Abstract
Nuclear power will play an essential role in the decarbonisation of global power grids, as it provides low-carbon and non-weather-dependent baseload power in a cost- and land-efficient way. However, the radioactive waste formed, such as spent nuclear fuels, requires specialist handling and storage conditions to ensure the safety of humans, wildlife, and the environment. Plutonium (Pu) metal can react with hydrogen to form plutonium hydrides (PuHx). The presence of the hydrides in handling and storage conditions is a known hazard as they can react vigorously with air, presenting a pyrophoric hazard. Furthermore, the hydrides can catalyse the oxidation reaction of the metal, which is further enhanced in humid conditions. The resulting volume increase, from metal to oxide, could theoretically place mechanical strain on the storage container and subsequently lead to containment failure. Experimental investigations into Pu materials are hindered by their radioactivity and toxicity, making computational modelling methods invaluable. The aim of this thesis is to study the complex structure of the interface between Pu metal and its surroundings. In this work, the hydride bulk, hydride surfaces, and the defect chemistry of the oxide are investigated using quantum mechanical modelling methods.Chapter 1 provides the general context for the work presented in this thesis with backgrounds to nuclear power, the UK nuclear industry, plutonium metal, and the storage of nuclear materials. This is followed by outlines for each of the three research topics presented in this work: plutonium hydrides, interactions of small molecules with plutonium hydride surfaces, and intrinsic defects in cerium dioxide. A literature review of each research topic studied is presented at the beginning of their respective chapters.
Chapter 2 provides the background theories, methods, and techniques used in this computational work. The computational methodologies used to perform the simulations presented in each research topic are stated in their respective chapters.
Chapter 3 investigates the structure, electronic and magnetic properties of cubic PuH2 and PuH3 using density functional theory (DFT) methods. The magnetic ground-state of PuH2 is disputed in experimental and computational literature, with both ferromagnetic (FM) and antiferromagnetic (AFM) orders found. Here, nine magnetic orders, including FM, longitudinal AFM, and transverse AFM aligned in the [100], [110], and [111] directions, are examined and the performances of PBEsol+U+SOC (0 ≤ U ≤ 7 eV) and HSE06sol+SOC methods to represent the magnetism explored. Findings show, using PBEsol+U+SOC, that the magnetic ground-state of the structures is dependent on the magnitude of U, with transitions occurring from AFM to FM at U = 5 eV for PuH2 and from FM to AFM at U = 6 eV for PuH3. Using hybrid HSE06sol+SOC, the magnetic ground-states of PuH2 and PuH3 are FM aligned in the [110] and [111] directions, respectively. For both DFT methods, only small differences in energy are calculated between FM and AFM orders for both structures, implying the orders could coexist. The face-centred cubic (Fm−3m) structure was only retained when the longitudinal AFM order aligned in the [111] direction was examined, with the other magnetic orders imposing small distortions in the structure. The electronic density of states (eDOS) was obtained for PuH2 and PuH3, using PBEsol+U+SOC, showing metallic behaviour, however for PuH3 only a small number of electrons cross the Fermi level.
Chapter 4 investigates the surface speciation, temperatures of desorption, surface phase diagrams, and nanoparticle morphologies of the adsorption of molecular H2O and dissociative H-OH on PuH2 {100}, {110}, and {111} surfaces at 25–100% water coverages using a DFT method. Findings show the adsorption of dissociative H-OH is preferred over the adsorption of molecular H2O, except on the {100} surface at 100% water coverage where molecular H2O is preferred. Molecular H2O first desorbs from the {111} surface, followed by the {110} surface, and then from the {100} surface at all water coverages. While dissociative H-OH desorbs in the same manner as molecular H2O at 25–50% coverages, but at 75–100% coverages desorption occurs first from the {100} surface, followed by the {111} surface, and then from the {110} surface. An octahedral morphology, expressing the {111} surface, was retained under a variety of temperatures and partial pressures of water. The morphology of the nanoparticles changed to truncated octahedral and truncated nanocubes when the {100} surface was appropriately stabilised. When the {110} surface was appropriately stabilised the morphology of the nanoparticles changed to rhombic dodecahedron.
Chapter 5 investigates the impact that intrinsic Frenkel and Schottky defects have on the thermal conductivity and lattice dynamics of the oxide using a DFT method and the band-unfolding methodology. Cerium dioxide (ceria, CeO2) is used as a surrogate for plutonium dioxide (PuO2) as it is non-toxic, non-radioactive, shares the fluorite structure, has a similar thermal conductivity across a wide temperature range, and requires less computational resources to simulate. Findings show that the presence of the intrinsic defects reduces thermal conductivity by up to 88% and broadened the phonon spectra compared to the stoichiometric structure. While the infrared (IR) spectra of the anion Frenkel and Schottky defects are similar to the spectrum of the stoichiometric structure, however the peaks are less symmetric and small shoulder features are present. These features are associated with the vibrations localised around the defect site. The cation Frenkel could be clearly identified from its IR spectrum due to the distinct splitting of both the main peaks in the spectrum of the stoichiometric structure.
Chapter 6 concludes the thesis with a summary of the key findings presented in each research chapter and discusses future works.
| Date of Award | 26 Feb 2024 |
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| Original language | English |
| Supervisor | David Cooke (Main Supervisor) & Marco Molinari (Co-Supervisor) |