Behaviour of UK Specific Spent Fuels Under Conditions Relevant to Geological Disposal.

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The UK has traditionally reprocessed its nuclear fuel rods to extract and re-use the ~95% uranium and ~ 1% plutonium that remains once they are removed from the reactor. It is becoming clear after reviews by government bodies that some UK nuclear fuels will not now be re-processed and will need to be directly disposed of in a suitable geological repository, recommended as the best option for disposing of the UK's nuclear wastes.Because of the previous nuclear fuel re-processing policy, very little experimental research has been carried out in the UK on this direct disposal option. However, a large amount has been carried out internationally on fuels that come from pressurised water (PWR) or boiling water reactors (BWR). The nuclear fuels concerned in this proposal are unique to the UK and come from Advanced Gas-cooled Reactors (AGR). These differ in the reactor coolant (carbon dioxide) and the moderator (graphite) from the more common water-moderated reactors used internationally and also in the operating temperature (825 C for AGR compared with ~300 C for a PWR or BWR). These differences create uncertainties in the way that these fuels would behave in a repository when compared with the spent fuels studied internationally.The aim of this proposal is to bring together researchers from the country's leading research universities and nuclear research universities and both the National Nuclear Laboratory and the Nuclear Decommissioning Authority to build research capacity to investigate the effect of these different fuel characteristics on the fuel behaviour and compare it with that found internationally. The aim is to use the international research underpinned by UK specific research to understand the long term behaviour of spent AGR fuel in repository so contributing to the safety assessment for its disposal.To do this the project will employ 5 investigators and 6 PhD students to create simulated nuclear fuels that consist of specially prepared uranium dioxide (depleted) with the same microstructural characteristics expected for AGR fuels and the additional chemical components of AGR spent nuclear fuel that result from uranium and plutonium atoms that have fissioned (split) during its time in the reactor. This will be determined by a deep search of UK data on these characteristic microstructures and chemical compositions determined from difficult examinations of the highly radioactive spent fuel in the past.The team will predict the chemical composition of the fuel for ages between 1,000 and 100,000 years which is thought to span the lifetime of the disposal canisters as they are affected by corrosion. They will then determine the rates of dissolution of the simulated fuels under different water compositions. These would be those expected for some representative natural ground water compositions and to water compositions governed by the dissolution of the steel cladding of the fuel rods and the external containers.Eventually, uranium minerals will form on the surface of these fuels and they will probably incorporate plutonium and neptunium. The stability of these phases will determine the rate of release of these elements back into the environment and they will be tested for resistance to damage by the radiation that remains at long times into the future.The group will be guided by an international advisory board and undertake experiments at international facilities equipped for modern analytical methods enabled for work with radioactive samples. This will help establish the group internationally and build the UK capacity for this radiological experimental research so it can support the licensing of a nuclear waste repository in the UK.