Understanding the long-term behaviour of the UO2 spent fuel in terms of helium build-up and oxidation is a very important issue for safety aspects of storage and disposal. Although helium behaviour in stoichiometric UO2 has been studied by many authors, it has not been well established and there is a lack of experimental studies in non-stoichiometric UO2. In this study, an infusion technique was chosen to introduce helium in stoichiometric and hyperstoichiometric single and polycrystalline UO2 and polycrystalline U3O8 samples. Characterization of the samples before and after infusion and after thermal desorption measurements were performed by thermogravimetry, X-ray diffraction, laser flash technique and scanning electron microscopy. It was observed that the increase of stoichiometry in UO2 affects barely the dissolved helium quantity (under the same infusion conditions as in stoichiometric), while it has a more pronounced influence on the helium diffusion. These two effects are much more pronounced for U3O8.
