Numerical study on debris bed formation characteristics following the FCI of a nuclear reactor severe accident with the EMPS method

Debris bed formation during fuel-coolant interaction (FCI) is a key process in the analysis of severe nuclear reactor accidents. Existing numerical studies typically decouple melt jet breakup and debris bed formation, relying on idealized fragment shape assumptions. The present study conducts a continuous numerical simulation of the FCI process by coupling the Explicit Moving Particle Semi-implicit (EMPS) method with a Contoured Continuum Surface Force (CCSF) model, a fragment identification model, and a Passively Moving Solid (PMS) model. The integrated approach tracks the dynamic evolution from initial melt injection and solidification to final bed accumulation. The established numerical model is validated against experimental data, with macroscopic predictions showing consistency with physical measurements. Parametric analyses reveal the mechanisms governing debris bed morphology. Increasing the LBE temperature, jet velocity, and water temperature increases the spreading area. High LBE and water temperatures induce edge crust remelting and secondary expansion. Furthermore, the debris bed accumulation height decreases as these three parameters increase. The total porosity increases with elevated LBE temperatures and jet velocities, and decreases under higher cooling water temperatures. The proposed modeling approach provides quantitative insight into debris bed formation and may support the development of severe accident management strategies.