Hyphenated SEM-Raman Characterization for Matrix Graphite of HTGR Spherical Fuels

Abstract

Matrix graphite (MG) is an important material for the spherical fuel elements of high temperature gas-cooled reactors (HTGR). After carbonization, the matrix material is composed of three isomers of natural graphite, artificial graphite, and carbon derived from phenolic resin (resin carbon) with the same element but different structure. According to previous study, the three components have different effect on the fission products diffusion and adhesion in the irradiated fuel. However, the characterizing methods to discriminate natural graphite, artificial graphite and resin carbon are rare. In this study, Raman Imaging and Scanning Electron (RISE) microscopy was used to study the structure and distribution of natural graphite, artificial graphite, resin carbon and graphite oxides produced from electrochemical deconsolidation of MG in post-irradiation examination (PIE). Natural graphite and artificial graphite have only a couple of Raman-active bands visible in the spectra between 1100-2400 cm-1, including the strong G band at ~1580 cm-1 and the weak D band at ~1350 cm-1. The D band is activated once the defects are introduced into sp2 hybridized carbon networks, and the intensity of D band is proportional to the defect concentration. Resin carbon has a strong D band, which can be easily distinguished from natural graphite and artificial graphite. The results show that the resin carbon is evenly coated on the surface of the natural and artificial graphite particles, and the network frame is formed during the pressing process to fix the graphite particles. This work also revealed that the changes of surface morphologies of MG after electrochemical deconsolidation by In-situ SEM analysis, of which a lot of cracks and holes on the surface of MG. Meanwhile, Raman spectra suggest that crystalline graphite and resin carbon have different oxidation behaviors. This work will provide a new method for MG components discriminating.