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
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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 being analyzed. Usually, an item is considered risk significant when the risk associated with it exceeds a predetermined limit for contributing to the risk associated with the facility. Since the overall risk of a nuclear facility can be calculated in terms of core damage frequency (CDF) (Level 1 PRA), or releases (Level 2 PRA), or health effects (Level 3 PRA), risk significance can also be determined as related to these various risk measures. NUREG-2122 also states that the term “risk significant” does not have the same meaning as the term “safety significant,” and safety significance is not evaluated in a PRA. This distinction between risk significance and safety significance is used, in part, to emphasize the need to consider safety margins when an application uses a risk-informed approach since the application should not overly rely on changes in CDF and large early release frequency (LERF) alone. Section B.3.4.4 describes how the risk significance of a CCF, as opposed to the safety significance of an SSC, is used in part to determine appropriate means to address the CCF. Because a risk-informed approach considers other factors to determine acceptability, the reviewer should verify that an application using a risk-informed approach demonstrates that sufficient safety margins exist so that DI&C and associated D3 systems remain capable of performing their safety functions. Section C.2.1.2 of RG 1.174 provides guidance on ensuring that designs possess sufficient safety margins. With sufficient safety margins, (1) the codes and standards or their alternatives approved for use by the NRC are met, and (2) safety analysis acceptance criteria in the licensing basis (e.g., FSAR, supporting analyses) are met or proposed revisions provide sufficient margin to account