With the UK committed to reducing its reliance on fossil fuels by 2050, there must be a commensurate increase from other energy sources to maintain and provide for increased energy requirements, at the same time increasing the diversity in energy sources. As a mature and clean energy production method, nuclear power will no doubt make a significant contribution to the UKs energy portfolio. The UK currently operates a civil nuclear fleet consisting primarily of advanced gas reactors (AGRs). Any new nuclear build in the forseable future will likely be new Pressurised Water Reactor (PRW), such as the Westinghouse AP1000 or Areva European Pressurised Reactor (EPR). Both reactors are designed to run using UO2 fuel types enriched with fissile uranium to ~3-5%. In addition to these generation III and III+ type of reactors, some fourth generation (GenIV) reactor types are designed to operate using UO2 fuels (3 out of the 6 final designs). Furthermore it is becoming more desirable by plant operators to increase the level of burn-up in the fuel, not only increasing the efficiency of the reactor, but also reducing operating costs and minimising the volume of nuclear waste produced. One key factor limiting higher burn-up of nuclear fuel is the production and accumulation of fission products within the fuel that can significantly affect the physical properties and limit performance. Broadly speaking, there are four different characteristic fission products produced during burn-up: gaseous, metallic precipitates, oxide precipitates, and those in solid solution. The work to be undertaken here examines the behaviour of fission products under irradiation in both a non-radioactive model nuclear fuel simulant (ceria) and in simulated and real spent nuclear fuel. These materials will be fully characterised, both structurally and chemically, through a combination of transmission electron microscopy and atom probe tomography, with the results being fed back to modellers to validate and/or benchmark predictive models for in-core performance of the fuels, such as ENIGMA. The behaviour of nanocrystalline materials under irradiation will also be investigated. Nanocrystalline materials are viewed to be a viable route towards radiation tolerance, and may therefore improve safety, due to the high number of grain boundaries and interfaces present acting as efficient sinks for defects. It is therefore critical that information on the radiation response of nanocrystalline materials is obtained if this class of materials is to be used in nuclear reactors as a means to improve reactor safety. This characterisation of the materials in the as-received and irradiated state will be performed using the electron microscopy and atom probe capabilities at Oxford University. The ceria samples will be provided by the Universities of Nebraksa and Sheffield, and the real/synthetic/simulated fuels samples will be provided by the National Nuclear Laboratory. The NNL - who will benefit significantly from the results to be obtained from this project - will provide advice in the safe handling of radioactive material, and will provide access to the hot/active transmission electron microscope at the Sellafield central laboratory for characterisation of active samples.