High Temperature Gas-Cooled Reactors (HTGRs) use helium gas as the primary coolant and graphite as both the neutron moderator and structural material, achieving reactor outlet temperatures of 700-950 degrees Celsius, far above what water-cooled reactors can deliver. This high temperature capability enables both high-efficiency electricity generation through Brayton cycle gas turbines and direct supply of industrial process heat for applications such as hydrogen production, chemical manufacturing, desalination, and synthetic fuel synthesis. HTGRs exclusively use TRISO fuel, either in pebble bed form (loose fuel spheres) or prismatic block form (fuel compacts embedded in graphite blocks), providing an inherent safety barrier at the fuel particle level.
X-energy's Xe-100 is the leading Western HTGR program. Each Xe-100 module produces 80 MWe (200 MWth) using helium-cooled TRISO pebble fuel, with standard deployment as a four-pack generating 320 MWe. X-energy's TRISO-X subsidiary received NRC Part 70 Special Nuclear Material license approval for its TX-1 fuel fabrication facility in Oak Ridge, Tennessee in February 2026, three months ahead of schedule, with building construction underway since November 2025 and capacity to fuel up to 11 Xe-100 modules. The NRC accepted X-energy's construction permit application for four Xe-100 units at Dow's Seadrift, Texas manufacturing facility in May 2025, marking the first HTGR CP application at a U.S. industrial site. Amazon and Energy Northwest have agreed to deploy up to 12 Xe-100 SMRs at the Cascade Advanced Energy Facility near Richland, Washington. X-energy closed a $700 million Series D in November 2025 led by Amazon, Citadel, Ken Griffin, Ares Management, and Jane Street Capital.
China operates the world's only commercial HTGR: the HTR-PM at Shidaowan, Shandong Province, developed by China Huaneng, CNNC, and Tsinghua University. The HTR-PM uses twin pebble-bed reactor modules driving a single 210 MWe steam turbine and achieved commercial operation in 2023, making it the world's first Generation IV reactor in commercial service. Ultra Safe Nuclear Corporation is developing a Micro Modular Reactor (MMR) producing 5 MWe using helium coolant and Fully Ceramic Microencapsulated (FCM) TRISO fuel, targeting deployment at Chalk River, Ontario. Nano Nuclear Energy (NASDAQ: NNE) is advancing its KRONOS HTGR microreactor toward formal licensing in the U.S. and Canada, with a $577.5 million cash position and an MoU for Gulf region deployment signed in February 2026.