Document: NUREG-0800
Document ID: 4865fc6d-b85f-4173-9bca-162fa9fe67d2
Document Type: srp
Title: Appendix 7-A
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0525/ML052500555.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
CFR Part: 
CFR Title: 

Content:
Appendix 7-A Rev. 4 — June 1997 BTP HICB-19-1 NEW Branch Technical Position HICB-19 Guidance for Evaluation of Defense-in-Depth and Diversity in Digital Computer-Based Instrumentation and Control Systems A. Background Digital instrumentation and control (I&C) systems are vulnerable to common-mode failure caused by software error, which defeats the redundancy achieved by hardware architecture. In NUREG-0493, "A Defense-in-Depth & Diversity Assessment of the RESAR-414 Integrated Protection System," the Staff documented a defense-in-depth and diversity (D-in-D&D) analysis of a digital computer-based reactor protection system, in which defense against common-mode failures was based upon an approach using a specified degree of system separation between echelons of defense. Subsequently, in SECY 91-292, "Digital Computer Systems for Advanced Light-Water Reactors," the Staff included discussion of its concerns about common-mode failures in digital systems used in nuclear power plants. As a result of the reviews of ALWR design certification applications that used digital protection systems, the Staff documented its position with respect to common-mode failures in digital systems and defense-in-depth. This position was documented as Item II.Q in SECY 93-087, "Policy, Technical, and Licensing Issues Pertaining to Evolutionary and Advanced Light-Water Reactor (ALWR) Designs," and was subsequently modified in the associated Staff Requirements Memorandum. Based on experience in the detailed reviews, the NRC staff has established acceptance guidelines for D-in-D&D assessments as described in this branch technical position. 1. Regulatory Basis 10 CFR 50.55a(h), "Protection Systems," requires in part that protection systems satisfy the criteria of ANSI/IEEE Std 279, "Criteria for Protection Systems for Nuclear Power Generating Stations." Section 4.2 requires in part that "any single failure within the protection system shall not prevent proper protective action at the system