ML23338A052
| ML23338A052 | |
| Person / Time | |
|---|---|
| Site: | 99902100 |
| Issue date: | 12/04/2023 |
| From: | Brusselmans R NRC/NRR/DANU/UAL1 |
| To: | George Wilson TerraPower |
| References | |
| Download: ML23338A052 (1) | |
Text
Enclosure OFFICE OF NUCLEAR REACTOR REGULATION REGULATORY AUDIT
SUMMARY
REPORT TERRAPOWER, LLC TOPICAL REPORT AN ANALYSIS OF POTENTIAL VOLCANIC HAZARDS AT THE PROPOSED NATRIUM SITE NEAR KEMMERER, WYOMING, REVISION 0A PROJECT NO. 99902100
1.0 BACKGROUND
By letter dated April 25, 2023, TerraPower, LLC (TerraPower) submitted topical report (TR) TP-LIC-LET-0070 An Analysis of Potential Volcanic Hazards at the Proposed Natrium Site Near Kemmerer, Wyoming, Revision 0A (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23115A387) to the U.S. Nuclear Regulatory Commission (NRC) staff. The TR describes TerraPowers volcanic hazard characterization for the design and operation of the proposed Natrium reactor at the Kemmerer Power Station Unit 1 site. The NRC staff completed its completeness determination of the TR on June 28, 2023, and found that the material presented in the TR provided technical information in sufficient detail to enable the NRC staff to begin its detailed technical review (ML23167B211).
TerraPower has requested the NRC staffs review and approval of the volcanic hazard characterization methodology, as presented in the subject TR, for potential use by future applicants, where applicable. TerraPower stated that its overall licensing methodology follows the technology-inclusive, risk-informed, and performance-based approach outlined in Regulatory Guide (RG) 1.233 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, Revision 0 (ML20091L698).
The NRC staff provided its audit plan for the subject TR to TerraPower dated on September 1, 2023 (ML23248A199). The audit was conducted virtually from September 11, 2023, through October 19, 2023, using TerraPowers electronic reading room (eRR). The NRC staff held an audit exit meeting with TerraPower on October 19, 2023.
2.0 AUDIT REGULATORY BASES The regulatory bases for the audit included the following:
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix A, General Design Criteria for Nuclear Power Plants, as it relates to the consideration of the most severe natural phenomena historically reported for the site and surrounding area (General Design Criterion (GDC) 2). The GDC establish minimum requirements for the principal design criteria (PDC) for water-cooled nuclear power plants. However, they are also considered to be generally applicable to other types of nuclear power units and are intended to provide guidance in establishing the PDC for such other units.
10 CFR 100.23(c), as it relates to the investigation of all geologic and seismic factors, including volcanic activity, that may affect the proposed nuclear power plant.
RG 4.26, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites, Revision 0 (ML20272A168), provides guidance on how to assess volcanic hazards at proposed nuclear power sites.
NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR edition, Section 2.5.1 Geological Characterization Information, Revision 5 (ML13316C067), and Section 2.5.3 Surface Deformation, Revision 6 (ML19009A314), which provide information on the content for characterizing the local and regional geology and considering sources of potential surface deformation, respectively.
3.0 AUDIT PURPOSE AND OBJECTIVES The purpose of the audit was for the NRC staff to gain a more detailed understanding of TerraPowers volcanic hazard characterization methodology for the proposed Kemmerer Power Station Unit 1 site near Kemmerer, Wyoming. A secondary purpose of the audit was to identify any information that would require docketing to support the NRC staffs safety evaluation.
4.0 SCOPE OF THE AUDIT AND AUDIT ACTIVITIES The audit followed the guidance in the Office of Nuclear Reactor Regulations Office Instruction LIC-111, Regulatory Audits, Revision 1 (ML19226A274). Audit activities included virtual meetings to discuss questions, review of submitted files, and code demonstrations via videoconference.
Members of the audit team included the NRC staff listed below:
Jenise Thompson, Geologist, Audit Lead Jason White, Meteorologist Scott Stovall, Seismologist Stephanie Devlin-Gill, Senior Project Manager Roel Brusselmans, Project Manager The participants from TerraPower for this audit were Patrick Donnelly and Brittain Hill.
On October 19, 2023, the NRC staff held an audit exit meeting with TerraPower and summarized the audit purpose, activities, and high-level results. The NRC staff did not acquire any documents during the audit. The NRC staff reviewed the following documents during the audit using TerraPowers eRR:
ASHPLUME2 source code, graphs illustrating PVHA_YM Version 2.0 modeling results, and input files for both ASHPLUME2 and PVYA_YM Version 2.0.
5.0
SUMMARY
OF OBSERVATIONS As indicated in the NRC staffs audit plan, the audit was focused primarily on specific inquiries pertaining to the content of the TR. These included questions related to the models used in the assessment of volcanic hazards at the proposed site, the overall hazard assessment, and plans for mitigating actions.
The NRC staff and TerraPower discussed TerraPowers use of the PVHA_YM computer code to model the potential for a new vent opening in the Leucite Hills Volcanic Field. TerraPower conducted a demonstration of the PVHA_YM program via videoconference, as well as placed the PVHA_YM input file in the eRR for the NRC staff review. TerraPower clarified that other than transposing the geographic location in the program, no other modifications were made to the model and therefore no additional validation or verification was necessary. TerraPower provided higher resolution figures in the eRR showing the PVHA_YM modeling results. This information addressed the NRC staffs inquiries related to the use of the PVHA_YM model to analyze the potential for a new vent opening in the Leucite Hills Volcanic Field.
The NRC staff and TerraPower discussed the use of the ASHPLUME2 computer code to model tephra fall hazards at the Natrium site from the regional sources of tephra hazards. TerraPower conducted a demonstration on the use of the ASHPLUME2 model via videoconference and made the input file used in the analysis available in the eRR. TerraPower described the modifications it made to the ASHPLUME Version 1.0 code for use in this analysis, which included breaking down regional wind field data into discrete altitude intervals. Notably, TerraPower stated that the changes were all considered minor alterations that would not require a formal verification of the code, but that verification was performed to ensure the changes in wind speed distributions were observed at the correct altitude.
The NRC staff and TerraPower discussed the realization intervals used when running the ASHPLUME2 code, which differ between mafic and silicic compositions and individual sources.
TerraPower clarified that the difference in realization intervals is designed to capture epistemic uncertainty while keeping the number of realizations to a manageable number. TerraPower further clarified that changing the realization interval did not affect the tephra fall hazard results for the Natrium site. TerraPower stated that the difference in realization intervals between silicic and mafic compositions is reflective of the spatial distribution within these volcanic fields.
The NRC staff and TerraPower also discussed the gaps in the wind speeds that are apparent in the following figures in the TR:
Figure 23, Results of hazard calculations for [Leucite Hills volcanic field] eruption
- sources, Figure 27, Results of hazard calculations for [Blackfoot Reservoir volcanic field (BRVF)]
eruption sources, and Figure 33, Results of silicic tephra-fall hazard calculations for BRVF eruption sources.
TerraPower stated that these are not reflective of data gaps but of changes in wind speed between high and low altitude columns, a representation of the non-continuous function in altitude distributions and the influence of the jet stream. The gaps shown depend on the wind speeds for that specific sector of National Oceanic and Atmospheric Administration data and show large differences for certain sectors. TerraPower stated that the areas of these figures that do not show a demonstrable gap have a smaller change between sectors and therefore show a more continuous distribution of wind speed.
TerraPower referred to TR Section 7.1.1.5, Interpretation of Silicic Tephra Volumes, for an explanation of how the tephra volumes were estimated. The information provided in Section 7.1.1.5 addressed the NRC staffs inquiry related to estimating tephra volumes in the site region when deposits are not well preserved.
As described in the NRC staffs audit plan, the NRC staff asked questions related to mitigation actions described in TR Section 10, Considerations for Potential Tephra-Fall Events at the Proposed Facility. During the audit, the NRC staff informed TerraPower that the NRC staff plans to include one or more Limitation(s) & Condition(s) in the NRC staffs safety evaluation regarding this topic.
With respect to deviations from the audit plan, the NRC staff developed additional questions regarding how the ASHPLUME Version 1.0 and ASHPLUME2 model performed computations and how the code worked. These questions were resolved during the audit discussions.
6.0 REQUESTS FOR ADDITIONAL INFORMATION RESULTING FROM AUDIT As a result of the audit, the NRC staff did not identify any requests for additional information related to this TR.
7.0 OPEN ITEMS AND PROPOSED CLOSURE PATHS There are no open items resulting from this audit.