TRISO fuel

Tri-structural Isotropic (TRISO) fuel is a form of micro-particle nuclear fuel. Each particle consists of a kernel of UO2 fuel (sometimes UC or UCO), which has been coated with four layers of three isotropic materials deposited through fluidized chemical vapor deposition (FCVD). TRISO fuel particles are designed not to crack due to the stresses from processes (such as differential thermal expansion or fission gas pressure) at temperatures up to 1600 °C, and therefore can contain the radioactive fission products even during severe accidents.

The four layers are 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 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.

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