Document: NUREG-0800
Document ID: fe8ec6c2-e960-404c-854f-55c77bde1672
Document Type: srp
Title: NUREG-0800
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML2400/ML24005A077.pdf
Revision Date: 2024-05
Chapter: 7
Section ID: 7
CFR Part: 
CFR Title: 

Content:
n release exceeding the applicable dose guideline values, in violation of the integrity of the primary coolant pressure boundary, or in violation of the integrity of the containment. 3.4 Risk-Informed Defense-in-Depth and Diversity Assessment A risk-informed approach to address a CCF generally consists of (1) analyzing the functional impact of the CCF, (2) determining the risk significance of the CCF, and (3) determining appropriate means to address the CCF commensurate with its risk significance. The risk significance of a CCF is distinct from the risk significance of the DI&C systems it may affect. Point 1 of SRM-SECY-22-0076 refers to the risk significance of the DI&C system. Under Point 1, the D3 assessment must be commensurate with the risk significance of the DI&C system. It is possible for a system to be risk significant because of the combined effects of system functions, hidden interdependencies, corresponding interactions, and emergent behaviors, even if each system function alone may not be risk significant. Therefore, when there is potential for such behaviors, correspondingly more comprehensive modeling and evaluation of the plant are needed. BTP 7-19-29 Revision 9 — May 2024 Point 3 of SRM-SECY-22-0076, however, refers to the risk significance of a CCF. Under Point 3, one option is for the D3 assessment to demonstrate that a postulated CCF is not risk significant. While the risk significance of a DI&C system is different from the risk significance of a CCF, the same causes may degrade different safety functions, thus increasing the risk significance of a CCF beyond the increase in risk from the loss of a single safety function. Risk significance and safety significance are different concepts. NUREG-2122 states the following: A principal focus of a PRA is to determine the risk significance of the various “features,” i.e., the systems, structures, and components (SSCs), human actions or the accident sequences involving those SSCs, of the facility