Document: NUREG-0800
Document ID: 757b676a-0f71-4094-8677-ca3a50743281
Document Type: srp
Title: Revision 7 – August 2016
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1601/ML16019A344.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
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Content:
Digital instrumentation and control (DI&C) systems can be vulnerable to common-cause failure (CCF) caused by software errors or software developed logic, which could defeat the redundancy achieved by hardware architecture. In NUREG-0493, “A Defense-in-Depth and Diversity Assessment of the RESAR-414 Integrated Protection System,” the U.S. Nuclear Regulatory Commission (NRC) staff documented a diversity and defense-in-depth (D3) analysis of a digital computer-based reactor protection system (RPS) in which defense against software CCF (or simply CCF hereafter) was based upon an approach using a specified degree of system separation between echelons of defense. The RPS consists of the reactor trip system and the engineered safety features (ESF) actuation system (ESFAS). Subsequently, in SECY-91-292, “Digital Computer Systems for Advanced Light-Water Reactors,” the NRC staff included discussion of its concerns about CCF in digital systems used in nuclear power plants (NPPs). As a result of reviews of advanced light-water reactor (ALWR) design certification (DC) applications for designs using digital protection systems, the NRC staff documented its position with respect to CCF in digital systems and D3. This position was documented as Item 18, 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 (SRM). On the basis of experience in detailed reviews, the NRC staff has established acceptance guidelines for D3 assessments as described in this branch technical position (BTP). Further guidance reflected herein was established through the efforts of the DI&C Task Working Group No. 2 on D3 with the development of DI&C-ISG-02, “Task Working Group No. 2: Diversity and Defense-in-Depth Issues Interim Staff Guidance,” Revision 2. This interim staff guidance (ISG) was developed with extensive review of D3