Characterization and quality control of pyrolytic carbon layers in coated particle fuels for HTR : a review

The TRISO(tri-isotropic)-coated fuel particle for a HTR (High Temperature gas-cooled Reactor) is composed of a fuel kernel and four different outer coating layers, which consist of a buffer PyC (pyrolytic carbon) layer, inner PyC layer, SiC layer, and outer PyC layer. Among these coating layers, the SiC layer has been investigated mainly for its deposition mechanism and consequent properties as well as property changes during irradiation since its early utilization in coating materials for HTR fuel. A recent review on SiC has established a firm database. Early works indicated that the most important properties influencing a coated particle fuel performance in a TRISO-coated fuel design were based on the pyrolytic carbon layers since the SiC layer was shown to be extremely durable and stable during an irradiation, as pointed out by Harmon and Scott. In this work, important characteristics of the pyrolytic carbon layers are reviewed together with their quality control items to be inspected in view of the fuel fabrication and material properties