多孔二氧化铀/纳米碳化硅全陶瓷微密封核燃料快速制备及高温失效研究

Fully ceramic microencapsulated (FCM) fuel is considered as one of the most promising accident-tolerant fuels (ATF) due to its excellent properties, for example, high thermal conductivity, low coefficient of thermal expansion, stable high temperature performance, good oxidation-resistant, good irradiation stability and high fission products retention. In view of the low uranium loadage, complex producting process and high cost for the current FCM fuels, an innovative idea was put forward to rapidly preparing FCM fuels with good accident tolerant performance and significantly improved uranium loadage. Specifically, the tri-structural isotropic (TRISO) fuel particles embedded in SiC matrix were replaced by porous UO2 particles, dense nanostructual SiC matrix was applied to enhance the high temperature properties and abilities of isolating and detaining the fission products. High-energy fast sintering technology (Dual Field Assisted Spark Plasma Sintering, DFA-SPS) was used to maintain nanostructure of the SiC matrix ,increase the density, and produce porous UO2/nano-SiC FCM fuels. The study will focus on several critical scientific issues such as, influence of DFA-SPS treatment on densification, microstructure evolution processes of the FCM fuels, influence factors, mechanisms and prediction models of the thermophysical properties of the FCM fuels under ultra-high temperature (up to 2800 °C), stress distribution, failure process and mechanisms of the FCM fuels under ultra-high temperature. The research results will have guiding significance for designing and safety assessment for the ATF systems.

全陶瓷微密封（FCM）核燃料具有热导率高、热膨胀系数小、高温强度好、抗氧化性好、辐照稳定性好、裂变气体密封性好等优点，是最有潜力的事故容错核燃料（ATF）之一。本项目针对目前FCM燃料芯块铀装量低、制备工艺复杂、生产成本高等问题，提出用多孔UO2颗粒取代三结构各向同性多层包覆颗粒（TRISO），通过致密的纳米SiC基体来进一步提高芯块的高温性能和裂变产物隔离性能，采用双重电场辅助放电等离子体烧结（DFA-SPS）来保持SiC的纳米结构，提高致密度，快速制备高铀装量的多孔UO2/纳米SiC基FCM燃料芯块。重点研究DFA-SPS高能快速烧结处理对芯块致密化，结构演变等的影响、超高温（2800℃）条件下FCM芯块热物理性能的影响因素，作用机理和预测模型、FCM芯块在超高温环境中的应力分布，失效过程与机制等关键科学问题。本研究可望为新一代ATF燃料体系的设计与评估提供重要的理论依据和技术支持。

全陶瓷微密封燃料因为具有特殊的结构特征，安全性能相对于传统核燃料大幅上升。然而采用具有多层包覆结构的燃料颗粒使得燃料芯块中的铀装载量大幅下降。这也显著缩短了反应堆燃料组件的换料周期，反应堆运行和维护经济性大打折扣。针对该问题，本项目提出采用具有多孔结构的二氧化铀颗粒取代具有多层包覆结构的燃料颗粒，采用纳米级的碳化硅作为燃料芯块的陶瓷基体，先将碳化硅粉包覆到二氧化铀球形颗粒表面，再通过放电等离子体快速烧结技术得到具有多孔结构的二氧化铀燃料颗粒弥散分布于致密的碳化硅基体中的增强型全陶瓷微密封燃料。在燃料芯块体积不变的情况下铀装量可提升5倍以上。通过系统研究制备工艺与参数对芯块结构性能的影响来获得最优的工艺方法与相应的技术参数。通过深入研究芯块在室温、正常运行工况、以及近事故工况高温下的热导率、热膨胀系数等热物理性能来评估其在各种条件下的性能水平。通过强制高温失效处理等手段对芯块的高温失效过程与机理进行试验研究，为研究该类型芯块在严重事故工况下的失效过程以及事故容错性能提高重要参考。通过上述研究为开发高安全性、高经济性、高性能的新型事故容错燃料芯块提供一定的理论与实验依据。