Microstructural evolution of ultrafine-grained graphite studied by in situ electron irradiation

[Background]: Nuclear graphite is utilized as a moderator and structural material in molten salt reactors, where the graphite is exposed to elevated temperatures and substantial fluxes of fast neutron irradiation over extended periods. The consequence of fast neutron irradiation on nuclear graphite is the production of a large number of point defects, which undergo a series of processes including annihilation, diffusion and agglomeration, resulting in the formation of large dislocation structures at elevated temperatures. It is evident that high-temperature neutron irradiation exerts a profound influence on the microstructure of graphite, consequently altering its macroscopic properties. [Purpose]: This study aims to reveal the defect evolution and microstructural changes of nuclear graphite under irradiation. [Methods]: NG-CT-50 nuclear graphite was selected as the research object in this study, which is a microfine grained nuclear graphite candidate for Thorium Molten Salt Reactor. In-situ electron irradiation experiments on NG-CT-50 nuclear graphite were conducted at the range from room temperature to 750 °C using an electron beam of 200 keV in Transmission Electron Microscope. The dynamic changes in the microstructure of the graphite and the evolution of defects were observed by High Resolution Transmission Electron Microscopy. The HRTEM images were noise filtered by Winer and Fourier filtering for better basal plane images. [Results]: The study found that irradiation temperature plays a significant role in the evolution of the microstructure of graphite. High temperatures facilitate the annihilation of point defects induced by irradiation and the movement and recombination of irradiation-induced partial dislocations. The in-situ high-temperature experiments did not reveal the formation of irradiation-induced ‘ruck and tuck’ defects or fullerene-like structures as reported in the literature; instead, the generation of a large number of dislocation loops were observed, a phenomenon consistent with the classical theory of graphite irradiation defects. The evolution of defects at high temperatures can be described as the formation of interstitial or vacancy loops, the growth and decomposition of dislocation loops, the interlayer migration or glide of partial dislocations, and the interaction or recombination of partial dislocations with other defects. [Conclusions]: This research provides a theoretical basis for predicting the high-temperature irradiation performance of graphite and for the optimal design of irradiation-resistant graphite.