Document: NUREG-0800
Document ID: dc63e8aa-2a74-4078-9e19-9a26ed1b4d8c
Document Type: srp
Title: Revision 6 – July 2012
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1105/ML110550791.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
CFR Part: 
CFR Title: 

Content:
echelons of defense that may be affected. 1.4 Four-Point Position On the basis of reviews of the ALWR DC applications for designs that use digital safety systems, the NRC has established the following four-point position on D3 for new reactor designs and for digital system modifications to operating plants. The foundation of BTP 7-19 is the “NRC position on D3” from the SRM on SECY-93-087, Item 18, II.Q. The four points (i.e., SRM on SECY-93-087 items) are quoted below: Point 1 “The applicant shall assess the defense-in-depth and diversity of the proposed instrumentation and control system to demonstrate that vulnerabilities to common-mode failures have adequately been addressed.” Point 2 “In performing the assessment, the vendor or applicant shall analyze each postulated common-mode failure for each event that is evaluated in the accident analysis section of the safety analysis report (SAR) using best-estimate methods. The vendor or applicant shall demonstrate adequate diversity within the design for each of these events.” Revision 6 – July 2012 BTP 7-19-7 Point 3 “If a postulated common-mode failure could disable a safety function, then a diverse means, with a documented basis that the diverse means is unlikely to be subject to the same common-mode failure, shall be required to perform either the same function or a different function. The diverse or different function may be performed by a non-safety system if the system is of sufficient quality to perform the necessary function under the associated event conditions.” Point 4 “A set of displays and controls located in the main control room shall be provided for manual, system-level actuation of critical safety functions and monitoring of parameters that support the safety functions. The displays and controls shall be independent and diverse from the safety computer system identified in Items 1 and 3 above.” Concerning Point 2: The term “best-estimate methods” is more accurately referred