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Abstract
In a series of irradiation examination on VHTR reference fuels, failure fractions of the coated particles have been measured in the acid-leaching method. All of the failure fractions of the coated particles irradiated in the form of compact were under an allowable limit designed for VHTR fuel. Comparing the failure fractions of the particles in the compact and in the loose form, the former fraction was about an order of magnitude as low as the latter, suggesting that the graphite matrix of the compact might have an effect on protection of coating layers. The metallography of the post-irradiation examination revealed irradiation-induced densification of the buffer layer, which caused debonding of this layer from inner PyC layer or occurrence of radial cracks in this layer. It was found that in the limited case the crack extended to SiC layer, then leading to total coating failure.