Gd-doping effect on crystallization behaviour and valence state of Ce-bearing zirconolite glass-ceramic

Abstract

Zirconolite-based glass-ceramic is a waste matrix used for incorporating high-level waste (HLW) which is rich in minor actinides. In this study, Ce-bearing zirconolite glass-ceramics were prepared via a traditional two-step thermal treatment method, and gadolinium was selected as the trivalent actinide residual to investigate co-doping effects. Co-doped samples were then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD results indicated that zirconolite was the sole crystalline phase synthesized from a SiO2–B2O3–Na2O–CaO–ZrO2–TiO2–Al2O3–CeO2–Gd2O3 system. A slight lattice expansion in the Ce-bearing zirconolite was also found after Gd doping. However, the average size of zirconolite particles obtained empirically via SEM remained constant (~1.8 μm) and was within the error range. A progressive decrease of the volume fraction of zirconolite was also observed after doping. In addition, XPS results confirmed that the Ce3+ fraction incorporated into zirconolite decreased with the increase in Gd3+, while it still dominated the Ce species, even for a Gd-doping concentration of as much as 1.0 wt%. This study aimed to advance the current understanding of the immobilization of actinide residuals in zirconolite glass-ceramics as well as provide some basic technical data for the deep geological disposal of HLW.