Fragmentation behavior of molten metal drop in sodium pool with instantaneous contact interface temperature below its melting point

In order to predict the progression of core disruptive accidents in metal-fuel-type liquid-metal-cooled fast reactors, an investigation on the dispersion of molten metal fuel from a core region is necessary, because the dispersion gives a negative reactivity feedback. Based on this context, the authors have conducted a series of experiments in which a molten copper drop was dropped into a sodium pool under the condition that the formation of solid crust on the surface of the drop and nucleate boiling of the coolant were assumed. The conclusion is that: (1) Thermal fragmentation originating from the coolant entrapped within the molten drop was observed under the condition of instantaneous contact interface temperatures between the boiling point of sodium and the melting point of drop, where the formation of solid crust on the surface of the drop as well as the nucleate boiling of coolant were surely expected. (2) The degree of fragmentation is not strongly affected by the condition whether the instantaneous contact interface temperature between the molten metal and the coolant is higher than the melting point of the metal or not. This means that an effective fragmentation can be caused due to the entrapment of coolant within the molten drop even near melting point, where the solid crust formation is expected. (3)The shapes of fragments are dependent on the time period for the transformation before solidification. A molten drop is first transformed into the sheet-like shapes, then transformed further into mesh-like shapes, and finally broken into shapes with branches or spherical shapes according to the heat capacity of the drop