Thermo-mechanical property measurement of nuclear graphites at elevated temperatures

核石墨在高温（高达850摄氏度）下的热机械性能以及机械应变对其热膨胀行为的影响与当前和未来的核系统有关。目前的理解表明，这些结构决定的属性是相互关联的，两者都影响着非常重要的，但了解甚少的辐照蠕变现象（即辐照诱导的应力下的尺寸变化）。在这篇博士论文中，将研究现有和未来石墨在非辐照条件下的特性，目的是确定如何在高温下测试辐照石墨，以及通过这些测试可以获得哪些知识。这些石墨将包括Gilsocarbon（即AGR石墨用于目前在英国的反应堆）和细颗粒石墨等级，如可用于封装燃料的一些设计的高温反应堆和那些在小型模块化反应堆。（拉伸和压缩，通过2-D和3-D中的高精度图像相关测量）和石墨晶体中的弹性应变（通过同步加速器X射线和中子衍射测量）。这些新的数据将进一步提高对石墨微观结构中机械损伤的理解。牛津大学以前对室温Gilsocarbon的研究表明，损伤会改变弹性模量，影响机械应变能的积累。这些是石墨部件的断裂容限和它们对应力集中的敏感性的重要因素，从而导致石墨部件强度的降低。高分辨率的方法，如相关的聚焦离子束（FIB）断层扫描，电子显微镜和拉曼光谱，将用于检查微观结构应变，而石墨填料和基质的局部性能的演变将通过微机械测试，如纳米压痕和微悬臂梁测试，在环境温度和高温下测量。通过确定现有的“适应孔隙度”和损伤机制如何吸收变形，对热膨胀系数的影响以及应变微结构如何响应辐照效应，即辐照蠕变，这项研究还将通过热机械测试在批量规模上对石墨进行测试（高达250摄氏度），并进行现场实验室X射线断层扫描或光学成像和数字图像相关（DIC）分析。计划在Engin-X和IMAT（都是ISIS，英国）使用中子，在钻石光源（DLS）使用X射线进行更高温度的临界实验。所获得的数据将为石墨在高温下的行为的非线性有限元建模提供输入和支持信息，并有可能模拟工程部件的行为。为了为今后的辐照石墨工作提供基础，这种建模方法将包括石墨变形的微观力学模型。在NNL的借调期间，学生可以参与NNL的建模工作和/或辐照石墨的性能测量，以帮助加深他们对这一领域的理解。该项目福尔斯属于EPSRC能源研究主题（核电）案例学生与NNL公司

The thermo-mechanical properties of nuclear graphites at elevated temperatures (up to 850 Celsius), and the influence of mechanical strain on their thermal expansion behaviour, are relevant to current and future nuclear systems. Current understanding suggests that these structure-determined properties are linked, and both influence the highly significant, but poorly understood phenomenon of irradiation creep (ie irradiation-induced dimensional change under stress). In this PhD the properties of existing and future graphites in the non-irradiated conditions will be investigated, with the objective of identifying how irradiated graphites could be tested at elevated temperatures and what knowledge would be obtained by such tests. These graphites will include Gilsocarbon (ie AGR graphite used in current reactors in the UK) and fine-grained graphite grades such as may be used to encapsulate the fuel in some designs of high temperature reactor and those in small modular reactors.The key experimental studies will investigate the relationships between applied total strains (tensile and compressive, measured by high-precision image correlation in 2-D and 3-D) and the elastic strains in the graphite crystals (measured by synchrotron X-ray and neutron diffraction). These novel data will further improve understanding of how mechanical damage is accommodated in the graphite microstructure. Oxford's previous studies of room temperature Gilsocarbon have shown that damage changes the elastic modulus, affecting the accumulation of mechanical strain energy. These are significant factors in the fracture tolerance of graphite components and their sensitivity to stress concentrations causing a reduction in graphite component strength. High resolution methods, such as correlative Focussed-Ion Beam (FIB) tomography, electron microscopy and Raman spectroscopy, will be used to examine microstructural straining, while the evolution of local properties of the graphite filler and matrix will be measured by micromechanical testing, such as nano-indentation and micro-cantilever tests, at ambient and elevated temperatures. By determining how existing 'accommodation porosity' and damage mechanisms absorb deformations, the effect on the coefficient of thermal expansion and how strained microstructures would respond to the effects of irradiation, i.e. irradiation creep, will be explored.The study will also characterise graphites at the bulk scale by thermo-mechanical testing (up to 250 Celsius) with in situ laboratory X-ray tomography or optical imaging and digital image correlation (DIC) analyses. Critical experiments at higher temperatures are planned using neutrons at Engin-X and IMAT (both ISIS, UK), and X-rays at the Diamond Light Source (DLS). The obtained data will provide the inputs and supporting information for non-linear finite element modelling of the behaviour of graphites at elevated temperatures, with the potential to simulate the behaviour of engineering components. To provide the foundations for future work on irradiated graphites, this modelling approach will include micro-mechanistic models for graphite deformation. During a secondment at NNL, the student could engage with NNL's modelling work and/or property measurement of irradiated graphites to help deepen their understanding of this area.This project falls within the EPSRC Energy Research Theme (Nuclear Power)Case student with company NNL