Numerical analysis of freezing controlled penetration behavior of the molten core debris in an instrument tube with MPS

The freezing controlled penetration behavior of the molten core debris in the BWR instrument tube had been analyzed with a two dimensional geometry under severe accident conditions using Moving Particle Semi-implicit (MPS) method. The change of melt viscosity with temperature in the phase transition region was taken into account in the present MPS method. Accordingly, in order to improve the computation speed of such highly viscous fluid, the implicit calculation scheme was employed to solve the viscous term in this study. The surface tension model based on the inter-particle potential force was incorporated in the MPS method to track the melt leading front more accurately. The present MPS method was validated first by simulating the experiment of molten aluminum oxide penetrating in a prototypical PWR instrument tube which was performed by EPRI. The comparison of the predicted penetration length and the measured results showed a good agreement selecting a parameter. Then the penetration and solidification behaviors of molten stainless steel and uranium dioxide in the BWR instrument tube were simulated under a wide parametric range. The computational results showed that the melt penetration length increased with the melt superheat, and the melt had plugged the tube in all simulations. The melt flow resistance increased due to the formation of the crust on the tube surface and the increase of melt viscosity in the phase transition region. The present results indicated that the typical melt penetration and solidification behavior in the instrument tube was successfully revealed by MPS method.