Simulation of steam explosion in stratified melt-coolant configuration

• Strong steam explosions may develop spontaneously in stratified configurations.
• Considerable melt-coolant premixed layer formed in subcooled water with hot melts.
• Analysis with MC3D code provided insight into stratified steam explosion phenomenon.
• Up to 25% of poured melt was mixed with water and available for steam explosion.
• Better instrumented experiments needed to determine dominant mixing process.

A steam explosion is an energetic fuel coolant interaction process, which may occur during a severe reactor accident when the molten core comes into contact with the coolant water. In nuclear reactor safety analyses steam explosions are primarily considered in melt jet-coolant pool configurations where sufficiently deep coolant pool conditions provide complete jet breakup and efficient premixture formation. Stratified melt-coolant configurations, i.e. a molten melt layer below a coolant layer, were up to now believed as being unable to generate strong explosive interactions. Based on the hypothesis that there are no interfacial instabilities in a stratified configuration it was assumed that the amount of melt in the premixture is insufficient to produce strong explosions. However, the recently performed experiments in the PULiMS and SES (KTH, Sweden) facilities with oxidic corium simulants revealed that strong steam explosions may develop spontaneously also in stratified melt-coolant configurations, where with high temperature melts and subcooled water conditions a considerable melt-coolant premixed layer is formed.In the article, the performed study of steam explosions in a stratified melt-coolant configuration in PULiMS like conditions is presented. The goal of this analytical work is to supplement the experimental activities within the PULiMS research program by addressing the key questions, especially regarding the explosivity of the formed premixed layer and the mechanisms responsible for the melt-water mixing. To better understand these phenomena a comprehensive parametric analysis was performed with the MC3D code, varying the premixed layer melt fraction, void fraction, and thickness, the melt spreading area, the melt droplet size, the water height, the water subcooling and the trigger strength. The results are discussed and compared to available experimental data. It seems that both identified melt-water mixing processes contribute to the steam explosion, i.e. the premixture formation before the explosion and the melt-water layer mixing during the explosion propagation itself. Suggestions for further analytical and experimental work are also given.