TRISO Fuel
One of the unique features of the high temperature gas-cooled reactor is the TRISO fuel used for the fission reaction. All high temperature gas-cooled reactors use small (~1mm diameter) fuel particles to accomplish the nuclear reaction. These small fuel particles have a kernel of enriched uranium in the form of an oxide (or oxycarbide). The kernel is subsequently coated with a porous carbon layer (to absorb fission gases), a dense pyrolytic carbon layer, a silicon carbide layer and finally another pyrolytic carbon layer.
The coatings surrounding the kernel of TRISO particles produce a very robust fuel form by acting as the containment boundary for the radioactive material. These coatings work in much the same way as the massive reinforced concrete structure surrounding the light water reactors currently in service. The TRISO coatings confine the fission products within the fuel particle.