For the safety design of a Fast Breeder Reactor (FBR), Post Accident Heat Removal (PAHR) is required when a hypothetical Core Disruptive Accident (CDA) occurs. In PAHR, it is strongly required that the molten core material can be solidified and cooled down by the sodium coolant in a reactor vessel. In order to estimate whether the molten material jet is completely solidified by sodium coolant, it is necessary to understand the interaction between the molten core material and the coolant. The objective of the present study is to clarify the dominant factor of the jet breakup length and the size of the fragments experimentally. In this study, we injected molten material (Sn–Bi alloy) into coolant (water) at free fall speed. We can simulate an actual FBR system by using Sn-Bi alloy and water because the density ratio of them is similar to that of an actual FBR system. The jet breakup and the fragmentation behavior of the molten material jet were observed with a high speed video camera. In the previous study which we conducted, solidified crust is generated by the solidification on the molten material jet surface and affects the jet breakup and the fragmentation behavior. Then from the experimental results, in order to predict the size of fragments, it is constructed that the instability model based on hydrodynamic and material mechanics. Then in this paper, the surface close-up of the molten material jet was observed in order to investigate the effect of the solidification on the molten material jet surface.

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