Investigation of additives effects on the pellet properties at the synthesis of (Th-Ce)O2 via sol gel method

The aim of the present study is to obtain easily sinterable pellets with high homogeneity and density, coarse grained attaining an average dimension of 50 µm, via sol-gel method by the addition of metal oxides (such as MgO, CaO, V2O5, Ti2O5, Cr2O3, Al2O3 having low neutron sections) to nitrate solutions of the initial chemical materials of Th(NO3)4.5H2O and (NH4)2Ce(NO3)6 conveniently prepared to satisfy (Th0.95Ce0.05)O2 final mole ratio. The sols have been prepared as a result of NH4OH pre-neutralisation of the mother liqueur obtained with the addition of predetermined amount of metal oxide additives to 1.6 M of Th(NO3)4.5H2O and (NH4)2Ce(NO3)6 mixed solution, in a temperature and pH controlled system. By using vibration-nozzle system, prepared sols have been gelled to spheres in a gelation column with the help of organic and aqueous phases. Following of aging, washing and drying processes, thermal behaviors of the microspheres were determined with TG/DTA analyses and the metal compositions of the microspheres were determined with ICP-MS/ICP-OES analyses. Three different heat treatments such as 450˚C, 800˚C and 1150˚C have been applied to microspheres and X-rays diffraction patterns were taken to calculate the crystal parameters. BET surface area and BJH porosity analyses also have been realized for heat treated microspheres. The microspheres were pelletized with a hydraulic press under a pressure of 520 MPa and thermo-mechanical analyses of pellets were realized for 4 different temperatures such as 1000˚C, 1100˚C, 1200˚C and 1300˚C. The activation energies of additives free (Th0.95Ce0.05)O2 and with additives (Th0.95Ce0.05)O2 pellets including 6 different metal oxide additives, were calculated and their sintering kinetics was determined. After sintering, SEM images of the pellets were taken with a scanning electron microscope and the real densities of the microspheres derived pellets were calculated with m-Ksilen immersion method. The activation energies were calculated as 449 kj/mol for additive-free (Th0.95Ce0.05)O2 pellets and 252 kJ/mol, 202 kJ/mol, 107 kJ/mol, 90 kJ/mol, 190 kJ/mol, 171 kJ/mol for Mg-(Th0.95Ce0.05)O2, Ca-(Th0.95Ce0.05)O2, V-(Th0.95Ce0.05)O2, Ti-(Th0.95Ce0.05)O2, Cr-(Th0.95Ce0.05)O2, Al-(Th0.95Ce0.05)O2 pellets, respectively. Green densities of pellets were determined as around 40-43% of the theoretical density. The densities after sintering were around 92-98% of the theoretical density. In this study, in comparaison of additive-free pellets, mixed oxide pellets including additives, prepared via sol-gel microspheres process, were coarse grained materials being able to keep the fission gasses at higher rate in the fuel to increase the burning rate