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Molten Fuel-Coolant Interaction (MFCI) is especially focused in Sodium-cooled Fast Reactor (SFR) with the consideration of possible recriticality and predominant effect on resultant consequences. As the nuclear fuel cladding material of SFR, kilogram quantities of molten stainless steel jet are used in the present experiments to study the fragmentation behavior when discharged into sodium pool. During the interaction between molten stainless steel jet and liquid sodium, the great differences in temperature, velocity and thermophysical properties result in violent sodium boiling, hydrodynamic instability and viscous shearing at the contact interface. The molten stainless steel jet suffers from significant fragmentation under multiple interfacial forces, which increases contact interface area to enhance heat transfer. However, the accelerated solidification of molten stainless steel jet at the contact interface greatly raises the criteria of fragmentation, conversely decreasing the heat transfer. In the present study, the sodium pool depth is varied to study the effect of the dropping height of molten stainless steel and inventory of liquid sodium on the fragmentation characteristics. The temperature variations in sodium pool are measured to estimate energy release during molten stainless steel-liquid sodium interaction. The resultant stainless steel debris is recorded and measured against debris mass with respect to size intervals for morphology analysis and size distribution. Furthermore, experiments with kilogram quantities of molten copper jet are conducted in the present study. The results with molten stainless steel jet are compared to those with molten copper jet and those with molten stainless steel droplets to distinguish fragmentation characteristics. According to the present findings, adequate fragmentation of molten material can be caused by increased relative velocity, high superheat and considerable energy release capability, enhancing heat transfer and accelerating solidification of molten material. With few researches on MFCI in SFR, a fragmentation mechanism is proposed based on the growth and collapse of sodium bubbles to further understand the fragmentation behavior of molten material in sodium pool.