Reactor type 4 of 10
High-temperature gas reactor (HTGR)
by Joe Miller
High-temperature gas reactors use helium gas to carry heat away from ceramic-coated TRISO fuel. They run much hotter than water reactors, which makes them good for industrial heat, and their fuel can survive temperatures that would melt ordinary fuel.
Plain-English explainer by Joe Miller, a nuclear engineer. Not investment advice. Figures are the latest public numbers we could find and change fast; sources are at the bottom of each page.
What it looks like
How the coolant circulates
- The circulator pushes helium down through the hot core of pebbles or blocks.
- Helium leaves at about 750°C.
- It flows to the steam generator and makes steam at about 565°C, like a modern coal plant.
- Cooled helium returns to the circulator.
The main components
| Component | What it does |
|---|---|
| TRISO fuel particles | Poppy-seed-size uranium kernels wrapped in layers of carbon and silicon carbide that hold in radioactivity up to about 1,600°C. |
| Pebbles or prismatic blocks | Thousands of particles are packed into tennis-ball-size graphite pebbles (pebble bed) or into hexagonal graphite blocks. |
| Graphite moderator and reflector | Graphite slows the neutrons and surrounds the core. |
| Helium circulator | A large blower that pushes helium through the core. |
| Steam generator | Hot helium boils water for a conventional turbine; future designs may drive a gas turbine directly. |
| Fuel handling system | In pebble beds, pebbles drop out the bottom, get checked and go back in the top while the reactor runs. |
Key details
| Item | Detail |
|---|---|
| Coolant | Helium gas |
| Moderator | Graphite |
| Fuel | TRISO, about 8 to 20% enriched (HALEU for new designs) |
| Pressure | About 1,000 psi helium |
| Outlet temperature | About 750°C |
| Efficiency | About 40% |
| Refueling | Online (pebble bed) or batch (prismatic) |
How it is maintained
- Online refueling for pebble beds: pebbles are measured and recycled continuously.
- Helium purity and leak control: helium leaks easily, and moisture or air must be kept out of hot graphite.
- Circulator and steam generator maintenance; Fort St. Vrain in Colorado (1979 to 1989) struggled with circulator and moisture problems.
- Graphite inspection for radiation damage over the plant's life.
Where it runs
China's HTR-PM at Shidao Bay (commercial since 2023); the UK's older CO₂-cooled AGRs; X-energy's Xe-100 planned for Dow in Texas and for Amazon in Washington State; several U.S. microreactors that reached criticality in 2026. Joe Miller's first paper (1975) modeled HTGR fuel management.
Who's writing this? I'm Joe Miller, a nuclear engineer since 1974 and an ASME Fellow. I've run safety analyses for PWRs and BWRs, directed nuclear engineering at a working BWR, and reviewed new reactor designs for the NRC. This site explains every kind of reactor in plain English. My background and papers.