Thermodynamic and kinetic aspects of UO2 fuel oxidation in air at 400-2000 K
Proceedings of 11th Symposium on Thermodynamics of Nuclear Materials, Karlsruhe, 6-9 sept 2004. Journal of Nuclear Materials, numéro spécial à paraître.
Most nuclear fuel oxidation research can be categorized as either low-temperature (<700 K) air oxidation linked to fuel storage or high-temperature (>1500 K) steam oxidation linked to reactor safety issues. This paper attempts to unify modelling for air oxidation of UO2 fuel over a wide range of temperature. A longer-term objective is to incorporate such a modelling approach in 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 on the scale of individual fragments to U3O8 via UO2+x and/or U4O9 above ~1200 K. A broad "grey area" exists between about 700 and 1200 K, where 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, and the thermodynamic properties that may govern solid-state oxygen diffusion. Model development is limited by the scarcity of experimental data, especially at low oxygen partial pressures and at high temperatures. Some of the underlying needs for a high-temperature, mechanistic model are described.