Design and fabrication of Gd₂Zr₂O₇-based waste forms for U₃O₈ immobilization in high capacity

For the long-term radiotoxicity of uranium in nuclear waste, U₃O₈ were immobilized into Gd₂Zr₂O₇ by ceramics solidification technology. Systematic samples (Gd_1−4xU_2x)₂(Zr_1−xU_x)₂O₇ (0 ≤ x ≤ 0.25) were designed and prepared simultaneously via lattice substitution. The immobilization effects were identified by XRD, BSE, EDS, SEM, TEM, and XPS. The results show that 38.83 wt% U₃O₈ gets dissolved in Gd₂Zr₂O₇ as a single phase (corresponding to 0 ≤ x ≤ 0.14 in (Gd_1−4xU_2x)₂(Zr_1−xU_x)₂O₇), with U⁶⁺ occupies 56 at.% of the Gd³⁺ positions and U⁴⁺ occupies 14 at.% of the Zr⁴⁺ sites. The XRD patterns indicate that the solid solutions transform from pyrochlore to defective fluorite structure with enhanced uranium, and this is due to the difference in ionic radii between dopant ions (U⁶⁺ and U⁴⁺) and the host ions (Gd³⁺ and Zr⁴⁺). Grain coarsening and densification are observed with the increased uranium. Tetravalent and hexavalent uranium in waste forms are confirmed through XPS. This research reveals the good adaptability of Gd₂Zr₂O₇ in immobilizing U₃O₈.