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:
tem analysis models (fault trees). A.5 Determination of Success Criteria a. Areas of Review Guidance in the PRA policy statement and in Regulatory Guide 1.174 stipulates that realistic analysis should be used in PRA implementation. The following discussion is intended to sort out what is meant by "realistic" analysis of success criteria by reference to SAR analysis. In order to fulfill its intended purpose, SAR analysis is ordinarily based on a set of assumptions containing significant embedded conservatisms. SAR analysis also reflects a postulated single active failure, in addition to whatever event initiated the sequence. When an SAR analysis shows a successful outcome, there is good reason to believe that (apart from beyond-single-failure scenarios) the system will meet or exceed performance requirements for the initiating event considered. Applying the SAR mission success criterion in a PRA would be conservative, in the sense that the probability of failure to meet this performance standard would be greater than probability of failure to meet a more realistic performance standard. However, re-analyzing event sequences with conventional SAR tools would be too burdensome to apply to the large number of scenarios that are defined in the course of a PRA. In addition, the rather specialized computer codes used in SAR analysis may not be appropriate in beyond-single-failure scenarios. Traditionally, development of mission success analyses in PRAs has ranged from the use of faster running models that might not have the same level of quality assurance as the conventional SAR tools to the extrapolation of results from analyses performed on similar plants. In order to satisfy the Commission’s guideline, then, reviewers should find that the licensee has not distorted the PRA insights by using a systematically conservative bias in mission success criteria and that mission success criteria used to justify changes to the plant’s design or operations have a sound technical