Self-propagating high-temperature synthesis of CeO2 incorporated zirconolite-rich waste forms and the aqueous durability

Abstract

Zirconolite-rich Synroc waste forms were rapidly synthesized by self-propagating high-temperature plus quick pressing (SHS/QP) using Ca(NO3)2 as the oxidant and Ti as the reductant. 2M-zirconolite (CaZrTi2O7) was produced as the major phase with perovskite (CaTiO3) and pyrochlore (Ca2Ti2O6) as minor phases. As the surrogate of tetravalent actinide nuclides, up to 50 at% CeO2 was incorporated to substitute the Zr site of zirconolite. The weight fraction of zirconolite decreases gradually while the perovskite proportion increases when the CeO2 addition is promoted. Ce was mostly incorporated into the Ca sites of perovskite and Ca2Ti2O6 pyrochlore, rather than the assumed Zr site of zirconolite. The Ce-bearing Synroc waste form exhibits high hydrothermal stability. The MCC-1 leaching test shows the normalized leaching rates (42d) of Ca and Ce are 0.49gm−2d−1 and 9.47×10−6gm−2d−1.