HTGRs

HTGRs, or high-temperature gas-cooled reactors, are a class of nuclear reactors that use graphite as the moderator and helium as the primary coolant. They are designed to operate at high outlet temperatures, typically in the range of 700 to 950 degrees Celsius, and can be built in modular or larger configurations. The fuel is usually in the form of TRISO-coated particles embedded in pebbles (pebble bed) or in prismatic blocks, which provides robust fission-product containment and high burnup.

Core designs fall primarily into two formats: pebble bed and prismatic block. Both use graphite to slow neutrons and helium to transfer heat. The high-temperature capability enables electricity generation via indirect or direct gas cycles, including Brayton cycles with gas turbines, and makes HTGRs suitable for process heat applications such as hydrogen production, desalination, or chemical processing, in addition to electricity production.

Safety features are a key attribute. The combination of a strong negative temperature coefficient, solid graphite moderation, and the inert, chemically simple helium coolant contributes to inherent safety characteristics. In many designs, passive cooling and heat dissipation mechanisms reduce the likelihood of dangerous scenarios without active intervention.

History and status: HTGR concepts were developed in the mid-20th century in several countries, with notable projects and experiments in Germany, the United States, and Japan. China has pursued HTGR development with demonstration reactors and modular concepts, and other nations have pursued related high-temperature reactors. As of the 2020s, several prototypes and demonstration facilities exist, but large-scale commercial HTGR deployment remains limited.