On the explosivity of a molten drop submitted to a small pressure perturbation

Under some circumstances that we aim to determine, a hot molten fuel drop flowing into a volatile liquid coolant and submitted to a small pressure wave, can be destabilized and explode in a few milliseconds. We propose a new approach to address this problem: in contrast with previous studies, we do not try to model the complete phenomenon but concentrate on its initiation. This way, we can differentiate favourable and non-favourable conditions with applications to PWR/BWR safety. We do the hypothesis that the occurrence of contacts between the two fluids is the criterion of explosion and phenomena occurring up to the contacting event are modelled, including the vapour film oscillations and the amplification of Rayleigh-Taylor instabilities at its interface. The latter feature receives a particular attention with a transient modelling adapted for variable acceleration cases. The fragmentation process itself is not studied in details but we give arguments supporting the fact that contacts between both liquids should induce a strong destabilization of the drops and initiate fragmentation. In this way, we can characterize the explosivity, i.e. the ability for the drop to explode, as a function of the various physical properties (e.g. pressure, temperatures). The model so deduced is qualified by comparison with the explosivity maps provided by Nelson and Duda [Nelson, L.S., Duda, P.M., 1985. Steam explosion experiments with single drops of iron oxide: Part II: parametric studies, NUREG CR-2718, April 1985]. Results obtained with this model confirm the experimental trends regarding the role of ambient pressure and liquid temperature. The influence of other parameters as the drop and the trigger characteristics are also investigated. We conclude this paper with some consideration on the implications for nuclear safety.