overcoming the uncertainty in a research reactor loca in level-1psa; fuzzy based fault-tree/event-tree analysis

Probabilistic safety assessment (psa) which plays a crucial role in risk evaluation is a quantitative approach intended to demonstrate how anuclear reactor meets the safety margins as part of the licensing process. despite psa merits, some shortcomings associated with the finalresults exist. conventional psa uses crisp values to represent the failure probabilities of basic events. this causes a high level ofuncertainty due to the inherent imprecision and vagueness of failure input data. in this paper, to tackle this imperfection, a fuzzy approachis employed with fault tree analysis and event tree analysis. thus, instead of using the crisp values, a set of fuzzy numbers is applied asfailure probabilities of basic events. hence, in the fault tree and event tree analysis, the top events and the end-states frequencies are treatedas fuzzy numbers. by introducing some fuzzy importance measures the critical components which contribute maximum to the systemfailure and total uncertainty are identified. as a practical example, under redesign iranian heavy water research reactor loss of coolantaccident is studied. the results show that the reactor protection system has the largest index in sequences lead to a core meltdown. inaddition, the emergency core cooling system has a main role in preventing abnormal conditions.