The Evolution of Probabilistic Risk Assessment in the Nuclear Industry

The use of probabilistic methods for evaluating the performance of plant is now commonplace. In the nuclear industries it has undergone a vigorous period of development and is now considered by its aficionados to be a mature topic. It may be considered technically mature in that methods and data have been refined considerably and its positive and negative points are well understood. However, its breadth of application, especially in its most complex forms when risk rather than reliability is evaluated, has not been as wide as originally hoped, especially as an aid to regulation as in the evaluation of risk acceptance, or tolerability. This paper follows the development of that set of analytical techniques which together form probabilistic risk (or safety) assessment through itsmost formative years (1975-1985) by means of examples drawn from the definitive calculations of the period– in particular, the Reactor Safety Study (1975), the Zion and Indian Point studies in theUSA and the Sizewell Bstudy in the UK (all circa 1983). All of these studies contributed in specific ways to the development of the methods. In addition, the Sizewell Bstudy, throughits use in a public enquiry, also precipitateda debate on the use and interpretation of the results in the public domain. This evolution clearly shows both the power of themethods, and their extreme complication. These aspects have contributed to the current status of the methods, both for plant performance and regulatory interpretations, and to the prospects for further developments.