We developed the numerical simulation method for predicting the melting core behavior including solidification and relocation based on the three-dimensional multi-phase thermal-hydraulic simulation models. In this code, each of gas, liquid and solid phase are treated individually, and interface between two phases are simulated directly. In this paper, the developed code was applied to numerical simulations of the melting behavior of the simulated fuel assemblies and reactor structures. In the simulation, complicated structures in the BWR lower plenum was simply modeled. A decay heat in molten or solidified debris was also considered. Moreover, several different initial conditions were used to check performance of this code and to evaluate adequacy of the present numerical method. From the present numerical results, it was confirmed that relocation of molten debris in the BWR lower plenum can be simulated by the currently developed code including effects of melting and solidification of debris.

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