ML25112A101

Enclosure 4 - Final Safety Evaluation Report for CoC 1031 Amendment 15

ML25112A101
Person / Time
Site:	07201031
Issue date:	04/25/2025
From:	Storage and Transportation Licensing Branch
To:	NAC International
Shared Package
ML25112A096	List: ML25112A097 ML25112A098 ML25112A099 ML25112A100 ML25112A101
References
CAC 001028, EPID L-2023-LLA-0143
Download: ML25112A101 (1)

Text

Enclosure 4 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 FINAL SAFETY EVALUATION REPORT DOCKET NO. 72-1031 NAC INTERNATIONAL, INC.

MAGNASTOR STORAGE SYSTEM CERTIFICATE OF COMPLIANCE NO. 1031 AMENDMENT NO. 15 Summary This safety evaluation report (SER) documents the U.S. Nuclear Regulatory Commission (NRC) staffs review and evaluation of the request to amend Certificate of Compliance (CoC) No. 1031, NAC International, Incs (NAC, or the applicant) MAGNASTOR storage system. By letter dated August 29, 2023 (Agencywide Documents Access and Management System Accession No. ML23241B053), as supplemented on October 26, 2023 (ML23300A138), October 8, 2024 (ML24283A084 and ML24284A267), and December 9, 2024 (ML24344A171), NAC submitted an application in accordance with Title 10 of the Code of Federal Regulations (10 CFR) Part 72 to amend CoC No. 1031 for the MAGNASTOR storage system to make the following changes to the MAGNASTOR storage system:

Add a new variation of the Lightweight MAGNASTOR Transfer Cask (LMTC), Reduced Width LMTC.

Add a new concrete cask design known as CC8. CC8 is based on the CC7 configuration and uses high-density concrete for enhanced shielding.

Increase the maximum system heat load capacity.

Add new pressurized-water reactor (PWR) loading patterns L, M, and N, (pattern N is a short loading pattern).

Add a Thermal Shunt to allow for short loading patterns.

Add new boiling-water reactor (BWR) loading patterns E and F.

Remove 5 percent burnup "penalty."

Increase Passive MAGNASTOR Transfer Cask (PMTC) heat load (30kW to 35.5kW, including preferential loading pattern B).

Revise previously approved drawings for the concrete cask for CC8, PMTC, and LMTC.

Revise Technical Specification Appendix A to include increased heat loads and loading patterns.

Revise Technical Specification Appendix B to include increased heat loads and loading patterns and remove cool-time tables B2-13 through B2-43.

Add two new PWR fuel types to support future site operations, resulting in revisions to structural, thermal, shielding and criticality chapters of the final safety analysis report (FSAR).

Modify the transportable storage canister (TSC) lid to allow additional clearance near the top center of the basket.

Revise license drawings 71160-584, -585, -684 and -685.

Correct and clarify principal design criteria, operating procedures, and the acceptance criteria and maintenance program.

2 The amended CoC, when codified through rulemaking, will be denoted as Amendment 15 to CoC No. 1031. This SER documents the staffs review and evaluation of the proposed amendment. The NRC staff reviewed the amendment request using guidance in NUREG-2215, Standard Review Plan for Spent Fuel Dry Storage Systems and Facilities - Final Report, dated April 2020 (ML20121A190), and the chapter numbering and subjects of this SER coincide with the corresponding numbering and subjects of the chapters in NUREG-2215.

For the reasons stated below and based on the statements and representations in NACs application, as supplemented, and the conditions specified in the CoC and the technical specifications (TS), the staff concludes that the requested changes meet the requirements of 10 CFR Part 72, Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste.

Chapter 1 GENERAL INFORMATION EVALUATION The objective of the review of this chapter is to evaluate design changes made to the MAGNASTOR storage system to ensure that NAC provided a description that is adequate to familiarize reviewers and other interested parties with the pertinent features of the system, including the requested changes.

1.1 General Description and Operational Features Section 1.3.1 of the FSAR, revision 14, dated November 1, 2023 (ML23307A134), identifies and describes the major components of the MAGNASTOR spent fuel dry storage system. In the Amendment 15 application, the applicant proposed to revise FSAR section 1, General Description, of the FSAR.

1.1.1 Concrete Cask FSAR section 1.3.1.3 describes the concrete cask. The applicant revised the FSAR to identify a new design variation of the concrete cask, identified as CC8. The previously approved concrete casks are identified as CC1 through CC7. As detailed in FSAR table 1.3-1, Design Characteristics, and the license drawings in FSAR section 1.8, all concrete cask configurations share the same basic layout and features with many major component details. The FSAR identifies key features of the CC8 cask variation, including the dimensions, the heat shield, the lid assembly with integrated air outlets, liner thickness, inlet shield bars, increased lid thickness, and increased concrete density.

1.1.2 Transfer Cask FSAR section 1.3.1.5 describes the transfer cask, which is designed, fabricated, and tested to meet the requirements of American National Standards Institute (ANSI) N14.6 as a special lifting device. The transfer cask provides biological shielding and structural protection for a loaded TSC and is used to lift and move the TSC between workstations. The transfer cask is also used to shield the vertical transfer of a TSC into a storage cask or a transport cask. The transfer cask is available in multiple configurations, including the standard MAGNASTOR Transfer Cask (MTC), the PMTC, and the LMTC.

This amendment identifies a new variation of the LMTC, referred to as the Reduced Width LMTC. The Reduced Width LMTC is intended for use at facilities with limited crane capacity and for TSCs with high heat loads. The LMTC includes a demineralized water-filled neutron shield tank that can be drained for pool loading operations to reduce the hook wet weight, then refilled

3 to restore neutron shielding prior to performing canister draining, drying, and closure operations.

The LTMC structural components are all fabricated from stainless steel. The Reduced Width LMTC is a configuration that is designed to satisfy site-specific requirements for crane capacity and cask loading pit size limits. It is like the standard LMTC in nearly all respects except that its gamma shield thickness is set to 2.5 inches, its lifting trunnions are rotated 45 degrees from the door rail axis, and flats are added to the neutron shield to maintain an 86.5-inch width on the short axis. The principal dimensions and materials of fabrication of the transfer cask appear in FSAR table 1.3-1.

1.1.2 Thermal Shunt Although not one of the major components of the MAGNASTOR storage system, the thermal shunt is a new component that the applicant added to the system in this amendment and is described in FSAR section 1.3.1. The thermal shunt is a specially designed stainless-steel weldment designed to occupy specific storage locations in a fuel basket for certain short-loaded preferential loading patterns. Thermal shunts prevent fuel assemblies from being inadvertently loaded into storage locations that are not intended for fuel. In addition, thermal shunts provide a heat transfer function. The applicant provided a diagram and specifications of the thermal shunt in FSAR drawing no. 71160-L378 (proprietary).

1.2 Drawings In FSAR section 1.8, License Drawings, the applicant provided revised versions of those drawings affected by the proposed amendment, including both public (non-proprietary, NP) and nonpublic (proprietary, P) versions, listed below.

71160-656, Cask Body Weldment, Passive Transfer Cask, MAGNASTOR, revision 3NP 71160-656, Cask Body Weldment, Passive Transfer Cask, MAGNASTOR, revision 3P 71160-L257, Cask Assembly, Lightweight MAGNASTOR Transfer Cask (LMTC),

revision 1P 71160-L258, Cask Body Weldment, Lightweight MAGNASTOR Transfer Cask (LMTC),

revision 1P 71160-L362, Reinforcing Bar and Concrete Placement, Concrete Cask MAGNASTOR, revision 1P 71160-L363, Lift Lug and Details, Concrete Cask, MAGNASTOR, revision 1P 71160-L364, Upper Segment Assembly, Concrete Cask, MAGNASTOR, revision 1P 71160-L378, Thermal Shunt Weldment, MAGNASTOR, revision 2P 71160-L385, TSC Assembly, BWR, MAGNASTOR, revision 1P 71160-584, DETAILS, PWR TSC, MAGNASTOR, revision 12P 71160-584, DETAILS, PWR TSC, MAGNASTOR, revision 12NP 71160-585, TSC Assembly, MAGNASTOR, revision 16 71160-685, DF, TSC Assembly, MAGNASTOR, revision 11P 71160-685, DF, TSC Assembly, MAGNASTOR, revision 11NP 71160-684, DETAILS, DF CLOSURE LID, MAGNASTOR, revision 5P 71160-684, DETAILS, DF CLOSURE LID, MAGNASTOR, revision 5NP 1.3 Material to be Stored As described in section 1.2 of the FSAR, revision 14, the MAGNASTOR storage system is designed to safely store up to 37 undamaged PWR fuel assemblies or up to 89 undamaged BWR fuel assemblies. The storage capacity can be configured to include a limited number of

4 damaged PWR or BWR fuel assemblies in damaged fuel cans. PWR assemblies are stored in either standard or damaged fuel basket each having a 37-assembly maximum capacity. The FSAR also describes allowable contents involving fuel debris and non-fuel hardware.

In the Amendment 15 application, the applicant proposed to revise the FSAR and TS to include two new fuel assemblies, the Westinghouse CE16-NGF and Framatome CE16-HTP (referred to in the FSAR as CE16H2). These changes did not affect the high-level description of allowable contents in FSAR section 1.2.

1.4 Evaluation Findings

Based on the NRC staff's review of information provided by the applicant for Amendment 15 to the MAGNASTOR system, the staff determined the following:

F1.1 A general description and discussion of Amendment 15 to MAGNASTOR system is presented in chapter 1 of the associated FSAR, with special attention to design and operating characteristics, unusual or novel design features, and principal safety considerations, and the description is sufficient to familiarize a reviewer or stakeholder with the design.

F1.2 Drawings for structures, systems, and components (SSCs) important to safety presented in section 1.8 of the FSAR were reviewed. Details of specific SSCs are evaluated in chapters 4 through 8 and 10 of this SER.

F1.3 The specifications for the material to be stored in the MAGNASTOR cask are sufficient to familiarize a reviewer or stakeholder with the contents to be stored. Additional details concerning these specifications are presented in FSAR section 2.2.

Chapter 2 SITE CHARACTERISTICS EVALUATION FOR DRY STORAGE FACILITIES Analysis of this topic was not included in this evaluation since it is only applicable to a specific license application.

Chapter 3 PRINCIPAL DESIGN CRITERIA EVALUATION The changes associated with principal design criteria are discussed and evaluated in chapters 4, 5, 6, 7, 8, and 10 of this SER.

Chapter 4 STRUCTURAL EVALUATION This section evaluates the structural design in Amendment 15 of the MAGNASTOR FSAR for the structural components of the MAGNASTOR storage system. The structural design features and design criteria are reviewed together with an evaluation of the analyses performed to demonstrate the structural acceptability of these changes in the storage system under normal, off-normal, accident, and natural phenomena events. The staff reviews the applicants analysis of each modification and associated information.

The applicants August 29, 2023, amendment proposed to modify the MAGNASTOR system with the following changes relevant to the structural evaluation:

1.

Add an eighth concrete cask variant (CC8) using highdensity concrete (208 pounds per cubic foot). Section 4.1.1 of this SER summarizes the staff review of structural design

5 features of the CC8. Section 4.1.3 of this SER summarizes the staff review of weight and centers of gravity of the CC8. Section 4.2.4 of this SER summarizes the staff review of structural evaluation of the CC8.

2.

Add Reduced Width LMTC. Section 4.2.3 of this SER summarizes the staff review of structural evaluation of the LMTC.

3.

Evaluate the TSC, concrete cask, and LMTC for an improved high heat load, which is defined as a heat load over 35.5 kW (up to 53 kW) for the PWR configurations and a heat load over 33 kW (up to 46 kW) for the BWR configurations. Evaluate the PMTC for an increased PMTC heat load. Section 4.1.4.2 of this SER summarizes the staff review of the impacts of the heat load changes on the lifting device. Impacts on other components are summarized throughout the structural review of section 4.0 of this SER.

The applicants October 26, 2023, supplement of Amendment 15 proposed the following additional changes:

4.

Add two new 16x16 fuel assembly types including the Westinghouse CE NGF and Framatome HTP 16x16 fuel assemblies. Section 4.2.5 of this SER summarizes the staff review of structural evaluation of the two new fuel assembly types.

5. Add a lid recess to the TSC closure lid. Section 4.2.6 of this SER summarizes the staff review of the structural evaluation of the added lid recess.

4.1 Structural Design Features and Design Criteria 4.1.1 Structural Design Features The FSAR provides a general description of the MAGNASTOR system, which includes three principal components: (1) the TSC; (2) the concrete cask (CC); and (3) the transfer cask. The concrete cask and transfer cask are also designed in two different lengths. The long storage cask and transfer cask configurations are designed to accommodate the two TSC lengths, and the short concrete storage cask and transfer cask configurations are designed to accommodate only the short TSCs. Based on the TSC length and arrangement of the fuel basket tube array, the system is configured to store up to 37 PWR or 89 BWR fuel assemblies.

The concrete cask is provided in various configurations designated CC1 through CC8. All concrete casks are designed with a carbon steel shell around the internal cavity. The CC3/CC5/CC7/CC8 configurations include a heat shield option to reduce the temperature of the concrete.

For the CC1 through CC5 configurations, the top end of the concrete cask is closed by a lid assembly composed of a carbon steel top plate and a cylindrical concrete plug that is encased in a carbon steel plate. For the CC6 through CC8 configurations, the top end of the concrete cask is enclosed by an upper segment which is comprised of a 1-inch thick 136.0-inch diameter top plate and a 5.5-inch thick 110-inch diameter concrete disc. For CC7/CC8, the upper segment is also available in an enhanced shielding version which adds an additional 3.5-inch of concrete thickness above the 1-inch-thick steel top plate.

6 The transfer cask is provided in three designs: standard MTC, PMTC, and LMTC. The standard MTC is fabricated from high-strength carbon steel (MTC1) or a shortened stainless-steel version (MTC2). Both the PMTC and LMTC are made of stainless steel. The Reduced Width LMTC is a modified version of the LMTC with a 2-inch shorter trunnion, a 31-inch longer expansion tank, and two parallel flat surfaces compared to the LMTC.

4.1.2 Structural Design Criteria Chapter 2 of the FSAR presents the structural design criteria for the MAGNASTOR system.

The criteria define, in general, the applicable codes and standards, individual loads as related to environmental conditions and natural phenomenon events, load combinations, and stress allowable, for normal, off-normal, and accident-level conditions. The applicant did not identify any revisions to the structural design criteria including codes, standards, and material specifications, nor proposed any deviations in the use of regulatory guidance used in the previously approved MAGNASTOR system. The staff did not perform any additional reviews of these for this amendment and considered the design criteria to remain consistent with the guidance provided in NUREG-2215.

4.1.3 Weights and Centers of Gravity FSAR tables 3.2.1-1 and 3.2.1-2 list the weights of individual components and relevant centers of gravity for the MAGNASTOR system under various operating configurations, including the new CC8 concrete cask configuration introduced in Amendment 15. The weights of the loaded concrete casks and corresponding centers of gravity provide the basis for selecting a lower or upper bounding configuration with the least resistance to tip-over for the cask seismic stability evaluation and the lifting lug design.

4.1.4 General Standards for Cask Design The design features of the MAGNASTOR system for meeting general cask design standards, including maintaining positive closure and allowing safe lift operations, are reviewed as follows.

4.1.4.1 Positive Closure The applicant did not identify any revisions to the positive closure design used in the previously approved MAGNASTOR system. The staff did not perform any additional reviews of these for this amendment.

4.1.4.2 Lifting Devices Analysis FSAR section 3.4.3 evaluates the various lifting devices and components of the MAGNASTOR system. The concrete cask is lifted by embedded lift anchor and lug assemblies or lift lug assemblies bolted to the top of the cask liner weldment. The loaded and closed TSC is lifted through the hoist ring threaded into the closure lid at three lift points. The transfer cask is lifted with two trunnions. The applicant evaluated these lifting devices for a lower yield and ultimate stress of the material due to the increased temperatures associated with improved high heat load or increased PMTC heat load in structural change 3.

7 The applicant did not identify any revisions for the fuel basket lifting design because the fuel baskets are evaluated for a higher temperature than experienced with the improved high heat load. The staff did not perform any additional reviews of these for this amendment.

4.1.4.2.1 Concrete Cask Lift The concrete cask configurations CC1 through CC5 are lifted using lift lugs through embedded lift anchors. The temperature increase is different at various locations on the concrete cask; therefore, the applicants evaluations are based on the reduced material stress at the temperature at the location where the structure component is located.

For lifting devices for CC1 through CC5, the applicant evaluated the lift lugs using a conservative weight of 340,000 pounds (lb.) with an added 10 percent dynamic load factor (DLF) (374,000 lb.) for a loaded concrete cask. The applicant checked the lugs for the failure modes in bearing, shear-tear-out, or hoop tension. The applicants factors of safety (FS) satisfy the criteria of a minimum FS of 5 on ultimate strength and 3 on yield strength. The applicant used same FS criteria for the lift anchor check. The applicant checked the concrete anchor portion for the lift anchor for the applicable concrete shear failure mode.

The CC6 through CC8 configurations are lifted using two lift lug assemblies bolted to the top of the cask liner weldment. The applicant chose CC8 as a representative case for the evaluation of the lifting device due to its heavier weight from highdensity concrete and provided an evaluation in FSAR section 3.14.3. The applicant used the same design criteria for checking CC1 through CC5 as those used for CC6 through CC8.

4.1.4.2.2 TSC Lift The applicant checked the hoist ring and sling for a 120,000 lb. weight with an added 10 percent DLF (132,000 lb.) for a loaded TSC. The hoist rings are rated at 50,000 lb. with a FS of five on ultimate strength. Using this 50,000 lb. as input, the applicant derived the minimum sling length. The applicant checked the shear force for the hoist ring hole threads in the closure lid based on a 2-inch engagement length for ultimate strength and yield strength with FS values of 5 and 3, respectively. The applicant used a boundary case for developing the finite element model for the TSC closure lid assembly and closure weld. The applicant checked the ultimate strength and yield strength using FS values of 5 and 3, respectively.

4.1.4.2.3 Transfer Cask Lift The transfer cask lifting trunnions are designed for an FS of 6 on material yield strength and an FS of 10 on material ultimate strength. The applicant used a finite element analysis (FEA) for the fully loaded transfer cask to calculate the stress in the transfer cask forgings, shells, and the trunnion region for the operational vertical lift condition.

The applicant used a total weight of 240,000 lb. for the loaded transfer cask with an added DLF of 10 percent (264,000 lb.) as the input for the MTC lift analysis. The applicant extracted and checked the results at several key locations on the transfer cask. The results show that the minimum FS for yield strength is larger than 6 and the FS for ultimate strength is larger than 10.

For the PMTC, the applicant considered a bounding weight of 257,000 lb. with an added DLF of 10 percent (282,700 lb.) in the evaluation. The applicant used a quarter-symmetry finite

8 element model for the vertical lift evaluation of the PMTC. The shear, bending, and combined stress results are calculated and listed in FSAR section 3.4.3.3.3 as a function of the vertical distance from the centerline of the trunnion at 0.5-inch intervals. The applicant implemented similar checking steps for other components including the top forging, inner shell, outer shell, intermediate shell, bottom forging, shield doors, shield door rails, and shield door rail weld.

The results show that the minimum FS for yield strength is larger than 6 and the FS for ultimate strength is larger than 10 For the LMTC, the applicant considered a bounding weight of 216,000 lb. with an added DLF of 10 percent (238,000 lb.) in its evaluation. The applicant used a quarter-symmetry finite element model for the vertical lift evaluation of the LMTC. The checks performed by the applicant for the LMTC components are similar to those performed for the PMTC and use the same criteria. The results show that that the minimum FS for yield strength is larger than 6 and the FS for ultimate strength is larger than 10.

For the Reduced Width LMTC per structural change 2, the applicant considered a bounding weight of 216,000 lb. with an added DLF of 10 percent (238,000 lb.) in its evaluation. The applicant included a 2-inch shorter trunnion, a 31-inch longer expansion tank, and 2 parallel flat surfaces in the quarter-symmetry finite element model used for its vertical lift evaluation.

The applicant performed checks for the Reduced Width LMTC components similar to those performed for the LMTC, using the same criteria. The results show that that the minimum FS for yield strength is larger than 6 and the FS for ultimate strength is larger than 10.

For off-normal events of inadvertent lift of the transfer cask by the TSC, the applicant considered lifting the PMTC, LMTC, and Reduced Width LMTC by the TSC instead of transfer cask lifting trunnions. The stresses associated with this condition are set to satisfy allowable stress limits from American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), Service Level C condition, which requires the FS to be larger than 1.

The applicant checked the reduced allowable stress for the retaining ring due to the increased temperature at 225 degrees (°F) at the top of the cask for the PMTC, and at 200°F at the top of the cask for the LMTC and Reduced Width LMTC. The applicant checked the mounting bolt bearing, bolt tensile stress, and bolt thread shear stress against allowable values. The results show that all FS values are above 1.

4.1.4.2.4 Lifting Devices Evaluation Conclusion NUREG-2215 references NUREG-0612 and ANSI N14.6, which provide guidance for stress design factor of safety criteria for lifting devices for heavy loads. The evaluations by the applicant described above in sections 4.1.4.2.1 through 4.2.3.2.3 show that the lifting devices for different concrete cask, TSC, and transfer cask configurations of the package meet or exceed these criteria. Based on the above evaluations in sections 4.1.4.2.1 through 4.2.3.2.3, the staff concluded that the lifting devices for different components of the package are structurally adequate.

4.2 Structural Evaluation The applicant performed the structural evaluation for the TSC, fuel basket, and concrete cask for applicable normal, off-normal, and accident conditions with the impact of the improved high heat load in section 14.3 of the FSAR. Therefore, the staff conducted the review pertinent to the structural evaluation for the TSC, fuel basket, and concrete cask for applicable normal, off-normal, and accident conditions under the improved high heat load.

9 4.2.1 TSC Evaluation An evaluation of the TSCs with single-piece closure lid assembly (TSC1/TSC2/TSC5) for the effect of increased temperatures for the improved high heat load is performed in section 13.4.1 of the FSAR.

The internal pressures used for the TSC evaluation without improved high heat load in the FSAR for normal, off-normal, and accident conditions are 110 psi, 130 psi, and 250 psi, respectively. Considering the improved high heat load, the internal pressure is raised to 120 psi for the normal conditions, while the original design pressure for the off-normal (130 psi) and accident conditions (250 psi) is still the bounding design pressure. The stresses due to the improved high heat load are scaled for a factor of 1.09 (120 psi / 110 psi) for all thirteen evaluated sections on the TSC for normal conditions. These stresses due to internal pressure are further combined with the stresses due to other applicable normal, off-normal, and accident loading conditions to calculate the stress result for all applicable load combinations. The results are shown in tables 3.14-1 through 3.14-8 of the FSAR. The results show that all FS values are above 1.

The staff reviewed the applicants evaluation and based on the above discussion, concluded that the TSCs are structurally adequate for normal, off-normal, and accident conditions.

4.2.2 Fuel Basket Evaluation In FSAR section 3.14.2, the applicant stated that the temperature gradient of 440°F from the existing thermal analysis is bounding for the basket with the improved high heat for the PWR and PWR Damaged Fuel (DF) fuel basket. The applicant stated that, therefore, no further analysis is required. The applicant applied the same method to the BWR fuel basket with a 440°F temperature gradient from the existing thermal analysis and to the BWR DF fuel basket with a 425°F temperature gradient from the existing thermal analysis. The applicant stated that, therefore, no further analysis is performed. Because the bounding thermal analysis is already performed in FSAR section 3.13.2.1 and 3.13.2.2 for PWR fuel basket and BWR fuel basket respectively, the staff agrees with the applicants approach.

4.2.3 Reduced Width LMTC Evaluation In FSAR section 3.4.3.3.5, the applicant evaluated the Reduced Width LMTC outer shell due to hydrostatic load using a methodology similar to its evaluation of the LMTC. This hydrostatic load is generated by the pressured water between the outer shell and middle shell for neutron shielding. The resulting maximum primary membrane plus primary bending at the base is 10.4psi for the PMTC, 8.43psi for the LMTC, and 16.05psi for the Reduced Width LMTC. With the increased heat load and corresponding allowable stress, the applicant checked these demands against allowable values using the same criteria defined in the previously approved FSAR. The result shows that all FS values are above 1.

The staff reviewed the applicants evaluation and based on the above discussion, concluded that the structural design for the Reduced Width LMTC under improved high heat load is acceptable.

10 4.2.4 Concrete Cask Evaluation Concrete cask configurations CC3/CC5/CC7/CC8 with a heat shield are used for high heat load. In FSAR section 3.14.3, the applicant presented its structural evaluation, which used CC8 as an enveloping case for evaluation of the improved high heat load due to its high concrete density. The staff agrees with the applicants approach because the high density of CC8 leads to a larger weight compared to all other concrete cask configurations and further leads to more critical stress analysis results under loading conditions due to self-weight.

4.2.4.1 Concrete Cask CC8 Lift Evaluation The applicant performed the concrete cask lift evaluation of all components and connections along the lifting load for CC8. A bounding maximum loaded cask weight of 411 kips with an additional 10 percent DLF is considered for all lift evaluations. The applicant calculated the FS of the lift lug assembly for applicable bearing and tensile stress and checked them using the criteria of minimum FS of 5 on ultimate stress and 3 on yield stress. The same criteria are used for checking the lift ring weldment, lift pin, lift lug bolt, the inner weld and outer weld of the lift lug, and the upper segment lifting tabs. The applicant checked the bearing and shear stress of the concrete at its interface with the top surface of the lift ring against allowable concrete stresses using an FS of 1.

The staff reviewed the applicants evaluation and based on the above discussion, concluded that the structural design for the lifting components and connections under improved high heat load is acceptable.

4.2.4.2 Concrete Cask CC8 Normal Operating Conditions Evaluation The applicant modified the finite element model used for concrete cask analysis with higher density and improved high heat load. The applicant performed a thermal stress sensitivity study to determine that the thermal stress result for CC8 under improved high heat load is bounded by the existing thermal analysis which is shown in FSAR section 3.14.3.2.1. This stress is further combined with the stress due to the dead and live load. The resulting vertical stress on the outer surface, inner surface, and circumferential stress on the inner surface of the concrete cask are used to check the rebar design. For all design conditions, the applicant calculated a minimum FS value for concrete allowable strength of 1.22, for concrete ultimate strength of 1.30, and for rebar strength of 1.96.

The staff reviewed the evaluation and based on the above discussion concluded that the structural design for the concrete cask design during normal operating conditions under the improved high heat load is acceptable.

4.2.4.3 Concrete Cask CC8 Off-Normal Conditions Evaluation In FSAR section 3.14.3.3, the applicant noted that the thermal analysis for off-normal conditions under improved high heat load is bounded by thermal analysis for normal conditions and stated that no additional thermal stress evaluation for off-normal events (106°F ambient temperature) is required. Because the bounded analysis is already performed in FSAR section 3.5.3.1, the staff agrees with the applicants CC8 off-normal conditions evaluation.

11 4.2.4.4 Concrete Cask CC8 Accident Conditions Evaluation In FSAR section 3.14.3.4, the applicant summarized a bounding structural evaluation of the concrete CC8 with a heat shield for accident events. The applicant noted that the extreme temperature events (133°F ambient), tornado and tornado-driven missiles, flood, and tip-over accident conditions are all bounded by the existing analysis. Because the bounded analysis is already performed in FSAR section 3.7.3.1 for CC7 and CC7 has identical geometry as CC8, the staff agrees with the applicants CC8 evaluation for extreme temperature events, tornado and tornado-driven missiles, flood, and tip-over accident conditions.

The applicant analyzed the CC8 for an earthquake condition by applying a 0.5g acceleration in both vertical and horizontal directions. These accelerations are more conservative compared to the maximum design-basis earthquake acceleration of 0.37g in the horizontal direction (without cask sliding) and 0.25g in the vertical direction at the independent spent fuel storage installation (ISFSI) pad top surface which do not result in cask tip-over. The resulting combined concrete stresses are listed in table 3.14.3.4-1 and table 3.14.3.4-2 of the FSAR.

The reinforcement design of the CC8 provided a larger capacity compared to the stress demand listed in these two tables.

The applicant performed a concrete cask 24-inch drop analysis in section 3.14.3.4.6 of the FSAR. The result of 0.178-inch concrete crush depth shows that the concrete casks steel shell will not experience any significant damage during a 24-inch drop.

The applicants concrete cask tip-over study is summarized in section 3.14.3.4.7 of the FSAR.

The applicant performed a FEA using LS-DYNA simulation software for CC8. The result shows that the peak accelerations for the top of the basket and the TSC are determined to be 25.8g and 27.5g, respectively. These accelerations are bounded by the existing design value of 40g.

The staff reviewed the evaluation and based on the above discussion, concluded that the structural design for the concrete cask design during accidental conditions under improved high heat load is acceptable.

4.2.5 Fuel Rods Evaluation The applicant added two new fuel assembly types in the supplement of Amendment 15, including Westinghouse CE NGF and Framatome HTP 16x16 fuel assemblies. The applicant evaluated the fuel rod cladding stresses for these two fuel assemblies for cask tip-over and 24-inch end-drop events in proprietary calculation 71160-2054, MAGNASTOR Structural Evaluation of the CE16-NGF and CE16-HTP Fuel, (proprietary) in the October 26, 2023, supplement of Amendment 15. Stresses are compared against the cladding yield strength computed at a bounding maximum fuel temperature during dry storage. The evaluation considers two fuel support conditions including all grids intact along the fuel assembly and missing grids condition which give a fuel rod maximum unsupported span of 60 inches.

For tip-over analysis, two ANSYS static analyses use a bounding acceleration of 30g for the missing grid condition and 45g for all grids intact condition and conservatively applied uniformly along the full length of the fuel rod. These two accelerations are determined from the response spectrum data derived from the acceleration time histories resulting from a tip-over simulation using LSDYNA for CC8 and CC1 as representative cases. A single fuel rod is modeled using beam elements and supported at the grid support locations. The maximum

12 membrane plus bending cladding stress is summarized in table 8.1-2 of calculation 71160-2054. As shown in table 9.0-1 of this calculation, the minimum FS is 1.20 for M5-type cladding, which is larger than the FS of 1.08 for the existing tip-over analysis result.

For the 24-inch end-drop analysis, two LS-DYNA transient analyses are performed for all grids intact condition and missing grids condition. For missing grids condition, the rods maximum unsupported span of 60 inches is defined at the bottom of the rod to capture the worst condition. A row of 16 fuel rods and a bounding bow of 0.55 inches as an initial condition is considered. These rods are modeled with shell elements located at the cladding mean radius.

The LS-DYNA solution is solved for a time of 0.080 seconds to capture the response of the fuel rod to the drop impact. The maximum shear stresses computed by LS-DYNA in the fuel rod cladding are factored by two to give the maximum stress intensity, which is then compared to the cladding yield strength for determining the FS. The maximum stress is summarized in table 8.1-2 of calculation 71160-2054. The minimum FS is 3.02 for M5-type cladding which is larger than the FS of 1.77 for the existing end-drop analysis result.

The staff reviewed the evaluation and based on the above discussion, concluded that the structural design for the fuel rod design is acceptable.

4.2.6 TSC Lid Recess Evaluation The applicant added a 3-inch deep, 5-inch diameter round optional lid recess to the 9-inch thick 70.75-inch diameter TSC lid in the October 26, 2023, supplement to Amendment 15. The applicant determined that, because of its small size, adding the recess would have no significant impact on the resulting stresses in the structural analysis of the TSC lid, and no evaluation is required. The staff agree with the evaluation based on the geometry of the recess and TSC lid.

4.3 Evaluation Findings

Based on the review of the evaluations and representations contained in the application for Amendment 15 and the supplements to Amendment 15, the staff finds that the capabilities of the structural components of the MAGNASTOR storage system meet the regulatory requirements of 10 CFR Part 72.

Chapter 5 THERMAL EVALUATION The NRC staffs thermal review of Amendment 15 for the MAGNASTOR cask system ensures that the cask components and fuel material temperatures will remain within the allowable values under normal, off-normal, and accident conditions. This review includes confirmation that the fuel cladding temperatures for fuel assemblies stored in the MAGNASTOR cask system will be maintained below specified limits throughout the storage period to protect the cladding against degradation that could lead to gross ruptures. This portion of the review also confirms that the cask thermal design has been evaluated using acceptable analytical techniques and/or testing methods. This review was conducted under the regulations described in 10 CFR 72.236, which identify the specific requirements for the regulatory approval, fabrication, and operation of spent fuel storage cask designs. The unique characteristics of the spent fuel to be stored in the MAGNASTOR cask system are identified, as required by 10 CFR 72.236(a), so that the design-basis and the design criteria that must be provided for the SSCs important to safety can be assessed under the requirements of 10 CFR 72.236(b). This application was also reviewed to determine whether the MAGNASTOR design fulfills the acceptance criteria listed in

13 sections 2, 5, and 15 of NUREG-2215, Standard Review Plan for Spent Fuel Dry Storage Systems and Facilities.

The following changes under Amendment 15 to the MAGNASTOR cask system are applicable to the thermal evaluation:

1.

Addition of a new variation of the LMTC, Reduced Width LMTC.

2.

Addition of a new concrete cask design known as CC8.

3.

Increased the maximum system heat load capacity.

4.

Added new PWR loading patterns L, M, and N.

5.

Addition of a Thermal Shunt to allow for short loading patterns.

6.

Added new BWR loading patterns E, and F.

7.

PMTC heat load increase (30 kW to 35.5 kW including preferential loading pattern B).

5.1 MAGNASTOR System Thermal Model The applicant used ANSYS and ANSYS FLUENT computer-based analysis programs to evaluate the thermal performance of the MAGNASTOR spent fuel storage system. ANSYS is a FEA program with capabilities to predict heat transfer phenomena in two and three dimensions.

ANSYS FLUENT is a finite volume computational fluid dynamics (CFD) program with capabilities to predict fluid flow and heat transfer phenomena in two and three dimensions.

Chapter 1 of the FSAR provides a general description of the PMTC, the LMTC, and the CC8, and FSAR section 4.14.1 provides a general description of the FEA and CFD thermal models.

The modeling methodology for the concrete cask and TSC is the same as described in FSAR section 4.11.1. The model represents a quarter of the loaded cask, with the air inlets and air outlets at two symmetry planes modeled. To enhance the heat transfer performance of the loaded basket for pattern 37P-N (PWR configuration) with a maximum heat load of 53 kW, aluminum blocks (same length as the basket) are placed in the developed slots.

Three-dimensional quarter-symmetry transfer cask and TSC models are used to perform steady state and transient analyses for both PWR and BWR configurations. The CFD models of the storage cask are used to perform steady state analysis for the normal, off-normal and accident conditions. The CFD models of the transfer cask are used to perform steady state or transient analyses for the water-or helium-backfilled phases of the TSC. The FEA models of the transfer cask are used to perform transient analyses for the vacuum drying conditions of the transfer operation. The applicant updated the thermal models by including changes 1 through 7, described at the beginning of this chapter of this SER, and performed the thermal evaluation of the MAGNASTOR storage system for normal conditions of storage, transfer operations, and off-normal and accident conditions. The analysis results and the NRC staffs evaluation are described in sections 5.2 and 5.3.

The NRC staff reviewed the applicants description of the MAGNASTOR storage system thermal model. Based on the information provided in the application regarding the thermal model, the staff determined that the application is consistent with guidance provided in

14 NUREG-2215, section 5.4.4, Analytical Methods, Models, and Calculations, which states that the applicant should present a thermal analysis clearly demonstrating the storage systems ability to manage specified heat loads and have the various materials and components remain within temperature limits. The applicant provided a detailed description of the thermal models used to perform the evaluation of the storage cask and the results of the model, as confirmed by the NRC staff. The applicant demonstrated the storage systems ability to manage specified heat loads and have the materials and components remain within temperature limits. Therefore, based on the staffs review of the applicants descriptions and evaluations, the staff concludes that the description of the thermal model is acceptable, as the description is consistent with NUREG-2215 and satisfies the requirements of 10 CFR 72.236(b), 10 CFR 72.236(f), 10 CFR 72.236(g), and 10 CFR 72.236(h).

5.2 Thermal Evaluation for Normal Conditions of Storage and Transfer Operations The applicant used the thermal models described in section 4.14.1 of the FSAR to determine temperature distributions under long-term normal storage and transfer conditions. Preferential loading patterns 37P-L, 37P-M, and 37P-N shown in FSAR figures 4.14-1, 4.14-2, and 4.14-3, respectively, are considered for the analyses for storage conditions in the FSAR for PWR fuel type. All predicted temperatures (including the maximum fuel cladding temperature) remain below the allowable limits provided in table 4.14.2.1-1 of the FSAR. For BWR fuel type, preferential loading patterns 89B-E, 89B-F, 81B-D, and 81B-E shown in FSAR figures 4.14-4, 4.14-5, 4.14-6, and 4.14-7, respectively, are considered for the analyses for storage conditions in the FSAR. All predicted temperatures (including the maximum fuel cladding temperature) remain below the allowable limits provided in tables 4.14.2.1-2 and 4.14.2.1-3 of the FSAR. The NRC staff examined predicted temperatures and conducted audits of key thermal analysis models. The staff verified that all component temperatures remain below the allowable limits described in the FSAR for normal conditions of storage and transfer operations.

The maximum average helium temperature in the TSC for all heat load patterns is bounded by the maximum average helium temperature calculated for PWR 37P-N heat load pattern, for internal pressure calculation. The applicant calculated maximum normal condition pressure for the TSC using the methods described in FSAR section 4.4.4, based on the average helium temperature obtained from the thermal analysis of the 37P-N heat load pattern. As reported in FSAR section 4.14.2.3, the calculated pressure is less than the internal pressure provided in FSAR section 3.14.1 for the improved high heat load configurations.

Using the thermal models described in section 5.1 of this SER, the applicant calculated maximum component temperatures during transfer operations. Transfer operations include the following phases: water phase, vacuum drying phase, cooling/helium phase, and transfer phase. FSAR tables 4.14.2.2-1 through 4.14.2.2-18 provide predicted maximum temperatures and duration for vacuum drying phase and cooling phases. All predicted temperatures remain below the allowable limits provided in the FSAR for the different phases during transfer operations.

The NRC staff reviewed the applicants thermal evaluation of the MAGNASTOR storage system during normal conditions of storage and transfer operations for the added PWR and BWR heat load patterns. Based on the information provided in the application regarding the thermal models and evaluation, the staff determined that the application is consistent with guidance provided in NUREG-2215, section 5.4.4, which states that the applicant should present a thermal analysis that clearly demonstrates the storage systems ability to manage FSAR specified heat loads and have the various materials and components remain within

15 temperature limits. The staff found that the applicant has provided a thermal evaluation showing that calculated maximum temperatures remain below the recommended limits described in the application and, therefore, meets the requirements of 10 CFR 72.236(f).

5.3 Off-Normal and Accident Events 5.3.1 Off-Normal Events The applicant evaluated the following off-normal storage events: severe ambient temperature and partial blockage of air inlets conditions. The off-normal event for an increase in the ambient temperature only requires a change to the boundary condition temperature. For the partial blockage of air inlets condition, the air inlet condition is modified to permit air flow through half of the inlet area. The applicant used the preferential loading pattern used for normal conditions of storage to perform these analyses since it is the bounding heat load pattern. The temperatures of cask components for off-normal storage conditions are provided in FSAR section 4.14.3.1.

The NRC staff examined predicted temperatures and conducted audits of key thermal analysis models. The staff verified that all component temperatures remain below the allowable limits described in the FSAR for off-normal conditions.

5.3.2 Accident Events The applicant evaluated the following storage accident events: maximum anticipated ambient temperature, fire, and full blockage of air inlet vents. The applicant performed steady state analysis using the thermal model described in FSAR section 4.14.1 but modified the thermal model accordingly to reflect the different event conditions. The temperatures of cask components for accident conditions during storage are provided in FSAR section 4.14.4.1. The staff examined predicted temperatures and conducted audits of key thermal analysis models.

The staff verified that all component temperatures remain below the allowable limits described in the FSAR for accident conditions.

The NRC staff reviewed the applicants thermal evaluation during off-normal and accident events. Based on the information provided in the application regarding the applicants thermal evaluation, the staff determined that the application is consistent with guidance provided in NUREG-2215, section 5.4.4, that states that the applicant should present a thermal analysis that clearly demonstrates the storage systems ability to manage specified heat loads and have the various materials and components remain within temperature limits. The applicant demonstrated this ability by performing the calculations and demonstrating that the analysis results for the maximum cladding temperature are lower than the recommended limit of 1058°F.

Therefore, the staff concludes that the thermal evaluation during off-normal and accident events is acceptable because the thermal evaluation is consistent with NUREG-2215, and therefore, meets the requirements of 10 CFR 72.236(f).

5.4 Confirmatory Analyses The NRC staff reviewed the applicants thermal models used in the analyses. The staff checked the code input in the calculation packages and confirmed the material properties and boundary conditions that were used. The staff verified that the applicants selected code models and assumptions were adequate for the flow and heat transfer characteristics prevailing in the MAGNASTOR cask system geometry and analyzed conditions. The engineering drawings were also reviewed to verify that geometry dimensions were representative in the analysis models. The material properties presented in the FSAR were reviewed to verify that they were appropriately referenced and used.

16 The staff performed audit reviews of bounding configurations to verify that key thermal design parameters have been appropriately determined and correctly expressed as inputs into the thermal analysis, performed checks of energy and mass balances, and verified that the applicants thermal analyses have adequately converged. In addition, the staff performed several sensitivity analyses by varying key assumptions to verify that the applicant had correctly characterized the heat transfer and flow type used in the analysis. The staff determined that the FSAR reported maximum temperatures are below the allowable temperature limits, but the reported margin was very small. Staffs sensitivity analysis results shows that some of the solution parameters used by the applicant are conservative and, based on the sensitivity analysis results, the staff determined that additional margin exists in the reported FSAR values.

Finally, through a request for additional information (see ML24226B217, dated September 6, 2024, and ML24283A084, dated October 8, 2024), the staff verified that the applicant provided complete and accurate information in the FSAR. Therefore, the staff made a safety determination on the adequacy of MAGNASTOR cask system thermal design.

5.5 Evaluation Findings

F5.1 FSAR chapter 2 describes SSCs important to safety to enable an evaluation of their thermal effectiveness. Cask SSCs important to safety remain within their operating temperature ranges.

F5.2 The MAGNASTOR storage system is designed with a heat removal capability having verifiability and reliability consistent with its importance to safety. The cask system (TSC, transfer cask and concrete overpack) is designed to provide adequate heat removal capacity without active cooling systems.

F5.3 The spent fuel cladding is protected against degradation leading to gross ruptures under long-term storage by maintaining cladding temperatures below 752°F (400 degrees Celsius [°C]). Protection of the cladding against degradation is expected to allow ready retrieval of spent fuel for future processing or disposal.

F5.4 The spent fuel cladding is protected against degradation leading to gross ruptures under off-normal and accident conditions by maintaining cladding temperatures below 1058°F (570°C). Protection of the cladding against degradation is expected to allow ready retrieval of spent fuel for future processing or disposal.

F5.5 The staff finds that the thermal design of the MAGNASTOR storage system complies with the design requirements in 10 CFR 72.236 and that the applicable design and acceptance criteria have been satisfied. The evaluation of the thermal design provides reasonable assurance that the cask will allow for safe storage of spent nuclear fuel. This finding is reached based on a review that considered the NRC regulations, appropriate regulatory guidance, applicable codes and standards, and accepted engineering practices.

Chapter 6 SHIELDING EVALUATION The staffs shielding review evaluates the ability of the proposed shielding features in MAGNASTOR Amendment 15 to provide adequate protection against direct radiation from the

17 storage system contents. The shielding features must limit the dose to the operating staff and members of the public so that the dose remains within regulatory requirements during normal operating, off-normal, and design-basis accident conditions. The review seeks to ensure that the shielding design is sufficient and reasonably capable of meeting the operational dose requirements of 10 CFR 72.104 and 72.106 in accordance with 10 CFR 72.236(d). The staffs shielding review evaluates the proposed changes requested in this revision in conjunction with the findings from previous staff analyses to determine whether, with the requested changes, the system continues to provide adequate protection from the radioactive contents within the system.

The staff reviewed the applicants safety analyses for the requested changes to the CoC following the guidance provided in NUREG-2215, Standard Review Plan for Dry Cask Storage Systems. This SER documents the staffs evaluation of the following proposed changes related to shielding design:

1.

The addition of a new variation of the LMTC, Reduced Width LMTC.

2.

The addition of a new concrete cask design known as CC8. CC8 is based on the CC7 cask configuration and uses high density concrete for enhanced shielding.

3.

The addition of new PWR loading patterns L, M, and N (pattern N is a short loading pattern, i.e., not all of the spaces in the fuel basket are filled).

4.

The addition of new BWR loading patterns E and F.

5.

An increase in the PMTC heat load (from 30kW to 35.5kW, including the uniform pattern A and preferential loading pattern B).

6.

The addition of a recess in the TSC closure lid.

7.

The addition of two new PWR fuel types.

Change 1. The addition of a new variation of the LMTC, Reduced Width LMTC.

In FSAR section 5.15.9, submitted on August 29, 2023, the applicant evaluated the Reduced Width LMTC, which has a neutron shield width of 86.5 inches outside diameter (OD) compared to the corresponding measurement of 89.6 inches OD in the existing LMTC. The Reduced Width LMTC appears in proprietary drawing no. 71160-L258 of the FSAR. The MCNP VISED

[Visual Editor] slice of the LMTC model is shown in figure 5.14.9-1 of FSAR. The evaluation used the minimum lead shield of 2.5 inches. The applicants model used a uniformly loaded 42 kW source at 52 gigawatt days per metric ton of uranium (GWd/MTU) for comparison to the non-reduced LMTC neutron shield. The dose rates for side and midplane azimuthal for both the LMTC and the Reduced Width LMTC appear in FSAR figures 5.14.9-2 and 5.14.9-3.

The side dose rate for the Reduced Width LMTC increased by 5 percent compared to the existing LMTC. The midplane azimuthal dose rate for the Reduced Width LMTC increased by 20 percent along localized peaks. The applicant stated that similar results will be observed for preferential patterns, and that dose rates during cask transfer will be dominated by welding and closure operations occurring at the top of the cask.

The staff performed a confirmatory evaluation using the Microshield dose assessment program and the same source terms as those used by the applicant. The confirmatory dose rates are similar to those from the applicants evaluation.

18 Change 2. The addition of a new concrete cask design known as CC8. CC8 is based on the CC7 configuration and uses high density concrete for enhanced shielding.

The material density of the high-density concrete used in CC8 is given in FSAR table 5.14.8-1, provided by the applicant in the August 29, 2023, submittal. The applicant evaluated the dose rate for the CC8 using spent fuel having a burnup of 55 GWd/MTU and 4.1 percent U-235 enrichment in an undamaged PWR fuel basket. The calculated dose rates for CC8 are lower than those calculated using CC7 due to higher density concrete. FSAR table 5.14.8-2 shows a comparison between CC7 and CC8 dose rates. The radial and top axial dose rates are significantly lower for CC8. The CC8 dose rates are bounded with corresponding CC7 dose rates.

The applicant performed a similar dose rate evaluation for BWR spent fuel having a burnup 50 GWd/MTU and 4.1 percent enrichment for undamaged fuel. FSAR table 5.16.8-1 shows a comparison of dose rates between CC7 and CC8. Dose rates for the radial and top axis locations decrease with the CC8.

Staff reviewed the applicants evaluation and concluded that dose rates are lower due the higher concrete density since higher density concrete has higher hydrogen content and, therefore, is more effective in radiation shielding.

Change 3. Shielding Evaluations for PWR Loading Patterns L, M, and N with CC3/CC7 and LMTC The applicant proposed new loading patterns L, M, and N for PWR fuel assemblies that allow for cool times as low as 2 years. The response function method, which the NRC previously approved in amendment 3 (ML13207A245), from section 5.8.2 of the FSAR was used in this amendment to evaluate PWR fuel using a modified eight-zone pattern. As described in FSAR section 5.14.4, the minimum cool times are 4 years for pattern I and 2 years for patterns J through N, which are intended to support continued operations and decommissioning use, respectively. Included in the evaluations is an increase in burnup to 70 GWd/MTU versus the 60 GWd/MTU evaluated previously. Evaluations are performed for the contents the CC3 and CC7. Like the CC3, the CC7 cask design contains a 3-inch liner, versus the 1.75-inch liner in the base cask design. The CC8 is a variation of the CC7 with high density concrete intended for use in cases of higher heat load. A revised lid/top cask section design is implemented within the CC7 design. The revised lid/top cask section incorporates the outlet vent structure and has a variable thickness concrete section. Only the minimum lid thickness section is evaluated for the CC7 design.

FSAR section 5.14.4 presents the eight zones of the response function for the six PWR patterns. Pattern I with a 51 KW heat load, patterns J and K with a 42-kW heat load, patterns L and M with a 48 kW, and pattern N with a 53-kW heat load.

In conclusion, the staff found these PWR loading patterns acceptable to be stored in CC3/CC7 and LMTC. The reason for this acceptance is due to the thickness of the liner, which is thicker than the base cask design. This liner will allow the CC3 and CC7 with the loading patterns mentioned above to reduce dose rates at the surface of the package and at 1-meter.

19 Fuel Content FSAR section 5.2 describes the hybrid content containing the maximum fuel and hardware masses used in these evaluations, which were approved in previous amendments to the MAGNASTOR CoC. The geometry of the hybrid fuel is described in FSAR section 5.8.1.

Model Specification and Shielding Densities The three patterns L, M, and N, which include up to 37 PWR assemblies, appear in figures E-1 through E-3 of appendix E of proprietary calculation 71160-5031, revision 5, submitted by the applicant on August 29, 2023. Pattern N is a checkerboard pattern having aluminum heat shunts in the vacant positions, as shown in the Figure E-3. The heat shunts are not modeled for heat removal. Table E-1 shows the heat loads for the uniform loading pattern and preferential patterns L, M, and N for eight heat zones with an undamaged basket. Amendment 15 did not propose to change the materials composition and dimensions for the fuel and the TSC.

Shielding Model Specification Configuration of the Shielding and Source In proprietary calculation 71160-5031, revision 5, the applicant calculated maximum dose rates through a combination of response function and direct dose rate calculations using a methodology previously approved in amendment no. 3. The applicant computed response functions and dose rates using MCNP6.2. Three-dimensional MCNP shielding evaluations provide dose rates for transfer and concrete casks. MCNP models of the fuel assembly, tube, basket, and canister are imported from the previously approved methodology in calculation 71160-5030. The applicant also included statistical uncertainty in the reported dose rates. The statistical uncertainty of the calculated dose rate is part of the computational results of the MCNP code that employs the Monte Carlo method for solving neutron and gamma shielding problems.

PWR Models for LMTC The applicant evaluated four PWR heat load patterns for undamaged PWR fuel, including the uniform pattern I and three preferential heat load patterns (L, M, and N). These patterns allow loading of PWR fuel with cooling times as low as 2 years. The applicant modeled multiple combinations of flooded (wet, W) and empty (dry, D) conditions in the canister and neutron shield regions. For example, a wet canister and dry neutron shield is labeled W/D. Based on the operations for the LMTC, the possible combinations are W/W, W/D, and D/W, and a D/D combination is not permitted. For determining maximum dose rates, the applicant states that the only relevant conditions are W/D and D/W. The W/W condition will always be bounded by W/D and D/W, but one case is included for comparison. Pattern N produced bounding dose rates for the LMTC in the Dry/Wet (D/W) but varies for W/D configuration.

The staff agrees with the applicants conclusion that the W/D and D/W configurations are bounding, as the W/W configuration includes significantly more water for shielding.

Table E-2 of calculation 71160-5031, revision 5 compares the average dose rates for the undamaged fuel basket for the uniform pattern and patterns L, M, and N. Table E-3 shows the bounding source terms for the undamaged fuel basket for LMTC W/D and D/W configurations.

20 Table E-4 summarizes the dose rates for patterns L, M, and N for surface dose and 1 meter dose rate.

The applicants three-dimensional MCNP shielding evaluations provide dose rates for the LMTC at distances up to 4 meters. Figures E-4 through E-11 present the calculated dose rates for the bounding dry canister (D/W) for patterns L, M, and N.

Damaged Basket Results.

For damaged fuel, the applicant presented dose rates for radial and bottom detectors calculated with source terms from table E-3 of calculation 71160-5031. Table E-5 presents the maximum surface and 1 meter dose rates for minimum and maximum lead for fuel at a 75 percent damaged fuel packing fraction. For the bottom detectors, the calculated dose rates are for minimum lead shielding configurations. Figures E-12 through E-17 present the calculated dose rates for the bounding dry canister with radial doses based on maximum lead shielding configuration.

MAGNASTOR PWR-37 High Heat Storage System for Concrete Casks CC3/CC7 There are seven heat load patterns (uniform pattern I and preferential patterns J through N) that have been evaluated by the applicant for PWR fuel. The preferential patterns allow loading of PWR fuel with cooling times as low as 2 years. Pattern I was designed as an ongoing operations pattern with fully burned fuel with typical cool times of 4 years or greater and is evaluated as a uniform pattern, while patterns J through N are primarily designed to allow flexibility in decommissioning scenarios and contain heat load variations (preferential loads).

Evaluations were performed by the applicant for the contents using CC3 and CC7.

The preferential loading pattern method from section 5.9 of the FSAR was used by the applicant to evaluate PWR fuel, modified to an eight-zone pattern.

The maximum and average dose rates for flexible uniform pattern I (4-year minimum cool time) analysis in CC3 and CC7 are shown in section 5.14.4.2 of the FSAR and proprietary calculation 71160-5032, appendix F. The maximum and average surface dose rates for preferential patterns are presented in FSAR table 5.14.4-2. Pattern N is bounding for the radial and top surfaces, as well as the inlet and outlet locations. CC7 is bounding for the air outlet maximum dose rate. CC3 is bounding for radial, top, and inlet maximum dose rates and the radial and top average dose rates. Calculation 71160-5032, table F-2 compares surface dose rates for the uniform pattern and preferential patterns L, M, and N using CC3 and CC7.

Change 4. Shielding Evaluations for BWR Load Patterns E and F Section 5.16 of the FSAR shows an evaluation for high heat load and ultra-high heat patterns for CC3/CC5/CC7 and LMTC for BWR system.

BWR Loading Patterns E and F Assembly Fuel Basket The applicant proposed new loading patterns E and F for damaged BWR fuel assemblies besides the previously approved preferential heat loads A, B, and C. Patterns E and F are for the undamaged fuel basket and patterns D and E are for the damaged fuel basket. The method, which has been previously approved, from section 5.9 of the FSAR was used in this amendment

21 to evaluate BWR fuel using a modified nine-zone pattern. The minimum cool time is 2 years.

Pattern A is for ongoing operations for fully burned fuel from ongoing operations with a minimum cooling time of 4 years or longer. Patterns B through F are for flexibility in decommissioning scenarios with variations of heat load and shorter cooling times. There are five unique patterns summarized in section 5.16.4 of the SAR. The analyzed pattern F, which has the highest heat load of 50.175 kW, was conservatively modeled. The pattern F heat load is significantly higher than the previously analyzed pattern A heat load of 47.437 kW.

There are three existing BWR heat load patterns (patterns A, B, and C) that were previously evaluated by the applicant for the LMTC and CC3, CC5, CC7 casks. The three unique patterns are summarized in section 5.16.4 of the FSAR. As noted above, pattern A has a total heat load of 47.437 kW/cask. Patterns B and C both have total heat loads of 42kW per cask.

In Amendment 15, the applicant calculated dose rates for the newly designated patterns E and F using CC3, CC5, and CC7. These load patterns require the use of the LMTC due to its liquid neutron shield providing for enhanced heat transfer. The CC7 is a variable height cask to support loading of either BWR/2-3 fuel (which includes a shorter fuel assembly) or BWR/4-6 fuel. For BWR/4-6 fuel, the CC7 cask height increases to 197.8 inches. The BWR hybrid fuel assemblies were used by the applicant in this evaluation. Hybrid means that the cask contents include maximum fuel and hardware masses to maximize the source terms. The staff finds this approach acceptable mainly because the fuel assemblies may contain less fuel and less hardware than the hybrid used by the applicant in the evaluation, which makes the evaluation a conservative assumption. Section 5.8.1 of the FSAR, which the applicant did not change in this amendment request, contains geometry data for the BWR hybrids. The applicant used a combination of direct solution cases and cases using the response function method to evaluate the loading of BWR fuel assemblies in the CC3, CC5, and CC7.

Section 5.16.4 of the FSAR presents the preferential loading patterns for undamaged BWR fuel in high heat and ultra heat using 9 heat zones. Figure 5.16.4-1 shows a sketch of the loading pattern.

Appendix E of proprietary calculation 71160-5033, revision 3, submitted August 29, 2023, shows the applicants shielding evaluation of BWR-89 high heat storage patterns E and F using concrete casks CC3 and CC7. The preferential loading patterns are presented in the figures E-1 and E-2. The heat loads for nine zones of response function are shown in table E-1 for a uniform pattern and preferential patterns E and F. The CC3 and CC7 surface doses for the uniform pattern and preferential patterns E and F are given in table E-2 for radial, top, inlet, and outlet locations.

For the BWR-89 high heat storage patterns E and F, the applicant used CC3 to compute dose rates on the side surface and air inlet locations and used CC7 to compute dose rates on the top and air outlet locations. Table E-4 shows the summary of dose rates at the surface and at 1 meter for pattern F. Figures E-3 through E-6 provide graphical representations of these dose rates.

Appendix E of proprietary calculation 71160-5034, revision 2, submitted August 29, 2023, shows the applicants shielding evaluation of BWR-81 high heat storage patterns D and E with concrete casks CC3, CC5, and CC7. The preferential loading patterns are presented in the figures E-1 and E-2. The heat loads for nine zones of response function are shown in table E-1 for the uniform pattern and preferential patterns D and E. The CC3 and CC7 surface doses for patterns D and E are given in the table E-2 for radial, top, inlet, and outlet locations.

22 For the BWR-81 high heat storage patterns D and E, the applicant used CC3 to compute dose rates on the side surface and air inlet locations and used CC7 to compute dose rates on the top and air outlet locations. Table E-4 shows the summary of dose rates at the surface and at 1-meter for pattern D and E. Figures E-3 through E-8 provide graphical representations of these dose rates.

Appendix E of proprietary calculation 71160-5035, revision 2, submitted August 29, 2023, shows the applicants shielding evaluation of patterns E and F with BWR-81 and BWR-89 high heat storage fuel contents using the LMTC.

For the 89-assembly basket, figures E-1 and E-2 show two unique preferential patterns E and F having 46 kW heat loads. The heat loads for nine zones of response function are shown in table E-1 for the uniform pattern and preferential patterns E and F. Table E-2 shows a comparison of maximum dose rates for the three loading patterns.

For the 81-assembly basket, figures E-3 and E-4 show two unique preferential patterns E and F having 46 kW heat loads. The heat loads for nine zones of response function are shown in table E-5 for the uniform pattern and preferential patterns D and E. Table E-6 shows a comparison of maximum dose rates for the three loading patterns.

In conclusion, the staff found these BWR loading patterns acceptable to be stored in CC3/CC7 and LMTC. The reason for this acceptance is due to the facts that these patterns are conservatively bounded by the previously loading patterns A, B, and C. Also, with the use of BWR hybrid fuel assemblies by the applicant in this evaluation, the cask contents include maximum fuel and hardware masses that maximize the source terms. The staff finds this approach acceptable mainly because the fuel assemblies may contain less fuel and less hardware than the hybrid used by the applicant in the evaluation, which makes this the evaluation a conservative assumption.

Change 5. PMTC Heat Load Increase (30kW to 35.5kW Including Uniform Pattern A and Preferential Loading Pattern B).

The applicant calculated the dose rates for the PMTC transporting a TSC containing 37 CE 16x16 PWR fuel assemblies. In the applicants evaluation, the TSC is placed inside the PMTC and sealed before it is moved to a concrete cask placed on the ISFSI pad. The CE 16x16 fuel is an authorized fuel to transfer using the PMTC. The applicant evaluated both uniform and preferential loading. The applicant considered both the TSC closure operations and transfer operations in evaluations of dose rates. The proprietary calculation 30032-5002, revision 0 and FSAR section 5.11, both submitted August 29, 2023, show the applicants shielding evaluation of a PMTC with high heat load of 35.5 kW. The previous results from MAGNASTOR FSAR revision 9 (ML17293A428) for PMTC for CE16x16 were updated for increased cask heat load.

Dose Rates for PMTC Containing Undamaged and Damaged Fuel As stated in FSAR section 5.11.1.1, the applicant used a three-dimensional Monte Carlo MCNP 6.2 analysis using surface detectors and superimposed mesh tally detectors. The mesh tally encompasses all streaming paths.

The applicant performed the dose rate calculations with focus on the streaming paths above and around the annulus between the TSC and PMTC. The applicant performed the dose rate

23 calculations for locations at the vent shields and bottom forging. The applicant considered both the TSC closure and transfer operations in evaluations of dose rates.

The three-dimensional MCNP shielding analysis model includes an explicit representation of the PMTC and TSC structure with the following assumptions:

1.

Dry canister cavity and TSC-to-PMTC annulus.

2.

Homogenization of the fuel assembly into four source regions.

3.

Damaged fuel loaded in the corner basket locations located between the vent shields.

The authorized contents for the PMTC include intact or damaged CE 16x16 PWR fuel assemblies and burnable poison rods used as replacement rods in the CE core. These rods are constructed with a zirconium alloy and do not contain a significant amount of activated material.

Therefore, they are enveloped by the fuel rods that they replaced. Only four damaged fuel assemblies are allowed to be loaded into the four corner locations, and the damaged fuel assemblies must be placed in damaged fuel cans. The maximum PMTC dose rates for undamaged fuel are depicted in tables 6.1 and 6.2 of calculation 30032-5002 revision 0 and FSAR tables 5.11.1-1 and 5.11.1-2. The damaged fuel dose rates are shown in FSAR tables 5.11.1-6 and 5.11.1-7.

Source Specification FSAR section 5.11.3 discusses the source specification. To determine the bounding radiation source terms of the CE 16x16 fuel assemblies to be loaded in the MAGNASTOR system, the spent fuel assemblies are sorted into groups according to the assembly types, i.e., PWR, and fuel and hardware masses. A hypothetical bounding fuel assembly is created for each assembly type. Each hypothetical assembly is based on the maximum allowable fuel and hardware masses and presents a conservative bounding value of fuel and hardware mass of that group.

The applicant used the SAS2H sequence of the SCALE 4.4 computer code system to evaluate the source terms of each spent fuel assembly group. The 44GROUPNDF5 library, which is composed primarily of ENDF/B-V cross-sections with limited ENDF/B-VI data for a limited number of isotopes, is employed to improve the calculation accuracy.

As discussed in FSAR section 5.11.3, the applicant evaluated source terms for the CE 16x16 spent fuel assemblies having the following characteristics:

Average assembly burnup from 10 to 70 GWd/MTU Fuel initial enrichment from 1.3 to 4.9 weight percent Cooling time from 4 to 90 years.

The applicant used the response function approach to perform the shielding evaluation. The applicant calculated the dose rates based on previous calculations for transfer and storage casks using combinations of cooling time, initial enrichment, and maximum assembly burnup.

Minimum Cool-Time Tables In FSAR section 5.11.7, the applicant evaluated PMTC system performance for a cask heat load of 35kW. The applicant used the same method as is used in FSAR section 5.8.3.2 to evaluate the minimum cool times. The evaluation considered uniform patterns and preferential loading patterns using the same three zone pattern described in FSAR section 5.8.7. The

24 evaluation calculated minimum cooling times for a matrix of burnup and enrichment levels.

Similar to the MTC, the results showed that the maximum and average dose rates of the PMTC using the preferential pattern are less than those for the uniform pattern.

The applicants evaluation produced the maximum dose rates shown in the sections 5.11.8 and 5.11.10 of the FSAR. The applicant calculated all dose rates using the high heat load source terms. This adds extra conservatism in the calculated dose rates. The source terms generating maximum dose rates for the PMTC all occur at lower burnups and shorter cool time combinations.

Model Specification In the MCNP shielding models used by the applicant for the PMTC transfer cask, the fuel and hardware source regions are homogenized within fuel basket cells of the TSC canister. The axial volume of the fuel is divided to active fuel, upper and lower plenum, and upper and lower fitting source segments. The three-dimensional MCNP model includes shield regions and streaming paths. The MCNP calculations used a biasing technique based on window adjustment in mesh cells. This technique is used to determine dose rates at the PMTC radial surface and vent shield locations. Axial biasing is used to determine the cask top and bottom dose rates.

PMTC Dose Rates The applicant provided PMTC dose rates in FSAR figures 5.11.8-1 through 5.11.8-4 as a function of distance and figures 5.11.8-5 through 5.11.8-8 as a function of source type. The peak dose rates are on the radial cask surface near top and bottom forging locations. The dose rate distribution profile follows the burnup profile over the active fuel region. Due to radiation streaming, the dose rates increase in the TSC-to-PMTC annulus area. During TSC closure operations, the shield and seal provide some shielding at the annulus location. The retaining ring used during the transfer provides shielding in streaming paths. The maximum dose rates occur during TSC closure without additional auxiliary shielding. The applicant calculated the dose rates at the vent shield and bottom forging locations by using superimposed mesh tallies.

FSAR figure 5.11.8-10 presents a dose rate contour map of the area around Vent B. Vent B is the location of the maximum dose rate, which results from streaming through the vent.

Figure 5.11.8-11 of the FSAR provides dose rate contour map for PMTC bottom steel doors.

The dose rates at the vent shield are higher than those at the bottom forging. This is because the bottom steel doors do not extend the full radius of the cask and the TSC-to-PMTC annulus.

Staff Confirmatory Review and Analysis The staff reviewed the applicants shielding analysis and found it acceptable. The maximum dose rates meet the limits defined by 10 CFR Part 72. The staff reviewed the applicants radiation shielding evaluations, including the calculations of the sources, the dose rates for the transfer cask, the concrete overpack, and the annual dose at the controlled area boundary. The staff independently calculated source terms for the bounding PWR CE16x16 fuel assemblies using combinations of different enrichments, burnups, and cooling times. The staff also performed confirmatory analyses of the dose rates for the PMTC. The staff performed confirmatory analyses of source term evaluations using the SCALE 6.2 computer code with the ORIGEN/ARP isotopic depletion and decay sequence with the 238-group ENDF/VII cross section library. Using irradiation parameter assumptions similar to the applicants, the staffs analyses resulted in bounding source terms that were similar to, or bounded by, those

25 determined by the applicant, and therefore the staff finds the applicants result acceptable. The staff finds the applicant has correctly determined the bounding dose rates for proposed payloads.

Change 6. TSC Closure Lid Recess Effect on Top Dose Rates (PMTC and CC5).

To allow for neutron source assemblies longer than the TSC cavity height, the applicant included a recess in the TSC closure lid. Proprietary calculation 30032-5003, revision 0, submitted October 26, 2023, evaluates the effect of adding the recess to the closure lid on the top dose rates for PMTC and CC5 using a cask heat load of 35.5 kW.

The applicant modified the existing model for PMTC and CC5 by including the recess in closure lid and compared the calculation results for both with and without the recess in the closure lid.

The recess is 3 inches deep with an OD of 5 inches as shown in FSAR figure 5.17.1-1 and proprietary drawing 71160-684. The results show that the effect on the dose rate at the top location is governed by the contribution from the fuel assemblies and not by the contribution from the neutron source which is inserted between fuel assemblies into middle of active fuel.

PMTC Results As described in FSAR section 5.17.2, submitted October 26, 2023, the applicant evaluated two configurations for the PMTC at the bounding 35.5 kW source terms. Configuration 1 models the vacuum drying and sealing process. The configuration installs a weld shield at the top of the TSC and leaves out the port covers. Configuration 1 also includes a shield ring that provides shielding in annulus between TSC and passive transfer cask. Configuration 2 models the transfer of TSC to a CC5 concrete cask. Results for the PMTC are shown in FSAR figure 5.17.2-1 and figure 5.17.2-2, which show comparisons of dose rates with and without the recess for configurations 1 and 2, respectively.

When adding the recess, dose rates increase for both models at cask centerline in comparison to the no recess (baseline) results. When adding the recess, dose rates at the TSC to cask annulus and over surface decrease for configuration 1 and increase for configuration 2, in comparison to the baseline. The TSC to cask annulus peak for configuration 2 is within 2 sigmas of the (no recess) baseline. Surface dose rates increase by 2 percent which is not significant considering the uncertainty in the calculation. The peak location, which is within 2 sigmas and is not significant, bounds the occupational exposures.

In conclusion, the staff found this PMTC results acceptable due to the facts that increment of the dose rates are within the uncertainty area which is about 2-sigma from the nominal value. This increment on dose rates does not affect the surface dose rates and bounds the occupational exposures CC5 Results Results for CC5, which are presented in FSAR figure 5.17.3-1, show an increase in dose rate at the cask centerline that is within 2 sigmas (not statistically significant) of the corresponding dose for the no recess closure lid.

In conclusion, the dose rates are within the uncertainty area (2-sigma), which does not affect the occupational exposures.

26 Change 7. Add Two New Fuels, CE16-NGF and CE16-HTP.

FSAR table 5.2.3-1 contains the key characteristics for these fuel assembly types evaluated for shielding performance in the MAGNASTOR SAR. At a given burnup (in GWd/MTU), cool time, and initial enrichment, a fuel assembly with increased mass fuel had an increased number of fission and absorption events and therefore an increased source term. Table 6.2.1-1 of the FSAR shows that the new fuel types CE16NGF and CE16HTP contain lower fuel mass than the current design-basis CE16 fuel type. Therefore, the new fuel assemblies CE16NGF and CE16HTP are bounded with the previous CE16 fuel and are acceptable.

Staff Evaluation The staff reviewed the applicants shielding analyses for proposed Amendment 15 to the MAGNASTOR dry storage system design. The staff finds that the approaches and methodologies used in these calculations and the results are acceptable for the LMTC and CC5/CC7 for PWR, CC3 and CC5 for BWR, and CC7 system design; new loading patterns L, M, and N for the PWR fuel basket; new loading patterns E and F for the undamaged BWR 89-assembly fuel basket; and the damaged BWR 81-assembly fuel basket.

The applicant used approaches and methodologies previously found to be acceptable by the staff in similar applications and used bounding conservative assumptions. The staff independently calculated source terms for the bounding PWR WE14x14 fuel assemblies using combinations of different enrichments, burnups, and cooling times using ORAGAMI, SCALE 6.3. Staff evaluated dose rates using MCNP6.2 for the CC3 for PWR and, CC3/CC5 for BWR, and CC7 storage casks. Using irradiation parameter assumptions similar to the applicants, the staff obtained bounding source terms that were similar to or bounded by those determined by the applicant and therefore finds the applicants result acceptable. The staff finds that the applicant has correctly assessed the bounding dose rates for all proposed contents. Based on this review and analyses, the staff concludes that the applicant has demonstrated that the MAGNASTOR dry cask storage system meets the radiation protection requirements of 10 CFR 72.104, 72.106, 72.126, and 72.128.

The staff concludes that the shielding and radiation protection design features of the MAGNASTOR system comply with 10 CFR Part 72, and that the applicable design and acceptance criteria continue to be satisfied. The evaluation of the concrete top lid in terms of shielding and radiation protection design features provides reasonable assurance that the system will still provide shielding and radiation protection from the spent fuel. This finding is based on the appropriate regulatory guides, applicable codes and standards, the applicants analyses, the staffs evaluations, and acceptable engineering practices.

Based on the information provided by the applicant on how the shielding evaluation was conducted, the staff concludes that the requested changes meet the regulatory limits, and the acceptance criteria specified in NUREG-2215 and provides reasonable assurance of the safe transfer and storage of the spent fuel and non-fuel hardware as specified in the TS for the MAGNASTOR system. On these bases, the staff finds:

F6.1 Chapter 5 of the MAGNASTOR FSAR sufficiently describes the shielding design bases and design criteria for the SSCs important to safety.

27 F6.2 The MAGNASTOR system radiation shielding features of CC3, CC5, and CC7 and the LMTC and their associated confinement features are sufficient to meet the radiation protection requirements of 10 CFR Part 20 and 10 CFR 72.236(d).

F6.3 The shielding and radiation protection design features of the MAGNASTOR system, including the concrete cask, the transfer cask, and the TSC, are in compliance with 10 CFR Part 72, and the applicable design and acceptance criteria have been satisfied.

The evaluation of the shielding and radiation protection design features provides reasonable assurance that the system will provide safe transfer and storage of spent fuels. This finding is based on a review that considered applicable regulations, the appropriate regulatory guides, applicable codes and standards, the applicants analyses, the staffs confirmatory analyses, and acceptable engineering practices.

Chapter 7 CRITICALITY EVALUATION The staff reviewed the amendment request to determine if the MAGNASTOR system continues to maintain its authorized contents in a subcritical configuration under all credible normal, off-normal, and accident events encountered during the handling, loading, transfer, and storage of spent nuclear fuel. The staff reviewed the applicants criticality safety analysis to ensure that all credible bounding scenarios were adequately identified and any potential consequences on the criticality safety of the MAGNASTOR dry cask storage system continues to meet the regulatory requirements of 10 CFR 72.124 and 72.236. The conclusions of the staff are based on the information provided by the applicant including the supporting calculations for the addition of a new fuel hybrid (CE16H2) with fuel rods having a slightly reduced rod OD, reduced clad thickness, and a reduced pellet diameter.

7.1 Criticality Design Criteria and Features The major components of the MAGNASTOR storage system include a storage canister, a concrete storage case, and a lead-shielded transfer cask. Criticality safety of the MAGNASTOR system is provided by a combination of fissile mass and enrichment controls, geometry control, and fixed neutron absorbers in the basket. The MAGNASTOR system may also contain damaged fuel cans to store damaged PWR fuel. Fixed neutron absorber sheets are attached to the walls of the fuel assembly tubes and sit between each fuel assembly in the basket. PWR fuel requires the use of soluble boron in the water that is used to flood the canister during loading and unloading operations. The minimum soluble boron content is based on the assembly type and the maximum initial assembly enrichment.

Since the previously approved cask design is not altered by this amendment, the staff evaluated the addition of the CE16H2 assemblies, the decreased OD, clad thickness, and pellet diameter.

The applicant made changes to the CoC and the TS to allow these additions to the MAGNASTOR storage system.

The staff reviewed the model descriptions provided by the applicant and their assumptions and finds that they are consistent with the description of the design and contents provided in the FSAR. The staff also evaluated the information the applicant provided in the amended FSAR and found the criticality calculations were sufficiently detailed to support the staff evaluation.

Based on this review, the staff finds that the applicant adequately met the requirements of 10 CFR 72.236.

28 7.2 Fuel Specifications Consistent with previous amendments, the applicant identified the proposed new fuel contents based on the specified fuel type and identified the conservative bounding values for the criticality significant parameters for each fuel. As described in proposed FSAR table 6.7.3-11 and TS table B2-3, the new hybrid CE16H2 fuel assemblies have the same number of fuel rods and guide tubes as the previously approved CE16H1 fuel. However, they are slightly different than those previously approved and have a minimum fuel rod OD of 0.3740 inches, a minimum clad thickness of 0.0225 inches, and a maximum pellet OD of 0.3225 inches. The previously approved maximum enrichment of 5.0 weight percent 235U is the same as that allowed for the CE16H2 fuel, as is the bounding PWR fuel assembly loading criteria for soluble boron. The staff reviewed the FSAR and the proposed fuel specifications in the TS and found the applicant adequately specified the proposed fuel specifications that could impact the criticality safety of the MAGNASTOR.

7.3 Model Specifications The applicant has evaluated the storage of both damaged and undamaged fuel assemblies in previous, NRC-approved amendments to the MAGNASTOR system (as documented in calculation 30032-6001), and these are unchanged in this amendment. The applicant used the same criteria used in the calculation to evaluate the new models to support the additional fuel types specified.

7.4 Criticality Analysis The applicant performed a criticality evaluation (calculation 30032-6001, proprietary, submitted October 26, 2023) that demonstrated the proposed new fuel assembly type for both damaged and undamaged fuel continue to remain under the established upper subcritical limit (USL) of 0.9376. The applicant evaluated the fuel under the same general conditions and assumptions used in previous amendments. The staff determined this evaluation to be acceptable due to the conservative assumptions used by the applicant, and in all instances the applicant-calculated keff was below the USL, and the staff found this acceptable.

The applicant evaluated each of the new hybrid fuel assemblies for both the undamaged and damaged configurations. For the undamaged configuration assemblies were modeled at an enrichment of 5.0 weight percent 235U. The applicant calculated the most reactive configuration for the CE16H2 assemblies to have keff + 2 = 0.93400. The applicant evaluated a wide range of configurations to find the maximum reactivity of fuel contents for the new fuel type. In all cases, the applicant demonstrated the reactivity was below the USL.

The applicant used MCNP5, a three-dimensional Monte Carlo code, with continuous neutron energy cross-sections to conduct its criticality analysis. This code was developed by the Los Alamos National Laboratory for performing criticality analyses and was used in calculations in previous amendments to the MAGNASTOR cask system. The staff finds its use acceptable for this system since the MCNP code and cross-section libraries are benchmarked by comparison to a wide range of critical experiments for light water reactor fuel in storage and transportation packages. The applicant also performed calculations showing that the MAGNASTOR system will continue to meet the design criterion of keff + 2 < USL when loaded with the authorized contents as specified in the FSAR and proposed TS.

29 7.5 Criticality Evaluation Summary All the applicants models for the new hybrid fuel type are based on the engineering drawings in the FSAR and builds on the previously reviewed models submitted as part of supplemental calculations. The design-basis, off-normal, and accident events do not affect the design of the cask with regards to maintaining the MAGNASTOR cask system in a subcritical configuration.

This means the calculation models for the normal, off-normal, and accident conditions applicable in this amendment are the same as those previously reviewed and accepted by the staff. The staff imported sample input files provided by the applicant in support of its supplemental calculations to confirm the results provided by the applicant. For these reasons, the staff finds that the applicants evaluation of the criticality design demonstrates that the MAGNASTOR cask storage system will continue to allow for the safe storage of spent nuclear fuel. This finding is reached based on a review that considered the regulation itself, appropriate regulatory guides, applicable codes and standards, and accepted engineering practices.

7.6 Evaluation Findings

The staff reviewed the information provided in Amendment 15 of the MAGNASTOR cask system application and determined that it complies with the requirements in 10 CFR 72.124, and 10 CFR 72.236(c). The staff also determined that the results of the applicants evaluation of the new hybrid CE16H2 fuel assembly, as described in this application, remain less than the USL for each of the evaluated cases. The applicant incorporated several conservative assumptions and evaluated the fuel over a range of bounding credible scenarios. Limits are imposed for each fuel in regard to the minimum 10B concentration in the absorber sheets, soluble boron concentrations in the pool, and allowable enrichments. As a result, the staff has reasonable assurance that the MAGNASTOR spent fuel dry cask storage system containing hybrid CE16H2 fuel, as described in this amendment to the application, will remain safe while in storage. Specifically, the applicants nuclear criticality safety evaluation demonstrates that the MAGNASTOR spent fuel dry cask storage system will continue to meet the relevant regulatory requirements, and the staff finds the following:

F7.1 The applicant described SSCs important to criticality safety in sufficient detail in the FSAR to enable an evaluation of their effectiveness.

F7.2 The cask and its spent fuel transfer systems are designed to be subcritical under all credible conditions.

F7.3 The criticality design is based on favorable geometry, fixed neutron poisons, and soluble poisons of the spent fuel pool. An appraisal of the fixed neutron poisons has shown that they will remain effective for the term requested in the application and there is no credible way for the fixed neutron poisons to significantly degrade during the requested term in the application; therefore, there is no need to provide a positive means to verify their continued efficacy as required by 10 CFR 72.124(b).

F7.4 The applicants analysis and evaluation of the criticality design and performance have demonstrated that the cask will enable the storage of spent fuel for the term requested in the application.

Chapter 8 MATERIALS EVALUATION The purpose of the staffs materials evaluation is to determine whether the amendment application adequately describes and evaluates how proposed changes affect material

30 properties and material performance characteristics for the MAGNASTOR storage system and to ensure compliance with regulatory requirements in 10 CFR Part 72. The NRC staff performed its materials evaluation for the MAGNASTOR Amendment 15 application by following the technical guidance in NUREG-2215, Standard Review Plan for Spent Fuel Dry Storage Systems and Facilities, April 2020 (ML20121A190).

The MAGNASTOR Amendment 15 application includes the following proposed changes to the design, construction, operating conditions, and associated safety analyses for the MAGNASTOR storage system:

1.

Addition of a new concrete cask variant with higher density concrete; 2.

Addition of a new variant of the LMTC with reduced width; 3.

Addition of new spent fuel loading patterns with improved high heat load performance; 4.

Increase in the heat load limit for the PMTC; 5.

Removal of the 5 percent burnup penalty on heat load for high-burnup fuel; 6.

Deletion of cool time tables from appendix B of the CoC TS; 7.

Addition of two new spent fuel assembly types to support future site operations; 8.

Addition of an optional recess into the TSC closure lid to provide additional clearance near the top center of the fuel basket.

Based on its review of the application, the staff determined that the scope of changes in the MAGNASTOR Amendment 15 application affecting material properties and performance includes just items 1 through 4 above, and not items 5 through 8. The staff identified that the proposed changes listed in items 5 through 8 involve adjustments to certain safety analysis inputs and results, changes to operating specifications and conditions, and changes to component geometry and dimensions. However, the staff identified that the changes listed in items 5 through 8 do not affect any intrinsic material property or material performance characteristic for the MAGNASTOR storage system. Therefore, the proposed changes listed in items 5 through 8 are not addressed as part of the staffs materials review. Specifically, the staffs materials evaluation is solely concerned with the following changes proposed in the MAGNASTOR Amendment 15 application:

1.

Addition of a new concrete cask variant with higher density concrete; 2.

Addition of a new variant of the LMTC with reduced width; 3.

Addition of new spent fuel loading patterns with improved high heat load performance; 4.

Increase in the heat load limit for the PMTC.

The staff identified that the above four changes include effects on the mechanical and thermal properties of materials for storage system components.

New Concrete Cask Variant - High-Density Concrete for Enhanced Shielding The Amendment 15 application adds an eighth concrete cask (CC) variant, CC8. CC8 is similar to previous approved CC designs, but with higher density concrete for enhanced shielding. CC8 also includes revised CC lug design to support the higher CC weight.

The staff reviewed the proprietary revised license drawings in the FSAR, Drawing Nos. 71160-L362, 71160-L363, and 71160-L364, and the cask material information in FSAR chapter 8 in the Amendment 15 application to determine whether the high-density concrete is adequately specified for CC8. The staff confirmed the new concrete density for CC8 is adequately specified in the drawings and in chapter 8 of the application. The staff identified that

31 the application proposes no changes to other non-concrete CC8 materials besides the revised CC lug design. The staff confirmed that the changes to the CC lug design for CC8 include adjustments to lug dimensions and the associated structural evaluation to accommodate the higher weight for CC8, but there are no changes to intrinsic material properties for the CC8 lugs.

The staff considered whether additional changes are needed for concrete mechanical and thermal properties based on the higher density of concrete for CC8. The staff noted that the concrete modulus of elasticity, E, generally increases for higher density concrete; however, this is not reflected in the table of concrete mechanical properties in FSAR chapter 8 in the Amendment 15 application. With respect to the effect of concrete density on modulus of elasticity, FSAR section 3.14.3.2.1 in the Amendment 15 application describes a sensitivity study to demonstrate that previously calculated concrete cask thermal stresses remain bounding for CC8. The application indicates that this sensitivity study accounts for the difference in concrete modulus of elasticity corresponding to high-density concrete. However, staff identified that the effects of the higher concrete density on concrete modulus of elasticity are not documented in the mechanical property tables in FSAR chapter 8 of the application, and these tables also do not address the potential effects of the higher concrete density on other mechanical and thermal properties of concrete. The staff noted that mechanical and thermal properties of concrete may be affected by the higher concrete density. Structural and/or thermal evaluations for storage system components may also be affected considering the changes to the thermal and mechanical properties associated with the higher density concrete. The staff considered that structural and/or thermal evaluations for storage system components should address changes to applicable material properties to ensure acceptable performance.

Based on the foregoing considerations, the staff issued a request for additional information (RAI) requesting the applicant to address whether the thermal and mechanical properties of the concrete used in the thermal and structural evaluations need to be updated to ensure they are valid for the high-density concrete for CC8. The RAI also requested the applicant to update the thermal and mechanical properties of the concrete, if needed, to ensure that are valid to the high-density concrete used for CC8.

In its October 8, 2024, RAI response (ML24283A085, ML24283A087), the applicant revised FSAR table 8.3-19 (mechanical properties of concrete) in the Amendment 15 application to include new values for the modulus of elasticity for the high-density concrete. The applicant stated that the added values of the modulus of elasticity are determined based on the most bounding allowable value of the specified concrete density for the high-density concrete, as opposed to the average value of the specified concrete density. The applicant stated that the new values for modulus of elasticity for high-density concrete are considered in the thermal stress sensitivity analysis in FSAR section 3.14.3.2.1 in the amendment application. The applicant updated the footnotes to FSAR table 8.3-19 to state that the existing values of compressive strength and coefficient of thermal expansion that are already listed in the table for the normal density concrete are applicable for the structural evaluation of concrete cask configurations with either normal or high-density concrete. The applicant added a footnote to FSAR table 8.3-26 (thermal properties of concrete) in the Amendment 15 application to specify that the existing values of the thermal properties listed in this table are applicable for thermal evaluations of concrete cask configurations with either normal or high-density concrete. The applicant stated that it is conservative to use existing thermal properties in FSAR table 8.3-26 for thermal analyses of CC8 with high-density concrete since the thermal conductivity increases with the concrete density.

32 The staff reviewed the applicants RAI response and determined that the updates to FSAR tables 8.3-19 and 8.3-26 in the Amendment 15 application adequately address the effects of the higher concrete density for CC8 on the thermal and mechanical properties of concrete.

Specifically, the staff confirmed that the new modulus of elasticity values for high-density concrete are included in table 8.3-19. The staff reviewed these values and found them to be consistent with the established values from technical literature. The staff also confirmed that the updates to the footnotes for table 8.3-19 adequately address how the existing values of the compressive strength and coefficient of thermal expansion that are listed in this table for the normal density concrete are applicable for structural evaluation of concrete casks with either normal or high-density concrete. Therefore, the staff determined that existing values of compressive strength and coefficient of thermal expansion are acceptable for use in the structural evaluation of concrete casks with both normal and high-density concrete. The staff also confirmed that the new footnote for table 8.3-26 adequately specifies that the existing values of the thermal properties in this table are applicable for thermal evaluations of concrete casks with either normal or high-density concrete. The staff noted, in particular, that thermal conductivity and specific heat capacity generally increase with higher concrete densities, and such increases are associated with better thermal performance for concrete casks. Therefore, the staff determined that the updates to table 8.3-19 and table 8.3-26 and associated footnotes in FSAR chapter 8 in the Amendment 15 application are acceptable for documenting the effects of the higher concrete density for CC8 on the concrete mechanical and thermal properties.

Reduced Width LMTC Design The MAGNASTOR Amendment 15 application includes a Reduced Width LMTC design. The changes to the LMTC design include changes to LMTC subcomponent geometries, dimensions, and updates to associated LMTC safety analyses, including the structural evaluation, thermal evaluation, and shielding evaluation. The staff checked the proprietary revised license drawings in the FSAR, Drawing Nos. 71160-L257 and 71160-L258, for the Reduced Width LMTC design, and the material property tables in FSAR chapter 8 in the application and verified that there are no changes to specifications for materials used to fabricate the LMTC and there are no changes to material properties associated with the Reduced Width LMTC design. The staff identified that the intrinsic properties and performance characteristics of the LMTC materials are not affected by any of the changes to LMTC subcomponent geometries and dimensions. The staff also identified that none of the updates to LMTC safety analyses associated with Reduced Width LMTC design, including the structural evaluation, thermal evaluation, and shielding evaluation, involve changes to intrinsic material properties.

The staff noted that the applicant added a footnote to FSAR table 8.3-2 in the Amendment 15 application regarding the required minimum yield strength (Sy) for certain LMTC subcomponent materials at 70°F. However, the staff confirmed that the subject required minimum Sy for these LMTC subcomponent materials at 70°F was already incorporated into the previous version (revision 14) of the application in other locations, specifically the structural evaluation of LMTC in FSAR section 3.4.3.3.4 and in the LMTC body weldment drawing (FSAR drawing no. 71160-L258). The staff determined that the addition of the required minimum Sy at 70 degrees F for the subject LMTC subcomponents in the FSAR table 8.3-2 footnote in the Amendment 15 application is just a cross reference to the specification of this property that was already included in LMTC license drawings and already documented in the LMTC structural evaluation in revision 14 of the application. Therefore, the staff determined that this change is acceptable.

33 New Spent Fuel Loading Patterns with Improved High Heat Loads The Amendment 15 application includes several new spent fuel loading patterns with improved high heat loads. Details of the loading patterns are proprietary. With respect to the impact of the higher heat loads on material properties, the staff identified that higher heat loads result in increased component temperatures. Increases in component temperatures generally result in lower values of tensile properties, including design stress intensity, yield strength, and ultimate tensile strength (Sm, Sy, Su respectively) for component materials and therefore lower design allowable stresses for various loading conditions.

The staff reviewed the application to determine if the application adequately considers the reductions in design allowable stresses based on reductions in tensile properties for component materials considering the increased material temperatures associated with higher heat load. For the TSC confinement boundary and the LMTC, the staff determined that the application adequately identifies the reductions in design allowable stresses based on lower tensile properties for the TSC confinement boundary and LMTC materials considering their increased material temperatures.

For the fuel basket, the staff noted that the application shows that the previous basket structural evaluations for MAGNASTOR Amendment 11 are bounding for the improved high heat load patterns based on a demonstration that thermal stress analyses used bounding temperature gradients, and the design allowable stresses are based on bounding material temperatures.

Thermal stress analyses are reviewed as part of the NRC staffs structural evaluation, which is covered in section 4 of this SER. For the design allowable stresses, staff confirmed that the allowable stresses are based on tensile properties at sufficiently high material temperatures that are bounding for calculated basket component temperatures from the thermal evaluation of the improved high heat load patterns. For the new high-density concrete cask (CC8), the application describes a sensitivity study to demonstrate that the previously calculated concrete cask thermal stresses remain bounding for CC8.

The staff confirmed that the thermal evaluation in the application shows that the calculated maximum temperatures of the fuel cladding, basket, TSC shell, and concrete associated with the higher heat loads are acceptable for normal conditions of storage, off-normal storage events, and design-basis accident storage events since these thermal evaluation tables show that the maximum temperatures of these components are within their allowable temperature limits. The staff also confirmed that the thermal evaluation demonstrates that the calculated maximum temperatures of cladding and basket associated with the higher heat loads are acceptable for vacuum drying of the TSC and transfer of the TSC to the concrete cask, since these thermal evaluation tables show that the maximum temperatures of these components are within their allowable temperature limits.

The staff identified that one of the new fuel loading patterns for the Amendment 15 application includes the use of thermal shunts that are loaded into certain cells in the basket in lieu of spent fuel assemblies. The application states that a thermal shunt is a weldment of a specified material that is designed to occupy specific storage locations in a fuel basket for certain short-loaded preferential loading patterns. The application states that thermal shunts prevent fuel assemblies from being inadvertently loaded into storage locations that are not intended for spent nuclear fuel. In addition, thermal shunts provide a heat transfer function.

The FSAR includes a new thermal shunt weldment drawing (pdf p. 164), which is drawing no. 71160-L378. The criticality evaluation of the basket with cells containing thermal shunts is

34 addressed in FSAR chapter 6 in the Amendment 15 application. The staff noted that the amendment application does not include thermal properties for the thermal shunt materials.

Also, the applicants thermal evaluation, as described in FSAR chapter 4 in the amendment application, does not address the effect of the heat transfer function of the thermal shunts on the thermal performance of the storage system components.

Therefore, the staff issued an RAI requesting the applicant to specify the thermal properties of the thermal shunt materials that are used as inputs into the thermal evaluation of their heat transfer performance, if applicable, and update the thermal property tables in FSAR chapter 8 in the amendment application, as needed, to include the thermal properties of the thermal shunt materials that are used for performing a heat transfer function. The staff also requested the applicant to identify whether the thermal evaluation of the storage system includes or credits the heat transfer function of the thermal shunts, and if so, staff requested the applicant to identify the location in the application or the associated calculation package where the heat transfer function of the thermal shunts is evaluated. If the heat transfer function of the thermal shunts is not specifically evaluated, the staff requested the applicant to provide information to demonstrate that such an evaluation is not needed to ensure adequate thermal performance of the storage system.

In its October 8, 2024, RAI response (ML24283A085, ML24283A087), the applicant provided the thermal properties of the thermal shunt material that is used for performing a heat transfer function. These properties, which are used in the thermal analyses, include thermal conductivity, specific heat capacity, density, and emissivity. The applicant updated FSAR table 8.3-36 in the Amendment 15 application to include these thermal properties. The staff reviewed the thermal shunt material properties and confirmed that the thermal conductivity, specific heat capacity, and density are consistent with established reference values in the ASME BPVC, section II, Part D. The staff noted that the emissivity value, while not available in the ASME BPVC, section II, Part D, is generally consistent with established values from technical literature. Therefore, the staff determined that the thermal properties for the thermal shunt heat transfer material are acceptable.

In its October 8, 2024, RAI response, the applicant also stated that the discussion in FSAR section 4.14.1.1.1 in the Amendment 15 application has been updated to provide clarification of the thermal modeling details associated with the thermal shunts. The applicant identified that the thermal shunts are included in the thermal model used to evaluate the new spent fuel loading pattern that includes thermal shunts, and FSAR section 4.14.1.1.1 of the application is updated accordingly. The staff reviewed the updated information in FSAR section 4.14.1.1.1 of the application and confirmed that it addresses the use of the thermal shunts to enhance the heat transfer performance of the loaded basket for the applicable spent fuel loading pattern that incorporates thermal shunts. The staff determined that the updated information in FSAR section 4.14.1.1.1 provides a reasonable explanation of the incorporation of the thermal shunt material into the thermal model for this spent fuel loading pattern. The NRC staffs detailed evaluation of the thermal modeling for all new spent fuel loading patterns is covered in section 5 of this SER.

Increased PMTC Heat Load The Amendment 15 application proposes to increase the maximum heat load limit for the PMTC by a specified amount. The staff reviewed proprietary FSAR drawing no. 71160-656 to confirm that there are no changes to actual materials and fabrication methods used for constructing the PMTC. The staff also confirmed that a reduction in the tensile properties of the PMTC materials

35 and associated design allowable stresses are adequately addressed in the structural evaluation based on the increase in the PMTC material temperatures associated with the increase in the PMTC heat load limit.

The applicant added footnote d to FSAR table 8.3-2 in the Amendment 15 application; table 8.3-2 provides mechanical properties for forged stainless-steel parts used in the construction of the storage system components. Footnote d specifies requirements for the minimum yield strength at 70°F for the trunnions of the PMTC and for the trunnions and top forging of the LMTC. The staff identified that the required minimum yield strength for the PMTC trunnions at 70°F in footnote d of table 8.3-2 is inconsistent with the corresponding value of the required minimum yield strength for PMTC trunnions in the structural evaluation of the PMTC in FSAR section 3.4.3.3.3 in the application; however, the staff noted that the required minimum yield strength for the PMTC trunnions in footnote d is consistent with the corresponding information in the PMTC body weldment drawing (FSAR drawing no. 71160-656). The staff also confirmed that the required minimum yield strength for the trunnions and top forging of the LMTC at 70°F in footnote d of table 8.3-2 is consistent with the corresponding yield strength requirement for these items in the structural evaluation of the LMTC in FSAR section 3.4.3.3.4 in the application and the LMTC body weldment drawing (FSAR drawing no. 71160-L258).

Therefore, considering the inconsistent requirement for the minimum yield strength for the PMTC trunnions between footnote d of table 8.3-2 and the structural evaluation of the PMTC in FSAR section 3.4.3.3.3 of the application, the staff issued an RAI requesting the applicant to update the Amendment 15 application to reconcile this discrepancy for the PMTC trunnion material. In its RAI response, the applicant stated that this discrepancy is corrected by updating the requirement for the minimum yield strength of the PMTC trunnions in FSAR section 3.4.3.3.3 in the application to be consistent with footnote d of FSAR table 8.3-2 and drawing no. 71160-656. The applicant noted that the structural calculations for the PMTC trunnions are based on the correct value of the required minimum yield strength for these items.

The staff confirmed that FSAR section 3.4.3.3.3 in the application is updated to include the correct value of the required minimum yield strength for the PMTC trunnions at 70°F, consistent with footnote d of FSAR table 8.3-2 and drawing no. 71160-656. Therefore, the staff determined that applicants RAI response is acceptable.

The staff identified that there are no other changes relevant to material properties for the PMTC or other storage system components associated with the proposed increase in PMTC heat load limit. Therefore, the staff determined that the updated information in the Amendment 15 application regarding the PMTC materials is acceptable.

Materials Evaluation Findings Based on the foregoing evaluation, the staff finds that the applicant has met the requirements in 10 CFR 72.236(b) because the applicant described the materials design criteria for SSCs important to safety in sufficient detail to support a safety finding. The staff also finds that the applicant has met the requirements in 10 CFR 72.236(g) because the properties of the materials in the storage system design have been demonstrated to support the safe storage of spent nuclear fuel.

36 Chapter 9 CONFINEMENT EVALUATION There were no requested changes to confinement. As such, no evaluation of this section is included.

Chapter 10 RADIATION PROTECTION EVALUATION The staff reviewed changes to the radiation protection design features, design criteria, and operating procedures in proposed Amendment 15 of the MAGNASTOR storage system to ensure that it will continue to meet the regulatory dose requirements of 10 CFR Part 20, 10 CFR 72.104(a), 10 CFR 72.106(b), 10 CFR 72.212(b), and 10 CFR 72.236(d). The staff also reviewed this proposed amendment to determine whether the MAGNASTOR storage system continues to fulfill the acceptance criteria listed in Section 10 of NUREG-2215, Standard Review Plan for Dry Cask Storage Systems and Facilities. The staff's review is based on information provided in proposed Amendment 15 to the MAGNASTOR system CoC, including the proposed changes to the FSAR and TS. This SER documents the basis for the staff's approval for the proposed changes for radiation protection and meeting the applicable dose limits in 10 CFR Part 72. The applicant requested the following changes relevant to the staffs evaluation of radiation protection, which correspond to changes the staff evaluated from a shielding perspective in chapter 6 of this SER.

Change 1.

The applicant expressed that there is a maximum of 5 percent on the side of the cask and increased to almost 20 percent more along the localized peak in calculation 71160-5031 appendix B for the Oconee LMTC. There is no significant increase to the occupational dose for either uniform or preferential loading patterns due to reduced neutron shield since dose rates during transfer cask is dominated by closure operation and welding time at the top of the cask.

Change 2.

The higher doses allowed by CC7 bound those of the CC8 due to the higher density of concrete used in CC8. Therefore, the occupational and site boundary doses are bounded with the previous evaluation of CC7.

Changes 3, 4, 5 and 6 MAGNASTOR High Heat Occupational Exposure Evaluation -

PWR/BWR Transfer and MAGNASTOR CC high heat site boundary dose rate analysis. This addresses the following 4 changes discussed in the shielding evaluation of this SER: (3) new PWR loading patterns L. M. and N, (4) new BWR loading patterns E and F, (5) increased heat load for the PMTC, and (6) the addition of a recess in the TSC closure lid.

10.1 Radiation Protection Design Criteria and Design Features 10.1.1 Design Criteria The applicant based the estimated exposures for operations and storage on the PWR or BWR contents that result in the highest dose rates. The applicant based the transfer cask exposures on the cask configuration documented in section 5.1.1 of the FSAR. Similarly, the LMTC exposures are based on the configuration documented in section 5.14.2 of the FSAR, which was submitted in a supplement dated October 26, 2024, and assume a dry canister cavity with a

37 supplemental (weld) shield in place for both the maximum and minimum radial lead shield thicknesses of 4 inches and 2.5 inches, respectively.

10.1.2 Design Features As noted in FSAR section 11.2, the principal radiation protection design is based on the placement of penetrations near the edge of the TSC lid to reduce operator exposure and improve access, as well as using the weld shield for work on and around the closure lid. The welding shield reduces operator exposure during the welding, inspection, draining, drying, and helium backfilling operations. The radiation exposure rates at various work locations within the vicinity of a single transfer and concrete cask were determined using the MCNP code and the NAC-CASC (a modified SKYSHINE-III version) code for the dose at the controlled area boundary for a hypothetical ISFSI (i.e., an array of concrete casks) as well as the dose rate as a function of distance from the ISFSI to the controlled area boundary. The applicant based the estimated operator exposures for loading and routine operations using these codes generated bounding dose rate profiles at various distances from the transfer and concrete cask.

10.2 Occupational Dose Evaluations The applicants proprietary calculation 71160-5053, revision 1, included in the applicants August 29, 2023, submittal, provided the estimated occupational radiation exposures to personnel (person-rem) during the loading and transfer to the pad for the MAGNASTOR storage system. Dose rates are taken from the high heat LMTC and concrete overpack (CC) shielding evaluations in proprietary calculations 71160-5031, revision 5; 71160-5032 revision 4; 71160-5033, revision 3; and 71160-5035, revision 2, also included in the applicants August 29, 2023, submittal. Specifically, the evaluation estimates the occupational radiation exposures incurred during fuel loading, TSC sealing, TSC transfer, and cask pad placement.

These estimates appear in FSAR table 11.3-1.

10.2.1 Estimated Dose Due to Loading Operations The applicant estimated doses for loading the high heat and ultra-high heat LMTC using the same method and operations that produced the results appearing in FSAR table 11.3-3.

Table 11.3-3 shows the exposure estimates for the PWR and BWR high heat LMTC systems.

The applicant used the uniform loading patterns (PWR pattern I outlined in FSAR section 5.14.4 and BWR pattern A outlined in FSAR section 5.16.4) to calculate the exposures due to LMTC loading operations, which used higher than allowable total cask heat load. Adding all the subtasks, exposure duration, and average dose rates, table 11.3-3 shows the total person-mrem for loading operations for PWR and BWR spent fuel. The applicant used the estimated dose exposures for the maximum 4-inch and minimum 2.5-inch radial lead shield.

Adding all the subtasks, exposure duration, and average dose rates, table 11.3-3 shows the total estimated exposures for loading operations to be 424, 667, 543, and 1,022 person-mrem for maximum and minimum lead thickness for PWR spent fuel and maximum and minimum lead thickness for BWR spent fuel, respectively, for high heat exposure. For ultra-high heat exposure, the applicant estimated exposures for maximum and minimum lead thickness for PWR spent fuel and maximum and minimum lead thickness for BWR spent fuel to be 658, 1,060, 877, and 1,250 person-mrem, respectively. The staff found that the estimated dose exposures envelop the dose exposures from the 37-assembly undamaged and damaged fuel PWR baskets and the 89-assembly undamaged BWR and 81-assembly damaged fuel BWR baskets. The NRC finds this acceptable because the number of persons allocated to task completion is generally the minimum number of actual operators required for the task and excludes supervisory, health physics, security, and other non-operating personnel.

38 10.2.2 Estimated Dose Due to Routine Operations The applicant considered the anticipated tasks to represent an operational facility in the annual dose evaluations. Exposure due to specific events, such as clearing the material and blocking the air vents, was also considered. FSAR table 11.3-6 shows the storage operation exposures for a 2x10 array of either PWR or BWR concrete casks loaded with TSCs containing bounding high heat and ultra-high heat fuel assembly sources. In table 11.3-6, the applicant estimated the total person-rem average dose per cask to be 46 mrem.

The staff reviewed the estimated dose due to routine operations and found them acceptable based on the applicant's estimated occupancy times for personnel involved in these functions, including the maximum expected total hours per year for any individual and total person-hours per year for all personnel. Also, the applicant estimated the annual collective doses associated with each significant function and each radiation area and showed that the individual doses to workers are below the dose limits specified in 10 CFR 20.1201, Occupational dose limits for adults.

10.3 Off-Site Dose Evaluation In FSAR section 5.1.3, the applicant stated that contributions from concrete casks to site radiation dose are either from the radiation emitted from the concrete cask surface (via skyshine) or a hypothetical release of surface contamination from the TSC. The applicant used NAC-CASC, a modified version of the SKYSHINE-III code, to calculate site boundary dose rates for a single cask or cask array. FSAR section 5.6 provides more detail on the shielding codes.

The analysis presented in section 5.6.5 of the FSAR estimates a total dose of less than 0.1 mrem at 100 meters from a design-basis concrete cask. The analysis demonstrates that the off-site radiological consequences from the release of TSC surface contamination are negligible, and all applicable regulatory criteria are satisfied for an ISFSI array. For any given ISFSI, the specific site will calculate ISFSI-specific allowable dose rates to conform to 10 CFR Part 72. As documented in section 5.6.5 of the FSAR, there is no significant site dose effect from the expected surface contamination of the system. The analyses showed that there is no credible leakage from the system. Therefore, no considerable effluent source can be released from the TSC contents because the TSCs comprise a welded shell, bottom plate, and lid structure, and redundant welded plates cover the vent and drain ports in the lid.

The NRC staff reviewed the evaluations presented in proprietary calculation 71160--5065, revision 1 for MAGNASTOR CoC, which the applicant provided in its submittal dated August 26, 2023. For the high heat site boundary dose rate analysis, the applicant found the reported dose rate calculations and the MCNP shielding analysis model for undamaged fuel, damaged fuel, and non-fuel hardware to be acceptable mainly because site boundary and restricted area boundary dose rates are dominated by the total emissions from the side of the concrete cask, which the average dose can characterize. The applicants evaluations for site exposure evaluations in Appendix C of calculation 71160-5065, revision 1, in tables C1 through C4, included a single cask and a 2x10 array of casks. Each cask in the array is assigned the maximum dose source allowed by the cask loading tables. Combining the maximum cask side and top dose cases estimates the controlled area boundary exposure since the different fuel types produce the highest cask surface dose components. Off-site dose calculations for the initial MAGNASTOR CoC had higher doses and bound the off-site dose rates for this amendment. Therefore, the staff finds that accident dose analyses are bounded by the off-site dose rates determined for the initial MAGNASTOR CoC and are adequate.

39 10.4 As Low as Reasonably Achievable (ALARA)

The onsite collective dose assessment estimates allow the user to perform ALARA evaluations on MAGNASTOR implementation and use and to establish personnel exposure guidelines for operating personnel. The applicant presented the personnel exposure estimates associated with loading and routine operations in FSAR table 11.3-1 through table 11.3-6. FSAR section 11.3 notes that the estimated durations, task sequences, and personnel requirements are based on the MAGNASTOR design features, operational experiences in loading systems of similar design, and operational and equipment improvements based on previous experience.

The applicant provided these estimates to allow the user to perform ALARA evaluations on MAGNASTOR implementation and use and to establish personnel exposure guidelines for operating personnel.

The staff found the ALARA evaluations acceptable because the applicant identified the collective and individual doses associated with all operations involved with placing one full storage container in the storage position according to the related function. Also, the applicant provided estimates of the annual collective and individual doses by multiplying the single-storage container dose by the maximum annual placement rate of containers into storage. This estimation made by the applicant assumed that the same personnel would be involved in the same operations for each container to ensure that the doses do not exceed those allowed by 10 CFR 20.1201(a).

10.5 Evaluation Findings The FSAR sufficiently describes the radiation protection design bases and design criteria for the proposed changes to the MAGNASTOR storage system.

Radiation shielding features are sufficient to meet the radiation protection requirements of 10 CFR Part 20, 10 CFR 72.104, and 10 CFR 72.106 and, therefore, meet the design requirements in 10 CFR 72.236(d).

The FSAR sufficiently describes the means for controlling and limiting occupational exposures within the dose and ALARA requirements of 10 CFR Part 20.

10.6 Conclusion Based on its review of the information presented in the application, the staff concludes that the design of the radiation protection system for the MAGNASTOR storage system with the proposed changes described in the FSAR and TS complies with 10 CFR Part 72 and the applicable design and acceptance criteria have been satisfied. Evaluating the radiation protection system design provides reasonable assurance that the MAGNASTOR storage system will safely store spent fuel. This finding is based on the staffs review considering applicable regulations, appropriate regulatory guides, applicable codes and standards, and accepted engineering practices.

Chapter 11 OPERATION PROCEDURES AND SYSTEMS EVALUATION Chapter 9 of the MAGNASTOR FSAR describes the operating procedures for loading spent fuel, for removing a loaded TSC from a concrete cask, and for the wet unloading of fuel from a TSC. The operating procedures described in the FSAR provide general guidance; the system

40 user (i.e., the ISFSI licensee) will develop more detailed, site-specific procedures for the actual loading, handling, transfer, storage, and unloading of the system at any given site.

In FSAR sections 9.2 and 9.3, the applicant proposed changes to the operating procedures. In addition to minor text flow changes, the applicant added a thermal shunt to allow for short loading patterns. Another addition proposed by the applicant is, should the dryness verification be not met within the first vacuum drying cycle time as defined in the TS, the applicant revised limiting condition for operation 3.1.1 to require that TSCs having certain decay heat levels shall be subjected to specific requirements for vacuum drying and helium backfilling, as well as specific cooling time limits. Further, following closure lid installation of the TSC, there is a restricted time limit applicable when using certain PWR and BWR heat loadings with the LMTC.

The time limit applies following installation of the TSC closure lid to commence operation of the Annulus Circulating Water System or an alternative site-approved annulus flow system operation. The applicant also specified maximum temperature limits of the LMTC annulus outlet flow.

In a supplement to the initial amendment request, the applicant proposed to add two new PWR fuel types to support future site operations that resulted in a change to the FSAR Operating Procedures. Specifically, the applicant proposed to revise the operating procedures to address installation and verification of damaged fuel can and thermal shunt loading (as applicable for the approved loading configuration) with the approved content provisions of the CoC when loading a canister using the LMTC. The staff determined that the changes are acceptable because they include the necessary steps for the described operations and are consistent with the technical analyses in the FSAR.

The staff concludes that the generic procedures and guidance for the operation of the MAGNASTOR system comply with 10 CFR Part 72 and that the applicable acceptance criteria have been satisfied. The evaluation of the operating procedure descriptions provided in the FSAR offers reasonable assurance that the system will enable safe storage of spent fuel. This finding is based on a review that considered the applicable regulations, appropriate regulatory guides, applicable codes and standards, and accepted practices.

Chapter 12 CONDUCT OF OPERATIONS EVALUATION There were no requested changes to conduct of operations. As such, no evaluation of this section is included.

Chapter 13 WASTE MANAGEMENT EVALUATION Analysis of this topic was not included in this evaluation since it is only applicable to a specific license application.

Chapter 14 DECOMMISSIONING EVALUATION Analysis of this topic was not included in this evaluation since it is only applicable to a specific license application.

41 Chapter 15 QUALITY ASSURANCE EVALUATION There were no requested changes to NACs quality assurance program, and none of the changes requested by NAC affect the quality assurance program.

Chapter 16 ACCIDENT ANALYSES EVALUATION The changes associated with accident analysis are discussed and evaluated in chapters 4, 5, 6, 7, 8, and 10 of this SER.

NAC made minor changes in chapter 12 of the FSAR to ensure the new and changed components and specifications are incorporated into the evaluation of accidents. Since NAC showed in its FSAR, and as NRC describes above in this SER, the evaluation results for these changes are either similar to or bounded by previous amendments. Therefore, the revisions requested by NAC do not affect the accident analysis evaluation for the system and do not alter the staffs previous evaluation of the accident analyses for the MAGNASTOR storage system.

Chapter 17 TECHNICAL SPECIFICATIONS AND OPERATING CONTROLS AND LIMITS EVALUATION In its application, NAC proposed incorporation of the changes listed in the Summary section above and associated changes in the TS. The staff reviewed the TS and the associated operating controls and limits to ensure they meet the requirements of 10 CFR Part 72. The staff evaluation of the adequacy of the TS is provided in chapters 5, 6, 7, and 10 of this SER.

Chapter 18 CONCLUSIONS The staff has performed a comprehensive review of the Amendment 15 application, during which it evaluated the following requested changes to the MAGNASTOR storage system:

Add a new variation of the LMTC, Reduced Width LMTC.

Add a new concrete cask design known as CC8. CC8 is based on the CC7 configuration and uses high-density concrete for enhanced shielding.

Increase the maximum system heat load capacity.

Add new PWR loading patterns L, M, and N, (pattern N is a short loading pattern).

Add a Thermal Shunt to allow for short loading patterns.

Add new BWR loading patterns E, and F.

Remove 5 percent burnup "penalty."

Increase PMTC heat load (30kW to 35.5kW including preferential loading pattern B).

Revise previously approved drawings for the concrete cask for CC8, PMTC, and LMTC.

Revise Technical Specification, Appendix A to include increased heat loads and loading patterns.

Revise Technical Specification, Appendix B to include increased heat loads and loading patterns and removal of cool-time tables B2-13 through B2-43.

Add two new PWR fuel types to support future site operations, resulting in revisions to structural, thermal, shielding and criticality chapters of the FSAR.

Modify the TSC lid to allow additional clearance near the top center of the basket.

Revise license drawings 71160-584, 585, 684, and 685.

Correct and clarify principal design criteria, operating procedures, and the acceptance criteria and maintenance program.

42 Based on the statements and representations provided by the applicant in its amendment application, as supplemented, the staff concludes that the changes described above to the MAGNASTOR storage system will meet the requirements of 10 CFR Part 72. Amendment 15 for the MAGNASTOR storage system should be approved.

Issued with CoC No. 1031, Amendment 15 on April 25, 2025.