Adsorption/desorption behaviour of CO2 and H2O on ThO2 powders

Thorium dioxide has recently regained interest as an alternative to uranium-based fuels due to its less long-lived transuranium element production or its higher natural abundance, among others. Though, ThO2 pellet production faces many challenges such as reaching high theoretical densities. This can be improved by sintering the pellets, but this treatment is altered by many factors. In fact, a release of adsorbed gases was observed which can negatively change the grain growth. This thesis presents a systematic study on the adsorption of CO2 and H2O on two types of ThO2 powders reproducing storage conditions, one derived from thorium oxalate (ThO_OX) and one from a novel 2-step alkali precipitation route (ThO_TSA). Both materials were characterized by means of N2 adsorption isotherm at 77K, SEM and TEM. Moreover, CO2 and H2O desorption was experimentally studied by TGA coupled to mass spectrometry. Surface analysis indicates that both materials are non-rigid aggregates with mesoporosity attributed to interparticular cavities which agrees with the SEM and TEM images. However, SEM/TEM also shows that the particle shapes differ significantly, being plate-like for ThO_Ox, and sphere-like for ThO_TSA. The powders retained CO2 adsorbed well above 600°C, and it seems the adsorbed water has a significant effect on the CO2 adsorption. ThO_TSA reached the saturation point earlier than ThO_Ox, suggesting a lower gas adsorption capacity. Therefore, the 2-step alkali precipitation route is recommended for long ThO2 powder storage periods.