ML25139A529
| ML25139A529 | |
| Person / Time | |
|---|---|
| Issue date: | 05/21/2025 |
| From: | Roberts T Advisory Committee on Reactor Safeguards |
| To: | Walter Kirchner Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML25139A529 (1) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001 May 21, 2025 MEMORANDUM TO:
Walter L. Kirchner, Lead NuScale Design-Centered Subcommittee Advisory Committee on Reactor Safeguards FROM:
Thomas E. Roberts, Member NuScale Design-Centered Subcommittee Advisory Committee on Reactor Safeguards
SUBJECT:
INPUT FOR ACRS REVIEW OF THE NUSCALE US460 STANDARD DESIGN APPROVAL APPLICATION -SAFETY EVALUATION FOR CHAPTER 8, ELECTRIC POWER In response to the Subcommittees request, I have reviewed the NRC staffs safety evaluation report (SER), with no open items, provided to support ACRS review of the standard design approval application (SDAA), and the associated sections of the applicants Final Safety Analysis Report (FSAR), for Chapter 8, Electric Power, Revision
- 1. 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 8 of the SER documents the NRC staffs review of the NuScale US460 SDAA for the design of NuScale reactor structures, systems, and components as presented in Chapter 8, Electric Power, of the NuScale US460 FSAR. The US460 reactor design was based on the US600 design, for which the NRC granted a design certification in 2020. The US460 electrical power systems were not substantively changed from those previously approved for the US600 reactor design. For example, NuScale renamed the direct current power system that powers important instrumentation and control system loads from Highly Reliable Direct Current Power System (EDSS) to Augmented Direct Current Power System (EDAS) and added description of the augmented design and quality features included to provide the requisite reliability for this important, non-safety, system.
SER Summary The NRC staff SER evaluated and documented discussions of US460 FSARs electric power systems, with special attention to changes that are being made from the US600 design. As none of the changes were deemed substantive, the draft SER is very similar to Chapter 8 of the US600 SER that was issued as part of the design certification.
W.
Discussion I agree that the changes made to the previously reviewed US600 design are minor and do not appear to change the prior conclusion that the electric power systems described in Chapter 8 support the overall demonstration of safety for the design.
There is discussion in FSAR Chapter 15 (Transient and Accident Analyses) regarding acceptability of a specific scenario where loss of EDAS is assumed to occur coincident with a reactor transient. This scenario relates to elimination of the inadvertent actuation block (IAB) feature on the emergency core cooling system (ECCS) reactor vent valves (RVVs). Specifically, NuScale states that elimination of the IAB function creates a scenario where a postulated complete loss of the redundant EDAS power system would lead to inadvertent opening of the RVVs. If this occurred during a transient (such as inadvertent reactivity addition) that increases plant temperature and power, the combination of the increased temperature/power and the RVV actuation could violate conservative minimum critical heat flux ratio (MCHFR) limits by a small amount for a short time. NuScale states that fuel temperature failure limits would not be exceeded, and no core damage is expected, on this MCHFR limit violation.
I observe that loss of EDAS is not the only failure that could lead to untimely actuation of an RVV. For example, the proposed Technical Specifications do not restrict plant operation after a failure that trips one of the two trip valves that operate to open an RVV. In this case, one trip valve would be open, and inadvertent RVV actuation would occur on failure of the control circuitry for the other remaining in-service trip valve. While extended operation in this condition is not precluded, NuScale has included a separate requirement (in FSAR Section 15.0.0.6.3) to ensure that the expected frequency of inadvertent ECCS initiation remains less than once in the life of a module (60 years). NuScale stated that operational programs (e.g., maintenance rule) necessitate evaluating the increased risk from a degraded condition whenever redundancy is reduced in a way that increases the expected frequency of inadvertent ECCS initiation. This requirement adequately addresses the potential for single failures to increase the risk of untimely RVV actuation while operating with reduced redundancy.
Recommendation As lead reviewer for NuScale Chapter 8, I recommend that the Committee not perform any additional review of this chapter.
References
- 1. NuScale Power, LLC, Standard Design Approval Application, Part 2, Chapter 8, Electrical Power, Revision 1, October 31, 2023 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23304A354)
- 2. U. S. Nuclear Regulatory Commission, Safety Evaluation of NuScale SDAA Chapter 8, Electrical Power, October 7, 2024 (ML24221A015).
- 3. NuScale, Standard Design Approval Application, Part 4, US460 Generic Technical Specifications, Vol. 1, Revision 1 (ML23304A387)
W.
May 21, 2025
SUBJECT:
INPUT FOR ACRS REVIEW OF THE NUSCALE US460 STANDARD DESIGN APPROVAL APPLICATION -SAFETY EVALUATION FOR CHAPTER 8, ELECTRIC POWER Package Accession No: ML25091A091 Accession No: ML25139A529 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 (MSnodderly for)
TRoberts DATE 5/20/25 5/20/25 5/20/25 5/21/25 OFFICIAL RECORD COPY