Uranium Silicide/Uranium Diboride Composite Fuel Development

轻水反应堆(LWR)约占世界当前核电装机容量的90%。事故容忍燃料(ATF)的使用可以显著提高高温能力和操作安全性。实现这一点的一种方法是通过用陶瓷复合材料取代锆包覆层来降低与高温蒸汽发生反应的可能性。这就需要用硅化物取代现有的UO2燃料；U3Si2，这是协调一致的国际研究工作和积极的商业部署计划的重点\(例如，见“未来的燃料”，“国际核工程”，2018年)。这项研究工作包括美国国内的一个国家研究计划，包括爱达荷州国家实验室、橡树岭国家实验室、南卡罗来纳大学、西屋电气和通用原子公司等的大量工作。它包括一系列ATF概念，其中一些非常接近当前的燃料设计(UO2，设计为具有更大的颗粒结构和涂层覆层)，以及更密集的研究工作，以部署真正的阶梯变化(U3Si2燃料颗粒与复合碳化硅覆层)。国际燃料供应商(西屋电气、通用电气、阿海珐)对开发ATF特别感兴趣，因为它们也可能通过消除对目前昂贵和多冗余的安全系统的需求，提供降低核资本成本的关键。这项研究的目的是确定在U3Si2中引入硼化物相是否可以改善材料的高温水性能。这是ATF社区目前面临的一个关键挑战。最近由橡树岭国家实验室的Sooby-Wood等人所做的研究表明，当暴露在高温蒸汽中时，U3Si2迅速和能量地粉碎(E.Sooby Wood等人(2018)。《核材料杂志》，501,404-412)。班戈大学的原子尺度模拟表明，这是材料内部氢反应的结果，这导致体积迅速变化，并增加了可供氧化的表面积。班戈大学对UB2系统进行的类似原子模拟(为支持本工作，目前正准备联合发表)表明，它将不会受到这种破坏机制的影响，因此可以在保持ATF燃料材料(高导热系数、高铀密度等)的优点的同时保护U3Si2。初步工作已通过粗略混合不同重量百分比的U3Si2和UB2粉末生产了几个试验球团。经研究，硼化物和硅化物相在其周围没有可检测到的相互作用层，UB2相似乎已在U3Si2相中成功烧结。对这些球团进行的初步蒸汽测试表明，与纯U3Si2相比，复合球团的水蒸气反应温度明显更高，而且反应速度似乎要慢得多。这表明U3Si2中UB2的存在可能不仅仅是限制U3Si2被水蒸气攻击的物理途径，因为它似乎以某种方式改变了反应机理，如在500℃以下没有任何反应所指出的那样。拟议的工作将建立在这些初步研究的基础上，并评估在U3Si2燃料材料中使用UB2的可行性，以将其抗蒸汽腐蚀性能提高到可接受的水平。这将包括球团制造和表征、蒸汽测试、质子辐照测试、高压锅研究和温度循环影响的表征。

Light Water Reactors (LWRs) comprise around 90% of the world's current nuclear generating capacity. The use of Accident Tolerant Fuels (ATFs) could offer significantly improved high temperature capability and operational safety. One way this can be achieved is by reducing the potential for reactivity with high temperature steam by replacing zirconium cladding with a ceramic composite material. This necessitates the replacement of existing UO2 fuel with a silcide; U3Si2, which is the focus of a concerted international research effort and aggressive commercial deployment plan \(see for example "Fuel of the Future", Nuclear Engineering International 2018). This research effort includes a national research programme within the United States, including large efforts at Idaho National Laboratory, Oak Ridge National Laboratory, The University of South Carolina, Westinghouse and General Atomics (amongst others). It includes a range of ATF concepts, some very close to current fuel designs (UO2, engineered to have a larger grain structure and coated claddings) as well as more research intensive efforts to deploy a true step-change (U3Si2 fuel pellets with composite silicon carbide cladding). International fuel vendors (Westinghouse, GE, Areva) are particuarly interested in devloping ATFs, as they may also provide the key to reducing nuclear capital cost by removing the need for currently expensive and multiply-redundant safety systems. The aim of this research is to establish if the introduction of a boride phase to U3Si2 could improve the materials high temperature water performance.This is a key challenge currently facing the ATF community Recent work performed by Sooby-Wood et al at Oak Ridge National Laboratory has demonstrated that U3Si2 rapidly and energetically pulverises when exposed to high temperature steam (E. Sooby Wood et al (2018). Journal of Nuclear Materials, 501, 404-412). Atomic scale modelling at the Universities of Bangor suggests that this is the result of a hydrogen reaction within the material, which leads to a rapid volume change and increases the surface area available for oxidation.Similar atomic modelling of the UB2 system performed at the University of Bangor (in support of the present work and currently being prepared for joint publication) suggests it will not be susceptible to this damage mechanism, and so could provide protection to U3Si2 while maintaining the benefits of an ATF fuel material (high thermal conductivity, high uranium density etc). Preliminary work has produced several test pellets by coarse mixing U3Si2 and UB2 powders in various weight percentages. On investigation, the boride and silicide phases do not have a detectable interaction layer around them, and the UB2 phase appears to have been successfully sintered within the U3Si2 phase. Preliminary steam tests on these pellets shows the steam reaction with composite pellets onsets at significantly higher temperatures than pure U3Si2 and appears to react much more slowly. This suggests that the presence of UB2 within U3Si2 may do more than simply limit the physical pathways by which U3Si2 is attacked by steam, as it appears to change the reaction mechanism in some way as noted by the lack of any reaction below 500C. The proposed work will build upon these preliminary studies and assess the feasibility of utilising UB2 within U3Si2 fuel material to improves its steam-corrosion resistance to acceptable levels. This will include pellet manufacture and characterisation, steam testing, proton irradiation tests, autoclave studies and characterisation of the effects of temperature cycles.