A nuclear reactor that uses nuclear fuel in a graphite matrix and helium as a coolant, which allows it to reach an output temperature of more than 700 °C. This makes it possible to use a more efficient gas turbine to generate electricity, or to use the output heat for other processes such as hydrogen production. Uranium fuel for HTGR takes the form of small spheres coated with several layers of graphite, pyrolytic graphite, or other compounds. The spheres are embedded in a graphite moderator in the shape of balls (pebble bed type reactor) or hexagonal blocks (prismatic block type reactor). In 2023, only one HTGR reactor was in operation.