An innovative, multi-scale, real-time approach to the understanding of deformation and fracture in irradiated nuclear reactor core graphites

石墨是目前英国反应堆中使用的最吸引人的材料之一，也是设计运行60到100年的新一代高温反应堆(第四代)的候选材料。石墨在辐照下具有复杂的微观结构和行为；它是先进气冷反应堆(AGR)中不可替代的堆芯部件，因此具有寿命限制。这种材料已经引起了学术界和工业界的广泛关注，以帮助确保核裂变反应堆的安全运行。目前，英国有16座反应堆的发电量约占其发电量的20%，其中除一座反应堆外，所有反应堆都计划在2023年退役。然而，已经计划将AGR装置的平均寿命延长7年。距离这些AGR最早的“寿命终结”情景还有8年的时间，这为这项关于石墨的工作方案奠定了基础。延长AGR的使用寿命具有重要的战略意义，不仅关系到EDF Energy及其商业利益，也关系到英国在新一代反应堆上线之前满足电力需求的能力。对AGR慢化剂组件中的石墨结构的进一步了解将继续确保其安全性。关键的挑战仍然存在，必须在提高对核石墨的基本机制的理解方面加以解决。虽然在这些领域的研究是困难和具有挑战性的，但本项目提案建立在国际和平研究所在这一专题领域的专门知识的基础上，并结合使用新兴的新技术，以解决这一关键问题。核石墨的多尺度表征为了在适当的长度尺度上生成基于微观结构的描述，并对辐照后的核反应堆堆芯石墨的显著变形、断裂机制和一般力学性能进行量化，布里斯托尔大学的PI开发了一种新的方法来研究局部损伤。该方法将计算结果与计算机模拟相结合，为核石墨的结构完整性分析和寿命预测奠定了坚实的基础。基于微结构的核石墨在温度下的变形和断裂提供三维的、原位的、在温度(超过1000℃。C用于第四代反应堆)使用计算机同步辐射X射线微层析技术对石墨的变形和断裂进行表征。还没有对核石墨进行过这样的试验。这一目标将考虑到英国AGR反应堆产生的微观结构梯度，从而对延长寿命的决策产生直接影响。这项工作的一部分将与美国加州大学伯克利分校的罗伯特·里奇教授一起进行。核石墨在应力作用下基于微观结构的热蠕变提供对石墨在使用寿命期间尺寸变化的力学理解，即评估从环境到反应堆温度(超过1000℃)载荷下原始和辐照石墨对蠕变的热贡献。C用于第四代反应堆)。英国国家物理实验室的Bryan Roebuck教授将提供实现这些研究的设备。优化项目输出上述三个方面的输入将有助于生成修订后的寿命评估方法。在完成上述三个关键领域的项目后，对石墨及其所代表的材料类别的机械理解将直接使相关学术界受益。在项目结束时以研讨会的形式传播结果将向工业界提供投入，从而对英国现有反应堆的持续安全运行和全球未来反应堆的验证的决策产生直接影响。

Graphite is one of the most fascinating materials used in the current UK reactors and is a candidate for the new generation of high temperature reactors (Gen IV) designed to operate for 60 to 100 years. Graphite has complex microstructure and behaviour under irradiation; it is a non-replaceable reactor core component in Advanced Gas-cooled Reactors (AGRs) and, hence, is life-limiting. This material has attracted extensive academic and industrial scrutiny to assist in underwriting the safe operation of nuclear fission reactors. Currently, the UK has 16 reactors generating about 20% of its electricity and all but one of these is scheduled to retire by 2023. However, life extension averaging 7 years for AGR units has been planned. There is 8 years before the earliest "end-of-life" scenarios for these AGRs is reached and this has set the horizon for this work programme on graphites. Lifetime extension of the AGRs is of strategic importance, not only for EDF Energy and its commercial interests but also for the UK's ability to meet electricity demand before the new generation of reactors are able to come online. Further understanding of the graphite structure in the moderator components of AGRs continues to ensure their safety. Key challenges remain, and have to be addressed in terms of improving the fundamental mechanistic understanding of nuclear graphite. Although research in these areas is difficult and challenging, the present project proposal builds on the PI's expertise in this topic area, combined with the use of emerging novel techniques, to attack this critical problem.1. Multi-scale characterisation of nuclear graphiteTo generate microstructure-based descriptions at appropriate length-scales - with quantification of damage evolution - of the salient deformation, fracture mechanisms and general mechanical properties of irradiated nuclear reactor core graphites, a novel approach to investigate local damage has been developed by the PI at the University of Bristol. This approach, and combining the outcomes with computer modelling, has the advantage of establishing a solid fundamental base for structural integrity analysis and lifetime prediction of nuclear graphite.2. Microstructure-based deformation and fracture of nuclear graphite at temperatureTo provide three-dimensional, in situ, at-temperature (over 1000 deg. C for Gen IV reactors) characterisation of the deformation and fracture of graphites using computed synchrotron X-ray micro-tomography. No such tests have been undertaken on nuclear graphite. This objective will take into account the microstructural gradient created in AGR reactors in the UK and, hence, provide direct impact on life extension decision making. Part of this work will be undertaken with Prof. Robert Ritchie at the University of California, Berkeley, U.S.3. Microstructure-based thermal creep in nuclear graphite under stressTo provide mechanistic understanding of the dimensional change of graphite over service life, i.e. to evaluate the thermal contribution to creep of virgin and irradiation graphite under load from ambient to reactor temperature (over 1000 deg. C for Gen IV reactors). Prof. Bryan Roebuck, of the National Physical Laboratory in the UK, will provide access to equipment that allows the realisation of these investigations.4. Optimisation of project outputInputs from the above three aspects will assist in generating a revised life evaluation methodology. On completion of the project with the above three key areas addressed, mechanistic understanding of the graphite, and the class of materials it represents, will directly benefit the related academic community. Dissemination of the results at the end of the project in the form of workshops will feed the input to industry and, thus, allow direct impact on the decision making for the continued safe operation of current reactors in the UK and validation for future reactors globally.