Résumé
The defect chemistry and thermal oxidation of lanthanide (Ln)
incorporated-UO2 are critical for understanding and predicting their behavior
as enhanced fuels, mixed oxide (MOX) fuels, spent nuclear fuels (SNF), and
particles for safeguard purposes. In this study, we independently controlled
the Ln type (Ce4+, Nd3+, and Gd3+) and the preparation condition (reduced and
nonreduced) to investigate their correlations to the generated non-equilibrated
defects correspondingly. From early to late lanthanides: Ce and U formed
close-to-ideal solid solutions in Fm-3m and oxidized to (Ce, U)4O9, Nd and U
mixing under the reducing condition formed solid solutions with oxygen
vacancies aggregating near Nd, and the mixing of smaller Gd with U resulted in
short-range subnano-domain segregations with Ia-3 region embedded in the global
Fm-3m matrix. Both trivalent Ln-incorporated UO2 oxidized to a mixture of (Ln,
U)4O9 and (Ln, U)3O8. From these signature defect structures resulting from
both Ln type and preparation condition, we proposed kinetic model and
thermodynamic hypothesis for explaining the oxidation resistance of (Ln, U)O2.
Although originated from f-block oxides, the discovery of long-range disorder
short-range ordering may be not uncommon in other metal oxide systems, which
can strongly influence their functionalities and properties.