To characterize the relationship between hydrodynamic and thermal effects on fragmentation of molten core structural material, which mainly includes cladding material, with the interaction of the coolant of sodium under a wide range of thermal and hydrodynamic conditions, this paper focuses on a series of fragmentation characteristics of a single molten Type 304 stainless steel droplet (5 g) with an ambient Weber number Wea from 199 to 586 and superheat conditions from 23 to 276°C, which penetrates into a sodium pool at an initial temperature from 301 to 313°C.

In our experiments, fine fragmentations of single molten stainless steel droplets with high Wea were clearly observed, even under a supercooled condition that is well below its melting point of 1427°C. The dimensionless mass median diameters (Dm/D0) of molten droplets with high Wea are less than molten droplets with low Wea under the same thermal condition. When Wea is approximately >250, the hydrodynamic effect on fragmentation becomes predominant over the thermal effect under a relatively low superheat condition. For a higher Wea range, the comparisons indicate that the fragment sizes of the molten stainless steel droplet and jet have similar distributions to those of molten metallic fuel jets even with different thermophysical properties and a thousandfold mass difference, which implies the possibility that the fragment size characteristics of molten metal jets could be evaluated by the interaction of a single droplet with the sodium coolant without the consideration of dropping modes and mass.