Document: NUREG-0800
Document ID: 106978f1-fb10-4fa9-ad95-601f3c16bd36
Document Type: srp
Title: Revision 5 - March 2007
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0705/ML070550072.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
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a type of failure to be considered when applying the single-failure criterion. NUREG/CR-6303, "Method for Performing Diversity and Defense-in-Depth Analyses of Reactor Protection Systems," summarizes several D3 analyses performed after 1990 and presents a method for performing such analyses. The SRM on SECY 93-087 describes the NRC position on D3. 3. Purpose The purpose of this BTP is to provide guidance for evaluating an applicant/licensee's D3 assessment and the design of manual controls and displays to ensure conformance with the NRC position on D3 for I&C systems incorporating digital computer-based reactor trip systems (RTS) or engineered safety features actuation systems (ESFAS). This BTP has the objective of confirming that vulnerabilities to common-cause failures have been addressed in accordance with the guidance of the SRM on SECY-93-087, specifically: • Verify that adequate diversity has been provided in a design to meet the criteria established by the NRC's requirements. • Verify that adequate defense-in-depth has been provided in a design to meet the criteria established by the NRC's requirements. • Verify that the displays and manual controls for critical safety functions initiated by operator action are diverse from computer systems used in the automatic portion of the protection systems. B. BRANCH TECHNICAL POSITION 1. Introduction The Staff has identified four echelons of defense against common-cause failures: • Control System - The control system echelon consists of non-safety equipment that routinely prevents reactor excursions toward unsafe regimes of operation and is used in the normal operation of the reactor. • Reactor Trip System - The RTS echelon consists of safety equipment designed to reduce reactivity rapidly in response to an uncontrolled excursion. • Engineered Safety Features Actuation System - The ESFAS echelon consists of safety equipment that removes heat or otherwise assists in maintaining the integrity of the three