The Development of Advanced Technologies and Modelling Capabilities to Improve the Safety and Performance of Nuclear Fuel

限制核燃料性能的主要因素是与燃料颗粒和反应堆冷却剂相互作用导致的包层失效。提高我们对包层失效机制的理解将增强我们预测其影响的能力，从而改善当前燃料的安全性和操作，以及将产生改进的燃料、包层和涂层材料的技术发展。下面的研究任务试图从当前和先进的燃料设计的角度依次解决这些问题。同名模拟研究将集中在原子尺度模拟在描述微结构缺陷的行为方面所作的输入。这将加深我们对降低燃料性能的基本物理过程的理解，并将导致对当前半经验燃料性能模型的改进。模拟将集中在裂变产物、辐射损伤和位错之间的相互作用，以及导致宏观观测的过程，如裂变气体释放和辐射诱导蠕变。热机械和辐射条件引起的燃料和包层尺寸变化产生复杂的弹丸-包层机械相互作用(PCMI)，已知会导致燃料失效，特别是在事故条件下。基于斜坡测试数据的PCMI失效模型已经开发出来，但这些模型都是高度经验性的，因此适用性有限。然而，有限元(FE)建模的进步现在允许开发详细的模型，扩展有限元方法等技术可以应用于精确地模拟裂纹扩展和裂纹尖端的应力和应变，同时考虑残余应力和施加的应力重新分布。这项研究将研究正常和非正常条件下的弹丸裂纹形态和弹丸-包层界面应力的发展。对复合覆层的研究将探索碳化硅复合材料与现有覆层材料相比提供显著更好的性能的潜力。我们将探索一种新的方法，即在实心的碳化硅内管上包裹碳化硅纤维，然后用碳化硅蒸气浸渗法进行粘结。这项研究将涉及这一新概念的基本方面，包括：描述设计和制造参数与机械强度之间的关系；管子保持对裂变产物的不渗透能力；以及在高温下抵抗氧化和裂变产物侵蚀的能力。尽管UO2作为燃料材料已经使用多年，但有希望的新材料已经开发出来，可以在安全和性能方面提供优势。这项研究的目的是确定替代燃料材料和燃料形式；评估它们的物理性质，如导热系数；使用高压灭菌器测试评估它们与水的反应性；以及评估工业上可行的制造路线。候选材料包括U3Si2、U-Mo、U-Zr等合金以及碳化物和氮化物等共价化合物(在后一种情况下，添加添加剂以降低在水中的反应速度)。化学气相沉积(CVD)法制备的Triso包覆燃料颗粒表现出优异的性能，但众所周知，它容易受到裂变产物如PA的攻击。这项研究将提供对这些问题的基本了解，并将研究替代材料和工艺，以提供更好的性能。将测试涂层的高温机械性能，以了解制造条件的影响。将研究裂变产物传输的机制，以期引入材料和微观结构变化，以改进这方面的性能。

The main factors that limit the performance of nuclear fuels are related to cladding failure due to interactions with the fuel pellet and reactor coolant. Improvements to our understanding of the cladding failure mechanisms will enhance our ability to predict their effects, leading both to improvements in the safety and operation of current fuels, and to technological developments that will produce improved fuel, cladding, and coating materials. The research tasks below seek to address each of these issues in turn, from the perspective of both current and advanced fuel designs.Aomistic modelling research will focus on the input that atomic scale simulations make in describing the behaviour of micro-structural defects. This will refine our understanding of the fundamental physical processes that degrade fuel performance, and will result in improvements to current semi-empirical fuel performance models. The simulations will focus upon the interaction of fission products, radiation damage and dislocations, processes responsible for macroscopic observables such as fission gas release and irradiation induced creep.Fuel and cladding dimensional changes resulting from thermo-mechanical and irradiation conditions produce complex pellet-clad mechanical interactions (PCMI) that are known to cause fuel failure, especially under accident conditions. Models for PCMI failure based on ramp-test data have been developed, but these are highly empirical and therefore of limited applicability. However, advances in finite element (FE) modelling now permit the development of detailed models, and techniques such as the extended FE method can be applied to model crack growth and crack tip stresses and strains accurately whilst taking into account residual and applied stress redistribution. This research will investigate the development of pellet crack patterns and pellet-clad interface stresses under both normal and off-normal conditions. Mechanistic models for pellet failure and cladding damage will be developed.Research into composite cladding will investigate the potential for silicon carbide composites to provide significantly better performance compared with existing cladding materials. A new approach will be investigated, based on a solid SiC inner tube wrapped with SiC fibers and bonded using SiC vapour infiltration. The research will address fundamental aspects of this new concept, including: characterisation of the relationship between both design and manufacturing parameters and mechanical strength; ability of the tube to remain impermeable against fission products; and resistance to oxidation and fission product attack at high temperatures. Although UO2 has been used for many years as a fuel material, promising new materials have ben developed that could offer advantages in terms of safety and performance. The objective of this research is to identify alternative fuel materials and fuel forms; to evaluate their physical properties such as thermal conductivity; assess their reactivity with water using autoclave testing; and to assess industrially-feasible manufacturing routes. Candidate materials include alloys such as U3Si2, U-Mo, U-Zr and covalent compounds such as carbides and nitrides (in the latter case with additives to reduce the reaction rate in water). TRISO coated fuel particles manufactured by chemical vapour deposition (CVD) have demonstrated remarkable performance, but are known to be susceptible to attack by fission products such as Pa. This research will provide a fundamental understanding of these issues and will investigate alternative materials and processes to provide improved performance. The high temperature mechanical properties of coatings will be examined to understand the effects of manufacturing conditions. The mechanisms of fission product transport will be studied with a view to introducing materials and microstructural changes that will improve performance in this respect.