A novel multiphase MPS algorithm for modeling crust formation by highly viscous fluid for simulating corium spreading

Corium (lava-like mixture of fissile material) spreading prediction is of great significance in the severe accidents of nuclear power plants. Crust formation due to solidification distinguishes corium spreading from common isothermal spreading. The Lagrangian moving particle semi-implicit (MPS) method is potential for such spreading flow with both free surface and crust-melt interface. Crust formation is usually represented by viscosity escalation, but crust creeping is an associated problem. In the original MPS algorithm, creeping velocity cannot be reduced steadily by the continuous increase of viscosity, owing to the numerical creeping. A new solution algorithm is proposed for particle methods to eliminate such numerical creeping, so that creeping velocity decreases proportionally with viscosity rise. In this situation, high enough viscosity can effectively represent crust behaviors. Three numerical examples, leakage flow with high viscosity, dam break flow with low viscosity and the VULCANO VE-U7 corium spreading experiment with both high and low viscosities simultaneously, are investigated to contrast the performance difference between the original and new algorithms. It is demonstrated that the current algorithm is suitable for crust formation in corium spreading.