OXIDATION STUDIES ON IRRADIATED UO2 FUELS

UO2 oxidation has been examined since the early age of nuclear energy civilian applications. Most studies refer to unirradiated analogues or low burnup spent nuclear fuels. Unirradiated UO2 or low burn up fuel follow the scheme UO2→U3O7→U3O8 having all oxides present during the process, while fuels with burnup > 35 GWd/tHM are initially converted to U4O9 showing a certain resistance to further transformation. The final product U3O8 has a different crystal structure (orthorhombic) than the cubic (fcc) UO2 and U4O9 phases; its formation is associated with a 36% volume expansion. Oxidation to U3O8 would negatively affect the structural integrity of the fuel rod, compromising storage and handling of the fuel after discharge from the reactor. An originally small clad defect can turn over to big crack due to fuel oxidation, swelling and powdering. Larger fuel surface areas would become available for further corrosion processes and radionuclides release. Information on the oxidation behaviour of high burnup UO2 fuel, representative of the current burnup trend in nuclear power plants, is relatively limited in the literature. This paper reports on oxidation ex-periments carried out on BWR UO2 with nominal burnup of 50 and 65 GWd/t, respectively. Fuel frag-ments were oxidized in air in a hot cell autoclave at 300, 350 and 400°C, to characterize the kinetics of the transformation of UO2 into U3O8. Weight gain data were combined with x-ray diffraction analysis for phase identification. High burnup UO2 oxidises initially to a cubic phase resembling U4O9, but with a composition beyond its stoichiometry, closer to UO2.4. The new phase, a derivative of the initial cubic UO2 with slightly higher density, forms very rapidly along grain boundaries and grows into the grains. The process is controlled by oxygen diffusion to the grain interior; the reaction rate follows a parabolic kinetic law. The U4O9+x is stabilised by the fission products present in the matrix, with a characteristic plateau (whose duration depends on the oxidation temperature) occurring at bulk oxygen to metal (O/M) ratio of ~2.45. Afterwards, the oxidation proceeds with the formation of the final and, at tempera-tures < 500°C stable U3O8. A nucleation-and-growth mechanism displaying sigmoid reaction kinetics was observed. Selected specimens were subjected to microscopy examination, as well as oxygen po-tential measurements, to determine extent and effects of the oxidation process on phase distribution and overall morphology of the fuel.

PAPAIOANNOU Dimitrios;  HOLLAS Simon;  RONDINELLA Vincenzo;  SASAHARA Akihiro; 

2013-03-18

European Nuclear Society

JRC69115

978-92-95064-16-4,