Document: NRC Regulatory Guide
Document ID: 2980f2d9-2e7d-4afa-9e1f-a8abe5fb7507
Document Type: regulatory_guide
Title: Software Requirement Specifications for Digital Computer Software and Complex Electronics Used in Safety Systems of Nuclear Power Plants + HISTORY - HISTORY 08/2012 – DG-1209 , Proposed Revision 1 08/1996 – DG-1058 , Proposed Revision 0 (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1030/ML103080963.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.172
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CFR Title: 

Content:
t specification of a requirement. The NRC uses the term in this regulatory guide to mean both internal and external consistency. External consistency implies that the SRS is consistent with associated software products and system products such as safety system requirements and design. Internal consistency means that no requirement in the requirements specification conflicts with any other requirement in the specification. d. Ranking for Importance or Stability. For safety system software, this characteristic means that software requirements important to safety must be identified as such in the SRS. Criterion 20 of Appendix A, among others, describes the function that reactor protection systems must perform. Subclause 4.3.5.2 of IEEE Std. 830-1998 suggests three degrees of necessity for requirements: (1) essential, (2) conditional, and (3) optional. As used in IEEE Std. 830-1998, the terms “conditional” and “optional” refer to requirements that are not necessary for the acceptability of the software. For safety system software, unnecessary requirements should not be imposed. There may be documented variations in essential requirements, but the SRSs need to link the variations either to site and equipment variations or to specific plant design bases and regulatory provisions. e. Verifiability. Subclause 4.3.6 of IEEE Std. 830-1998 recommends the removal or revision of unverifiable requirements. The NRC believes that all requirements should be verifiable and should be modified or restated as necessary to allow for the verification of each one. f. Modifiability. This term is closely related to the style (form, structure, and modularity), readability, and understandability of the SRS. With respect to these characteristics, precise definitions of technical terms must be available either in the SRS itself or in a glossary. g. Traceability. In accordance with GDC 1 of Appendix A to 10 CFR Part 50 and as described in Subclause 4.3.8 of IEEE Std. 830-1998, each