ML25037A123
| ML25037A123 | |
| Person / Time | |
|---|---|
| Site: | 99902100 |
| Issue date: | 02/23/2025 |
| From: | Gregory Bowman Office of Nuclear Reactor Regulation |
| To: | Walter Kirchner Advisory Committee on Reactor Safeguards |
| Mallecia Sutton, NRR/DANU | |
| References | |
| Download: ML25037A123 (1) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 Dr. Walter L. Kirchner, Chair Advisory Committee on Reactor Safeguards U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
SUBJECT:
RESPONSE TO DRAFT SAFETY EVALUATION OF THE TERRAPOWER TOPICAL REPORT, PLUME EXPOSURE PATHWAY EMERGENCY PLANNING ZONE METHODOLOGY, REVISION 3
Dear Dr. Kirchner:
By letter dated November 26, 2024 (Agencywide Documents Access and Management System Accession No. ML24324A305), the Advisory Committee on Reactor Safeguards (ACRS) reported on its review of the U.S. Nuclear Regulatory Commission (NRC) staffs safety evaluation report (SER) and TerraPower, LLC (TerraPower) Topical Report (TR), Plume Exposure Pathway Emergency Planning Zone Methodology, Revision 3 (ML24304B034). The NRC staff briefed the ACRS on the SER on October 30, 2024.
In its letter to the NRC Executive Director for Operations, the ACRS noted in Conclusion and Recommendation 1 that the proposed methodology is expected to result in an EPZ size that is consistent with the principles historically used as part of the emergency preparedness (EP) framework. The ACRS identified questions regarding potential cliff-edge effects, and in Conclusion and Recommendation 3 stated, the SER should not be issued until the rationale for the use of the frequency threshold when assessing the potential for cliff-edge effects is addressed.
Conclusion and Recommendation 2 in the ACRS letter states that:
The TR proposal to assess potential cliff-edge effects for emergency planning zone (EPZ) sizing only when accident event sequence frequency exceeds a threshold (1x10-8/plant-year) is not aligned with more recent regulatory guidance provided for the risk-informed, performance-based, technology inclusive licensing methodology for non-light water reactors (non-LWRs). The EPZ regulatory guidance recommends retention of event sequences below a frequency cutoff (for Natrium this value is 1x10-7/plant-year) when assessing for cliff-edge effects.
However, there is no specific guidance on how to further screen the large number of sequences that must be considered in practice. Given this lack of specificity, the threshold used in the TR should have been justified As described in ACRSs letter, this guidance is from appendix B of Regulatory Guide (RG) 1.242, Revision 0, Performance-Based Emergency Preparedness for Small Modular Reactors, Non-Light Water Reactors, and Non-Power Production or Utilization Facilities, which provides guidance for development of source terms. However, since the scope of the TR is February 23, 2025
W. Kirchner specific to the establishment of the site-specific EPZ size, guidance in appendix A of RG 1.242 was used in the development and review of the TR. Appendix A allows for the use of probabilistic risk assessment (PRA) results to screen out accident sequences for EPZ sizing as long as the PRA is acceptable for that use in risk-informed decision-making and the uncertainty associated with the frequency estimates is quantified. In response to Conclusion and Recommendation 2, the NRC staff plans to incorporate additional information regarding the cliff-edge effects evaluation into SER Section 1.5, Selection of Non-seismic Release Sequences, prior to its issuance.
The NRC staff notes that Limitation and Condition 1 in the SER will require TerraPower to use RG 1.247, Acceptability of Probabilistic Risk Assessment Result for Non-Light Water Reactor Risk-Informed Activities, and appendix A to RG 1.253, Guidance for a Technology-Inclusive Content of Application for Licenses, Certifications, and Approvals for Non-Light-Water Reactors to establish the acceptable PRA in support of implementing the EPZ methodology. This condition provides confidence to the NRC staff that appropriate PRAs are used and, accordingly, that cliff-edge effects will be appropriately considered.
The NRC staff acknowledges Conclusion and Recommendation 4 in the ACRSs letter that additional clarifying guidance is warranted regarding selection criteria for the spectrum of events to consider for determination of the source term that is to be applied for EPZ sizing. The NRC staff will take the ACRSs observations into account and evaluate what enhancements or clarifications could be made in a future revision to the guidance for determination of a plume exposure pathway EPZ.
The NRC staff appreciates the ACRSs timely and thorough review of the TerraPower TR and the staffs SER and looks forward to working with the Committee in the future.
Sincerely, Gregory T. Bowman, Deputy Director Office of Nuclear Reactor Regulation Project No.: 99902100
Enclosure:
As stated cc: TerraPower Natrium via GovDelivery Signed by Bowman, Gregory on 02/23/25
ML25037A123 NRR-106 OFFICE NRR/DANU/UAL1:PM NRR/DANU/UAL1:LA NRR/DANU/UAL1:BC NAME MSutton DGreene JBorromeo DATE 02/06/2025 02/10/2025 02/10/2025 OFFICE NRR/DANU/UTB2:BC NRR/DANU/D NRR/DD NAME CdeMessieres JBowen GBowman DATE 02/12/2025 02/19/2025 02/23/2025