Volatile fission product behaviour during thermal annealing of irradiated UO2 fuel oxidised up to U3O8

The behaviour and release of fission products during high-temperature annealing of irradiated UO2 samples have been studied as a function of the oxidation state. The behaviour of a sample pre-oxidised to U3O8 was compared to that of non-pre-treated fuel from the same pellet radial location. The Knudsen cell mass spectrometer technique was used up to 1900 K for the pre-oxidised sample and up to 2800 K for the untreated sample. Both types of tests were run in vacuum. The possible chemical forms of the different fission products in the bulk and in the vapour phase have been estimated from the release curves and microprobe analysis. This study concerns essentially iodine, tellurium, caesium, rubidium, strontium, barium, technetium and molybdenum, whose effusion behaviour was strongly affected by the pre-oxidation treatment, resulting in an almost complete release by 1900 K. Release of zirconium, the lanthanides and actinides was observed at temperatures >1900 K, reached only in the case of the non-pre-treated UO2 experiments.