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:
accounted for in evaluating of the change in risk, as illustrated by the following examples: • Reviewers should ensure that the licensee accounted for model asymmetries during the application of the PRA models. Asymmetries could result from modeling assumptions (e.g., assuming one train to be the operating train, and the second train to be the standby train), from differences in support system alignment, or from actual differences in system design or operating procedures. The licensees should have accounted for these asymmetries when evaluating changes to the 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