ML23117A010
| ML23117A010 | |
| Person / Time | |
|---|---|
| Site: | 99902069, Hermes File:Kairos Power icon.png |
| Issue date: | 04/27/2023 |
| From: | Walter Kirchner Advisory Committee on Reactor Safeguards |
| To: | David Petti Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML23117A010 (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 Advisory Committee on Reactor Safeguards
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 5, HEAT TRANSPORT SYSTEMS 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 5, Heat Transport Systems. 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===
The primary heat transport system (PHTS) transfers heat from the reactor core by circulating a chemically stable salt coolant (Flibe) between the pebble bed core and reflectors in the reactor vessel (RV) and the heat rejection subsystem during normal operations. The PHTS includes a primary salt pump (PSP), heat rejection subsystem, and associated piping. The heat rejection subsystem includes a heat rejection radiator (HRR), heat rejection blower, and associated ducting.
The Flibe reactor coolant supports reactivity control (negative moderator/coolant reactivity coefficient) and accumulates fission products (from pebble release) and transmutation products (from Flibe and impurities), the Flibe serving as an additional barrier to release of radionuclides (part of functional containment). The PHTS piping is austenitic stainless steel designed to the ASME B31.3 Code and the HRR is designed to the ASME BPVC,Section VIII.
The PHTS outside the RV is considered non-safety-related and is not functionally required to achieve safe, stable shutdown. This is predicated on the functioning of the fluidic-diodes and anti-siphon features of the core internals within the RV (Chapter 4) in preventing excess loss of coolant inventory in the vessel in event of an ex-vessel pipe rupture (i.e., maintaining the coolant level above the active pebble bed core). Also, the heat rejection blower and the PSP are tripped in event of an HRR tube failure to prevent forced air ingress to the system.
The PHTS also includes thermal management features to maintain the reactor coolant in the liquid phase when the reactor core is not generating heat, and the capability to drain external piping and the HRR to allow cooldown, inspection, and maintenance. Connected auxiliary systems (Chapter 9) include chemistry control, inert gas, inventory management, and thermal management systems (i.e., thermophysical property and purity control, limits on air ingress and circulating radionuclide inventory, fill and drain functions, and preventing freezing of coolant).
Chapter 14, Technical Specifications, provides for associated limiting conditions of operations for the primary coolant systems.
SE Summary The staff evaluated the sufficiency of the preliminary information on the design of the Hermes PHTS, as described in PSAR Chapter 5 and other relevant portions of the PSAR, using the guidance and acceptance criteria from Section 5.2 of NUREG-1537, Parts 1 and 2. As part of its review, the staff evaluated whether PSAR Section 5.1 identified the appropriate PDCs and offered sufficient information and design description to provide reasonable assurance that the design bases will be met at the operating license (OL) stage. Based on the information provided by Kairos and the staffs evaluation, the staff found that the applicant provided sufficient preliminary information in accordance with 10 CFR 50.34 to develop a primary coolant system design that staff has reasonable assurance will be able to accomplish the design functions of fuel integrity and sufficient heat removal, coolant loss prevention, conversion to passive natural-convection flow, limited corrosion of essential components, and sufficient radiation shielding for limiting personnel exposures. Accordingly, the staff found that there is reasonable assurance that Hermes reactor will comply with applicable requirements.
Discussion/Observations While I did not identify any specific deficiencies in my review, I would observe that there is a major premise, if not precedent, that is predicated on successful design and to be demonstrated performance of several novel and unique features of the Hermes design. That is in Chapter 3 the applicant proposes to redefine part of the safety-related definition (10 CFR Part 50.2) regarding integrity of the reactor coolant pressure boundary to integrity of portions of the reactor coolant boundary relied upon to maintain the reactor coolant level above the active core.
While this revised definition applied to design basis events involving a major break in the PHTS would address the continued cooling of the pebble bed core and passive decay heat removal (assuming the function of the RV anti-siphon features and demonstration of the fluidic diode device design), it potentially weakens overall defense-in-depth and independence of barriers/safety functions.
The staff accepted this novel safety classification on the basis that the piping performs no safety role and the functional containment capability of the TRISO fuel pebbles and Flibe coolant in the RV remains largely intact even during a spill. Kairos has committed to protecting safety-related equipment in the reactor building and precluding Flibe/concrete interactions.
For PHTS salt coolant spills and unlimited air ingress to the RV (Chapter 13.1.3), exposed Flibe and graphite structures could lead to uncontrolled and unmitigated release of radionuclide volatiles and aerosols, possible chemical reaction with the coolant, and oxidation of graphite over a 7-day accident span. (The staff addresses this in their SE for Chapter 13 in part by suggesting that the coolant and metallic/graphite materials qualification programs will need to bound the postulated air ingress scenarios through a 7-day period to meet the requirements of PDC 70.)
Secondly, classifying the PHTS piping as non-safety-related suggests that the system may not survive a design-basis earthquake, potentially endangering the integrity of the RV at the nozzle/piping interface with a break at that location. Kairos stated that the design of the PHTS piping at the RV connection is of sufficiently small wall thickness such that loads beyond the elastic limits would result in an inelastic response (hence break/rupture?) prior to any impact on the RV or decay heat removal system (DHRS). If this is indeed the design approach, further evaluation will have to wait until the OL stage, when detailed design information is available for the PHTS, and associated seismic analyses of support, restraint, and structural layout are completed. In Chapter 6, Engineered Safety Features, the applicant also identifies Flibe leaks as an external hazard to the DHRS in the reactor cell.
Classifying the PHTS as safety-related does not preclude a major pipe break, just as it does not in a light water reactor, but for this First-of-a-kind reactor, designing, analyzing, and constructing the PHTS to the same quality level as the RV (ASME BPVC,Section III, Division 5 design and construction, and Section 11 testing and inspection) and seismic criteria would enhance confidence in the low probability of significant pipe rupture (thermally or seismically induced).
Finally, there is no credit for a confinement function provided by the reactor compartment design, therefore unmitigated air ingress and interaction with Flibe raises airborne toxicity (Be primarily) and radiotoxicity concerns. Potential occupational hazards of such a release or spill will also need to be addressed.
Several of these items, primarily coolant thermophysical properties, purity, and chemistry, circulating radionuclides, and entrained air and gases can be addressed or mitigated by limiting conditions of operation (LCOs) as called for in Chapter 14 (TBD).
Recommendation As lead reviewer for Hermes SE Chapter 5, I recommend that the staff document clearly their logic in accepting the applicants revised safety-related definition and its application to the Hermes design, specifically the PHTS, and detail more explicitly expectations (i.e., fuel and coolant qualification, novel component and equipment test demonstrations, test and inspection requirements, instrumentation, etc.) for the operating license phase. Appendix A of the staffs draft SE does contain pointers to a number of these matters.
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 Chapters 2,3, 4, 5, 6, and 13, January 2023 (ML23017A120 and 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.
SECY-18-0096, Functional 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, 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, Fuel Qualification Methodology for the Kairos Power Fluoride Salt-Cooled High Temperature Reactor (KP-FHR), Revision 2, June 2022 (ML22186A212).
- 10. Kairos Power, Topical Report KP-TR-012, KP-FHR Mechanistic Source Term Methodology, Revision 1, August 2021 (ML21231A292).
- 11. Kairos Power LLC, Metallic Materials Qualification for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor, Topical Report KP-TR-013, Revision 4, September 30, 2022 (ML22263A456).
- 12. Kairos Power LLC, Graphite Material Qualification for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor, Topical Report KP-TR-014-NP, Revision 4, September 2022 (ML22259A145).
April 27, 2023
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 5, HEAT TRANSPORT SYSTEMS Package No: ML23117A000 Memo Accession No: ML23117A010 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