ML20049A046
| ML20049A046 | |
| Person / Time | |
|---|---|
| Issue date: | 02/19/2020 |
| From: | Dimitrijevic V, David Petti Advisory Committee on Reactor Safeguards |
| To: | Matthew Sunseri Advisory Committee on Reactor Safeguards |
| Lui, C, ACRS | |
| Shared Package | |
| ML20044D595 | List: |
| References | |
| Download: ML20049A046 (4) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001 February 19, 2020 MEMORANDUM TO:
Matthew W. Sunseri, Chairman Advisory Committee on Reactor Safeguards FROM:
Vesna Dimitrijevic and David Petti, Members /RA/
NuScale Subcommittee Advisory Committee on Reactor Safeguards
SUBJECT:
PROPOSED RECOMMENDATION FOR ACRS REVIEW OF NUSCALE POWER, LLC, DESIGN CERTIFICATION APPLICATION - ADVANCED SAFETY EVALUATION WITH NO OPEN ITEMS FOR CHAPTER 19, PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR NEW REACTORS In response to the Committees request, we have reviewed the NRC staffs advanced safety evaluation report (SER) with no open items for Chapter 19, Probabilistic Risk Assessment and Severe Accident Evaluation for New Reactors, dated December 12, 2019 (ML19143A106).
The following is our recommended course of action concerning further review of this chapter of the design certification application and the staffs associated safety evaluation.
Applicable Concerns from ACRS Phase 3 Letter Report 19.1 Probabilistic Risk Assessment (PRA)
To help identify risk insights in this unique design, there are technical issues in the PRA that merit additional consideration:
(1) A further examination of the design of emergency core cooling system (ECCS) valves and the associated PRA model to help build confidence that the plant risk is accurately represented.
(2) To ensure that the risk measures reported in the PRA include more realistic uncertainty results (and associated mean values), the uncertainty analysis merits further investigation.
(3) More complete sensitivity analyses would increase confidence that the plant risk is accurately represented.
(4) Even though we agree with the reason for excluding detailed failures from the reactor building crane analysis (due to lack of the final design details), we believe that this operator error (of commission) should be identified and included as a risk-significant human action in Table 19.1-70 and in Chapter 18.
19.2 Severe Accident Evaluation The Committee did not identify any specific concern with regard to 19.2 in Phase 3. The largest change from the Phase 2 to Phase 4 SER in this section is associated with equipment survivability related to postaccident sampling to provide information on the conditions in containment during severe accidents.
Staff Response to ACRS Letter Report The NRC staff acknowledged the technical issues raised by the ACRS in its letter and noted that the scope of the staffs review of a design certification PRA is to ensure the applicant has adequately addressed the Commissions objectives regarding consideration of severe accidents and the use of PRA in the design of the facilities.
These objectives are focused on identification and reduction of potential vulnerabilities, identification of risk-informed safety insights, comparison to the Commissions safety goals, and support for other programmatic objectives (e.g., regulatory treatment of non-safety systems and the reliability assurance program).
The staff performed an extensive review of the NuScale DCA and audits of the supporting notebooks, including an assessment of sensitivity studies and uncertainty quantification, to verify that the PRA will support the intended uses of a design certification PRA once all open items are resolved. The staff utilized the agency commissioned enhanced safety focused review approach and took into consideration the unique design features to limit its review to the information needed to make reasonable assurance findings that the Commissions objectives were met.
In particular, the staff noted that a COL holder will be required by 10 CFR 50.71(h) to develop a Level 1 and a Level 2 PRA covering those initiating events and modes for which NRC-endorsed consensus standards on PRA exist one year prior to the scheduled date for initial loading of fuel. The staff stated that it will consider revisiting the technical issues raised by the ACRS as it prepares its Phase 4 advanced SER with no open items.
Open Items from Phase 3 Requiring Further ACRS Review We consider Issue (1) is still opened; it will be evaluated further as part of the ECCS & Valve Performance focus area.
The staff made minor changes in the SER considering Issue (2), for example:
the staff finds that the uncertainty could be larger than indicated; however, even with greater uncertainty, the low CDF estimate reflects deliberate engineering and design effort to reduce or eliminate the contributors to CDF found in previous designs.
We recognize, but not necessarily agree with, the argument made above. We plan to elaborate further on Issues (2) & (3) as part of Committees upcoming PRA focus area review.
We will also elaborate further on Issue (4) in cooperation with the considerations identified in Chapter 18.
As a focus area itself, the PRA review is an important part of all ongoing focus area reviews.
Applicable findings from all these reviews, and their impact on the PRA model and results, will be considered as part of our PRA focus area review.
Based on our review of Chapter 19.2 and the Committees source term focus area review, a meeting with the staff is planned to discuss the topic of post-accident sampling in March 2020.
RECOMMENDATION As lead reviewers for NuScale Chapter 19, we recommend that the Committee not perform any additional Phase 5 review of this chapter, beyond the scheduled focus area reviews.
February 18, 2020
SUBJECT:
PROPOSED RECOMMENDATION FOR ACRS REVIEW OF NUSCALE POWER, LLC, DESIGN CERTIFICATION APPLICATION - ADVANCED SAFETY EVALUATION WITH NO OPEN ITEMS FOR CHAPTER 19, PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR NEW REACTORS Package No.: ML20044D595 Accession No: ML20049A046 Publicly Available Y Sensitive N Viewing Rights:
NRC Users or ACRS Only or See Restricted distribution *via email OFFICE ACRS/TSB SUNSI Review ACRS/TSB ACRS NAME CLui CLui LBurkhart (CBrown for)
WKirchner (CBrown for)
DATE 2/18/2020 2/18/2020 2/18/2020 2/18/2020 OFFICIAL RECORD COPY