Nuclear power is arguably the only option for large-scale baseload electricity generation that is compatible with the UK Government's commitment to an 80% reduction in greenhouse gas emissions by 2050. The safe operation of current and future generations of nuclear reactors requires the development and refinement of materials to be used in the construction of reactors and in materials (glass and glass-ceramic wasteforms) to be used for the long-term safe disposal of radioactive wastes. The inevitable irradiation of such materials with energetic particles such as neutrons and alpha particles can have extremely deleterious effects on their structural strength and even their physical dimensions. Ballistic effects cause atoms to be knocked off their normal positions creating vacant sites (vacancies) and displaced atoms (interstitials). Nuclear reactions induced by neutron irradiation can create alpha particles (which are just helium nuclei) causing a build-up of helium gas in these materials. Helium has very little solubility in most materials and will generally combine with vacancies (or accumulate in other regions of lower than average electron-density) to form bubbles. These can have very significant unwanted effects on the properties of the materials by, for instance, building up at the boundaries between grains in polycrystalline materials and making them much more brittle and likely to fracture. Bubbles will also result in highly undesirable changes to the physical dimensions of components. The high temperatures at which reactors operate, and to which wasteforms will be subjected for the first 500 years-or-so of storage, can greatly exacerbate these problems, particularly in the reactor materials by enabling the vacancies, interstitials and helium atoms to combine in different ways and form extended defects such as voids, dislocations and stacking faults.
This project aims to explore systematically the effects that varying the amount of displacement damage, the helium concentration and the temperature has on the damage that develops in a range of structural materials and wasteforms. Different combinations of these parameters pertain to different types of material (both structural and wasteforms), different reactors and even different locations within a reactor. In addition, aspects of the waste glasses, such as alkali content and the presence of glass ceramic interfaces will also be varied in order to determine their role in the development of bubbles and other defects. The project exploits the unique attributes of the MIAMI facility (constructed with EPSRC funding) that permit the ion irradiation of thin foils of materials in-situ within a transmission electron microscope. By varying the ion energy, the ratio of injected helium to the amount of displacement damage can be varied over the range of values relevant to reactor and wasteform materials without the necessity of using two separate ion beams. The ability to irradiate at a range of temperatures from -150 to +1000 degrees Celsius means the that the entire relevant parameter space (helium content, damage and temperature) can be explored.
In this way, transmission electron microscopy (and also electron energy-loss spectroscopy for the nuclear glasses) will be used to build up a comprehensive dataset of the form and structure of defects (defect morphologies) resulting from the various combinations of these parameters. The main aim is then to develop a phenomenological picture of the processes occurring. For the structural materials, the dataset will be calibrated and validated by comparisons with neutron-irradiated materials which will give the dataset greater power to predict defect morphologies likely to result under reactor conditions.
Finally, through collaboration with computer modellers, we will seek to obtain a fundamental understanding of the underlying physical processes which drive the behaviour of these materials under irradiation.