ML23117A008
| ML23117A008 | |
| Person / Time | |
|---|---|
| Site: | 99902069, Hermes File:Kairos Power icon.png |
| Issue date: | 04/27/2023 |
| From: | David Petti Advisory Committee on Reactor Safeguards |
| To: | David Petti Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML23117A008 (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:
David Petti, Member Advisory Committee on Reactor Safeguards
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - SAFETY EVALUATION FOR CHAPTER 4, REACTOR DESCRIPTION In response to the Subcommittees request, I have reviewed the NRC staffs safety evaluation (SE) with no open items, and the associated section of the applicants Preliminary Safety Analysis Report (PSAR), for Chapter 4, Reactor Description. 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 4 describes the Hermes reactor. It is a 35 MWth reactor that uses TRISO fueled pebbles in a low pressure chemically inert molten salt, Flibe, coolant. Inlet temperature is 550°C and outlet temperature is 650°C. The core is 2 m3 in volume and has a power density of 17.5 MWth/m3. PSAR Chapter 4 describes various important aspects of the reactor design, including the reactor core (fuel, control and shutdown system, neutron startup source), reactor vessel and internals, biological shield, nuclear design, thermal-hydraulic design, and reactor vessel supports and their associated design criteria.
SE Summary The SE reviewed the available details of the reactor design including the fuel, control and shutdown systems, and neutron startup source. The staff evaluated the Kairos design against the relevant Principal Design Criteria (PDC). The staff found the fuel performance conditions were bounded by the DOE Advanced Gas Reactor (AGR) testing envelope for TRISO fuel.
They evaluated the response of the fuel to overpower transients and found through assessment of the Kairos results and independent scoping calculations that the fuel was not challenged by such events in the design. Monitoring of the cover gas and the Flibe coolant for fission products will provide assurance of fuel integrity during operation. Monitoring/visual examination of the physical characteristics of the pebbles as they traverse the pebble handling system and destructive testing of a select number of fuel pebbles is also planned to evaluate in-pile performance. Kairos assumes a SiC failure fraction of 2.3E-03 under normal operation, 100 times above the values found in the DOE AGR program. No incremental failures under
accidents are assumed given the small changes in temperature from normal operation. The reactivity control and shutdown systems were found to provide fail safe protection in the event of loss of power. The systems could control the reactivity within proper limits and protect the fuel during anticipated operational occurrences. The staff also reviewed the structural design of the control and shutdown rods against the requirements of ASME BPVC Section III Division 5 and had reasonable assurance that the final design will conform to its design bases.
The staff reviewed the reactor vessel and its internals including the graphite reflector, core barrel and fluidic diodes against the relevant PDCs and ASME BPVC Section III Division 5. For seismic safety, these structures will be designed as SDC-3 components. Much of the technical basis for the graphite and stainless-steel vessel design is based on the two topical reports we recently reviewed on these materials. Novel features in and testing associated with these systems include (1) the need to have a restraint/hold down subassembly welded to the underside of top of the reactor vessel to counter upward buoyancy forces of the graphite reflector, (2) anti-siphon features to limit loss of coolant in the event of breaks in the primary heat transport system cold leg, (3) four fluidic diodes in the upper plenum to enable natural circulation when forced circulation is lost, and (4) planned thermal mapping of the graphite reflector using thermocouples to assure the temperature and fluence response of the graphite remains inside the testing envelope. The bioshield is concrete shielding that surrounds the reactor to protect workers from radiation. It will also be designed as seismic category SDC-3.
The nuclear and thermal hydraulic design of Hermes was reviewed. In the current configuration, the core is slightly under-moderated. The staff performed independent scoping calculations that were generally consistent with the calculations performed by Kairos. They evaluated the nuclear design including power distribution, shutdown margin, and individual reactivity coefficients of the pebbles, coolant, and reflector. Peaking factors will be confirmed using ex-core detectors during initial operation. The shutdown margin is adequate in all core states even with the most reactive shutdown rod removed from the core. All reactivity coefficients were negative except the reflector which was conservatively assessed as slightly positive. Staff has not approved the codes used by Kairos, but this is adequate at this stage in the licensing process. Testing is planned at zero power to confirm these values. Monitoring of the flux in the reactor via ex-core detectors and burnup of the pebbles as they exit the core is also planned. The thermal hydraulic analyses used several heat transfer and pressure drop correlations from testing of previous pebble bed reactors and electrically heated pebble beds.
The details of the thermal response of the core in terms of peak vessel, reflector and pebble temperatures are a function of the bypass flow through the reflector. Validation of the physics and thermal hydraulic tools (e.g., Serpent2, Star CCM+, and SAM) and assessment of uncertainties have not yet been performed and are planned by Kairos prior to the operating license (OL). Of concern is ensuring that the peak temperatures of metallic structures in the upper part of the core remain within their code allowable.
The core will contain both moderator (pure graphite) and fueled pebbles. They have the same density so both will be similarly buoyant in the Flibe coolant. During startup, moderator pebbles and fueled pebbles with natural uranium fill the core. The natural uranium fueled pebbles are replaced with the nominal fuel pebbles as they ascend to power. Once all natural uranium pebbles have been removed from the core, the core can begin its transition to equilibrium. This process will take multiple years, typical of previously operated pebble bed reactors. Careful analysis of the passive heat removal system during startup when equilibrium decay heat has not yet been reached is above the target value will be required to assure no overcooling occurs.
The reactor vessel support system is designed to prevent uplift and shear during seismic events. The entire reactor vessel and its supports sits on seismic isolators to limit seismic effects. The staff had reasonable assurance that the final design of the system will provide adequate structural support under static and dynamic loadings and will meet the relevant PDCs for the system.
Discussion I did not identify any specific deficiencies in my review. As noted in the SE, because this is a preliminary safety analysis report, many of the details of the design and the associated analyses are not complete (or may not have yet started), planned qualification testing to demonstrate the adequacy of novel features in the design is not yet complete, in-reactor testing planned during startup and monitoring and inspection details are not yet available, the analytic tools are not fully validated, and uncertainties are not fully assessed. The staff noted these items in their review and are tracking them to closure prior to the OL review or during initial reactor startup.
Recommendation As lead reviewer for Hermes SE Chapter 4, I recommend no further action on the scope of this chapter.
References
- 1.
USNRC, Draft Safety Evaluation for Hermes NonPower Reactor Preliminary Safety Analysis Report Chapter 4, February 7, 2023 (ML23065A010)
- 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.
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)
- 4.
Kairos Power LLC, Metallic Materials Qualification for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor, Topical Report, Revision 4, September 30, 2022 (ML22263A458).
- 5.
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)
- 6.
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)
- 7.
Kairos Power LLC, KPTR003NP-A, Revision 1, Principal Design Criteria for the Kairos Power Fluoride Salt-Cooled, High-Temperature Reactor, July 2019 (ML19212A756).
April 27, 2023
SUBJECT:
INPUT FOR ACRS REVIEW OF KAIROS NON-POWER REACTOR HERMES CONSTRUCTION PERMIT APPLICATION - SAFETY EVALUATION FOR CHAPTER 4, REACTOR DESCRIPTION Package No: ML23117A000 Memo Accession No: ML23117A008 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) DPetti DATE 4/27/2023 4/27/2023 4/27/2023 4/27/2023 OFFICIAL RECORD COPY