2D MPS method analysis of ECOKATS-V1 spreading with crust fracture model

Highlights: • A novel timestep control has been developed for the MPS method based on particle number density. • This new technique can efficiently prevent numerical instability and reduce the calculation cost. • Crust formation and breach condition in ECOKATS-V1 have been investigated in detail. • Solidification Viscosity Threshold (SVT) and Crust Fracture Stress Threshold (CFST) have an important influence on crust formation and breach condition. Understanding the ex-vessel spreading behavior of molten core materials (corium) on the reactor containment floor is of great importance for designing, managing, and evaluating severe accidents of light water reactors. Among some simulated ex-vessel corium spreading experiments, temporal termination and restart of the flow (stop-and-go) were highlighted in the ECOKATS-V1 experiment, in which the simulant oxide melt (consisted of 41% of $A{l}_2{O}_3$, 24% of $Si{O}_2$, 19% of $CaO$, and 16% of $FeO$) was released into a 1-D channel, and the spreading behavior seemed to have temporarily stopped and restarted following crust formation and breach at the spreading leading edge. The preceding study developed crust formation and breach models for the Lagrangian-based Moving Particle Semi-Implicit (MPS) method to analyze such spreading. However, the applicability of the models was limited due to numerical instability. In this study, the timestep control of the MPS method is revised by newly introducing particle number density control, which can efficiently reduce timestep to prevent instability when the crust is formed or breached in low viscous flow. The new method is successfully applied to ECOKATS-V1 spreading with different crust formations and breach conditions. Namely, with different Solidification Viscosity Threshold (SVT) and the Crust Fracture Stress Threshold (CFST) of the melt. The former determines the threshold viscosity above which the fluid is regarded as solid (crust). The latter determines the mechanical strength of the crust at the leading edge of the spreading front. There remain issues for future studies to further develop the method for more quantitative discussions.