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
CFR Part: 
CFR Title: 

Content:
lfills the intent of the General Design Criteria of 10 CFR Part 50, Appendix A. Reviewers can assess fulfillment of the above guidelines by using qualitative or traditional engineering arguments or by using PRA results contained in the accident sequences or cutsets. III.2.1.1.2 Role of PRA in Review of Defense in Depth In addition to the usual quantitative risk indices, PRAs provide important qualitative results, namely, the contributors to accident sequences. For PRAs that use the fault tree linking approach, these contributors are described by the accident sequence minimal cutsets. Each accident sequence minimal cutset is a combination of passive and active SSC failures and human errors 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 19.2-10 June 2007 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