Study On Preparation And Stability Of Zircon-based Simulated Actinide Nuclides

Zircon?ZrSiO₄,I41/amd,Z=4?has long been considered as a potential candidate for fixing high-level radioactive waste due to its high thermal decomposition temperature,small thermal expansion coefficient,excellent chemical stability and good radiation stability.In this paper,zirconium dioxide?ZrO₂?and silica?SiO₂?are used as raw materials for the synthesis of zircon ceramics,and using yttrium oxide?Nd₂O₃?to simulate+3 valence nuclides,using cerium oxide?CeO₂?to simulate+4 valent actinides.The zircon-based simulated actinide nuclides solidified bodies Zr_1-xCe_xSiO₄ and Zr_1-xNd_xSiO_4-x/2 were prepared by high temperature solid phase method,and the structural stability and chemistry of zircon after immobilization of Nd³⁺and Ce⁴⁺were studied by static leaching experiments.XRD,SEM,backscattered electron imaging and ICP-MS were used to test the phase structure,morphology and chemical stability of the samples.ZrSiO₄ maintains structural stability well after accommodating Ce⁴⁺ions.The density of Zr_1-xCe_xSiO₄ ceramics is experimentally and theoretically higher than that of ZrSiO4 ceramics.Based on first-principles structural calculations and charge density distributions,Ce-O bonds are longer and weaker than Zr-O bonds,which results in relatively weaker Ce-O bonds and easier separation in water.Due to the high symmetry of[CeO₈]polyhedron,the defect and chemical stability of ZrSiO₄ ceramics are hardly affected after Ce⁴⁺replaces part of Zr⁴⁺,indicating that ZrSiO₄ ceramics are very suitable for fixing low-dose?An?act elements.As the Nd content and acid concentration increase,LR_Zr and LR_Nd increase.This result is attributed to the formation of oxygen vacancies resulting in the weakening of the crystal lattice due to Nd incorporation.After 42 days,the LR_Nd values in all leachates were found to be higher than the LR_Zr values due to the difference in energy of their respective oxygen atom bonds.LR_Zr and LR_Ce rapidly decreased at the initial stage of leaching and then stabilized at a constant value(10^-66 g m^-2d^-110^-5 g m^-2d^-1),showing excellent chemical stability.LR_Zr and LR_Ce are the highest in acidic leachates,followed by alkaline leachate and deionized water systems.According to the structural calculation and charge density distribution of the first principle,the reason why LR_Ce is slightly higher than LR_Zr is explained,that is,the Ce-O bond is longer and weaker than the Zr-O bond,resulting in a relatively weaker Ce-O bond and easier to be in the water.Separation.It is shown that ZrSiO₄ ceramics are very suitable for the immobilization of actinides at low dose ratios.