ML25119A006
| ML25119A006 | |
| Person / Time | |
|---|---|
| Issue date: | 05/08/2025 |
| From: | Harrington C Advisory Committee on Reactor Safeguards |
| To: | Walter Kirchner Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML25119A006 (1) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001 May 08, 2025 MEMORANDUM TO:
Walter L. Kirchner, Chairman Advisory Committee on Reactor Safeguards FROM:
Craig Harrington, Member Advisory Committee on Reactor Safeguards
SUBJECT:
INPUT FOR ACRS REVIEW OF THE NUSCALE STANDARD DESIGN APPROVAL APPLICATION - SAFETY EVALUATION WITH NO OPEN ITEMS FOR CHAPTER 6, ENGINEERED SAFETY FEATURES In response to the Committees request, I have reviewed the NRC staffs safety evaluation report (SER) provided to support ACRS review of the standard design approval application (SDAA), with no open items for Chapter 6, Engineered Safety Features. The following is my recommended course of action concerning further review of this chapter of the SDAA and the staffs associated SER.
SER Summary Chapter 6 of the SER documents the staffs review of Revision 1 of Chapter 6, Engineered Safety Features, of the NuScale SDAA, Final Safety Analysis Report (FSAR). Additionally, this advance SER also reflects updated text submitted by letter which will be incorporated into Revision 2 and docketed later.
The engineered safety features (ESFs) that are part of an individual NuScale Power Module (NPM) include the containment systems, the emergency core cooling system (ECCS), and fission product removal and control systems. Also addressed in Chapter 6 are control room habitability systems, and in-service inspection and testing of related ESF systems and components.
The staff completed a thorough review of this chapter and concluded that the NuScale design elements therein and associated combined license (COL) items met regulatory criteria.
Discussion Although the US460 design is fundamentally the same as the US600 approved in the design certification, notable changes have been made, including to the reactor building pool (ultimate heat sink) and within the NPM-20 module itself. This evolution to a more robust design as reflected in the US460 plant is a positive development and, in several instances, has effectively addressed areas of prior Committee concern.
W.
6.1 ESF Materials: The NPM-20 upper portion of the containment vessel (CNV) from above reactor vessel flange elevation (thus including both the lower and upper CNV flanges), will be fabricated from martensitic stainless steel (F6NM). The remainder of the lower CNV will be fabricated from FXM-19 austenitic stainless-steel forgings, with a dissimilar metal weld between the F6NM and FXM-19 components. These are notable changes in materials as compared to the NPM-160 design where the upper CNV was fabricated from low alloy steel with all wetted surfaces clad with weld deposited 3xx stainless steel, and the lower CNV, including the flange, was FXM-19. There are no open items or confirmatory/COL items for this section and the advance SER concludes these changes are acceptable.
6.2 Containment Systems: The NPM-20 design incorporates several notable containment system design improvements relative to the NPM-160 design. The CNV design pressure and temperature were increased which establishes improved containment response design margins. Of the spectrum of primary and secondary mass and energy releases evaluated, peak containment pressure (919 psia) and temperature (532 deg. F) in the NPM-20 design result from a chemical and volume control system discharge line break.
These values are modestly different from NPM-160, and the limiting events are different, but they are fully accommodated in the design and remain acceptable.
NPM-20 incorporates a safety related passive autocatalytic recombiner within the CNV for combustible gas control by maintaining an acceptably low oxygen concentration. This change addresses a previous Committee concern in the NPM-160 design regarding post-accident sampling of the CNV atmosphere for combustible gases and supports an NPM-20 exemption request from Title 10 of the Code of Federal Regulations (10 CFR) 50.34(f)(2)(xvii)(C) for combustible gas monitoring. Additionally, the applicant has proposed Principal Design Criterion 41 to meet the combustible gas control intent of General Design Criterion (GDC) 41. The advance SER approves these exemptions.
Regarding containment heat removal, the reactor building pool (ultimate heat sink) elevation control band has been lowered in the US460 design to better match the passive heat transfer rate to the pool with the available residual and stored energy. Given the unique NuScale passive design of a CNV largely immersed in a large pool of water, the advance SER approves an exemption to the GDC 40 requirement for periodic testing of the containment heat removal systems for NPM-20, as for NPM-160. To support long-term cooling, the COL item requiring a containment cleanliness program limiting debris within the CNV is also carried forward from NPM-160.
The NPM-20 containment isolation valve design configuration has also been modified to include a containment isolation test fixture to better support periodic containment isolation valve (CIV) local leak rate testing.
NuScale sought exemptions for NPM-20 to GDC 52 and 10 CFR 50, Appendix J, regarding periodic integrated leak rate testing, because the CNV is designed as a ASME Section III, Class 1 pressure vessel and will be hydro-tested as per the ASME code; it is 100% inspectable (every 10 years); and all bolted flanges and connections will be locally leak tested or inspected at each refueling. Exemptions to GDCs 55 and 56 to place both isolation valves outside the CNV, and GDC 57 to credit a closed system (DHRS), were also proposed by the applicant.
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An exemption was also requested to the 10 CFR 50.34(f)(2)(xiv) item (E) requirement for external release paths to be isolated on a high containment radiation indication because the NPM-20 design ensures that isolation will occur without explicitly meeting this requirement. The staff approved these proposed exemptions with one COL item related to the containment leak rate.
Finally, the staff reviewed the materials selected for the CNV and found that the applicant met the fracture toughness requirements of GDC 51.
6.3 Emergency Core Cooling System: The ECCS is a passive system that includes four valves with independent hydraulic actuation systems. When actuated, ECCS vents steam through two reactor vent valves (RVVs) mounted at the top of the reactor pressure vessel (RPV) to the CNV where condensate accumulates and is then returned through the two reactor recirculation valves to the downcomer region of the RPV. Notably, the NPM-160 design employed three RVVs and included inadvertent actuation block valves in the RVV control system which have been removed from the NPM-20 design.
The ECCS does not provide additional coolant to the system but is instead designed to retain sufficient inventory in the RPV to keep the core covered during all postulated events.
However, the NPM-20 design does incorporate an ECCS supplemental boron system and a set of lower riser flow holes to mitigate boron dilution within the flow and retention volumes of the reactor coolant system and CNV during extended passive cooling. These are significant improvements relative to NPM-160 as boron dilution questions and a narrowly defined scenario potentially leading to a return to criticality were thoroughly vetted by the Committee during the design certification review and were noted in the Committees final letter. For NPM-20, shutdown margin during the passive cooling phase is further evaluated in NuScale TR-124587, Extended Passive Cooling and Reactivity Control Methodology. The Committee expressed concerns during review of TR-124587 (March 4, 2025) that under certain conservatively defined scenarios, analytical uncertainties might negate the predicted, moderately positive shutdown margin. This issue was discussed further during the Chapter 15 review on April 1, 2025.
In the final design certification letter, the Committee also noted performance of the unique ECCS valve systems as an important risk contributor to the PRA. The Committee letter stated NuScale will perform extensive qualification testing to provide confidence in the ability of the valves to maintain their required performance after extended periods in an operational environment and concluded these additional actions should address the underlying safety concerns. For the SDAA review, residual Committee concerns regarding reliable valve operation (opening) on demand were discussed on April 2, 2025 following the Chapter 15 review and are considered resolved.
6.4 Control Room Habitability: The control room habitability system is not a safety function because the NPM-20 design does not credit any control room operator actions to mitigate design-basis events. NuScale requested exemption to GDC 19 and proposed Principal Design Criterion 19 as meeting the underlying purpose of the remote shutdown portion of GDC 19. The advance SER approves the requested GDC 19 exemption.
The staffs review of these and other changes reflected in the US460 design, as compared to the US600 certified design, was thorough with well supported conclusions throughout. Member questions and concerns were addressed by both the applicant and staff during the February 18, 2025, subcommittee meeting on Chapter 6. However, certain lines of questioning were deferred
W.
to the remaining FSAR Chapter and Technical Report reviews, including Chapters 15 and 19.
Notably, these included further review of shutdown margin under relevant postulated extended passive cooling scenarios and ECCS valve performance.
Recommendation As lead reviewer for NuScale Chapter 6, I recommend that the Committee not perform any additional review of this chapter.
References
- 1. U. S. Nuclear Regulatory Commission, Safety Evaluation of NuScale SDAA Chapter 6, Engineered Safety Features, February 12, 2025 (ADAMS Accession Nos.
ML25041A202 (Public) ML24327A060 (Non-Public)).
- 2. NuScale Power, LLC, Standard Design Approval Application, Part 2, Chapter 6, Engineered Safety Features, Revision 1, October 31, 2023 (ADAMS Accession No. ML23304A345).
- 3. NuScale Power, LLC, TR-124587, Extended Passive Cooling and Reactivity Control Methodology, Revision 0, January 5, 2023 (ADAMS Accession Nos. ML23005A308 (Public) ML23005A309 (Non-Public)).
W.L. Kirchner May 08, 2025
SUBJECT:
INPUT FOR ACRS REVIEW OF THE NUSCALE STANDARD DESIGN APPROVAL APPLICATION - SAFETY EVALUATION WITH NO OPEN ITEMS FOR CHAPTER 6, ENGINEERED SAFETY FEATURES Package Accession No: ML25091A091 Accession No: ML25119A006 Publicly Available (Y/N): Y Sensitive (Y/N): N If Sensitive, which category?
Viewing Rights:
NRC Users or ACRS only or See restricted distribution OFFICE ACRS SUNSI Review ACRS ACRS NAME MSnodderly MSnodderly LBurkhart CHarrington DATE 4/28/25 4/28/25 4/28/25 5/01/25 OFFICIAL RECORD COPY