Document: NUREG-0800
Document ID: ac43f632-9db1-4857-9f56-bfba45965456
Document Type: srp
Title: * These are modifications to a plant’s design, operations, or other activities that require NRC approval. These modifica
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0232/ML023250195.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19
CFR Part: 
CFR Title: 

Content:
ors that would cause core damage or a release of radioactivity. The cutsets therefore directly show one particular aspect of defense in depth, in that they reveal how many failures must occur in order for core damage or radiological release to occur. Thus, the minimal cutsets show the effective redundancy and diversity of the plant design. For analysis approaches that use event trees with boundary conditions, the results take the form of accident sequence descriptions and typically include elements representing unavailabilities of systems (or trains of systems) rather than components. However, in most cases, cutsets providing a component level decomposition of the system (or train) unavailabilities are provided, and an equivalence to the minimal cutset description can be established if necessary. In most cases, events that appear in each minimal cutset are targeted by programmatic activities to ensure the reliability of the associated SSC. Specific activities that are important to maintain the reliability of a component include IST, ISI, periodic surveillance required 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, SRP 19-11 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