ORCID Identifier(s)

0000-0002-2957-9787

Graduation Semester and Year

2017

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Muhammad N Huda

Abstract

Plutonium oxides is of widespread significance due its application in nuclear fuels, space missions, as well as the long-termed storage of plutonium from spent fuel and nuclear weapons. The processes which refine and store plutonium bring many other elements in contact with the plutonium metal and thereby affect the chemistry of the plutonium. Pure plutonium metal corrodes to an oxide in air and the most stable form of this oxide is the stoichiometric plutonium dioxide, PuO2. Defects such as impurities and vacancies can form in the plutonium dioxide before, during and after the refining processes as well as during storage. Studying the interaction between transition metals and plutonium dioxide is critical for better, more efficient storage plans as well as gaining insights to provide a better response to potential threats of exposure to the environment. Using density functional theory, first the bulk and then the PuO2 (111) surface of the pristine system have been investigated. The bulk results show that magnetic configurations of PuO2¬ are still debatable and need to be tested further in surface slabs. The pristine periodic slab models of the 110 and the 111 surface were examined to find surface properties and slab size effects. Surface modelling of up to six molecular layers were conducted. Ferromagnetic (FM) and anti-ferromagnetic (AFM) configurations were considered with and without spin-orbit coupling for the 1x1 slab. Furthermore, the effects of periodicity were explored between 1×1 and 2×2 super cell of the anti-ferromagnetic surface slabs. Results based on surface energies, work function, band gaps and density of states show that 5-6 molecular layer is sufficient for surface modelling. Except for the outer layer, the surface in general retains the Mott-insulator property. Secondly the effects of impurity and defects in the bulk and then the PuO2 (111) surface have been investigated. Our study explores the interaction of a few key metals within the plutonium dioxide structure which have a likelihood of being exposed to the plutonium dioxide powder. We explored a doped system of substituted metal impurity within PuO2 supercell. We repeated the calculations with an additional oxygen vacancy. An impurity defect manifests itself at the bottom of the conduction band and affects the band gap of the unit cell. Our results reveal interesting volume contraction of PuO2 supercell when one plutonium atom is substituted with a metal atom. The volume of the doped system was further contracted with the addition of a single nearby oxygen vacancy. Extending the impurity dopant to a 5 molecular layer 2x2 surface, three layers were highlighted. The outer layer, exposed to the vacuum, the bulk layer in the middle of the slab and the subsurface layer sandwiched between the bulk and the outer layer. Oxygen vacancy in the outer layer is less favorable than the bulk-layer and effects the Pu-Pu distances beyond the layer that it resides in. Zirconium impurity tested on each highlighted layer, has negative formation energy and has minimal impact on the slab beyond its nearest neighbor atoms. The non-pristine slab contracts due to dopants and defects as well but retains the Mott-insulating character with a zirconium impurity within the PuO2 (111) surface slab.

Keywords

plutonium dioxide, ab initio calculation, . bulk, surface, impurity, impurities, DFT+U, WIEN2K, VASP, Moten

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Included in

Physics Commons

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