稀土锆酸盐复相陶瓷模拟固化^(241)Am和^(90)Sr的化学机理

Chemical mechanism of immobilizing^(241)Am and^(90)Sr in rare-earth zirconate composite ceramic:a simulation study

为实现将锕系核素和裂变产物同步固化至稳定的介质材料中,达到简化高放射性废液处理工艺和提高治污效率的目的,以^(144)Nd^(3+)和^(89)Sr^(2+)分别为锕系核素^(241)Am^(3+)和裂变产物的模拟核素,采用溶胶-喷雾热解法制备出能同时固化Am和Sr的稀土锆酸盐复相陶瓷固化体。利用X射线衍射(XRD)、拉曼光谱(Raman)、扫描电子显微镜(SEM)和背散射电子显微镜(BSEM)技术手段研究稀土锆酸盐复相陶瓷的物相组成和烧结体的显微结构。热力学理论计算结果表明,Ani的模拟固化体Nd2Zr207的吉布斯自由能明显高于Sr的固化体SrZr03。试验结果表明:在1200 t条件下,Ln_(1.5)-Zr0_(2)(Ln=La、Nd、Eu和Gd)二兀系合成的单一稀土错酸盐Ln_(2)Zr_(2)0_(7)均为纯的烧绿石结构,且Ln2Zr207的晶格常数随着三价稀土离子(Ln^(3+))的离子尺寸增大而线性增加;SrO-Zr02二元系合成的产物为钙钛矿正交结构的SrZr03,其晶格常数分别为a=0.5797 nm、6=0.5817 nm、c=0.8204 nmiNdO^-SrO-Zr02三兀系反应的产物是由烧绿石Nd_(2)Zr_(2)0_(7)和正交结构SrZrO_(,)两相组成。该复合固化体中Nd_(2)Zr_(2)0_(7)与SrZrO,两相的相对含量可以根据高放射性废液中不同的241 Am/^Sr比例进行相应调节,这与热力学理论计算结果相一致。经1550 T;烧结48h后,Nd_(2)Zr_(2)O_(7)-SrZrO_(3)复合模拟固化体中的晶粒尺寸处于微米尺度,内部未有任何显微裂纹出现,相对密度达95%以上。且模拟固化体中的Nd_(2)Zr_(2)0_(7)和SrZrO_(7)两相分布较均匀,化学相容性良好。

This paper aims to simultaneously immobilize actinide nuclides and fission products in stable curing medium material,achieving the goal of simplifying the treatment process of high-level liquid waste(HLLW),thereby improving the pollution abatement efficiency of nuclear waste.The ^(144)Nd^(3+)and ^(89)Sr^(2+)ions were used as rare earth analogues for actinide nuclide ^(241) Am^(3+)and fission product ^(90)Sr^(2+)ions,respectively.The rare-earth zirconate composite ceramics for immobilizing Am and Sr were prepared by using the sol-spray pyrolysis method.The phase composition and microstructure of the rare earth zirconate composite ceramics were investigated by XRD,Raman,SEM,and BSEM techniques.The thermodynamic theory calculation result showed that the Gibbs free energy of Nd_(2)Zr_(2)0_(7) for simulating Am immobilization was significantly higher than that of SrZr0_(3) for curing Sr.The experimental results indicated that compounds of single rare-earth zirconate Ln_(2) Zr_(2)0_(7) synthesized from LnO,5-Zr02(Ln=La,Nd,Eu,and Gd)binary mixtures at 1200℃ had a pure pyrochlore structure.The lattice constants of Ln_(2) Zr_(2)0_(7) increased linearly with increasing the ion size of trivalent rare-earth ions(Ln^(3+)).The product synthesized from SrO-Zr0_(2) binary system was single phase SrZr0_(3) with orthogonal perovskite structure.The lattice constants of SrZr0_(3) were calculated to be a=0.5797 nm,6=0.5817 nm,c=0.8204 nm.The reaction products of the NdO,5~SrO-Zr02 ternary system composed of pyrochlore Nd_(2) Zr_(2)0_(7) and orthorhombic SrZr03 phases.The relative contents of Nd_(2) Zr_(2)0_(7) and SrZr03 phases in the composite waste forms could be adjusted according to different 241 Am/wSr ratios in the HLLW,coinciding with the calculation result of thermodynamics theory.After sintering at 1550℃ for 48 h,the simulating waste form Nd_(2)Zr_(2)0_(7)~SrZr03 had a grain size range in micron-scale and a dense microstructure without micro-cracks.The measured relative density of the Nd_(2)Zr_(2)0_(7)-SrZrO,bulk was higher than 95%.Besides,the Nd_(2) Zr_(2)0_(7) and SrZrO_(7)phases distributed uniformly in the sintered simulating waste form and showed well chemical compatibility.