Self-propagating High-temperature Synthesis Of Zirconolite-rich Synroc Waste Forms For The Immobilization Of Simulated Actinide

In order to solve the problem that the preparation temperature was relatively high and the process was complex, as well as the raw materials was expensive, the Nd/Ce-doped zirconolite-rich synroc were rapidly synthesized by self-propagating high-temperature plus quick pressing ?SHS/QP? using CrO3, Fe2O3, Ca?NO3?2, MoO3 as the oxidant and Ti as the reductant. The adiabatic temperature was calculated, while the combustion temperature was measured. Different TiO2/Ti molar ratios were explored to investigate combustion temperature of the designed SHS reactions. The mechanism of simulated actinide in SHS-ed zirconolite-rich synroc was investigated. The leaching performance of simulated actinides in the Ce/Nd-doped zirconolite-rich synroc waste forms were studied as well.The results show that zirconolite could be prepared when Ca?NO3?2 was employed as the oxidants. Meanwhile, the decrement of TiO2/Ti molar ratio was beneficial to the higher combustion temperatures and lower reactivity. As the surrogate of tetravalent/trivalent actinide nuclides, CeO2 was incorporated to substitute the Zr site of zirconolite while Nd2O3 was doped to simultaneous replace the Zr and Ca sites of zirconolite in Ca?NO3?2 system. However, the proportion of zirconolite declines gradually while that of perovskite increases when the CeO2/Nd2O3 incorporation was promoted. The raw CeO₂ partially was reduced to trivalent state. The incorporated Ce substitutes the Ca site of perovskite or pyrochlore rather than the Zr site of zirconolite. Nd was mostly incorporated in the Ca sites of zirconolite, perovskite or pyrochlore. Meanwhile, Ce was also employed to replaced the Zr site of zirconolte in MoO3 system. The raw CeO2 was partially reduced to trivalent state. Also, Ce was incorporated into the Ca sites of zirconolite anf perovskite. Nd/Nd-Al-doped zirconolite-rich synroc waste form was also synthesized by SHS method using MoO3 as the oxidant. The result show that Nd was incorporated into the Ca site of zirconolite.The chemical durability of the waste form was characterized by standard MCC^-1 leaching test method. The results show that the Ce/Nd-doped zirconolite-rich ceramics exhibit high hydrothermal stability. The leaching rates of Ce, Ca and Nd reach low values of 10^-1,10^-6 10^-4 g·m^-2·d^-1 relatively after 42 days in Ca?NO3?2 system. The leaching rates of Ce and Mo reach 8.09 g·m^-2·d^-1 and 1×10^-2 g·m^-2·d^-1 relatively after 42 days in MoO3 system. Nd, Zr and Ca exhibite excellent aqueous durability. Leaching rates reach low values of 10^-4,10^-5,10^-5g·m^-2·d^-1 after 42 days.