Atomistic Simulations of the Uranium / Oxygen System
Atomistic Simulation techniques have been used to model surface and bulk properties of UO2, the most common nuclear fuel.
After an overview concerning nuclear power technology, the characteristics, fabrication and applications of UO2 are reviewed. Subsequently, the theoretical base for this work is explained, including the shell model, Mott-Littleton method, Born model, Ewald sum and a summary of the five potential models that describe interactions within the UO2 lattice.
Chapter 3 describes previous experimental and theoretical determinations of the bulk properties of UO2, presenting oxygen migration studies performed as part of this work, and chapter 4 explains basic concepts of surface simulation and the methodology of MARVIN’s program, which was used for all surface simulations.
In chapter 5, the morphology of stoichiometric UO2 is calculated for the five predefined potential models, and the subsequent chapter presents a detailed study of the (200) surface of UO2. Particular attention is given to the enlargement of the simulation unit cell and relaxation of the surface atoms.
Chapter 7 presents calculated morphologies of UO2 for different degrees of hydroxylation on the (111), (110) and (200) surfaces, and chapter 8 outlines the change of surface energy for the (111) and (110) surfaces of UO2+x.
Chapter 9 presents a general examination of the correlation between bulk and surface properties by comparing the Schottky and surface energies for a range of binary oxide materials with different crystal structures. A clear relation between these two energies is found for all these materials.
The concluding chapter provides a summary of the achievements in this thesis and outlines possible future work.
Appendix A presents results of a molecular dynamics study of CaF2, which has the same crystal structure as UO2. Appendix B provides details of the program written to list the unique configurations of surface oxygen for an enlarged simulation unit.