Tri-structural isotropic (TRISO) fuel is a form of micro-particle nuclear fuel. Each particle consists of a kernel of uranium dioxide (UO2) fuel (sometimes UC or UCO), which has been coated with four layers of three isotropic materials deposited through fluidized-bed chemical vapor deposition. TRISO fuel particles are designed not to crack from thermal or mechanical stresses at temperatures up to 1600 °C, and therefore can contain the radioactive fission products even during severe accidents. Each particle is coated in a porous buffer layer made of carbon that absorbs fission product recoils, followed by a dense inner layer of protective pyrolytic carbon (PyC), followed by a ceramic layer of silicon carbide (SiC) to retain fission products at elevated temperatures and to give the TRISO particle more structural integrity, and sealed by a dense outer layer of PyC. The finished TRISO particles are then embedded into a graphite matrix to form spherical or cylindrical fuel elements. Historically, TRISO has been used in high-temperature gas-cooled reactors (HTGRs), both prismatic-block and pebble-bed. The first reactor to use TRISO was the Dragon reactor, while the first commercial station was the Fort Saint Vrain Nuclear Power Plant, a prismatic-block HTGR. As of 2026, TRISO fuel compacts are being used in some experimental reactors, such as the HTR-10 in China and the high-temperature engineering test reactor in Japan, as well as commercially in the 100 MWe HTR-PM pebble-bed HTGR.
History Coated-particle ceramic fuels were initially developed in the United Kingdom as part of the Dragon reactor project. During the development of the Dragon reactor, its designers became concerned by the need to purge gaseous fission products from the reactor core and their potential migration to other parts of the reactor. This concern led to the choice of coated-particle fuel, where the fuel would be formed from small particles of uranium then coated with pyrolytic carbon. The inclusion of silicon carbide as a diffusion barrier was first suggested by D. T. Livey in 1961, in order to better retain fission products.
Work on coated-particle fuels also took place at the same time in the United States at the Atomic Energy Commission. Peach Bottom Unit 1, a 40 MWe demonstration HTGR, used prismatic coated-particle fuel consisting of highly enriched uranium (HEU) carbide mixed with thorium carbide and coated in a single layer of pyrolytic carbon in its first core. Due to fracturing of the pyrolytic carbon layer, a low-density porous carbon buffer layer was added before the dense PyC layer to absorb fission product recoils and accommodate fission gas swelling. This new design was used in the reactor's second core, and the two-layer particle design was called buffer-isotropic or bistructural-isotropic (BISO) fuel. In Germany, the experimental AVR reactor used 232ThO2-235UO2 BISO fuel, but in a spherical pebble form rather than as prismatic blocks. This was replaced with TRISO in the late 1970s. The later commercial THTR-300 reactor used similar oxide BISO fuel as AVR and ran from 1983 to 1988. The first commercial HTGR, and the first commercial reactor to use TRISO, was the 330 MWe Fort Saint Vrain Nuclear Power Plant. It used prismatic-block 232ThC2-235UC2 fuel similar to Peach Bottom, along with fertile 232ThC2 elements in preparation to investigate a full thorium fuel cycle using 232Th-233U. This fuel used a full four-layer TRISO coating, and the fuel elements performed better than its designers anticipated. However, the plant suffered serious issues with its mechanical components, notably its helium circulators, and achieved an availability of only 14.6%. The experience in the US program with carbide fuel led to the transition to a mixture of 80%-UO2, 20%-UC2, known as uranium oxycarbide (UCO), due to its superior fission product retention compared to pure uranium carbide (UC2). The experimental High Temperature Test Reactor in Japan, constructed in 1998, uses prismatic UO2 TRISO fuel. Tsinghua University constructed a 10 MWth prototype pebble-bed HTGR, the HTR-10, in 2000. It used UO2 TRISO pebbles containing low-enriched uranium and was used as a prototype for the larger 100 MWe HTR-PM small modular reactor, which came online in December 2021. As of 2026, it is the only TRISO-fueled reactor in commercial operation. In the United States, TRISO is being explored for use in the very-high-temperature reactor concept, one of the six classes of reactor designs in the Generation IV initiative that is attempting to reach higher HTGR outlet temperatures. The X-energy Xe-100 pebble-bed HTGR is planning to use spherical pebbles containing TRISO particles containing UCO, while Kairos Power is constructing a 50 MWe pebble-bed molten-salt reactor using UCO TRISO fuel containing high-assay low-enriched uranium.
QUADRISO fuel
QUADRISO fuel is a concept based on TRISO that incorporates a burnable neutron poison (europium oxide or erbium oxide or carbides) layer surrounding the fuel kernel of ordinary TRISO particles to better manage the excess of reactivity. During reactor operation, neutron irradiation of the poison causes it to "burn up" or progressively transmute to non-poison isotopes, depleting this poison effect and leaving progressively more neutrons available for sustaining the chain-reaction. This mechanism compensates for the accumulation of undesirable neutron poisons which are an unavoidable part of the fission products, as well as normal fissile fuel depletion. The concept was conceived at Argonne National Laboratory.
Production TRISO fuel is most commonly fabricated using the sol-gel process, developed at Oak Ridge National Laboratory in the United States. First, uranium or thorium is dissolved using nitric acid, and ammonia is used to precipitate UO2 or ThO2 ("sol"). The sol is then sprayed through a heated organic liquid, where the surface tension forms tiny gel spheres. To form UCO, carbon is dispersed through the gel to promote formation of UC2. Fluidized-bed chemical vapor deposition is then used in several steps to apply the porous carbon, inner PyC, SiC, and outer PyC coatings. The finished TRISO particles are then embedded in a matrix of graphite and resin, then heated and pressed.
See also Power Reactor Demonstration Program Thorium-based nuclear power
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