Experimental investigation of Kr diffusion in UO$_{2+x}$: Slight deviations from stoichiometry, significant effects on diffusion kinetics and mechanisms - Archive ouverte HAL Access content directly
Journal Articles J.Nucl.Mater. Year : 2023

Experimental investigation of Kr diffusion in UO$_{2+x}$: Slight deviations from stoichiometry, significant effects on diffusion kinetics and mechanisms

Abstract

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.
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Dates and versions

hal-03919884 , version 1 (13-11-2023)

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D. Horlait, J. Domange, M.-L. Amany, M. Gérardin, M.-F. Barthe, et al.. Experimental investigation of Kr diffusion in UO$_{2+x}$: Slight deviations from stoichiometry, significant effects on diffusion kinetics and mechanisms. J.Nucl.Mater., 2023, 574, pp.154191. ⟨10.1016/j.jnucmat.2022.154191⟩. ⟨hal-03919884⟩
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