Molten salt synthesis of Ce doped zirconolite for the immobilisation of pyroprocessing wastes and separated plutonium

Abstract

Molten salt mediated synthesis of zirconolite Ca0.9Zr0.9Ce0.2Ti2O7 was investigated, as a target ceramic matrix for the clean-up of waste molten salts from pyroprocessing of spent nuclear fuels and the immobilisation of separated plutonium. A systematic study of reaction variables, including reaction temperature, time, atmosphere, reagents and composition, was made to optimise the yield of the target zirconolite phase. Zirconolite 2M and 3T polytypes were formed as the major phase (with minor perovskite) between 1000 – 1400 °C, in air, with the relative proportion of 2M polytype increasing with temperature. Synthesis under 5% H2/N2 or Ar increased the proportion of minor perovskite phase and reduced the yield of the zirconolite phase. The yield of zirconolite polytypes was maximised with the addition of 10 wt.% TiO2 and 5 wt.% TiO2, yielding 91.7 ± 2.0 wt.% zirconolite, primarily as the 2M polytype, after reaction at 1200 °C for 2 h, in air. The particle size and morphology of the zirconolite product bears a close resemblance to that of the TiO2 precursor, demonstrating a dominant template growth mechanism. Although the molten salt mediated synthesis of zirconolite is effective at lower reaction temperature and time, compared to reactive sintering, this investigation has demonstrated that the approach does not offer any clear advantage with over conventional reactive sintering for the envisaged application.