Document: NRC Regulatory Guide
Document ID: 2459a562-8984-421c-8058-d096dbb9617c
Document Type: regulatory_guide
Title: Guidelines for Categorizing Structures, Systems, and Components in Nuclear Power Plants According to Their Safety Significance
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0314/ML031430373.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.201
CFR Part: 
CFR Title: 

Content:
ety significance (i.e., the margin it provides in preventing core damage) of an SSC, whether it be evaluated for a single SSC or a group of SSCs. The RAW value provides the factor that the CDF or LERF increases when the SSC enters a failed state or, for a group of SSCs, when a common cause failure (CCF) 5 degradation mechanism manifests itself to the point that multiple SSCs are in a failed state. NEI 00-04 excludes the RAW of the CCF probability associated with a SSC from the importance measure calculations. The NRC believes it is appropriate to include the RAW of the CCF probability to assess the RAW associated with a component since the CCF contribution is a distinct contribution resulting from a specific failure mechanism not represented in the other basic events. The consequences of common cause events are of concern and as such, the risks from these types of events need to be fully assessed. NUREG/CR-5485, Guidelines on Modeling Common-Cause Failures in Probabilistic Risk Assessment, notes that more than 2,500 common cause events involving the majority of significant PRA SSCs are documented in the NRC common cause data base. If a CCF is modeled in the PRA, a plausible CCF mechanism has been identified that could cause the simultaneous failure of more than one nominally identical SSC. If there are no plausible mechanisms known (because of diversity or reliance on only passive functions) that could cause a simultaneous failure of more than one SSC, no CCF is modeled in the PRA. If a CCF is modeled in the PRA, the RAW associated with that CCF must be included in the safety-significance determination of the affected SSCs. Section 5 discusses a number of sensitivity studies in Tables 5-2, 5-3, 5-4, and 5-5 where the unavailability of types of PRA events (e.g., human errors, CCFs, and maintenance unavailabilities) are simultaneously modified and the importance measures of the SSCs recalculated. These studies are performed to ensure that assumptions in these types of