ABSTRACT
Probabilistic risk assessment (PRA) is a systematic procedure for investigating how complex
systems are built and operated. The PRAs model how human, software, and hardware
elements of the system interact with each other. Also, they assess the most significant
contributors to the risks of the system. The PRA procedure involves quantitative application
of the triplets discussed by (2.1), in that probabilities (or frequencies) of scenarios of events
leading to exposure of hazards are estimated, and the corresponding magnitude of health,
safety, environmental, and economic consequences for each scenario are predicted. The risk
value (i.e., expected loss) of each scenario is often measured as the product of the scenario
frequency and its consequences. The main result of the PRA is not the actual value of the risk
computed (the so-called bottom-line number); rather it is the determination of the system
elements that substantially contribute to the risks of that system, uncertainties associated with
such estimates, and effectiveness of various risk reduction strategies available. That is, the
primary value of a PRA is to highlight the system design and operational deficiencies
and optimize resources that can be invested on improving the design and operation of the
system.
