Document: NUREG-0800
Document ID: b8d2b8f5-432a-4c0a-81ab-20231f7d0f28
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1508/ML15089A068.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
the beneficial impacts of these features are only credited for appropriate failure modes and that the limitations, including failure of the design feature itself, is considered in the model). An issue associated with including a design feature such as fault tolerance in a DI&C system modeled in a PRA is that its design may be such that it can only detect, and hence mitigate, certain types of failures. A feature may not detect all the failure modes of the associated component, but just the ones it was designed to detect. The PRA model should only give credit to the ability of these features to automatically mitigate these specific failure modes; it should consider that all remaining failure modes cannot be automatically tolerated. Those failure modes that were not tested should not be considered in the fault coverage, and should be included explicitly in the logic model. When a specific datum from a generic database, such as a failure rate of a digital component, is used in a DI&C risk assessment, the risk analyst, in conjunction with the I&C reviewer, should assess whether the datum was adjusted for the contribution of design features specifically intended to limit those postulated failures. If so, the failure rate may be used in the PRA, but no additional fault coverage should be applied to the component, unless it is demonstrated that the two fault coverages are independent. Otherwise, applying the same or similar fault coverages would generate a non-conservative estimate of the component’s failure rate. A fault-tolerant feature of a DI&C system can be explicitly included either in the logic model or in the PRA data, but not both. 19.0-31 Revision 3 – December 2015 With respect to the above design features, the concept of fault coverage is used to express the probability that a failure will be tolerated for the types of failures that were tested. Fault coverage is a function of the failures that were used in testing. It is essential that the reviewer be aware