Event Tree Analysis for Material Relocation on Core Catcher in a Sodium-Cooled Fast Reactor

Abstract For an in-vessel retention strategy, severe accident analysis needs to be input the condition of loading on a core catcher based on physically compatible assumptions. This study has conducted event tree analyses emphasizing key phenomena in the lower plenum based on available experimental knowledge as well as design measure effectiveness. Criticality assessment has also been performed for the event tree analysis. In this paper, the approach to event tree development and the scope of the event tree analysis are described with key points on the loading conditions on the core catcher. For the analytical conditions, two loading conditions on core catcher were given as bounding and conservative cases. For the important heading of the event tree, key important phenomena were included: strong back design, fuel-coolant interaction and quench in the sodium plenum design, jet attack, criticality and coolability of molten core materials on the core catcher. In this study, preliminary trial quantification was attempted using a probability ranking table which is based on engineering judgement. This event tree analysis has identified the dominant sequence, and clarified the effect of the loading condition on the core catcher and effectiveness of design measures. This study suggests that the criticality measure is very important for the core catcher study.