TRISO (Tri-structural Isotropic) fuel particles are a revolutionary nuclear fuel form consisting of a uranium oxycarbide or uranium oxide kernel approximately 0.5 mm in diameter, surrounded by four concentric layers: a porous carbon buffer, an inner pyrolytic carbon layer, a silicon carbide barrier coating, and an outer pyrolytic carbon layer. The resulting particle, roughly the size of a poppy seed, functions as its own miniature containment system, retaining fission products at temperatures exceeding 1,600 degrees Celsius, well beyond the operating conditions of any reactor design employing them. This inherent fuel-level safety is a cornerstone of the licensing case for several advanced reactor concepts.
TRISO fuel is central to the design of high-temperature gas-cooled reactors (HTGRs) and fluoride salt-cooled high-temperature reactors (FHRs). X-energy's Xe-100 uses proprietary TRISO pebble fuel manufactured by its subsidiary TRISO-X, which received NRC Part 70 Special Nuclear Material license approval in February 2026 for its TX-1 fuel fabrication facility in Oak Ridge, Tennessee. Building construction began in November 2025, with a planned capacity of 5 metric tons of uranium per year, enough to fuel up to 11 Xe-100 reactor modules. Kairos Power's Hermes demonstration reactor and commercial KP-FHR design also employ TRISO pebble fuel in a fluoride salt coolant environment. China's HTR-PM, the world's first Gen IV reactor in commercial operation since 2023, uses TRISO pebble bed fuel with helium cooling, demonstrating the technology at commercial scale at the Shidaowan plant in Shandong Province.
TRISO fuel has also been adopted for microreactor applications. Radiant Industries' Kaleidos portable microreactor uses HALEU TRISO fuel with helium coolant, targeting first criticality at Idaho National Laboratory's DOME test bed on July 4, 2026. Ultra Safe Nuclear Corporation has developed Fully Ceramic Microencapsulated (FCM) fuel, an evolution of TRISO that embeds particles in a silicon carbide matrix rather than a graphite compact, further enhancing structural integrity and fission product retention. BWXT manufactures TRISO fuel for defense and NASA space nuclear applications. The TRISO fuel supply chain is tightly linked to HALEU availability, as most TRISO-fueled reactor designs require enrichment levels between 5% and 20% U-235.