Investigation on the mechanism of the immobilization of CeO₂ by using cullet-based glass (CBG)

Highlights: • CBG was chosen to vitrify Pu directly by using Ce as a surrogate for Pu. • Two synergistic immobilization mechanism to make Ce difficult to release. • The formation of Ce-O-Si bonds increased the NBO proportion in the glass structure. • The 7-PCT results indicated that vitrification treatment of Ce by CBG is effective. - Abstract: The safe disposal of nuclear waste is a key factor restricting the sustainable use of nuclear energy. Exploring safe and reliable disposal methods for nuclear waste has become a research hotspot. In this paper, cullet-based glass (CBG) was chosen as a new immobilization agent to host the plutonium (Pu). The immobilization mechanism of Pu in the CBG was investigated by using cerium (Ce) as a surrogate for Pu, and the interaction was characterized by using XRD, XPS, FTIR and SEM-EDS analyses. The results indicate that there are two synergistic immobilization mechanisms by which Ce is hosted in the vitrified products. One mechanism is doping of the Ce into the crystal lattice of the glassy matrix to form CeOSi bonds, which increases the proportions of non-bridging oxygen (NBO) bonds in the glass structure. The other mechanism is Ce co-existing in the glass structure gaps as individual crystals of CeO₂. The synergistic immobilization makes Ce hard to release. The PCT results show that the normalized leach rate of Ce decreases with the Ce partitioning ratio in the CeO₂ crystalline phase. These data show that CBG can be used as a good immobilization agent to vitrify the Pu presented in high-level radioactive wastes.