Under Hypothetical Core Disruptive Accidents (HCDA) in Sodium-cooled Fast Reactor (SFR), Molten Fuel-Coolant Interaction (MFCI) will cause intensive fragmentation of molten fuel materials and violent sodium boiling, possibly resulting in recriticality due to sodium void effect and reactor core compaction. And the formation and sufficient cooling of core debris bed are significantly affected by the fragmentation of molten fuel materials in sodium pool. Due to approximate ambient Weber Number (We(a)) with molten metallic fuel, kilogram-level molten copper jet is dropped into sodium pool to study the fragmentation characteristics under varied experimental parameters in the present experiments. The nucleate boiling of liquid sodium is estimated to occur at the instantaneous contact interface between molten copper and liquid sodium. The molten copper jet suffers from intensive fragmentation and solidification to produce many fragments under thermodynamic and hydrodynamic effects. Besides, experiment with molten aluminum jet injected into sodium pool is conducted without occurrence of sodium boiling to independently study the effect of hydrodynamic interaction on interphase penetration and fragmentation. During the experiments, temperature variations and dynamic pressure change in sodium pool are monitored to evaluate energy release. The resultant fragments are recorded and measured for morphology analysis and size distribution to examine the fragmentation characteristics. Based on experimental results, the fragmentation mechanism is discussed against hydraulic and thermal factors. The present findings contribute to further understanding the fragmentation characteristics of molten corium in sodium pool and optimization of mitigation systems against MFCI for SFR. (C) 2019 Elsevier Ltd. All rights reserved.