Modelling long timescale effects of irradiation damage of nuclear graphite

目前，英国有14个先进的气体冷却反应堆(AGR)，由EDF运营。这些AGR中的反应堆核心由几吨合成形式的石墨组成，通常被称为核石墨。这种核石墨有两个目的：使中子减速以促进核裂变，并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结构。这种微观结构可以通过制造条件来定制。一个关键的问题是，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)。这会在砖块中产生应力，最终导致裂缝。破裂和碎裂的砖块最终会导致反应堆堆芯通道堵塞，这将阻止燃料棒和控制棒的插入或移除。因此，抗辐射损伤、不容易在辐照下膨胀或破裂的材料(和特殊的微结构)是可取的，因为它们将导致未来第四代核反应堆更长的运行时间。本项目的主要目的是加深对石墨砖的微结构如何在反应堆堆芯的高温和高辐照条件下影响其性能(如尺寸变化)的理解。这一理解将通过以下项目目标获得：(1)模拟小的和大的原子缺陷结构的相互作用，以期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，揭示了尺寸变化的关键机制。我们将使用先进的计算机模拟技术，以原子级别的细节对这些石墨结构进行建模。我们将使用长时间刻度技术来延长原子模拟的模拟时间。(2)对含有真实石墨中缺陷密度的微米级薄石墨晶片结构的力学性能进行建模。对压痕和划痕过程进行了计算机模拟，并与实验结果进行了比较。(3)研究了具有真实缺陷和晶界分布的石墨结构的热性能。将对一系列此类结构进行非平衡分子动力学模拟，以确定微结构对石墨导热系数的影响。还将模拟一系列具有代表性的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)计算机模拟的效果仅与在模型中拟合通常有限的数据集时所做的基本近似一样好。在这个项目中产生的数据将被用来评估文献中现有的石墨模型的适用性。这些现有的模型将得到改进，确定每个模型的最佳成分，并将它们组合成一个新的混合模型，能够准确预测各种石墨材料的性能。

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.