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

Thermo-Desorption Spectrometry (TDS) was employed to study the release of fission gases from a Kr implanted UO$_2+x$ during 1050–1300 °C isotherms. The stoichiometry of the sample has been regularly increased by O$_2$ 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 UO$_2$ 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 UO$_2$. A model was developed from the experimental results and well-accepted data for UO$_2$ to estimate D$_Kr$ (and D$_Xe$ 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 UO$_2$, 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 UO$_2+x$ for similar x ≥ 0.001 threshold, makes these defects prime suspects in controlling Kr (and Xe) diffusion in hyperstoichiometric uranium dioxide.