disposal structure (i.e., SDS 1 through SDS 12) from the water table to the 100-meter boundary, which included a range of concentrations. H owever, the response did not address several pieces of requested information. The DOE response did not include peak concentration results for IHI Well 1 through IHI Well 7 from the 1,000-year and 10,000-year (see Table 6.1-2 and 6.1-4 in the 2020 SDF PA) provid ing the percent concentration obtained from different hydrogeological units (i.e., UTRA -UAZ, UTRA -LAZ, Gordon Aquifer Unit). Nor did the DOE either, (1 ) provide the results of an analysis of an exposure scenario with the receptor dependent on the UTRA -UAZ as the main water source (e.g., receptors water well is screened exclusively in the UTRA -UAZ ) or (2) demonstrate that the proposed alternative conceptual model is not plausible, or (3 ) demonstrate that the UTRA-UAZ as a main water source for a receptor is not plausible.

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The DOE did provide useful results from an alternative flow model using the alternative regional groundwater flow model named GSA_2021 (SRR -CWDA -2021-00065 (ML21217A082)), which was developed to have a higher-than -expected water table and faster lateral transport in the UAZ, such that nearly all plume migration occurs within the UTRA -UAZ. The modified GSA_2021 model, named GSA_2021_UAZ, incorporated modified tops of the TCCZ and UTRA-LAZ hydrostratigraphic units near SDS 4 and the moisture characteristic curves to increase the down component of unsaturated zone flow. That model was further modified by increasing hydraulic conductivity of the Transmissive Zone within the UTRA -UAZ by two-ti mes and by decreasing the hydraulic conductivity of the TCCZ and the UTRA -LAZ by four -times to increase lateral transport in the UTRA-UAZ and hinder downward migration. The DOE Response did not make clear if the hydraulic conductivities that were changed were horizontal or vertical. The results of the GSA_2021_UAZ model did not reflect hydraulic head calibration targets and the DOE did not attempt to simulate the past behavior of the SDS -4 plume and use concentrations and locations of the plume as calibrati on targets, as recommended in the NRC staff TRR entitled Groundwater Monitoring at and Near the Planned Saltstone Disposal Facility (ML18117A494). The DOE presented GSA_2021_UAZ model results that included simulated hydraulic head over the Z-Area, reverse particle tracking with cross-sections, and steady -state tracer plume simulation for all the disposal structures. In addition, the DOE presented Table CM&FSU-5.6 in SRMC -CWDA-2022-00016, which clearly demonstrated the GSA_2021_UAZ model was successfully transporting most of the contaminants in the UTRA -UAZ by showing the mass balance percentage for the hydrogeologic units for the Z-Area: 90% within the UTRA -

UAZ, 9.3% within the UTRA-LAZ, and 1.7% within the Gordon Confining Unit. That compares with trace mass fractions for the 2020 SDF PA: 11% within the UTRA -UAZ, 87.5% within the UTRA-LAZ, and 1.5% within the Gordon Confining Unit.

Some of the information and results in the DOE Response to CM&FSU-5 is not clear to the NRC staff. For example, Figure CM&FSU-5.21(a) presented simulated hydraulic heads from the original GSA_2018 and clearly showed the upper UTRA downgradient from SDS 4 as being dry so that it was not clear how GSA_2018 modeling results of the UTRA-UAZ pertaining to SDS 4 were obtained. In addition, the reverse particle tracking cross -sections from the GSA_2021_UAZ model (Figures CM&FSU-5.23(b) and CM&FSU -5.24(b)) clearly showed reverse particle tracking not originating from SDS 4. The NRC staff is not certain if findings discussed above are significant to the overall results. Regardless, the GSA_2021_UAZ model successfully transported the majority of the contaminants with in the UTRA-UAZ.

Figures CM&FSU-5.55 and CM&FSU -5.56 compared dose results for several alternative transport simulations. However, the doses were based on the highest concentrations observed anywhere along the 100-meter perimeter and at any depth whether in the UTRA -UAZ, TCCZ, or the UTRA-LAZ. The DOE results d id not include the results of an exposure scenario with the receptor dependent on the UTRA-UAZ as the main water source nor did the DOE results demonstrate that the proposed alternative conceptual model (i.e., lateral flow in the UTRA-UAZ) was implausible nor did the DOE results demonstrate that the UTRA -UAZ as a main water source for a receptor was unrealistic (e.g., water volume is insufficient at the 100-meter perimeter). Therefore, the NRC staff will monitor a DOE analysis of the groundwater yield of the UTRA-UAZ in the Z-Area to support a hypothetical receptor. Evidence for lateral flow in the UTRA-UAZ is convincing and sufficient for the development of an alternative conceptual model within the Central Scenario and sufficient water may be available in the UTRA -UAZ to be the main source of water for a hypothetical receptor. Therefore,

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Recommendation FSC M-0 7 The NRC staff recommends opening a new high priority monitoring factor entitled Groundwater Yield of the UTRA -UAZ in the Z-Area under the new Future Scenarios and Conceptual Models monitoring area under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Groundwater Yield of the UTRA-UAZ in the Z-Area after the staff has observed an analysis of groundwater yield of the Upper Three Run Aquifer - Upper Aquifer Zone (UTRA-UAZ) in the Z-Area has been performed to determine the plausibility of an exposure scenario with the UTRA-UAZ as the main source of water.

5.0 Teleconference or Meeting

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

6.0 Follow-up Actions

Besides recommendations to revise the NRC SDF Monitoring Plan, there are no specific Follow-up Actions related to this TRR.

7.0 Conclusions

In general, the NRC staff determined that the DOE process es for developing future scenarios and conceptual models for the 2020 SDF PA wa s comprehensive and well documented.

Exceptions to that determination are indicated in Section 4.0 in this TRR and are addressed in recommended changes to the NRC SDF Monitoring Plan below. However, the NRC staff also found that the DOE addressed the implementation of several risk significan t FEPs (i.e.,

seismicity, subsidence, erosion, weathering, and mass wasting) by indicating that a future design of the closure cap would limit the consequences of the FEPs. The NRC staff determined that the risk significance of those FEPs and the uncertainty of the future design and implementation of that design limited the NRC staffs ability to assess the projected performance of the SDF.

The NRC staff found the DOE process for developing an initial list of FEPs to be acceptable because it was thorough and well documented. The NRC staff determined that several parts of the DOE process for screening FEPs were acceptable because they were transparent and suited for the purpose. However, the NRC staff found that the DOE FEPs auditing process was not completely transparent because the DOE did not indicate why the judgement of the analysts who excluded FEPs in the final step of the FEPs auditing process superseded the judgement of the FEPs screening team members who screened thos e FEPs in.

The NRC staff determined that some of the DOEs alternative scenarios lacked plausibility. The DOE appears to have developed those scenarios to evaluate the sensitivity of particular parameters or FEPs, and specific processes and features that should have been screened in and been part of thos e alternative scenario were absent (e.g., seismic disruption of the closure cap without disruption of other engineered features). That is, the DOE alternative scenarios mostly did not reflect alternative scenarios with a pl ausible evolution of the site different from that of the Central Scenario.

Relatedly, the NRC staff determined that m ost of the alternative conceptual models appeared to

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describe how specific FEPs would behave or would be impacted within the bounds of implausible future alternative scenarios. That is, many of the alternative conceptual models did not account for the interdependencies and interrelationships between FEPs that would occur in a plausible alternative future scenario and frequently not all plausible FEPs for a specific alternative scenario were included in the conceptual model.

In addition, the NRC staff determined that the DOE did not evaluate all plausible alternative conceptual models for the Central Scenario. For example, the NRC staff has determined that the DOE has not evaluated an exposure scenario based on a plausible alternative conceptual model involving potentially significant contaminant transport to the 100-meter boundary within the UTRA-UAZ and the UTRA-UAZ as the hypothetical receptors main water source.

The NRC staff recommends making the following changes to the NRC SDF Monitoring Plan after the Technical Evaluation Repor t for the 2020 SDF Performance Assessment is complete:

Recommendation FSCM-0 1 Future Scenarios and Conceptual Models

• The NRC staff recommends opening a new monitoring area entitled Future Scenarios and Conceptual Models under the performance objectives of §61.41 and §61.42.

Recommendation FSCM-0 2 Scenario Development and Defensibility

• The NRC staff recommends closing the medium priority MF 10.14 and opening a new monitoring factor entitled Scenario Development and Defensibility under the new Future Scenarios and Conceptual Models monitoring area (from Recommendation FSCM-0 1) under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Scenario Development and Defensibility after the DOE updates the performance assessment and the NRC staff determines that future scenarios are appropriate.

Recommendation FSCM-0 3 Defensibility of Conceptual Models

• The NRC staff recommends closing the high priority MF 10.02 and opening a new monitoring factor entitled Defensibility of Conceptual Models under the new Future Scenarios and Conceptual Models monitoring area (from Recommendation FSCM -01) under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Defensibility of Conceptual Models after DOE updates the performance assessment and the NRC staff determines that the conceptual models are appropriate.

Recommendation FSC M-0 4 Identification and Screening of Features, Events, and Processes

• The NRC staff recommends opening a new medium priority monitoring factor entitled Identification and Screening of Features, Events, and Processes under the new Future Scenarios and Conceptual Models monitoring area (from Recommendation FSCM -01) under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Identification and Screening of Features, Events, and Processes after the staff determines that the identification and screening of features,

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events, and processes (FEPs) has resulted in no risk -significant FEPs being excluded from consideration in the alternative scenario or conceptual model development process.

Recommendation FSC M-0 5 Future Designs and Analyses as They Pertain to Potential Degradati on Processes and Performance

• The NRC staff recommends opening the new high priority monitoring factor Future Designs and Analyses as They Pertain to Potential Degradation Processes and Performance in the new Future Scenarios and Conceptual Models monitor ing area (from Recommendation FSCM-0 1) under the performance objectives of §61.41 and

§61.42. The NRC staff expects to close the new monitoring factor Future Designs and Analyses as They Pertain to Potential Degradation Processes and Performance after the staff determines that DOEs future designs and analyses can exclude or minimize the occurrence of degradation processes previously identified by the NRC staff.

Recommendation FSC M-0 6 Modifying Scenario Development and Defensibility

• The NRC staff recommends modifying the text of the new moni toring factor for Scenario Development and Defensibility, which the NRC staff recommended opening in recommendation FSCM-0 2. The NRC staff recommends updating the text under the performance objectives of §61.41 and §61.42 to include monitoring how the processes and impacts related to potential climate change are incorporated into the performance assessment. In addition, the NRC staff recommends monitoring the development of model support for the excepted performance of the SDF in the case of episodic fl ow, and for the performance of the SDF due to erosion, weathering, and mass wasting. The NRC staff expects to close the new MF after the DOE updates the performance assessment, and the NRC staff determines that future scenarios are appropriate.

Recommendation FSC M-0 7 Groundwater Yield of the UTRA-UAZ in the Z-Area

• The NRC staff recommends opening a new high priority monitoring factor entitled Groundwater Yield of the UTRA -UAZ in the Z-Area under the new Future Scenarios and Conceptual Models monitoring area (from Recommendation FSCM-01) under the performance objectives of §61.41 and §61.42. The NRC staff expects to close the new monitoring factor Groundwater Yield of the UTRA -UAZ in the Z-Area after the staff has observed an analysis of groundwater yield of the Upper Three Run Aquifer - Upper Aquifer Zone (UTRA-UAZ) in the Z-Area has been performed to determine the plausibility of an exposure scenario with the UTRA -UAZ as the main source of water.

8.0 References

International Atomic Energy Agency (IAEA), Safety Assessment Methodologies for Near Surface Disposal Facilities, Results of a Co-ordinated Research Project, Vol 1. July 2004. ISBN 92-0 -104004-0

Miller, B., Savage, D., McEwen, T., and White, M. SKI Report 02:35, Encyclopedia of Features, Events, and Processes (FEPs) for the Swedish SFR and Spent Fuel Repositories, Preliminary Version, August 2002. ML22271A279

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Nuclear Waste Management Organization, NWMO DGR -TR -2009-05, Deep Geologic Repository for OPGs Low & Intermediate Level Waste, Postclosure Safety Assessment (V1):

Features, Events, and Processes, July 2009. ML22271A193

U.S. Department of Energy (DOE), UCRL-76419, Rev. 0, Resuspension and Redistribution of Plutonium in Soils, January 1975. ML22010A042

___, ANL-WI S-MD -000027, Rev. 0, Features, Events, and Processes for the Total System Performance Assessment: Analyses, March 2008. ML090710324

___, FCRD-USED-2011 -000297, Rev. 0. Jones, R.H. Features, Events and Processes for the Disposal of Low-Level Radioactive WasteFY 2011 Status Report, Rev 0. September 2011.

ML13053A280

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

___, SRR-CWDA -2013- 00062, Rev. 2, FY2013 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, Sep tember 2014. ML15097A366

___, SRNS-RP-2015-00902, Z-Area Groundwater Characterization Data Report, January 2016.

ML16057A135

___, SRR-CWDA 2016 00004, Rev. 1, Comment Response Matrix for NRC Staff Request for Additional Information on the Fiscal Year 2014 Special Analysis for the Saltstone Disposal Facility at the Savannah River Site, March 2016. ML16105A043

___, SRR-CWDA 2016 00052, Rev. 1, Presentation for Savannah River Site Salt Waste Disposal NRC Onsite Observation Visit, April 2016. ML16134A185

___, SRR-CWDA -2016- 00072, Rev. 0, Fiscal Year 2016 Special Analysis for the Saltstone Disposal Facility, October 2016. ML18081A262

___, SRR-CWDA -2017- 00057, Rev. 0, Features, Events, and Processes for the Saltstone Disposal Facility Performance Assessment, August 2017. ML18170A253

___, SRR-CWDA -2018- 00006, Rev. 0, Conceptual Model Development for the Saltstone Disposal Facility Performance Assessment, May 2018. ML18143B265

___, SRR-CWDA -2018- 00036, Rev. 0, Evaluation of Soil and Groundwater Contamination from Saltstone Disposal Unit 4, July 2018. ML20206L238

___, SRR-CW DA-2019- 00001, Rev. 0, Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site. March 2, 2020. ML20190A056

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___, SRR-CWDA -2021- 00036, Evaluation of the Potential for Erosion in the Vicinity of Z-Area,

Savannah River Site, Aiken, SC, June 2021. ML21160A063

___, SRR-CWDA -2 021- 00040, Evaluation of the Uncertainties Associated with the SDF Closure Cap and Long-Term Infiltration Rates, Savannah River Site, Aiken, SC, June 2021.

ML21160A064

___, SRR-CWDA -2021- 00047, Rev. 1, Comment Response Matrix for the First Set of U.S.

NRC Staff Requests for Additional Information on the Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, July 2021. ML21201A247

___, SRR-CWDA -2021- 00065, Rev. 0, Response to NRC Request for Supplemental Information #8: Upper Three Runs Aquifer - Upper Aquifer Zone Lateral Flow Analysis,

August 2021. ML21217A082

___, SRR-CWDA -2021- 00066, Evaluation of the Combined Uncertainties Associated with the Long-Term Performance of Saltstone Disposal Facility Fl ow Barriers, Savannah River Site, Aiken, SC, August 2021. ML21217A083

___, SRR-CWDA -2021- 00072, Rev. 0, Comment Response Matrix for the Second Set of U.S.

Nuclear Regulatory Commission Staff Requests for Additional Information on the Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, August 2021.

ML21231A102

___, SRMC-CWDA-2022-00016, Rev. 0, Comment Response Matrix for the Fourth Set of U.S.

NRC Staff Requests for Additional Information on the Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, April 2022. ML22118A297

U.S. NRC, NUREG/CR-5512, Vol. 3, Residual Radioactive Contamination from Decommissioning: Technical Basis for Translating Contamination Levels to Annual Total Effective Dose Equivalent, Final Report, October 1992. ML082460902

___, NUREG-1563, Branch Technical Position on the Use of Expert Elicitation in the High-Level Radioactive Waste Program, November 1996. ML033500190

___, NDAA-Waste Incidental to Reprocessing Monitoring Plan for the Savannah River Site Saltstone Disposal Facility, Rev. 1, September 2013. ML13100A113

___, Technical Review: U.S. Department of Energy Documentation Related to Features, Events, and Processes in the F-Area Tank Farm Performance Assessment, April 2014.

ML13277A063

___, U.S. Nuclear Regulatory Commission, May 27 - 29, 2014, Onsite Observation Visit Report for the Savannah River Site Saltstone Disposal Facility, August 2014. ML14199A219

___, NUREG-2175, Guidance for Conducting Technical Analyses for 10 CFR Part 61, Draft Final Report, October 2016. ML15056A516

___, Technical Review of Dose Calculation Methodology for Liquid Waste Performance

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Assessments at the Savannah River Site, December 2016. ML16277A060

___, Technical Review of Groundwater Monitoring at and Near the Planned Saltstone Disposal Facility, May 2018. ML18117A494

___, U.S. Nuclear Regulatory Commission, July 9 - 11, 2018, Onsite Observation Visit Report for the S Savannah River Site Saltstone Disposal Facility, November 2018. ML18219B859

___, Preliminary Review of the U.S. Department of Energy 2020 Performance Assessment for the Savannah River Site Saltstone Disposal Facility, October 2020. ML20254A003

___, Second Set of U.S. NRC Staff Request for Additional Information Questions Regarding the 2020 Savannah River Site Saltstone Disposal Facility Performance Assessment, June 2021.

ML21133A296

___, Fourth Set of U.S. NRC Request for Additional Information Regarding the 2020 Savannah River Site Saltstone Disposal Facility Performance Assessment, February 2022. ML22026A391

___, Technical Review: Hydrogeology, Groundwater Monitoring, and Far -Field Modeling for the 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, Rev. 1, April 18, 2023. ML23017A084

___, Technical Review: Intrusion Analysis for the 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, Rev. 1, April 18, 2023. ML23017A085

___, Technical Review: Model Integration for the 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, Rev. 1, April 18, 2023. ML23017A090

___, Technical Review: Percolation Through and Potential Erosion near the Closure Cap for the 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, Rev. 1, April 18, 2023. ML23017A083

___, Technical Review: Site Stability for the 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, Rev. 1, April 18, 2023. ML23017A114

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