ML25260A002
| ML25260A002 | |
| Person / Time | |
|---|---|
| Site: | Kemmerer File:TerraPower icon.png |
| Issue date: | 09/17/2025 |
| From: | George Wilson TerraPower |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| TP-LIC-LET-0448 | |
| Download: ML25260A002 (1) | |
Text
15800 Northrup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889 September 17, 2025 TP-LIC-LET-0448 Docket Number 50-613 U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Attn: Document Control Desk
Subject:
Response to NRC Audit Question 3-85 on Kemmerer Power Station Unit 1 Preliminary Safety Analysis Report Pursuant to Title 10 of the Code of Federal Regulations, Part 50 (10 CFR 50), Domestic Licensing of Production and Utilization Facilities, TerraPower, LLC (TerraPower), on behalf of US SFR Owner, LLC (USO), submitted a construction permit application (CPA) to construct a Natrium Reactor Plant1 at Kemmerer Power Station Unit 1 (Reference 1).
To support preparation of the NRCs safety evaluation report, TerraPower submits this letter to provide supplemental information to NRC audit question 3-85 on the Kemmerer Power Station Unit 1 Preliminary Safety Analysis Report (PSAR) provided as of the CPA.
Chapters 3, 5, and 7 of the PSAR provides a description of the licensing basis events (LBEs), safety-significant probabilistic risk assessment (PRA) safety functions (PSF), and system descriptions of the pool immersion cell (PIC) and the bottom loading transfer cask (BLTC). This letter provides design progression information pertaining to these descriptions and a timeline for PRA model updates.
For the RFH-ESWR LBEs, the PSAR stage PRA model assumed that the PIC and BLTC were mated during the PIC transfer and cleaning process. Therefore, the PRA model utilized to develop the CPA was such that:
1 Natrium is a TerraPower and GE Vernova Hitachi Nuclear Energy Technology.
September 17, 2025 Page 2 of 5 15800 Northrup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889 The BLTC mates to the fueling floor valve that is attached to the PIC to transfer a core assembly through the PIC receiver down into the PIC cleaning vessel (CV).
The cleaning process takes place where the residual sodium is removed from the core assembly via humidified inert gas and then the core assembly is rinsed to remove all byproducts from removing the residual sodium. In this configuration, the radionuclide retention barrier is being performed by the DL3-RR5a PSF, which includes the PIC and BLTC.
Once the core assembly is cleaned, the PIC CV is lowered down into the spent fuel pool.
Due to the assumed configuration of the PIC and BLTC during the initial transfer and cleaning process, the processes were grouped together for potential excessive sodium-water reactions (ESWRs) as one family of RFH-ESWR LBEs. An ESWR in the PIC while the BLTC is not mated to the PIC was not considered as a postulated initiating event in the PSAR stage PRA model.
Since the CPA submittal, it was recognized that the PSAR stage PRA model for the PIC and BLTC configuration differed from the design of the fuel assembly cleaning process, which is described below:
The BLTC mates to the fueling floor valve that is attached to the PIC to transfer the core assembly down into the PIC CV.
The fueling floor valve (FFV) closes, isolating the PIC from the BLTC, and the BLTC is released from the PIC.
Once the PIC has been isolated, the cleaning process takes place to remove the residual sodium.
Once clean, the core assembly is lowered down into the spent fuel pool (SFP).
While the PIC and BLTC are mated for the transfer process, the radionuclide retention barrier function, PSF DL3-RR5a, is being performed by the BLTC, FFV, PIC CV, PIC Receiver, and the associated PIC processing skid components including piping, filter housing, and high efficiency particulate air (HEPA) filters. Once the BLTC is released and the PIC is isolated, the radionuclide retention barrier function, PSF DL2-RR2, is being performed by the SSCs in DL3-RR5a except the BLTC. Since an ESWR in the PIC while the BLTC is not mated to the PIC was not considered as a postulated initiating event in the CPA PRA model, DL2-RR2 was not credited in any LBE analyses.
September 17, 2025 Page 3 of 5 15800 Northrup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889 With design progression of the PIC and its subsequent support systems, there have been changes since submittal of the PSAR to reduce the frequency of a potential ESWR occurring inside the PIC during a core assembly transfer and throughout the cleaning process as described below:
The design of the PIC includes pressure and flow sensors to provide positive indication that the CV has been purged of excess water prior to transferring a core assembly into the PIC.
The PIC controls design includes an interlock to prevent inadvertent rinse water pump operation that could result in the introduction of excess water into the CV prior to completion of sodium removal.
The PIC receiver assembly includes a lock to secure the PIC basket to the receiver.
The lock disengagement involves an interlock to prevent lowering of the CV, including the core assembly, into the SFP prior to the removal of the residual sodium.
A Process Hazard Analysis will be performed to inform the PRA event trees and the associated source terms. The risk and consequence information will be used to define the system LBEs following the process depicted in NEI 18-04, Figure 3-2 (Reference 2) as endorsed by RG 1.233 (Reference 3). In accordance with NEI 18-04, the PRA Safety Functions impact on the event will be used to classify the associated SSCs, which will confirm or drive revision to the design codes and standards and special treatments applied to the PIC SSCs for fabrication and construction. In accordance with RG 1.87 (Reference 4), justification for the design codes and standards applied to the PIC SSCs will be provided in support of the operating license (OL).
In accordance with RG 1.247 (Reference 5), prior to the operating license application a full-scope peer review of the PRA, reflecting the operating license plant design, will be performed against the applicable requirements of ASME/ANS RA-S-1.4-2021 following the guidance provided in NEI 20-09 (Reference 6). In support of the OL, event specific source term models will be developed that represent the detailed system configuration of the PIC and an analysis methodology specific to events involving the sodium-water reaction with considerations for the impact of pressure and temperature effects driving radionuclide releases from fuel will be developed. Additionally, mechanistic source term evaluation model treatment of sodium pool scrubbing and iodine releases will be
September 17, 2025 Page 4 of 5 15800 Northrup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889 further examined for the OL stage to confirm the implementation and supporting methodology is appropriate for the application.
In support of the OL, specific PRA models will be developed, including configurations where the PIC and BLTC are not mated, that represent the detailed system design of the PIC with associated support systems.
The frequency of the RFH-ESWR LBEs reported in the PSAR are expected to decrease due to the PIC system interlocks discussed above once they're incorporated into the PRA model. The interlocks that act to reduce the frequency will be evaluated as PRA safety functions or as part of the preventative measure defense-in-depth (DID) adequacy evaluation to verify the safety classification of the interlocks at the OL stage.
This letter makes no new or revised regulatory commitments.
If you have any questions regarding this submittal, please contact Nick Kellenberger at nkellenberger@terrapower.com or Ian Gifford at igifford@terrapower.com.
I declare under penalty of perjury that the foregoing is true and accurate.
Executed on September 17, 2025.
Sincerely, George Wilson Senior Vice President, Regulatory Affairs TerraPower, LLC cc:
Mallecia Sutton, NRC Josh Borromeo, NRC Nathan Howard, DOE
September 17, 2025 Page 5 of 5 15800 Northrup Way, Bellevue, WA 98008 www.TerraPower.com P. +1 (425) 324-2888 F. +1 (425) 324-2889
References:
- 1. TerraPower, LLC, Submittal of the Construction Permit Application for the Natrium Reactor Plant, Kemmerer Power Station Unit 1, March 28, 2024 (ML24088A059).
- 2. NEI 18-04, "Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development,"
Nuclear Energy Institute, August 2019.
- 3. 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, June 2020.
- 4. RG 1.87, Acceptability of ASME Code,Section III, Division 5, High Temperature Reactors, January 2023.
- 5. RG 1.247, TRIAL - Acceptability of Probabilistic Risk Assessment Results for Non-Light Water Reactor Risk-Informed Activities, March 2022.