Synthesis and thermodynamic characterization of thoria-ceria solid solutions

Thorium dioxide finds extensive application in the nuclear industry. Pellets of thoria are used in PHWRs for flux flattening and in FBRs as a blanket material. The Indian Advanced Heavy Water Reactor (AHWR) would be using solid solutions of thoria and plutonia as driver fuels. Selective partitioning of plutonium and the minor actinides followed by their transmutation is being considered as a potential method for the reduction of long term radiotoxicity of the spent fuel. Thoria is one of the matrices that are being considered for this application. Studies on the synthesis and sintering of the solid solutions (Th1-xPux)O2 are relevant to the above applications. The system ThO2-CeO2 is relevant in this context for ceria serves as a surrogate for plutonia. The results obtained in the studies on the synthesis, sinterability and thermodynamic characterization of powders of (Th1-xCex)O2 that are pertinent to the above are presented here. Nanocrystalline powders of the solid solutions (Th1-xCex)O2 (X= 0,0.1, 0.25, 0.5, 0.75, 0.9, 1) were synthesized by using citrate gel-combustion synthesis. The as prepared and calcined powders were characterized by using IR spectra, analysis of residual carbon present in them as well by determining their X-ray crystallite size, specific surface area, particle size distribution and bulk density. High density compacts were obtained by cold pelletizing these powders without binder or lubricant followed by sintering in air at 1473, 1673 and 1873 K. The reactivity of the calcined powders was examined with the help of these sintering experiments. These powders could be sintered to high densities (96-99% TD) at relatively low temperatures (1473 K). The thermodynamic functions such as the enthalpy increment and heat capacity of the thoria-ceria solid solution have not been measured experimentally. Hence, we measured the enthalpy increments of thoria-ceria solid solutions, Th0.75Ce0.25O2 and Th0.50Ce0.50O2 by inverse drop calorimetry in the temperature range 523-1723 K. The measured enthalpy increments were fitted in to polynomial functions by using the least squares method and the other thermodynamic functions such as heat capacity, entropy and Gibbs energy functions were computed in the temperature range 298-1800 K. The reported thermodynamic functions for Th0.75Ce0.25O2 and Th0.50Ce0.50O2 forms the first experimental data and the heat capacity of (Th,Ce)O2 solid solutions was shown to obey the Neumann-Kopp's rule. (author)