ML24291A115
| ML24291A115 | |
| Person / Time | |
|---|---|
| Issue date: | 10/22/2024 |
| From: | Roberts T Advisory Committee on Reactor Safeguards |
| To: | Walter Kirchner Advisory Committee on Reactor Safeguards |
| References | |
| Download: ML24291A115 (1) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001 MEMORANDUM TO:
Walter Kirchner, Chair NuScale Subcommittee Advisory Committee on Reactor Safeguards FROM:
Thomas Roberts, Member NuScale Subcommittee Advisory Committee on Reactor Safeguards
SUBJECT:
INPUT FOR ACRS REVIEW OF NUSCALE US460 REACTOR STANDARD DESIGN APPROVAL APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 7, INSTRUMENTATION AND CONTROLS In response to the Subcommittees request, I have reviewed the NRC staffs draft safety evaluation report (SER) with no open items, and the associated sections of the applicants Final Safety Analysis Report (FSAR), for Chapter 7, Instrumentation and Controls, Revision 1. The following is my recommended course of action concerning further review of this chapter and the staffs associated safety evaluation.
Background
Chapter 7 of the SER documents the NRC staffs review of the NuScale US460 Standard Design Approval Application (SDAA) for the design of NuScale reactor structures, systems, and components (SSCs) as presented in Chapter 7, Instrumentation and Controls, of the NuScale US460 Final Safety Analysis Report (FSAR), Revision 1. The US460 reactor design was based on the US600 design, for which the NRC granted a design certification in 2020. The US460 instrumentation and control (I&C) systems were not substantively changed from those previously approved for the US600 reactor design. For example, NuScale replaced digital pressure sensors with analog sensors, which allowed them to simplify the assessment of digital I&C diversity documented in the FSAR.
SER Summary The NRC staff SER evaluated and documented discussions of US460 FSARs I&C 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 7 of the US600 SER that was issued as part of the design certification.
Discussion I agree that the changes made to the previously reviewed US600 design are minor and do not change the prior conclusion that the I&C SSCs described in Chapter 7 support the overall 10/22/24 Thomas E. Roberts Digitally signed by Thomas E.
Roberts Date: 2024.10.22 15:09:03 -04'00'
W. Kirchner demonstration of safety for the design. In my view, there is one portion of the safety case that warrants clarification before being applied as a precedent for a different reactor that would leverage the NuScale safety case. The rest of this discussion identifies that clarification and suggests questions that the NRC staff should pursue if another applicant were to cite the NuScale justification.
The clarification relates to the following statement provided in Section 7.1.1.1 (Design Bases) regarding the approach to meet the intent of the Anticipated Transient Without Scram (ATWS) regulation stated in 10 CFR 50.62: The design meets the intent of 10 CFR 50.62 by demonstrating the redundancy and diversity of the MPS design, which avoids common cause failures and reduces the probability of a failure to scram (Section 15.8). This statement implies that diversity within the module protection system (MPS) architecture itself is sufficient to meet the intent of the ATWS regulation regardless of the consequences of a postulated ATWS event.
In support of this statement, Section 7.1.5 of the FSAR provides a detailed assessment of the protection provided against common cause failures (CCFs) within the digital portions of the MPS. This assessment does not specifically address potential CCFs associated with the non-digital portions of the system. For example, the US600 FSAR discussion of CCFs involving pressure signals was deleted from the US460 FSAR since the pressure sensors were modified from digital to analog. Additionally, neither the US600 nor US460 FSAR addresses the potential for CCFs in the Neutron Monitoring System (NMS), since this system is analog. It should be noted that 10 CFR 50.62 was promulgated in 1984, in advance of the adoption of digital I&C, based on concerns with CCFs regardless of the technology used.
In addition to diversity within the MPS, NuScale also documents transient analyses that demonstrate that the consequences of an ATWS event would not be catastrophic were such an event to occur. For example, a NuScale technical report from 2013 provided two justifications to support meeting the intent of the ATWS regulation: diversity within the MPS, and the consequences of an ATWS event not being catastrophic were it to happen. This second justification is validated by discussion in Section 19.2.2.1 of the FSAR. A similar two-part justification is also provided in Section 3.2.1 of NuScale US460 SDAA Part 7, Exemptions, which lists both MPS Diversity and ATWS Response as parts of the technical basis for an exemption to the ATWS regulation. In the Chapter 7 SER draft (as in section 7.1.5.4.5.1.2 of the issued US600 design certification SER), NRC staff cites the sensitivity study in FSAR 19.2.2 in their assessment of the basis for the NuScale approach to ATWS.
My observation is that, since neither Chapter 7 nor Chapter 15 (section 15.8) include discussion of the role of ATWS consequences, the FSAR is not entirely clear on the complete technical basis to accept MPS diversity as sufficient to meet the intent of the ATWS regulation. It is evident to me that the NuScale approach to ATWS is sufficient given the limited consequences of the event and the considerable diversity within the MPS design. In response to a question at our August 22, 2024, subcommittee meeting, NuScale personnel stated they can't really speculate on how they would have presented the ATWS justification without the analysis of ATWS consequences, but what they presented in the SDAA includes the analysis that was done. The role of the consequence of an ATWS event should be considered when using the NuScale approach as a precedent by subsequent applicants.
Recommendation:
The NRC staff should issue Chapter 7 of the SER for the US460 FSAR. Additionally, the staff should continue to consider the role of ATWS consequence for any future applicant who, consistent with the text of NuScale FSAR section 7.1.1.1, concludes the degree of diversity
W. Kirchner within a single integrated system is sufficient to meet the intent of the ATWS regulation. The staff should also evaluate the need to request the applicant to revise the FSAR to more clearly document the role of ATWS consequences in the justification for their approach to ATWS.
References:
- 1. NuScale Power, LLC, US 460 Final Safety Analysis Report (FSAR), Revision 1, November 16, 2023, (Agencywide Documents Access and Management System (ADAMS) Package No. ML23306A033).
- 2. U. S. Nuclear Regulatory Commission, Safety Evaluation of NuScale SDAA Chapter 7, Instrumentation and Controls, July 22, 2024 (ADAMS Accession No. ML24080A181).
- 3. U. S. Nuclear Regulatory Commission, Title 10 of the Code of Federal Regulations, Part 50, paragraph 50.62, Requirements for reduction of risk from anticipated transients without scram (ATWS) events for light-water-cooled nuclear power plants, https://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-0062.html.
- 4. NuScale Power, LLC, Technical Report, NuScale Power Plant Design for ATWS and 10 CFR 50.62 Regulatory Compliance, (NP-ER-0000-2196-P), September 2013 (ADAMS Accession Nos. ML13263A109 (Public); ML13263A110 (Non-Public)).
- 5. NuScale Power, LLC, US460 Plant Standard Design Approval Application, Part 7, Exemptions, Revision 1 (ADAMS Accession No. ML23304A389).
- 6. U. S. Nuclear Regulatory Commission, US600 FSAR SER Chapter 7, (ADAMS Accession No. ML20204B028), Section 7.1.5.4.5.1.2, No Undue Risk to Public Health and Safety.
W. Kirchner
SUBJECT:
INPUT FOR ACRS REVIEW OF NUSCALE US460 REACTOR STANDARD DESIGN APPROVAL APPLICATION - DRAFT SAFETY EVALUATION FOR CHAPTER 7, INSTRUMENTATION AND CONTROLS Package Accession No: ML24291A111 Accession No: ML24291A115 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 CAntonescu CAntonescu LBurkhart TRoberts DATE 10/09/24 10/09/24 10/09/24 10/17/24 OFFICIAL RECORD COPY 10/22/24