Design and fabrication of Gd sub(2)Zr sub(2)O sub(7)-based waste forms for U sub(3)O sub(8) immobilization in high capacity

For the long-term radiotoxicity of uranium in nuclear waste, U sub(3)O sub(8) were immobilized into Gd sub(2)Zr sub(2)O sub(7) by ceramics solidification technology. Systematic samples (Gd sub(1-4x )U sub(2x )) sub(2)(Zr sub(1-x )U sub( x )) sub(2)O sub(7) (0 less than or equal to x less than or equal to 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% U sub(3)O sub(8) gets dissolved in Gd sub(2)Zr sub(2)O sub(7) as a single phase (corresponding to 0 less than or equal to x less than or equal to 0.14 in (Gd sub(1-4x )U sub(2x )) sub(2)(Zr sub(1-x )U sub( x )) sub(2)O sub(7)), with U super(6+) occupies 56 at.% of the Gd super(3+) positions and U super(4+) occupies 14 at.% of the Zr super(4+) 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 (U super(6+) and U super(4+)) and the host ions (Gd super(3+) and Zr super(4+)). 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 Gd sub(2)Zr sub(2)O sub(7) in immobilizing U sub(3)O sub(8).