Crystal chemistry and synthesis of pyrochlore structured CaAti2O7 and brannerite, ATi2O6 (A = Ce, U, Th, Pu) phases for Pu immobilisation

Full text: Ceramics designed for the geological immobilisation of excess weapons Pu are based on pyrochlore-structured Ca(U,Pu)Ti2O7, with minor brannerite, (U/Pu)Ti2O6, and rutile phases being present. To make samples for leaching and drop calorimetry, the individual phases have been synthesised where possible by the alkoxide/nitrate route involving liquid-state cation mixing. The synthesis of single-phase Ca(U/Pu)Ti2O7 has not been easy, using a variety of sintering, melting and hot-pressing treatments and fabrication atmospheres. No evidence for CaThTi2O7 was observed. CaCeTi2O7 can be synthesised in air at 1200 deg C. U-brannerite is readily synthesised under mildly reducing conditions at ∼ 1450 deg C, and the addition of Ca or rare earths stabilises this phase in an air atmosphere, with concomitant formation of higher U valence states. Ce/Th/Pu brannerites can be synthesised in air. Both the pyrochlore-and brannerite structures can accommodate considerable amounts of Hf and Gd neutron absorbers for criticality control of the ceramics