Document: NUREG-0800
Document ID: 9a64419a-34ff-49d2-87dd-c6f10c12014f
Document Type: srp
Title: or 7.3 of the SAR.
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0525/ML052500501.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7.2
CFR Part: 
CFR Title: 

Content:
goals as a sole means of meeting the NRC's regulations for the reliability of digital computers used in safety systems. The NRC staff's acceptance of the reliability of the computer system is based on deterministic criteria for both the hardware and software rather than on quantitative reliability goals. This topic is discussed further in Reg. Guide 1.152 and SRP Appendix 7.1-C. Nevertheless, qualitative reliability estimation using a combination of analysis, testing, and operating experience can provide an added level of confidence in a system's reliable performance. Qualitative estimation of software reliability should address the fact that software failures that are not the consequence of hardware failures are caused by design errors and, therefore, do not follow the random failure behavior used for hardware reliability. Software that complies with the quality criteria of item 2 above and that is used in safety systems that provide measures for defense against common mode failures as described below are considered by the Staff to comply with the fundamental reliability requirements of GDC 21, ANSI/IEEE Std 279, and IEEE Std 603. 7. Defense against common-mode failures — Experience shows that flaws in requirements and design can be expected to exist in even the highest quality software despite good engineering processes and testing. These residual flaws pose the concern that the use of common software has the potential to propagate Rev. 4 — June 1997 SRP 7.1-8 common-cause or common-mode failures that can defeat the redundancy provided by hardware architectural structure. To address this issue, designs that incorporate digital computer-based protection systems must comply with the NRC position on defense-in-depth and diversity as described in the Staff Requirements Memorandum on SECY-93-087. BTP HICB-19 describes acceptable means for complying with this position. 8. Use of emerging software methods — Software engineering is a maturing field. Certain