Study On The Microstructure Of Nuclear Graphite And Its Evolution Under Ion Beam Irradiation

Nuclear graphite is an important candidate material for the moderator and reflector in Molten Salt Reactor(MSR)due to its high neutron moderator ratio,high-temperature strength,and excellent chemical compatibility.Due to the porous nuclear of artificial graphite,it may be infiltrated while contacting with the molten fuel salt in the molten reactor,which may lead to hot spots in the component and degradation of the material.Hence,it is important to study the pore structure and the characteristics of the pore structure,so as to provide a reference for the selection of nuclear graphite materials in the MSR.On the other hand,nuclear graphite undertakes high doses of neutron irradiation during its service in the MSR,which changes the volume and properties of graphite greatly.These changes are closely related to the microstructures of nuclear graphite and their evolution in the irradiation environment,which are worthwhile for further study to understanding the behavior of nuclear graphite in reactors.In this work,the pore structure of several graphite grades has been studied.The pore network of different grades of nuclear graphite was obtained by using micron 3D imaging technology.It was found that the pore throat size and coordination number of nuclear graphite are important factors for the fluid flow in nuclear graphite.Due to the small pore throat size of NG-CT-50 nuclear graphite,it has a low molten salt permeation rate according to the simulation.Furthermore,by using nanometer 3D imaging technology based on synchrotron radiation facility,the 3D image of the microcracks in nuclear graphite was obtained,which enhanced our understanding of the microcracks in nuclear graphite.Two grades of nuclear graphite,IG-110 and G1,were irradiated by 30Me V Ni⁵⁺ion beam at high temperature(400?),and the microstructure evolution of these two graphite grades under irradiation was studied.The electron probe micro-analyzer(EPMA)was used to measure the distribution of the implanted Ni element in the samples and determine the ion ranges of the ion beam in these two graphite grades.The cross-sections of the irradiated samples were investigated by metallographic microscope,scanning electron microscope(SEM)and transmission electron microscope(TEM).It was found that the size of the microcracks in the irradiated region contracted and the number of nano-sized pores decreased.The amorphization degree of nuclear graphite was characterized by selected-area diffraction,and it was increasing with the increase of depth and reached the maximum at the end of the ion range.Besides,Raman spectroscopy was also used to study the microstructure evolution of nuclear graphite under irradiation.The Raman 2D mapping of the sample cross-sections showed that the full width at half maximum(FWHM),peak position and intensity of the D and G peaks of the Raman spectrum varied with the irradiation depth.The intensity ratios of peaks D and G(I_D/I_G)in Raman spectra increases with the increase of irradiation depth within the ion ranges,consistent with the calculated defect distribution in graphite.Meanwhile,the I_D/I_Gincreases faster in the shallow region than that in the deep region,indicating that the irradiation defects in graphite irradiated at high temperature will be saturated at a high dose.The Raman spectra of the samples irradiated at different temperatures(400?and 600?)were compared.The results suggest that the radiation damage of graphite can be relieved to a certain extent at high temperatures.