The Preparation Of Zirconolite By SHS With CuO As The Oxidant And Their Immobilization Of Simulated Actinide Nuclides

The disposal of nuclear waste is an important factor to promote the development of nuclear science and technology.Especially,the safe disposal of actinide nuclides is the most important but difficult aspect of radioactive waste treatment.In this study,zirconolite-rich ceramics were prepared by self-propagating high-temperature synthesis?SHS?technology with Cu O was used as the oxidant and Ca O,Zr O ₂,Ti,Ti O₂ were used as the raw materials.The adiabatic temperatures(T_ad)of each designed reaction equations were calculated as well.The SHS experiments were carried out and the actual combustion temperatures were recorded to determine the optimum system.The compact bulk was obtained by quick pressure at different time after combustion.On the basis of this,Hf,Gd,Sm,which are the important neutron absorption elements,were employed to simulate tetravalent and trivalent actinide nuclides.Their immobilization mechanisms in this prepared ceramics were explored as well using some analytical tests such as XRD,BSE and SEM-EDX.In addition,MCC-1 leaching test was conducted to characterize the chemical durability of samples.The results show that zirconolite-rich Synroc was successfully prepared by SHS with Cu O as the oxidant and the Ti O₂/Ti ratio was 1:3.Three major phases,namely zirconolite?Ca Zr Ti₂O₇?,copper?Cu?and perovskite?Ca Ti O₃?,were observed in the XRD pattern.The most optimum sample with density of 4.28g/cm³ and vicker hardness of 1135.74 HV was obtained when it was pressed at30 s after be ignited.When different contents of Hf O₂ were added,Ca Zr_1-x-x Hf_x Ti₂O₇ was prepared with Cu and a little amount of Ca Ti O ₃.It was also found that Hf substitute the Zr site in Ca Zr Ti₂O₇.Gd₂O₃ was added to the raw materials of Ca Hf Ti₂O₇ to realize double immbolization,and its solution limitation is 41.81-50.86%.The EDX analysis suggests that the Ca and Hf sites in Ca Hf Ti₂O₇ are substituted by Gd at the same time.When Sm₂O₃ was added with Al₂O₃ as the charge-compensator ion,the activity of SHS reaction decreased and the raw materials did not react completely.On the other hands,composites Sm-doped zirconolite and Cu were prepared when only Sm₂O₃ was added.The EDX result illustrates that Sm replaced the Ca site and Zr site of Ca Zr Ti₂O₇ dividually.In addition,the hydrothermal stability was investigated as well by the characterization of normalized leaching rate.The results show that the leaching rates of Ca and Cu decrased gradually with the increment of duration time,and reached 10^-1 and 10^-2 order of magnitude after 42 days,respectively.Moreover,Hf,Gd,Sm exhibit leaching resistant with their normalized elemental leaching rates reached 10^-7 or 10^-8 order of magnitude after 42 days.In summary,the zirconolite-rich ceramics prepared in this experiment could be an ideal material for the immobilization of actinide nuclides.