Document: NRC Regulatory Guide
Document ID: c7a40fcc-fc9d-4eb2-ad86-f9f5b0f04c82
Document Type: regulatory_guide
Title: Plant-Specific, Risk-Informed Decisionmaking:  Technical Specifications (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1920/ML19206A489.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.177
CFR Part: 
CFR Title: 

Content:
Test-Limited Risks When multiple SFs are modified or defined, the total test-limited risk from the multiple SF changes or definitions should be properly evaluated. Instead of using the PRA to evaluate all the changes in a given run, the individual test-limited risks can be evaluated one at a time, provided that the updated SFs are used for the other relevant components. An iterative procedure can then be used, in which individual SFs are successively updated using the methods described above for individual component SF risk contributors. These one-at-a-time, or “iterative,” evaluations are useful if acceptable guidelines on test-limited risks are defined and the SFs are selected to satisfy the risk guidelines. A-1.3.2 Appropriate Calculation of Conditional Core Damage Frequency A-1.3.2.1 Conditional Core Damage Frequency for Failure of a Component To calculate the conditional CDF when a component is failed (typically represented by R1 in this RG), the component unavailability is changed to the “true,” or “T,” state. However, the component unavailability may be modeled in terms of many contributors: random failure, maintenance downtime, test downtime, and CCF. The CCF term represents the failure probability of two or more redundant components that include the failed component in question. The CCF term is modeled as a product of multiple terms (e.g., using the β-factor model for two redundant components, the CCF term is β times the component unavailability from random failures) but may be represented by one parameter. Consider a component Q in train A of a safety system and let QLA, QMA, and QTA represent the component’s unavailability from random failures, maintenance downtimes, and test downtimes, respectively. Also, let QC = βQL be the term for CCF of the redundant components in trains A and B, where QL is numerically equal to QLA and represents QLA or QLB. QLB is the unavailability of a component in train B from random failure. Usually, the terms