Synthesis and chemical durability of U-doped sphene ceramics

U-doped sphene ceramics with and without Al as charge compensation, i.e. Ca1−xUxTi1−2xAl2xSiO5 and Ca1−2yUyTiSiO5, were prepared by solid-state reaction method. The effects of U content on the phase structure of the ceramics were investigated. The chemical durability of U-doped sphene ceramic waste forms was also examined. The results show that the optimum sintering temperature of the U-doped sphene ceramics is about 1260 °C. For the ceramics with x ⩽ 0.065 or y ⩽ 0.045, the primary crystalline phase is sphene, and a small amount of CaTiO3, SiO2 phases are also oberved. UO2 phase appears when x ⩾ 0.07 or y ⩾ 0.05. The solubility of U in Ca1−2yUyTiSiO5 is lower than that in the samples with Al as charge compensation. Moreover, the normalized elementary release rate of U in both the Ca1−xUxTi1−2xAl2xSiO5 and Ca1−2yUyTiSiO5 ceramics decreases with increasing time and remains almost unchanged (∼2 × 10−3 g m−2 d−1) after 28 days, exhibiting high chemical durability. These results suggest that the sphene-based ceramics are promising candidate for the immobilization of actinides.