High-temperature reactor and very high temperature nuclear reactor

A key attribute of the very high temperature reactor (VHTR) concept is its ability to produce the higher temperatures (up to 1 000 °C) needed for large scale hydrogen production using thermo-chemical cycles and some high temperature process heat applications. Also, the high temperatures allow for very high efficiencies together with a Brayton cycle. However, VHTRs would not permit use of a closed fuel cycle. Reference designs are for around 250 MW of electricity, or 600 MW of heat, with a helium coolant and a graphite-moderated thermal neutron spectrum. Fuel would be in the form of coated particles, formed either into rods or pebbles according to the core design adopted. VHTR designs are based on prototype high-temperature gas-cooled reactors built and operated in the United States and Germany, and much R&D has been completed. Key challenges for the VHTR (temperature 1 000 °C) include developing improved high temperature-resistant materials, and the fuel design and manufacture. In the meantime, the technology exists to build and operate High Temperature Reactors (with outlet temperatures up to 750-900 °C). China has constructed the HTR-PM (750 °C, with Rankine cycle) to be connected to the grid in 2021. Japan has a test reactor (HTTR) which operated for several hours with 950 °C outlet temperatures and was shutdown after the great earthquake in 2011. The reactor was restarted in 2021.