The high temperature thorium fuelled reactor is a special type of the gas cooled reactors. Only one of this type operated until now, between 1985 and 1989 in Germany. The thermal power of the reactor was 760 MW while the electrical power was 307 MW, with an efficiency of 40.5%. (This is very high taking into account the light water moderated reactors' 32 to 33 percent efficiency.) The other advantage of this type is the outstanding safety.

The fuel elements of THTR-300 were balls of 6 cm diameter, in each of which 35 000 smaller balls (diameter between 0.5 and 0.7 mm) were found. Each small ball contained a certain amount of U-235 and ten times this amount of Th-232, as breeder material. Upon neutron capture, U-233 is produced from Th-232, which is fissionable for slow neutrons. Correspondingly, during its operation the reactor itself produced part of its fuel. The moderator was graphite, in the form of layers between the small balls.

The heat produced in the reactor was conveyed using helium, which entered the reactor at the top at 250 oC and exited at the bottom at 750 oC-on (the name is related to this high temperature, which is responsible for the high efficiency). The helium gave its heat to a water-steam loop in six heat-exchangers (only two are visible in the figure). In order to control and shut down the reactor, 51 control rods could be inserted between the balls from the top.