Experimental investigation of Kr diffusion in UO2+x: Slight deviations from stoichiometry, significant effects on diffusion kinetics and mechanisms

International audienceThermo-Desorption Spectrometry (TDS) was employed to study the release of fission gases from a Kr implanted UO2+x during 1050–1300 °C isotherms. The stoichiometry of the sample has been regularly increased by O2 additions in the vacuum setup to cover an hyperstoichiometry x range from ∼1 × 10−6 to ∼0.1. We used an analytical model based on Fick's second law to determine krypton diffusion kinetics (DKr) from the Kr cumulated release profiles. We demonstrate that even little x deviations from stoichiometric UO2 down to the 10−6 order are sufficient to quantitatively enhance fission gas diffusion. This likely explains a fair amount of data scatter in literature for experimental fission gases diffusion rates in (supposedly) perfectly stoichiometric UO2. A model was developed from the experimental results and well-accepted data for UO2 to estimate DKr (and DXe equivalently) as a function of T and x in the ranges of the present study. For moderate hyperstoichiometries, the present results suggest a fission gas diffusion mechanism identical (or similar to the very least) to that largely accepted for stoichiometric UO2, with the increased concentration of oxygen interstitials in the material kinetically enhancing Kr diffusion. At x ≈ 0.001 the diffusion-limiting mechanism changes, materialized by an increase of the activation energy (Ea). This Ea increase seemingly rules out a diffusion of fission gases controlled by VU (U vacancies). Instead, the rise of oxygen interstitial clusters, becoming the dominant type of defects in UO2+x for similar x ≥ 0.001 threshold, makes these defects prime suspects in controlling Kr (and Xe) diffusion in hyperstoichiometric uranium dioxide