Determination of the safety importance of systems of the Tehran research reactor using a PSA method

This paper presents the results of a research project performed to study the ranking of systems and components of the Tehran research reactor, important to safety, using a level 1 probabilistic safety assessment (PSA). The occurrence of an imbalance condition, i.e. a lack of equilibrium between heat generation and heat removal without reactor scram, was defined as the “Top Event”. A fault tree technique was used to study all scenarios leading to the top event, the probability of which was shown to be 9.5E-5. It has been concluded that, objects falling onto the reactor core; operator error to shutdown the reactor while safety flapper open; and simultaneous opening of water outlet valves, are the most important initiating events. The results of this study led to some back-fittings in the reactor sub-systems.