Study On Fabrication And Immobilisation Mechanisms And Chemical Stability Of Gd₂Zr₂O₇ Ceramics Incorporated With Simulated Actinides

With the development of nuclear industry,the output of high radioactive nuclear waste is increasing.Currently,the temporary storage of highly radioactive nuclear waste through storage tanks is facing the dilemma of full capacity.Moreover,the high radioactive nuclear waste can not be sealed safely and steadily for a long time by this way of treatment.The long-term safe and reliable high radioactive waste disposal technology requires nuclear radioactive waste to be sealed in the nuclear waste form and then geologically disposed The stability of Gd₂Zr₂O₇ ceramics as actinide and minor actinide radionuclide waste forms is discussed.Gd₂Zr₂O₇ ceramics with different simulated actinides capacity were prepared by chemical co–precipitation calcination method.The optimized calcining temperature of powders was determined by T G–DSC.The structure and immobilization mechanism of Gd₂Zr₂O₇ ceramics were investigated by XRD,SEM,EDS and XPS.Vickers hardness,compressive strength,thermal conductivity,thermal expansion coefficient,and chemical stability of the Gd₂Zr₂O₇nuclear waste form were measured by Vickers hardness tester,universal testing machine,laser–flash thermal conductivity apparatus,high temperature dilatometer and leaching and aqueous corrosion test.The optimized calcination temperature for Gd₂Zr₂O₇ precursor by chemical co–precipitation method is 1073 K.The Gd₂Zr₂O₇ waste form immobilizing Nd and Ce were single–phase pyrochlore structures and single–phase defect fluorite structures,respectively.with Ce existing as trivalent and tetravalent chemical state in the Gd₂Zr₂O₇ waste form.The mechanical properties test results show that the Vickers hardness of Gd_2–x–x Nd_x Zr₂O₇ waste form is between 10.12 to 11.19 GPa,and the compressive strength value is between 237.1 to 282.9MPa;the Vickers hardness of the Gd₂Zr₂O₇waste form immobilizing Ce is between 8.73 to 9.59 GPa and the compressive strength is between 159.7 to 252.4MPa.The indentation cracks show a significant transgranular extension and the failure morphology in the compression test show a clear cleavage fracture.The results of thermophysical properties tests show that the thermal diffusivity of Gd_2–x–x Nd_x Zr₂O₇ waste form is between 0.473 to 0.759 mm²·S^–1and the thermal conductivity is between 1.41 to 2.06W·m^–1·K^–1from room temperature to 1273 K.The average coefficient of thermal expansion is in the range of?10.34–11.93?×10^–6K^–1from room temperature to 1023 K.The thermal diffusivity of the Gd₂Zr₂O₇ waste form immobilizing Ce is between 0.464 to 0.880mm²·S^–1and the thermal conductivity is between 1.44 to 2.33W·m^–1·K^–1.The average of thermal expansion is in the range of?10.05–10.67?×10^–6·K^–1from room temperature to1023 K.Short–term accelerated leaching test results show that only Gd ions can be detected in the leachate.When the leaching temperature and the chemical composition of the Gd₂Zr₂O₇ waste form remain unchanged,the leaching amount of Gd³⁺is proportional to the leaching time.The cumulative cation leaching concentration of the Gd₂Zr₂O₇ waste form with different simulated nuclear nuclides varies,and the cumulative cation leaching concentration is in propotion to the simulated nuclides content.The Gd₂Zr₂O₇ waste form can be corroded under the aqueous environment at 473 K.The crystal particles dissociate at the surface defect first and eventually develope into etch pits.Vapor corrosion experiments show that the Gd₂Zr₂O₇ waste form can be corroded by Na Cl vapor,which will cause dissociated crystal particles and pits on the surface.However,only a slight corrosion can be observed in the present work.After corrosion,the Gd₂Zr₂O₇ waste form is still a complete ceramic bulk with a bright luster and a certain strength.The Gd₂Zr₂O₇ waste form can be severely corroded by HCl vapor with chloride formation,the cations of which partially move to the environment.Eventually,Gd₂Zr₂O₇ waste form collapse and form a honeycomb structure,results in the lost of strength and the powderization in the end.