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令人着迷的材料之一，也是新一代高温反应堆（第四代反应堆）的候选材料，设计运行时间为60至100年。石墨在辐照下具有复杂的微观结构和行为；它是先进气冷堆（agr）中不可更换的堆芯部件，因此具有寿命限制。这些材料吸引了广泛的学术和工业审查，以协助核裂变反应堆的安全运行。目前，英国有16座反应堆，发电量约占其总发电量的20%，除一座外，其余都计划在2023年前退役。然而，AGR机组的平均寿命延长了7年。距离这些agr最早的“寿命终结”情景还有8年的时间，这为石墨的工作计划设定了地平线。agr的寿命延长具有重要的战略意义，不仅对法国电力公司及其商业利益，而且对英国在新一代反应堆上线之前满足电力需求的能力也有重要意义。进一步了解agr慢化剂组分中的石墨结构将继续确保其安全性。关键的挑战仍然存在，并且必须在提高对核石墨的基本机制理解方面加以解决。尽管在这些领域的研究是困难和具有挑战性的，但目前的项目建议建立在PI在该主题领域的专业知识基础上，结合使用新兴的新技术，来解决这个关键问题。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-与损伤演化的量化-辐照核反应堆堆芯石墨的显著变形，断裂机制和一般力学性能，布里斯托尔大学的PI开发了一种研究局部损伤的新方法。该方法将结果与计算机建模相结合，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。利用计算机同步加速器x射线微断层扫描技术，提供三维的、原位的、温度下（超过1000摄氏度的第四代反应堆）石墨的变形和断裂特征。没有对核石墨进行这种试验。这一目标将考虑到英国AGR反应堆中产生的微观结构梯度，因此，对延长寿命的决策提供直接影响。这项工作的一部分将由美国加州大学伯克利分校的Robert Ritchie教授承担。为了对石墨在使用寿命期间的尺寸变化提供机理上的理解，即评估从环境温度到反应堆温度（第四代反应堆超过1000℃）负载下原始石墨和辐照石墨对蠕变的热贡献。英国国家物理实验室的Bryan Roebuck教授将提供设备，使这些研究得以实现。优化上述三个方面的项目产出将有助于产生一种订正的寿命评价方法。在解决上述三个关键领域的项目完成后，对石墨的机理理解及其所代表的材料类别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，将为工业界提供投入，从而对英国当前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.