Document: NUREG-0800
Document ID: 4cca74ec-66e0-4765-9c61-cf547e17c308
Document Type: srp
Title: of the plant safety analysis report (SAR).  Although the NRC did not endorse the annexes of
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1601/ML16019A114.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7.6
CFR Part: 
CFR Title: 

Content:
communication link failure. This is one, but not the only possible method of deterministic communications. The objective in the review is to determine that the applicant or licensee has satisfactorily demonstrated that the applicable requirements of 10 CFR 50.55a(h) and General Design Criterion 24 are met. Additional guidance on communications independence is provided in SRP Appendix 7.0-A, “Review Process for Digital Instrumentation and Control Systems,” SRP Appendix 7.1-C, and SRP Section 7.9, “Data Communication Systems.” 5.7 Capability for Test and Calibration (IEEE Std 7-4.3.2, Clause 5.7) Sub-Clause 5.5.2 of IEEE Std 7-4.3.2 recommends that test and calibration functions should not adversely affect the ability of the computer to perform its safety function. The reviewer should check to ensure this has been accomplished. Sub-Clause 5.5.3 of IEEE Std 7-4.3.2 recommends that fault detection and self-diagnostics are one means that can be used to assist in detecting partial system failures that could degrade the capabilities of the computer system, but may not be immediately detectable by the system. The reviewer should carefully examine the capability of the software to test itself. From experience with a number of digital failures, the failures were not in the operational code but in the diagnostic code. One of the reasons for this may be that the diagnostic code may be much more complex than the operational code. The reviewer should examine the portion of the analysis in the Failure Mode and Effects Analysis on diagnostic code failure. Assertions that failure of the operation code is not credible because the system and software diagnostics will find every failure should be carefully examined. The total amount of software code should be compared to the amount of operational code. Large amounts of test and diagnostic software increase the complexity of the total software, and this increase in complexity should be balanced against the potential gain in