Oxidation behaviour of thorium-uranium mixed oxides in controlled oxidation experiments

Oxidation experiments in Ar/H ₂O/H ₂ gas mixtures were performed using a coulometric titration technique to investigate the oxidation behaviour of single-phase thorium–uranium mixed oxides under a controlled oxygen potential relevant to defective fuel operating conditions. The oxidation behaviour of the mixed-oxide (Th,U)O ₂ solid solutions with 8, 30, 60 and 100 wt% UO ₂ was investigated through oxygen-to-metal (O/M) ratio measurements combined with X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS) analysis, and neutron diffraction analysis (NDA). The local structure around U examined by EXAFS did not show any evidence of local clustering. The U L ₃ EXAFS results of (Th,U)O ₂ show that the local structures do not appear to depend on the UO ₂ content. This is an indication that Th and U are randomly distributed in the mixed oxides, which is in good agreement with the long-range structural properties determined by XRD and NDA. The XPS results combined with XANES results of the U L ₃ edge confirmed the oxidation of U ⁴⁺ for all compositions. An examination of the U L ₃ EXAFS spectra from the oxidized samples shows a split in the first coordination shell peak, which indicates the presence of different U–O bond-lengths in the oxidized materials. The long-range structure of the oxidized materials was characterized with NDA. Rietveld refinement revealed that the oxidized (Th,U)O ₂ materials with high UO ₂ content were biphasic. The quantitative phase analysis demonstrated the co-existence of two closely cubic structures: a stoichiometric (Th,U)O ₂ phase and a UO _2.06 phase. These observations suggest that the oxidation of uranium in the (Th,U)O ₂ mixed oxides occurred in the bulk of the samples as well as at the surface as revealed by XPS analysis.