ML25351A003

Non-Proprietary Final Safety Evaluation Nuclear Design Methodology Topical Report

ML25351A003
Person / Time
Site:	99902079
Issue date:	01/09/2026
From:	Alissa Neuhausen NRC/NRR/DANU/UAL2
To:	Schoedel A Westinghouse
Vechioli-Feliciano L
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ML25351A006	List: ML25351A003 ML25351A004
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Download: ML25351A003 (0)

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OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION WESTINGHOUSE ELECTRIC COMPANY, LLC. EVINCITM - SAFETY EVALUATION OF TOPICAL REPORT NUCLEAR DESIGN METHODOLOGY TOPICAL REPORT, REVISION 0 (EPID L-2024-TOP-0020)

SPONSOR AND SUBMITTAL INFORMATION Sponsor:

Westinghouse Electric Company, LLC.

Sponsor Address:

51 Bridge Street Pittsburgh, PA 15223 Docket/Project No.:

99902079 Submittal Date: May 15, 2024 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML24137A244 Supplement and Request for Additional Information response letter Date(s) and ADAMS Accession No(s): N/A Brief Description of the Topical Report: By letter dated May 15, 2024 (Reference 1),

Westinghouse Electric Company, LLC (Westinghouse) submitted the EVR-LIC-RL-002-P/NP, Nuclear Design Methodology Topical Report, Revision 0, for the U.S. Nuclear Regulatory Commission (NRC) staffs review. The topical report (TR) describes the methodology intended to evaluate neutronics-related characteristics of the eVinci' microreactor. The methodology relies on the Serpent Monte Carlo code (Serpent) to simulate the behavior of the reactor core for steady-state and depletion analyses. It is intended to contribute to eVinci's important core parameters for the safety analysis evaluation model, which is to be described in a future licensing submittal. Serpent is a computer code used for Monte Carlo simulation of neutronics that incorporates continuous-energy cross-sections, reactor component and fuel geometries, temperature effects, and fuel depletion effects, amongst others, to model the neutron interactions in the core and determine criticality and kinetics parameters of interest. The subject TR provides an overview of the eVinci' microreactor design, a core design description, modeling and calculational details and assumptions used in Serpent, and plans for its verification and validation (V&V).

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION REGULATORY EVALUATION Regulatory Basis:

The regulations under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of Production and Utilization Facilities, Appendix A, General Design Criteria for Nuclear Power Plants, provide general design criteria for water-cooled nuclear power plants similar to those historically licensed by the NRC. Under the provisions of 10 CFR Part 50 and Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants, applicants for a construction permit (CP), operating license (OL), design certification (DC), combined license (COL), standard design approval (SDA), or manufacturing license (ML) must include the principal design criteria (PDC) for the proposed facility.

Regulatory Requirements The following regulatory requirements are applicable to the NRC staffs review of EVR-LIC-RL-002-P:

Paragraph 50.34(a)(4) of 10 CFR requires, in part, that each application for a CP shall include a preliminary safety analysis report. The minimum information to be included shall include a preliminary analysis and evaluation of the design and performance of structures, systems, and components (SSCs) of the facility with the objective of assessing the risk to public health and safety resulting from operation of the facility and including determination of the margins of safety during normal operations and transient conditions anticipated during the life of the facility, and the adequacy of the SSCs to provide for the prevention of accidents and mitigation of the consequences of accidents.

Paragraph 50.34(b)(4) of 10 CFR requires, in part, that applications for a CP include PDCs for the facility. An OL would reference a CP and therefore would include the PDCs.

Paragraph 50.34(a)(8) of 10 CFR requires, in part, that an applicant for a CP describe the research program to resolve any safety questions associated with safety features or components. Such research and development may include obtaining sufficient data pertaining to the safety features of the design to assess the analytical tools used for safety analysis in accordance with 10 CFR 50.43(e)(1)(iii).

Paragraph 50.43(e)(1)(iii) of 10 CFR requires, in part, that sufficient data exist on the safety features of the design to assess the analytical tools used for safety analyses over a sufficient range of normal operating conditions, transient conditions, and specified accident sequences, including equilibrium core conditions.

Paragraph 52.47(a)(3)(i) of 10 CFR requires, in part, that the application for a DC includes the PDC for the facility.

Paragraph 52.79 (a)(4)(i) of 10 CFR requires, in part, that the application for a COL includes the PDCs for the facility.

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION Paragraph 52.137(a)(3)(i) of 10 CFR requires, in part, that the application for an SDA includes the PDCs for the facility.

Paragraph 52.157(a) of 10 CFR requires, in part, that the application for an ML includes the PDCs for the reactor to be manufactured.

The eVinciTM microreactor core design methods and analyses provide input to the eVinciTM microreactor safety analyses.

Regulatory Guide (RG) 1.231, Revision 0, Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Safety-Related Applications for Nuclear Power Plants, (ML16126A183) provides methods that the NRC staff considers acceptable in meeting the regulatory requirements for acceptance and dedication of commercial-grade design and analysis computer programs used in safety-related applications for nuclear power plants. This RG endorses Revision 1 of Electric Power Research Institute (EPRI) Technical Report 1025243, Plant Engineering: Guideline for the Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Nuclear Safety-Related Applications, with respect to the acceptance of a commercial-grade design and analysis computer programs associated with basic components for nuclear power plants.

Guidance associated with the performance demonstration requirements of 10 CFR 50.43(e) for advanced non-light water reactors is provided by the NRC in an enclosure to A Regulatory Review Roadmap for Non-Light Water Reactors, (ML17312B567) and Interim Staff Guidance (ISG) DANU-ISG-2022-01, Review of Risk-Informed, Technology-Inclusive Advanced Reactor Applications - Roadmap (ML23277A225).

Principal Design Criteria The TR EVR-LIC-RL-001-A, Revision 1, Principal Design Criteria Topical Report, (ML24353A097) dated December 17, 2024, provides PDCs for the eVinciTM microreactor design that were reviewed and approved by the NRC staff in the associated safety evaluation (SE)

(ML24283A133). The PDCs that have been identified as relating to the methodologies described in TR EVR-LIC-RL-002 include:

PDC 1, Quality Standards and Records - Safety significant SSCs shall be designed, fabricated, erected, and tested to quality standards commensurate with the safety significance of the functions to be performed. Where generally recognized codes and standards are used, they shall be identified and evaluated to determine their applicability, adequacy, and sufficiency and shall be supplemented or modified as necessary to assure a quality product in keeping with the safety significant function. A quality assurance program shall be established and implemented in order to provide reasonable assurance that these SSCs will satisfactorily perform their safety significant functions. Appropriate records of the design, fabrication, erection, and testing of safety significant SSCs shall be maintained by or under the control of the nuclear power unit licensee for an appropriate period of time.

PDC 10, Reactor Design - The reactor system and associated heat removal, control, and protection systems (along with any SSCs supporting the reactor system and associated heat removal, control, and protection systems safety function(s)) shall be

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION designed with appropriate margin to ensure that specified acceptable system radionuclide release design limits are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences (AOOs).

PDC 11, Reactor Inherent Protection - The reactor core and associated SSCs that contribute to reactivity feedback shall be designed so that, in the power operating range, the net effect of the prompt inherent nuclear feedback characteristics tends to compensate for a rapid increase in reactivity.

PDC 12, Suppression of Reactor Power Oscillations - The reactor core; associated structures; and associated coolant, control, and protection systems shall be designed to ensure that power oscillations that can result in conditions exceeding specified acceptable system radionuclide release design limits are not possible or can be reliably and readily detected and suppressed.

PDC 16, Functional Containment - A functional containment shall be provided to control the release of radioactivity to the environment and to ensure that the safety significant functional containment design conditions are not exceeded for as long as licensing basis event (LBE) conditions require.

PDC 25, Protection System Requirements for Reactivity Control Malfunctions - The protection system shall be designed to ensure that specified acceptable system radionuclide release design limits are not exceeded during any AOO, accounting for a single malfunction of the reactivity control systems.

PDC 26, Reactivity Control - Reactivity control shall be provided. Reactivity control shall provide:

(1) A means of inserting negative reactivity at a sufficient rate and amount to assure, with appropriate margin for malfunctions, that the specified acceptable system radionuclide release design limits and the reactor helium pressure boundary design limits are not exceeded, and safe shutdown is achieved and maintained during normal operation, including AOOs.

(2) A means, which is independent and diverse from the other(s), shall be capable of controlling the rate of reactivity changes resulting from planned, normal power changes to assure that the specified acceptable system radionuclide release design limits and the reactor helium pressure boundary design limits are not exceeded.

(3) A means of inserting negative reactivity at a sufficient rate and amount to assure, with appropriate margin for malfunctions, that the capability to cool the core is maintained and a means of shutting down the reactor and maintaining, at a minimum, a safe shutdown condition following an LBE.

(4) A means for holding the reactor shutdown under conditions that allow for interventions such as fuel loading, inspection, and repair.

PDC 28, Reactivity Limits - Any SSCs that provide reactivity control shall be designed with appropriate limits on the potential amount and rate of reactivity increase to ensure

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION that the effects of postulated reactivity accidents can neither: (1) result in damage to the reactor system greater than limited local yielding nor (2) sufficiently disturb the core, its support structures, or other reactor system components to significantly impair the capability to cool the core.

The NRC staffs SE on the eVinciTM microreactor PDCs includes a condition that applications referencing the TR must confirm that the PDCs remain appropriate for the design (ML24283A133). Therefore, the NRC staff determined that the list of PDCs identified above also needs to be confirmed to ensure conformance with the TR on eVinciTM microreactor PDCs.

Accordingly, the NRC staff imposed Condition 1 requiring an applicant referencing EVR-LIC-RL-002 to confirm or update the regulatory basis relevant to the use of the described methods.

TECHNICAL EVALUATION Scope of NRC Review As reflected in TR section 1.4, Request for NRC, this SE focuses on the NRC staffs review of the software and analysis methods described in TR section 5.0, Analysis Methods and Assumptions, the code and solution verification plan described in TR section 6.1, V&V Plan Verification, and the existing benchmark data combined with the proposed experiments discussed in TR section 6.2, V&V Plan Validation. Specifically, the NRC staff evaluated the acceptability of Serpent, its application, and the plans for V&V as appropriate means to perform nuclear physics analysis and to provide inputs, as described, for future eVinciTM microreactor safety analyses. The methods described in the TR are preliminary and V&V of these models has not been completed at the time of this review. Therefore, the NRC staff makes no finding concerning the results of the analyses performed using these methods, including the use of those results to demonstrate conformance with PDCs or any other regulatory requirements. The NRC staff will review those, as requested, as part of future TRs or license applications. As such, the NRC staff imposes Limitation 1 in the SE section, Limitations and Conditions. Further, TR section 1.0, Introduction; section 2.0, Summary of the eVinciTM Microreactor Design and Facility Description; section 3.0, Background on Nuclear Design Analysis; section 4.0, eVinciTM Microreactor Core Design Description; section 7.0, Summary; appendix A, Demonstration of Core Analysis; appendix B, Demonstration of Verification; and appendix C, Demonstration of Validation; which include the description of the eVinciTM microreactor design, were considered by the NRC staff to inform aspects of the review of section 5.0, Analysis, Methods, and Assumptions, and section 6.0, V&V Plan, but were not within the scope of this SE.

eVinciTM Microreactor Design Overview As discussed in TR section 2, the proposed conceptual design for the eVinciTM microreactor is a high temperature, heat pipe-cooled, thermal spectrum, 15 MWth reactor. The reactor core is fueled with high-assay, low-enriched uranium tri-structural isotropic (TRISO) fuel and consists of horizontal hexagonal graphite blocks with channels for fuel, burnable absorbers, alkali metal heat pipes, and shutdown rods. The core is surrounded by a radial reflector that houses control drums designed to manipulate core reactivity and allow the otherwise subcritical core to achieve criticality when the drums are specifically oriented. The control drums and shutdown rods provide independent, diverse means of achieving sub-criticality (shutdown).

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION The reactor core and reflector are contained within a canister that makes up an element of the functional containment design, with the layers of the TRISO fuel particles representing the other physical barriers. The vessel is filled with helium gas to enhance decay heat removal, which can be accomplished through the core block, radial reflector, core containment system (vessel), and shielding. Reactor heat produced for power generation (15 MWth) will be removed through alkali metal heat pipes and a primary heat exchanger (PHX) and will be converted to electric power

(~5MWe) through an open-air Brayton cycle power conversion system (PCS).

The eVinciTM microreactor concept is designed such that the reactor canister and core, and the support and PCS, can be transported in shipping containers by truck, rail, or waterway to an approved reactor site that has been appropriately constructed and prepared. Following installation at the site, criticality testing and subsequent operation will commence and continue under remote monitoring with limited on-site operations, maintenance, and security staff until the core reaches end-of-life. Following a reactors operation, a replacement reactor can be shipped to and installed at the site in its place, while the spent reactor is allowed to cool until it is ultimately removed from the site for refurbishment, refueling, and/or decommissioning, as appropriate.

1.0 Overview of Nuclear Design Methodology Section 5 of the TR presents the nuclear design methodology proposed for analyzing the eVinciTM microreactor. The Serpent code, which is used for analysis of the eVinciTM microreactor, is introduced and the Serpent eVinciTM microreactor simulation configuration is described. Details for calculating important reactor core physics parameters and key safety parameters (e.g., keff, control drum worth, reactivity coefficients, and power distribution) are described.

The NRC staff reviewed the information provided and assessed TR section 5.1, Serpent Monte Carlo Computer Code; section 5.2, eVinciTM Microreactor Serpent Simulation Configuration; section 5.3, Types of Analyses Performed; and consolidated subsections. The following SE sections detail the NRC staffs review of the methodology described.

1.1 Serpent Monte Carlo Computer Code The NRC staff reviewed the use of Serpent to perform reactor physics calculations, as well as to perform neutron and photon transport calculations for dose rate and shielding calculations.

Particularly, the NRC staff assessed Serpents capabilities for geometry and particle tracking, interaction physics, and depletion calculation.

With respect to geometry and particle tracking, the NRC staff considered that Serpent allows for flexibility in 2D and 3D spaces, and that it contains an independent routine to randomly distribute fuel particles within a fixed volume, which enables modeling of fuel components containing TRISO fuel. The NRC staff finds that Serpent provides modeling flexibility with adequate spatial specificity to accurately represent the eVinciTM microreactor core design.

Serpent aligns with well understood principles of nuclear physics and engineering. Therefore, Serpent will provide accurate core performance calculations.

The interaction physics in Serpent are based on classical collision kinematics, evaluated nuclear data file (ENDF) reaction laws, and probabilistic tables. Continuous-energy cross-sections are read from an ACE (A Compact ENDF) formatted data library. The NRC staff noted that Serpent

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION CONCLUSION The NRC staff has determined that Westinghouses TR provides an acceptable methodology for steady-state and depletion analysis of the eVinci' microreactor because: (1) the underlying calculational methods and techniques used in the model are acceptable approaches to represent the physical phenomena of interest to be modeled because the calculational methods and techniques rely on sound principles of engineering and adequately reflect the underlying physics being modelled, as appropriate; (2) the discussion includes appropriate consideration of model and design uncertainty; and (3) subsequent development, finalization, verification, validation, and implementation of the described methods and models will be completed in accordance with applicable technical knowledge and expertise. This approval is subject to the limitations and conditions discussed above. Accordingly, the NRC staff concludes that Westinghouses TR can be used to support reactor licensing applications for permits, licenses, certifications, or approvals under 10 CFR Parts 50 or 52.

OFFICIAL USE ONLY PROPRIETARY INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION REFERENCES

1. Westinghouse Electric Company (Westinghouse). Submittal of the Westinghouse Nuclear Design Methodology Topical Report for the eVinci' Microreactor (EVR-LIC-RL-002-P/NP), Revision 0, (ML24137A245), May 15, 2024.

2. Westinghouse Submittal of Westinghouse Principal Design Criteria Topical Report (EVR-LIC-RL-001-P-A/NP-A), Revision 1, (ML24353A097), December 17, 2024.

3. U.S. Nuclear Regulatory Commission (NRC), Report for the Regulatory Audit regarding Westinghouse Electric Companys Nuclear Design Methodology Report (ML25184A372), August 29, 2025.

4. U.S. NRC, Westinghouse Electric Company, LLC. - Final Safety Evaluation for Topical Report EVR-LIC-001-P/NP, Westinghouse Principal Design Criteria Topical Report for the eVinci' Microreactor (ML24283A133), October 16, 2024.

5. U.S. NRC, Regulatory Guide 1.231, Revision 0, Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Safety-Related Applications for Nuclear Power Plants, (ML16126A183), January 2017.

6. Electric Power Research Institute, 2013 Technical Report Revision 1 of 1025243, Plant Engineering: Guideline for the Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Nuclear Safety-Related Applications, (ML14085A084),

December 2013.

7. U.S. NRC, A Regulatory Review Roadmap for Non-Light Water Reactors, (ML17312B567), December 2017.

8. U.S. NRC, Interim Staff Guidance: Review of Risk-Informed, Technology-Inclusive Advanced Reactor Applications - Roadmap, (ML23277A225), 88 FR 33924, May 25, 2023.

9. Westinghouse EVR-RXS-GL-008, Commercial Grade Dedication of Serpent 2 for Nuclear Design Analysis, Revision 0, (ML25323A314), November 19, 2025.

Principal Contributors: Ayesha Athar, NRR Dan Beacon, R-1 Robert Mikouchi-Lopez, NRR Andrew Bielen, RES Date: January 9, 2026