Experimental research on the fragmentation of continuous molten 316SS droplets penetrating into sodium pool

•The fragment sizes distribution of the continuous molten droplets is studied.•Thermal and hydrodynamic effects on fragmentation are investigated.•The fragmentation regime map of the continuous molten droplets are developed.•The fragmentation mechanisms of the molten stainless steel droplet are proposed. Stainless steel (316SS) is commonly used as the cladding and structural material in sodium-cooled fast reactors (SFRs), and the molten 316SS always appears along with the molten metallic fuel during core disruptive accidents (CDAs) in SFRs. Since the research on the thermal and hydrodynamic fragmentation mechanism, and fragment size distribution characteristics of the continuous molten 316SS droplets is insufficient, the experiments of 20 g continuous molten 316SS droplets penetrating into the sodium were conducted under the wide thermohydraulic conditions in this paper. The results show that the fragmentation of the continuous 316SS droplets can be divided into three fragmentation zones of solidification and deformation zone, rapid breakage growth zone and slow breakage growth zone. Based on the analysis of the fragment size distribution and the fragment appearances, the schematic fragmentation mechanisms of the 316SS droplet are proposed under different thermohydraulic conditions.