Zirconolite is recognized as one of the most durable waste matrices for the disposal of high-level radioactive wastes (HLWs). In this study, HfO₂ was employed as the surrogate of tetravalent actinides. Hf-bearing zirconolite-based composite waste forms (CaZr_1-xHf_xTi₂O₇) were rapidly prepared by combustion synthesis (CS) using CuO as the oxidant, where quick pressing (QP) was introduced to obtain densified samples. Similar as solid state reaction process, the Zr site of zirconolite can be totally occupied by Hf (x = 1.0) under the CS reaction. The original 2M zirconolite structure was maintained and a small amount of perovskite impurity phase was generated in the final products. The aqueous durability of representative sample (Cu-Hf-0.6) was tested, where the 42-day normalized leaching rates (LR_i) of Ca, Cu, and Hf are 0.25, 3.10×10^-2, and 1.11×10^-8 g·m^-2·d^-1.

In this research, Zr-doped Gd₂Ti₂O₇ pyrochlores, with the composition of Gd₂(Ti_1-xZr_x)₂O₇, were firstly synthesized by self-propagating high-temperature synthesis plus quick pressing (SHS/QP) using CuO as the oxidant and Ti as the reductant. To improve the radiation resistance of titanate–pyrochlore, up to 35 at% Zr was incorporated to substitute the Ti site of Gd₂Ti₂O₇ pyrochlore (Gd₂(Ti_0.75Zr_0.35)₂O₇). XRD and SEM microstructural characterizations showed the formation of a composite ceramic with the major pyrochlore phase and the minor Cu phase. The generated temperature of samples decreased from 1702 to 1011 ℃ with increasing Zr content. The effects of sintering temperature and pressure time on phase composition and microstructure were systematically studied. Besides, the influence of thermal transmission on the whole combustion process was also explored. The pyrochlore-based waste form possessed high bulk density of 6.25 g/cm³ and Vickers hardness of 10.81 GPa. The MCC-1 leaching test showed the normalized elemental leaching rates (42 d) of Cu, Gd, and Zr are 1.27×10^-2, 1.33×10^-3, and 8.44×10^-7 g·m^-2·d^-1, respectively.