The fully ceramic core of a High-Temperature Gas-Cooled Reactor (HTR) consists of graphite internals and graphitic fuel elements that contain coated particles. These are fissile ker nels that are surrounded by several coating layers: in the TRISO (tri-isotropic coated) particle there is a SiC interlayer interspersed between two dense pyrocarbon layers. While coatings originally had only been introduced for the handling of the fuel, they have been optimized in the meantime to withstand operational gas pressures, to retain fission products generated during irra diation and to keep them inside in accidents. The latest development showed the dense coating layers also to be efficient in long-term storage of HTR fuel. Graphite is used for the core structure of HTRs as a stable and strong material, with standing temperatures of well over 2000°C and a complete lack of interaction with the fuel and with the helium coolant.
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