The Preparation Of Gd₂(Ti_2-xZr_x)O₇ Pyrochlore Waste Forms By Self-propagating High-temperature Synthesis

In recent years,nuclear energy has been widely used as a new and efficient clean energy.But its nuclear wastes aslo poses a serious threat to human living environment.Therefore,nuclear waste needs to be effectively solidified to avoid radioactive release and endanger public safety.Appropriate disposal of nuclear waste is one of the key factors of nuclear industry.According to the different preparation methods of pyrochlore,this study can be divided into two parts:self-propagating synthesis?SHS?and self-propagating chemical furnace synthesis plus quick pressing?SCF/QP?.Gd₂(Ti_2-xZr_x)O₇ pyrochlore ceramics and nanograin Gd₂Zr₂O₇ceramic were studied.In the first part,Gd₂(Ti_2-xZr_x)O₇ pyrochlore was synthesized using Gd₂O₃,Ti,CuO,TiO₂ and ZrO₂ as the raw materials.Zr⁴⁺and Nd³⁺doped Gd₂Ti₂O₇ have been prepared by SHS/QP.The results show that Gd₂(Ti_2-xZr_x)O₇ pyrochlore was successfully prepared by SHS/QP method.The maximum content of Zr-doped pyrochlore is 70%.On the basis of this,Nd₂O₃was added to the raw materials of Gd₂Ti_1.3Zr_0.7O₇ and the solid solution content of Gd₂Ti_1.3Zr_0.7O₇ pyrochlore is 70%.The effect of pressure time on mechanical properties of pyrochlore was explored.The bulk density of Gd_0.6Nd_1.4Ti_1.3Zr_0.7O₇ pyrochlore sample is4.93 g/cm³ and its Vickers hardness is 14.90 GPa.Moreover,the chemical stability of Gd_0.6Nd_1.4Ti_1.3Zr_0.7O₇ pyrochlore sample was characterized by MCC-1 method.The results show that the leaching rates of Gd and Nd reached 10^-55 order of magnitude after 42 d,which exhibit excellent chemical stability.In the second part,taking advantage of the characteristics of high temperature and high reaction rate of SHS reaction,it acts as a chemical furnace instead of the traditional sintering furnace.Nano-ZrO₂ and nano-Gd₂O₃ were used as the raw materials.Gd₂Zr₂O₇ compact was loaded inside the combustion reactants.After reaction,the temperature of Gd₂Zr₂O₇ compact was raised with the highest temperature.3Ti+6CuO+2Gd₂O₃+TiO₂ system was selected as the chemical furnace.The single Gd₂Zr₂O₇ nanograin ceramic was rapidly synthesized by SCF/QP.The results demonstrate Gd₂Zr₂O₇ nanograin ceramic with an average grain size of84 nm was successfully obtained under the ultra-high heating rate.The Gd₂Zr₂O₇ ceramic shows reasonable high Vickers hardness of 11.4 GPa and bulk density of 6.54 g/cm³.The aqueous durability was also evaluated.The LR_Gdd and LR_Zrr values of Gd₂Zr₂O₇ sample are as low as 5.4×10^-66 and 8.2×10^-77 g·m^-2·d^-11 after 42 d leaching.Therefore,Gd₂Zr₂O₇ pyrochlore has good chemical stability,which can be used as an ideal base material for incorporating actinide nuclides.