High temperature reactors (HTRs) operate at 750 to 950 C and are normally helium-cooled. Their fuel takes the form of TRISO (TRi-structural ISOtropic) particles, each less than a millimetre in diameter.
Each TRISO particle consists of:
- A kernel (~0.5 mm) of uranium oxycarbide (or uranium dioxide), enriched up to 20% U-235
- Surrounded by layers of carbon and silicon carbide, providing containment for fission products that is stable to very high temperatures (confirmed up to ~1800 C)
These particles are arranged in one of two configurations:
- Prismatic blocks: hexagonal prisms of graphite
- Pebbles: billiard-ball-sized spheres of graphite encased in silicon carbide, each containing about 15,000 fuel particles and 9 g of uranium
In both designs, the moderator is graphite. HTRs can potentially use thorium-based fuels.