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Helium behaviour in stoichiometric and hyper-stoichiometric UO2

Understanding the long term behaviour of the UO2 spent fuel in terms of helium behaviour and oxidation is a very important issue for the safety aspects of storage and disposal. Although helium behaviour in stoichiometric UO2 has been studied by many authors, the helium solubility in UO2 is not well established and there is a lack of experimental studies for helium behaviour in non-stoichiometric UO2. Therefore this study aims to improve the understanding of the helium behaviour in stoichiometric and hyperstoichiometric UO2. First of all infusion, experiments were performed on stoichiometric and hyperstoichiometric single and polycrystalline UO2 samples as well as on polycrystalline U3O8 samples. After infusion, the samples were annealed in a laser heated Knudsen cell until all the helium was released, then helium release profiles were obtained and total helium quantities were measured using a mass spectrometer and a quantitative gas measurement system, respectively. Isothermal helium release measurements at low temperature were also performed on stoichiometric and hyperstoichiometric UO2 single crystal samples to understand the role of stoichiometry on helium diffusion, using a high resolution magnetic sector mass spectrometer coupled with a furnace. Characterization of the samples before, 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 increase of stoichiometry in UO2 does affect barely the helium solubility, while it has a more pronounced influence on the helium diffusion. Those two effects are much more pronounced forU3O8.