Oxygen diffusion in liquid (over)stoichiometric corium

Highlights: • Liquid corium oxidation may impact FCI phenomena like steam explosion. • Modelling of 1-D oxygen diffusion in liquid corium droplets is performed. • CALPHAD-based thermodynamic calculations are coupled with oxidation kinetics. • Typical oxidation transient times are determined for various corium droplets. • Transient analysis highlights original internal mass transfer phenomena. - Abstract: Within the framework of pressurized water reactor severe accidents, the ICE research program is devoted to studies on a variety of fundamental mechanisms involved in Fuel-Coolant Interaction (FCI). Among them, the oxidation of liquid corium is expected to impact the premixing/fragmentation stage, thus possibly influencing the subsequent steam explosion. As a first step towards the exhaustive description of oxidation phenomena occurring in typical FCI layouts, the 1-D oxygen diffusion in liquid (over)stoichiometric spherical corium droplets is investigated in this article. A mesoscopic description is adopted, based on the evolution of oxygen concentration $c_O$, depending on space, time, corium composition and temperature. The underlying physical model is coupled with CALPHAD-based thermodynamic calculations. An asymptotic analytical solution is first determined, assuming that the apparent diffusion coefficient $D_O$ does not depend on $c_O$. Then, the general variational form of governing equations is derived, leading to the implementation of a related numerical approach using the finite-element method, in COMSOL® software. This numerical approach is first benchmarked with the analytical calculation, and then extrapolated to the more general case where $D_O$ varies with $c_O$. Finally, some case studies representative of FCI issues are selected, in order to determine typical oxygen diffusion transient times within a variety of liquid corium droplets.