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

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 codoping 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 (similar to 1.8 mu m) 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.