Document: NRC Regulatory Guide
Document ID: 086612c4-a8a7-4f50-a166-6f1cb05bcdaf
Document Type: regulatory_guide
Title: Guidelines for Environmental Qualification of Safety-Related Computer-Based Instrumentation and Control Systems in Nuclear Power Plants
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0630/ML063040591.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.209
CFR Part: 
CFR Title: 

Content:
ndard Criteria for Digital Computers in Safety Systems of Nuclear Power Generating Stations,” issued in 1993 and revised in 2003. This standard emphasizes that the application of computers in safety systems needs to address reliability and environmental compatibility. In particular, Annex F.2.3 to IEEE Std. 7-4.3.2 states that analyses must be performed to ensure both that the system has a high “correct response probability” and that the probability of common-cause failure is reduced to an acceptable level. Addressing qualification requirements for safety-related computer-based I&C systems is one method of ensuring that the probability of common-cause failure attributable to environmental stressors is reduced to an acceptable level. Specifically, Section 5.4.1 of IEEE Std. 7-4.3.2 provides criteria for the equipment qualification of computer-based safety systems, including performing testing under environment stress with the full range of safety-related software functioning. DG-1142, Page 5 Computer-based I&C systems present unique characteristics that must be considered in the qualification process. These characteristics include both functional and hardware considerations. One significant difference between analog and digital equipment is the higher functional density that is possible with computer-based I&C systems. Because of the expanding single-chip capabilities, many safety-related installations involve replacement of multiple functional modules with a multifunction microprocessor-based module. Another difference involves the sequential function execution that typifies computer-based I&C systems compared to the essentially parallel execution of analog modules. The effect of this behavior can be compounded for multiple systems that rely on either successful completion of digital data communication or error detection before continuation of discrete functional steps. The capability of digital system design accommodates the potentially cumulative effects of