[en] In the U.S. the new-generation HTGRs employ fertile fuel particles consisting of ThO₂ microspheres encapsulated in miniature pressure vessels produced by chemical vapor deposition of carbon and SiC coatings. More than 58 accelerated irradiation capsule experiments have provided data on ThO₂ performance in both BISO and TRISO coated particles irradiated unbonded or bonded with carbonaceous matrix into fuel rods. Temperatures up to 1500⁰C with burnups as high as 20% FIMA have been experienced during irradiation, and postirradiation tests to 2500⁰C have been conducted with the ThO₂ kernel consistently showing stable properties. Fission gas and oxygen release rates from ThO₂ have been characterized in order to quantify the performance margin for coated particle pressure vessel design and to predict release through failed coatings