Unfortunately, the amount of naturally occurring uranium-235 and plutonium-239 is very small, while the amount of uranium-235 is considerable, but it is also quite rare. In any specimen of natural uranium, only seven out of every 1 uranium atoms are uranium 235, and the rest are uranium 238.
uranium-238 is the most common kind of uranium, but it is not a practical nuclear fuel. Uranium 238 can also undergo fission under the action of neutrons, but only fast neutrons can do this. Those uranium-238 that split in half will produce some slow neutrons, which are not enough to cause further fission. Uranium 238 can be compared to wet wood: you can burn it, but it will go out eventually.
However, assuming that uranium 235 is separated from uranium 238 (which is a rather arduous task) and an atomic nuclear reactor is built with uranium 235, at this time, the uranium 235 atoms that make up the reactor fuel will split
and emit countless slow neutrons in all directions. If the reactor is wrapped in a shell made of ordinary uranium (most of which is uranium-238), the neutrons injected into this shell will be absorbed by uranium-238. These neutrons can't force uranium 238 to fission, but they will make other changes in uranium 238, and finally plutonium 239 will be produced. If these plutonium-239 are separated from uranium (a fairly easy task), they can be used as practical nuclear fuel.
a reactor that can generate new fuel to replace the used fuel in this way is a breeder reactor. A properly designed breeder reactor produces more plutonium-239 than uranium-235. In this way, all uranium on the earth-not just the rare uranium 235-can be turned into a potential fuel source.
naturally occurring thorium is completely composed of thorium 232. Thorium-232, like uranium-238, is not a practical nuclear fuel, because it needs fast neutrons to fission.
however, if thorium 232 is placed in the shell surrounding the nuclear reactor, thorium 232 will absorb slow neutrons, and although it will not fission, it will eventually become uranium 233. Because uranium-233 is a practical fuel that can be easily separated from thorium, the result of this is another breeder reactor, which will turn the existing thorium resources on the earth into potential nuclear fuel.
The total amount of uranium and thorium on the earth is about 8 times more than that of uranium-235. That is to say, if the breeder reactor is used properly, the potential energy on the earth can be increased by 8 times through the nuclear fission power plant.