An investigation into the influence of implanted oxygen on krypton behaviour in uranium dioxide during annealing

Abstract

Abstract It is generally agreed that fission gas release in hyperstoichiometric uranium dioxide is strongly influenced by the excess oxygen. However, the exact mechanisms have not been studied at the fundamental level. The aim of the work reported here was to obtain information on this topic by controlled experiments using an ion implantation approach rather than working with irradiated nuclear fuel. Krypton implants were used to create krypton bubbles in UO 2 samples to which oxygen was then added, again by implantation, to induce local changes in stoichiometry. The effect of the oxygen on subsequent krypton desorption was studied during annealing to high temperatures using transmission electron microscopy and thermal desorption spectroscopy. This combination of techniques on individual samples allowed bubble substructure changes to be followed and directly matched to the gas release measurements. Two clear effects of oxygen were seen. At around 1300 K, a new gas release stage was found; it appeared to be related to the 1600 K peak seen after Kr-only implants and could be attributed to the loss of near-surface krypton gas atom clusters, with accelerated migration in hyperstoichiometric UO 2 . However, at higher temperatures the coarsening of bubbles in (Kr + O) regions was surprisingly suppressed with the bubbles appearing more uniform and smaller than in Kr-only areas. It is suggested that excess oxygen might form a complex bubble coating effectively pinning the bubbles to slow bubble migration.