Document: NUREG-0800
Document ID: c90016c6-e03f-4224-8ff3-87a6f7d3ca42
Document Type: srp
Title: Appendix 7-A
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0525/ML052500547.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
CFR Part: 
CFR Title: 

Content:
Safety Traceability Security Unambiguity Timing Verifiability a. Requirements Activities & Software Requirements Specification An SRS that exhibits the functional and the software development process characteristics listed below should be produced. Reg. Guide 1.172, "Software Requirements Specifications for Digital Computer Software Used in Safety Systems of Nuclear Power Plants," which endorses IEEE Std 830, "IEEE Recommended Practice for Software Requirements Specifications," describes an acceptable approach for describing software requirements. Functional Characteristics For each of the functional characteristics, the requirements imposed by the system requirements and system design on the software for that characteristic should be contained in the SRS. Functional characteristics addressed by the SRS include accuracy, functionality, reliability, robustness, safety, security, and timing. Accuracy requirements should be provided for each input and each output variable. Accuracy requirements should be stated numerically, and appropriate physical units and error bounds should be supplied. Accuracy requirements should include a description of data type and data size for each input and output variable. Functionality requires that the operations that must be performed for each mode of operation be completely specified. Functions should be specified in terms of inputs to the function, transformations to be carried out by the function, and outputs generated by the function. Reliability requires that all requirements for fault tolerance and failure modes be fully specified for each operating mode. Software requirements for handling both hardware and software failures should be provided, including requirements for analysis of and recovery from computer system failures. Requirements for on-line in-service testing and diagnostics should be provided. Robustness requires that the behavior of the software in the presence of unexpected, incorrect, anomalous and improper (1)