Numerical Simulation of Microstructure Evolution during Molten Zirconium Oxidation Solidification Process

In the severe accidents in pressurized water reactors (PWR), the oxidation behavior of zirconium alloy cladding melts will increase the apparent viscosity of the melt, which significantly influences the migration and relocation process of core melts and is a key phenomenon during core degradation. Published experimental mechanism studies on the oxidation kinetics of molten zirconium alloys have indicated that the microstructural characteristics of molten zirconium alloys during oxidative solidification have an important influence on their oxidation kinetics. In this study, a multi-physics coupling model integrating solidification, temperature and oxygen concentration fields, targeting the oxidation and solidification process of molten zirconium was established. The validity and accuracy of the model were verified by the analytical approximate of the flow through regular arrays of parallel solid cylinders and the experimental results of the dendrite growth kinetics of zirconium alloys under electrostatic levitation condition. Based on the model, the effects of temperature and melt flow on the microstructural evolution of molten zirconium alloy during solidification were investigated. The simulation results show that: The melt flow will significantly affect the microstructure, the dendrite growth process in the direction of the on-flow can be enhanced, and the direction of the dendrite growth will be deflected to the direction of the on-flow. The solid-state zirconium crystal structure and the physical parameters of phase diagrams affected by temperature have a significant effect on the microstructural morphology of the solidification process. Temperature affects the crystal structure of molten zirconium oxidative solidification products. In the temperature range of 1 855-1 968 ℃, the solidification of the β-Zr phase with body-centered cubic structure forms a prismatic microstructure, while the solidification of the α-Zr(O) phase with densely-arranged hexagonal structure forms a hexagonal microstructure in the temperature range of 1 968-2 129 ℃. Temperature also affects the liquid-phase line equilibrium oxygen mole fraction and equilibrium binary partition coefficient parameters in the binary phase diagram, resulting in different melt oxygen concentration inhomogeneities during oxidative solidification at different temperatures. By establishing a fine model for the evolution of microstructure in the solidification process of molten zirconium alloys, a theoretical tool is provided for the analysis of the oxidation phenomenon of molten zirconium alloys.