Document: NUREG-0800
Document ID: 9a64419a-34ff-49d2-87dd-c6f10c12014f
Document Type: srp
Title: or 7.3 of the SAR.
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0525/ML052500501.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7.2
CFR Part: 
CFR Title: 

Content:
4.6 and 4.7 of ANSI/IEEE Std 279 require independence between redundant channels of the protection system and between safety systems and non-safety systems. IEEE Std 603, Section 5.6 contains similar requirements as do GDC 21, 22, and 24. Evaluation of digital systems with respect to these requirements should consider the effect of data communications on independence and the isolation of safety and non-safety portions of computer software. This is in addition to the need to consider electrical and physical independence within any I&C system as discussed elsewhere in this chapter. Annex G of IEEE Std 7-4.3.2 describes acceptable approaches to computer communication independence. The preferred approach to communication independence ensures that (1) redundant safety-grade equipment communicate via one-way communications paths, (2) safety-grade systems do not receive information from non-safety-grade systems except when under test, (3) if two-way communications are used, failure of coordination or handshaking between sending and receiving systems does not prevent either system from functioning correctly, and (4) the control of communications links resides in the sending system. SRP Appendix 7.1-C provides guidance for the review of communications independence. 6. Reliability — GDC 21 and ANSI/IEEE Std 279 require that protection systems be designed with high functional reliability. IEEE Std 603 requires the analysis of system design to confirm that safety systems achieve the reliability goals established by the design basis. As discussed in Section 5.15 of IEEE Std 7-4.3.2, when reliability goals are established at the system level, the proof of meeting the goals must address software reliability. The Staff does not endorse the concept of quantitative reliability goals as a sole means of meeting the NRC's regulations for the reliability of digital computers used in safety systems. The NRC staff's acceptance of the reliability of the computer system is based on