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:
ion levels in evaluating R1 and R0 are of concern. R1 is the increased CDF, with the component assumed to be inoperable (or equivalently, the component unavailability set to “true”), and R0 is the reduced CDF, with the component assumed to be operable (or equivalently, the component unavailability set to “false”). If the component in question appears in the cutsets near the truncation limit (e.g., all appearances are in cutsets within a factor of 10 of the truncation limit), it may be necessary to reduce the truncation limit. If R1 is marginally larger than the base case value, then one order of additional cutsets should be generated to ensure that any underestimation did not take place. When considering risk from plant configurations involving multiple components, a cutset with a relatively small frequency can become a significant contributor to the CDF. This is because more than one of the affected components may appear in the same minimal cutset, and the unavailability (increased by the TS change) of more than one of these components could cause a significant increase in the cutset’s frequency. For such cases, truncation levels must be reduced by a larger amount than would be the case for single components. Particular care should be taken if the evaluation of R1 is based on requantification of pre-solved cutsets, as the events related to the component of concern may not even appear in the cutsets. 2.3.4 Assumptions in Completion Time and Surveillance Frequency Evaluations When using PRA to evaluate TS changes, the evaluation should consider the assumptions made within the PRA that could have a significant influence on the ultimate acceptability of the proposed changes. The submittal requesting the TS changes should discuss such assumptions. Assumptions that CT change evaluations should consider include the following: a. If CT risk evaluations are performed using only the PRA for power operation (i.e., to calculate the risk associated with (1) the