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:
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 QLA, QMA, QTA, and QC will be part of the PRA input data. To calculate the conditional CDF, given that the component is failed, the component unavailability should be represented by the “T” state. This means that QLA, QMA, and QTA should be changed to the “T” state and QC should be divided by QLA since the component is down because of failure. In principle, changing one of the three conditions (QLA, QMA, QTA) to the “T” state should suffice. However, in many cases, truncated cutsets are used to calculate the conditional CDF, and changing all three will ensure that the failed state of the component is represented. For this example, QC will be changed to β, which represents the conditional failure probability of the redundant component. When QC represents the failure of more than two components, QC will be converted to the failure probability of the remaining components—in this case, two components. A-1.3.2.2 Conditional Core Damage Frequency When a Component Is Down (but Not Failed) for Preventive Maintenance To calculate the conditional CDF when a component is taken down for PM (R1 for PM analyses), the CCF term should be treated differently from that described above for the failure of the component. Considering the same example as above, the down state of the component is represented by changing QLA, QMA, and QTA to “T” and by changing QC to QL, which is numerically the same as QLB or QLA. The CCF term is changed to represent the unavailability of the remaining component and not β because the initial component is already down for PM and not because of failure. If the redundant component is successfully tested before taking the component down for PM, QC can then be equated to zero for a short-duration PM (i.e., when the duration of the PM is much