Document: NRC Regulatory Guide
Document ID: 22ccfd5e-c5d8-4615-a02c-32369aa9f533
Document Type: regulatory_guide
Title: Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1930/ML19308B636.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.200
CFR Part: 
CFR Title: 

Content:
ange in the PRA that results in the applicability of one or more supporting requirements that were not previously included within the PRA (e.g., performing qualitative screening for Part 4 of ASME/ANS Level 1/LERF PRA standard when the related high-level requirement was previously not applicable, or adding a new hazard model), an implementation of a PRA method in a different context, or the incorporation of a PRA method not previously used. realism an accurate representation (to the extent practical) that reflects the expected response of the as-built and as-operated plant. risk significance Design or operational features of the plant, including operator actions, that are important contributors because of their ability to either increase or decrease the risk. With regard to a risk significant contributor (e.g., risk significant accident sequence, risk significant basic event, risk significant HFE, etc.), significance (or contribution) is measured with respect to the degree to which the contributor impacts the decision under consideration. For the base PRA model, significance can be measured with respect to the contribution to the total CDF or LERF, or it can be measured with respect to the contribution to the CDF or LERF/LRF for a specific hazard group or POS, depending on the context. For example, for the purposes of defining capability categories, the ASME/ANS Level 1/LERF PRA standard defines significance at the hazard group level. Whatever the context, the numerical criteria in the definitions of significant accident sequence and significant basic event/contributor are recommended. significant accident sequence An accident sequence is considered significant when it is part of the set of sequences that, when ranked by risk contribution from highest to lowest, compose 95 percent of the CDF or the LERF/LRF, or that individually contribute more than 1 percent to the CDF or LERF/LRF. This set of risk significant accident sequences may be defined functionally or in