Molten-fuel coolant interaction: an investigation of the likelihood of a steam explosion event

There are considerable uncertainties in the thermal and hydraulic behavior of CANDU core material under severe accident conditions. An important issue in the analysis of a hypothetical nuclear power reactor accident is core overheating leading to fuel melting and subsequent interaction of molten fuel with coolant. The fuel melting is possibly resulting from a loss of coolant accident (LOCA). Consequently, a thermohydraulic energy release, namely a steam explosion can occur from the molten-fuel coolant interaction. This has been known to cause serious consequences. There are several hypothetical severe accidents (core damage states), due to this thermal interaction of the molten material with the moderator which can potentially lead to steam explosion. Steam explosion phenomenon refers to the rapid conversion of the cold liquid into vapor by heat transfer from the hot fuel with subsequent explosion of the vapor into the environment doing mechanical work and generating shock waves. There are a number of studies, theoretical and experimental research work, which are quite extensive, covering the details of steam explosion phenomenon. Steam explosion have been taken into consideration under reactor safety research programs, in several countries. The present study summarizes only the essential mechanisms and provides a global understanding of the phenomenon and presents a tentative conclusion about the likelihood of steam explosion occurrence in severe accidents. (author)