Fundamentals of current and future uses of nuclear graphite

石墨是大多数英国运行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产生清洁燃料——氢。石墨起到了减缓中子速度的作用，使它们更有效地进行核裂变。它也是一种结构成分，所以不使用光滑而脆弱的单晶石墨，而是使用多晶成分（就像岩石由许多不同的相互连接的矿物晶体组成一样）。在中子缓和的过程中，它被破坏，变得更多孔，单个晶体改变了形状。这些变化是被仔细监测的，但我们需要能够预测这些变化，这样我们才能更好地衡量我们反应堆的预期寿命。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，高温气冷反应堆（htr）在国际上被视为一种重要的电力来源，特别是在制氢方面，因此我们同样需要证明，未来的国际高温气冷反应堆能够运行60-100年。由于为英国agr提供寿命数据的辐照实验提前终止，核石墨的试验反应堆数据不完整。当石墨的原始理论在六七十年代形成时，人们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单独分离和研究（2004年由曼彻斯特的Andre Geim和同事发现石墨烯），它们可以卷成管（1991年由饭岛发现纳米管），它们可以形成球（1985年由Kroto和同事发现富勒烯）。因此，现有的理论没有考虑到石墨层的弯曲或折叠，而我们已经证明这在辐射损伤中是重要的。此外，当时的电子显微镜还没有现在这么强大：我们可以得到石墨层的原子细节照片。我们可以通过拉曼光谱和电子能谱检测不同结构的光谱特征，甚至可以实现纳米精度的多晶石墨全息成像。最后，计算机软件和硬件的进步意味着我们可以通过求解使原子结合在一起的电子的运动方程来精确计算中子与碳原子碰撞所产生的结构。碳-碳键的长度约为七分之一纳米，而典型石墨成分的长度约为一米，两者之间的比较大得令人难以置信：70亿！所以我们必须在不同的长度尺度上使用不同的理论，这样我们才能把我们从测量和模拟中得到的理解结合起来。因此，我们使用多尺度方法来计算石墨组分的形状、强度和刚性，同时考虑中子对单个原子、对它们所处的层、对晶体以及对整个组分的影响。研究结果将为核电公司和下一代本质上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.