Moving particle semi-implicit (MPS) method is the particle (girdles) method for incompressible medium. Particles are used for discretization of fluids, and governing equations are transformed to particle interaction models. Phase changes are treated as changing particle dynamics. The interface is treated accurately. In the present study, two-dimensional code is created for molten core concrete interaction (MCCI) by using MPS method. Each particle has enthalpy, and three types of heat transfer are treated. Conductive heat transfer is calculated with Laplacian model of MPS. Nucleate boiling and radiation heat transfer is calculated by removing enthalpy from surface particles. Liquid particles can be changed to solid particles depending on their enthalpy. These particles are fixed to space. Semi-implicit method is used in original MPS method, but in this study, explicit method is introduced in order to increase calculation speed. The SWISS-1 and SWISS-2 experiments are analyzed. Gas release from concrete is ignored, and melting of concrete is treated as disappearing of particles. This generate void between crust bridge and liquid debris in SWISS-2, and crust bridge is heated mainly by radiation heat transfer. Calculated ablation rate of concrete agrees well with experimental results of SWISS-1 and SWISS-2, but calculated heat flux from crust bridge to the water pool of SWISS-2 is lower. It is because the water penetration through the crust is ignored, and the amount of penetrated water is estimated by the difference between calculation and experimental result.

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