Atomistic studies of structure-property relationships in actinide and transition metal alloys
Startt, Jacob K.
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In this thesis, structure-property relationships are investigated for a series of nuclear material systems using density functional theory (DFT). Each chapter presents a unique problem within the field of nuclear materials engineering and can demonstrate the application of computational methods in support of existing experimental framework or in the guidance or suggestion of new experimental investigation. In the first project, point defect formation energies in Th and Th-U metal are modeled using DFT. These defects often have a large effect on a material’s mechanical properties and in the highly radioactive environment of a reactor core defects are readily created near the displacement cascades of primary knock-on atoms (PKAs). The second project comprises an investigation of Cr depletion in Ni-Cr surfaces in molten salt reactors. The depletion of Cr atoms in the surface regions of these materials has been observed as the dominant form of corrosion but the underlying mechanisms and driving forces are not well understood. In this project, DFT is used to model the segregation behavior of Cr in fcc Ni surfaces. The oscillatory nature of the segregation energy profile is then discussed in terms of charge transfer and lattice distortion effects. In the third project, DFT is again used to extend the investigation of Cr segregation behaviors, with a focus aimed at understanding the effects of adsorbed salt atoms on segregation behavior near the surface of a Ni-Cr alloy. An emphasis is placed on the relative effects of cations and anions in the molten salt, and on the relative effects of natural salt components versus salt impurities. In the fourth and final project, an understanding is sought for the atomic ordering behavior in U-Zr, an alloy under investigation for use as a metallic fuel in advanced fast nuclear reactors. In these metals there is a potential for phase decomposition and a redistribution of fissile U atoms and so a complete understanding of the atomic ordering behavior in U-Zr is therefore needed. DFT is employed to investigate this ordering behavior.