Abstract
In liquid-metal-cooled fast reactors (LMFRs), the occurrence of hypothetical core disruptive accidents (HCDAs) has been considered. Although the probability of occurrence of HCDAs is quite low, it is indispensable to evaluate of the potential risk of HCDAs. To reduce further the potential risk of HCDAs, the core concept that molten fuel will be discharged from the core region before occurrence of an energetic re-criticality is desirable. As a basic study to establish the core concept, we have previously reported a mechanism on breakup of molten metal jet penetrating a sodium pool at instantaneous contact interface temperatures below its freezing point. In the present study, we carried out a series of experiments to confirm a mechanism of thermal fragmentation of a single molten drop penetrating a sodium pool, which is important to understand the fragmentation of molten metal drops after breakup of molten metal jet. A single molten copper drop from 5g to 0.25g was dropped into the sodium pool through the argon gas atmosphere. Thermal fragmentation originating inside the molten copper drop with a thin solid crust at its lower surface was clearly observed at all runs of 5.0g molten drop and 1.2g molten, drop. At runs of 0.5g molten drop and 0.25g molten drop with almost its freezing point, a single debris which was a shell structure of irregular shape with a large open mouth, was obtained even at low sodium temperatures less than 433K. In this way, even the initial temperatures are almost its freezing point, the molten drop fragments. In the present experimental condition, latent heat contributes to fragmentation. Thermal fragmentation of a single molten drop is caused by boiling of sodium entrapped into the molten drop and absorbed latent heat. According to our previous study, this sodium entrapment occurs because of sodium micro jet driven into the upper surface of the molten drop. In order to confirm this action, an experiment to measure impact of the micro jet driven into the upper surface of the molten drop penetrating a water pool was carried out using a strain gauge. The instantaneous pressure rise from 0.03 atm to 0.04 atm by the micro jet driven into the upper surface of the drop was detected. The dissolution of argon gas into molten copper drop was also investigated. The cross section of the frozen drops without fragmentation showed no trace of argon gas bubbles. Therefore, the dissolution of argon gas essentially does not affect fragmentation observed.
