The progression of hypothetical core disruptive accidents in metallic fuel fast breeder reactors is strongly affected by the exclusion of molten metallic fuels from the core region due to the molten fuel-coolant interaction (FCI). As a basic simulation study of FCI, the present paper focuses on the fragmentation of a single molten copper droplet from 1g to 5g, which penetrated a sodium pool at instantaneous contact interface temperatures Ti from 995°C to 1342°C. Intensive fragmentation of single molten copper droplet was clearly observed in all runs even if Tis are below the melting point (1083°C) of copper except low Tis. In the low Tis condition the mass median diameters Dm of droplet fragments both the same and different mass scatter widely. When Tis are somewhat below and above the melting point, the Dms of copper droplet with the different mass of 5 times or the same mass differ very little, nearly the same. The present Dms show somewhat larger distribution than the Dms of molten copper jets with large mass in the low superheating condition, which were previously reported by the authors. The correlation between molten metal droplets and jets found in the present study is very useful to conservatively predict the fragment size in the FCI of molten jets.

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