The characterization of uncertainty in Probabilistic Risk Assessments of complex systems

This paper discusses the issue of the characterization of uncertainty in a Probabilistic Risk Assessment (PRA) of a complex system, such as a nuclear power plant. The significance to the interpretation of the results of a PRA of maintaining the distinction between the aleatory and epistemic components of uncertainty is illustrated using a simple example. The point of view presented here is that the degree to which it is necessary to invoke both aspects of uncertainty to characterize an event in a PRA model is as much a function of the way the analyst chooses to model the event of interest as it is of the nature of the event itself.