Modelling long timescale effects of irradiation damage of nuclear graphite

目前，英国有14座先进气冷堆（AGR）由EDF运营。这些AGR中的反应堆堆芯由数吨合成石墨组成，通常称为核石墨。这种核石墨有两个用途：一是减缓中子的速度，以增强核裂变;二是形成反应堆堆芯的主要结构。核石墨不是单晶，而是由许多取向错误的石墨颗粒组成的高度复杂的微观结构。这种微结构可以通过制造条件来定制。关键问题是核反应堆堆芯的高温和高辐照条件导致石墨砖显著膨胀（尺寸变化）。这在砖中引起应力，最终导致裂缝。破裂和破碎的砖块最终会导致反应堆堆芯通道堵塞，从而阻止燃料棒和控制棒的插入或取出。因此，我们认为，材料该项目的主要目的是了解石墨砖的微观结构如何影响其性能（1）模拟大小原子缺陷结构的相互作用，以了解石墨尺寸变化的主要原因。通过研究石墨中空位和间隙原子的运动以及石墨烯层的屈曲，揭示尺寸变化的关键机制。我们将使用先进的计算机模拟技术，可以模拟这些石墨结构与原子水平的细节。长时间尺度技术将被用来延长我们的原子模拟的模拟时间。(2)模拟石墨中含有真实缺陷密度的微米级薄石墨晶片结构的机械性能。压痕和划痕过程的计算机模拟将与实验结果进行比较。(3)研究含有真实分布的缺陷和晶界的石墨结构的热性能。非平衡分子动力学模拟将在这些结构的范围内进行，以确定石墨的导热性的微观结构的效果。还将模拟一系列代表性石墨结构的热膨胀系数。这些结果也将与实验结果进行比较。(4)计算机模拟只有在模型中拟合通常有限的数据集时才能与基本近似值一样好。本项目中生成的数据将用于评估文献中现有石墨模型的适用性。这些现有的模型将通过识别每个模型的最佳组件并将其组合成能够准确预测广泛的石墨材料特性的新混合模型来进行改进。

There are currently 14 Advanced Gas-cooled Reactors (AGRs) in the UK which are operated by EDF. The reactor cores in these AGRs are composed of several tonnes of a synthetic form of graphite, commonly called nuclear graphite. This nuclear graphite serves two purposes: to moderate (slow down) neutrons to enhance nuclear fission, and to form the main structure of the reactor core.Nuclear graphite is not a single crystal but is instead composed of a highly complex microstructure consisting of many mis-orientated graphite grains. This microstructure can be tailored by the manufacturing conditions.A key problem is that the high temperature and high irradiation conditions of a nuclear reactor core causes the graphite bricks to swell considerably (dimensional change). This induces stress in the bricks which eventually results in cracks. Cracked and crumbling bricks would eventually lead to blocked channels in the reactor core which would prevent the insertion or removal of fuel rods and control rods. Therefore, materials (and particular microstructures) that are resistant to radiation damage and do not readily swell or crack under irradiation are desirable since they would lead to longer run times of future generation 4 nuclear reactors.The key aim of this project is to develop understanding of how the microstructure of a graphite brick influences its properties (such as dimensional change) in the high temperature and high irradiation conditions of a reactor core.This understanding will be gained by the following project objectives:(1) Model the interaction of small and large atomic defect structures with a view to understanding the main causes of dimensional change in graphite. Uncover the key mechanisms for dimensional change by studying the motion of vacancy and interstitial atoms within graphite as well as the buckling of the graphene layers. We will use advanced computer simulation techniques that can model these graphite structures with atomic level detail. Long timescale techniques will be used to extend the simulation time of our atomistic simulations.(2) Model the mechanical properties of micrometre sized thin graphite wafer structures containing realistic densities of defects in graphite. Computer simulations of indentation and scratching processes will be performed and compared with experimental results.(3) Investigation of the thermal properties of graphite structures containing realistic distributions of defects and grain boundaries. Non-equilibrium molecular dynamics simulations will be performed on a range of these structures to determine the effect of the microstructure upon the thermal conductivity of the graphite. The coefficient of thermal expansion will also be simulated for a range of representative graphite structures. These results will also be compared to experimental results.(4) Computer simulations are only ever as good as the underlying approximations made in fitting a usually limited data set in the model. The data generated in this project will be used to assess the suitability of existing models for graphite available in the literature. These existing models will be improved upon by identifying the best components of each model and combining them into a new hybrid model capable of accurately predicting a wide range of graphite material properties.