Document: NRC Regulatory Guide
Document ID: 82659041-98b0-4721-b25d-c4fb2ea394d0
Document Type: regulatory_guide
Title: An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1635/ML16358A153.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.174
CFR Part: 
CFR Title: 

Content:
uclear plant that leverages the implementation of the defense-in-depth philosophy in the design of the plant. For example, testing and maintenance of SSCs or corrective action to restore an engineered safety system might be allowed for short periods while remaining at power consistent with established technical specifications. The NRC recognizes and allows these temporary configurations within these established programs. If a licensee proposes a licensing basis change that permits new or extended entry into a temporary condition, the licensee should demonstrate that the new or extended entry into that temporary condition is justified and that consistency with the defense-in-depth philosophy is maintained as described in this section. Defense-in-depth is often characterized by varying layers of defense, each of which may be representative of conceptual attributes of nuclear power plant design and operation or tangible objects such as the physical barriers between fission products and the environment. The NRC implements defense-in-depth as four layers of defense that are a mixture of conceptual constructs and physical barriers (See NUREG/KM-0009 for further detail). As such, for the purposes of this RG, nuclear power plant defense-in-depth is taken to consist of layers of defense (i.e., successive measures) to protect the public: • Robust plant design to survive hazards and minimize challenges that could result in an event occurring. • Prevention of a severe accident (core damage) should an event occur. DG-1285, Page 14 • Containment of the source term should a severe accident occur. • Protection of the public from any releases of radioactive material (e.g., through siting in low population areas and the ability to shelter or evacuate people, if necessary). Factors for Evaluating the Impact of the Proposed Licensing Basis Change on Defense- in-Depth Any one or more of the layers of defense discussed above might be adversely impacted by the proposed licensing