Document: NUREG-0800
Document ID: e32f0820-4e33-476e-aa36-4ca8c2c64af0
Document Type: srp
Title: Use of Probabilistic Risk Assessment in Plant-Specific, Risk-Informed Decisionmaking:
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0119/ML011940192.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
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e affected systems. Reviewers should ensure that, if cutset/sequence deletion is performed during quantification, these are correctly addressed in the assessment of risk change. In some quantification processes, cutsets that contain combinations of maintenance actions that are disallowed by the Technical Specifications are deleted from the accident sequence equations after the merging of functional cutset equations. This is done to avoid undue conservatism. If the PRA application deals with Technical Specification allowed outage issues, reviewers should confirm that any impacts on such deletions have been 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 SRP chapter 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 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. SRP 19-A19 Sequence Truncation The staff prefers that licensees 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