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Probabilistic risk assessment (PRA) (or probabilistic safety assessment/analysis) is a systematic and comprehensive methodology to evaluate risks associated with a complex engineered technological entity (such as airliners or nuclear power plants).

Risk in a PRA is defined as a feasible detrimental outcome of an activity or action.

In a PRA, risk is characterized by two quantities:

1. the magnitude (severity) of the possible adverse consequence(s), and
2. the likelihood (probability) of occurrence of each consequence.

Consequences are expressed numerically (e.g., the number of people potentially hurt or killed) and their likelihoods of occurrence are expressed as probabilities or frequencies (i.e., the number of occurrences or the probability of occurrence per unit time). The total risk is the sum of the products of the consequences multiplied by their probabilities. The spectrum of risks across classes of events are also of concern, and are usually controlled in licensing processes - (it would be of concern if rare but high consequence events were found to dominate the overall risk.)

Probabilistic Risk Assessment usually answers three basic questions:

1. What can go wrong with the studied technological entity, or what are the initiators or initiating events (undesirable starting events) that lead to adverse consequence(s)?
2. What and how severe are the potential detriments, or the adverse consequences that the technological entity may be eventually subjected to as a result of the occurrence of the initiator?
3. How likely to occur are these undesirable consequences, or what are their probabilities or frequencies?

Two common methods of answering these questions are Event Tree Analysis and Fault Tree Analysis - for explanations of these, see safety engineering.

In addition to the above methods, PRA studies require special but often very important analysis tools like human reliability analysis (HRA) and common-cause-failure analysis (CCF). HRA deals with methods for modeling human error while CCF deals with methods for evaluating the effect of inter-system and inter-component dependencies which tend to cause simultaneous failures and thus significant increases in overall risk.

PRA studies have been successfully performed for complex technological systems at all phases of the life cycle from concept definition and pre-design through safe removal from operation. For example, the Nuclear Regulatory Commission required that each nuclear power plant in the US perform an Individual Plant Examination (IPE) [1] to identify and quantify plant vulnerabilities to hardware failures and human faults in design and operation. Although no method was specified for performing such an evaluation, the NRC requirements for the analysis could be met only by applying PRA methods.

• WASH-1400
• SAPHIRE PRA software
• Transportation safety in the United States

• PRA Software used by the U.S. Department of Energy, Nuclear Regulatory Commission, and NASA
• NASA Paper on PRAs
• Nuclear industry PRA software (CAFTA)
• GoldSim: Simulating future performance while quantitatively representing the uncertainty and risks inherent in complex systems