Modeling of Water Ingression Mechanism for Corium Cooling with MELCOR

The water ingression mechanism can enhance the coolability of a pool of molten corium in containment during a severe accident. A water ingression model was added to the MELCOR code in 2015. The purpose of this work was to test the new model. It was found that the water ingression model performed satisfactorily in core-concrete-interaction experiments in which gas bubbles were released to the melt from decomposing concrete. The new model had little effect in the Small-Scale Water Ingression and Crust Strength (SSWICS) experiments that were done without gas bubbling through the melt. When applied to the Fukushima Daiichi Unit 1 accident, the water ingression model slowed down concrete ablation by 19% but did not quench the melt. Because the water ingression model was added to MELCOR so recently, the default treatment is still to use multipliers for the boiling heat transfer coefficient and thermal conductivity instead of the proper water ingression model. These default parameters significantly overestimated melt coolability in all the experiments that were calculated.