ML23117A011
| ML23117A011 | |
| Person / Time | |
|---|---|
| Site: | 99902069, Hermes File:Kairos Power icon.png |
| Issue date: | 04/27/2023 |
| From: | Jose March-Leuba Advisory Committee on Reactor Safeguards |
| To: | David Petti Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML23117A011 (1) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001
April 27, 2023
MEMORANDUM TO: David Petti, Lead Kairos Power Licensing Subcommittee Advisory Committee on Reactor Safeguards
FROM: Jose March-Leuba, Member Advisory Committee on Reactor Safeguards
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 6, ENGINEERED SAFETY FEATURES
In response to the Subcommittees request, I have reviewed the NRC staffs draft safety evaluation (SE) with no open items and the associated section of the applicants Preliminary Safety Analysis Report (PSAR) for Chapter 6, Engineered Safety Features. The following is my recommended course of action concerning further review of this chapter and the staffs associated safety evaluation.
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Background===
Chapter 6 of the SE report documents the staffs review of the performance of the engineered safety features (ESF) that are credited in PSAR Chapter 13, Accident Analysis. ESFs are those systems, structures, or components (SSCs) that prevent and/or mitigate the consequences of potential accidents described in the PSAR, including the bounding design basis accidents.
SE Summary
Kairos identifies only two ESF s in their design: functional containment and the decay heat removal system (DHRS). Functional containment is a feature that serves a safety function equivalent to that of the hardened structures in large light water reactors that limit radioactive release to the public in case of accidents. Guidance to evaluate it is contained in NUREG-1537, SECY-18- 0096, and SRM-SECY-18- 0096. DHRS is an ex-vessel passive system whose safety function is to remove residual heat after shutdown for up to seven days after postulated events that assume unavailability of the normal heat rejection system. It is evaluated based on the guidance in NUREG-1537.
The SE documents the staffs evaluation of the applicants design for compliance with applicable regulations and standards as they apply to a construction permit (CP) application. The NRC staff evaluated the descriptions and discussions of Kaiross and found that they are sufficient and meet the applicable regulatory requirements, guidance, and acceptance criteria for the issuance of a construction permit.
D. Petti
Discussion
Kairos has implemented the f unctional containment concept by crediting the effective retention of radionuclides by the multiple layers in TRISO fuel. Additional barriers are the spherical pebble and the Flibe coolant, which retains a large fraction of the radionuclides that are not aerosolized or evaporated during events. The spherical pebble containment capability is not credited, and its main function is to provide physical protection for the TRISO particles. TRISO fuel is qualified for a design temperature of 1,600°C. Substantial margin to this limit is shown in the preliminary calculations of technical report KR-TR-018P, which estimates maximum accident temperatures below 1,100°C, even for the postulated maximum hypothetical accident (MHA). For steady state operation, TRISO manufacturing defects and in-service failures are assumed to be <0.23 % of the total particle population. These particles will increase radioactive releases from the fuel. Limiting condition for operation 3.3 (PSAR Table 14.1-1 Proposed Variables and Conditions for Technical Specifications) ensures that the functional containment performance is continuously monitored, and the radionuclide inventory of the reactor coolant remains within expected limits. The staff concludes that the Kairos functional containment approach is consistent with applicable guidance, and it meets regulatory requirements for the issuance of a CP. I concur.
The staff has reviewed the DHRS safety function against the principal design criteria (PDC),
including performance, inspection, and testing requirements. DHRS is a passive cooling system with four independent trains. The ultimate heat sink is provided by water tanks that feed, by gravity, thermosyphon thimbles that remove vessel heat and reject steam to the atmosphere.
We note that the steam rejected to the atmosphere is not contaminated because it is never in contact with the in-vessel coolant; the decay heat is transferred by conduction through the vessel and DHRS walls. The water tanks are sized to provide passive cooling for up to seven days, and the staff has verified the sizing with confirmatory calculations. The staff concludes that the DHRS design provides reasonable assurance it will perform its safety function of passive residual heat removal. Testing is planned prior to operation. The staff audit found that the effects of corrosion and fouling, flow instability during transitions, and dynamic loads during operation need to be examined during the testing. This preliminary information is adequate at the CP stage of the design.
The staff SE report notes that all safety calculations start from full power conditions, which correctly challenge high temperature limits for fuel and vessel. Flibe does not expand when freezing, however, freezing by overcooling could challenge the integrity of vessel and piping components by blocking flow. Thus, for the final evaluation at the operating license (OL) stage, the staff intends to identify initial conditions and event scenarios that could challenge overcooling. This evaluation should include startup conditions of unirradiated cores. A potential conclusion from this study is that the Reactor Thermal Management System may be required to prevent salt freezing and, thus, may have some safety function at least for a fraction of time of reactor operation.
We note that the safety calculations in KP-TR-018-P are preliminary and are based, in part, on a preliminary review of the core design analysis methodology documented in KP-TR-017-P.
These two technical reports do not document, at this stage, all the validation efforts against available experimental data. The staff concludes that these preliminary calculations are of sufficient quality to issue a CP, and final and complete review, including code validation with the chosen nodalization level, will be conducted at the OL stage.
D. Petti
I did not identify any specific deficiencies in my review of the CP stage. I would observe that the application was well documented, and the staffs evaluation thorough. I provided some insights that should be considered at the OL stage.
Recommendation
As lead reviewer for Hermes SE Chapter 6, I recommend no further action for the PSAR.
References
- 1. USNRC, Draft Safety Evaluation for Hermes Non Power Reactor Preliminary Safety Analysis Report Chapter 6, February 2023 (ML23065A 010)
- 2. Kairos Power LLC, Submittal of the Preliminary Safety Analysis Report for the Kairos Power Fluoride Salt-Cooled, High Temperature Non-Power Reactor (Hermes), Revision 2, February 2023 (ML23055A672)
- 3. USNRC, NUREG-1537, Part 1, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Format and Content, issued February 1996 (ML042430055)
- 4. USNRC, NUREG-1537, Part 2, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Standard Review Plan and Acceptance Criteria, issued February 1996 (ML042430048)
- 5. SECY-18- 0096, Functi onal Containment Performance Criteria for Non-Light-Water-Reactors (ML18114A546).
- 6. SRM-SECY-18- 0096, Staff Requirements - SECY-18- 0096 - Functional Containment Performance Criteria for Non-Light-Water-Reactors (ML18338A502).
- 7. Kairos Power LLC,KP-TR-003-NP-A, Revision 1, Principal Design Criteria for the Kairos Power Fluoride Salt-Cooled, High Temperature Reactor, July 2019 (ML19212A756)
- 8. Kairos Power LLC, KP-TR-017, KP-FHR Core Design and Analysis Methodology, September 29, 2021 (ML21272A375)
- 9. Kairos Power LLC, KP-TR-018, Postulated Event Analysis Methodology, February 28, 2023 (ML23055A672)
D. Petti - 4 -
April 27, 2023
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 6, ENGINEERED SAFETY FEATURES
Package No: ML23117A000 Memo Accession No: ML23117A011 Publicly Available Y SensitiveN Viewing Rights: NRC Users or ACRS Only or See Restricted distribution *via e-mail OFFICE ACRS/TSB* SUNSI Review* ACRS/TSB* ACRS*
NAME WWang WWang LBurkhart (WWang for) JMarch -Leuba DATE 4/ 27/2023 4/ 27/2023 4/ 27/2023 4/ 27/2023 OFFICIAL RECORD COPY