12.206. We may define reliability as the probability that a system or component will perform a specified function (or not fail) for a prescribed time. Now, if in addition, we consider the consequences of failure, i. e., a financial loss or injury to people, we have the concept of risk. Mathematical reliability models as design tools evolved from statistical sampling proce­dures used for quality control and biological research. The underlying probability theory, of course, is centuries old.

12.207. The aircraft industry provided an early incentive for the de­velopment of reliability engineering since in aircraft, one cannot accept overly conservative design if additional weight would be required. In the German World War II missile program, the concept of integrated system reliability analysis was pioneered. In the United States during subsequent years, the requirements of both the space and military missile programs led to the development of sophisticated systems studies which included the fault tree concept.

12.208. The application of probabilistic risk assessment (PRA) to nu­clear reactor safety began in the 1960s in the United Kingdom with the study of advanced gas-cooled reactors [28]. As we shall see shortly, PRA is an analytical technique for modeling the possible failure of subsystems and components where there are complex interactions. Following the Brit­ish work, the U. S. Atomic Energy Commission sponsored a number of studies of reliability techniques applied to U. S. reactors. Next, a major advance in PRA development was made in a З-year effort known as the Reactor Safety Study, the Rasmussen Report, or WASH-1400, published in 1975 [6]. The performance of a PRA as a licensing requirement was instituted in 1982. Also, ongoing PRA studies applicable to existing plants have resulted in various modifications to improve reliability. We will discuss various PRA efforts after we have presented additional introductory material.