The Microwave Sintering And Stability Of Zircon-Based Wasteform For Immobilizing Simulation Actinides

Since Ewing and Weber put forward the idea of using zircon as the material to immobilize weapons-grade nuclear waste in 1990s, zircon has been received intensive attention because of its fantastic properties, such as high thermal decomposition temperature, low thermal expansion coefficient, good chemical stability as well as the ability to immobilize actinides in the lattices. However, in the practice, high singtering temperature and long holding time are aways essential in order to obtain zircon with high formation rate, which restrict its application for immobilizing the radioactive nuclear waste. In this paper, the Zr1-xCexSiO4(x=0.01-0.10) and Zr1-xNdxSiO4-0.5x(x=0-0.10) series wasteforms were synthesized with the help of the microwave sintering methods, in which the SiO2 and ZrO2 were used as the raw materials, and the CeO2 and Nd2O3 were used to simulate the trivalent and tetravalent actinide. The pahse composition and microstructure were characterized by X-Ray diffraction (XRD), scan electron microscope (SEM), element mapping and backscattered electron imaging. The structure stability of zircon wasteform for dealing with Pu (Zr1-xPuxSiO4 (x=0-0.156)), together with the anti-irradiation properties of zircon for a and 235U particles, were computer simulated by employing density function theory and Monte Carlo Method respectively. And the results show as following:The zircon and zircon wasteform for immobilizing simulation actinide Zr1-xCexSiO4(x=0.01-0.10) and Zr1-xNdxSiO4-0.5x (x=0-0.10), with high formation rate, could be synthesized with microwave sintering at 1500℃ for 12h. For the Zr1-xCexSiO4(x=0.01-0.10) series wasteforms, there weren’t Ce concentration when x<5%, and the cell parameters increased with the increase of Ce content, while the Ce2Si2O7 appeared when the x≥5%. However, for the Zr1-xNdxSiO4-0.5x (x=0-0.10) series wasteforms, the change of their cell parameters didn’t show any obvious relationship to the addation of Nd, the Nd concentration appeared when the x≥4%. The crystal grain size overall increased with the increase of Nd and Ce content for both Ce-dooping ind Nd-dooping zircon wasteform.The results of strcture stability of zircon wasteforms for immobilizing Pu (Zr1-xPuxSiO4 (x=0-0.156)), which were obtained with computer simulation, shown that:the cell parameters increased with the increase of Pu content, agreeed with the Vegard’s Law. The final energy of wasteform reached its lowest value when the molar ratio of Pu substituting the Zr site was 6.25%, corresponding to the experiment result (～5mol%). The occurance of Si-O, Zr-O and Pu-O bands was comfired, and the existence of Pu in zircon had evident effect on the zircon system.235U particle of energy 85keV and a particle of energy 5.155 MeV vertically incidented upon the zircon, whose density is 4.36g/cm3, the radiation damage had been computer simulated, and the results shown as following:the depth of 5.155 MeV a particle in zircon was 15.4um, and most of its energy lost via the ionization of target atoms, while the depth of 85keV 235U particle in zircon was 24.5nm, and the recoil particles, which generated form the collision of U particles with target atoms, ionized the target atoms and producted phonons, the energy loss in this process were the main energy loss of the 235U particles.