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Technical Review: Inventory for the 2020 Saltstone Disposal Facility Performance Assessment - CR
	ML23017A087
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Site:	PROJ0734
Issue date:	04/18/2023
From:	Christianne Ridge; Division of Decommissioning, Uranium Recovery and Waste Programs
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ML23090A081	List: ML23017A083; ML23017A084; ML23017A085; ML23017A086; ML23017A087; ML23017A088; ML23017A089; ML23017A090; ML23017A113;
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Technical Review: Inventory for the Department of Energy 2020 Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site

Date

((date:long

Reviewer

Christianne Ridge, Sr. Risk Analyst, U.S. Nuclear Regulatory Co mmission

1.0 Purpose and Scope

The purpose of this U.S. Nuclear Regulatory Commission (NRC) st aff Technical Review Report (TRR) is to document the NRC staff review of the radiological i nventory used in the U.S. Department of Energy (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 demonstr ated that radioactive waste disposal activities at the SDF are in compliance with the perfo rmance objectives of Title 10 of the Code of Federal Regulations (10 CFR) Part 61. This technical review also supports NRC monitoring of the SDF under Monitoring Factor (MF) 1.01, Inven tory in Disposal Structures, and MF 1.02 Methods Used to Assess Inventory, as detailed in the NRC staffs current plan for monitoring the SDF (NRCs Agencywide Documents Access and Manag ement System [ADAMS] under Accession No. ML13100A113).

The NRC staff reviewed both the radionuclide inventory projections the DOE used in models to support the 2020 PA for the SDF (SRR-CWDA-1019-00001, Rev. 0) ( referred to as the 2020 PA in this document) and the processes the DOE used to d evelop the inventory. Those processes included radionuclide screening, calculation of best estimates of radionuclide activities, development of probabilistic distributions for inve ntory values, and performance of uncertainty and sensitivity analyses. The scope of the review included the concentrations and volumes of waste emplaced in the SDF, which directly affect the projected dose to a member of the public 100 meters (m) (330 feet) from the SDF boundary (ref erred to in this document as a member of the public). The radionuclide concentrations in sa ltstone are an input into the model the DOE used to project a dose to an individual who inadv ertently intrudes into the SDF 100 years or more after site closure (referred to in this docum ent as an inadvertent intruder). The NRC staff will review the geometric inputs to calculating t he inventory encountered by an inadvertent intruder in a separate TRR because the geometric as sumptions are part of the conceptual model for inadvertent intrusion.

2.0 Background

2.1 Overview

This is the first NRC TRR on the SDF inventory. The NRC staff previously addressed the SDF inventory in the following documents:

2005 NRC Technical Evaluation Report (TER) for the SDF (ADAMS Accession No. ML053010225);

Enclosure

2005 NRC Request for Additional Information (RAI) on the 2005 DOE Draft Waste Determination for the SDF (ADAMS Accession No. ML051440589);
2007 NRC Monitoring Plan for the SDF (ADAMS Accession No. ML070730363);
2012 NRC TER for the SDF (ADAMS Accession No. ML121170309);
2012 NRC Letter from M. Satorius to T. Spears Acknowledging th e DOE Response to the NRC Letter of Concern Regarding Disposal Activities at the SDF (ADAMS Accession No. ML12213A447); and
2013 NRC Monitoring Plan for the SDF (ADAMS Accession No. ML13100A076).

The 2012 NRC TER for the SDF (referred to as the 2012 TER in this document) concluded that the NRC staff did not have reasonable assurance that the D OE disposal activities at the SDF would meeting the 10 CFR 61.41 performance objective, Prot ection of the General Population from Releases of Radioactivity. On April 30, 2012, the NRC issued a letter to the DOE and the South Carolina Department of Health and Environment al Control (SC DHEC) to inform those agencies of the NRC conclusion (referred to as the NRC Letter of Concern in this document). In response, the DOE provided the NRC with addition al information that included revised projections of the inventory of technetium-99 (Tc-99) i n Saltstone Disposal Structure (SDS) 1, 2 A, 2B, 3 A, 3B, 4, 5 A, and 5B.

After the NRC issued the 2013 Monitoring Plan for the SDF (refe rred to as the Monitoring Plan in this document), the NRC staff reviewed the DOE Fiscal Year ( FY) 2013 and FY 2014 Special Analyses for the SDF (ADAMS Accession Nos. ML14002A069 and ML15 097A366). However, the NRC staff did not write TERs about those DOE analyses or ad dress inventory in its RAI questions on those documents. Therefore, the 2012 TER and 2013 Monitoring Plan represent the most recent documented NRC staff review of the DOE SDF inve ntory. The remainder of this section provides key points from the documents listed abov e and the DOE FY 2013 and FY 2014 Special Analyses to provide background information on t he DOE SDF inventory development.

2.2 The 2005 NRC TER, 2005 NRC RAI, and 2007 NRC Monitoring Plan

Before the DOE began operating the Salt Waste Processing Facili ty (SWPF) at SRS in 2020, the DOE used two interim treatment processes to treat salt wast e. The first process, called Deliquification, Dissolution, and Adjustment, (DDA) did not r emove any dissolved radionuclides from the waste and also allowed a small amount of sludge that was entrained in the solid salt waste to be transferred in the salt waste and in corporated into saltstone. The second interim treatment process, which the DOE called the Act inide Removal Process/Modular Caustic Side Solvent Extraction Unit (ARP/MCU), had a decontamination factor of approximately 200 for cesium (Cs) and removed essenti ally all of the entrained sludge from the salt waste.

The main issues related to inventory identified in the 2005 RAI and 2005 NRC TER were: (1) uncertainty in the contribution of entrained sludge from wa ste treated with the DDA process and (2) traceability of individual radionuclide inventory value s. The uncertainty related to the contribution of entrained sludge decreased when the DOE finishe d treating waste with the DDA process and instead treated salt waste with the ARP/MCU. Altho ugh the DOE began using the

ARP/MCU to treat waste in 2008, the DOE completed the final DDA treatment in September 2009. Therefore, inventory uncertainty due to potent ial sludge entrainment is no longer an active issue. The NRC staff monitors the traceabilit y of individual radionuclide inventory values under MF 1.01 and MF 1.02 in the Monitoring Pl an.

In addition to sludge entrainment and data traceability, the 20 07 Monitoring Plan also indicated that the staff would monitor removal efficiencies to assess whe ther doses were maintained as low as reasonably achievable, as required by the 10 CFR 61.41 p erformance objective for protection of the general population from releases of radioacti vity.

2.3 The 2009 DOE PA, 2012 NRC TER, and 2013 NRC Monitoring Plan

In the 2009 DOE SDF PA, the DOE identified Tc-99, iodine-129 (I -129), radium-226 (Ra-226), neptunium-237 (Np-237), and protactinium-231 (Pa-231) as key ra dionuclides. In response to an NRC RAI question, the DOE indicated that it had significantl y overestimated the inventory of Ra-226 and its parent, thorium-230 (Th-230) because the actual concentrations of those radionuclides were below their detection limits (SRR-CWDA-2011- 00044, Rev. 1). In the 2012 NRC TER, the NRC staff determined that it was acceptable for th e DOE to develop the Ra-226 and Th-230 inventories based on the ingrowth from U-234 if ther e had not been significant inputs of thorium-bearing waste.

The NRC Monitoring Plan indicates that the NRC staff will track the inventory of disposed waste into each disposal structure and evaluate the effects on dose f rom any exceedance of the projected inventories in the 2009 DOE PA. The structure-by-str ucture inventory comparison envisioned in the Monitoring Plan is not possible because the D OE significantly changed the number and design of the disposal structures at the SDF since t he NRC issued the 2012 TER. Therefore, the NRC staff reviewed the changes in the projected SDF closure inventory in its entirety rather than on a structure-by-structure basis. Table 1 provides a comparison between the base case inventory the DOE used in its 2009 PA for the S DF and the most probable and defensible (MPAD) case the DOE used in the 2020 PA because the DOE identified both of those cases as the compliance case in their respective analys es. Table 1 includes both the original values provided in the 2009 SDF PA, which are based on an expected closure date of October 1, 2030, and values that reflect decay an ingrowth to t he current projected closure date of January 1, 2037 to provide an equal basis for comparison to the 2020 PA inventory. The NRC staff calculated the revised inventory values using the Gol dSim modeling platform and decay constants from the International Committee on Radiologica l Protection (ICRP) Report No. 107.

The inventory estimates of several radionuclides increased by m ore than an order of magnitude between the 2009 and 2020 PA: americium-241 (Am-241), Am-242 m, californium-249 (Cf-249), chlorine-36 (Cl-36), curium-247 (Cm-247), potassium-4 0 (K-40), niobium-93 m (Nb-93 m), plutonium-238 (Pu-238), Pu-239, and strontium-90 (Sr -90) (see Table 1). Despite the significant increases in the modeled inventory of these rad ionuclides, none made a significant contribution to the modeled dose in any of the Cent ral Scenario cases in the 2020 PA. The modeled inventory of three radionuclides decrease d by an order of magnitude or more between the 2009 and 2020 PA: lead-210 (Pb-210), Ra-226, and Th-230. The decrease in the modeled inventory of those three radionuclides is relate d to changes the DOE made to its model of the inventory of the uranium decay series. That chang e is discussed further below, in the context of the FY 2013 Special Analysis.

Table 1. Comparison of 2037 inventories the DOE used in the 2009 and 202 0 PAs for the SDF. 2009 PA Base 2009 PA 2020 PA (Ci) a, d Case, Base Case, Radionuclide October 1, Decayed to 2030 Closure January 1, Realistic MPAD Pessimistic Curies (Ci)a, b 2037 (Ci) a, c Ac-227 2.7x10-5 5.4x10-5 2.89x10-4 3.41x10-4 3.53x10-4 Al-26 1.3x101 1.3x101 2.52x101 3.65x101 3.92x101 Am-241 2.2x102 2.6x102 1.40 x104 2.06x104 2.21x104 Am-242 m 1.0x10-1 9.7x10-2 6.81 9.98 1.08x101 Am-243 4.2 4.2 6.17 8.84 9.45 Ba-137 m 2.8x105 (e) (e) (e) (e) Bk-249 1.2x10-26 (e) (e) (e) (e) C-14 1.6x102 1.6x102 5.39x102 7.86x102 8.41x102 Ce-144 2.5x10-8 (e) (e) (e) (e) Cf-249 4.4x10-11 4.3x10-11 3.38x10-1 3.38x10-1 3.38x10-1 Cf-251 1.2 1.2 1.48x10-1 1.48x10-1 1.48x10-1 Cf-252 1.2x10-16 (e) (e) (e) (e) Cl-36 3.1x10-2 3.1x10-2 9.02x10-1 1.31 1.42 Cm-242 6.7x10-2 (e) (e) (e) (e) Cm-243 2.2x10-1 1.9x10-1 3.29x10-2 4.51x10-2 4.80x10-2 Cm-244 1.9x102 1.5x102 1.16x102 1.63x102 1.75x102 Cm-245 9.4x10-1 8.5x10-1 9.20x10-1 9.56x10-1 9.65x10-1 Cm-247 3.9x10-6 4.2x10-6 1.77x10-1 1.77x10-1 1.77x10-1 Cm-248 4.9x10-12 (e) (e) (e) (e) Co-60 3.9 1.7 7.54 1.11x101 1.19x101 Cs-134 5.2x10-1 (e) (e) (e) (e) Cs-135 5.4 5.4 3.12 3.67 3.81 Cs-137 3.0x105 2.6x105 3.44x105 4.47x105 4.71x105 Eu-152 6.4 4.6 7.88 1.16x101 1.24x101 Eu-154 1.3x102 7.8x101 7.32x101 1.07x102 1.15x102 Eu-155 9.0 (e) (e) (e) (e) H-3 2.2x103 1.5x103 3.07x103 4.50x103 4.84x103 I-129 2.5x101 2.5x101 1.59x101 1.66x101 2.44x101 K-40 3.1x10-2 3.1x10-2 9.03x10-1 1.32 1.42 Na-22 4.6 (e) (e) (e) (e) Nb-93 m 3.2x101 3.2x101 2.02x103 2.06x103 2.07x103 Nb-94 3.3x10-1 3.3x10-1 2.34x10-1 2.97x10-1 3.12x10-1 Ni-59 5.8 (e) (e) (e) (e) Ni-63 1.8x102 1.7x102 1.99x102 2.91x102 3.12x102 Np-237 3.8 3.8 1.23x101 1.77x101 1.89x101 Pa-231 1.6x10-4 2.5x10-4 4.51x10-4 5.81x10-4 6.17x10-4 Pb-210 no initial inventory 7.3x10-1 6.06x10-5 7.08x10-5 7.38x10-5 Pd-107 4.1x10-1 (e) (e) (e) (e)

2009 PA Base 2009 PA 2020 PA (Ci) a, d Case, Base Case, Radionuclide October 1, Decayed to 2030 Closure January 1, Realistic MPAD Pessimistic Curies (Ci)a, b 2037 (Ci) a, c Pm-147 5.3 (e) (e) (e) (e) Pr-144 2.5x10-8 (e) (e) (e) (e) Pt-193 8.1x101 7.4x101 8.13x101 1.12x102 1.21x102 Pu-238 2.0x104 1.9x104 1.43x105 2.10x105 2.26x105 Pu-239 1.3x103 1.3x103 8.88x103 1.30x104 1.40x104 Pu-240 3.8x102 3.8x102 1.91x103 2.78x103 2.98x103 Pu-241 5.1x103 3.8x103 1.68x104 2.47x104 2.65x104 Pu-242 1.1 1.1 8.94 1.03x101 1.07x101 Pu-244 1.7x10-2 1.7x10-2 3.91x10-2 4.55x10-2 4.71x10-2 Ra-226 4.1 4.1 1.65x10-4 1.94x10-4 2.02x10-4 Ra-228 5.6x10-3 5.0x10-2 2.56x10-1 3.77x10-1 4.0510-1 Rh-106 7.8x10-5 (e) (e) (e) (e) Ru-106 7.8x10-5 (e) (e) (e) (e) Sb-125 2.1x101 (e) (e) (e) (e) Sb-126 7.8x101 (e) (e) (e) (e) Sb-126 m 5.2x102 (e) (e) (e) (e) Se-79 1.4x102 1.4x102 1.00x102 1.42x102 1.52x102 Sm-151 3.8x103 3.6x103 3.86x103 5.66x103 6.09x103 Sn-126 5.3x102 5.3x102 3.56x102 5.18x102 5.59x102 Sr-90 2.4x105 2.1x105 3.60x106 5.30x106 5.70x106 Tc-99 3.5x104 3.5x104 2.24x104 3.29x104 3.55x104 Te-125 m 5.2 (e) (e) (e) (e) Th-229 2.8x101 2.8x101 3.71 3.73 3.74 Th-230 2.0x101 2.0x101 1.29x10-2 1.50x10-2 1.55x10-2 Th-232 9.0x10-2 9.0x10-2 2.56x10-1 3.77x10-1 4.06x10-1 U-232 6.4x10-2 6.0x10-2 1.86x10-1 2.02x10-1 2.06x10-1 U-233 2.7x101 2.7x101 2.51x101 3.04x101 3.18x101 U-234 3.5x101 3.5x101 3.12x101 4.01x101 4.23x101 U-235 6.7x10-1 6.7x10-1 5.67x10-1 7.84x10-1 8.38x10-1 U-236 1.8 1.8 1.16 1.65 1.77 U-238 7.0 7.0 1.53x101 2.24x101 2.41x101 Y-90 2.4x105 (e) (e) (e) (e) Zr-93 3.2x101 3.2x101 9.85x101 1.39x102 1.50x102 (a) To convert Ci to Becquerel (Bq), multiply by 3.7x10 10. (b) From Table 3.3-7 of SRR-CWDA-2009-00017, Rev. 0. (c) The NRC staff decayed the values in column 1 to the current pr ojected closure date using the GoldSim modeling platform with decay constants from the ICRP Re port No. 107. (d) From the 2020 PA, Tables 3.3-5 (realistic), 3.3-6 (MPAD), and 3.3-7 (pessimistic). (e) Screened out of the 2020 PA (See Section 3.2 of this TRR).

2.4 The NRC 2012 Letter of Concern, 2012 DOE Response, and NRC 2012 Letter of Acknowledgment

In the 2012 TER, the NRC staff concluded that it did not have r easonable assurance that the DOE disposal activities at the SDF would meet the 10 CFR 61.41 performance objective, Protection of the General Population from Releases of Radioact ivity. On April 30, 2012, the NRC issued a Letter of Concern to the DOE and the SC DHEC to in form those agencies of the NRC conclusion. In response, the DOE provided the NRC with a r evised model of SDF performance and a revised projection of the inventory of techne tium-99 (Tc-99) in SDS 1, 2 A, 2B, 3 A, 3B, 4, 5 A, and 5B (SRR-CWDA-2012-00002, Rev. 0; SRR-C WDA-2012-00095, Rev. 1). In a letter dated August 31, 2012 (ADAMS Accession No. ML1 2213A447), (referred to as the NRC Letter of Acknowledgement in this document), the NRC staff indicated that the revised Tc-99 inventories the DOE provided for SDS 2 A, 2B, 3 A, 3B, 5 A, and 5B alleviated the NRC staff concern related to the 10 CFR 61.41 performance objec tive at that time, and that the NRC staff would continue to monitor several technical issues re lated to projected SDF performance. Specifically, the NRC stated that if the projecte d inventories for those disposal structures was correct, the disposal structures would be unlik ely to cause an off-site peak dose exceeding the requirements of §61.41 (i.e., 0.25 mSv/yr (25 mre m/yr)).

In the FY 2014 Special Analysis, the DOE increased the modeled inventories of Tc-99 in SDS 2B, 3 A, 3B, 4, 5 A, and 5B, as compared to the values the DOE projected in its response to the NRC Letter of Concern. In the transmittal letter for the FY 20 14 Special Analysis (ADAMS Accession No. ML14322A259), the DOE stated:

Disposal activities at the SDF during FY 2012 through FY 2014 h ave been at these lower Tc-99 inventory values [that the DOE projected in respons e to the NRC Letter of Concern] and DOE will continue to supply the quarterly Tank 50 sample results to NRC for your information. Based upon the results of the FY 2014 [S pecial Analysis], DOE has authorized disposal of Tc-99 at the analyzed inventory (see Tab le 3.4.1 of the FY 2014 [Special Analysis]).

Table 2, below, provides the Tc-99 inventories from the DOE FY 2014 Special Analysis in comparison to the lower values the DOE projected in response to the NRC Letter of Concern and the values the DOE used in models supporting the 2009 and 2 020 PAs. Section 2.6 of this TRR addresses the remaining inventory for the FY 2014 Special A nalysis.

The 2020 PA indicates that the DOE considers SDS 1, 2 A, 2B, 3B, 4, 5 A, and 5B to be operationally filled, and that SDS 3 A is 15 percent (%) full. Therefore, the Tc-99 inventories for those disposal structures in Table 2 represent emplaced sal tstone and the inventory for SDS 3 A represents a new inventory projection. Because the inv entories for all the disposal structures except for SDS 3 A are based on emplaced saltstone, they are the same in all the DOEs Central Scenario projections (i.e., the realistic, MPAD, and pessimistic projections). The value for SDS 3 A is from the MPAD projection.

Table 2. Comparison of projected Tc-99 inventories in the DOE 2009 PA and 2012 DOE response to the NRC Letter of Concern with emplaced and projected inventories modeled in the 2020 PA for SDS 1, 2 A, 2B, 3 A, 3B, 4, 5 A, and 5B. DOE Response DOE FY 2014 Disposal 2009 PA Base to the NRC Special Analysis 2020 PA (Ci) (a, c) Structure Case (Ci) (a) Letter of (Ci) (a, c) Concern (Ci) (a, b) 1 110 55 55 49.3 2 A 540 105 to 120 120 114 2B 540 105 to 120 140 137 3 A 540 98 to 117 540 363 3B 540 98 to 117 540 380 4 580 530 640 634 5 A 540 98 to 117 540 175 5B 540 98 to 117 540 120 (a) To convert Ci to Bq, multiply by 3.7x1010. Because of the long half-life of Tc-99, the NRC staff did not account for differences in the assumed year of site closure in these values. (b) Values for SDS 1 and SDS 4 are from SRR-CWDA-2012-00002, Rev. 0. The remaining values are from SRR-CWDA-2012-00095, Rev. 1. (c) Except for the value for SDS 3 A, values represent emplaced saltstone in operationally filled disposal structures. The value for SDS 3 A is from the MPAD projection.

2.5 The DOE Fiscal Year 2013 Special Analysis

In the FY 2013 Special Analysis, the DOE identified Tc-99, I-12 9, and Cs-135 as key radionuclides. The DOE FY 2013 Special Analysis used the same inventory as the DOE 2009 PA with four exceptions:

Pu-238 decreased by 40%
Ra-226 decreased by 99.96%
Th-230 decreased by 96%
U-234 decreased by 46%.

The DOE provided the basis for the adjustment to the inventorie s of Ra-226 and its parents in response to an NRC RAI on the 2009 PA (SRR-CWDA-2011-00044, Rev. 1) and a supporting reference (SRR-CWDA-2011-00115, Rev. 0). The largest modeled d ecrease occurred for Ra-226 and Th-230 because those radionuclides had concentration s below detection limits in most salt waste samples. The DOE revised its calculated inventories for Ra-226 and Th-230 by assuming their inventories could be determined based on ingrowt h from U-234 instead of by assuming Th-230 and Ra-226 were present at their detection limi ts (SRR-CWDA-2011-00044, Rev. 1).

2.6 The DOE Fiscal Year 2014 Special Analysis

The modeled inventory of most radionuclides in the FY 2014 Spec ial Analysis was significantly greater than the modeled inventories in the 2009 SDF PA, FY 201 3 Special Analysis, or 2020 PA. For radionuclides that the DOE did not consider to b e risk-significant in the initial phases of the FY 2014 Special Analysis, the DOE assigned the en tire SRS Tank Farms (i.e., F-Tank Farm and H-Tank Farm) inventory in supernate and salt waste (i.e., all phases except sludge solids) to each of the 15 disposals structures mo deled in that assessment with two exceptions:

the DOE assumed a decontamination factor of 200 for Cs-137; an d
the DOE reduced the modeled disposal structure inventories of Pu-238, Pu-239, and Pu-240 so that the concentrations would be less than the Low-Le vel Waste Class C limit in 10 CFR Part 61 because the DOE would not allow the act ual concentrations to exceed the Class C limit.

In the FY 2014 Special Analysis, the DOE identified Tc-99, I-12 9, Ra-226, and K-40 as key radionuclides because they caused a projected dose greater than 0.0025 millisieverts per year (mSv/year) (0.25 millirem per year (mrem/year)) to a member of the public within 20,000 years of SDF closure. However, the DOE also indicated that it assign ed overly conservative projected inventories to K-40 and Ra-226 because it did not consider them to be risk-significant during inventory development. In the analysis of results in the FY 20 14 Special Analysis, the DOE excluded Ra-226 and K-40 from the list of risk significant ra dionuclides. The DOE instead listed Tc-99 and I-129 based on their projected dose to a membe r of the public and listed Cs-135 based on its projected dose to an inadvertent intruder.

3.0 Inventory Development for the 2020 PA

3.1 Overview

For the 2020 PA, the DOE developed three inventory projections as part of its Central Scenario: realistic, MPAD, and pessimistic. The DOE desc ribed the realistic projection as its best estimate of the SDF inventory at the time of closure. The DOE described the MPAD inventory as more easily defensible than the realistic case, an d used it in the deterministic compliance case in the 2020 PA. Finally, the DOE described the pessimistic projection as biased toward increasing dose results and maximizing defensibil ity. The NRC staff used the terms realistic, MPAD, and pessimistic throughout this TR R to facilitate comparison with cited DOE tables and figures. However, the purpose of this NRC staff review is to determine whether the MPAD inventory is acceptable for use in models supp orting the 2020 PA. The NRC staff has not determined whether the MPAD inventory is the mos t probable and defensible inventory, or whether the projections the DOE refers to as the realistic and pessimistic are realistic or pessimistic, respectively.

The DOE considered three sources of inventory: (1) the existin g SDF inventory as of March 31, 2018, (2) the Tank Farms inventory in certain waste p hases as of January 8, 2018, as modified by Cs removal, and (3) the projected transfers from H-Canyon from FY 2019 through FY 2026, as modified by Cs removal. For the realistic, MPAD, and pessimistic inventories, the DOE used the same projections of the existing SDF inventory and the projected contributions from H-Canyon. Therefore, all the difference bet ween the realistic, MPAD, and pessimistic inventories results from differences in how the DOE treated the uncertainty in the

projected contribution to the SDF from waste in the Tank Farms. Section 3.3 of this TRR addresses the general process the DOE used to develop radionucl ide inventories for all radionuclides. Section 3.4 of this TRR addresses additional ca lculations the DOE used to refine the inventory projection for I-129 and Tc-99. The 2020 PA indi cates that the DOE performed those calculations because I-129 and Tc-99 dominate the dose to a member of the public and an inadvertent intruder in the 2020 PA.

3.2 Radionuclide Screening

To determine which radionuclides to include in detailed technic al analyses in the 2020 PA, the DOE began with an initial list of 80 radionuclides based on pro cess knowledge of the waste streams entering the tank farm and historical measurements. Fi gure 1 shows the DOE screening process to narrow down the list of 80 radionuclides i nto a list of 52 radionuclides of concern to include in detailed analyses (SRR-CWDA-2018-00044, Rev. 3).

Figure 1. Flow chart of the DOE screening process for the 2020 PA (Figure 1 in SRR-CWDA-2018-00044, Rev. 3).

As shown in Figure 1, the screening process considered the radi onuclide half-life, progeny, volatility, and the potential dose in conservative screening ca lculations based on the expected inventory to determine which radionuclides to screen out from f urther analysis. The screening scenarios included scenarios relevant to both a member of the p ublic and an inadvertent intruder. In those screening calculations (i.e., third and fou rth diamonds from the top on the left side of Figure 1), the DOE assumed that an individual directly consumes, inhales, showers in, or is directly exposed to soil, water, air with radionuclide conce ntrations equal to saltstone grout. The DOE then compared the results to a 0.25 mSv (25 mrem) annua l dose limit for a member of the public and a 5 mSv (500 mrem) annual dose limit for an inad vertent intruder. The DOE used the screening calculations for waste burial developed by t he National Council on Radiation Protection and Measurement (NRCP) in NCRP Report No. 123 to ide ntify radionuclides to deliberately screen in and did not use the NRCP by itself to screen out any radionuclides (see Figure 1).

In addition to the screening process described above, the DOE s creened out molybdenum-93 (Mo-93) based on analyses with the GoldSim model that supported the FY 2014 Special Analysis for the SDF that showed low dose projections for a mem ber of the public and an inadvertent intruder (SRR-CWDA-2015-00020, Rev. 0). Finally, t he DOE omitted palladium-107 (Pd-107) from detailed analyses although it did not meet the sc reening criteria. However, based on a comparison of the inventory, dose conversion factors, and transport properties of Pd-107 and I-129, the DOE expects the projected dose to either a membe r of the public or an inadvertent intruder from Pd-107 would be at least four orders of magnitude less than the projected dose from I-129 (SRR-CWDA-2021-00047, Rev. 0). Table 3 shows the result of the result of the screening process.

Table 3. Screening decisions in the 2020 PA for radionuclides the DOE determined could potentially be in saltstone (adapted from Tables 1, 2, and 3 of SRR-CWDA-2018-00044, Rev.3). Radionuclide Screening Decision H-3 C-14 Al-26 Cl-36 Screened in. K-40 Co-60 Ni-63 Se-79 Sr-90 Zr-93 Nb-93 m Nb-94 Tc-99 Pd-107(a) Sn-126 I-129 Cs-135 Cs-137 Sm-151 Eu-152 Eu-134 Pt-193 Pb-210 Ra-226 Ac-227 Ra-228 Th-229 Th-230 Pa-231 Th-232 U-232 U-233 U-234 U-235 U-236 Np-237 Pu-238 U-238 Pu-239 Pu-240 Pu-241 Am-241 Pu-242 Am-242 m Am-243 Cm-243 Cm-244 Cm-245 Pu-244 Cm-247 Cf-249 Cf-251 Na-22 Ru-106 Rh-106 Te-125 m Screened out because of short half-life Sb-125 Sb-126 Cs-134 Pr-144 without significant progeny. Ce-144 Pm-147 Lu-174 Am-242 Cm-242 Bk-249 Cf-252

Screened out because of short half-life and insufficient inventory (compared to progeny inventory) to significantly contribute to progeny inventory. Radionuclide Screening Decision Y-90 Ba-137 m Sb-126 m

Screened out because radionuclide is accounted for in modeling by assuming it is in secular equilibrium with parent. Ca-41 Ni-59 Sm-147 Gd-152 Screened out because of low dose potential Eu-155 Cm-248 in screening scenarios based on available inventory, absence of significant progeny, and lack of volatility. Ag-Bi-210 m Cm-246 Screened out because no inventory data 108 m was available and because one or more members of the decay chain are tracked in the radionuclide or chemical inventory. Mo-93 Screened out because of low dose potential in calculations performed for the FY 2014 Special Analysis. (a) Although the DOE initially screened Pd-107 in, the DOE later d etermined Pd-107 had low dose potential (SRR-CWDA-2021-00047, Rev. 0).

3.3 General Process for Inventory Development

The first contribution to the projected SDF closure inventory i s the existing inventory in the SDF. The DOE tracks the waste disposed of in the SDF with the Waste Inventory Disposed Estimator (SDF-WIDE) model (SRR-CWDA-2015-00003, Rev. 0). Section 3.7 of this TRR provides more information about that DOE model. For the 2020 PA, the DOE rev ised its estimate of the emplaced inventory in the SDF by reevaluating some of the alter native methods the SDF-WIDE model uses to calculate radionuclide concentrations when direct concentration measurements are not available (e.g., ingrowth and decay calculations, ratio s to other radionuclides). For the 2020 PA, the DOE used the SDF-WIDE model with historical inform ation about waste transfer volumes, historical measured radionuclide concentrations, and r evised alternative methods to recalculate the emplaced SDF inventory as of March 31, 2018 (SR R-CWDA-2018-00041, Rev. 3). As noted in Section 3.1 of this TRR, the DOE used the same estimate of the existing inventory in the SDF for the realistic, MPAD, and pessimistic i nventory projections.

The second source of waste that the DOE considered was the exis ting inventory in the Tank Farms. As in the 2009 SDF PA, the DOE used the Waste Character ization System (WCS) as the primary source of information about the Tank Farms inventor y. The WCS uses a combination of sample data, materials accounting, assumptions, and special calculation methods to track radionuclide inventories in each phase of wast e (i.e., supernate, interstitial liquid, salt, and sludge) in each tank. Like the alternative m ethods used in the SDF-WIDE model, the special calculations used in the WCS include using d etection limits, calculating ingrowth from measured ancestor radionuclides, using solubility information to estimate supernate concentrations, applying partitioning between sludge and supernate observed in some waste tanks to other tanks, or using correlations to more easily detectable radionuclides. For the 2020 PA, the DOE used data taken from the WCS on Januar y 8, 2018. The DOE supplemented that information with reports of waste transfers t o, within, and from the Tank Farms through March 1, 2018 (see SRR-CWDA-2018-00041, Rev. 3, T able A-2).

Unlike the SDF-WIDE model, which uses data from samples of wast e that have undergone Cs removal, the WCS tracks radionuclide inventories in untreated t ank farm waste. Therefore, to use the information in the 2020 PA, the DOE applied a decontami nation factor of 200 to the projected inventories of all isotopes of Cs. That assumption i s consistent with operating experience from the ARP/MCU and the DOE expects that it will be conservative for waste treated in the SWPF. Although the DOE indicated that it did no t assume any solids filtering would occur during ARP/MCU or SWPF treatment (SRR-CWDA-2018-000 41, Rev. 3), the DOE also did not include any sludge inventory in the contributions to the SDF inventory for radionuclides other than I-129 and Tc-99. Instead, the DOE con sidered the radionuclide inventory the soluble phases of tank waste (i.e., supernate, in terstitial liquid, and dry salt) because those are the phases of waste that the DOE plans to sen d to the SDF. However, for both I-129 and Tc-99, the DOE included a contribution from slud ge in addition to the soluble phases (see Section 3.4 of this TRR).

As noted in Section 3.1 of this TRR, the difference between the realistic, MPAD, and pessimistic inventory projections results from differences in the DOE proje ction of the contribution from the existing Tank Farms waste. For radionuclides other than I-120 and Tc-99, the DOE based the realistic, MPAD, and pessimistic inventory projections on the f ollowing assumptions about the contributions from existing waste in the Tank Farms (SRR-CWDA-2 018-00041, Rev. 3):

the realistic projection includes the DOE estimate of the inve ntory in supernate, interstitial liquid, and dry salt, as modified by cesium (Cs) r emoval;
the MPAD projection includes the DOE estimate of the inventory in interstitial liquid plus 1.5 times the DOE estimate of the inventory in supernate and so lid salt, as modified by Cs removal; and
the pessimistic projection is 1.6 times the realistic projecti on.

The final component of the projected closure inventory for the SDF was the contribution to the SDF (i.e., via the Tank Farms) from future effluents from H-Can yon. The DOE estimated that contribution by multiplying projected radionuclide concentratio ns in H-Canyon waste by the anticipated volume of annual transfers from H-Canyon to the Tan k Farms. For the 2020 PA, the DOE used the same projected radionuclide concentrations in H-Ca nyon waste that the DOE used in the 2009 SDF PA. As it did for the calculation of the SDF inventory contribution from the existing Tank Farms waste, the DOE applied a decontaminatio n factor of 200 to the projected contribution from H-Canyon to account for salt waste treatment. The DOE projected a total transfer volume of 9,270 cubic meters (m 3) (2.45 x 106 gallons), based on the annual transfer volume estimates in the DOE Liquid Waste System Plan ( SRR-LWP-2009-00001, Rev. 20) for FY 2018 through FY 2026, when the DOE anticipated transfers from the H-Canyon to the Tank Farms would end.

After the DOE established the projected inventory for the SDF f or use in the 2020 PA, the DOE issued a revision of the Liquid Waste System Plan (SRR-LWP-2009 -00001, Rev. 21), which extended the duration of transfers from H-Canyon to the Tank Fa rms until FY 2030. Accounting for the lower-than-projected transfers from H-Canyon to the Tan k Farms in FY 2018 through FY 2020 (SRR-CWDA-2021-00147, Rev. 1), the revised Liquid Waste Treatment Plan increased the projected volume transferred from H-Canyon to the Tank Farms from FY 2018 to FY 2030 to 11,800 m3 (3.11x106 gallons) (i.e., a 27% increase).

3.4 Additional Calculations for I-129 and Tc-99

In addition to the general approach for inventory development d escribed in the previous section, the DOE performed additional calculations for I-129 and Tc-99. The 2020 PA indicates the DOE performed those additional calculations because I-129 and Tc-99 dominate the projected dose to a member of the public and an inadvertent intruder within 10,000 years of site closure.

To reduce the uncertainty in the projected SDF closure inventor y of I-129 and Tc-99, the DOE re-evaluated its algorithms for I-129 and Tc-99 concentrations in Tank Farms waste. The DOE has many more measurements of Cs-137 concentrations in Tank Far ms waste than it has of either I-129 or Tc-99 concentrations. In addition, I-129 and T c-99 are sometimes present in tank waste at concentrations below their respective detection l imits. To reduce uncertainty in the I-129 and Tc-99 inventories, the DOE developed relationship s between Cs-137 and both I-129 and Tc-99 to supplement measured concentrations of I-129 and Tc-99. In addition to using the relationship with Cs-137 to estimate I-129 and Tc-99 concentrations in samples in which the DOE measured Cs-137 but did not analyze for I-129 or Tc-99, the relationships were used to estimate I-129 and Tc-99 concentrations in samples of t ank farm waste when I-129 and Tc-99 concentrations were less than the detection limit.

Prior to the 2020 PA, the DOE calculated both I-129 and Tc-99 c oncentrations as constant fractions of measured Cs-137 activity. However, because Cs-137 has a much shorter half-life than either I-129 or Tc-99 does (i.e., 30 years for Cs-137, 15. 7 million years for I-129, and 112,000 years for Tc-99), the relationships between Cs-137 and the other two radionuclides changed as the waste aged. In 2014, the DOE adjusted the relat ionship between measured Cs-137 and estimated concentrations of the other two radionucli des; however, the DOE still represented the I-129 and Tc-99 concentrations as a constant fr action of Cs-137 concentration and did not account for Cs-137 decay.

To revise the relationships between measured Cs-137 and project ions of I-129 and Tc-99 concentrations in soluble tank farm waste for the 2020 PA, the DOE re-analyzed data from tank farm samples taken between 1976 and 2017 (SRR-CWDA-2015-00077, Rev. 2 and SRR-CWDA-2015-00123, Rev. 2). The DOE used samples with detect able concentrations of either I-129 or Tc-99 to develop relationships with measured Cs -137 concentrations that could be used to project I-129 or Tc-99 concentrations in samples in which I-129 or Tc-99 was not measured or when the I-129 or Tc-99 concentrations were below t heir detection limits. Using a different process than it did in previous analyses, the DOE fir st calculated decayed values for all the measured Cs-137 concentrations accounting for decay to Octo ber 1, 2032. By accounting for the effects of Cs-137 decay before evaluating the relations hip between I-129 and Cs-137, the DOE should be able to avoid the need for further adjustment s to the relationship between I-129 and Cs-137 unless tank waste characteristics change (e.g., introduction of different waste streams with significantly different I-129, Tc-99, or Cs-137 co ncentrations).

Before quantifying the relationships between I-129, Tc-99, and the decayed values of Cs-137, the DOE removed four types of outliers from the data:

concentrations in samples from Tank 50, which had been treated to remove Cs-137 but not I-129 or Tc-99;
data from samples taken after bulk waste removal (i.e., resid ual waste samples), which the DOE expected did not accurately represent waste that will b e sent to the SDF;
measurements from samples with chemical properties that the DO E did not expect would represent waste to be sent to the SDF, including waste th at had undergone chemical treatment and waste in which I-129 might have been bou nd to mercury; and
data with reporting issues, including values that did not ap pear to account for sample dilution or that did not match the underlying reference.

After removing the outliers, the DOE made three additional adju stments to the data:

removal of values that represented either I-129 or Tc-99 detec tion limits instead of measured concentrations;
averaging values for samples that represented multiple measure ments of essentially the same waste (i.e., samples of the same waste phase from the same tank on or near the same date); and
removal of very low concentration samples.

The DOE removed data points that represented I-129 or Tc-99 det ection limits because they did not represent the relationship between the true concentration o f those radionuclides in the samples and the measured Cs-137 concentration. The DOE average d values that represented samples of the same phase of waste from the same tank on or nea r the same day to avoid giving those samples undue weight in the relationship. Finally, the DOE removed several samples with Cs-137 concentrations below 3.7x10 5 becquerel per liter (Bq/L) (1x10 7 picocuries per liter [pCi/L]) for two reasons: (1) the dilute samples rep resent waste that will not add significantly to the SDF inventory, and (2) the data lowered th e projected values of I-129 and Tc-99 for a given Cs-137 concentration (i.e., eliminating those va lues caused a conservative increase in projected I-129 and Tc-99 inventories). Although t he DOE observed one additional apparently anomalous Tc-99 data point, the DOE retained the val ue in its analysis because the underlying reference showed no clear justification for removing it and retaining it was conservative (i.e., increased projected Tc-99 concentrations).

After normalizing the data as described above, the DOE used the remaining values to derive relationships between the measured Cs-137 and I-129 or Tc-99 co ncentrations in aqueous phases (i.e., supernate and interstitial liquid). The DOE foun d the data varied logarithmically and used linear regression between the log-transformed concentr ations to determine the best-fit lines to project either I-129 or Tc-99 as a function of measure d Cs-137.

For I-129, after determining a best-fit line with Cs-137, the D OE found that the relationship tended to systematically underestimate the I-129 concentrations in higher concentration samples. To avoid underestimating the I-129 in the more concen trated samples and to provide a margin for uncertainty, the DOE multiplied the projected I-12 9 values by a factor of 1.25. The resulting relationship is given in Equation 1 below (Equation 3 -6 in SRR-CWDA-2015-00077, Rev. 2):

projected I-129 concentration = 1.25 (3.64 x 10 x y. ) Equation 1

where y is the measured Cs-137 concentration decayed to October 1, 2032. For Tc-99, the DOE did not observe any systematic variation in the fit of the data to the regression line and, therefore, used the relationship given in Equation 2 below (Equ ation 2-4 in SRR-CWDA-2015-00123, Rev. 2) without further adjustment:

projected Tc-99 concentration = 0.0115 x y. Equation 2

The DOE used Equations 1 and 2 above to project values for I-12 9 and Tc-99 in liquid phases (i.e., supernate and interstitial liquid) for all samples with measured Cs-137 values. For samples of aqueous tank waste that had measured I-129 or Tc-99 concentrations, the DOE then replaced the projected I-129 values with the measured valu es.

The relationships in Equations 1 and 2 do not apply to I-129 or Tc-99 in solid salt because differences in the solubility of all three radionuclides change their relative concentrations. For I-129, Equation 1 does not apply to I-129 in solid salt because I-129 is more soluble than Cs-137, and the DOE expects there to be very little I-129 in so lid salt. For the inventory estimate for the 2020 PA, the DOE assumed that all solid salt h as an I-129 concentration of 96 Bq/L (2.6 pCi/mL) because I-129 is highly soluble and the DO E does not expect there to be a

significant contribution of I-129 from the dry salt. For Tc-99, Equation 2 does not apply because Tc-99 is less soluble than Cs-137 and Equation 2 would underest imate the Tc-99 concentration in the solid salt waste. For the 2020 PA, the DOE calculated t he Tc-99 concentration in salt waste by using the relationship between Tc-99 and Cs-137 in aqu eous phases given by Equation 2 and increasing the Tc-99 concentration by a factor o f 1.5:

projected Tc-99 concentration = 1.5(0.0115 x y. )

Finally, the DOE used the relationships developed in this secti on, with measured I-129 and Tc-99 concentrations when available, to develop inventories of I-129 and Tc-99 in the Tank Farms for use in projecting closure inventories for the SDF. F or I-129, the DOE used the following contributions from the Tank Farms to the projected SD F inventory (SRR-CWDA-2015-00077, Rev. 2):

5.55x1011 Bq (15.0 Ci) for the realistic projection (i.e., 5.51x10 11 Bq [14.89 Ci] in aqueous phases, 4.07x10 9 Bq [0.11 Ci] in solid salt);
5.81x1011 Bq (15.7 Ci) for the MPAD projection (i.e., 5.51x10 11 Bq [14.89 Ci] in aqueous phases, 4.07x109 Bq 0.11 Ci in solid salt, 2.66x10 10 Bq [0.72 Ci] in sludge); and
8.73x1011 Bq (23.6 Ci) for the pessimistic projection (i.e., the MPAD pr ojection increased by 50%).

For Tc-99, the DOE accounted for the waste phases slightly diff erently. For Tc-99, the DOE used the following contributions from the Tank Farms to the pro jected SDF (SRR-CWDA-2015-00123, Rev. 2):

7.84x1014 Bq (2.12x104 Ci) for the realistic projection (i.e., the inventory in super nate, interstitial liquid, and salt);
1.17x1015 Bq (3.17x104 Ci) for the MPAD projection (i.e., the realistic value multipl ied by 1.5); and
1.27x1015 Bq (3.43x104 Ci) for the pessimistic projection (i.e., the realistic value plus half the sludge inventory).

The two other sources of I-129 and Tc-99 in the projected SDF i nventory are the existing inventory in emplaced saltstone and projected future additions from H-Canyon. For both radionuclides, the DOE updated the emplaced SDF inventory by re trieving data from the SDF-WIDE model that reflected transfers to the SDF through Sept ember 2017. The DOE calculated that 3.2x10 10 Bq (0.85 Ci) of I-129 and 4.59x10 13 Bq (1.24x103 Ci) of Tc-99 had been emplaced in the SDF by that time. Because of the long half-liv es of I-129 and Tc-99, those value can be used without adjustment as the contributions of em placed saltstone to the projected SDF inventories of I-129 and Tc-99.

In response to an NRC RAI question, the DOE indicated that the inventories for I-129 and Tc-99 used in the 2020 PA inadvertently omitted future effluent from H-Canyon (SRR-CWDA-2021-00047, Rev. 0). Since the DOE developed the Tan k Farms inventory as of the end of 2017, the DOE expected the SDF inventory should incl ude eight years of future transfers from H-Canyon, from FY 2018 through FY 2025. The DOE provided two different estimates of the potential I-129 and Tc-99 transfers from H-Can yon.

The first estimate used the projected radionuclide concentratio ns and transfer volumes provided in Tables 4.2-1 and 4.2-1 of SRR-CWDA-2018-00041, Rev. 3. That DOE document provides an estimate of 1,140 m3/year (300,000 gallons/year) of effluent from H-Canyon to the T ank Farms with an estimated I-129 concentration of 19.4 Bq/mL (5.23 x 10 2 pCi/mL) and estimated Tc-99 concentration of 4.33x10 3 Bq/mL (1.17x105 pCi/mL). Using those volume and concentration values, the DOE calculated that the projected SDF inventory sho uld increase by 1.79x1011 Bq (4.85 Ci) I-129 and 4.03x1013 Bq 1.09x103 Ci Tc-99. For the MPAD case, those additions would be a 29% increase in the total I-129 inventory and a 3.3% incre ase in the total Tc-99 inventory.

The DOE based the second estimate on revised projections of the volume of future H-Canyon transfers and the concentrations of I-129 and Tc-99 in those tr ansfers (SRR-CWDA-2021-00047 Rev. 0). To revise the volume estimate, the DOE provided a his tory of annual transfers from H-Canyon to the tank farm and demonstrated that the average ann ual volume transferred from 2010 through 2019 was 473 m 3 (125,000 gallons) rather than 1,140 m 3 (300,000 gallons/year). The DOE stated that the smaller volume would be a reasonable a ssumption for future transfers from H-Canyon. In addition, the DOE indicated that t he concentrations of I-129 and Tc-99 that it assigned to H-Canyon effluents in SRR-CWDA-2018-0 0041, Rev. 3 also were conservative estimates. Based on measurements from Tank 39, wh ich receives H-Canyon effluents, the DOE indicated that a more realistic I-129 concen tration for future H-Canyon effluents would be 1.1 Bq/mL (30 pCi/mL) rather than 19.4 Bq/mL (523 pCi/mL) and a more realistic Tc-99 concentration would be 910 Bq/mL (2.46x10 4 pCi/mL) rather than 4.33x103 Bq/mL (1.17x105 pCi/mL). Based on those revised volume and concentration projections, the DOE indicated that the projected additions fro m H-Canyon would increase the projected SDF inventory of I-129 by less than 3% and the increa se in the Tc-99 inventory would be negligible. Therefore, the DOE did not change the realistic, MPAD, or pessimistic projections of the I-129 or Tc-99 inventories (Table 1).

3.5 Probabilistic Sensitivity and Uncertainty Analyses

The DOE assessed the effect of inventory assumptions on the pro jected dose to a member of the public and an inadvertent intruder with both probabilistic and deterministic analyses. This section addresses the development of probabilistic distribution s to represent uncertainty in inventory and the results of the probabilistic analyses. Secti on 3.6 addresses the deterministic sensitivity analyses related to inventory in the 2020 PA.

In the 2020 PA, the DOE created three truncated, log-normal dis tributions to represent uncertainty in inventory: one for I-129, one for Tc-99, and on e representing the uncertainty in the inventory of all other radionuclides (see Table 4 below). Because the uncertainty in the volumes of each soluble waste phase in each tank in the Tank Fa rms was much smaller than the uncertainty in the concentration data, the DOE based the un certainty distributions on variability in the concentration data and assumed the waste vol umes did not contribute to the

uncertainty in inventory. To apply distributions of concentrat ion data to inventory, the DOE normalized the concentration distributions by dividing by the m ean concentration for the appropriate radionuclides. That step created distributions of unitless values. The DOE then modeled uncertainty in inventory by multiplying the realistic i nventory values in Table 1 by the appropriate distribution of unitless multipliers from Table 4.

Table 4. Parameters defining the truncated log-normal distributions for the unitless multiplier applied to the realistic inventory values in probabilistic an alyses in the 2020 PA (from Tables 3, 6, and 12 of SRR-CWDA-2018-00076, Rev. 0).

Radionuclide Minimum Mean Standard Maximum Deviation I-129 0.079 1.0 0.26 9.78 Tc-99 0.13 1.0 0.11 13.71 All other radionuclides 0.10 1.19 3.11 14.4

For I-129 and Tc-99, the DOE based the distributions on the mea sured and calculated concentration values referred to in Section 3.4 of this TRR. T he DOE used the percentile function in Microsoft Excel to generate 1 st, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 99th percentiles for the concentration data, which created an empirical probabil ity distribution for each radionuclide. The DOE then tested the fit of uniform, triangul ar, normal, log-normal, and gamma distributions to the portion of the empirical distribution betw een the 10th and 90th percentiles of the empirical distribution. Figure 2 shows an example of the f it of different distribution shapes to the empirical distribution for I-129 concentration values. For both I-129 and Tc-99, the DOE determined that a log-normal distribution provided the best-fit to the empirical distribution (SRR-CWDA-2018-00076, Rev. 0).

Figure 2. Comparison of fits of different distribution types to the em pirical distribution (dotted line) of I-129 concentration data (Figure 2 of SRR-CWDA-2018-00 076, Rev. 0).

The DOE used the mean and standard deviation from fitting the l og-normal distribution to the empirical distributions to define the probabilistic distributio ns for inventory uncertainty for I-129 and Tc-99 (Table 4). The DOE normalized both the mean and stan dard distribution by dividing by the mean concentration (i.e., either of I-129 or Tc-99) to c reate a distribution of unitless multipliers. Similarly, the DOE also calculated unitless minim um and maximum multipliers by taking the ratio of the smallest or largest concentration value s for I-129 and Tc-99 and dividing each by the appropriate mean concentration value.

The DOE used a similar process to develop the probabilistic dis tribution for inventories of radionuclides other than I-129 and Tc-99, except that the proce ss was slightly more complex because the DOE first took steps to combine concentration data for different radionuclides. The DOE considered the 18 radionuclides that made the greatest cont ribution to dose to a member of the public in the FY 2014 and FY 2016 Special Analyses for t he SDF. The DOE evaluated concentration data for those 18 radionuclides from samples of s alt waste, primarily taken from salt batch qualification reports (SRR-CWDA-2018-00009, Rev. 0). Of the 18 radionuclides, two (I-129 and Tc-99) were considered individually, six had an inad equate number of samples to reliably quantify variability, and three had concentrations tha t were all below the detection limits.

The DOE used the variability in the data from the remaining sev en radionuclides (Am-241, C-14, Np-237, Pu-238, Pu-239, Pu-241, and U-234) as the basis for the distribution to represent the uncertainty in the inventory all radionuclides other than I-129 and Tc-99. The DOE first decayed all the concentration data to October 1, 2032. That step allow ed the DOE to remove variation in measured concentrations caused by sampling on different dates f rom the true variability of the waste. The DOE then normalized the concentration data from the seven radionuclides by dividing the concentrations of each radionuclide by the median concentration for that radionuclide (SRR-CWDA-2018-00076, Rev. 0). That step allowed the DOE to compare the variability in the data from different radionuclides that had v ery different median concentrations.

Using the scaled data, the DOE determined that C-14, Np-237, an d U-234 showed much less variation than Am-241, Pu-238, Pu-239, and Pu-241 (SRR-CWDA-201 8-00076, Rev. 0). To avoid underestimating the variability for all radionuclides, th e DOE combined the normalized data for C-14, Np-237, and U-234. The DOE then had five data s ets: one each for Am-241, Pu-238, Pu-239 and Pu-241 and one additional data set represent ing C-14, Np-237, and U-234. The DOE used the percentile function of Microsoft Excel to find the 1st, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 99th percentiles of each of the five normalized data sets. Then, t he DOE used the geometric mean of the normalized data for each percentile value to combine the data into one empirical probability distribution. The DOE set the minimum an d maximum multiplier values for radionuclides other than I-129 and Tc-99 (Table 4) to the 10 th and 90th percentile values of that combined empirical distribution (Tables 10 and 12 of SRR-CWDA-2 0018-00076, Rev. 0).

To develop a probabilistic distribution, the DOE tested the fit of normal, log-normal, gamma, uniform, and triangular distributions to the 10 th, 25th, 50th, 75th, and 90th percentiles of the combined empirical distribution. The DOE determined that the l og-normal distribution had the best-fit and used the mean and standard deviation from fitting the log-normal distribution to the empirical distribution to define the probabilistic distribution for inventory uncertainty for radionuclides other than I-129 and Tc-99 (Table 4).

Section 5.7 of the 2020 PA addresses the results of probabilist ic uncertainty and sensitivity analyses for both a member of the public. Section 6.6 of the 2 020 PA focuses on the results of probabilistic analyses for an inadvertent intruder who drills o r uses a well 1 m from a disposal structure. Section 5.7.3.3 demonstrates that high inventory mu ltipliers for I-129 appear to have

a significant effect on dose in several probabilistic realizati ons with relatively high projected doses to a member of the public. The other two inventory multi pliers (i.e., for Tc-99 or the remaining radionuclides) did not appear to significantly affect those realizations. Similarly, Section 5.7.4.2.4 of the 2020 PA indicates that the projected d ose to a member of the public was sensitive to the inventory multiplier for I-129 in SDS 9. The projected dose to a member of the public was not sensitive to the inventory multiplier for Tc -99 or the other radionuclides and the projected dose to an inadvertent intruder was not sensitive to any of the inventory multipliers.

3.6 Deterministic Sensitivity Analyses

In addition to the probabilistic analyses discussed in the prev ious section, the DOE used the following deterministic sensitivity analyses to evaluate the ef fects of alternative inventory assumptions on the projected dose to a member of the public:

two Central Scenario cases that evaluated the effects of using the realistic and pessimistic inventories shown in Table 1 for all radionuclides;
one case assuming the same total I-129 inventory with an alter native distribution among disposal structures;
two Extreme Inventory Values cases that used the minimum and maximum multiplier values in Table 5 multiplied by the realistic inventory in Tabl e 1 to create minimum and maximum inventories for all radionuclides except for Tc-9 9; and
four cases that considered the realistic, pessimistic, minimum, and maximum values for the Tc-99 inventory alone.

For each of sensitivity cases listed above, the values of all o ther (non-inventory related) parameters were set to their MPAD values.

Section 5.8.5.1 of the 2020 PA provides the results of the DOE sensitivity cases that used the realistic and pessimistic inventories. Figure 3 below (Figure 5.8-55 of the 2020 PA) shows the results of those two sensitivity cases with three other cases f or context. The two dotted lines show the dose projections for the cases that used the realistic and pessimistic inventory values from Table 1 with MPAD values of all other model parameters (e. g., parameters related to infiltration, wasteform performance, far-field flow, and biosph ere). The solid black line represents the compliance case, which uses the MPAD inventory f or all radionuclides and MPAD values for all other model parameters. The two additional solid lines represent the overall realistic and pessimistic Central Scenario cases, i n which the DOE used realistic and pessimistic values for uncertain parameters throughout the mode l. As shown in Figure 3, the variation in inventory accounts for a small fraction of the var iation in the dose projections for the overall realistic and pessimistic cases. For the cases that on ly varied the inventory projections, there is little variation between the projected dose in the MPA D and realistic cases, and approximately a 50% increase in dose in the case that used the pessimistic inventory as compared to the MPAD inventory. That pattern is similar to the difference between the realistic, MPAD, and pessimistic inventories of I-129, even though there i s more difference between the realistic and MPAD inventories of other radionuclides, because the I-129 inventory has a greater effect on the projected dose than the inventories of other radi onuclides.

Figure 3. Comparison of the projected dose to a member of the public in sensitivity cas es that vary inventory alone (dotted lines) versus cases that vary all uncertain parameters between the realistic, compliance (MPAD), and pessimistic cases (Figure 5.8 -55 of the 2020 PA).

Section 5.8.5.2 of the 2020 PA provides the results of an analy sis that used the MPAD total I-129 inventory (i.e., 6.14x1011 Bq [16.6 Ci]) with alternative assumptions about the spatial distribution of that inventory in individual disposal structure s. The DOE based the alternative assumptions on a preliminary system planning estimate. In ge neral, the alternative assumptions shift inventory away from SDS 9-12 and into SDS 6-8 (see Table 5 below; Table 5.8-17 in the 2020 PA). That case resulted in an approxi mate 5% decrease in the projected dose within 6,000 years of site closure and an approx imately 20% increase in the projected dose within 10,000 years of site closure (see Figure 5.8-56 in the 2020 PA).

Section 5.8.5.3 of the 2020 PA provides the results from cases that used values the DOE referred to as extreme inventory values. For that case, the DOE used the realistic inventory values in Table 1 multiplied by the minimum and maximum val ues in Table 4 to generate upper and lower bounding values for the inventory of all radion uclides except Tc-99. Tc-99 was tested separately, as described below. As shown in Figure 4 (F igure 5.8-57 of the 2020 PA), varying the radionuclide inventories to the maximum bounding va lues in Table 4 above causes approximately as much of an effect on the projected dose to a m ember of the public as changing parameters to their pessimistic case values throughout the model. In contrast, using the minimum inventory multipliers in Table 4 has a significantl y smaller effect than changing parameters to their realistic values through the model (note th e log scale of the y-axis in Figure 4). The smaller difference between the dose projections for the realistic and MPAD inventory cases as compared to the difference between the MPAD and pessimistic inventory cases is consistent with the differences between the realistic, MPAD, and pessimistic inventory projections for I-129 (Table 1). Although other radionuclides have a greater difference between the realistic and MPAD cases than I-129 does, the I-129 invento ry has a greater effect on dose projections, as addressed in Section 3.5 of this TRR.

Table 5. Alternative arrangement of I-129 inventory in the SDF (Table 5.8-17 in the 2020 PA). Disposal MPAD Case I-129 Alternative Case Structure Inventory (Ci) (a) I-129 Inventory (Ci) (a) 1 0.20 0.20 2 A 0.07 0.07 2B 0.07 0.07 3 A 0.19 0.19 3B 0.19 0.19 4 0.28 0.28 5 A 0.14 0.14 5B 0.09 0.09 6 2.20 2.81 7 2.20 3.28 8 2.20 3.11 9 2.20 1.19 10 2.20 1.63 11 2.20 1.24 12 2.20 1.39 Total 16.6 16.6 (a) To convert Ci to Bq, multiply by 3.7x10 10.

Figure 4. Comparison of the projected dose to a member of the public in s ensitivity cases that vary inventory alone (dotted lines) to bounding values versus c ases that vary all uncertain parameters between the realistic, compliance (MPAD), and pessim istic cases (Figure 5.8-57 of the 2020 PA).

Section 5.8.5.4 of the 2020 PA provides the results from cases that varied the inventory of Tc-99 alone. This group included four sensitivity cases: two cases that used the realistic and pessimistic values for Tc-99 in Table 4 and two cases that used the realistic inventory values for Tc-99 in Table 1 multiplied by the minimum and maximum multipli ers for Tc-99 in Table 4. As shown in Figure 5 (Figure 5.8-59 of the 2020 PA), the change in the inventory values for Tc-99 caused a much smaller change in the projected dose. For exampl e, the maximum multiplier for Tc-99 in Table 4 is 13.71, but the peak dose within 10,000 year s in the maximum bounding case that used that inventory multiplier increases by approxima tely 15%. The 2020 PA indicates that the sub-linear relationship between an increase in modeled inventory and the projected peak dose occurs because Tc-99 is solubility limited when it is chemically reduced, so that increasing Tc-99 inventory does not increase its release r ate from chemically reduced parts of saltstone.

Figure 5. Comparison of the projected dose to a member of the public in s ensitivity cases that vary Tc-99 alone while using MPAD values for all other paramete rs (Figure 5.8-59 of the 2020 PA).

3.7 The DOE Methods for Tracking SDF Inventory

Since 2015, the DOE has tracked the emplaced inventory in the S DF with the SDF-WIDE model (SRR-CWDA-2015-00003, Rev. 0). The SDF-WIDE model calculates t he inventory inputs to the SDF based on measured or calculated radionuclide concentrations in each batch of salt waste and the volume of waste in the batch. When possible, the model uses measured radionuclide concentrations from the feed tank for the Saltstone Production Facility (i.e., currently Tank 50). For radionuclide concentrations that are below detection limits, the model uses alternative calculations, including using detection limits, calculating ing rowth from measured ancestor

radionuclides, using solubility information to estimate superna te concentrations, applying partitioning between sludge and supernate observed in some wast e tanks to other tanks, or using correlations to more easily detectable radionuclides. Th e SDF-WIDE model also calculates radionuclide ingrowth and decay.

4.0 NRC Staff Evaluation

4.1 Overview

The NRC staff conducted a risk-informed review that focused on the development of the inventories of I-129 and Tc-99 because those two radionuclides dominate the projected dose to both a member of the public and an inadvertent intruder in the 2020 PA. Sensitivity analyses and uncertainty analyses in the 2020 PA demonstrated that the p eak projected dose to a member of the public is more sensitive to the inventory of I-12 9 than it is to any other radionuclide, including Tc-99 (see Sections 3.5 and 3.6 of this TRR). That result is consistent with NRC staff expectations because the Tc-99 release rate is l imited by solubility when saltstone is chemically reducing. However, the NRC staff focus ed on Tc-99 as well as I-129 because the inventory of Tc-99 affects the duration of the peak release from saltstone. In addition to reviewing the development of the inventory of I-129 and Tc-99 in detail, the NRC staff reviewed the following DOE processes: (1) screening radi onuclides for detailed analysis, (2) developing the inventory for all radionuclides, (3) develop ing uncertainty distributions for all radionuclides, (4) developing sensitivity and uncertainty analy ses, and (5) tracking the SDF inventory.

The NRC staff conducted this review to assess the radionuclide inventories the DOE used in modeling to support the 2020 PA. In addition, the NRC staff wi ll use insights from this technical review to develop recommended changes to the Monitoring Plan. In this section, the NRC staff has included recommended additions to the Monitoring Plan in it alics. The NRC staff summarized those additions in the Conclusions section of this T RR (i.e., Section 7). As described in the current Monitoring Plan, the NRC staff routine ly monitors several documents related to SDF inventory, including the SDF PA Annual Review, S altstone Permit Website Reporting Data (http://sro.srs.gov.saltstone.htm ), and the Quarterly Tank 50 Waste Acceptance Criteria Sample Analysis (see the Monitoring Plan, Table 1-1). Therefore, the NRC staff recommended additional specific monitoring activities in this s ection with the expectation that they would supplement the ongoing routine monitoring activities the NRC staff performs under MF 1.01 and MF 1.02 of the Monitoring Plan. The Monitoring Plan includes a table that provides the DOE expected inventory of the SDF at the time of closure. T he DOE updated that inventory in the 2020 SDF PA. Therefore,

INVT-01 The NRC staff recommends updating Table A-1 of the Monitoring Plan to reflect the MPAD inventory in the 2020 PA under MF 1.01.

4.2 Radionuclide Screening

The NRC staff determined that the DOE process for radionuclide screening is acceptable because it began with a comprehensive list of radionuclides tha t was consistent with process knowledge and it used technically defensible reasons for screen ing out radionuclides. The DOE considered the potential dose from longer-lived progeny of radi onuclides with half-lives less than five years before screening out the short-lived parents. The D OE also accounted for short-lived radionuclides that it assumed to be in secular equilibrium with a parent. The NRC staff agreed with the DOE that it is appropriate to screen radionuclides bas ed on conservative stylized dose

calculations. Although the DOE used the whole dose limit (i.e., either 0.25 mSv/year (25 mrem/year) for a member of the public or 5 mSv/year (500 mr em/year) for an inadvertent intruder) instead of using a fraction of the dose limit for scr eening, the NRC staff found the radionuclide concentration assumptions that the DOE used in its screening dose calculations were sufficiently conservative that the calculations were appro priate for screening. Although Pd-107 did not meet the formal screening criteria, the NRC agreed with the DOE conclusion that omitting it did not impact the 2020 PA results. The NRC staff agreed with the DOE technical basis for determining that it would have a projec ted dose at least four orders of magnitude less than that of I-129 for both a member of the publ ic and an inadvertent intruder. However, the NRC staff expects it will be included in future DO E analyses for completeness unless it meets the screening criteria in future analyses.

The DOE screened out Ag-108m, Bi-210m, and Cm-246 because there was no inventory data available for those radionuclides. All three radionuclides hav e half-lives greater than five years. The NRC staff determined that lacking inventory data is not a s ufficient technical basis to screen out the radionuclides because it does not limit the potential d ose from those radionuclides. However, the DOE did not include any of those three radionuclid es in previous analyses of the SDF that the NRC staff reviewed. Furthermore, the DOE did not indicate that new process information indicated that those radionuclides should be evalua ted. Instead, the DOE indicated that it added the radionuclides for consistency with the SDF-WI DE model (SRR-CWDA-2018-00044, Rev. 3). Therefore, the NRC staff does not expect that Ag-108m, Bi-201m, or Cm-246 will make a significant contribution to the projected dose to a n offsite member of the public or an individual who inadvertently intrudes on the SDF.

4.3 General Process for Inventory Development

The NRC staff determined that the DOE process for inventory dev elopment was acceptable because it accounted for the major projected contributions to t he SDF (i.e., the existing SDF inventory, the existing Tank Farms inventory, and future transf ers from H-Canyon) and used technically defensible methods to determine the contributions f rom those sources.

The NRC staff calculated that the existing SDF inventory makes a relatively small contribution to the projected SDF inventory for most, but not all, radionuclide s (Table 6). Two exceptions for which the existing SDF inventory contributes more than 15% of t he projected SDF closure inventory are cesium-135 (Cs-135) and Cs-137. The NRC staff ex pects those isotopes are exceptions because the ARP/MCU and SWPF remove Cs from salt waste, in contrast with the DDA process, which did not. Therefore, it is reasonable that C s isotopes would have a significant fractional contribution from the waste already empl aced in the SDF. Other exceptions include isotopes of actinium (Ac), californium (Cf), curium (Cm), lead (Pb), niobium (Nb), plutonium (Pu), protactinium (Pa), radium (Ra), thorium ( Th), and uranium (U). That observation is consistent with the NRC staff expectation becaus e those elements are associated with sludge, and, as discussed in Section 3.2, the t ransfer of sludge entrained in solid salt waste to the SDF diminished significantly after comp letion of the DDA process.

The DOE process to calculate the existing SDF inventory of radi onuclides other than I-129 and Tc-99 is acceptable for two reasons: (1) whenever possible, th e DOE based the inventory calculation on direct measurements of radionuclide concentratio n in the tank that receives treated salt waste prior to incorporation into saltstone (i.e., Tank 50) and (2) for radionuclides that have concentrations below detection limits in Tank 50, the DOE used technically justified alternative methods to estimate the concentrations. As noted i n Section 3.3, those alternative

methods included using detection limits, calculating ingrowth f rom measured ancestor radionuclides, using solubility information to estimate superna te concentrations, applying partitioning between sludge and supernate observed in some wast e tanks to other tanks, or using correlations to more easily detectable radionuclides. In general, the NRC staff found the results of special methods agreed with available detected conce ntrations or detection limits for each radionuclide in Tank 50 (see SRR-CWDA-2018-00041, Rev. 3, Appendix B). The NRC staff also found that the DOE special methods used in the devel opment of the existing SDF inventory appropriately accounted for the different characteris tics of the wastes generated by the DDA and ARM/MCU processes.

Table 6. Comparison of the DOE inventory projections at closure (i.e. Ja nuary 1, 2037) compared to the inventory in the SDF on March 31, 2018 decayed to January 1, 2037. SDF Inventory as of Projected Contribution 2020 PA 3/31/2018 Decayed to from H-Canyon Radionuclide MPAD 1/1/2037 Inventory Percent of Percent of (Ci) (a, b) (Ci) (a, c) MPAD (Ci) (a, e) MPAD Inventory(d) Inventory(d) Ac-227 3.41x10-4 1.78x10-4 52 (f) (f) Al-26 3.65x101 1.24 3.4 (f) (f) Am-241 2.06x104 2.28x101 0.11 3.34x101 0.16 Am-242 m 9.98 3.39x10-2 0.34 (f) (f) Am-243 8.84 5.41x10-1 6.1 5.57 63 C-14 7.86x102 2.02x101 2.6 1.31 0.17 Cf-249 3.38x10-1 3.38x10-1 100 (f) (f) Cf-251 1.48x10-1 1.48x10-1 100 (f) (f) Cl-36 1.31 1.92x10-2 1.5 (f) (f) Cm-243 4.51x10-2 7.10x10-3 16 (f) (f) Cm-244 1.63x102 1.59x101 10 1.75x101 11 Cm-245 9.56x10-1 8.45x10-1 88 (f) (f) Cm-247 1.77x10-1 1.77x10-1 100 (f) (f) Co-60 1.11x101 7.97x10-3 0.072 3.09x10-2 0.28 Cs-135 3.67 1.92 52 2.29x10-3(g) 6.2x10-4 Cs-137 4.47x105 1.26x105 28 4.91(g) 1.1x10-5 Eu-152 1.16x101 2.87x10-2 0.25 (f) (f) Eu-154 1.07x102 6.78x10-1 0.63 2.09 1.9 H-3 4.50x103 2.91x101 0.49 9.46 0.21 I-129 1.66x101 8.80x10-1 5.3 1.19x10-1(h) 0.72 K-40 1.32 2.02x10-2 1.5 (f) (f) Nb-93 m 2.06x103 8.84x102 19 (f) (f) Nb-94 2.97x10-1 9.98x10-2 34 1.34x10-1 45 Ni-63 2.91x102 3.01 1.0 1.95 0.67 Np-237 1.77x101 1.01 5.7 2.50 14 Pa-231 5.81x10-4 1.65x10-4 28 (f) (f) Pb-210 7.08x10-5 3.70x10-5 53 (f) (f) Pt-193 1.12x102 1.34x101 12 (f) (f)

SDF Inventory as of Projected Contribution 2020 PA 3/31/2018 Decayed to from H-Canyon Radionuclide MPAD 1/1/2037 Inventory Percent of Percent of (Ci) (a, b) (Ci) (a, c) MPAD (Ci) (a, e) MPAD Inventory(d) Inventory(d) Pu-238 2.10x105 3.87x102 0.18 5.68x101 0.027 Pu-239 1.30x104 6.29x101 0.48 7.48x101 0.57 Pu-240 2.78x103 7.70x101 2.8 7.50x101 2.7 Pu-241 2.47x104 5.50x101 0.22 2.74x102 1.1 Pu-242 1.03x101 5.96 58 7.46x10-2 0.72 Pu-244 4.55x10-2 2.53x10-2 56 (f) (f) Ra-226 1.94x10-4 1.02x10-4 53 2.39x10-5 12 Ra-228 3.77x10-1 2.87x10-4 0.076 (f) (f) Se-79 1.42x102 1.08x101 7.6 4.83x10-1 0.34 Sm-151 5.66x103 1.77x101 0.31 1.54x102 2.7 Sn-126 5.18x102 7.93 1.5 4.66x101 9.0 Sr-90 5.30x106 1.94x103 0.037 8.53x102 0.016 Tc-99 3.29x104 1.27x103 3.9 9.78x101(h) 0.30 Th-229 3.73 3.66 98 (f) (f) Th-230 1.50x10-2 8.49x10-3 57 3.54x10-6 0.025 Th-232 3.77x10-1 2.87x10-4 0.076 3.54x10-6 0.0010 U-232 2.02x10-1 1.50x10-1 74 (f) (f) U-233 3.04x101 1.36x101 45 1.05x10-1 0.34 U-234 4.01x101 1.21x101 30 4.18 10 U-235 7.84x10-1 1.01x10-1 13 8.78x10-2 11 U-236 1.65 1.21x10-1 7.4 5.68x10-1 35 U-238 2.24x101 2.04x10-1 0.91 2.12x10-3 0.0095 Zr-93 1.39x102 1.02x101 7.3 (f) (f) (a) To convert Ci to Bq, multiply by 3.7x10 10 (b) Inventory projections from the 2020 PA, Table 3.3-6 (c) The NRC staff calculated the inventories as of the closure dat e (January 1, 2037) based on the sum of the individual disposal structure inventories in SRR -CWDA-2018-00041, Rev.3, Table 3.5-1. Staff decayed the totals for each radionuclide wi th the GoldSim modeling platform using the decay constants from the ICRP Report No. 107. (d) Precents calculated by the NRC staff based on the inventory va lues in this table. (e) Unless otherwise noted, the NRC staff calculated the projected contribution using the radionuclide concentrations and projected transfer volumes prov ided in Tables 4.2-1 and 4.2-2 of SRR-CWDA-2018-00041, Rev. 3, with the equation in Sect ion 4.2.1 of that document. (f) Concentrations to calculate inventories were not included in S RR-CWDA-2018-00041, Rev.3, Table 4.2-1. (g) Staff applied a decontamination factor of 200 to Cs isotopes, per the DOE description in SRR-CWDA-2018-00041, Rev. 3, Section 4.2.1. (h) The DOE calculated alternative H-Canyon contributions for I-12 9 and Tc-99 (Section 3.4).

In the Monitoring Plan, the NRC staff indicated that it was acc eptable for the DOE to base Ra-226 and Th-230 concentrations on ingrowth from U-234 if the Tank Farms did not receive significant inputs of thorium-bearing waste. The NRC staff fou nd the assumption to be acceptable for the emplaced saltstone because Th-230 measuremen ts (i.e., both detectable concentrations of Th-230 and upper bounds determined by detecti on limits) were consistent with the values calculated from ingrowth from U-234 (SRR-CWDA-2018-0 0041, Rev. 3, Appendix B). Furthermore, because the DOE has been able to detect Th-230 in some samples at concentrations similar to the detection limit, the NRC staff ex pects that waste with significantly greater Th-230 concentrations would have measurable Th-230 conc entrations. The NRC staff therefore expects that it would observe any increases in Th-230 contributions to the SDF as part of its routine monitoring processes. The NRC staff will contin ue to monitor the addition of all radionuclides to the SDF by reviewing the DOE salt waste accept ance criteria sample analysis reports, which the DOE issues approximately quarterly (see Tabl e 1-1 in the Monitoring Plan).

INVT-02 The NRC staff recommends monitoring additions to the SDF of radionuclides that are at or near the total projected SDF inventory, including thorium-229 (Th-229), curium-245 (Cm-245), Cm-247, californium-249 (Cf-249), and Cf-251 under MF 1.01. The NRC staff recommends assessing the impact on dose to a member of the public and an inadvertent intruder if the SDF inventories exceed the MPAD inventory in the 2020 PA.

The NRC staff found the development of the projected contributi on of the existing tank farm inventory to the future SDF inventory to be acceptable for two reasons: (1) whenever possible, the DOE based the inventory calculation on direct concentration measurements of samples from supernate and solid salt waste and volumes of those phases in w aste tanks and (2) for radionuclides that have concentrations below detection limits i n Tank 50, the DOE used technically justified alternative methods to calculate the conc entrations. As noted in Section 3.3, those methods were essentially the same as the alternative meth ods the DOE used in the SDF-WIDE model to track the disposed inventory in the SDF. Bec ause the inventory in the Tank Farms represents future disposals to the SDF, the NRC staf f will assess the agreement between the inventory additions to the SDF from the Tank Farms and the DOE projections by monitoring periodic reports the DOE produces related to SDF inv entory (see Table 1-1 of the Monitoring Plan). The NRC staff will continue to monitor radio nuclide concentrations in Tank 50 (see, e.g., SRR-STI-2021-00165, Rev. 0) to ensure that the conc entrations are consistent with or less than the concentrations the DOE assumed in the calculat ion of tank farm contributions to the SDF. If radionuclide concentrations in Tank 50 are signifi cantly greater than the concentrations the DOE assumed in the development of the projec ted SDF inventory, the NRC staff will assess whether the differences affect the conclusion s of the 2020 PA.

To calculate the projected contribution to the SDF of radionucl ides other than I-129 and Tc-99 from future H-Canyon transfers to the Tank Farms, the DOE used the same radionuclide concentration estimates that the DOE used to represent H-Canyon waste in the 2009 PA, multiplied by the projected volumes of transfers from FY 2018 t o FY 2026 in the DOE Liquid Waste System Plan (SRR-LWP-2009-00001, Rev. 20). Based on the DOE projected radionuclide concentrations and transfer volumes provided in SR R-CWDA-2018-00044, Rev. 3, the NRC staff calculated that transfers from H-Canyon accounted for a small percentage of the DOE projected SDF MPAD inventory for most radionuclides (Table 6). The two exceptions are Am-243 and Nb-94, for which the projected contributions from H-Canyon account for 63% and 45% of the MPAD SDF inventory, respectively.

As discussed in Section 3.3, the most recent revision of the DO E Liquid Waste System Plan (SRR-LWP-2009-00001, Rev. 21) projects 29% more waste being tra nsferred from H-Canyon to the Tank Farms than the DOE anticipated when developing the inv entory for the 2020 PA.

Despite that increase, the NRC staff determined the volume esti mate the DOE used to develop the inventory for all radionuclides other than I-129 and Tc-99 to be acceptable because the annual transfer volumes that the DOE used to calculate the inve ntory contribution from future H-Canyon transfers overestimated the actual average transfer volu me from FY 2010 to FY 2019 by a factor of 2.4.

INVT-03 The NRC staff recommends monitoring significant changes in waste characteristics and annual volumes of waste sent from H-Canyon to the Tank Farms to assess the potential effects on SDF inventory under MF 1.02.

4.4 Additional Calculations for Inventory Development for I-129 and Tc-99

The NRC staff determined the DOE overall approach to developing the projected inventory for I-129 and Tc-99 is acceptable because it accounted for the majo r projected sources of I-129 and Tc-99 (i.e., the existing SDF inventory, the existing Tank Farms inventory, and future transfers from H-Canyon) and used technically defensible method s to determine the contributions from those sources.

The DOE process to calculate the existing SDF inventory of I-12 9 and Tc-99 is acceptable for two reasons: (1) it was based on measurements from the Saltstone Production Facility feed tank when possible and (2) the DOE expects the existing invento ry of both I-129 and Tc-99 is 5% or less of the projected inventory of those radionuclides. As noted in Section 2.4 of this TRR, in the NRC Letter of Acknowledgment (ADAMS Accession No. M L12213A447), the NRC staff determined that the lower Tc-99 inventories in SDS 2B, 3 A, 3B, 4, 5 A, and 5B that the DOE projected in response to the NRC Letter of Concern alleviat ed the NRC staff concern regarding Tc-99 releases from those disposal structures. After that NRC evaluation, the DOE increased the authorized Tc-99 inventory in SDS 2B, 3 A, 3B, 4, 5 A, and 5B based on the results of the FY 2014 Special Analysis. As indicated in Secti on 2.1 of this TRR, the NRC staff did not write a TER or make conclusions based on the FY 2014 Sp ecial Analysis. The NRC staff will evaluate the effects of the emplaced Tc-99 inventori es in SDS 2B, 3 A, 3B, 4, 5 A, and 5B in the context of the revised models the DOE used to support the 2020 PA.

The DOE process for developing the projected contributions of I -129 and Tc-99 from the current Tank Farms inventory demonstrated a risk-informed focus on the largest projected source of I-129 and Tc-99 and a focus on the radionuclides that cause the majority of the projected dose for a member of the public and an inadvertent intruder. The da ta analysis the DOE used to improve the I-129 and Tc-99 inventory estimates is acceptable f or four reasons: (1) the sources of data the DOE used to develop the relationship between Cs-137 and I-129 or Tc-99 were appropriate and traceable, (2) the process the DOE used to deve lop a relationship between measured Cs-137 and I-129 or Tc-99 was clear and appropriately accounted for radioactive decay of Cs-137, (3) the DOE provided a clear and justified tec hnical basis for removing any data points that were not included in the derived mathematical relationships, and (4) the application of the relationship between Cs-137 concentrations a nd I-129 or Tc-99 was clear and technically justified.

The DOE proposed realistic, MPAD, and pessimistic inventory est imates for I-129 and Tc-99 are acceptable because they are based on the best available data an d they adequately represent

the uncertainty in the existing Tank Farms inventory of I-129 a nd Tc-99. Although the inventory recommendations only included uncertainty in the current tank f arm inventories of I-129 and Tc-99 (i.e., the DOE treated the current SDF inventory and H-Ca nyon inventory as certain), the NRC staff determined the recommendations were acceptable for tw o reasons: (1) the NRC staff calculated that 95% or more of the DOE MPAD projected SDF inven tory of both radionuclides is in the current Tank Farms inventory (see Table 6 above); and (2 ) for both radionuclides, the proposed variation in the realistic, MPAD, and pessimistic inve ntory estimates is larger than the entire current inventory of the SDF and the revised projected i nventory from projected transfers from the H-Canyon.

For I-129 and Tc-99, the NRC staff found the calculation of the inventory contribution from projected additions of waste to the tank farm from H-Canyon to be acceptable because the concentration estimates were based on recent measured values fr om the tank that receives H-Canyon waste (i.e., Tank 39) and the volume estimates are equ al to the average transfer volume from FY 2010 to FY 2019 (SRR-CWDA-2021-00047, Rev. 1). However, the NRC staff also determined that the revised estimates of the H-Canyon tran sfer volumes and radionuclide concentrations provided in the DOE document SRR-CWDA-2021-00047, Rev. 0 appeared to remove the allowance for uncertainty that was included in the o riginal volume and concentration measurements. The DOE also could require some of that margin t o account for the additional years of H-Canyon inputs to the tank farm and 29% increased vol ume of H-Canyon contributions discussed in Section 3.3. Therefore,

INVT-04 The NRC staff recommends monitoring significant changes in waste characteristics and annual volumes of waste sent from H-Canyon to the Tank Farms to assess the potential effects on I-129 and Tc-99 inventory in the SDF under MF 1.01.

4.5 Probabilistic Distributions

The probabilistic distributions the DOE developed for I-129 and Tc-99 inventories are acceptable for two reasons: (1) the data the DOE used to devel op the distributions was traceable and represented the tank farm waste that the DOE plan s to send to the SDF and (2) the method the DOE used to develop the distributions was clear and technically justified. The NRC staff determined that the concentration data the DOE based the distributions on were representative of the soluble phases of tank farm waste because the data included direct measurements of I-129 and Tc-99 concentrations in supernate fro m approximately half of the waste tanks (see Table 3-1 of SRR-CWDA-2015-00077, Rev. 2) supp lemented with concentrations calculated from Cs-137 measurements using Equati ons 1 and 2 (above) from all of the operating waste tanks (see Table 3-2 in SRR-CWDA-2015-00 077, Rev. 2). The NRC staff found that the lower number of concentration measurements from samples of dry salt were acceptable because both I-129 and Tc-99 are highly soluble and the NRC staff expects the dry salt phase to make a very small contribution to the SDF invento ry. Similarly, the NRC staff determined that the DOE choice to base the distributions on the variability of I-129 and Tc-99 concentrations in soluble tank farm waste was acceptable becaus e the existing tank farm inventory is the largest input to the projected SDF inventory ( i.e., 95% of the projected I-129 inventory and 96% of the projected Tc-99 inventory) and (2) the NRC staff expects the SDF disposal structure inventory to be less variable than the tank farm inventory because the DOE intentionally combines lower and higher activity waste in salt waste batches to moderate radionuclide concentrations in saltstone.

The DOE process for developing probabilistic distributions for the inventory for radionuclides other than I-129 or Tc-99 is acceptable for four reasons: (1) the DOE based the distributions on

the variability of measured radionuclide concentrations in salt batch verification reports, which the NRC staff expects to represent waste that will be emplaced in saltstone, (2) the DOE used a technically justified process to select the radionuclides that it used to develop the distribution, (3) the NRC staff expects the SDF disposal structure inventory to be less variable than the tank farm inventory because the DOE combines lower and higher activi ty waste in salt waste batches to minimize variability, and (4) the lower dose contributions o f radionuclides other than I-129 and Tc-99 do not justify a more detailed approach.

4.6 Deterministic Sensitivity Analyses

The scope of the DOE deterministic sensitivity analyses for inv entory is acceptable because the cases are well-suited to demonstrate key features of the 2020 P A, including: (1) the relative importance of realistic, MPAD, and pessimistic assumptions abou t inventory as compared to realistic, MPAD, and pessimistic assumptions made throughout th e model, (2) the effects of bounding assumptions about inventory, and (3) the relative impo rtance of the inventory of I-129, Tc-99, and all other radionuclides combined. The deterministic analyses are acceptable because the bases for the alternative inventory assumptions is transparent and the range of inventory assumptions the DOE used covers plausible inventory v alues. The NRC staff agrees the values the DOE refers to as extreme inventory values are likely to bound the SDF inventory projection because the values represent the extremes of large sets of concentration measurements (and calculated values) for tank farm waste, and i t is unlikely that the average concentration of any radionuclide in tank farm waste would be m ore extreme than the variation seen in individual values.

In the deterministic Central Scenario cases, the DOE considered the uncertainty in the inventory in emplaced saltstone to be negligible compared to the uncertai nty in projected additions to the SDF from the Tank Farms. That approach differs from the approa ch the DOE took in the probabilistic analyses described in Section 3.5, in which the s ame probabilistic distribution of unitless multiplier values was applied to the realistic invento ry values for all radionuclides other than I-129 and Tc-99, irrespective of how much of the inventory had already been emplaced.

In the deterministic Central Scenario cases, the projected inve ntories of radionuclides that are attributable mainly to the existing SDF inventory have little o r no variation between the realistic, MPAD, and pessimistic estimates. For example, by decaying the current SDF inventory values that the DOE provided in the 2020 PA to the expected closure da te of January 1, 2037, the NRC staff calculated that the DOE has already emplaced 100% of the MPAD closure inventory of Cf-251 in the SDF (see Table 6 of this TRR) and Table 1 of this TRR shows identical values for the realistic, MPAD, and pessimistic projections for Cf-251. S imilarly, Table 1 shows only slight variation in the realistic, MPAD, and pessimistic projections f or U-232, for which Table 6 shows that 74% of the projected inventory has been emplaced in the SD F. In contrast, Table 1 shows a factor of 1.6 difference between the pessimistic and realisti c projections for Cl-36, for which the DOE estimates less than 3% of the projected inventory has b een emplaced in the SDF. Those results are consistent with the DOE assumption that the e mplaced saltstone inventory does not contribute uncertainty to the projected closure invent ory. The NRC staff determined that assumption is acceptable in the Central Scenario cases for two reasons: (1) radionuclide concentrations and waste volumes in salt waste in the feed to t he Saltstone Production Facility is significantly less uncertain than radionuclide concentration s and waste volumes in each waste phase in each tank in the Tank Farms; and (2) the DOE conducted additional sensitivity analyses to demonstrate the effects of using bounding inventory values for all radionuclides.

Similarly, the DOE assumed that future contributions to the SDF inventory from H-Canyon transfers of waste to the Tank Farms would not contribute uncer tainty to the inventory in the Central Scenario cases. In contrast with the inventory emplace d in the SDF, future inputs from H-canyon appear to be uncertain. For example, as noted in Sect ion 3.3 of this TRR, the most recent Liquid Waste System Plan (SRR-LWP-2009-00001, Rev. 21) i ncreases the projected transfer of H-Canyon waste to the Tank Farms by 25% compared to the volume the DOE assumed when developing the inventory for the 2020 PA. In addi tion, the radionuclide concentrations in the waste from H-Canyon may depend on process inputs that may not all be known at this time. Despite this uncertainty, the NRC staff de termined that the DOE assumption that future waste transfers from H-Canyon will not add uncertai nty to the Central Scenario cases to be acceptable for two reasons: (1) the DOE projected that t he contribution of inventory from H-Canyon to the SDF will be small (see Table 6 of this TRR) and the DOE does not expect that the two exceptions (i.e., Am-243 and Nb-94) will cause a signif icant dose to a member of the public or an inadvertent intruder; and (2) the DOE conducted ad ditional sensitivity analyses to demonstrate the effects of using bounding inventory values for all radionuclides. As noted in Section 4.3, the NRC staff recommends monitoring significant ch anges in waste characteristics and annual volumes of waste sent from H-Canyon to the Tank Farm s to assess the potential effects on SDF inventory.

As noted in Section 3.6 of this TRR, the alternative spatial ar rangement for I-129 that the DOE modeled based on a preliminary system planning estimate resul ted in an increase of approximately 20% in the projected dose to a member of the publ ic within 10,000 years of site closure. Therefore,

INVT-05 The NRC staff recommends monitoring the inventory of I-129 in individual disposal structures in comparison to the projected inventories in Table 5 of this TRR under MF 1.01. The NRC staff recommends assessing the impact on the dose to a member of the public and an inadvertent intruder if one or more individual disposal structures exceed the MPAD inventory in the 2020 PA.

4.7 The DOE Process for Tracking SDF Inventory

The NRC staff monitors the DOE processes for tracking the SDF i nventory under MF 1.02, Methods Used to Assess Inventory. The DOE currently uses the SDF-WIDE model (SRR-CWDA-2015-00003, Rev. 0) to track the SDF inventory. That model tracks inventory disposed in the SDF based on radionuclide concentrations and vo lumes of treated salt waste in the feed tank for the Saltstone Production Facility (i.e., curr ently Tank 50). The NRC staff finds use of the SDF-WIDE model to be acceptable for the purpose of t racking SDF inventory for three reasons: (1) the model accounts for decay and ingrowth, which limits the potential for calculation and transcription errors, (2) the model facilitates tracking of data references, which improves data traceability, and (3) the DOE routinely updates t he model with data from samples from Tank 50.

As documented in the June 2008 Onsite Observation Visit Report (ADAMS Accession No. ML081290367), the NRC staff previously found the DOE method s for sampling and analyzing waste from Tank 50 to be acceptable (ML081290367). T he NRC staff will continue to evaluate the sampling and analysis methods to determine if ther e are changes due to the recent start of operations at the SWPF. Staff will evaluate those met hods both during Onsite Observation Visits and by monitoring the description of samplin g and analysis methods in the DOE quarterly Tank 50 Salt Solution Sample reports (e.g., SRR-S TI-2021-00165, Rev. 0). The NRC staff also will continue to use the quarterly Tank 50 Salt Solution Sample reports to track

the inventory added to the SDF in comparison to the values in T able 1. The NRC staff will evaluate whether values that exceed the MPAD values the DOE use d in the 2020 PA significantly affect the conclusions of the PA.

5.0 Teleconference or Meeting

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

6.0 Follow-up Actions

There are no specific Follow-up Actions related to this TRR. T he NRC staff will continue to monitor the radionuclide inventory in emplaced saltstone under MF 1.01 and the DOE methods to determine inventory under MF 1.02 until the DOE completes sa ltstone emplacement. The NRC staff recommends updating the Monitoring Plan as described in the Conclusions.

7.0 Conclusions

The NRC staff concluded that the radionuclide inventories in the 2020 PA are acceptable for modeling the projected dose from the SDF for the purpose of the DOE demonstrating compliance with the 10 CFR 61.41 performance objective, Protec tion of the General Population from Releases of Radioactivity. In addition, the NRC staff de termined that the resulting radionuclide concentrations in saltstone are acceptable as one input to the inventory for modeling the dose to an individual who intrudes on the SDF for the purpose of the DOE demonstrating compliance with the 10 CFR Part 61.42 performance objective, Protection of Individuals from Inadvertent Intrusion.

The NRC staff did not change the status (open) or priority (per iodic) of MF 1.01, Inventory in Disposal Structures, based on this TRR. The NRC staff also di d not change the status (open) or priority (medium) of MF 1.02, Methods Used to Assess Invent ory, based on this TRR. The NRC staff will continue to monitor the radionuclide inventory i n emplaced saltstone under MF 1.01 and the DOE methods to determine inventory under MF 1.0 2 until the DOE completes saltstone emplacement. The NRC staff will continue the followi ng monitoring activities, which are included in the Monitoring Plan (ADAMS Accession No. ML1310 0A003):

review waste sampling reports and other documents the DOE issu es periodically related to inventory, as listed in Table 1-1 of the Monitoring Plan (MF 1.01 and MF 1.02);
monitor waste sampling methods and frequency to ensure reporte d radionuclide concentrations in salt waste are representative of waste emplac ed in the SDF (MF 1.02);
monitor the methods the DOE uses to track inventory emplaced i n the SDF (MF 1.02); and
monitor inputs of thorium-bearing waste to ensure additions of Th-230 are small compared to the ingrowth of Th-230 from U-234 (MF 1.01 and MF 1.02).

The NRC staff recommends updating the Monitoring Plan to add th e following monitoring activities after the NRC staff completes the Technical Evaluation Report:

INVT-01 The NRC staff recommends updating Table A-1 of the Monitoring Plan to reflect the MPAD inventory in the 2020 PA under MF1.01.
INVT-02 The NRC staff recommends monitoring additions to the SDF of radionuclides that are at or near the total projected SDF inven tory under MF 1.01.

The NRC staff recommends assessing the impact on dose to a memb er of the public and an inadvertent intruder if the SDF inventories exceed the M PAD inventory in the 2020 PA;

INVT-03 The NRC staff recommends monitoring significant changes in wast e characteristics and annual volum es of waste sent from H-Canyon to the Tank Farms to assess the potential effects on SDF inventory under MF 1.02;
INVT-04 The NRC staff recommends monitoring significant changes in wast e characteristics and annual volum es of waste sent from H-Canyon to the Tank Farms to assess the potential effects on I-129 and Tc-99 inventory in the SDF under MF 1.01;
INVT-05 The NRC staff recommends monitoring the inventory of I-129 in individual disposal structures in comparison to the projected inventories in Table 5 of this TRR under MF 1.01. The NRC staff recommends assessing the impact o n the dose to a member of the public and an inadvertent intruder if one or more individual disposal structures exceed the MPAD inventory in the 2020 PA.

8.0 References

National Council on Radiation Protection (NCRP) Report No. 123, Screening Models for Releases of Radionuclides to the Atmosphere, Surface Water, and Ground, January 1996.

U.S. Department of Energy (DOE), SRR-CWDA-2011-00115, Rev. 0, Saltstone Disposal Facility Case K Inventory Determination, August 16, 2011. ML113320433

___, SRR-CWDA-2011-00044, Rev 1, Comment Response Matrix for NRC RAI-2009 Second Request for Additional Information on the Saltstone Disposal Facility Performance Assessment at the Savannah River Site, August 2011. ML113320303

___, SRR-CWDA-2012-00002, Rev. 0, Revised Methodology for Determination of Inventories in SDF Vaults 1 and 4 through 9/30/2011, January 2012. ML12171A395

___, SRR-CWDA-2012-00095, Rev. 1, Projected Technetium-99 Inventory in Saltstone Disposal Facility Units 2, 3, and 5, July 2012. ML1219A8307

___, SRR-CWDA-2013-00147, Rev. 0. SDF Inventory Estimates for Transport Modeling, Savannah River Site, Aiken, SC, December 9, 2013. ML21181A117

___, SRR-CWDA-2014-00006, Rev. 2, Letter from J. Folk to L. Camper Transmitting the DOE Fiscal Year 2014 Special Analysis for the Saltstone Disposal Facility at the Savannah River Site, October 29, 2014. ML14322A259

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

___, SRR-CWDA-2015-00003, Rev. 0, Saltstone Disposal Facility Waste Inventory Disposed Estimator Model Report, January 2015. ML15335A099

___, SRR-CWDA-2015-00020, Rev. 0, Memorandum: Consideration of Mo-93 as a Constituent in SDF Inventory, February 17, 2015. ML21187A295

___, SRR-CWDA-2016-00072, Rev. 0, Fiscal Year 2016 Special Analysis for the Saltstone Disposal Facility, October 2016. ML18081A262 ___, SRR-CWDA-2015-00003, Rev. 0. Saltstone Disposal Facility Waste Inventory Disposed Estimator Model Report, January 2015. ML20206L216

___, SRR-CWDA-2015-00020, Rev. 0. Consideration of Mo-93 as a Constituent in SDF Inventory, February 2015. ML21187A295

___, SRR-LWP-2009-00001, Rev. 20. Liquid Waste System Plan, January 2016. ML21201A040

___, SRR-CWDA-2015-00077, Rev. 2, Evaluation of I-129 Concentration Data to Improve Liquid Waste Inventory Projections, February 2018. ML20206L219

___, SRR-CWDA-2015-00123, Rev. 2, Evaluation of Tc-99 Concentration Data to Improve Liquid Waste Inventory Projections, March 2018. ML18170A279

___, SRR-CWDA-2018-00009, Rev. 0, Memorandum: Recommended Sampling Distribution for SDF Performance Assessment Inventory, May 2, 2018. ML21176A027

___, SRR-CWDA-2018-00076, Rev. 0, Memorandum: Recommended Implementation of Inventory Sampling Distributions for the Saltstone Disposal Facility Performance Assessment GoldSim Model, October 24, 2018. ML20206L249

___, SRR-CWDA-2018-00072, Rev. 0, Determination of the SDF Inventory through 9/30/2018, January 7, 2019. ML19179A042

___, SRR-LWP-2009-00001, Rev. 21, Liquid Waste System Plan, January 2019. ML21201A046

___, SRR-CWDA-2018-00044, Rev. 3. Memorandum: Inventory Screening Methodology and Application to the FY2019 Saltstone Disposal Facility (SDF) Performance Assessment (PA) Inventory, February 14, 2019. ML20206L241

___, SRR-CWDA-2018-00041, Rev. 3. Determination of Inventory for FY 2019 PA Modeling, July 2019. ML20206L240

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

___, SRR-CWDA-2021-00047, Rev. 0. Comment Response Matrix for the First 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, May 2021. ML21147A104

___, SRR-STI-2021-00165, Rev. 0. Results for the First Quarter Calendar Year 2021 Tank 50 Salt Solution Sample, May 2021. ML21148A002

U.S. Nuclear Regulatory Commission (NRC), U.S. Nuclear Regulatory Commission Technical Evaluation Report for the U.S. Department of Energy Savannah River Site Draft Section 3116 Waste Determination for Salt Waste Disposal, December 2005. ML053010225

___, Request for Additional Information on the Draft Section 3116 Determination for Salt Waste Disposal at the Savannah River Site, May 26, 2005. ML051440589

___, U.S. Nuclear Regulatory Commission Plan for Monitoring the U.S. Department of Energy Salt Waste Disposal at the Savannah River Site in Accordance with the National Defense Authorization Act for Fiscal Year 2005, May 3, 2007. ML070730363

___, Onsite Observation Visit Report for the Savannah River Site Saltstone Production and Disposal Facilities, June 5, 2008. ML081290367

___, Technical Evaluation Report for the Revised Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, Rev. 1, April 2012. ML121170309

___, Letter from M. Satorius to T. Spears Acknowledging the DOE Response to the NRC Letter of Concern Regarding Disposal Activities at the SDF, August 31, 2012. ML12213A447

___, U.S. Nuclear Regulatory Commission 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, September 2013. ML13100A113

___, U.S. Nuclear Regulatory Commission Staff Comments and Requests for Additional Information on the Fiscal Year 2013 Special Analysis for the Saltstone Disposal Facility at the Savannah River Site, SRR-CWDA-2013-00062, Rev. 2, June 13, 2014. ML14148A153

___, U.S. Nuclear Regulatory Commission Staff Request for Additional Information on the Fiscal Year 2014 Special Analysis for the Saltstone Disposal Facility at the Savannah River Site, SRR-CWDA-2014-00006, Rev. 2, June 26, 2015. ML15161A541}}

ML23017A087

Text

1.0 Purpose and Scope

2.0 Background

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