Document: NRC Regulatory Guide
Document ID: 22ccfd5e-c5d8-4615-a02c-32369aa9f533
Document Type: regulatory_guide
Title: Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1930/ML19308B636.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.200
CFR Part: 
CFR Title: 

Content:
depth. To provide the risk perspective for use in decision-making, a Level 1 PRA assesses the CDF risk metric and a limited Level 2 PRA assesses the LERF risk metric. DC and COL applicants should meet certain requirements in most recent NRC-endorsed ASME/ANS Level 1/LERF PRA standard, ASME/ANS RA-Sa-2009 (Ref. 21), such as requirements to perform plant walkdowns if that information is available. Similarly, COL holders should meet certain requirements in ASME/ANS RA-Sa-2009 (e.g., requirements to use plant-specific operating experience) when plant-specific experience becomes available after operating for some time. Plant operating states (POS) 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 operation of the reactor, which includes not only at-power operation, but also LPSD conditions. For some applications, 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 the effect on the plant. A hazard is category of similar challenges to plant operations that poses some risk to a facility. For example, internal events is a hazard group and loss-of-coolant accidents (LOCAs) are a hazard within the internal events hazard group. A hazard group is characterized