Thorium 4 - fuel cycle
The thorium fuel cycle is based on the use of the common isotope Th-232 as what is called the fertile material in the reactor. Natural thorium is not fissile meaning that it cannot create self-sustaining reactor. The Th-232 must be transmuted into the fissile material, U-233, via neutron capture. The U-233 is the actual nuclear fuel for the reactor.