Compositional and Structural Evolution of Plutonium Dioxide: Underpinning Future Decisions

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Plutonium dioxide is a very dynamic material. Radioactive decay damages the lattice and also forms other elements in the material. Helium, an inert gas, may be localised or trapped in the lattice, or maybe released. Uranium isotopes (formed from the decay of plutonium-238, 239, 240) and americium-241 (formed from decay of plutonium-241) are formed atom-by-atom within the plutonium dioxide lattice. The UK has 140 tonnes of separated plutonium in the form of plutonium dioxide, the World's largest civil stockpile. This has been separated over the last half century and will need to be stored for several decades into the future before its end use. Currently, Government intends most of this material to be made into nuclear reactor fuel ('mixed oxide fuel'), with a small proportion, which cannot be made into fuel, being disposed of as waste, although policy changes could lead to more of it being designated as waste. Whatever the final fate of the plutonium, the material will need to be processed into a suitable form for its end use, and its evolution while it is being stored will affect its suitability for processing. We therefore need to be able to predict how plutonium dioxide will change in storage, so we know whether it will be suitable for its final use. The purpose of this project is to understand how plutonium dioxide changes so we can make these predictions. We will make experimental measurements with plutonium dioxide to define the effects of radiation damage, helium formation and decay product formation on the material over timescales up to several decades. The evolution of plutonium dioxide will be explored using both a series of model samples and materials drawn from the UK stockpile. Behaviour of decay products will be determined using the stockpile materials. We will use synchrotron techniques (X-ray absorption spectroscopy, diffraction and tomography), electron microscopy and specific surface area measurements to characterise the materials. The results of these experiments will be used to develop computational models of plutonium dioxide evolution. Because decay products form atom-by-atom, and decay processes affect the electronic structure of the material, we need to model all these processes at the scale of individual atoms and small aggregates of atoms, but because the properties we are interested in are manifest at the lattice scale, we also need to understand how the atomic-scale effects carry across to this larger scale, and we will also develop models which we can use at this larger scale.