Development of Advanced Ceramic Breeder Materials for Fusion Energy

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Nuclear fusion offers the promise of abundant, clean, low cost energy. The fusion process involves fusing together two nuclei releasing large amounts of energy that can be harnessed for electricity generation. Future power stations will employ the reaction between two isotopes of hydrogen, tritium and deuterium, creating a helium atom and a neutron. Deuterium is available from seawater, however, tritium does not occur naturally due to its short half-life. Therefore, tritium will be created, or bred, in the reactor from lithium in a process called transmutation. Transmutation will occur immediately outside the main chamber of the reactor, in a region called the breeder blanket. One of the leading breeder blanket designs will use lithium containing pebbles, such as lithium metatitanate and lithium orthosilicate. Solid breeder materials are attractive as they have high lithium densities that will ensure excellent tritium production and their low reactivity with other reactor materials means they are safe. However the use of a solid breeder material means that following transmutation the tritium will be trapped in the pebbles and must be extracted from the crystal. For recovery the tritium must diffuse to the pebble surface where it can be carried away by the coolant. The rate at which the tritium can escape from the pebbles is a very important parameter to consider when designing a fusion reactor because if the rate drops too low and tritium is retained in the pebble the fusion reaction will be unsustainable. Therefore, the main goal of this research to understand the process of tritium diffusion in lithium ceramics to design materials that have high tritium release rates. The exact mechanism of tritium release will depend on the microstructure of the host material. All crystals contain defects, such as missing atoms (called vacancies), and these defects can either promote tritium release or act as traps and inhibit it. The types and concentrations of defects in a material depend on the exact conditions (i.e. temperature) and will evolve over time. Therefore, to understand the tritium release process we must first understand the microstructure of the ceramics and what defects are presentPrevious studies of tritium release have adopted a top down approach where experimentally observed tritium release rates under different conditions are used to infer the exact atomic level mechanism responsible. By contrast this proposal adopts a novel bottom up approach that uses advanced electronic structure calculations to build a tritium release model from first principles. A key advantage of this approach is that the calculations provide detailed understanding of the atomic rearrangement processes that constitute tritium diffusion and allow a rate to be determined for each process. Initially the intrinsic defect chemistry of the host materials will be examined. This will allow the identification of the defects present in the ceramic under different conditions. Once the intrinsic defect populations are established the interaction of tritium atoms with the defects will be studied. By examining the bonding between tritium and the defects it is possible to determine exactly where the tritium will sit in the crystal and to identify which defects will act as traps. The information gathered so far considers where tritium will sit in the crystal but it does not provide information about how quickly the tritium can move through the crystal. Therefore, the next step in the process is to understand how tritium hops between the defects available and to determine which types of hop are most likely under certain conditions. Finally, all of this information will be used to create a tritium release model from lithium ceramics. This model will be used to optimise the microstructure of the ceramics to deliver maximum tritium release to ensure the fusion process is sustainable.