Computational simulations in materials for energy applications 1. Crystal and electronic structure in <i>Ln</i>-U-O compounds. 2. Dynamics of point defect interaction with dislocations in bcc iron.

Nuclear energy is a viable solution to the world’s energy demands. Nuclear energy applications involve rich and complex physics, with high energy events, the incorporation of fission products, and the production of point and extended defects. All these phenomena have an impact on the microstructure of the constituent materials and represent efficiency and safety concerns. A mature understanding of the microstructural evolution of the component materials in the nuclear reactor core is essential to have a safe and reliable process. Experimental investigation of materials in radiation environments is difficult and expensive, making computational simulations a suitable alternative. In this dissertation, employ computational methods to study the microstructural evolution of both nuclear fuel and the iron based reactor structural components, and the impact on their material properties. In the nuclear fuel side, we investigate the crystallographic and electronic structure of Ln-U-O compounds that may be formed inside nuclear fuel operational life by the incorporation of lanthanide fission products using density functional theory (DFT). We used a layered atomic model to propose ordered structures and compared their stability to disordered phases. We also employed the atom-in-molecule approach to study the oxidation state of uranium atoms, and the iconicity/covalency of the U-O bonds. In the structural components side, we studied the migration mechanisms of self-interstitial dumbbells and vacancies around single edge or screw dislocations. The actual saddle point energy and configuration as a function of position with respect of the dislocation core was calculated with the self-evolving atomistic kinetic Monte Carlo (SEAKMC) method, and used this data as an input for KMC calculations. This allowed the analysis of the migration paths, the range of interaction of point defects with dislocations, and the preferential absorption of self-interstitial dumbbells over vacancies, known as dislocation bias, which is responsible for swelling in irradiated materials. The understanding of the mechanism responsible for the microstructural changes, and how these changes impact the material properties is a key aspect to be able to develop materials with enhanced radiation resistance, and achieve high performance under extreme conditions that are vital for nuclear energy generation with improved efficiency and safety.