Solidification of Uranium and Thorium in Nd2Zr2O7 Pyrochlore

In order to study the solidification behavior of actinides uranium in pyrochlore, a series of Nd2-xThxZr2O7 and Nd2-xUxZr2O7 samples were prepared by the solgelspray pyrolysishigh temperature sintering method. These samples were characterized by XRD, SEM, EDS, Raman and XPS. For Nd2-xThxZr2O7, the solubility of thorium is estimated up to 40%, the structure of immobilization remains single pyrochlore structure phase. Further addition of thorium in the Nd2Zr2O7 pyrochlore leads to the formation of multiphase structure with pyrochlore, ZrO2 and ThO2. The solubility of uranium in the Nd2Zr2O7 pyrochlore is found only 10%, the pyrochlore structure shows the transformation of disorder. With the increase of uranium content, the structure transforms from disordered pyrochlore to fluorite structure, the solubility of uranium in fluorite structure is estimated to about 20%. The solubility of uranium and thorium in the Nd2Zr2O7 pyrochlore shows the significant difference. Compared with Th(Ⅳ) ion, uranium exhibits +4 and +6 valence states in the immobilization. For Nd2-xUxZr2O7, more oxygen are incorporated in the lattice of pyrohlore due to the higher average valence of uranium, the coordination structure of pyrochlore is changed. The Nd2-xUxZr2O7 pyrochlore quickly transforms into the disordered fluorite structure with uranium content increasing, the solubility of uranium decreases significantly.