A methodology study on event-tree analysis for ULOF sequences considering passive safety features

In order to establish a method of probabilistic safety analysis for passive safety features, the event-tree (E/T) of ULOF accident sequences in the early stage of accident progression was constructed for an 600 MWe LMFBR model plant equipped with passive safety features such as Self Actuated Shutdown System (SASS) and Gas Expansion Modules (GEM). The development of this E/T was based on the results of some ULOF accident sequence analyses considering the effect of GEM. Even if the negative reactivity introduced by the GEM could not be enough to terminate the accident progression completely, there is some possibility to make the accident progression slower and to terminate the accident by manual reactor scram procedures with successfully starting of the pony motors in primary coolant loops. This accident mitigation pass was introduced into the E/T. Using this E/T and some fault tree (F/T) models related to the reactor shutdown function and the pony motor, the accident sequences were quantified and the conditional probability of coolant boiling when ULOF accidents occur was evaluated. Though the evaluation was in a preliminary stage, the conditional probability of coolant boiling when ULOF accidents occur was evaluated in the order of 10-3 due to the effect of the passive safety features such as GEM and SASS. Through the preliminary evaluation, system analysis models such as E/T and F/Ts for ULOF sequence with considering the effect of passive safety features were developed. (author)