Microstructure of a pyrolytic carbon coating on a nuclear graphite substrate IG-110

Abstract

The molten salt reactor (MSR) is one of the six Generation IV reactors that is being reexamined today, owing to its unique fuel cycle capabilities and safety characteristics. IG-110 nuclear graphite a candidate material for constructing a MSR. However, the existence of large pores at its surface is a big problem due to the impregnation of molten salts and the diffusion of fission product gases into the graphite through the pores. A pyrolytic carbon (PyC) coating can act as a barrier coating on the nuclear graphite. Investigation of the microstructure and growth characteristics of PyC is very important for an understanding of the relationship between microstructure and performance. In this study, polarized light microscopy, scanning electron microscopy, transmission electron microscopy and synchrotron based grazing incidence X-ray diffraction were used to study the microstructure and growth characteristics of the PyC coating. Results show that the PyC coating shows three growth cones (a large cone, a smaller one and a regenerative cone) and exhibits a wave-like layered structure. The resulting structure is fairly dense. There are two kinds of textures in the PyC coating: smooth laminar and regenerative laminar, each of which contains two crystal structures with different interlayer spacings. The smooth laminar carbon has mostly a low degree of graphitization, while the regenerative laminar structure mainly has a high degree of graphitization. The PyC coating is a perfect barrier to gas infiltration due to its compact structure and it containing only nanopores rather than large pores.