Oxygen difusion in liquid (over)stoichiometric corium
Abstract
Within the framework of pressurized water reactor severe accidents, the ICEresearch program puts the focus on a variety of fundamental mechanisms involvedin Fuel-Coolant Interaction (FCI). Among them, the oxidation of liquidcorium is expected to impact the premixing-fragmentation stage, thus possiblyinfluencing the subsequent steam explosion. As a first step towards the exhaustivedescription of oxidation phenomena occurring in typical FCI layouts, the1-D oxygen diffusion in liquid (over)stoichiometric spherical corium dropletsis investigated in this article. A mesoscopic description is adopted, based onthe evolution of oxygen concentration cO, depending on space, time, coriumcomposition and temperature. The underlying physical model is coupled withCALPHAD-based thermodynamic calculations. An asymptotic analytical solutionis first determined, assuming that the apparent diffusion coefficient DO doesnot depend on cO. Then, the general variational form of governing equationsis derived, leading to the implementation of a related numerical approach usingthe finite-element method, in COMSOL software. This numerical approach isfirst benchmarked with the analytical calculation, and then extrapolated to themore general case where DO varies with cO. Finally, some case studies representativeof FCI issues are selected, in order to determine typical oxygen diffusiontransient times within a variety of liquid corium droplets.