To estimate the state of Reactor Pressure Vessel (RPV) of Fukushima Daiichi nuclear power plant, it is important to clarify the breakup and the fragmentation behavior of molten material jet in BWR lower plenum by a numerical simulation. To clarify the effects of complicated structures on jet breakup and fragmentation behavior experimentally and construct the benchmarks of the simulation code, we conduct the visualized experiments simulating the severe accident in the BWR. In this study, the jet breakup behavior, the fragmentation behavior and internal/external velocity profiles of the jet were observed by the backlight method and the particle image velocimetry (PIV). From experimental results, it is clarified that the complicated structures prolong the jet breakup length or make the fragments fallen together to the lower plenum similar to the bulk state. In addition, it is clarified that strong shearing stress occurs at the crest of interfacial waves at side of the jet when fragments are generated. Finally, the fragment diameters measured in the present study well agree with the theory suggested by Kataoka et al. (1983) by changing the coefficient term at each experimental condition. Thus, it is suggested that the fragmentation mechanism is mainly controlled by shearing stress and the fragment diameter can be estimated by adjusting the constant term.

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