Document: NUREG-0800
Document ID: fea61ff0-f344-4101-adfb-590e3d9dfafe
Document Type: srp
Title: Draft Revision 6 – August 2015
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1515/ML15159A946.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
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of all elements taken together, have only one interpretation. (See also the definition of “Unambiguous” in Clause 4.3.2 of IEEE Std 830, which is endorsed by RG 1.172.) Verifiability - The degree to which a software planning document, implementation process document or design output is stated or provided in such a way as to facilitate the establishment of verification criteria and the performance of analyses, reviews, or tests to determine whether those criteria have been met. (See also the definition of “Verifiable” in Clause 4.3.6 of IEEE Std 830, which is endorsed by RG 1.172.) B. BRANCH TECHNICAL POSITION B.1 Introduction Digital I&C safety systems must be designed, fabricated, installed, and tested to quality standards commensurate with the importance of the safety functions to be performed. Implementation of an acceptable software life cycle provides the necessary software quality. Digital I&C systems can share code, data transmission, data, and process equipment to a greater degree than analog systems. Although this sharing is the basis for many of the advantages of digital systems, it also raises a key concern: a design using shared data or code has the potential to propagate a common-cause or common-mode failure via software errors, thus defeating the redundancy achieved by the hardware architectural structure. Greater sharing of process equipment among functions within a channel, that is, the same processing equipment will be performing a number of different safety functions, increases the consequences of the failure of a single hardware module and reduces the amount of diversity available within a single safety channel. Because of these concerns, the staff review of digital I&C systems emphasizes quality, diversity, and defense-in-depth as protection against common-cause failures within and between channels. Software quality is an important element in preventing the propagation of common- cause failures. The development of safety system software