A study of molybdenum behaviour in UO2 by X-ray absorption spectroscopy

Molybdenum is one of the most abundant fission products created in UO2 fuel. Post-irradiation examinations show that molybdenum is mainly associated with rhodium, ruthenium, palladium and technetium in metallic precipitates. However, for low burn-ups, these five metal particles cannot be detected. In order to determine the chemical state of molybdenum in UO2, XANES experiments were carried out on fresh UO2 pellets implanted with Mo ions. The measurements performed at the oxygen K and molybdenum LIII edges showed a correlation between the chemical state of molybdenum and the O/U ratio of the UO2 matrix. Indeed, the molybdenum located at the grain boundaries is present in both metallic and +IV oxidation states. To our knowledge, this study is the first experimental evidence of the solution annealing of Mo+IV in the cationic lattice of UO2. Thus, when molybdenum is oxidised at the +IV state, it remains in solution in the UO2 matrix.