Document: NUREG-0800
Document ID: 8dd882f4-a34f-4415-acd1-ebb441c72786
Document Type: srp
Title: PROBABILISTIC RISK ASSESSMENT AND SEVERE ACCIDENT
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1213/ML12132A481.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19.0
CFR Part: 
CFR Title: 

Content:
fensive measures meant to protect against CCF (e.g., separation and independence, operational testing, maintenance, diagnostics, self-testing, fault tolerance, and software/hardware design/development techniques and processes). Failures of system modules common across multiple applications should be identified (e.g., CCF of common function modules). If the safety functions of a DI&C system (and the redundancy within safety functions) use common software, dependency should be identified for software faults. That is, when common software is used for different safety functions (or in the redundancy within a safety function) it may fail each function. Hardware CCF between different safety functions using the same hardware should be identified. Dependencies between hardware and software should be identified. The applicant should provide the rationale for the degree of dependency assumed for DI&C CCF. An important expectation is that the applicant included sufficient equipment in the CCF groups. The evaluation should address why various channels, trains, systems, etc. were or were not placed in each CCF group. The justification should discuss common software/hardware among the equipment considered 19.0-31 Draft Revision 3 – September 2012 and the level(s) of dependency among them. The reviewer should work with the I&C reviewer to evaluate the applicant’s justifications. D. It is important to evaluate claims by applicants regarding the credit that should be given for defensive design features. Design features such as fault tolerance, diagnostics, and self testing are intended to increase the safety of DI&C systems, and therefore are expected to have a positive effect on the system’s safety. However, these features may also have a negative impact on the safety of DI&C systems if they are not designed properly or fail to operate appropriately. The potentially negative effects of these features should be included in the probabilistic model. The PRA should account for