Probabilistic Risk Assessment (PRA) is a method to estimate risk by computing real numbers to determine what can go wrong, how likely is it, and what are its consequences. Thus, PRA provides insights into the strengths and weaknesses of the design and operation of high value assets.
PRA can estimate three levels of risk.
A Level 1 PRA estimates the frequency of accidents that cause damage to the nuclear reactor core. This is commonly called core damage frequency (CDF).
A Level 2 PRA, which starts with the Level 1 core damage accidents, estimates the frequency of accidents that release radioactivity from the nuclear power plant.
A Level 3 PRA, which starts with the Level 2 radioactivity release accidents, estimates the consequences in terms of injury to the public and damage to the environment.
The results of the Individual Plant Examinations of External Events (IPEEE) program and actual fire events indicate that fire can be a significant contributor to nuclear power plant risk, depending on design and operational conditions. In particular, failures of fire protection defense-in-depth, (i.e. failure to prevent fires, failure to rapidly suppress fires, or failure to protect plant systems to provide stable, safe shutdown) can lead to risk significant conditions. Fire PRA (probabilistic risk assessment) provides a structured, integrated approach to evaluate the impact of failures in the fire protection defense-in-depth strategy on safety.
Those technical issues directly addressed in fire PRA are fire ignition frequency, detection and suppression, fire damage to diverse and redundant trains of core cooling equipment, circuits (i.e. spurious actuations), and plant response including manual operator actions
The Nuclear Regulatory Commission (NRC) and Electric Power Research Institute (EPRI) embarked upon a cooperative program to improve the fire PRA state of the art. This joint program culminated in 2005 in the production of the report entitled “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities,” (NUREG/CR-6850).