Breeder reactors can create as much or more fissile material (atoms that readily split) than they use. These special reactors are designed to have extra neutrons flying around, so that some can convert U-238 to Pu-239 (see above) and the others can run the reactor. Often, these special reactors are deemed "fast" reactors because the neutrons are moving through the reactor at higher speeds, on average. In a full breeder fuel cycle, we get the maximum use of the Uranium resources on Earth, and what we already know exists can last tens of thousands of years. The cycle has cost and proliferation concerns associated with any closed cycle. Additionally, we have significantly less operational experience with breeder reactors, so we would need to train builders and operators for such a machine. Using a Thorium cycle instead of a Uranium-Plutonium cycle may allow breeding in less exotic reactors. Using this kind of fuel cycle, nuclear power can
truly be considered sustainable.