Uranium-Incorporated Pyrochlore La<sub>2</sub>(U<sub><i>x</i></sub>Mg<sub><i>x</i></sub>Zr<sub>1–2<i>x</i></sub>)<sub>2</sub>O<sub>7</sub> Nuclear Waste Form: Structure and Phase Stability
Zhe Tang, Zhangyi Huang, Wei Han, Jianqi Qi, Yanli Shi, Nannan Ma, Yutong Zhang, Xiaofeng Guo, Tien‐Chang Lu
Abstract
As efficient and stable nuclear waste forms, single-phase uranium (U6+)-incorporated La2Zr2O7 nanoparticles were designed and synthesized in an air atmosphere. To obtain a high U loading, divalent magnesium (Mg2+) was introduced to balance the extra charge from the substitution of tetravalent zirconium (Zr4+) by U6+ with a minimized impact to the lattice. There is a composition-driven phase transition from order pyrochlore to defect fluorite as the U concentration increases from 10 to 30 mol %, demonstrating both good solubility and stability of the La2Zr2O7 host for U and potentially for other actinides. La2(UxMgxZr1–2x)2O7 (x = 0–0.3) nanoparticles showed good dispersity and crystallinity with an average particle size of ∼48 nm. Furthermore, X-ray photoelectron spectroscopy, Raman spectroscopy, and emission spectroscopy revealed that U was stabilized in the hexavalent state in the form of a UO22+ ion. Spectroscopic methods also demonstrated that our samples caused a scintillating response with an orange emission (597 nm) by 230 nm excitation. In addition, density functional theory simulations were employed to investigate the atomic structures and electronic properties of the U-incorporated pyrochlores. The calculated bond lengths, atomic charges, and charge density confirm the existence of UO22+ ions. Supported by both experimental and computational results, a novel geometrical structure was proposed to explain the Mg2+–U6+ substitution. This work demonstrated the successful development of U-incorporated La2Zr2O7 nanoparticles and provided an efficient way to immobilize U in these ceramic waste matrixes.