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:
ered. Some test and maintenance activities can contribute to some transients. Initiating event frequencies used in the PRA do not typically separate out this contribution, but such a separation may be needed during TS change evaluations. For example, the effect of test-caused transients may be evaluated in determining an SF. Initiating event frequencies from conduct of the test (i.e., test-caused transients) could then be modeled separately to evaluate the risk contribution from test-caused transients. Section A-2 of the appendix to this RG discusses data needs for estimating initiating event frequencies from test-caused transients. 2.3.3.3 Screening Criteria The main qualitative consideration for the screening of sequences in TS change evaluations is the inclusion of sequences directly affected by the TS change that would have been truncated by frequency-based screening alone. For example, if the TS change involves accumulators in a pressurized-water reactor, qualitative considerations imply the inclusion of sequences that contain the accumulators, even if these sequences do not meet the frequency criteria. Excluding these sequences would result in an underestimate of the risk impact of the proposed TS changes; and therefore, could have a significant influence on the ultimate acceptability of these changes. 2.3.3.4 Truncation Limits Truncation levels should be used appropriately to ensure that significant underestimation caused by truncation of cutsets does not occur, as discussed below. Additional precautions relevant to the cutset manipulation method of analysis are needed to avoid truncation errors in calculating risk measures. DG-1287, Page 15 When failure or outage of a single component is considered, as in the case of a CT or SF risk evaluation, the truncation 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