BREEDER REACTORS:
Another element found in nature is 90Th232 .
This can be used in a breeder reactor, since
on1 + 90Th232 90Th233 which decays to 92U233 which is a fissile material with half life of 162,000 years!
To run a breeder reactor, we run a normal reactor and put the Th-232 or the U-238 around the reactor to absorb neutrons. This otherwise useless material then becomes a fissile material (a fuel).