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

核电可以说是大规模基本负荷发电的唯一选择，这与英国政府到2050年减少80%温室气体排放的承诺是一致的。当前和未来几代核反应堆的安全运行需要开发和改进用于建造反应堆的材料以及用于长期安全处置放射性废料的材料（玻璃和玻璃陶瓷废料）。这些材料不可避免地会受到高能粒子（如中子和α粒子）的照射，这会对它们的结构强度甚至物理尺寸产生极其有害的影响。弹道效应导致原子被撞离其正常位置，产生空位（空位）和移位的原子（间隙）。中子辐照引起的核反应可以产生α粒子（即氦核），导致这些材料中氦气的积聚。氦在大多数材料中溶解度很小，通常会与空位结合（或积聚在低于平均电子密度的其他区域）形成气泡。这些可能会对材料的性能产生非常显著的不良影响，例如，在多晶材料的晶粒之间的边界上积聚，使它们更脆，更容易断裂。气泡还会导致组件物理尺寸发生非常不希望发生的变化。反应堆运行时的高温，以及废物在最初500年左右的储存中所经受的高温，会极大地加剧这些问题，特别是在反应堆材料中，空位、间隙和氦原子会以不同的方式结合，形成空洞、位错和堆积错误等延伸缺陷。该项目旨在系统地探索不同位移损伤量、氦浓度和温度对一系列结构材料和废物形式的损伤的影响。这些参数的不同组合适用于不同类型的材料（结构和废料形式）、不同的反应堆，甚至反应堆内的不同位置。此外，废玻璃的各个方面，如碱含量和玻璃陶瓷界面的存在也会发生变化，以确定它们在气泡等缺陷发展中的作用。该项目利用了迈阿密设施（由EPSRC资助建造）的独特属性，该设施允许在透射电子显微镜下对材料薄箔进行离子照射。通过改变离子能量，注入的氦气与位移破坏量的比值可以在与反应堆和废物材料相关的值范围内变化，而无需使用两个单独的离子束。在-150到+1000摄氏度的温度范围内辐照的能力意味着可以探索整个相关参数空间（氦含量，损坏和温度）。通过这种方式，透射电子显微镜（以及核玻璃的电子能量损失光谱）将用于建立由这些参数的各种组合产生的缺陷的形式和结构（缺陷形态）的综合数据集。然后，主要目的是发展发生过程的现象学图像。对于结构材料，数据集将通过与中子辐照材料的比较进行校准和验证，这将使数据集更有能力预测在反应堆条件下可能产生的缺陷形态。最后，通过与计算机建模人员的合作，我们将寻求获得对驱动这些材料在辐照下行为的潜在物理过程的基本理解。

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.