Document: NUREG-0800
Document ID: ac43f632-9db1-4857-9f56-bfba45965456
Document Type: srp
Title: * These are modifications to a plant’s design, operations, or other activities that require NRC approval. These modifica
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0232/ML023250195.pdf
Revision Date: 2023-06
Chapter: 19
Section ID: 19
CFR Part: 
CFR Title: 

Content:
pletely relaxing the defense-in-depth posture at points at which the plant design has the least overall functional independence, redundancy, and/or diversity. On the other hand, in areas where a plant has substantial redundancy and diversity, defense-in-depth arguments used to justify relaxations should be given appropriate weight. As part of the defense-in-depth evaluation, reviewers should consider the effects of multiple component failures and common cause failures that could result from the proposed change. For example, if the licensee proposes to reduce the requirements for all events in a cutset, reviewers should ensure that the effect of the change is properly modeled and that the change does not have an adverse effect on defense in depth. Finally, in assessing the accident sequence cutsets, reviewers should devote attention to potential over-reliance on programmatic activities or operator actions that compensate for weaknesses in the plant design. For example, proposed maintenance and surveillance activities should complement and not replace proper plant design. III.2.1.2 Safety Margins In the determination of the design performance characteristics of a system, safety margin represents an allowance for uncertainty in SSC performance. Current safety analysis practices incorporate consideration of margin in most areas. As examples, many engineering standards, licensing analyses, and technical specifications take margin into account. Incorporating margin can result in over-designing of components, incorporation of extra systems or system trains, or in conservative operating requirements for systems and components. Therefore, some licensee applications will seek to reduce this margin in some areas. Such reductions should appropriately reflect the current understanding of existing uncertainties and the potential impact of the proposed change. Therefore, in evaluating a proposed change request, reviewers should establish that the proposal fulfills the following