Document: NRC Regulatory Guide
Document ID: 8a2332d3-66ca-40af-84e1-507db8b26559
Document Type: regulatory_guide
Title: TRIAL - Acceptability of Probabilistic Risk Assessment Results for Non-Light Water Reactor Risk-Informed Activities
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2123/ML21235A008.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.247
CFR Part: 
CFR Title: 

Content:
ght differ from one specific design class to another specific design class to account for different mitigating concepts (e.g., confinement instead of containment). However, the Large Release Guideline applies to all current as well as all future designs. • The NLWR PRA standard addresses the development of a PRA that characterizes risk with cumulative risk metrics but also allows the user to define intermediate metrics if justified. Accordingly, the NLWR PRA standard does not provide specific requirements related to the use of risk surrogates. Each application should define and justify all cumulative risk metrics and risk surrogates used to characterize risk. POSs are used to subdivide the plant operating cycle into unique states, such that the plant response can be assumed to be the same within the given POS for a given initiating event. Operational characteristics (such as reactor power level; in-vessel temperature, pressure, and coolant level; equipment operability; and changes in decay heat load or plant conditions that allow new success criteria or reactor coolant system or containment configuration) are examined to identify those relevant to defining POSs. These characteristics are used to define the states, and the fraction of time spent in each state is estimated using plant-specific information. The risk perspective is based on the total risk associated with the RG 1.247, Page 17 operation of the reactor, which includes not only at-power operation but also LPSD types of POSs; however, the risk impact may affect some modes of operation but not others. A hazard group is a group of similar hazards that are assessed in a PRA using common approaches, methods, and likelihood data for characterizing their effect on the plant. A hazard is a category of similar challenges to plant operations that poses some risk to a facility. For example, internal events are a hazard group, whereas a reactor containment building (RCB) breach is a hazard within the internal events