Document: NUREG-0800
Document ID: dd4f84a9-8976-44c4-b631-d7c0254f4efc
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT EVALUATION FOR
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1416/ML14161A594.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
o 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 19.0-35 Draft Revision 3 –November 2014 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. 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 of the types of failures that were used in testing to apply a value of fault coverage to a PRA model. How fault coverage is measured and defined should be evaluated by the risk analyst in conjunction with the I&C reviewer. E. If a DI&C system shares a communication network with other DI&C systems, the effects on all systems due to failures of the network should be modeled jointly. The impact of communication faults on the related components or systems should be evaluated, and any failure considered relevant should be included in the probabilistic model. F. If hardware, software, and system CCF probabilities are treated together in the PRA and if the applicant uses standard methods such as the multiple Greek letter method, alpha factor method, or beta factor method to model DI&C system CCFs, an NRC audit of these calculations, their bases, and the modeling assumptions may be warranted. G. The data for hardware failure rates (including CCF) will likely be more robust than the software failure data. Review of applicant claims regarding data should be proportional to the use made of the PRA results. If limited use is made, limited review is necessary. If