High capacity crystal immobilization of U5+ and U6+ by ZrSiO4 ceramics

ZrSiO4-based ceramics have been considered as one of the candidate nuclear waste forms, while the immobili-zation behavior of mixed-valent uranium in the structure of the ceramics was unclear. Herein, ZrSiO4-U ceramics with a general formula of Zr1-4x(U-x(6+) U-2x(5+))SiO4 were designed and synthesized. The evolutions of phase and microstructure depending on the content of U were investigated. The ceramics with 0 <= x < 0.04 showed a single ZrSiO4 phase, while U3O8 phase was detected when x >= 0.04. The results demonstrated that the solubility limit of U in ZrSiO4 ceramic was up to 1.51 at.% or 11.06 wt%. The calculated lattice parameters increased with the increase of U content. The increased parameters provided the evidence of the immobilization of both U5+ and U6+ into ZrSiO4 crystal lattice by replacing Zr4+. Furthermore, the obtained ZrSiO4-U ceramics exhibited good aqueous durability (similar to 10(-5) g m(-2) d(-1)). The results indicated that ZrSiO4-based ceramics can be employed to effectively immobilize mixed-valent U.