A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes^[1] because its neutron economy is high enough to breed fissile fuel from fertile material like uranium-238 or thorium-232. Breeders were at first considered attractive because of their superior fuel economy compared to light water reactors. Interest in breeders declined after the 1960s as more uranium reserves were found,^[2] and new methods of uranium enrichment reduced fuel costs. In more recent decades, breeder reactors are again of research interest as a means of controlling nuclear waste and closing the nuclear fuel cycle.

A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes^[1] because its neutron economy is high enough to breed fissile fuel from fertile material like uranium-238 or thorium-232. Breeders were at first considered attractive because of their superior fuel economy compared to light water reactors. Interest in breeders declined after the 1960s as more uranium reserves were found,^[2] and new methods of uranium enrichment reduced fuel costs. In more recent decades, breeder reactors are again of research interest as a means of controlling nuclear waste and closing the nuclear fuel cycle.

Breeder reactors could in principle extract almost all of the energy contained in uranium or thorium, decreasing fuel requirements by a factor of 100 compared to traditional once-through light water reactors. Conventional Light Water Reactors extract less than 1% of the energy in the uranium mined from the earth.^[3] The high fuel efficiency of breeder reactors could greatly dampen concerns about fuel supply or energy used in mining. In fact, with seawater uranium extraction, there would be enough fuel for breeder reactors to satisfy our energy needs for as long as the current relationship between the sun and Earth persists, about 5 billion years at the current energy consumption rate (thus making nuclear energy as sustainable in fuel availability terms as solar or wind renewable energy).^[4][5]

Nuclear waste became a greater concern by the 1990s. Breeding fuel cycles became interesting again because they can reduce actinide wastes, particularly plutonium and minor actinides.^[6] After "spent nuclear fuel" is removed from a light water reactor, it undergoes a complex decay profile as each nuclide decays at a different rate. Due to a physical oddity referenced below, there is a large gap in the decay half-lives of fission products compared to transuranic isotopes. If the transuranics are left in the spent fuel, after 1000 to 100,000 years, the slow decay of these transuranics would generate most of the radioactivity in that spent fuel. Thus, removing the transuranics from the waste eliminates much of the long-term radioactivity of spent nuclear fuel.^[7]

Today''s commercial Light Water Reactors do breed some new fissile material, mostly in the form of plutonium. Because commercial reactors were never designed as breeders, they do not convert enough uranium-238 into plutonium to replace the uranium-235 consumed. Nonetheless, at least one-third of the power produced by commercial nuclear reactors comes from fission of plutonium generated within the fuel.^[8] Even with this level of plutonium consumption, light water reactors consume only part of the plutonium and minor actinides they produce, and nonfissile isotopes of plutonium build up, along with significant quantities of other minor actinides.^[9] Even with reprocessing, reactor-grade plutonium is normally recycled only once in LWRs as mixed oxide fuel, with limited reductions in long-term waste radioactivity.^{[citation needed]}