Document: NUREG-0800
Document ID: f4b8f0c1-9a14-46be-88c9-a6cb74d64dff
Document Type: srp
Title: Rev. 4 — June 1997
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0525/ML052500499.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
CFR Part: 
CFR Title: 

Content:
the safety system is designed to provide a protective function are assumed to occur if the failure adversely affects the safety system performance. In general, the lack of equipment qualification may serve as a basis for the assumption of certain failures. After assuming the failures of non-safety-grade, non-qualified equipment and those failures caused by a specific event, a random single failure is arbitrarily assumed. With these failures assumed, the safety system must be capable of performing the protective functions required to mitigate the consequences of the specific event. Digital computer-based I&C systems share data, data transmission, functions, and process equipment to a greater degree than analog systems. Although this sharing forms the basis for many of the advantages of digital systems, it also raises a key concern with respect to I&C system vulnerability to a different type of failure. The concern is that a design using shared databases and process equipment has the potential to propagate a common-mode failure of redundant equipment. Another concern is that software programming errors can defeat the redundancy achieved by the hardware architectural structure. Because of these concerns, the NRC staff has placed significant emphasis on defense-in-depth against common-mode failures within and Rev. 4 — June 1997 SRP 7.1-C-6 between functions. The principle of defense-in-depth is to provide several levels or echelons of defense to challenges to plant safety, such that failures in equipment and human errors will not result in an undue threat to public safety. A detailed defense-in-depth and diversity study should be made to address common-mode failures in digital computer-based systems. The NRC's position for providing defense against common-mode failures in digital I&C systems for future light-water reactors is given in the Staff Requirements Memorandum on SECY-93-087, "Policy, Technical, and Licensing Issues Pertaining to Evolutionary and Advanced