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:
ponent event in the PRA models to a true state. The truncation limit of the minimal cutset can be reduced by at least a factor of 10 to give added assurance of sufficient coverage. The minimal cutsets that are generated using the reduced truncation limit can then be used to determine R1 by setting the down unavailability to zero. Contributions from common-cause failures (CCFs) need special attention when calculating the increased risk level R1. If the component is down because of a failure, the common-cause contributions involving the component should be divided by the probability of the component being down because of failure since the component is given to be down. If the component is down because it is being brought down for maintenance (but not failed), the CCF contributions involving the component should be modified to remove the component and to only include failures of the remaining components (also see Regulatory Position 2.3.3 of this RG). If other components are reconfigured while the component is down, these reconfigurations can be incorporated in estimating R1 or ΔR using the PRA. If other components are tested before repair or if maintenance is carried out on the downed components, the conduct of these tests and their outcomes also can be modeled. If other components are more frequently tested when the component is down for the conditions addressed by the CT, this increased frequency of testing also can be incorporated into the PRA. These modeling details are sometimes neglected in the PRA because of their apparently small contribution. However, when isolating the CT risk contributions and in justifying modified CTs, these details can become significant. DG-1287, Appendix A, Page A-4 A-1.3.1.2 Appropriate Use of Probabilistic Risk Assessment Minimal Cutsets As indicated, a PRA computes the yearly CT risk contribution to the yearly CDF. Basically, the yearly CT risk contribution is the sum of the minimal cutset contributions containing the