tristructuralisotropic

Tristructural-isotropic (TRISO) is a coating system applied to microscopic nuclear fuel kernels to produce TRISO-coated fuel particles used in certain high-temperature gas-cooled reactor (HTGR) designs and other advanced reactors. The coating provides fission-product containment and structural stability under irradiation and high temperatures.

Structure and materials: The coating stack usually consists of a porous carbon buffer layer that absorbs gaseous fission products and provides strain relief, followed by three structural layers: an inner pyrolytic carbon (IPyC), a silicon carbide (SiC) diffusion barrier, and an outer pyrolytic carbon (OPyC). The term tristructural isotropic refers to the three primary structural layers and the isotropic microstructure of the carbon films.

Fabrication: TRISO particles are produced by sequential chemical vapor deposition processes that deposit each layer onto a small fuel kernel, with layer thicknesses measured in micrometers. The resulting particle is a compact, multi-layer coated fuel entity designed to retain fission products under demanding conditions. Quality control involves imaging and leak-testing to verify coating integrity.

Function and performance: The multi-layer coating confines fission products at high temperatures and during transient events. SiC provides a robust diffusion barrier, while the pyrolytic carbon layers supply mechanical strength and further containment. The design enables sustained operation at elevated temperatures and improves safety margins in hypothetical accidents.

Applications and research: TRISO-coated fuel particles are central to several HTGR concepts, including pebble-bed and modular reactors, and have been explored for high-temperature process heat and hydrogen production. Ongoing research focuses on manufacturing consistency, burnup performance, and long-term fission-product retention.