熔盐增殖堆用纳米孔石墨辐照稳定性研究

Molten salt reactor (MSR) is the important high temperature reactor proposed by Generation IV International Forum and our government. Realizing the breeder of the molten salt reactor puts a constraint on the neutron moderator graphite with a pore diameter requirement of less than 100 nm. Mesocarbon microbeads (MCMBs) were used to prepare binderless nanopore-isotropic graphite (NPIG) to inhibit liquid fluoride salt and fission products penetration. Compared to the traditional nuclear graphite using calcined cokes as aggregates and pitch as binder, the pores of the NPIG are much smaller. The amounts and sizes of the graphite microcracks are smaller. There is less space for absorbing the graphite interlayer spacing expansion and the larger interlayer spacing leads to the prone to forming graphite intercalation compound and C-F bonds under irradiation. However, the microstructure and mesoscopic structure uniformity of the NPIG is excellent, which inhibits the structure difference and stress concentration on the interface of the two phases (aggregates and binder) under irradiation. And the small crystallite layers form the micron balls, which leads to the chemical stability of the graphite. It is required to obtain the irradiation behaviors and stability of the NPIG for improving the graphite preparation process and the different theoretical understanding of the MCMBs structure. In the present work, low cost xenon ion irradiation is performed on NPIG and IG-110 graphite. Microstructure, crystal structure, defects and properties of the ion-irradiated graphite under different irradiation dose will be studied to identify the physical structure irradiation stability of the NPIG. Graphite intercalation compounds and C-F bonds in the interface of the graphite and molten salts under irradiation will be studied to obtain the chemical stability of the graphite. It provides scientific basis for the improvement of pores, crystal structure and properties of the nanopore graphite.

熔盐堆是我国重点发展的第四代反应堆。为防止熔盐和裂变产物扩散，实现熔盐堆增殖，慢化体石墨孔径须小于100 nm。项目组用中间相炭微球开发了均质纳米孔石墨，与以焦粉和沥青为原料的传统核石墨差异巨大。该石墨孔径小，微裂纹少且尺度低，辐照下缺少吸收石墨层间变宽的间隙；层间距大，辐照下易与熔盐产生插层和化学成键。但石墨介观均质结构可避免辐照结构差异和界面应力集中；小的微晶片层构成微米级球，化学稳定性好。该石墨是否耐辐照尚存理论认识分歧，且其辐照稳定性亟待研究以指导石墨工艺。本课题拟采用低成本氙离子辐照石墨，对比传统核石墨辐照数据，研究不同辐照剂量石墨晶体结构、缺陷、孔隙和性能等变化，揭示石墨辐照损伤规律，得到其辐照物理结构稳定性；研究辐照下，熔盐与石墨界面化学成键和石墨层间化合物及其对石墨性能损伤，得到石墨辐照化学稳定性。课题旨在研究石墨与熔盐接触前后辐照行为，指导石墨孔径、晶体结构和性能调控。

为防止石墨作为熔盐堆慢化体和反射体，熔盐和气体裂变产物在石墨中的扩散，通过中间相炭微球、生焦、酚醛树脂为原料，分别采用压制和浸渍工艺制备了不同孔径的纳米孔石墨。对比国外成熟的微米孔经IG-110核石墨，研究了新型纳米孔石墨的微观结构、热物理性能和其它性能。通过高压浸渗熔盐和真空透气性测试装置，测试了新型纳米孔石墨的熔盐浸渗特性和气体阻隔特性。结果表明，不同工艺制备的纳米孔石墨能够很好的阻止熔盐和裂变产物的扩散。本研究采用不同注入剂量的氙离子为主，高能氦离子辐照为辅，对不同孔径和微观结构的纳米孔石墨进行了研究，获得了不同辐照剂量下，石墨形貌、缺陷种类及密度、结构和性能变化。中间相炭微球纳米孔石墨石墨化度和热导率较低，热膨胀系数较高，整体辐照性能较IG-110石墨差，低剂量辐照即可导致石墨辐照膨胀。通过生焦、酚醛树脂等制备的纳米孔石墨，由于均采用高石墨化度基体或原料，相对IG-110石墨可获得更高的石墨化度和热导率，更低的热膨胀系数，辐照实验也表明，该类石墨拥有良好的辐照稳定性。