The progression of hypothetical core disruptive accidents in metallic fuel fast breeder reactors is strongly affected by the fragmentation of molten metallic fuels due to the molten fuel-coolant interaction (FCI). As a basic study of FCI, the present paper focuses on the fragmentation of a single molten copper droplet with mass from 1 to 5 g, which penetrated a sodium pool at instantaneous contact interface temperatures (T i ) from 995 to 1,342 C. Intensive fragmentation of a single molten copper droplet was clearly observed even if T i values are below the melting point (1,083 C) of copper besides the higher T i range. The intensive fragmentation shows that the mass median diameters (D m ) of copper droplets with a fivefold difference in mass or the same mass have little difference, i.e., they are nearly the same. Under the lower T i condition, the D m data of droplet fragments of both the same and different masses scatter widely. It is found that the present D m =D 0 data of mass median diameter normalized by the diameter before touching sodium (D 0 ) give a distribution with larger values than those of molten copper jets with large mass from 20 to 300 g under the lower T i condition, which were previously reported by the authors, because of the limited amount of heat of droplets. The present D m =D 0 data under the higher T i condition are found to show an effective fragmentation compared with those of molten copper jets with a large mass of 4 kg.
To clarify the fragmentation mechanism of a molten metallic fuel jet in a sodium pool under highejection-velocity conditions that correspond to the medium-and high-burnup conditions in the metallic fuel core of liquid-metal-cooled fast breeder reactors, a series of experiments with molten copper as a metallic fuel simulant and a sodium pool was carried out. Under low-ejection-velocity conditions in the range of an ambient Weber number ðWe a Þ < 200, the fragmentation of the molten copper jet depends on the initial superheating of the jet. The size of copper fragments decreases with increasing initial superheating. Under high-ejection-velocity conditions in the range of We a ! 200, the size of the fragments is confirmed to be almost independent of the initial superheating of the jet. Furthermore, the size of the fragments agrees well with that evaluated using the Rayleigh-Taylor instability model, in which the fragment size is assumed to be equal to half the fastest growing wavelength. This result is qualitatively consistent with the characteristics that the molten jet column with large inertia force owing to the high ejection velocity, which transports enthalpy downwards, can penetrate the decelerated leading edge and can directly come into contact with sodium successively.
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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