ML23117A052
| ML23117A052 | |
| Person / Time | |
|---|---|
| Site: | 99902069, Hermes |
| Issue date: | 04/27/2023 |
| From: | Walter Kirchner Advisory Committee on Reactor Safeguards |
| To: | David Petti Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML23117A052 (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: Walter L. Kirchner, 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 13, ACCIDENT ANALYSIS 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 13, Accident Analysis. The following is my recommended course of action concerning further review of this chapter and the staffs associated safety evaluation.
Background
This accident analysis chapter provides information and analyses to evaluate the potential radiological consequences in the event of malfunctions and the capability of the facility to accommodate disturbances such that no postulated credible event leads to unacceptable radiological consequences to the public. (The health and safety of the workers will be addressed at the application for an Operating License (OL).) The Kairos Hermes design relies primarily on a functional containment approach (TRISO pebble fuel/Flibe molten salt coolant) to retain most of the radioactive material available for release during a postulated event. This chapter provides material to demonstrate that the facility design features, and bounding values for parameters expected to be controlled by technical specifications, will ensure that no postulated event in the facility design basis leads to unacceptable radiological consequences to the public or the environment.
The applicants accident analysis presented in this chapter seeks to bound all potential accident releases in the reactors design basis by evaluating the dose consequences of a Maximum Hypothetical Accident (MHA). The MHA analysis is mainly built off the analysis methodology in KP-TR-012, Mechanistic Source Term Topical Report, and the assumptions that the reactor protection system and reactor/primary heat transport system function as designed, most importantly keeping the reactor core covered with Flibe.
Chapter 13 references KP-TR-018-NP, Postulated Event Methodology Technical Report, Revision 1, which provides analysis details to support the conclusions in the various PSAR sections. The potential radiological releases from analyzed initiating events or groups of events
D. Petti for this test reactors design basis are then compared using figures of merit (time and temperature of the reactor fuel and internals - see PSAR Figure 13.2-1, radioactive material at risk for release (MAR), etc.), as surrogates for dose. The events analyzed included: (a) insertion of excess reactivity; (b) salt spills; (c) loss of forced circulation (heat-up and cool-down); (d) mishandling/malfunction of the pebble handling and storage system; (e) radioactive release from subsystems and components (e.g., MAR in auxiliary systems for salt coolant cleanup); (f) general challenges to normal operation; and (g) internal/external events (e.g.,
seismic and flooding). Conservative assumptions were selectively applied to analyses of each group and specific events. These included single failure criteria, assumption of highest worth control rod stuck out, conservative assumptions of radioisotope releases and rates (i.e., from fuel, structure, and FLIBE, tritium and Argon-41 releases, etc.), and three out of four decay heat removal system trains available. The accident analysis results will inform the final the technical specifications, limiting conditions for operation (LCOs) and limiting safety system settings, and design specifications for safety-related components and systems. The suite of accident analyses also included those prevented or precluded by design and the rationale for their exclusion (e.g., failure to shut down the reactor, significant ingress of air, etc.). Each analysis was compared to selected figures of merit to assure that the results are bounded/enveloped by the MHA. The dose consequences of the MHA analysis then are used to demonstrate the acceptability of the design when compared to regulatory dose limits in 10 CFR 100.11 (at the exclusion area boundary and low population zone) for siting of a non-power reactor. The MHA analysis bounding dose consequences presented are significantly lower than those specified in 10 CFR 100.11.
SE Summary The staff evaluated the sufficiency of the information on the preliminary design of the Hermes test reactor, accident analysis as described in the Chapter 13, and other relevant portions of the PSAR, against applicable regulations and the guidance and acceptance criteria from NUREG-1537, Parts 1 and 2. The staff reviewed the MHA analysis and its application in bounding all credible accident scenarios and releases, and in demonstrating that the reactor siting criteria in 10 CFR Part 100.11 are met. The staff also evaluated the applicants deterministic safety analyses for postulated equipment failures and malfunctions. The analyzed event scenarios and figures of merit used as surrogates for detailed dose calculations were thoroughly reviewed.
The staffs review emphasized the importance of identifying all MAR throughout the facility; the role of temperature and time in the retention or transport of radionuclides, and bounding assumptions or conservative selection of inputs for initiating events and scenarios analyses.
While the SE notes that Kairos did not request approval of any specific safety feature, nor was any approved, the staff cited that the functional containment approach is consistent with the approved SECY-18-0096 on the topic. The staffs review highlighted several of the aforementioned considerations in its review: the importance of identifying MAR throughout the facility; temperature versus time for evaluating retention and release of fission products and radioisotopes; and the application of conservative inputs to event analyses.
The staff also performed scoping calculations of several of the events to gain confidence in Kaiross calculational results. The results presented showed good agreement on key parameters (i.e., peak fuel and reactor vessel wall temperatures) with those of the applicant for selected important scenarios such as reactivity insertion and loss of forced circulation events.
The staff found that the preliminary design of the Kairos Hermes reactor complies with applicable requirements. At the operating license application review stage, subject to qualification testing and research and development program demonstrations and confirmation
D. Petti that the final design conforms to the design basis, the staff concluded that safety questions can be adequately resolved and that, taking into consideration the site criteria (10 CFR Part 100),
the reactor can be constructed and operated at the proposed location without undue risk to the health and safety of the public.
Discussion/Observations I did not identify any specific deficiencies in my review. The applicants documentation was reasonably thorough and complete at this preliminary design stage in identifying initiating events, assumptions, and limitations, supporting methodologies, and presenting bounding results, as was the staffs evaluation. Several of the event groups identified warrant further attention and detailed review at the OL stage including, but not limited to: design of the primary heat transport system piping, pipe breaks, and assumptions about salt coolant inventory loss (potentially leading to uncovered fuel pebbles); functioning of the fluidic diodes to support natural circulation to enhance decay heat removal (probably more important in concept for eventual application in a power reactor); features to preclude salt spill sprays and salt interaction with safety-related components and structures (i.e., concrete - the applicant has committed to design measures to preclude these interactions); air ingress to the reactor vessel (i.e., oxidation of fuel and graphite, generation of combustible gases, and fuel/graphite/metallic materials qualification for a 7-day period following such an event as Kairos does not take credit for the confinement structure of the reactor building in mitigating air ingress); pebble handling system malfunctions; and seismic events (e.g., core/reflector disruptions and impact on reactivity control and shutdown).
Of historic note, the potential for air and water ingress into a high-temperature reactors vessel, core, and structures was and remains a safety concern, and was addressed in the modular high temperature gas-cooled reactor (MHTGR) programs. At high temperatures (about 1000 degrees Celsius), air (oxygen) interaction with carbon (graphite) results in an exoergic reaction and produces carbon monoxide and dioxide. The type of graphite and how it was manufactured and machined are significant factors, as well as temperature-time history, in the rate and yield of these reactions. Introduction of water by internal flooding, although the ensuing reaction is endoergic, potentially results in significant combustible gas generation (carbon monoxide and hydrogen) and concerns about corrosion and fuel integrity. Resolution of these potential scenarios will depend on the final design choices, details, and plant layout, and consideration of internal and external initiating events.
Recommendation As lead reviewer for Hermes Chapter 13 PSAR and associated SE, I recommend no further review at this time of Chapter 13, Accident Analysis. I do note that Appendix A to the staffs draft SE is a useful compendium of items important to safety to be addressed at the time of the review of an OL application.
D. Petti References
- 1. 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).
- 2. USNRC, Draft Safety Evaluation for Hermes NonPower Reactor Preliminary Safety Analysis Report Chapter 13, January 2023 (ML23065A010).
- 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. UNNRC, SECY-18-0096, Functional Containment Performance Criteria for Non-Light-Water-Reactors, September 28, 2018 (ML18114A546).
- 6. SRM-SECY-18-0096, Staff Requirements - SECY-18-0096 - Functional Containment Performance Criteria for Non-Light-Water-Reactors, December 4, 2018 (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, letter KP-NRC-2001-001, Topical Repot KPTR005 Submittal, Reactor Coolant for the Kairos Power Fluoride Salt-Cooled High Temperature Reactor, Revision 1, January 16, 2020 (ADAMS Accession No. ML20016A486 and Proprietary version ML20016A487).
- 9. Kairos Power LLC, KP-TR-011-NP-A, Fuel Qualification Methodology for the Kairos Power Fluoride Salt-Cooled High Temperature Reactor (KP-FHR), Revision 2, June 2022 (ML22186A212 and ML23089A398 (redacted version)).
- 10. Kairos Power, Topical Report KP-TR-012, KP-FHR Mechanistic Source Term Methodology, Revision 3, March 2022 (ML22136A291).
- 11. Kairos Power, KP-TR-013-NP-A, Revision 4, Metallic Materials Qualification for the Kairos Power Fluoride SaltCooled HighTemperature Reactor, September 2022 (ML23102A179 (redacted version)).
- 12. Kairos Power, KP-TR-014-NP-A, Revision 4, Graphite Material Qualification for the Kairos Power Fluoride SaltCooled HighTemperature Reactor, September 2022, (ML23108A317 (redacted version)).
- 13. Kairos Power, KP-TR-018-NP, Revision 2, Postulated Event Methodology for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor, February 28, 2023 (ML23055A672).
D. Petti April 27, 2023
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 13, ACCIDENT ANALYSIS Package No: ML23117A000 Memo Accession No: ML23117A052 Publicly Available Y Sensitive N 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) WKirchner DATE 4/27/2023 4/27/2023 4/27/2023 4/27/2023 OFFICIAL RECORD COPY