Accident management measures on steam explosion and debris coolability for light water reactors

Two series of experiments to investigate Fuel Coolant Interactions (FCI) for LWRs have been performed as a part of ALPHA program at JAERI. One of the major objectives of the experiments is to assess the effectiveness of the possible accident management measures on steam explosions and debris coolability in a RCV. In the melt drop steam explosion experiments, a melt simulating a molten core was dropped into a pool of water. Effects of pre-dispersion of the melt on FCI phenomena were examined based on the evaluation of void faction in a coarse mixing region, melt setting velocity in a water pool, propagation velocity, expansion velocity, energy conversion ratio and debris characteristics. It was found that the probability of the occurrence of spontaneous steam explosions could be reduced by pre-dispersing the melt. However, the results showed that pre-dispersion of the melt could cause a more energetic steam explosion. Knowledge of the parametric effects of melt mass, ambient pressure and water temperature on the occurrence of spontaneous steam explosion was extended and confirmed under a melt drop geometry. In the melt coolability experiments, water was poured onto a simulated core melt. Heat transfer characteristics between the melt and the overlying water were investigated. Berenson's correlation for a flat plate film boiling could be applicable for the prediction of the heat transfer from the melt to the overlying water. An explosive interaction, which might have been resulted from melt eruptions into the overlying water, occurred in one out of seven experiments. The melt eruptions were suppressed when water was near the saturation temperature or supplied through a spray nozzle. The explosive interaction in a stratified geometry was found to be much smaller in magnitude than steam explosions in a melt drop configuration. Water addition onto the melt was thus found to be generally effective