In the 2011 Fukushima nuclear power plant accident,the zirconium alloy claddings in the reactor fuel were oxidized violently by high temperature water vapor generated under accident conditions,resulting in a large amount of hydrogen and heat, which eventually led to reactor core melt down and hydrogen explosion.The event delivered severe impacts on the human society and the environment.From then on,the development programs of accident-tolerant fuels (ATF)have been initiated worldwide.Com-pared with the traditional UO2-Zr system,ATF can sustain or improve the fuel performance at normal operating conditions,as well as maintain the core integrity for an extended time after accidents to provide temporal safety margins for the accident management. The environment of the reactor core is very harsh,as the claddings are exposed lastingly to high temperature and high pressure corrosion medium,and moreover,neutron irradiation.As a result,if any new materials were to qualify as cladding materials,they would need excellent radiation stability and corrosion resistance.According to the research and evaluation of different researchers,the accident-tolerant fuel cladding materials which has displayed the potential of substituting for Zr alloy can be classified into ceramic materials and metallic materials,in which the ceramic materials are mainly represented by SiC/SiC composite materials,and the me-tallic materials mainly include Fe-based alloy exemplified by FeCrAl and refractory metals exemplified by Mo/Mo alloy. Each of the three materials has its own advantages and disadvantages,and still needs improvements to attain the requirement of engineering application,and all of them have some unresolved key issues.The research and development of the FeCrAl alloy has reached the second generation model alloy.The thermophysical and mechanical properties,corrosion resistance and radiation resis-tance of FeCrAl alloy are outstanding,while the research on hydrogen permeability and industrial processing and welding are still in progress.In the case of SiC/SiC composites,the high brittleness of SiC material leads to inadequate mechanical strength.Hence re-searchers have proposed various structural design schemes to reduce the failure probability of cladding tubes,but the final structural design of the cladding remains undetermined.The sharp reduction of thermal conductivity of SiC/SiC composites caused by irradiation and the joining and fabrication are still under investigation.Mo and Mo alloys possesses excellent mechanical properties and radiation resistance,but exhibits poor corrosion resistance.The present countermeasures are mainly focused on improving the purity of molyb-denum,adjusting the elemental composition of the alloys and adopting surface coating techniques.The processability of these three kinds of cladding tube materials has not reached the level of industrial manufacturing of thin-wall long tubes.For these candidate ma-terials,a property database and an integrated series of standards are of great necessity to evaluate the quality of the materials.In addi-tion,the requirement of in-core behavior assessment also necessitates the development of the corresponding fuel performance code. This paper summarizes the latest research progress on the candidate ATF cladding materials,including their physical properties, corrosion resistance,mechanical behaviors,radiation resistance,pellet-cladding mechanical and chemical interactions,behaviors un-der accident conditions and engineering applications.It also gives critical discussions about the current research situation and the po-tential key issues of each candidate material.%2011年福岛核电站事故中,反应堆堆芯燃料中的锆合金包壳在事故工况下与高温水蒸汽发生剧烈氧化反应继而产生大量的氢气和热量,最终导致反应堆堆芯熔化和氢气爆炸,对社会和环境造成极大负面影响.自此之后,国内外纷纷展开对事故容错燃料的研究开发.相较于传统的 UO2-Zr合金燃料体系,事故容错燃料能够在反应堆正常运行工况下维持或提高燃料性能,并在事故发生后相当长的一段时间内维持堆芯完整性,提供足够的时间裕量来采取事故应对措施.反应堆堆芯环境非常极端,包壳长期处于高温高压腐蚀介质中,同时还受到中子辐照的影响,因此新型包壳材料需要较好的耐腐蚀性和辐照稳定性.经不同研究者的研究评估,目前能够替代 Zr 合金的事故容错燃料包壳材料可分为陶瓷材料和金属材料两类:陶瓷材料主要以 SiC/SiC复合材料为代表;金属材料主要有以 FeCrAl 为代表的 Fe 基合金和以 Mo 为代表的难熔金属及其合金.上述三种替代Zr包壳的材料各有其利弊,均未达到工程应用水平,并且都存在待解决的关键性问题.其中,FeCrAl 合金的研发进展最快,目前在热学性能、力学性能、抗腐蚀性能、抗辐照性能等方面表现较好,但在工业加工和焊接等方面仍有待进一步改善.就 SiC/SiC复合材料而言,由于 SiC自身的高脆性而导致力学强度不足,不同的研究者提出了不同的结构设计思路试图降低包壳管失效概率,但包壳最终的结构设计仍未确定,而辐照引起的热导率急剧降低及连接密封和加工制造等方面还在不断研究中.Mo及 Mo合金的力学性能和抗辐照性能较好,但自身抗腐蚀性较差,解决思路主要集中在提高钼纯度、调整合金的元素成分、进行表面涂层等方面.目前,对后两种材料包壳管的加工能力均未达到薄壁长管的工业制造水平.对于这几种候选包壳材料,需要建立属性数据库和一套完善的标准来衡量材料的质量.此外,还需开发相应的程序来评估包壳在堆内的行为.本文主要综述了 SiC/SiC复合材料、FeCrAl合金、Mo及 Mo合金三种候选包壳材料的研究进展,包括候选包壳材料的物理性质、耐腐蚀性能、力学性能、抗辐照性能、芯块-包壳力学与化学相互作用、在事故工况下的行为和工程应用等,综合分析了事故容错燃料包壳材料当前的研究现状,指出了各事故容错燃料包壳未来需集中解决的关键性问题.
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