Enhancing nuclear fuel efficiency through improved understanding of irradiation damage in zirconium cladding

这个项目的重点是能源，更具体地说是核裂变。从核反应堆开启的那一刻起，燃料组件等核心材料就暴露在辐射中。用中子轰击物质会产生级联碰撞，立即在材料中产生点缺陷和位错。这导致与未辐照材料相比，材料性能发生了非常显著的变化。轻水反应堆的核燃料是由所谓的包层管盛装的，包层管由锆合金制成，因为锆合金具有优异的耐腐蚀性、足够的机械性能和低的中子吸收系数。核燃料最初用5%的235U浓缩。然而，由于包层材料降解的不确定性和燃料组件尺寸的不稳定性，燃料不能完全燃烧。尺寸不稳定性与锆合金的辐照生长和蠕变有关。辐照生长发生在锆合金中，而不施加任何外部载荷，这是由于锆的六方紧密排列的晶体结构。由于辐照材料中空位密度的增加，辐照蠕变明显快于热蠕变。核燃料组件的安全运行需要尺寸稳定性，以确保足够的冷却剂流量和控制棒在需要时的安全运行。辐照生长和蠕变会导致燃料组件的弯曲和屈曲，这是目前核电站关注的问题，更令人关注的是核燃料燃耗的增加。因此，我们需要详细了解导致这些现象的机制，以及它们如何受到材料化学和辐照过程中微观结构演变的影响。传统上，辐照材料的微观结构和损伤表征主要是通过电子显微镜进行的。然而，在过去的十年中，已经建成了非常强大的第三代同步辐射源，这为开发辅助工具或辐射损伤和微观结构演变的定量表征提供了巨大的机会。在20世纪60年代和70年代，包括英国在内的许多国家都有试验反应堆，使科学家能够对辐照物质进行研究。然而，这些试验反应堆中的大多数现在已经消失，英国或其他国家不太可能建造许多新的试验反应堆。出于这个原因，政府已经投资于质子/离子加速器来模拟中子辐照。这种设施的优势在于，它们的运行成本比试验反应堆低很多个数量级。然而，我们对中子诱导损伤与质子/离子诱导损伤之间的关系的理解是有限的。由于Zr合金在中子照射时具有相对温和的活性，因此它们也是校准质子/离子对抗中子辐照的理想材料。在奖学金期间，我的研究小组将：-确定合金化学和微观结构对燃料包层的辐照生长和蠕变的作用，-首次广泛使用同步辐射来表征辐照损伤，-校准质子/离子辐照和中子辐照包层材料，以便使用前者（非活性材料）的便利。很容易在短时间内照射到不同的水平)，以确定导致分离生长的环路形成的路线

This project focuses on energy and more specifically on nuclear fission. Core material such as fuel assemblies are exposed to irradiation from the moment a nuclear reactor is switched on. The bombardment of material with neutrons creates collision cascades that immediately produce point defects and dislocations in the material. This results in very significant changes of the material properties compared to non-irradiated material.Nuclear fuel for light water reactors is contained by so-called cladding tubes, which are made from zirconium alloys because of their excellent corrosion resistance, sufficient mechanical properties and their low neutron absorption coefficient. Nuclear fuel is enriched initially with 5% 235U. However, the fuel cannot be fully burned due to the uncertainty of clad material degradation and dimensional instability of fuel assemblies. The dimensional instabilities are related to irradiation growth and creep of zirconium alloys. Irradiation growth occurs in zirconium alloys without applying any external load and is due to the hexagonal close packed crystal structure of zirconium. Irradiation creep is significantly faster than thermal creep due to the increased density of vacancies in irradiated material. The safe operation of nuclear fuel assemblies requires dimensional stability to ensure sufficient coolant flow and the safe operation of control rods when needed. Irradiation growth and creep can lead to bowing and buckling of fuel assemblies, which is of concern with current plants and even more a concern for increased burnup of the nuclear fuel. Consequently, we need to develop a detailed understanding of the mechanisms leading to these phenomena and how they are affected by material chemistry and the microstructure evolution during irradiation.Traditionally, microstructure and damage characterisation of irradiated material is mainly carried out by electron microscopy. However, in the last decade, very powerful 3rd generation synchrotron radiation sources have been built, which represent a tremendous opportunity to develop complementary tools or quantitative characterisation of irradiation damage and microstructure evolution.During the 1960s and 70s many countries including the UK had test reactors that allowed scientists to undertake research on irradiated material. However, most of these test reactors are gone now and it is unlikely that the UK or other countries will build many new test reactors. For this reason, governments have invested in proton/ion accelerators to simulate neutron irradiation. The advantage of such facilities is that they are by many order of magnitudes cheaper to run than a test reactor. However, our understanding of how well neutron induced damage is related to proton/ion induced damage is limited. Since Zr alloys are relatively mildly active when irradiated by neutrons, they represent also an ideal material to calibrate proton/ion against neutron irradiation.During the fellowship my research group will:- identify the role of alloy chemistry and microstructure on irradiation growth and creep of fuel clad,- for the first time extensively use synchrotron radiation to characterise irradiation damage and- calibrate proton/ion irradiated against neutron irradiated cladding material in order to use the convenience of the former (non-active material, easily irradiated to different levels in a short time) to identify the route cause for loop formation resulting in breakaway growth