Abstract: In a probable scenario for core disruptive accidents (CDAs) of sodium-cooled fast reactors, massive molten core material will be discharged into the lower sodium plenum through the control rod guide tubes and might impose a considerable thermal burden on the lower structures. Nevertheless, such thermal burden will be reduced, provided that the discharged molten core material is fragmented into small particles well before reaching the lower structures. In the present study, in order to develop an evaluation method of the distance for fragmentation of molten core material discharged into the sodium plenum, fundamental experiments were carried out using a high-density melt (an alloy with low-melting temperature) and water as simulants for the molten fuel and coolant, respectively. In the experiments, the melt was discharged into a water pool through a nozzle (inner diameter: from 30 mm to 150 mm) under a simulated CDA condition in which formation of a stable vapor film is inhibited around the melt surface and thus a liquid-liquid direct contact is maintained between the melt and water. The present experimental results showed that measured distances for fragmentation of the injected melt were limited to approximately 10 percent of predictions by the existing representative correlation, and that vapor expansion with pressure buildup in the vicinity of the melt could facilitate the reduction of the distance for fragmentation. Through the fundamental experiments, useful knowledge were obtained for the future development of an evaluation method of the distance for fragmentation of molten core material.