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

Abstract

Oxidation experiments in Ar/H2O/H2 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)O2 solid solutions with 8, 30, 60 and 100 wt% UO2 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 L3 EXAFS results of (Th,U)O2 show that the local structures do not appear to depend on the UO2 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 L3 edge confirmed the oxidation of U4+ for all compositions. An examination of the U L3 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)O2 materials with high UO2 content were biphasic. The quantitative phase analysis demonstrated the co-existence of two closely cubic structures: a stoichiometric (Th,U)O2 phase and a UO2.06 phase. These observations suggest that the oxidation of uranium in the (Th,U)O2 mixed oxides occurred in the bulk of the samples as well as at the surface as revealed by XPS analysis.