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:
ve the result that each individual system could meet all quantitative guidelines to be categorized in the low safety-significance group. It would be prudent, and the licensee is expected, to designate at least one system associated with critical high-level functions as high safety significant. " Screening analyses are used to dismiss some functional failures as insignificant. In many cases, credit for the redundancy or reliability of plant systems or structures is taken to bolster the arguments that the functional failure need not be modeled. Thus, the importance of some systems, functions, and structures will not show up in the PRA results since the functional failure is screened out. (For example, screening out certain containment penetrations because of the number of isolation valves involved obscures the importance of the containment isolation function of the system.) " Risk insights from nonquantitative external event and shutdown risk studies should also be used. All the system functions credited in these studies should initially be categorized as "high safety- significant" candidates. Final categorization into a lower safety-significance category should include consideration of the initiating event's frequency or magnitude and the ability of the SSC to respond to the event. " Risk insights from the evaluation of the Maintenance Rule (10 CFR 50.65) should be incorporated to ensure the identification of functions that are (1) relied upon to mitigate accidents; (2) used in emergency operating procedures; (3) those whose failure could prevent a safety-related SSC from performing its safety- related function; and (4) those whose failure could cause a reactor scram or actuation of a safety- related system. * PRA importance measures do not fully address the significance of SSCs that support operator actions for emergency and severe accident management. Such systems can include environmental controls, lighting, alarms, communications, and annuncia- tors.