Topic summary
TRISO fuel
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 MWeHTR-PM pebble-bed HTGR.