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

Abstract

ZrSiO4-based ceramics have been considered as one of the candidate nuclear waste forms, while the immobilization behavior of mixed-valent uranium in the structure of the ceramics was unclear. Herein, ZrSiO4–U ceramics with a general formula of Zr1-4x(U6+xU5+2x)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 (∼10−5 g m−2 d−1). The results indicated that ZrSiO4-based ceramics can be employed to effectively immobilize mixed-valent U.