ML25238A082

Moab TER 2024 ET Cover Revision

ML25238A082
Person / Time
Site:	WM-00110
Issue date:	09/17/2025
From:	NRC/NMSS/DDUWP/URMDB
To:	US Dept of Energy, Grand Junction Office
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ML25238A078	List: ML25238A081 ML25238A082
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Download: ML25238A082 (1)

Text

Technical Evaluation Report of Revision 3 to Remedial Action Plan for Moab Remedial Action Project Docket No. WM-00110 September 2025

Table of Contents

1.0 INTRODUCTION

...................................................................................................1 1.1 EPA Standards..............................................................................................................1 1.2 Site History.....................................................................................................................1 1.3 Review Process.............................................................................................................2 1.4 TER Organization..........................................................................................................2 1.5 Conclusions....................................................................................................................2 2.0 GEOLOGY AND SEISMOLOGY...........................................................................3 3.0 GEOTECHNICAL STABILITY...............................................................................4 3.1 Introduction....................................................................................................................4 3.2 Characterization of Contaminated Materials and Disposal Site Stratigraphy.....4 3.3 Slope Stability................................................................................................................5 3.4 Settlement......................................................................................................................6 3.5 Liquefaction....................................................................................................................6 3.6 Cover Design.................................................................................................................7 3.7 Construction Considerations.......................................................................................9 3.8 Disposal Cell Hydraulic Conductivity.......................................................................10 3.9 Conclusions..................................................................................................................10 4.0 SURFACE WATER HYDROLOGY AND EROSION PROTECTION..................11 4.1 Introduction..................................................................................................................11 4.2 Hydrologic Description and Site Conceptual Design.............................................11 4.3 Flooding Determinations............................................................................................11 4.4 Erosion Protection.......................................................................................................13 4.5 Conclusions..................................................................................................................17 5.0 WATER RESOURCES PROTECTION...............................................................18 5.1 Introduction..................................................................................................................18 5.2 Conceptual Design Features.....................................................................................18 6.0 RADON ATTENUATION AND SITE CLEANUP.................................................19 6.1 Introduction..................................................................................................................19 6.2 Radon Attenuation......................................................................................................19

7.0 REFERENCES

....................................................................................................22

1

1.0 INTRODUCTION

The Floyd D. Spence National Defense Authorization Act for fiscal year 2001 identified that Moab site was to be remediated by the U.S. Department of Energy (DOE) under Title I of the Uranium Mill Tailings Radiation Control Act of 1978 (UMTRCA). UMTRCA requires, in part, that the U.S. Nuclear Regulatory Commission (NRC) concur with DOEs selection of remedial action, such that the remedial action meets appropriate standards promulgated by the U.S.

Environmental Protection Agency (EPA). In 2008, DOE submitted the Final Remedial Action Plan and Site Design for Stabilization of Moab Title I Uranium Mill Tailings at the Crescent Junction, Utah, Disposal Site (DOE, 2008), referred to as the 2008 RAP in the remainder of this document. The NRC staff reviewed and conditionally concurred on the surface remediation aspects of the remedial action plan in 2008 (NRC, 2008a); the staff noted that its concurrence on the groundwater cleanup would be based on a future submittal. In 2024, DOE submitted Revision 3 to the Final Remedial Action Plan (Revision 3 to the RAP) to alter the cross section of the final cover system at the Crescent Junction disposal site (DOE, 2024b). DOEs submittal modified the final cover system to be able to construct an evapotranspiration cover system on the top slope of the disposal cell at Crescent Junction. As the NRC staff previously concurred on the remedial action plan, this review focused on those aspects of the remedial action plan that DOE modified from the NRCs previous concurrence. This Technical Evaluation Report (TER) documents the NRC staffs review of DOEs 2024 revisions to the remedial action plan.

1.1 EPA Standards As required by UMTRCA, remedial action at the Moab site must comply with regulations established by EPA in 40 CFR Part 192, Subparts A-C. These regulations are summarized as follows:

1.

The disposal site shall be designed to control the tailings and other residual radioactive materials for 1000 years, to the extent reasonably achievable, and, in any case, for at least 200 years [40 CFR 192.02(a)].

2.

The disposal site design shall prevent radon-222 fluxes from residual radioactive materials to the atmosphere from exceeding 20 picocuries/square meter/second or from increasing the annual average concentration of radon-222 in air by more than 0.5 picocuries/liter [40 CFR 192.02(b)].

3.

The disposal site design shall provide reasonable assurance that identified constituents entering the groundwater from the site will not exceed established concentration limits in the uppermost aquifer [40 CFR 192.02 (c)].

4.

The remedial action shall ensure that radium-226 concentrations in land that is not part of the disposal site averaged over any area of 100 square meters do not exceed the background level by more than 5 picocuries/gram averaged over the first 15 centimeters of soil below the surface and 15 picocuries/gram averaged over any 15-centimeter-thick layer of soil more than 15 centimeters below the land surface [40 CFR 192.12(a)].

1.2 Site History Details about the history of Moab and the Crescent Junction disposal site can be found in DOEs 2008 RAP and the corresponding NRC staff TER (NRC, 2008). Since the NRCs 2008

2 concurrence on DOEs remedial action plan, DOE has relocated approximately 15 million tons of residual radioactive material to the Crescent Junction disposal site (DOE, 2024a).

1.3 Review Process The NRC review process followed the Final Standard Review Plan for UMTRCA Title I Mill Tailings Remedial Action Plans, Revision 1 (NRC, 1993) and consists of an evaluation of the modifications to DOEs 2008 RAP. DOE described the modifications in Revision 3 to the RAP (DOE, 2024b). In this review, the NRC staff focused on the modifications from the 2008 RAP.

DOEs Revision 3 to the RAP does not address the groundwater remediation requirements in 40 CFR 192.12(c). Similar to the approach followed at other UMTRCA Title I sites, DOE will address cleanup of contaminated groundwater at the Moab site separately. The NRC staff understands that DOE plans to submit its remedial action plan addressing groundwater contamination at a later date.

1.4 TER Organization The purpose of this TER is to document the NRC staff review of DOEs Revision 3 to the RAP for the Moab site. Following the outline of the NRC staffs 2008 TER (NRC, 2008b), the chapters of this report have been organized by technical discipline relative to the EPA standards in 40 CFR Part 192, Subparts A-C. Chapters 2 (Geology and Seismology), 3 (Geotechnical Stability),

and 4 (Surface Water Hydrology and Erosion Protection) provide the technical basis for the NRC staffs conclusions with respect to the long-term stability standard in 192.02(a). Section 5, Water Resources Protection, summarizes the NRC staffs conclusions related to the adequacy of DOEs compliance demonstration with EPAs groundwater protection requirements in 192.02(c). Chapter 6, Radon Attenuation and Site Cleanup, provides the basis for the staffs conclusions with respect to the radon control standards in 192.02(b) and soil cleanup standards in 192.12. As discussed above, this TER follows the format of the NRC staffs 2008 TER (NRC, 2008b) and, when appropriate, incorporates the previous conclusions for aspects of the RAP that have not changed. DOEs modifications contained in Revision 3 to the RAP focus on the final cover system.

1.5 Conclusions The NRC staffs review of DOEs Revision 3 to the RAP determined that the change to the top slope of the disposal cell to implement an evapotranspiration cover system and the change to the rock sizing on the side slope is consistent with the requirements in 40 CFR Part 192. Note that as was the case with the 2008 RAP, Revision 3 to the RAP does not address remediation of groundwater contamination at the Moab site. Although the NRC staff considers DOEs deferral of groundwater cleanup to be acceptable, it precludes us from fully concurring at this time. When DOE has adequately addressed groundwater cleanup, NRC will provide its final concurrence on the remedial action for the Moab site.

3 2.0 GEOLOGY AND SEISMOLOGY The NRCs 2008 TER documented the staffs review of the geology and seismology aspects of DOEs RAP (NRC, 2008b). The staffs 2008 review evaluated geologic and seismologic characterization, geologic stability, bedrock stability, geomorphic stability, and seismic stability.

As discussed in more detail in that document, the Crescent Junction disposal site appears to be geologically and tectonically stable. The staff did not identify faults or other mass movement processes that would likely impact the site. DOE adequately estimated the seismic design for ground motion, 0.22g. The staffs review concluded that DOE demonstrated that the geologic and seismologic processes present at the Crescent Junction disposal site would not jeopardize compliance with 40 CFR 192 requirements.

The NRC staff reviewed DOEs 2024 RAP, Revision 3 (DOE, 2024). This included a review of geologic characterization, as well as geologic, bedrock, geomorphic, and seismic stability. As the physical location of the Crescent Junction disposal cell remains unchanged, the geologic and seismologic conditions present today are consistent with the historical knowledge of the site. During the current review, the NRC staff did not identify any new information that would alter the staffs conclusions made in the 2008 TER. Therefore, the findings related to the RAP meeting the requirements of 40 CFR Part 192 remain valid and the staff is not re-reviewing these aspects of the design in this TER.

4 3.0 GEOTECHNICAL STABILITY 3.1 Introduction The staff has reviewed the geotechnical engineering aspects of DOEs modifications to the Final Remedial Action Plan and Site Design for Stabilization of Moab Title I Uranium Mill Tailings at the Crescent Junction, Utah Disposal Site, Revision 3 (DOE, 2024b). DOEs Revision 3 to the RAP consists of changes to the profile of the cover system on the top slope of the disposal cell and altering the rock sizing on the side slopes of the disposal cell. The side and top slopes of the disposal cell remain unchanged from the 2008 design.

Staff reviewed these modifications against the EPA requirements presented in 40 CFR Part 192, Health and Environmental Protection Standards for Uranium and Thorium Mill Tailings using Section 2.0 of the Final Standard Review Plan for the Review and Remedial Action of Inactive Mill Tailings Sites Under Title I of the Uranium Mill Tailings Radiation Control Act (NRC, 1993). The NRC staffs review focused on the modifications contained in Revision 3 to the RAP.

For aspects of the RAP that remain unchanged, the NRC staff incorporates the previous findings from its 2008 TER. This TER includes a review of the: 1) general information on the processing and disposal sites; 2) characterization data for materials associated with the remedial action activities, including the residual radioactive material (RRM) at the Moab site, and the disposal cell foundation and excavation materials; 3) disposal cell design and construction details; and 4) the long-term stability of the waste disposal cell and its cover.

3.2 Characterization of Contaminated Materials and Disposal Site Stratigraphy DOE performed geotechnical investigations at both the Crescent Junction Disposal Site and the Moab processing site to define the occurrence and engineering properties of contaminated materials and subsurface materials. A series of test pits, boreholes, geophysical investigations, and laboratory testing yielded information on contaminated and subsurface materials. The NRCs 2008 TER documented its review of subsurface investigations and found that based on the field investigations, sampling techniques, and laboratory procedures, that DOE had acceptably described the stratigraphy and geotechnical engineering characteristics of the disposal site, and the characteristics of the processing site RRM (NRC, 2008).

To support Revision 3 of the RAP, DOE conducted a limited geotechnical investigation focused on understanding parameters related to unsaturated soil modeling for the evapotranspiration (ET) cover system and for identifying potentially dispersive soils. DOEs efforts included collection of soil samples from the alluvium within the excavation area of the Crescent Junction disposal cell. DOE obtained the soil samples from areas planned for use in the interim cover, radon barrier, and surface admixture components of the final cover system. In testing the soils for being dispersive in nature, DOE followed the procedures for the Crumb Test (ASTM D6572) and Pinhole Test (ASTM D4747). DOEs analysis determined that dispersive soils are not a significant issue at Crescent Junction.

The NRC staff reviewed the results of DOEs limited geotechnical investigation, including the dispersive soil tests. DOE determined the engineering properties of the soils using acceptable sampling techniques. DOEs investigations and analyses followed with acceptable standards and engineering practices. Additionally, DOE adequately described the laboratory procedures and testing techniques. Therefore, information from DOEs 2008 RAP, supplemented by the information contained in DOEs 2024 Revision 3 to the RAP is sufficient to support engineering assessments related to waste isolation characteristics, permeability characteristics, and long-

5 term stability of the disposal cell for controlling radiological hazards. On the basis of the information presented in 2008 RAP, and supplemented by the information in Revision 3 to the RAP and the review conducted of the site characterization, the NRC staff concludes that the characterization information is sufficient and acceptable, and that its use with other information, such as the results of design analyses, provides an acceptable basis to enable the staff to make a finding on compliance with applicable criteria in 40 CFR Part 192.

3.3 Slope Stability The NRC staff reviewed the configuration of the disposal cell in Revision 3 of the RAP. Based on its review, the NRC staff understands that the slope lengths and angles presented in the 2008 RAP remain the same; Revision 3 to the RAP only alters the configuration of the layers of the final cover system. Therefore, the location of the critical cross section and slope stability analysis from the 2008 TER remains applicable to the arrangement of the disposal cell presented in Revision 3 of the RAP.

As discussed in the staffs 2008 TER, DOE identified the critical cross section location used in its analysis of the cells slope stability because it represents a combination of both the highest slope face of the disposal cell and a natural grade that slopes away from the disposal cell at the base of the side slope. DOE based the geotechnical properties of the clean fill dike and cover materials used in construction of the cell at the densities and moisture contents that are consistent with placement specifications. Additionally, DOE used geotechnical properties of the tailings materials based on available test results for the Moab tailings. The critical slope section analysis considered for both short-term (end of construction) and long-term conditions, under static and seismic loading. For the seismic loading, DOE arrived at a predicted Peak Horizontal Acceleration (PHA) of 0.22g (see Section 2.6 of this TER). Consistent with accepted practice, the seismic coefficient for a pseudo-static analysis is equivalent to half of the PHA (0.11g) for an end of construction analysis, and 2/3 of the PHA (0.15g) for the long-term analysis.

In the 2008 TER, the NRC staff determined that DOE acceptably presented the slope stability evaluation by 1) providing an appropriate cross section and profile to accurately represent slope and foundation conditions; 2) establishing a slope steepness of five horizontal (5h) to one vertical (1v); 3) providing measurement and selection of static and dynamic properties of soil and rock using acceptable assumptions, tests, and standards; and 4) selecting a location for slope stability analyses that considers the maximum slope angle and slope height, and the weakest materials and foundation conditions.

Based on the information presented in DOEs 2008 RAP, and the 2024 Revision 3 to the RAP, the staff concludes that, under static and dynamic conditions, the cell foundation, the slopes of the disposal cell, and the cover system will not adversely affect the cell performance due to slope failure. The staff has not identified any information in the 2024 Revision 3 to the RAP that alters the previous conclusions. The results of the slope stability analyses, and the associated models used provide an acceptable basis to demonstrate compliance with the applicable long-term stability criteria in 40 CFR Part 192.02(a), which requires that impoundment designs provide reasonable assurance of control of radiological hazards to be effective for 1,000 years, to the extent reasonably achievable, and, in any case, for at least 200 years.

3.4 Settlement As discussed in the NRC staffs previous TER, DOE placed and compacts RRM from Moab in a new disposal cell at the Crescent Junction site. In Addendum D, Appendix D of Revision 3 to the

6 RAP (DOE, 2024), DOE analyzed post-construction settlement of the ET cover system. While the cover system profile is slightly thinner than what DOE proposed in the 2008 RAP, the different layer configuration in Revision 3 to the RAP alters the stresses acting on the RRM and underlying soil. The NRC staff considers this a change to the design that necessitates a review.

Therefore, the NRC has reviewed DOEs settlement calculations in Revision 3 to the RAP and documents its findings below. DOEs analysis included consideration of differential settlement and its potential impact on the ability of the cover to maintain positive drainage off the cover.

As discussed in Revision 3 to the RAP, DOE will remove overburden soils and the excavation will extend at least two feet into the Mancos Shale. Therefore, settlement of the foundation soils will be negligible. DOE has not proposed altering the method of RRM placement in the disposal cell. DOE will continue to mix RRM at the Moab site prior to transport to Crescent Junction.

Once at Crescent Junction., Consistent with the technical specifications, DOE will place RRM in the cell as an unsaturated material near optimum moisture content, spread in lifts, and compacted to 90% of maximum dry density with conventional construction equipment. The NRC staff observes that the multi-year construction schedule for the disposal cell provides significant time for RRM drying and settlement prior construction of the ET cover system. The NRC staff recognizes that the approach to mixing and placing described in the utilized in Revision 3 to the RAP at Crescent Junction results in relatively uniform RRM within the disposal cell.

DOE presents its analysis of differential settlement in Addendum D, Appendix D of Revision 3 to the RAP. DOEs calculations anticipate 9.2 inches of primary settlement at the center of the disposal cell and 8.3 inches of primary settlement at the northern edge of the disposal cell.

According to DOE, the differential settlement of 0.9 inches will not result in ponding issues over the northern slope of the cell. When considering secondary settlement, DOE calculated 12.7 inches of settlement at the center and 6.9 inches of settlement at the northern edge of the disposal cell, respectively. According to DOE, the differential settlement of 5.8 inches will not result in a grade reversal or ponding issues on the final cover system.

The NRC staff reviewed DOEs calculations in Revision 3 to the RAP. In its review, the staff observed that material properties, thicknesses, and load increments used to calculate settlement are representative of the ET cover design and subsurface site conditions. DOE followed the Terzaghi one dimensional consolidation theory, which is an appropriate methodology for the disposal cell design and soil conditions at the site. Additionally, DOE properly documented the results of the settlement analyses. The NRC staff observed that DOEs calculations were based on the known material properties for the site. Additionally, the NRC staff did not identify any errors in DOEs calculations.

Based on the information presented in Revision 3 to the RAP (DOE, 2024) and the staff review conducted of the settlement resulting from the cover system construction, the NRC staff concludes that DOEs settlement analysis demonstrates compliance with the criteria in 40 CFR Part 192 applicable to stability and cover integrity.

3.5 Liquefaction The staff has evaluated DOEs analysis of the potential for liquefaction of the RRM in the disposal cell. In this regard, the RRM materials will not be saturated, and DOE will place the RRM as a structural fill. Therefore, liquefaction of the RRM is highly unlikely. However, DOE elected to perform a liquefaction analysis in the unlikely event that the RRM becomes saturated.

In Revision 3 to the RAP, DOE conducted analyses of this liquefaction potential based on the classic Seed and Idriss methodology (Day, 1999). This is consistent with the approach used in

7 the 2008 RAP and involves a comparison of the seismic stress ratio due to the design seismic event with the seismic stress ratio that would cause liquefaction of the RRM at a specific depth of analysis.

For the design Floating Earthquake of magnitude 6.2, DOE estimated the design PHA to be 0.22g. As discussed in Section 2 of this TER, this value remains representative for the Crescent Junction site. The DOE analysis assumed an extremely unlikely situation that the entire RRM thickness is saturated. For the relative density and the measured fines content of the compacted RRM, and for the depths analyzed (top and bottom of RRM), the stress ratio required to cause liquefaction was higher than the seismic stress ratio from the design earthquake, resulting in factors of safety of from 1.4 to 3.0. Based on this, DOE concludes that liquefaction is not a concern.

The staff reviewed DOEs liquefaction analysis. By following the same approach used in the 2008 RAP, DOE has acceptably evaluated liquefaction potential based on results from properly conducted laboratory tests and analyses. DOEs revised analysis incorporates the change to the ET cover system. The staff determined that DOEs methods used for interpretation of test data and calculation of stress ratios are consistent with current practice. Based on the information presented in Revision 3 to the RAP and the review conducted of the liquefaction potential at the Crescent Junction disposal cell, the NRC staff concludes that the results of DOEs evaluation of liquefaction potential demonstrate compliance with the criteria in 40 CFR Part 192 applicable to stability and cover integrity.

3.6 Cover Design The staff has completed a geotechnical engineering review of the modification to the disposal cell cover design. As previously discussed, DOE intends to construct an ET cover on the top slope of the disposal cell cover system. The ET cover used at this site would consist of the following layers, listed from bottom to top: 1) an interim cover of soil placed to a minimum thickness of 1-foot; 2) a 4-feet-thick compacted clay radon barrier constructed from conditioned on-site Mancos Shale; 3) a 2.375-foot-thick (28.5 inch thick) frost protection layer (consisting of the same material used to construct the interim cover); and (4) a 0.83-foot thick (10 inch thick) rock/soil admixture at the surface. The rock/soil admixture will consist of a blend of 60 percent rock to 40 percent soil. According to DOE, the rock/soil admixture limits erosion, but also to allow for establishment of vegetation. Taken together, the integrated components of the disposal cell cover control the release of radon from the contaminated materials, limit infiltration of precipitation, shed precipitation from the cover surface, and protect the cell from erosion and from freeze-thaw effects.

Interim Cover - DOE has not proposed any changes to this layer of the cover system. As discussed in the NRC staffs previous review (NRC, 2008), DOE will place interim on the contaminated RRM material where it has reached its full height of disposal. This layer will consist of a uniform fine-grained material produced on the site by modifying the existing overburden and weathered Mancos Shale excavated from the cell. The 1-foot-thick interim cover will protect the RRM from erosion by wind and water during construction and will minimize worker exposure to the RRM.

Radon Barrier - The first layer of final cover over the interim cover will be the radon barrier.

DOE will produce the radon barrier material by pulverizing, mixing, and wetting the excavated Mancos Shale to form a uniform fine-grained fill soil near its optimum moisture content for compaction. The purposes of the radon barrier are to limit the emanation of radon gas from the

8 RRM into the atmosphere, and to minimize the infiltration of precipitation into and through the RRM. DOE updated its radon flux modeling results to account for the ET cover system. DOEs model results indicate that a 4-foot-thick Mancos Shale radon barrier will control radon flux to levels below the EPA standard in 40 CFR Part 192.02(b). Section 6.2 of this TER discusses this in more detail.

Frost Protection - Above the radon barrier will be a 2.375-foot-thick (28.5 inch thick) frost protection layer. This layer serves two purposes. First, it provides protection to underlying cover layers from degradation due to environmental factors such as freezing and thawing. Second, it also serves as a layer to store water and help facilitate plant growth in the overlying layer. In the 2008 RAP, DOE compiled climate data, established soil thermal properties, calculated annual frost depths using the modified Berggren formula, and extrapolated beyond the record of observed data for added conservatism. With this process, DOE established a design extreme frost penetration of 45 inches for a rock cover and 38.5 inches for a vegetated cover. The total depth from the ground surface to the top of the Mancos Shale radon barrier will be 38.5 inches, which is consistent with the frost depth identified in the 2008 RAP for a vegetated cover. The frost depth calculations acceptably demonstrate that this layer and the erosion protection layer above will provide adequate protection for the radon barrier from freeze-thaw effects.

Erosion Protection - On the top slope of the cover system, the uppermost layer of the cover consists of an erosion protection layer. DOEs design calls for this 0.83-foot thick (10 inch thick) layer to consist of a rock/soil admixture. The rock/soil admixture will be 60 percent rock to 40 percent soil. The rock component will have a D50 of 2-inches. The side slopes of the final cover will all have a 4-inch-thick bedding layer. The north and south facing side slopes consist of a 6-inch thick layer of rock with a D50 of 3-inches. The east and west facing side slopes consist of a 4-inch thick layer of rock with a D50 of 2-inches. See Section 4.0 of this TER for a detailed discussion of the erosion protection design.

DOE has acceptably defined the modification to the disposal cell cover design by presenting detailed descriptions of the material types and/or soil mixtures, and the basis for their use in the cover layers. In Figure 9 of Revision 3 to the RAP, DOE has provided an acceptable schematic diagram displaying the various layers and thicknesses of the ET cover on the top slope of the disposal cell. DOE also has provided a description of the applicable field and laboratory investigations and testing, and identification of design material properties for the cover layers.

DOE has determined the properties of the cover materials using appropriate methods.

Additionally, DOE has identified adequate amounts of borrow materials with the desired material characteristics (cell excavation material) to construct the ET cover.

The NRC staff concludes that the type and thickness of the cover materials will limit infiltration, control the release of radon, and protect the radon barrier from potential damage from freeze-thaw cycles. Section 4.4 of this TER presents the staffs conclusions regarding the adequacy of the cover design to protect the disposal cell from adverse effects. Sections 3.8 and 6.2 of this SER present additional conclusions regarding the radon emanation design and infiltration design. From a geotechnical engineering perspective, the cover design will facilitate compliance with 40 CFR Part 192.02, which establishes requirements for disposal cell stability and control of radon.

3.7 Construction Considerations As previously discussed, the NRC staff is focusing its review on the aspects of Revision 3 to the RAP that have changed compared to the 2008 RAP. With respect to this review, the staff has

9 reviewed the information provided on specifications, testing, and other construction considerations to the disposal cell cover at Crescent Junction. DOE updated the specifications to address construction aspects of the ET cover. For example, DOE includes a method specification for the surface admixture (rock/soil matrix) in Specification 31 00 50.

DOE has updated the relevant specifications for the changes in the final cover system of the disposal cell. During its review, the staff observed that the updated specifications include requirements for: 1) general earthwork; 2) material characteristics, placement, compaction, and testing of the final cover layers; 3) placement of the surface admixture; and 4) rock aggregate and riprap characteristics and placement. For the surface admixture, DOE plans to test different placement methods to achieve the desired ratio of rock to soil. Addendum H to Revision 3 of the RAP describes this aspect of the design. As discussed in more detail in Section 4.4 of this TER, placing the surface admixture layer at the desired rock to soil ratio is a risk significant aspect of the ET cover construction.

DOE will continue to use a third party quality assurance program for the duration of the project.

This function ensures that all construction by the Remedial Action Contractor is in accordance with project specifications, that proper testing and inspection are performed to ensure project compliance, and that the construction process is properly documented. DOEs Technical Assistance Contractor will perform quality assurance audits. The audits will review construction progress and review all inspection records and testing reports. DOEs Remedial Action Inspection Plan (RAIP) provides details of the methods, procedures, and frequencies for testing and inspection of construction materials to verify compliance with the design specifications. It addresses cell floor (Mancos) inspection, embankment construction, contaminated material placement, interim cover and final cover (radon barrier, frost protection, and surface admixture) construction, rock armoring on the side slopes, settlement monitoring, and records management.

DOE has acceptably described the construction considerations by: (1) providing complete engineering drawings showing all design features; (2) describing sources and quantities of borrow material, including acceptable field and laboratory testing; and (3) identifying methods, procedures, and requirements for excavations, haulage, stockpiling, and placement of materials and demonstrating that all are consistent with accepted engineering practices for earthen works.

The construction considerations and associated specifications, inspection, and testing plans are acceptable and provide input to compliance with the EPA standards. The RAIP will provide a framework for quality control to ensure that DOEs construction of the disposal cell is constructed in accordance with the approved design and will meet the regulatory requirements.

3.8 Disposal Cell Hydraulic Conductivity The staff has completed its review of the hydraulic conductivity aspects of the disposal cell cover. DOE will construct the cell cover using fine-grained silts, clays, and weathered Mancos Shale. The low-permeability Mancos Shale used in the radon barrier limits percolation into the underlying RRM. Engineered compaction of these materials lowers the hydraulic conductivity of the cover. With the shift to an ET cover for the top slope of the disposal cell, DOE is relying on both the low hydraulic conductivity of the Mancos Shale radon barrier and evapotranspiration from the frost protection and surface admixture to limit infiltration into the RRM.

In Addendum D of Revision 3 to the RAP (DOE, 2024), DOE presents its design report for the ET cover system. This includes unsaturated flow modeling to estimate the amount of water

10 infiltrating into the RRM. DOEs modeling used the UNSAT-H computer code, which the staff acknowledges is appropriate for use in this situation. DOEs calculations estimate that the infiltration is minimized with the proposed ET cover thickness of 8.2 ft. The staff reviewed DOEs calculations, which were developed based on material properties in the 2008 RAP, and published literature. The staff determined that DOE used parameter values that are consistent with the site conditions and used an acceptable modeling code. As discussed in more detail in the staffs 2008 TER and revisited in Section 5 of this TER, a thick layer of Mancos Shale underlies the Crescent Junction disposal site. Additionally, there are no groundwater users nearby. Therefore, the staff does not consider infiltration of meteoric water into the RRM as a significant risk driver at this site.

For the ET cover as described above, DOE has acceptably evaluated the disposal cell cover material hydraulic conductivity. DOEs design limits infiltration of water into the RRM with a combination of a resistive barrier provided by the low-permeability Mancos Shale radon barrier in combination with ET capabilities of the overlying soil layers.

On the basis of information presented in Revision 3 to the RAP and the review conducted of the unsaturated modeling of the cover system, the NRC staff concludes that the determination and design of hydraulic conductivity aspects of the cell includes the information necessary to demonstrate compliance with the criteria in 40 CFR Part 192 applicable to stability and water resources protection.

3.9 Conclusions DOE has adequately addressed the key geotechnical areas required for demonstrating an acceptable plan for altering the cover design of the disposal cell at Crescent Junction, Utah. The staff concludes that from a geotechnical engineering standpoint, DOE has provided an acceptable design and has demonstrated its compliance with applicable criteria in 40 CFR Part 192.

11 4.0 SURFACE WATER HYDROLOGY AND EROSION PROTECTION 4.1 Introduction This section of the TER describes the staff's review of surface water hydrology and erosion protection issues related to long-term stability of the proposed changes to the cover system at Crescent Junction. As the staff discussed previously in this TER, this review is focused on the aspects of Revision 3 of the RAP that have changed when compared to the 2008 RAP. The staff compared Revision 3 to the RAP against the EPA requirements presented in 40 CFR Part 192, Health and Environmental Protection Standards for Uranium and Thorium Mill Tailings using Section 3.0 of the Final Standard Review Plan for the Review and Remedial Action of Inactive Mill Tailings Sites Under Title I of the Uranium Mill Tailings Radiation Control Act (NRC, 1993). Review areas covered include: estimates of flood magnitudes; water surface elevations and velocities; sizing of riprap to be used for erosion protection; long-term durability of the erosion protection; and testing and inspection procedures to be implemented during construction.

4.2 Hydrologic Description and Site Conceptual Design The staff reviewed Revision 3 to the RAP. The site hydrologic description and conceptual design remains consistent with the 2008 RAP. DOE has based its design on the Probable Maximum Precipitation (PMP) and the Probable Maximum Flood (PMF) events, both of which are considered to have very low probabilities of occurring during the 1000-year stabilization period.

DOE configured the top surface of the cell to drain in various directions at a slope of about two percent, and the embankment side slopes will be constructed on a one vertical (V) on five horizontal (H) slope. To protect against erosion, DOEs design calls for a rock/soil admixture on the top slope and riprap on the side. The remainder of this section of the TER focuses on DOEs proposed changes in Revision 3 of the RAP. DOE has not proposed any changes to the drainage channels surrounding the disposal cell at Crescent Junction.

4.3 Flooding Determinations DOE performed the computation of peak flood discharges for various site design features in several steps. These steps included: (1) selection of a design rainfall event; (2) determination of infiltration losses; (3) determination of times of concentration; (4) determination of appropriate rainfall distributions and intensities, corresponding to the computed times of concentration; and (5) calculation of flood discharge. DOE derived input parameters from each of these steps to calculate the peak flood discharges used in the final determination of rock sizes for erosion protection.

4.3.1 Selection of Design Rainfall Event In the cover system design report, Addendum D of Revision 3 to the RAP, DOE uses a 1-hr PMP of 8.2 inches in its erosion protection design. The NRC staff observes that this is the same value used in the 2008 RAP. As described in the staffs 2008 TER, DOE used Hydrometeorological Report No. 49 (HMR 49) to determine the PMP. The staff considers this an appropriate methodology for estimating PMP values for the small drainage areas present at the site. During its review, the NRC staff has not identified any new information that would result in

12 a change to this PMP value. Therefore, the staffs previous 2008 determination related to use of the 8.2 inch 1-hour PMP event remains valid.

4.3.2 Infiltration Losses The determination of the peak runoff rate is also dependent on the amount of precipitation that infiltrates into the ground during its occurrence. If the ground is saturated from previous rain, very little of the rainfall will infiltrate and most of it will become surface runoff. The loss rate is variable, depending on the vegetation and soil characteristics of the watershed. With the switch to an ET cover with vegetation, DOE used the Rational Method (Chow, 1959) with a runoff coefficient of 0.3 in its calculations. This is a relatively low runoff coefficient; however, DOE has followed a conservative approach in its calculation of rainfall intensity. Section 4.3.4 of the TER discussed this in more detail. Therefore, the staff concludes that use of this runoff coefficient is acceptable in this situation.

4.3.3 Times of Concentration The time of concentration (tc) is the amount of time required for runoff to reach the outlet of a drainage basin from the most remote point in that basin. The peak runoff for a given drainage basin is inversely proportional to the time of concentration. For a smaller tc, the peak discharge will be larger. In Revision 3 to the RAP, DOE used the Kirpich Method to determine the tc. The staff recognizes this method is consistent with the guidance in NUREG 1623 (NRC, 2002).

Based on its review of the calculations provided, the staff concludes that the tc values used by DOE were acceptably derived.

4.3.4 Rainfall Distributions and Intensities After the PMP is determined, it is necessary to determine the rainfall intensities corresponding to shorter rainfall durations and tc. If the tc is less than one hour, it is necessary to extrapolate the data presented in the various hydrometeorological reports to shorter time periods.

To determine peak flood flows for the cell, DOE developed a rainfall depth-duration curve using guidelines in NUREG-1623 and calculated the rainfall intensities for the small drainage areas at the site to be between 35 and 54 inches per hour. Based on a review of this aspect of the flooding determination, the staff concludes that the computed peak rainfall intensities are acceptable.

4.3.5 Computation of PMF Discharges In Revision 3 to the RAP, DOE utilized a different methodology to estimate the PMF peak discharges for the top slopes of the disposal cell. DOE used the Rational Method (Chow, 1959) but assumed that a gully has formed. In DOEs analysis, the drainage area contributing flow to the gully is equal to the slope length in one direction and is equal to 25 percent of the slope length in the other direction. Figure F-1 in Appendix F of Addendum D of the design report shows the contributing drainage area. DOEs assumption around gully formation and contributing area is acceptable to the staff as it represents a conservative configuration. In using the Rational Method, DOE assumed a runoff coefficient equal to 0.3. For a maximum top slope length of about 1300 feet (with a slope of 0.025), DOE estimated the peak flow rates to be about 102 cubic feet per second within a gully formed on the south facing portion of the top slope. For the north facing portion, which has a slope length of approximately 548 feet at a slope of 0.02,

13 DOE calculated a peak flow rate of approximately 78 cubic feet per second. The staff determined that DOEs correctly implemented its calculations.

For the side slope of the disposal cell, DOE used the Rational Method to calculate the peak flow rate per unit width (cubic foot per second per foot, or cfs/ft). This is consistent with the methodology used in the 2008 RAP and is acceptable to the staff. For the side slopes of the disposal cell, DOE calculated unit flow rates that ranged from 0.59 to 1 cfs/ft. The staff reviewed DOEs calculations and determined they were implemented correctly.

Based on its review of DOEs calculations for the PMF discharges from the top and side slopes of the disposal cell, including the time of concentration, rainfall intensity, and runoff, the staff concludes that DOEs estimated flow rates are acceptable.

4.4 Erosion Protection The ability of a riprap layer to resist the velocities and shear forces associated with surface flows over the layer is related to the size and weight of the stones which make up the layer.

Typically, riprap layers consist of a mass of well-graded rocks which vary in size. For the top slope of the disposal cell, DOEs erosion protection layer consists of a rock/soil admixture. The intent of this layer is to both provide erosion resistance and a soil component to support plant growth. DOEs design in Revision 3 of the RAP calls for the admixture to consist of 60 percent rock to 40 percent soil, measured by volume. For the side slope of the disposal cell, DOEs design continues to call for a rock cover.

4.4.1 Sizing of Erosion Protection DOE proposed riprap layers of various sizes and thicknesses for use at this site, and the design of each layer is dependent on its location and purpose. As discussed above, DOEs design calls for the top slope of the cover system to consist of a rock/soil admixture. This section reviews the rock sizing calculations.

4.4.1.1 Top Slopes, Side Slopes, and Aprons As discussed above in TER Section 4.3.5, DOEs design for the top slope anticipates that some gullies will form within the rock/soil admixture. DOEs calculation methodology in Appendix F of Addendum D of Revision 3 to the RAP outlines an approach that assumes a gully shape based on the mean annual flow, which is equal to 10 percent of the peak PMF described in TER Section 4.3.5. Based on that gully shape, DOE calculates an incipient particle size, which is the particle size on the brink of movement based on the conditions present. The incipient particle size represents the maximum particle size expected to move based on the conditions. DOE then calculates the required depth of the admixture, which is related to the incipient particle size.

DOEs calculations determined that a rock/soil admixture placed at a ratio of 60 percent rock (with a D50 of 2 inches) to 40 percent soil placed to a depth of 10 inches will provide sufficient erosion protection. DOE performed a check of the calculations following the rocky soil slope approach in NUREG 1623 (NRC, 2002).

The NRC staff reviewed DOEs approach and detailed calculations for the top slope of the disposal cell. From the staffs perspective, considering gully formation in the erosion protection design represents a more likely scenario for the top slope of the disposal cell given the long slope lengths. The staff recognizes the uncertainty related to the size, shape, and corresponding flow in a gully that does form. As discussed in more detail in TER Section 4.4.2

14 below, DOEs gradation for the rock/soil admixture does include larger particles that can help mitigate the concentrated flow that would exist within a gully. With a 60 percent rock to 40 percent soil admixture, the staff expects the admixture to perform in a similar manner to what is envisioned in the rocky soil slope approach in NUREG 1623 (NRC, 2002). The staffs review determined that DOEs calculations were implemented correctly.

For the side slopes of the disposal cell, DOE used the Abt-Johnson method to calculate the required D50 size (median rock diameter). This methodology is recommended in NUREG 1623.

In Revision 3 to the RAP, DOE proposes to use a 6-inch layer of rock with a minimum D50 of about 3 inches on the north and south side slopes of the disposal cell. The east and west side slopes will be protected by 4-inch layers of rock with a minimum D50 of 2 inches. The staff reviewed DOEs calculations and determined they were implemented correctly. The staff observes that the reduction in rock size on the side slopes of the disposal cell results from a combination of the presence of the rock/soil admixture on the top slope, which is anticipated to retain more water and result in slightly less runoff to the side slopes.

To protect the toe of the disposal cell and to dissipate the energy as the side slopes transition to natural ground, DOE will construct aprons along the toe of the side slopes. This aspect of the design will remain unchanged from the 2008 RAP. The area along the base of the south side slope consists of a rock toe/apron with a minimum D50 of 12 inches, while the toe of the north side slope consists of rock with a minimum D50 of 8 inches. During its review, the staff did not identify any new information that would require a change to DOEs previous analysis. Therefore, the staffs findings related to the toe apron discussed in the 2008 TER remain valid.

Based on staff review of DOE's analyses and the acceptability of using design methods recommended by the NRC staff, the staff concludes that the proposed rock sizes for the rock/soil admixture on the top slope are adequate. Additionally, the staff concludes that the rock sizes on the side slopes and aprons are adequate.

4.4.1.2 Diversion Channels In Revision 3 to the RAP, DOE does not propose any changes to the diversion channels at various locations around the disposal cell. Therefore, the staffs previous findings its 2008 TER remain valid.

4.4.1.3 Channel Outlets In Revision 3 to the RAP, DOE does not propose any changes to the diversion channel outlets.

The channel outlets will convey flow in the same manner described in the 2008 RAP. DOEs design conveys flow to the east and west sides of the disposal cell before turning southward.

The staff has not identified any new information that would require a change to this aspect of the design. Therefore, the staffs previous findings in the 2008 TER related to the channel outlets being sufficient to resist gully intrusion remain valid.

4.4.1.4 Sediment Considerations In Revision 3 to the RAP, DOE does not propose any changes to the design with respect to sediment considerations. DOEs design will continue to utilize a wedge of excess excavated material that will act as a diversion berm to re-direct runoff away from the disposal cell. During its review, the staff did not identify any new information that would require a chance to this

15 aspect of the design. Therefore, the staffs previous findings in the 2008 TER related to sediment considerations and the wedge remain valid.

4.4.2 Riprap Gradations DOE selected riprap gradations for each of the different rock sizes and layers using basic gradation criteria. As described in Revision 3 to the RAP, the erosion protection layer will consist of a rock/soil admixture with 60 percent rock and 40 percent soil. DOE provided an overall gradation for the rock and soil components of the admixture in Addendum D, Appendix J, Table J-4, of Revision 3 to the RAP. Additionally, Table 3 in Section 2.1.2.1 of Technical Specification 32 11 22 provides gradation parameters for the rock/soil admixture as well as for the rock component of the side slopes. The rock/soil admixture has a D75 value of approximately 2.4 inches and a D50 value of approximately 1.2 inches. DOEs design for the rock/soil admixture contains larger particles that can help mitigate the higher flows within a gully, should one form. The staff observes that constructing the erosion protection layer with the desired 60 percent rock to 40 percent soil ratio is important in maintaining the function of that layer of the ET cover. In Addendum H of Revision 3 of the RAP, DOE discusses a field demonstration plan to evaluate different methods to construct the erosion protection layer at the desired rock to soil ratio. DOE plans to incorporate observations and results from this field demonstration into the specification for the admixture layer and into the quality assurance approval during full scale construction of the erosion protection layer. The staff reviewed DOEs field demonstration plan and concurs on the approach.

4.4.3 Rock Durability The staffs 2008 TER evaluated rock durability to determine if there is reasonable assurance that the rock itself is durable and will survive and remain effective for 1000 years. The staffs review included an evaluation of the rock type and source, rock durability testing and scoring, absence of adverse minerals and heterogeneities, and evidence of resistance to weathering. In the 2008 RAP, DOE selected a basalt as a rock source from a site approximately four miles east of Fremont Junction, Utah, which is approximately 95 miles west of the Crescent Junction site. DOE has used rock from this quarry in the construction at the Crescent Junction site to date. In Revision 3 to the RAP, DOE plans to continue using rock from the Freemont Junction quarry. During its review, the staff did not identify any information that would alter its findings in the 2008 TER. Therefore, the staffs review and findings in Sections 4.4.3.1 through 4.4.3.5 remain valid and are not repeated here. As discussed in more detail in the staffs 2008 TER, the staff considers that the Fremont Junction basalt is acceptable for use in the erosion controls at the Crescent Junction site.

4.4.4 Testing and Inspection of Erosion Protection The staffs 2008 TER evaluated DOEs testing and inspection of the rock components of the cover system and drainage channels at Crescent Junction. This included a review of DOEs methods for rock selection during production, durability testing, gradation testing, and riprap placement. This portion of the staffs review focuses on the changes DOE has made in Revision 3 of the RAP to facilitate construction of the ET cover system.

4.4.4.1 Rock Selection During Production As discussed in TER Section 4.4.3, DOE will continue to use the Fremont Junction basalt as it rock source. In Revision 3 to the RAP, DOE provided information to document the quality

16 assurance and quality control (QA/QC) procedures that will be implemented during rock production to address variability in the rock source to assure consistent production of rock of acceptable quality. During its review, the staff did not identify any information that would alter its findings in the 2008 TER related to rock selection during production. Therefore, the staffs review and findings from the 2008 TER remain valid.

4.4.4.2 Durability Testing The staff reviewed the durability testing requirements for the rock component of the erosion protection layer. Similar to the 2008 RAP, DOE proposes that rock durability testing will be performed a minimum of four times and/or at a frequency of one test for every 10,000 cubic yards of material produced for the rock/soil admixture. This testing frequency is recommended in NUREG-1623 and is equivalent to others approved by the staff and have been implemented at other reclaimed sites during construction. DOE's rock durability testing program for the erosion protection layer will follow the approach used in the 2008 RAP and consist of the following tests, shown with their ASTM International designation:

1. Bulk Specific Gravity - ASTM C 127

2. Absorption - ASTM C 127

3. Sodium Sulfate Soundness - ASTM C 88

4. L.A. Abrasion at 100 cycles - ASTM C 131

5. Rebound Hardness - ASTM 5873 Based on a review of the proposed procedures, the staff concludes that an acceptable durability testing program has been provided to ensure that rock of acceptable quality will be provided for the erosion protection layer. The testing program was developed using suggested staff guidance in NUREG-1623.

4.4.4.3 Gradation Testing As discussed above in TER Section 4.4.2, DOE plans a field demonstration test to verify construction techniques to achieve the desired rock to soil ratio for the erosion protection layer.

The purpose of the demonstration test is to verify construction of the rock/soil admixture at the desired 60% rock to 40 % soil ratio, by volume. At the conclusion of the field demonstration, the contractor will describe the installation requirements to maintain the desired ratio, considering different construction techniques. The staff reviewed DOEs field demonstration plan and concurs on the approach. The gradation testing requirements for the rock components on the side slopes of the disposal cell remain consistent with the 2008 RAP and are therefore acceptable to the staff.

4.4.4.4 Riprap Placement Similar to gradation testing, in the previous section, DOEs field demonstration test will also be used to verify placement techniques for the rock/soil admixture. The field test will verify layer thickness, uniformity, and compaction. The staff reviewed DOEs field demonstration plan and concurs on the approach. The riprap placement requirements for the side slopes of the disposal cell remain consistent with the 2008 RAP and are therefore acceptable to the staff. Based on its review of the information provided by DOE, the staff concludes that the proposed procedures are sufficient to ensure acceptable placement of the riprap, including the rock/soil admixture of the ET cover.

17 4.5 Conclusions Based on review of the information submitted by DOE and the staffs review as described above, the NRC staff concludes that sufficient information has been provided to justify that the erosion protection design is adequate to provide reasonable assurance of protection for 1000 years, as required by 40 CFR 192.

18 5.0 WATER RESOURCES PROTECTION 5.1 Introduction The NRCs 2008 TER documented the staffs review of the water resources protection aspects of DOEs 2008 RAP. The staffs 2008 review evaluated groundwater site characterization, hydrogeologic units, hydraulic transport properties, geochemical conditions, conceptual design features, and a discussion of how the Crescent Junction disposal site meets the criteria in 40 CFR Part 192.

The NRC staff reviewed DOEs Revision 3 to the 2024 RAP. As the physical location of the Crescent Junction disposal cell remains unchanged, the site characteristics described in the revision are consistent with the staffs previous approval. The staff reviewed the groundwater site characterization information and the location of any new groundwater wells drilled since 2008 in the vicinity of the disposal cell. The staff did not identify any new groundwater users near the disposal cell. During its current review, the NRC staff did identify that the conceptual design features of the cover system are different from the 2008 RAP. Therefore, the NRC staff is only updating that portion of this TER. The staffs findings related to the groundwater site characterization and disposal standards meeting the requirements of 40 CFR Part 192 remain valid and the staff is not re-reviewing these aspects of the design in this TER.

5.2 Conceptual Design Features DOE has designed the cell cover to limit infiltration into the cell. The 2008 RAP called for a resistive style rock cover to minimize erosion and protect the RRM. DOEs Revision 3 to the RAP calls for construction of an evapotranspiration (ET) cover on the top slope of the disposal cell. The lower portions of the ET cover remain the same as the 2008 RAP. Above the contaminated RRM, DOE will construct an interim cover of fine-grained material to protect the RRM prior to construction of the rest of the cover. DOE will construct a radon barrier of low-permeability pulverized Mancos Shale above the interim cover. Above the radon barrier, DOE will construct a soil layer, called the frost protection layer. According to DOE, this layer protects the underlying radon barrier from freeze/thaw effects and to store water to support vegetation growth. The uppermost layer of the ET cover is a mixture of rock and soil, referred to as the rock/soil admixture to provide erosion resistance. As discussed in the design, the combination of the frost protection layer and the rock/soil stores water and releases it through evaporation or transpiration through plants, which is more consistent with the natural conditions of the disposal cell. A more complete description of the cover can be found in Section 3.6 of this TER. Use of an ET cover for the Crescent Junction disposal cell is consistent with requirements in 40 CFR Part 192.

19 6.0 RADON ATTENUATION AND SITE CLEANUP 6.1 Introduction This section of the TER documents the staffs evaluation of the radon attenuation design and soil cleanup for the planned remediation action at the Moab, Utah Uranium Mill Tailings Remedial Action (UMTRA) Project. The staffs 2008 TER documented radon attenuation of the final cover at Crescent Junction as well as the site cleanup standards at the Moab processing site. In Revision 3 to the RAP, DOE has not proposed any changes to the site cleanup standards at the Moab processing site; the only changes are to the profile of the final cover system at Crescent Junction. Therefore, the staffs findings related to site cleanup documented in the 2008 TER remain valid. This section focuses on the radon attenuation aspects of the final cover. The staff reviewed Revision 3 to the RAP against the EPA requirements presented in 40 CFR Part 192, Health and Environmental Protection Standards for Uranium and Thorium Mill Tailings using Section 5.0 of the Final Standard Review Plan for the Review and Remedial Action of Inactive Mill Tailings Sites Under Title I of the Uranium Mill Tailings Radiation Control Act (NRC, 1993). The results of this review consist primarily of evaluations of the proposed remedial actions to ensure compliance with the applicable EPA standards.

6.2 Radon Attenuation Consistent with the 2008 RAM, DOE will relocate the RRM at the Moab processing site to the Crescent Junction, Utah, disposal site and placed in an engineered disposal cell. The disposal cell will have a radon barrier designed to limit the release of radon to less than 20 picoCuries per meter2 per second to meet the EPA radon flux standard. The staffs review of the disposal cell design for radon attenuation included evaluation of the pertinent design parameters for both RRM and the radon barrier soils, and the calculation of the radon barrier thickness.

The Moab UMTRA Project cover design consists of an interim cover of clean native alluvial materials of one-foot minimum thickness, a compacted clay radon barrier of conditioned on-site weathered Mancos Shale with a thickness of 4-foot, and a 2.375 -foot thick frost protection layer, and a 0.83-foot thick rock/soil admixture erosion protection layer. DOE evaluated the side slopes solely for erosion protection; there is no RRM located under the side slopes of the cover system. DOE calculated the thickness of the required Mancos Shale radon barrier for the disposal cell is four feet for a one-foot thick interim cover. This thickness of the radon barrier is consistent with the 2008 RAP.

The design parameters of the contaminated and cover materials that were evaluated for acceptability include the following: long-term moisture content, radon diffusion, radon emanation, density, porosity, material layer thickness, average radium-226 activity, and ambient radon concentration. In its calculation, DOE used the on-line Uranium Mill Tailings Cover Calculator (http://www.wise-uranium.org/ctc.html), which the NRC staff understands is a clone of the Radon Attenuation Effectiveness and Cover Optimization with Moisture Effect (RAECOM) code (Rogers 1984a, Rogers 1984b).

6.2.1 Evaluation of Parameters

20 The required thickness of the barrier depends on the properties of the radon barrier and the contaminated materials. NRC staff reviewed the validity of the physical and radiological parameters of the contaminated materials and the cover system used for input in the on-line calculator. In its review, the NRC staff observed that the values used for the moisture content, porosity, and specific gravity for the interim cover, Mancos Shale radon barrier, and frost protection layer are consistent with the 2008 design. As discussed in the staffs 2008 TER, the properties for the various layers were determined from laboratory testing of representative samples. During its review of Revision 3 to the RAP, the NRC staff did not identify any new information that would warrant using updated material properties for these parameters. While the function of the ET cover will be different from the previously approved resistive cover, the values that DOE used are conservative in nature and reflective of the anticipated long-term conditions. An example is with the use of a value of 12 percent for the long-term moisture content of the Mancos Shale radon barrier.

The NRC staff did identify differences in the radon diffusion coefficient used for the tailings, interim cover, and Mancos Shale radon barrier. The on-line Uranium Mill Tailings Cover Calculator that DOE used in Revision 3 to the RAP uses a slightly different methodology for determining the radon diffusion coefficient value. A comparison of the values used in the 2008 RAP and the 2024 Revision 3 to the RAP is provided in the table below.

Soil Layer Radon diffusion coefficient value used in 2008 RAP (cm2/s)

Radon diffusion coefficient value used in 2024 RAP, Revision 3 (cm2/s)

Tailings 0.01037 0.0121 Interim Cover and frost protection 0.01636 0.0162 Mancos Shale radon barrier 0.004636 0.002 The NRC staff reviewed these values and the methodology used to determine them. DOEs approach is acceptable to the NRC staff as the values are consistent with what was used in the 2008 RAP.

With respect to the radium-226 radioactivity levels in the soil layers, DOE used the values in the table below in the 2008 RAP and in Revision 3 to the RAP. Additionally, DOE also calculated the radon flux based on the actual, in place Ra-226 concentration based on sampling that has occurred during placement of RRM.

Soil Layer Ra-226 concentration used in 2008 RAP (pCi/g)

Ra-226 concentration used in Revision 3 to the RAP, (pCi/g)

Tailings 707 707 (Sensitivity Analysis 1) 455.5 (Sensitivity Analysis 2)

Interim Cover and frost protection 1.9 1.9 Mancos Shale radon barrier 2.3 2.3 DOE used a 500 cm tailings thickness for its calculation of radon flux. This is consistent with the approach used in the 2008 RAP. The NRC staff observes that Regulatory Guide 3.64 (NRC,

21 1989) indicates that a 500 cm tailings thickness an infinitely thick source for radon. Therefore, DOEs approach is acceptable to the NRC staff.

6.2.2 Evaluation of Radon Attenuation Model As discussed above in this section, DOE used the on-line Uranium Mill Tailings Cover Calculator (http://www.wise-uranium.org/ctc.html), to determine the required radon barrier thickness for the Moab UMTRA Project disposal cell. DOE performed two different sensitivity analyses based on the different Ra-226 concentration in the upper portion of the RRM. DOEs analyses resulted in a required radon barrier thickness of four feet using the parameters discussed above; this is consistent with the radon barrier thickness in the 2008 RAP.

The NRC staff reviewed DOEs calculations for the radon barrier thickness in Revision 3 to the RAP. The staffs review included evaluation of the input parameters DOE used that were different from the 2008 RAP - the radon diffusion coefficient, the Ra-226 concentration for sensitivity analysis accounting for the actual Ra-226 activity in the placed RRM, and the differences in soil thickness above the radon barrier. DOEs parameters reflect the known conditions at the Crescent Junction site and accurately incorporate the modifications to the final cover system that arise from the use of an ET cover. Based on its review, the NRC agrees that the proposed radon barrier thickness of four feet is adequate to meet the EPA radon flux standard.

6.2.3 Conclusions The NRC staff, having reviewed the parameters of the contaminated materials discussed above, concluded that the physical and radiological properties are representative of the configuration proposed with the use of an ET cover system. Additionally, the NRC staff determined that an acceptable methodology was used to determine the Ra-226 activity and emanation fraction of the contaminated materials at the site. NRC reviewed DOEs radon flux calculation and verified that the radon barrier thickness of 4 feet was sufficient to meet the EPA radon flux standard in 40 CFR Part 192.02(b)(1). DOEs calculated value of 2.1 pCi/m2s leaving the Mancos Shale radon barrier is less than the required value of 20 pCi/m2s in the regulation.

22

7.0 REFERENCES

40 CFR Part 192, U.S. EPA, Health and Environmental Protection Standards for Uranium and Thorium Mill Tailings, Code of Federal Regulations.

Chow, V.T., Open Channel Hydraulics, McGraw-Hill Book Co., New York, 1959.

Day, R.W., Geotechnical and Foundation Engineering, Design, and Construction, McGraw-Hill Book Co., New York, 1999.

DOE (U.S. Department of Energy), 2008, Final Remedial Action Plan and Site Design for Stabilization of Moab Title I Uranium Mill Tailings at the Crescent Junction, Utah, Disposal Site, ADAMS Accession Numbers ML080920250, ML080920251, ML080920252 and ML082180244).

DOE (U.S. Department of Energy), 2024a, Press Release entitled Moab UMTRA Project Safely Ships 15 Million Tons to Disposal Site accessed May 27, 2025 (https://www.energy.gov/em/moab/articles/moab-umtra-project-ships-15-million-tons-disposal-site)

DOE (U.S. Department of Energy), 2024b, Final Remedial Action Plan and Site Design for Stabilization of Moab Title I Uranium Mill Tailings at the Crescent Junction, Utah, Disposal Site, Revision 3. ADAMS Accession Numbers: ML24183A373, ML24183A374, ML24183A375, ML24183A376, ML24183A377, ML24183A378, ML24183A379, ML24183A380, and ML24183A381.

NRC (U.S. Nuclear Regulatory Commission), 1989, Calculation of Radon Flux Attenuation by Earthen Mill Tailing Covers, Regulatory Guide 3.64, June.

NRC (U.S. Nuclear Regulatory Commission), 1993, Final Standard Review Plan for the Review and Remedial Action of Inactive Mill Tailings Sites under Title I of the Uranium Mill Tailings Radiation Control Act, Revision 1. Accession Number ML110190562.

NRC (U.S. Nuclear Regulatory Commission), 2002, Design of Erosion Protection Covers for Long-Term Stabilization, NUREG-1623, NRC (U.S. Nuclear Regulatory Commission), 2008a, Concurrence in the U.S. Department of Energy's Proposed Remedial Action for the Moab, Utah Mill Tailings Site, ADAMS Accession Number ML082050189.

NRC (U.S. Nuclear Regulatory Commission), 2008b, Final Technical Evaluation Report for the Proposed Remedial Action of the Moab, Utah Uranium Mill Tailings Site, ADAMS Accession Number ML082060216.

Rogers and Associates Engineering, 1984a, Radon Attenuation Effectiveness and Cover Optimization with Moisture Effects (RAECOM), computer program, U.S. NRC, Washington, D.C.

Rogers and Associates Engineering, 1984b, Radon Attenuation Handbook for Uranium Mill Tailings Cover System, NUREG/CR-3533, U.S. NRC, Washington, D.C.