Synthesis and characterisation of brannerite compositions (U0.9Ce0.1)1−xMxTi2O6 (M = Gd3+, Ca2+) for the immobilisation of MOX residues

Abstract

A suite of uranium brannerites for the disposal of MOX residues, formulated (U_0.9Ce_0.1)_1−xM_xTi₂O₆ (M = Ca²⁺ and/or Gd³⁺), were prepared using a mixed oxide route under oxidising, inert and reducing atmospheres (air, argon and H₂/N₂). Gd³⁺ was added to act as a neutron absorber in the final Pu bearing wasteform and Ce added to function as a structural analogue for Pu. X-ray powder diffraction of the synthesised specimens found that phase distribution was strongly affected by the processing atmosphere and Gd content. In all cases prototypical brannerite was formed, accompanied by different secondary phases dependent on processing atmosphere. Microstructural analysis (SEM) of the sintered samples confirmed the results of the X-ray powder diffraction. Bulk XANES found that Ti remained in the Ti⁴⁺ oxidation state whereas Ce was uniformly reduced to the Ce³⁺ oxidation state regardless of processing conditions or stoichiometry. Micro-focus XANES was used to determine U oxidation in the brannerite phase and showed that U oxidised to higher U oxidation states to charge compensate. It was concluded that the charge balance mechanism was a combination of U oxidation and A-site vacancies.