Improving the long term durability of UO2 wasteforms through thoria doping

The long term stability of spent nuclear fuel (UO2) is an issue currently facing the nuclear industry. The concept of disposal mixed-oxides (MOx), where spent nuclear fuel is combined in a solid solution with another light actinide (Th, Pu) can provide non-proliferation benefits and has the potential to improve long-term stability as a waste form. In this study, we investigated how thorium can effect the dissolution of UO2 by utilising two different experimental methods to model radiolytic dissolution in water. Changes to the structural properties of a series of epitaxial [0 0 1]-oriented (UxTh1−x)O2+y thin films were measured using x-ray reflectivity (XRR) and x-ray diffraction (XRD), utilising both laboratory and synchrotron sources. Overall results from this study indicated that dissolution can be fully inhibited where x = 0.87, in a homogeneous solid solution. These results suggest that the use of U-Th MOx as a spent nuclear fuel disposal solution would provide significantly improved long-term stability as a waste form, preventing the continued dissolution of UO2 in oxidising solutions.