PROBABILISTIC DYNAMICS - A COMPARISON BETWEEN CONTINUOUS EVENT TREES AND A DISCRETE-EVENT TREE MODEL

The feeling that dynamics and their interaction with the random evolut ion of parameters was ill-treated in classical probabilistic safety as sessment methodologies led to the development of probabilistic dynamic s methodologies, These methods explicitly model the mutual influence b etween physical variables, operators and components, using different b asic assumptions. This paper is a first attempt at a systematic compar ison between two such methodologies, namely, DYLAM and the continuous event tree (CET) theory on a simple problem. The problem involves one bistate component and one physical variable whose evolution depends on the component current state and that should not, in any case, cross a prespecified threshold. The methods are briefly discussed. In particu lar, we show how DYLAM can be derived as a special case of the CET the ory. The numerical implementation of each method is also reviewed. Eac h method is then applied to the specific problem. The probability of s ystem failure over time is compared to its real, analytically derived, value. We focus on key issues such as exactness, stability and effici ency. We point out the main differences between the methods and draw a first set of conclusions as to their respective fields of application , recognizing, however, that the analysis should be carried further on more complex problems to reach definitive conclusions.