Document: NUREG-0800
Document ID: a2598df1-7ec6-43a6-9d2a-d8210d1f944f
Document Type: srp
Title: through 7.9. Additional information relevant to the review process can be found in the references in
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0525/ML052500461.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7.1
CFR Part: 
CFR Title: 

Content:
s. To address this issue, the Staff's approach to the review of design qualification for digital systems focuses, to a large extent, upon confirming that the applicant/licensee employed a high-quality development process that incorporated disciplined specification and implementation of design requirements. Inspection and testing is used to verify correct implementation and to validate desired functionality of the final product, but confidence that isolated, discontinuous point failures will not occur derives from the discipline of the development process. B.2. Defense Against Common-Mode Failure In digital I&C systems, code, data transmission, data, and hardware may be common to several functions to a greater degree than is typical in analog systems. Although this commonality 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 commonality or sharing of hardware among functions within a channel 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 this concern, the staff review of digital I&C systems emphasizes quality and defense-in-depth and diversity (D-in-D&D) as protection against propagation of common-mode failures within and between functions. B.3. System Aspects of Digital Instrumentation and Control Certain functional requirements that apply to I&C safety systems involve system aspects that pose new assurance challenges when applied to digital systems. These aspects include real-time performance, independence, and on-line testing. The review process for these topics must recognize the special characteristics of digital systems. C. Review Process C.1. Summary The overall process for reviewing the unique