Oxidation of Uranium Dioxide

Abstract

This thesis investigates the influence of the crystallographic orientation of the UO2 on the oxidation process and phase transition into U3O8. The samples were prepared by DC magnetron sputtering and a new sputtering system was constructed. A new method to produce poly-epitaxial columns of UO2 with controlled grain size was developed and used. X-ray characterisation of single crystal epitaxial thin films revealed the best long range order for the (001) orientated samples. The truly polycrystalline character of the poly-epitaxial thin films, without preferred orientation, was demonstrated by x-ray diffraction and electron backscatter diffraction.
Utilising these idealised systems, fundamental studies have been conducted to explore the oxidation behaviour of uranium dioxide. The in-situ oxidation studies performed on poly-epitaxial UO2 revealed a fastest oxidation rate for (001) orientation, in contradiction to the literature data. This discovery was further investigated using single crystal samples. In situ High Temperature- Environmental Scanning Electron Microscopy observations showed disintegration of the UO2 structure after phase transition to U3O8, for the (111) and (110) oriented single crystal, while there was no loss of integrity for the initially (001) oriented structure. Additionally, in-situ X-ray diffraction studies revealed an epitaxial relationship between the (130) plane of U3O8, rotated 45°, on the (001) plane of UO2. This was identified as a topotactical phase transition, with sigmoidal nucleation and growth kinetics.
In conclusion, this thesis has discovered a new epitaxial relationship between the (001) plane of UO2 and (130) plane of U3O8. This match was also attributed to the faster oxidation rate
observed for (001) oriented grains in polycrystalline systems, as possibly lower energy is required for phase transition in this oxidation route. This new insight into the oxidation process of the most common nuclear fuel can help improve the efficiency, reliability and safety throughout the nuclear fuel cycle, and might potentially apply to other oxide systems.

Date of Award	21 Jan 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Ross S Springell (Supervisor)