Computer simulations can predict the properties of materials that might be difficult to study experimentally. In particular, atomic-scale simulations continue to play a crucial role in understanding fundamental mechanisms governing the behaviour of nuclear fuel. Such
simulations can inform the development of fuel performance codes, which are essential tools used by industry and the regulator. In this thesis, molecular dynamics simulations have been used to investigate the properties of mixed oxide nuclear fuels. Two main themes will be addressed: i) properties of mixed oxide fuel in the solid and liquid state and ii) the behaviour of fission gas in nuclear fuel. The significance of this work is that it provides improved understanding of fuel performance, develops predictions where experiments are particularly challenging/expensive, and provides information on compositions for which no irradiation data exist. The first part of the study was to investigate the thermophysical and ion transport properties of (ThPu)O2 by predicting the thermal expansion, specific heat capacity and oxygen diffusivity. This was extended by considering the accommodation of the burnable poison gadolinium as Gd3+ ions compensated by oxygen vacancies. Enhanced oxygen diffusivity was predicted for particular compositions, however, this enhancement is not seen with the introduction of Gd to the system (although the system oxygen diffusivity is higher overall). Building on the method for generating (ThPu)O2 structures, solid to liquid transitions were investigated for UO2, ThO2, PuO2 both as end members and as components in mixed oxides.
Melting points were initially determined using a common moving interface method for (ThPu)O2. In addition, a new compositional moving interface method was used to identify the solidus and liquidus for (UTh)O2, (UPu)O2 and (ThPu)O2.
The accommodation of gas species in fuel is performance limiting. While much work exists on gas in end-member oxides, molecular dynamic simulations bring valuable insight on gas behaviour mechanisms. Thus, the behaviour and transport of He in UO2 and how it was effected approaching dislocations and grain boundaries was also investigated. The results show a clear enhancement in He diffusivity at dislocations or grain boundaries. Gas can also accumulate and so Xe bubble pressures in UO2 were also examined and compared to experimental datasets to determine the discrepancy between equilibrium bubble pressures and bubble pressures in nuclear fuel. It was found that bubble pressures in nuclear fuel are over-pressurised in comparison to pressures given by the Young-Laplace relationship and that important terms such as surface stress should not be ignored when using the Young-Laplace
relationship. This work lays the foundations for further simulations that need to be done to investigate it in mixed oxide fuel.