A simple model to understand physics of melt coolability under bottom flooding

Following a severe accident in a nuclear reactor, stabilization and termination of molten pool/debris is necessary to ensure safety of the reactor. In this regard, ex-vessel corium coolability still remains an unresolved issue in spite of several efforts being taken towards its understanding. Extensive research has been carried out in the field of corium coolability phenomenon, but the outcome is far from establishing a certainty of complete coolability of melt under all possible scenarios. Major thrust so far was on top flooding condition, which often results in uncoolable condition due to inability of water ingression deep into the solid crust. The research on coolability under bottom flooding condition started long back, but has gained lot of momentum recently. However, mainly the focus is on development of engineering system to enhance the coolability using bottom flooding. The understanding of the phenomenon is not yet totally brought out. In this paper, we have postulated the mechanism of melt coolability under bottom flooding. We have developed a simplified mechanistic model for prediction of coolability of molten corium under bottom flooding condition based on the postulations. The model has been validated with the experimental data from literature. Comparison of top flooding vis-à-vis bottom flooding has also been presented.