ABSTRACT
This chapter aims to describe in general terms how risks are estimated, what problems exist in doing this, how judgement enters the process and what the uncertainties are. It discusses the basic concepts involved in probabilistic safety assessment (PSA) and also describes how the method is implemented for nuclear reactors. The chapter provides some of the problems which exist with the PSA technique; it is the judgements taken to resolve these problems that have to be evaluated by the risk assessor. The fundamental objective of PSA is the estimation of probabilities for events of safety concern. PSA includes two types of analysis: a logical examination of the system; and physical models of the progression of accidents. A long-standing difficulty in the plant analysis stage of a PSA is where there is a causal link between two or more failures which means that they cannot be considered independent.
