Modeling corium pool stratification and focusing effect in APR1400 using a multiphase CFD framework

Accurately predicting molten corium behavior is critical for assessing severe accident scenarios in nuclear reactors, particularly under In-Vessel Corium Retention with Reactor Vessel External Cooling (IVCR-RVEC) strategies. This study presents a high-fidelity computational framework developed in OpenFOAM to simulate multiphase heat transfer and vessel ablation during corium pool formation. Two approaches are evaluated: the traditional Enthalpy-Porosity (EP) method and a newly implemented Multiphase Flow Heat Transfer (MFHT) model. The MFHT formulation resolves coupled mass, momentum, and energy transport with phase change across stratified oxide and metal layers. Model validation against Gallium melting experiments demonstrates good agreement with experimental data. For full-scale APR1400 simulations, a decay heat of 2.1 MW/m³ is applied to a stratified corium pool in the reactor lower head. The MFHT approach offers improved resolution of melt front propagation and localized vessel ablation, especially near angular zones 0-15° and 80-85°, where peak heat fluxes are observed. A mesh sensitivity study confirms grid independence, and a time step of 0.1s ensures numerical stability. Among the turbulence models tested, the SST k - ω model provided the most consistent predictions, making it suitable for natural convection flows in stratified molten pools. This work establishes a validated multiphase CFD tool for IVCR analysis, enhancing the simulation of phase-changing high-temperature systems under extreme thermal and geometric constraints.