Study Of Microstructure Evolution Of Ultrafine Grained Graphite

The Molten Salt Reactor (MSR) is one of the Generation IV reactors, with the advantage of inherent safety that based on physical principles. Nuclear graphite is mainly used as neutron moderator and structural material in MSR with the means that molten salt flows through the graphite channels. Nuclear graphite is faced with fast neutron irradiation environment in his lifetime in reactors and carbon atoms will be displaced from their lattice sites for many times, leading to property changes of graphite, such as dimension, thermal conductivity, electrical resistivity and strength. This will directly affect lifetime of graphite in reactors. The performance of the graphite under fast neutron irradiation is closely associated with the defects produced in the graphite such as interstitial clusters and vacancy clusters. Therefore, it is important to establish the relationship between characterization of the changes in the physical properties and radiation-induced defects, which will be helpful in understanding the mechanisms of ion irradiation and predicting the property changes in new graphites. As we know, a new kind of graphite cannot be called nuclear graphite but through the neutron irradiation test. It is essential for the application of a new nuclear graphite to pass through irradiation effect assessment. Although the irradiation effect of graphite materials evaluation is a must, the neutron irradiation is limited by many factors and it is also expensive. Relatively, ion irradiation can be easily achieved. So we use ion irradiation as a surrogate of neutron irradiation. The efficient characterization methods of ion irradiation (MeV) effects may be successful for the preliminary assessment on the newly developed super ultrafine grained isotropic graphite materials promising for the MSR.Therefore, xenon ion irradiation is used in this paper, making a discussion of the connection between microstructure and mechanical properties after ion irradiation, so as to provide a reference method and primary irradiation assessment for new graphite. Three parts were included.Firstly, the irradiation induced damage on the PyC was investigated by 7 MeV 129Xe26+irradiation. The irradiation effects were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and nano-indentation. The surface of PyC appeared slightly smoother after exposure to ion irradiation, and high irradiation dose lead pealing of the PyC coating, which indicating that the surface microstructure of the graphite was damaged by 129Xe26+bombardment. The Raman studies indicated a rapid decrease of in-plane "crystallite size", and increase in the interstitial and vacancy defects. The enhancement in the hardness and modulus can be attributed to the pinning of basal plane dislocations by lattice defects produced by 129Xe26+ irradiation.Secondly, the irradiation induced damage on the ultrafine grained isotropic graphite prepared from mesocarbon microbeads(NPIG)was investigated by 7 MeV 129Xe26+ irradiation. The changes of microstructure and mechanical properties were characterized using SEM, XRD, Raman spectroscopy, and nano-indentation. The surface of NPIG graphite changes from complete structure to spheroidal micro-particles. Raman studies indicated a continuous increase in the vacancy defects with a rapid increase in the initial stage of irradiation. The rapid increase in the modulus in the initial stage of irradiation is consistent with the rapid increase in defects, indicating that the initial rapid increase in modulus can be attributed to the pinning of basal plane dislocations by radiation induced lattice defects.Thirdly, although our graphite is made from mesocarbon microbeads and we have not added binder phase, fillers as well as QI particles were observed in TEM, the structure of QI particles is very complicated. The TEM images revealed clearly and convinced the evidence for the increase in defect clusters, basal plane bending, and basal plane dislocations after irradiation, which might be the main reason for the property changes of graphite.