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:
en correctly addressed. • Reviewers should ensure that, if operator recovery actions are incorporated after the initial quantification, these actions are still valid in light of the proposed change. Section A.8 of this appendix discusses this in more detail. • Circular logic in fault trees will cause the quantification process to abort. This is a problem for systems such as the emergency service water system, which provides cooling to the emergency diesel generators, but requires power from those diesel generators when offsite power is lost. Another example is the mutual dependency between the DC and AC power systems. In situations such as these (i.e., when the physical situation has embedded circular dependencies), analysts have to break this circularity to allow for model solution. For changes on systems that are affected by circular logic, reviewers should investigate the manner in which the circularity was 19.2-58 June 2007 broken (usually in the sequencing of functions in the event tree) and should verify that the dependency is still being accounted for in the evaluation of the risk change. Sequence Truncation Licensees may calculate the change in risk from the application by requantifying the base PRA model so that the potential effects of originally truncated events can be accounted for should they become important as a result of an application. If the licensee did not requantify the model, or if the application depended on the risk ranking of SSCs from a pre-solved equation, reviewers should use the guidelines provided below. Reviewers should be assured (either by documentation provided in the licensee’s submittal or by an independent staff analysis) that cutset or scenario truncation did not introduce errors into the application results or the logic of the PRA that affects the application. The staff review could also involve performing (or reviewing) sensitivity studies where the truncation limit is lowered for the dominant sequences and event