A Study of the Combined Effects of Displacement Damage and Helium Accumulation in Model Nuclear Materials

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.

Securing energy supplies is a key requirement for a prosperous economy. Electricity from nuclear power is an essential component of the energy mix for baseload generation in the UK. This is set to expand with the UK Government in 2008 approving eight sites for the development of new nuclear power stations and the announcement in 2013 of the development of the first of these with two new European Pressurised Reactors (EPRs) at Hinkley Point in Somerset.

This research project will generate scientific results and understanding which will enable the nuclear industry to safely operate the UK's fleet of nuclear power stations. Understanding how materials perform under the extreme conditions found in a nuclear reactor core over, for example, the 60 year operational lifetime of an EPR, will allow the industry and Office for Nuclear Regulation (ONR) to make informed decisions based on scientific evidence. This will support the safe generation of electricity in the UK from a secure low-carbon source and therefore will ensure the reliable supply of energy to power the rest of the economy.

As well as the structural materials used in the reactors, it is also vital that the materials which can be used to safely contain nuclear waste be developed and understood. Therefore a significant component of the project will focus on nuclear glasses and glass-ceramic composites to support regulators and industry in planning disposal which will be safe for centuries to millennia.

The UK has been a leader in nuclear energy for 60 years. In order for this world-class industrial sector to flourish it is vital to maintain the expertise and to continue to develop cutting edge nuclear technologies in the UK. Looking beyond the new nuclear power stations to be built in the UK over the next decade, Generation IV reactor concepts offer even greater gains in safety and cost. This research project will support the contribution of the UK to these emergent technologies laying the foundations for the domestic nuclear industry to develop these advanced reactor designs and thus concentrate economic activity in the UK. Similarly, the UK has been a pioneer in the development of fusion power through projects such as the Joint European Torus (JET) at the Culham Centre for Fusion Energy. As the global effort to realise this technology shifts focus to the International Thermonuclear Experimental Reactor (ITER) in France, maintaining the contribution of the UK to the fundamental science will keep the UK as a centre of excellence in this field and will enable UK businesses to exploit fusion when it becomes a practical source of energy. Finally, overcoming the materials challenges to ensure the safe storage of nuclear waste will benefit the economy not only in the short term as these solutions are implemented but also in the much longer term as potentially disastrous financial and environmental costs are avoided.

As described above, wealth will be generated in the UK through support for both UK nuclear energy electricity generation and the development of new nuclear technologies. With world demand for energy continually increasing, producing energy in the UK cheaply and sustainably will create wealth both in the generation sector and in all other sectors of the economy which depend on a reliable affordable energy supply. Furthermore, the creation of intellectual property and the manufacture of components for nuclear power stations can flourish if the research, development and exploitation of these technologies are supported in the UK.

Our quality of life is dependent on many factors including a clean environment and access to energy. The UK is committed to an 80% reduction in greenhouse gases by 2050 and this will require a shift away from our reliance on fossil fuels. Nuclear energy will be a vital component of this change but it is important that we continue to develop our understanding and technologies in order to facilitate a safe expansion in this area.