VHTRs

Very High Temperature Reactors (VHTRs) are a subset of high-temperature gas-cooled reactors within the Gen IV family, designed to operate at substantially higher outlet coolant temperatures than other reactors. They aim to supply heat suitable for electricity generation with high thermal efficiency and for industrial process heat, including hydrogen production, at temperatures around 750 to 1000 degrees Celsius.

VHTRs use helium as the primary coolant and a graphite-based core that contains TRISO-coated fuel particles embedded in a graphite matrix. The core can be arranged in prismatic blocks or in a pebble-bed configuration. The inherent safety features of HTGRs, including a low power density and a strong negative temperature coefficient, contribute to passive cooling capabilities in many design concepts. High outlet temperatures enable efficient conversion of reactor heat to electricity via a gas turbine Brayton cycle or to steam cycles, and they enable direct industrial heat applications.

Fuel in VHTRs consists of durable TRISO particles that retain fission products under high-temperature operation. This fuel form supports relatively high burnup and stable performance at the elevated temperatures of VHTRs. Operational schemes emphasize long refueling intervals and modular or larger-scale plant layouts.

Applications of VHTRs include electricity generation with potentially higher thermal efficiency and the supply of industrial process heat, notably hydrogen production through high-temperature electrolysis or thermochemical cycles. Development has been pursued in several countries, with demonstrator and research reactors such as China’s HTR program and various Gen IV studies in the United States and Europe. As of now, no commercial VHTR is in widespread operation, though ongoing research investigates materials, fuel performance, safety, and integrated energy systems.