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

Abstract

For the long-term radiotoxicity of uranium in nuclear waste, U3O8 were immobilized into Gd2Zr2O7 by ceramics solidification technology. Systematic samples (Gd1−4xU2x)2(Zr1−xUx)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 (Gd1−4xU2x)2(Zr1−xUx)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.