The aim of work described in this thesis is to gain insight into the role of interfaces on atomic transport processes in polycrystalline uranium dioxide at high temperatures. Previous studies have highlighted the importance of grain boundaries as fast diffusion pathways and their potential to significantly effect the oxidation of nuclear fuel samples. The effect of interfacial transport is also significant in terms of helium produced through the α-decay of actinides throughout the life-cycle of nuclear fuels. The resultant swelling can have important implications for the long term storage of nuclear materials and can even lead to radionuclide release.
Chapter 1 comprises a review of the relevant literature focussing on the uranium oxide system, oxide microstructure and the transport properties of both helium and oxygen. In Chapter 2 the computational methodologies used throughout this investigation are described in detail including both energy minimisation and molecular dynamics. Chapter 3 focuses on the development of a new potential for uranium dioxide derived to be computationally efficient and suitable for large simulation cells. Additionally the model needed to be capable of modelling the high temperature structures of tilt and twist grain boundaries and accurately representing oxygen transport. Having developed and validated the model, we explored protocols for generating grain boundaries and in Chapters 4 and 5 model a range of tilt and twist grain boundaries. In each case the structure, stability and transport properties of each were investigated in depth. Chapter 6 introduces a new helium potential model and uses it to evaluate the diffusion and segregation in a range of tilt grain boundaries. Also introduced is a new type of interface that models the interface where grain boundaries and surfaces meet. The approach of routinely generating such interfaces is explored and is then applied to look at the release of helium from bulk UO2 to the gas phase. The thesis concludes in Chapter 8 with a summary of the thesis and possible direction of future work.
|Date of Award
|6 Jun 2014
|Steve Parker (Supervisor)