Atomistic Simulations of the Uranium / Oxygen System

Abstract

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 UO_2+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.