An Investigation of the Structure, Stability and Transport of Hydrogen in Uranium Oxides

Abstract

The aim of this thesis is to develop an understanding of how hydrogen defects behave in uranium dioxide. As a major concern is the formation of pyrophoric UH3 at the uranium metal/uranium oxide interface. There is currently very little data, experimental or theoretical, available for hydrogen in UO2. However, due to uranium dioxide's prominence as a nuclear fuel, there is a vast repository of data for UO2 available in the literature. This has shown that UO2 is also highly susceptible to oxidation, this further complicates the problem of determining how hydrogen behaves as this is highly dependent on the oxygen stoichiometry. This thesis begins by outlining the background to nuclear power, the nuclear fuel cycle and the uranium-oxygen system (Chapter 1). This is followed by an explanation of the quantum mechanical methodology used (Chapter 2). A range of different GGA+U functionals (PBE+U, PBEsol+U, PW91+U and rPBE+U) are investigated to determine the most suitable for simulation of UO2 (Chapter 3). This is followed by an assessment of the functional dependence on the predicted behaviour of hydrogen in UO2 (Chapter 4) From the results of Chapters 3 and 4 the PBE+U functional was determined to be the most suitable for the simulation of UO2 and predicting hydrogen behaviour in UO2. The PBE+U functional was then used to investigate the behaviour of hydrogen as a function of oxygen content in UO2+x (Chapter 5), where hypostoichiometric UO2 was found to favour hydride formation and hyperstoichiometric UO2 favoured protonic defects (as part of a hydroxyl group). Having determined the preferred hydrogen species in UO2 the energy barriers to transport are the assessed (Chapter 6), with the lowest energy pathway predicted to involve the conversion of hydride defects to hydroxyl defects, with a corresponding U5+ to U3+ conversion. UO2 is particularly susceptible to oxidation, leading to a large range of proposed oxygen defect clusters in the literature. The effect of hydrogen on the stability on the smallest of these clusters (split di-interstitial and 2:2:2 Willis) is investigated (Chapter 7), where a single hydrogen atom is enough to stabilise the 2:2:2 Willis cluster. One of the motivations for investigating hydrogen behaviour in UO2 is the formation of UH3 at the metal oxide interface. The chosen methodology for hydrogen defects in UO2 is assessed for its suitability to simulate UH3 (Chapter 8) and the oxidation of the hydride is investigated. In the final results chapter (Chapter 9) a model is developed to assess the thermodynamic stability of all the different defects as a function of varying oxygen and hydrogen chemical potentials. Finally, the conclusions and possible directions for building on the work in this thesis are presented (Chapter 10).

Date of Award	13 Feb 2018
Original language	English
Awarding Institution	University of Bath
Sponsors	Atomic Weapons Establishment
Supervisor	Steve Parker (Supervisor)