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:
is, the component unavailability is set to “false.” A-1.3.1.1 Using Probabilistic Risk Assessment to Obtain Completion Time, Preventive Maintenance, and Configuration Risk Contributions R1 can be calculated by setting the component-down event to a true state in the PRA. Similarly, R0 can be calculated by setting the component-down event to a false state in the PRA. The component-down event in the PRA is the event describing that the component is down for repair or maintenance. If the existing minimal cutsets include the component-down event, these minimal cutsets can be used to determine R1 and R0 provided the minimal cutsets sufficiently cover the contribution of the down event. The existing minimal cutsets are sufficient if those containing the down event are not all near the truncation limit (i.e., are not all within a factor of 10 of the truncation limit). Alternatively, the minimal cutsets are sufficient if those containing the down event have a nonnegligible contribution (i.e., a contribution greater than or equal to 1 percent). If the existing minimal cutsets are sufficient, the increased risk level R1 can be determined by setting the component-down unavailability to 1 and deleting larger minimal cutsets that contain smaller minimal cutsets (i.e., those absorbed by the smaller minimal cutsets). If any minimal cutsets contain complementary events, they also should be removed if they are inconsistent with the component being down. The reduced risk level R0 can be determined analogously by setting the down unavailability to zero. If the existing minimal cutsets do not contain the component-down event, or if there is a question on the coverage of the existing minimal cutsets, the minimal cutsets should be regenerated. R1 is determined by setting the down-component 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