Fragmentation of a single molten metal droplet penetrating sodium pool. Comparisons of thermal and hydrodynamic fragmentation

In order to clarify the effects of principal factors dominating the thermal and hydrodynamic fragmentation and simulate the interaction of molten metallic fuel and structural materials mainly including cladding material with the coolant of sodium during hypothetical core disruptive accidents (HCDAs) in fast breeder reactors (FBRs), the present paper focuses on a series of fragmentation experiments of a single molten stainless steel (304SS) droplets (1-5g) at initial temperatures from 1470degC to 1820degC, which penetrated a sodium pool in the low and high Weber number condition. The intensive fragmentation of single molten stainless steel droplet was clearly observed in all runs even if the instantaneous contact interface temperature (Ti) is far below the melting point of stainless steel. The results also show that between stainless steel-sodium and copper-sodium experiments there is a very similar fragment distribution that the normalized size distribution (Dm/D0) of molten stainless steel and copper droplet with the different mass of 5 times differ very little when Ti is above the turning point (Ti>1050degC), while at Ti<1050degC the size distribution both the same and different mass scatter widely because of unstable relationship of a limited amount of heat from droplet vs. coolability of surrounding sodium. The Dms of molten 304SS droplet with high Weber number somewhat incline to be less than those with low Weber number. Moreover, the present Dm data link the data of Armstrong et al. (1971) in high superheating condition very well. Relationship on the fragment size between molten cladding material and metallic fuels is very useful to the conservative core design concept in FBRs. (author)