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:
he proposed digital I&C system to demonstrate that vulnerabilities to common-mode failures have been adequately addressed. In this assessment, the applicant/licensee should analyze design basis events (as identified in the safety analysis report). If a postulated common-mode failure could disable a safety function that is required to respond to the design basis event being analyzed, then a diverse means of effective response (with documented basis) is necessary. The diverse means may be a non-safety system, automatic, or manual if the system is of sufficient quality to perform the necessary function under the associated event conditions and within the required time. The methods and results of D-in-D&D assessments used in ALWR design certification submissions are documented in NUREG/CR-6303. This document describes an acceptable method for performing such assessments. In those cases where the RTS or ATWS mitigation system in an operating plant is modified, the requirements of the ATWS rule, 10 CFR 50.62, must be met. 10 CFR 50.62 requires that the ATWS mitigation system be composed of diverse equipment from the RTS. Therefore implementation of RTS digital modifications by a different manufacturer from the ATWS mitigation system satisfies the diversity requirements of 10 CFR 50.62. This is also true in the complementary case in which an existing ATWS system is modified by the inclusion of digital equipment and the RTS is already digital. If "sufficient" difference in manufacturer cannot be demonstrated, then a case-by-case assessment of the RTS and ATWS mitigation system designs should be conducted. This analysis should include differences such as manufacturing division (within a corporate entity), software (including implementation language), equipment (including CPU architecture), function, people (design and verification/validation team), and initiating events. 2. Information to be Reviewed The information to be reviewed is the D-in-D&D assessment conducted by the