Fragmentation Mechanisms of a Single Molten Copper Jet Penetrating a Sodium Pool—Transition from Thermal to Hydrodynamic Fragmentation in Instantaneous Contact Interface Temperatures below Its Freezing Point—

To clarify the fragmentation mechanism of a molten metallic fuel jet in a sodium pool under highejection-velocityconditions 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 (Wea) < 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 Wea≥ 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.