High Temperature Reactor (HTGR)
3 min read · Energy Atlas Editorial
In a High Temperature Gas-cooled Reactor (HTGR), the highly enriched uranium spheres are dispersed in graphite (moderator) balls. Helium is a coolant.
Modern High Temperature Gas-cooled Reactors (HTGRs) can achieve high thermal-to-electric conversion efficiencies of around 40%. Today, HTGR technology is represented mainly by experimental and demonstration reactors. China's commercial demonstration HTR-PM reactor at the Shidao Bay Nuclear Power Plant entered full commercial operation in 2023.
The fuel consists of TRISO particles containing uranium oxide or uranium oxycarbide. Each fuel kernel, approximately 0.5 mm in diameter, is surrounded by several protective layers of pyrolytic carbon and silicon carbide. After coating, the particles are about 1 mm in diameter and are embedded in graphite fuel elements. In pebble-bed reactors, these take the form of graphite spheres about the size of a tennis ball, known as fuel pebbles. The fuel is continuously loaded into the reactor from above and removed from the bottom after passing through the core. Another design uses hexagonal graphite blocks containing the fuel, an arrangement used particularly in American reactor designs.
Graphite is used as the moderator and also forms a protective matrix surrounding the fuel particles. Helium is circulated through the reactor core as the coolant. Depending on the reactor design, its outlet temperature may range from around 700 °C to 900 °C or more. The thermal energy carried by the helium can be used directly in a gas-turbine Brayton cycle or transferred in a steam generator to produce steam for a steam turbine. The high outlet temperature also makes HTGRs suitable for high-temperature industrial processes, including hydrogen production.
TRISO fuel is one of the most robust types of nuclear fuel. Each fuel kernel is enclosed within several protective layers that act as a miniature containment system and retain fission products even at very high temperatures. TRISO fuel has demonstrated excellent performance in accident testing at temperatures of around 1,600 °C. Together with the low power density and passive heat-removal characteristics of HTGRs, this provides a high degree of inherent resistance to severe fuel overheating.
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