Multiphysics study on phase separation and thermal aggregation effects in the melt pool of the reactor lower head under severe accidents

Abstract

In the progression of severe accidents in light water reactors, the retention of core melt in the reactor pressure vessel (RPV) lower head, known as In-Vessel Retention (IVR), constitutes a core strategy for ensuring nuclear power plant safety and preventing large-scale release of radioactive materials. Targeting the complex multi-component liquid-liquid phase separation phenomena within the melt pool and the resulting “thermal focusing effect,” a coupled phase-field-flow-heat transfer multiphysics model based on free energy functionals was established. By simultaneously solving the Cahn-Hilliard equation, incompressible Navier-Stokes equations, and energy conservation equation in a two-dimensional axisymmetric domain, the dynamic evolution of a U–Zr–Fe–O quaternary system from an initially random uniform mixture to a macroscopic stratified state within the RPV lower head was simulated. Results demonstrate that the melt pool, driven by decay heat and interfacial energy, undergoes a “labyrinthine” bicontinuous structure evolution initiated by spinodal decomposition, ultimately forming a stable bilayer structure with a light metal layer above and a heavy oxide layer below under gravitational effects. The flow field evolves from initial small-scale chaotic convection (maximum velocity 0.0541 m/s) to large-scale circulation driven by thermal buoyancy (maximum velocity 0.00192 m/s), forming a stable configuration with an active upper convection zone and a stagnant lower dead zone. Due to the significant thermal conductivity contrast between the two stratified layers, wall heat flux exhibits a notable increase at the metal-oxide interface region, with peak heat flux reaching approximately 0.9 × 10⁶ W/m² during mid-evolution, attaining 60% of the upper CHF limit from ULPU experiments, posing a potential threat to IVR safety margins. Quantitative analysis of the phase separation time scale and its impact on wall thermal loading was conducted, providing numerical evidence and theoretical support for IVR safety margin assessment.