Document: NRC Regulatory Guide
Document ID: a5ee4c78-1135-4bb6-8d54-e974a3402f87
Document Type: regulatory_guide
Title: An Approach for Plant-Specific, Risk-Informed Decisionmaking: Graded Quality Assurance
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1221/ML12216A017.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.176
CFR Part: 
CFR Title: 

Content:
riginal PRA logic model to minimize truncation effects. These sensitivity studies are desirable since human actions and CCF probabilities are derived from models requiring extensive interpretation and manipulation of observable data. When an SSC moves into the high safety-significant category as the result of a sensitivity study, the expert panel should consider the reasonableness of the recovery action or CCF event that caused the low safety significance in the original results and consider assigning the SSC into a higher safety-significant category. If the sensitivity studies are not performed, additional peer and NRC staff review of the human error and CCF probability development may be necessary to develop confidence that the quantitative results provided to the expert panel are sufficiently robust to support the categorization process. When each SSC is categorized, the safety significance of all the functions that SSC supports must be known. Therefore, the PRA model element most applicable to the SSC grading process described in this regulatory guide is a system function failure. System function importance provides the expert panel clear and documented information referencing individual component functions to plant safety functions. Developing system functional importance will assist in both the risk categorization process and the NRC staff review. If basic event (such as component failure) importance measures, rather than system function importance measures, are used to directly categorize SSCs at the component level, the categorization process becomes more dependent on PRA characteris- tics such as system success criteria, system modeling detail, and component modeling guidance. System functions generally require the proper operation of a group of SSCs and are represented in the PRA models as a set of logically linked basic events. Some PRA codes are not well suited to the development and quantification of system level importance measures. One alternative