Document: NUREG-0800
Document ID: 7916b088-fb90-4163-84fe-027bd315bcc5
Document Type: srp
Title: REVIEW OF RISK INFORMATION USED TO SUPPORT PERMANENT PLANT-
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0717/ML071700658.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.2
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Content:
by Technical Specifications, quality assurance, and maintenance. Therefore, when a review of the minimal cutsets reveals areas where redundancy or diversity are already marginal, it would arguably be inappropriate to reduce the level of activities aimed at ensuring SSC performance. (The exception would arise if the licensee can show that the activities have little or no effect on SSC performance, or if it can be shown that uncertainties in the performance of the elements in this cutset are well understood and quantified. It is also possible that the licensee could propose compensating or alternative activities to provide assurance of SSC performance.) The objective of this review is to avoid completely relaxing the defense-in-depth posture at points at which the plant design has the least overall functional independence, redundancy, and/or diversity. On the other hand, in areas where a plant has substantial redundancy and diversity, defense-in-depth arguments used to justify relaxations should be given appropriate weight. As part of the defense-in-depth evaluation, reviewers should consider the effects of multiple component failures and common cause failures that could result from the proposed change. For example, if the licensee proposes to reduce the requirements for all events in a cutset, reviewers should ensure that the effect of the change is properly modeled and that the change does not have an adverse effect on defense in depth. Finally, in assessing the accident sequence cutsets, reviewers should devote attention to potential over-reliance on programmatic activities or operator actions that compensate for weaknesses in the plant design. For example, proposed maintenance and surveillance activities should complement and not replace proper plant design. III.2.1.2 Safety Margins In the determination of the design performance characteristics of a system, safety margin represents an allowance for uncertainty in SSC performance. Current safety analysis practices