The progression of hypothetical core disruptive accidents in metal fuel cores is strongly affected by exclusion of molten metal fuel from the core region due to molten fuel-coolant interaction (FCI). As a basic study of FCI in metal fuel fast reactors, the present paper focuses on the fragmentation and the characteristics of the debris produced during a series of experiments of a single molten copper droplet from 1g to 5g and continuous droplets with a total mass from 16g to 26g, which penetrated a sodium pool at instantaneous contact interface temperatures from 1005 °C far below the freezing point: 1084°C to 1342°C. The results show that the mass median diameters (Dm) of different mass copper droplets both a single droplet and continuous ones penetrating a sodium pool differ very little, nearly the same when the instantaneous contact interface temperatures (Ti) are above the freezing point, and also the droplets are fragmented finely with their increasing Tis; but when Tis are below the freezing point, the Dms of different mass copper droplets scatter a little widely. These results basically show the fragmentation of molten fuel, which is important to assure the termination of accidents, is promising in the sodium space in the upper and lower plenums.

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