For the safety design of a Sodium-cooled Fast Reactor, it is strongly required that the molten material which is released from a core region has to be solidified and cooled down in a reactor vessel by breaking up in the sodium coolant during a Core Disruptive Accident. In this paper, in order to investigate the effects of the surface solidification on the fragmentation of a molten jet, the experimental results using a low melting point alloy and water are reported. After the molten jet is

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... ed into water, the fragments are collected from the test section. The shapes of the fragments are investigated, and the mass median diameter of the fragments, which indicates characteristic diameter of the fragments, is measured using sieves. From the results, it is found that fragments with several shapes such as sphere, filament and sheet are generated. The tendency of the mass median diameter of the spherical fragments is close to the Critical Weber number theory. However, the tendency of the mass median diameter of the fragments except the spherical fragments, which are mostly the filament and sheet fragments, is larger than the previous theories such as the Critical Weber number theory and the Kelvin-Helmholtz instability. Then we applied the interfacial instability considering the effects of the surface solidification to the jet system. As a consequence, it is found that the interfacial instability has a possibility of evaluating the tendency of the mass median diameter of the filament and sheet fragment.