Crystalline and amorphous structures have been investigated using a range of modelling methods within this body of work to understand the behaviour of disordered grain boundary regions. The oxide systems considered are ZrO₂ formed as a passivation layer on the Zr based nuclear fuel cladding, and UO₂, used as nuclear fuel. This work aims to analyse whether amorphous grain boundary regions may be stabilised due to the inclusion of dopants or impurities at the boundary, impacting the grain boundary properties and therefore the material performance during manufacture and/or operation.

Undoped and doped oxide systems were simulated successfully, and structural analysis found that amorphous structures were distinctly different to the crystalline counterpart, not only in their arrangement but also in other material properties. Native oxide bonds were retained in undoped systems, with coordination environments being similar. Dopant channels were formed upon introducing dopants into the amorphous systems, affecting the connectivity of the network. Oxygen was the fastest diffusing species in all systems analysed in Chapter 4 (ZrO₂) and Chapter 6 (UO₂). Diffusion of oxygen ions in amorphous undoped ZrO₂ and UO₂ systems was predicted to be much faster than that observed in the crystalline counterparts, over the temperature ranges simulated. This result highlights the difference in properties observed in amorphous regions that may be formed along the grain boundaries, in comparison to the crystalline bulk.

The diffusion mechanism in amorphous systems can be considered as a structural re-arrangement of ions to accommodate diffusion events, opposite to the more defined vacancy or interstitial diffusion mechanisms observed in crystalline systems. This relates well with the range of activation energies often observed in amorphous systems, due to the disorder in amorphous systems impacting the activation energy landscape of the system.

For the first time, the thermodynamic stability of undoped and doped amorphous oxide systems was investigated by assessing their configurational entropies. Crystalline and amorphous ZrO₂ systems were computed, and Voronoi tessellations were conducted on these systems to quantify the configurational entropy with increasing temperature. Enthalpy calculations were conducted using density functional theory, and by combining both enthalpy and entropy terms, the Gibbs free energy of reaction was computed successfully. This corroborates experimental literature reports of grain boundary complexions. Further utilisation of this novel approach will be useful in predicting fuel performance at high temperatures and understanding the corrosion mechanism along disordered grain boundary regions in ZrO₂.