HTRs

HTRs, or high-temperature reactors, are a class of nuclear reactors designed to operate at higher outlet temperatures than conventional light-water reactors. They typically use helium as a coolant and a graphite-based moderator, with fuel composed of TRISO-coated particles embedded in a graphite matrix. Core designs fall into pebble-bed and prismatic-block configurations, and the strong fuel containment allows higher thermal efficiency and potential for process heat applications.

The fuel forms in spherical pebbles or hexagonal blocks, each containing many TRISO particles that retain fission products at high temperatures. The high-temperature capability enables industrial process heat, including hydrogen production through thermochemical cycles or high-temperature electrolysis, as well as efficient electricity generation when combined with turbines.

HTRs emphasize safety through inherent features. They have negative temperature and sometimes negative power coefficients, low power density, and rely on passive heat removal by natural convection and radiation in many designs. Helium is chemically inert, reducing corrosion risk, and the modular or compact layout can limit radioactive inventory. However, graphite in the core introduces considerations about oxidation if air ingress occurs, and material performance at high temperatures remains a focus of research.

Historically, several countries pursued HTRs, including Germany with early pebble-bed and gas-cooled designs, Japan with the HTTR, and China with the HTR-10 and the demonstration HTR-PM program that couples multiple modules for electricity generation. Research continues in various nations to assess economics, fuel cycle implications, and integration of process heat with industrial sectors. Commercial deployment remains limited, with ongoing development and demonstration projects shaping the technology’s future.