Study On Solidification Mechanism And Irradiation Effect Of Zirconolite Glass-ceramic

The rapid growth and utilization of nuclear power has provided enormous energy to humanity,but it has also produced a considerable amount of radioactive waste.As the first engineering barrier for deep geological disposal high-level radioactive waste,the solidification material plays a crucial role in achieving the overall engineering objectives of the entire system.Therefore,the selection of a highly stable solidification matrix is particularly important for the long-term stable containment and separation of the high-level radioactive waste containing actinides.Glass-ceramics,with zirconolite as the basic crystalline phase,have the advantages of both glass and ceramics,making them an ideal solidification matrix that meets the requirements.During the solidification process,most of the actinides can be dissolved in the lattice of ceramic,while the remaining impurities and small amounts of actinides separated can be immobilized in the glass matrix,achieving the optimal solidification effect.Due to its advantages of high stability,low leaching rate,and large actinide containment capacity,zirconolite glass-ceramics are considered one of the most promising solidification matrices and a hot topic in the field of solidification matrix research.After a comprehensive introduction to the sources of high-level radioactive waste and the main forms of solidification currently used for research,this paper selects borosilicate glass,which has already been industrialized,and introduces the necessary elements Ca,Zr,and Ti that form zirconolite,as well as substitutions of different valences of actinide elements such as Ce and Gd,based on previous work.By changing the amount of simulated actinide doping,doped zirconolite glass-ceramics were prepared through a two-step heat treatment method.In addition,the irradiation effect was studied using an external ion irradiation method.The prepared glass-ceramics and the mechanisms behind the observed phenomena and changes during the research were characterized and analyzed using techniques such as X-ray diffraction（XRD）,scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,atomic force microscopy（AFM）,transmission electron microscopy（TEM）,and time-of-flight secondary ion mass spectrometry（TOF-SIMS）.Through the work of this paper,the following conclusions are drawn:1)Different amounts of Gd₂O₃ were introduced into Ce O₂-containing zirconolite glass-ceramics,and a single zirconolite phase glass-ceramic with a capacity of～10.0 wt%was fabricated.Both doping elements mainly existed in the zirconolite crystal,and a small amount was distributed in the glass matrix.Different Gd doping amounts would lead to polymorphic transformation of zirconolite in the glass-ceramics.The structural transformation was related to the solidification mechanism of co-doped elements in the zirconolite crystal.Further increasing the doping amount would cause the precipitation of pyrochlore phase with the same solidification ability in the glass ceramics.2)The solidification mechanism of the Ce-doped zirconolite crystal under co-doping conditions was elucidated based on the microstructure.For zirconolite,Ce⁴⁺directly replaces Zr sites to achieve solidification,and Ce³⁺and Gd³⁺replace Ca sites while Al enters Ti sites to complete the substitution.The influence of Gd³⁺introduction on the Ce⁴⁺valence state was studied.In the zirconolite glass-ceramic,Ce exists in two mixed valence states,+3 and+4,in the solidification,and the introduction of Gd³⁺causes the reduction of Ce⁴⁺to Ce³⁺to be inhibited and then undergoes a dynamic equilibrium process.This result is of great significance for understanding the behavior of Pu in co-doped zirconolite glass-ceramics.3)Experiments of heavy-ion irradiation were conducted on the zirconolite glass-ceramics doped with Ce O₂ to study its irradiation effects.Significant changes were observed in the glass-ceramic solidified body after irradiation,including the swelling of the glass matrix,the amorphization and deformation of the zirconolite crystals,and the alteration of the two-phase compoistion.The reasons for radiation-induced changes were analyzed using the thermal-spike model.The swelling of the glass was caused by Na migration on the surface induced by irradiation,forming larger ring structures within the glass.In contrast to nuclear collisions,the energy deposition by ionization processes at high doses leads to the repeated generation of amorphous tracks,resulting in the amorphization of zirconolite.The stress caused by the swelling of the glass phase can squeeze and deform the zirconolite crystals at high temperature melting.The change in the components is related to the migration of elements.All of these changes will affect the leaching rate of the glass-ceramic.