Thermodynamic and kinetic aspects of UO2 fuel oxidation in air at 400–2000 K

Most nuclear fuel oxidation research has addressed either low-temperature (<700 K) air oxidation related to fuel storage or high-temperature (>1500 K) steam oxidation linked to reactor safety. This paper attempts to unify modelling for air oxidation of UO2 fuel over a wide range of temperature, and thus to assist future improvement of the ASTEC code, co-developed by IRSN and GRS. Phenomenological correlations for different temperature ranges distinguish between oxidation on the scale of individual grains to U3O7 and U3O8 below ∼700 K and individual fragments to U3O8 via UO2+x and/or U4O9 above ∼1200 K. Between about 700 and 1200 K, empirical oxidation rates slowly decline as the U3O8 product becomes coarser-grained and more coherent, and fragment-scale processes become important. A more mechanistic approach to high-temperature oxidation addresses questions of oxygen supply, surface reaction kinetics, thermodynamic properties, and solid-state oxygen diffusion. Experimental data are scarce, however, especially at low oxygen partial pressures and high temperatures.