Ab initio structure determinations of high-performance radioactive waste forms

Immobilisation of high-level radioactive wastes in stable matrices for long-term storage or geological disposal is a key step in closing the nuclear fuel cycle. Titanate minerals have shown much promise in their capacity to take up radioactive elements into their crystal structures at regular lattice sites in pyrochlore and brannerite phases. Previously unreported uranyl titanates BaUTiO6 and Na2U3Ti2O11 have been synthesised under argon at 1400 and 1250 deg. C, respectively and their structures determined by ab initio techniques using a combination of electron, synchrotron and neutron powder diffraction. BaUTiO6 crystallises with a carnotite-type structure (P21/c; a=6.4463(1) A, b=8.5999(1) A, c=10.2532(1) A; >=75.936(1)o and V=551.36(1) A3); while Na2U3Ti2O11 forms an orthorhombic structure (Pnma; a=31.0421(8) A, b=7.2774(2) A, c=7.6111(2) A and V=1719.4(1) A3). Due to their potential value as a radioactive waste forms, the leach rates of uranium, barium and sodium were determined. Normalised uranium leach rates from BaUTiO6 and Na2U3Ti2O11, averaged over 7 days were orders of magnitude less than those for brannerite and for U-containing pyrochlore waste forms