Fundamentals of current and future uses of nuclear graphite

石墨是大多数英国运行的反应堆的关键组成部分，也是最令人兴奋的新型高温反应堆设计的关键组成部分，这些反应堆有朝一日将生产清洁燃料氢气。石墨作为一种减速剂，使中子减速，使它们更有效地进行核裂变。它也是一种结构成分，因此不使用光滑和脆弱的单晶石墨，而是使用多晶成分（就像岩石包含许多不同的联锁矿物微晶一样）。在中子慢化的过程中，它变得受损，多孔性增加，单个微晶改变了形状。这些变化受到仔细监测，但我们需要能够预测这些变化，以便我们能够更好地衡量我们的反应堆的预期寿命。这将是英国实现2020年及以后碳减排承诺的重要一步。从长远来看，高温气冷堆（HTR）在国际上被视为重要的电力来源，特别是用于制氢，因此我们同样需要证明未来的国际HTR能够运行60-100年。材料测试反应堆的核石墨数据是不完整的，因为旨在为英国AGR提供寿命数据的辐照实验提前终止。当石墨的原始理论在60年代和70年代形成时，对石墨层的六边形碳网知之甚少。我们现在知道这些网可以被分离出来并单独研究（2004年安德烈·海姆和他在曼彻斯特的同事发现了石墨烯），它们可以卷成管（1991年饭岛发现了纳米管），它们可以形成球（1985年克罗托和他的同事发现了富勒烯）。因此，现有的理论没有考虑到石墨层的弯曲或折叠，而我们已经证明了这在辐射损伤中是重要的。此外，电子显微镜当时没有现在那么强大：我们可以得到石墨层原子细节的照片。我们可以从拉曼和电子光谱中检测不同结构的光谱特征，甚至可以对多晶石墨进行纳米精度的全息照相。最后，计算机软件和硬件的进步意味着我们可以通过求解将原子结合在一起的电子的运动方程，精确计算中子与碳原子碰撞产生的结构。碳-碳键的长度约为七分之一纳米，与典型石墨组件的长度（约为一米）相比，其长度大得令人难以置信：70亿！所以我们必须对不同的长度尺度使用不同的理论，这样我们就可以在每一个尺度上将我们从测量和模拟中获得的理解结合起来。因此，我们使用多尺度方法来计算石墨部件的形状、强度和刚度，同时考虑中子对单个原子、它们所在的层、微晶以及整个部件的影响，结果将为核设施和下一代本质安全高效的超高温反应堆的设计者提供预测能力。

Graphite is a key component of most UK operational reactors and for the most exciting designs of new high temperatures reactors that should one day produce the clean fuel, hydrogen. Graphite acts as a moderator to slow neutrons down and make them more effective for nuclear fission. It is also a structural component, so the otherwise slippery and weak single crystal graphite is not used but rather the components are polycrystalline (in the same way that a rock comprises many different interlocking mineral crystallites). In the course of its neutron moderation it becomes damaged, more porous and the individual crystallites change their shape. These changes are carefully monitored but we need to be able to predict the changes so that we can better gauge the life expectancy of our reactors. It will be an important step towards meeting the UK's commitments to carbon emission reduction to 2020 and beyond. In the longer term, High Temperature gas-cooled Reactors (HTRs) are internationally seen as an important source of power, in particular for hydrogen production, so we need similarly to show that future international HTRs could be capable of operating for 60-100 years. Materials Test reactor data for nuclear graphite are incomplete due to the early termination of irradiation experiments aimed at giving lifetime data for UK AGRs.When the original theories of graphite were formulated in the 60's and 70's, less was known about the hexagonal carbon nets that are the layers of graphite. We now know these nets can be isolated and studied on their own (the discovery of graphene in 2004 by Andre Geim and co-workers at Manchester), they can be rolled into tubes (discovery of nanotubes by Iijima in 1991) and they can form into balls (discovery of fullerenes by Kroto and coworkers in 1985). Thus, existing theories did not think to account for buckling or folding of the graphite layers, which we have shown to be important in radiation damage.In addition, electron microscopes were not as powerful then as now: we can get pictures of the layers of graphite in atomic detail. We can detect spectroscopic signatures of different structures from Raman and electron spectroscopy and even perform holography of the polycrystalline graphite with nanometre precision. Finally, the progress in computer software and hardware means that we can calculate exactly the structures that will result from neutrons colliding with carbon atoms by solving the equations of motion of the electrons that hold atoms together. The comparison between the length of a carbon-carbon bond, which is about one seventh of a nanometre, and the length of a typical graphite component (about a metre) is unbelievably large: 7,000,000,000! So we must use different theories for different length scales so that we can combine our understanding from measurements and simulation at every scale in between. Thus we use a multiscale approach to calculate the shape, strength and rigidity of the graphite components taking into account what the neutrons do to individual atoms, to the layers they reside in, to the crystallites and then to the component as a whole.The result will give predictive power to the nuclear utilities and to the designers of the next generation of inherently safe and efficient very high temperature reactors.