ML25064A595
| ML25064A595 | |
| Person / Time | |
|---|---|
| Site: | 99902071 |
| Issue date: | 05/06/2025 |
| From: | Stephen Philpott NRC/NRR/DANU/UNPL |
| To: | Kalinousky D X-Energy |
| McGovern D | |
| References | |
| EPID L-2024-TOP?0019 | |
| Download: ML25064A595 (1) | |
Text
Mr. Douglas Kalinousky Licensing Manager, X Energy, LLC 530 Gaither Road, Suite 700 Rockville, MD 20850
SUBJECT:
U.S. NUCLEAR REGULATORY COMMISSIONS DRAFT SAFETY EVALUATION FOR X ENERGY LLCS XE100 LICENSING TOPICAL REPORT MECHANISTIC SOURCE TERM APPROACH, REVISION 3 (EPID NO: L-2024-TOP0019)
Dear Mr. Kalinousky:
By letter dated March 14, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25073A093), X Energy, LLC., (Xenergy) submitted Revision 3 of its Xe100 Licensing Topical Report Mechanistic Source Term Approach to the U.S. Nuclear Regulatory Commission (NRC) staff for review. This topical report describes a mechanistic methodology intended to evaluate the radionuclide source terms used in the preliminary safety analysis in support of the Xe100 design.
The enclosed draft safety evaluation for the aforementioned topical report is being provided to the Advisory Committee for Reactor Safeguards (ACRS) to support the upcoming ACRS Subcommittee meeting, scheduled for June 3, 2025.
If you have any questions, please contact Denise McGovern at (301) 4150681 or via email at Denise.McGovern@nrc.gov.
Sincerely, Stephen Philpott, Acting Chief Advanced Reactor Licensing Branch 2 Division of Advanced Reactors and Non-Power Production and Utilization Facilities Office of Nuclear Reactor Regulation Project No. 99902071
Enclosure:
As stated cc: Distribution via XEnergy Xe100 GovDelivery May 6, 2025 Signed by Philpott, Stephen on 05/06/25
ML25064A595 NRR043 OFFICE NRR/DANU/UAL2:PM NRR/DANU/UAL2:LA NRR/DANU/UTB1:BC NAME DMcGovern CSmith TTate DATE 3/7/2025 3/11/2025 4/29/2025 OFFICE OGC/NLO NRR/DANU/UAL2:BC NAME JEzell SPhilpott DATE 4/25/2025 5/6/2025
Enclosure XENERGY - DRAFT SAFETY EVALUATION OF TOPICAL REPORT XE100 LICENSING TOPICAL REPORT MECHANISTIC SOURCE TERM APPROACH, REVISION 3 (EPID L-2024TOP0019)
SPONSOR AND SUBMITTAL INFORMATION Sponsor:
X Energy, LLC. (Xenergy)
Sponsor Address:
X Energy, LLC.
530 Gaither Road, Suite 700 Rockville, MD 20850 Docket /Project No.:
99902071 Submittal Date:
March 14, 2025 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: Package ML25073A093 Supplement and Request for Additional Information response letter Date(s) and ADAMS Accession No(s): N/A Brief Description of the Topical Report: On May 10, 2024, Xenergy, LLC, (Xenergy) submitted topical report (TR) 000632, Xe-100 Licensing Topical Report Mechanistic Source Term Approach, Revision 2, (hereafter referred to as MST TR) for review by the U.S. Nuclear Regulatory Commission (NRC) staff (ADAMS Accession No. ML24131A146 (package)).
Xenergy submitted Revision 3 of this TR on March 14, 2025 (ML25073A093 (package)). The TR describes a mechanistic methodology intended to evaluate the radionuclide source terms used in the preliminary safety analysis in support of the Xe100 design described in Section 3 of the TR. The approach uses the XSTERM computer code to quantify the Xe100 radionuclide source terms that encompass a range of phenomena, including TRistructural ISOtropic (TRISO) particle failure, solid and gaseous fission product transport, dust production, helium pressure boundary radionuclide transport/deposition/liftoff, core corrosion, and tritium production and transport for dose calculations determined from analysis models. This mechanistic source term (MST) approach is part of the Xe100 transient and safety analysis methodology (TSAM) as described in the TSAM TR, Xe-100 Licensing Topical Report Transient Safety Analysis and Methods, Revision 2.1 Specifically, the MST uses input from the TSAM and provides predicted 1 The NRC staff notes that the TR references older revisions of certain TRs that the NRC staff is reviewing concurrently. In this SE, the NRC staff references the most recent revision of the TRs submitted to the NRC for review.
dose consequences used to support the licensing of the Xe100 design. This TR describes an overview of the Xe100 design, details regarding the MST approach and models, and plans for verification and validation (V&V) of the methods. On August 29, 2024, the NRC staff transmitted an audit plan to Xenergy (ML24236A768), and subsequently conducted an audit of materials related to the TR.
REGULATORY EVALUATION Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a)(1)(ii)(D) requires, in part, that an applicant for a construction permit (CP) perform an evaluation and analysis of a postulated fission product release to evaluate the offsite radiological consequences. This evaluation must determine that:
An individual located at any point on the exclusion area boundary for any 2-hour period following the onset of the postulated fission product release, would not receive a radiation dose in excess of 25 rem total effective dose equivalent (TEDE).
An individual located at any point on the outer boundary of the low population zone, who is exposed to the radioactive cloud resulting from the postulated fission product release (during the entire period of its passage) would not receive a radiation dose in excess of 25 rem TEDE.
Under 10 CFR 50.34(a)(4) an applicant for a CP must perform a preliminary analysis and evaluation of the design and performance of structures, systems, and components (SSCs) with the objective of assessing the risk to public health and safety resulting from the operation of the facility and including the determination of margins of safety during normal operations and transient conditions anticipated during the life of the facility. These analyses are associated with the principal design criteria (PDC) and associated SSC design bases, which are required by 10 CFR 50.34(a)(3). Revision 3 of the Xe100 PDC TR has been determined to be suitable for referencing in future licensing applications for the Xe100 design, subject to the limitations documented in the NRC staffs safety evaluation (ML24319A155). Based on the Xe100 design description, provided in Section 3 of the TR, Overview of the Xe-100 Plant Design, the NRC staff identified the following PDC, including required functional design criteria (RFDC), as applicable to its review of the MST TR:
Xe100 PDC 10, Reactor design, requires that the reactor system and associated heat removal, control, and protection systems be designed with appropriate margin such that specified acceptable system radionuclide release design limits are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences. Demonstrating adequate reactor design generally includes, in part, the use of safety analysis methodologies (potentially including source term and consequence analysis methodologies).
Xe100 PDC RFDC 16, Functional containment design, requires that the design of the reactor fuel particles and pebbles provide barriers as part of the reactor functional containment to control the release of radioactivity to the environment to ensure that the functional containment design limit is not exceeded during design basis events and design basis accidents (DBAs). Demonstrating the functional containment design generally includes, in part, the use of safety analysis methodologies (potentially including source term and consequence analysis methodologies).
Xe100 PDC 19, Control room, requires, in part, that adequate radiation protection be provided to permit access and occupancy of the control room during anticipated operational occurrences and design basis events without personnel receiving radiation exposures in excess of 5 rem TEDE. Demonstrating this criterion generally involves the use of transient and safety analysis methodologies (including source term and radiological consequence analysis).
Under 10 CFR 50.34(a)(8) an applicant for a CP must identify the systems, structures or components of the facility, if any, which require research and development to confirm the adequacy of their design and describe the research program that will be conducted to resolve any safety questions associated with such systems, structures, or components. Such research and development may include obtaining sufficient data regarding 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).
Regulatory Guide (RG) 1.233, Revision 0, Guidance for a Technology-Inclusive, Risk-Informed, and Performance-Based Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors, (ML20091L698) provides the NRC staffs guidance for one way that applicants can perform a safety analysis using a technology-inclusive, risk-informed, and performance-based methodology to inform the licensing basis and content of applications for non-light-water reactors (LWRs). The RG endorses Nuclear Energy Institute (NEI) 1804, Revision 1, Risk-Informed Performance-Based Guidance for Non-Light Water Reactor Licensing Basis Development, with clarifications as one acceptable method for informing the licensing basis and determining the appropriate scope and level of detail for parts of applications for licenses, certifications, and approvals for non-LWRs.
The approach in RG 1.233 and NEI 18-04 is often called the Licensing Modernization Project (LMP).
RG 1.253, Revision 0, Guidance for a Technology-Inclusive Content-of-Application Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors, (ML23269A222) endorses NEI 21-07, Revision 1, Technology-Inclusive Guidance for Non-Light Water Reactors, Safety Analysis Report Content: For Applicants Using the NEI 18-04 Methodology, with additions and clarifications as one acceptable methodology for use in developing certain portions of the Safety Analysis Report for a non-LWR application that follows the LMP approach.
TECHNICAL EVALUATION 1.
Scope of the NRC Staffs Review Xenergy requested the NRC staffs review and approval of MST TR which describes MST models used to determine radionuclide transport phenomena and estimates MSTs to support the preliminary analysis and evaluation of the Xe100 design described in the TR. MST TR, Section 4.2 states that the MST models are used to calculate dose consequences for licensing basis events (LBEs) identified by probabilistic risk assessment and to develop the frequency-consequence curves specified in NEI 18-04. The MST models are also used to calculate the dose consequence for deterministically evaluated DBAs.
Accordingly, the NRC staff reviewed the MST modeling approach used to address radionuclide transport phenomena and estimate the MSTs to support the preliminary analysis and evaluation of the Xe100 for LBEs including DBAs. Specifically, this SE documents the NRC staffs review of the XSTERM code to develop the radiological source terms for use in future Xe-100 licensing applications.
However, the NRC staff notes that because the Xe100 design is preliminary, and development and V&V of the methods described in the TR and other related TRs (including TSAM) by Xenergy is in progress or planned, the NRC staffs review is limited to and focuses on high-level physical phenomena of interest and whether the analysis approach and methods described can reasonably support future licensing actions for deployment of the Xe100 design.
The NRC staffs evaluation of the models within XSTERM for acceptability will be conducted during the review of an application that relies on the results of XSTERM evaluations.
The numbering of sections 2 through 6 below, in this SE, follows the numbering in the TR.
2.
Regulatory Requirements and Guidance Section 2, Overview of Regulatory Requirements and Guidance, of MST TR discusses the regulatory requirements and guidance relevant to the MST analysis. TR Section 2 identifies a wide spectrum of regulatory requirements (including all of 10 CFR 50.34(a) for CPs, all of 10 CFR 50.34(b) for operating licenses, rules under 10 CFR 52, Licenses, Certifications, and Approvals for Nuclear Power Plants, and 10 CFR 100, Reactor Site Criteria). The NRC staff identified the regulatory basis in the Section Regulatory Evaluation, above and focused on the requirements applicable to preliminary safety analyses for a CP application (including relevant PDCs). The NRC staff focused on requirements applicable to a CP because TR Section 1.5, Outcome Objectives, states that, Xenergy is requesting NRC review and approval [] to support the preliminary analysis and evaluation of the Xe100. The NRC staff does not make any determinations on the information in Section 2 of the TR.
3.
Xe100 Design Overview Section 3, Overview of the Xe-100 Plant Design, of MST TR provides an overview of the Xe100 preliminary design considered in this SE. The design features of particular interest for the source term analysis are the barriers to radionuclide release described in Section 3.2, Key Features of the Xe-100, as:
Fuel particle kernel (Uranium Oxycarbide (UCO)) within the TRISO fuel particles Silicon Carbide and Pyrolytic Carbon coatings applied to the fuel kernel Fuel matrix and fuel free zone of the fuel pebble Reactor Helium Pressure Boundary (HPB)
Reactor Building The NRC staff notes that, while the reactor building is part of the functional containment in the Xe100 design, Section 4.1 of the TR clarifies that the radionuclide deposition and holdup in the reactor building is not credited in the MST TR. The NRC staff considers the information describing the credited barriers to fission product release in its assessment of the MST methodology. While Section 3 provides important context to the TR, the NRC staff does not evaluate the design information provided in Section 3 or make determinations on the acceptability of the information.
4.
Mechanistic Source Term Approach MST TR, Section 4 states that Xenergy developed a comprehensive set of MST models for the quantification of the Xe100 source terms and dose calculations. Section 4.2, XSTERM, clarifies that the models are captured in a comprehensive software code, XSTERM, which has subroutines to address the following phenomena:
Thermal hydraulic modeling and transient analysis (e.g., reactor physics and thermal hydraulic simulations)
Radionuclide production, decay, transmutation, and transport in the fuel spheres Radionuclide retention, transport and release from the fuel pebbles into the helium coolant Transport and distribution of radionuclides, including dust effects, within the HPB Transport of radionuclides into the reactor building and the environment Point-source to point-receptor dose calculations The NRC staff concludes that MST TR, Section 4 provides a reasonable summary of high-level modeling requirements needed for the MST analysis because it describes phenomena associated with the fission product barriers that constitute the Xe100 functional containment described in Section 3 of the TR. The underlying methodology and theory upon which XSTERM subroutines are based is addressed in TR Section 5, Mechanistic Source Term Models, and its associated Appendixes. The NRC staffs review of the MST methodology as implemented in XSTERM, is provided in Section 5 of this SE.
5.
Mechanistic Source Term Models MST TR, Section 5 describes the Xe100 MST models associated with the barriers to radionuclide release. TR Section 5.1, Xe-100 MST Models, summarizes the following models:
TR Section 5.1.1 describes the TRISO Particle Failure Probability Model (FPM) as calculating TRISO-coated particle failures probabilities during normal LBE conditions.
TR Section 5.1.2 describes the Solids Product Transport Calculations Model (SOLM) as calculating the production, decay, transmutation, transport and leakage of gaseous and solid fission products from fuel pebbles to the helium pressure boundary under steady-state and LBE conditions.
TR Section 5.1.3 describes the Thermodynamics Calculation Model (THM) as calculating detailed temperature distributions in fuel pebbles, TRISO-coated fuel particles, and all core components (reflectors, core barrel, and reactor pressure vessel) during normal and LBE conditions.
TR Section 5.1.4 describes the Steady-State Gaseous Fission Products Transport Calculations Model (GASM) as calculating gaseous fractional release-tobirth ratios of fission product release from TRISO-coated fuel particles and pebbles into the primary circuit coolant gas.
TR Section 5.1.5 describes the Dust Production Rate Calculations Model (DUSTM) as calculating the graphite and metallic dust production rates for the Xe100. Dust generation sources include pebble-topebble and pebble-toreflector interactions, pebble-topipe interaction associated with the fuel handling system, and control-rod-toreflector interactions. The dust production model constant is calculated for German Arbeitsgemeinschaft Versuchs Reaktor (AVR) and the resultant calibration is assumed by Xenergy to be reactor independent and applied to the Xe100.
TR Section 5.1.6 describes the Helium Pressure Boundary Model (HPBM) as calculating the deposition and resuspension of fission products on helium pressure boundary surfaces and the release of fission products into the reactor building.
TR Section 5.1.7 describes the Core Corrosion Model (CORRM) as simulating the mass transport and chemical reaction aspects of the core corrosion phenomena encountered during water or air ingress into the core of the Xe100.
TR Section 5.1.8 describes the Tritium Production and Transport Model (TRITM) as calculating the overall tritium mass balance in the Xe100. TR Section 5.1.8 clarifies that this model is under development.
TR Section 5.1.9 describes the 2D Point Kinetics Core Simulation Model (KSIM) as a 2D axisymmetric geometry to simulate the transient behavior of the Xe100 core. TR Section 5.1.9 further states that the point kinetics approach is applied to each cell individually (there are many cells in the model) and that the flux profile between timesteps can be reshaped using a diffusion kernel.
As discussed above, the NRC staff did not perform a detailed evaluation of the models described in TR Section 5 and its associated Appendixes. However, the NRC staff notes the following evaluation considerations necessary for the MST methodology for CP applications:
The calculations performed in THM are of high importance to MST evaluations and the NRC staffs regulatory review because fission product release from the associated structures is expected to be diffusion dominant and diffusion is temperature dependent.
Therefore, use of THM for analyses supporting a Xe100 licensing application requires justification by the applicant.
The point kinetics model (KSIM) in the TR appears to be different than standard point kinetics approaches which are normally 0D, utilize a single eigenvalue, and lack diffusion coupling (Reference 1). Therefore, use of KSIM for analyses supporting a Xe100 CP application requires justification by the applicant.
The Xe100 tritium model (TRITM) is under development. The contribution of tritium to radiological consequence analysis is expected to be analyzed and justified by the applicant as part of a risk-informed Xe100 CP application.
For the remaining models (FPM, SOLM, GASM, DUSTM, HPBM, CORRM), the NRC staff determined that the models address phenomena needed to predict the MST to support the preliminary analysis and evaluation of the Xe100 design described in the TR. This observation is based on the following:
o The models rely on previous modeling and operational experience of gas-cooled reactors such as the German AVR.
o Based on the NRC staffs experience with LWR and non-LWR source term analysis, the NRC staff did not identify significant gaps in the MST models.
o TR Section 4.2 states that the source term modeling described in the TR may be revised to improve accuracy and performance, but the asdocumented base theory is not expected to change.
MST TR, sections 4.1 and 5.2 clarify that the Xe100 MST methodology does not credit decay (holdup) or deposition in the reactor building. TR Section 5.3 states that: (1) an atmospheric dispersion and dose calculation is used to calculate doses from DBAs, (2) a generic set of dispersion factors has been developed to provide conservative values, and (3) the calculation of atmospheric dispersion and dose is performed in accordance with TR 007116, Xe-100 Licensing Topical Report Atmospheric Dispersion and Dose Calculation Methodology, Revision 2, (ML23268A454). The NRC staffs SE for TR 007116 includes limitations and conditions requiring, in part, justification of the generic dispersion factors for applicability to the site (ML24242A251).
6.
MST Models V&V Plans MST TR, Section 6 describes the XSTERM preliminary V&V plans and states that: (1) code V&V effort is underway to ensure XSTERM is qualified to support the final safety analysis, (2) the validation plans are developed to cover high and medium ranked phenomena that are identified through a Phenomena Identification and Ranking Table (PIRT) process, and (3) the phenomena modeled by XSTERM as shown in the validation plan were extracted from an earlier version of the PIRT. TR Section 6 provides an overview of the V&V plan consisting of the following phases:
Phase 1: Activity release and transport Phase 2: Pebble bed and reactor structures temperature and power changes Phase 3: Dust production Phase 4: Exposure to oxidizing environments Based on the information described above, the NRC staff concludes that the process is acceptable because the identification of code assessment requirements (i.e., V&V) through the PIRT process is an established approach that is consistent with the guidance provided in RG 1.203, Section 1.1.4, Step 4 Identify and Rank Key Phenomena and Processes, (ML053500170). However, the NRC staff are unable to assess the adequacy of the V&V plan because: (1) the validation plan is not based on the latest PIRT information, (2) the TR does not contain information describing the knowledge level of the associated phenomena identified in the PIRT, and (3) the plan is preliminary and subject to change. To address requirements associated with 10 CFR 50.34(a)(8), the NRC staff expects that a completed validation plan, based on a PIRT that identifies importance and knowledge levels of key phenomena, will be provided prior to the issuance of a CP in accordance with 10 CFR 50.35(a)(3).
LIMITATIONS AND CONDITIONS The NRC staff has not identified any limitations or conditions because the scope of the NRC staffs review and approval is limited to determining that the topical report provides a reasonable plan for the development of the MST methodology and does not provide approval on the use of XSTERM as described in the TR.
CONCLUSION The NRC staff concludes that Xenergys TR, Xe-100 Licensing Topical Report Mechanistic Source Term Approach, Revision 2, provides a reasonable plan for the development of the MST methodology. This conclusion is based on the following:
The FPM, SOLM, GASM, DUSTM, HPBM, CORRM models in XSTERM appear to cover the phenomena needed to predict the MST to support the preliminary analysis and evaluation of the Xe100 design (see SE Section 5).
The TR describes an acceptable approach to V&V (see SE Section 6).
However, the NRC staff make no conclusions regarding the acceptability of the models in XSTERM for the MST analyses of the Xe100 because:
Models within XSTERM are still under development.
A detailed technical review of the individual models was not completed.
Details regarding key phenomena identification and associated knowledge levels are not provided in the TR.
The models and associated validation plans are preliminary and subject to change.
The NRC staff expects that a detailed technical review of XSTERM model applicability to the Xe100 reactor will be addressed as part of the review of a licensing application that references this TR.
REFERENCES 1.
K. Ott, R.J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, issued 1985.
Principal Contributors: Inseok Baek Jason Schaperow Tim Drzewiecki Michael Salay Date: May 6, 2025