Fabrication of crack-free oxidic thorium–cerium microspheres by an external gelation process

Abstract

Thorium dioxide as an excellent matrix with stable properties is available for the direct incineration of plutonium, while (Th,Pu)O2 is a promising advanced fuel, which can be applied to a wide range of reactor types. In this study, cerium was used to simulate plutonium owing to their similar physicochemical properties. (Th1-xCex)O2 kernels were prepared by an external gelation process, which employs thorium and ceric ammonium nitrate as source materials, ammonium hydroxide as a neutralizing agent, polyvinyl alcohol as a thickening agent, and an ammonia gas ring for preliminary forming. The effect of the reaction temperature, reactant concentration, and pH on the feeding solution gelation properties was investigated. After the dispersive process, the aqueous (Th1-xCex)(OH)y·nH2O gel microspheres were washed and dried at 200 °C, calcinated and sintered. It was found that more than 99 % of the sintered microspheres maintained an excellent degree of sphericity and integrality when the parameters were properly controlled. It was found that the feeding solution preparation process was the most important and complex step to obtain uniform microspheres of good sphericity during the whole kernel fabrication process. The (Th1-xCex)O2 microspheres produced were characterized using thermogravimetric analysis, X-ray diffraction (XRD), scanning electron microscopy, and energy dispersive spectroscopy (EDS). The XRD analyses showed that all products appeared as a solid solution with a cubic fluorite structure when doped with cerium in the selective compositional range. The elemental scanning by EDS showed an excellent uniform distribution of thorium and cerium in the sintered microspheres.