There has been considerable research over the years on whether steam explosions pose a risk to structural (containment) failure. Experimental programmes include FARO, KROTOS, ECO and BERDA (Jorge and Chaumont, 2001). There is evidence of pre­mixing of melt and water during the core relocation phase providing a mitigating effect. Experiments such as ECO (FZK) have shown that energy conversion factors now seem to be much lower than were originally envisaged. However, in, e. g., the French safety analysis process, in-vessel and ex-vessel steam explosion risks are still considered (Jorge and Chaumont, 2001). The R&D needs for in-vessel steam explosions are mainly concerned with gaining a better understanding of material effects and better characterisation of experiments (water, fuel and vapour fractions) (Seiler et al.). For ex­vessel melt, the main challenge is to establish the lack of explosive potential from realistic corium melt flow into water.