During the severe accident, the core melt could leak out of the reactor pressure vessel through the broken instrument tubes of the RPV lower head. Thus, the melt freezing behavior in an instrument tube is a key factor that concerns the failure of the reactor pressure boundary. In this study, the Moving Particle Semi-implicit (MPS) method was adopted to analyze the melt penetration and solidification behaviors in a tube. The change of melt viscosity with temperature was taken into account in the present MPS method. The influence of surface tension on the melt penetration behavior was also considered. The numerical results had been compared with the upward melt injection experiments. The comparative results showed that the melt penetration lengths were in good agreement with the experiments. The typical melt freezing behaviors were successfully reproduced by MPS method. The crust formed on the surface of the tube increased the melt flowing resistance. The melt velocity also decreased due to the increase of its viscosity. The present results indicate that MPS method has the capacity to analyze the melt freezing behavior in the lower head penetration tubes.

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