事故容错燃料包壳候选材料的研究现状及展望

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. Compared 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 metallic 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 resistance 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 researchers 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 molybdenum, 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 materials, a property database and an integrated series of standards are of great necessity to evaluate the quality of the materials. In addition, 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 under accident conditions and engineering applications. It also gives critical discussions about the current research situation and the potential key issues of each candidate material.