Some nuclear reactors are called fast breeders because: 1. they use fast neutrons, which means the neutrons have more energy than "thermal neutrons", that have the same energy as the surrounding material. 2. they are called "breeders" because they "breed" fuel. U238 is not a fissile atom, but by absorbing a neutrons and spitting out two electrons out of the nucleus (so that two neutrons can turn into protons), it can be turned into Pu239, which is fissile. Fast breedeers are not so common. Though there is a large one in Belayarsk Russia currently working and another in Monju Japan, France stopped its own (Superphenix) some years ago. The trouble with breeders is you need more neutrons. If your reaction is based on U235, a naturally occurring atom, you just need one neutron to make it fission and get energy. If it is based on U238, you'll need two neutrons, one to turn the U238 into Pu239, another to fission the Pu239. The problem is solved by working with fast neutrons, because high energy fissions produce more neutrons. Thorium 232, an extremely abundant element, is fertile too. If it absorbs a neutron and spits two electrons out, it becomes U233, which is fissile. So I naively supposed that the possible reactor of the future, the utterly fascinating LFTR (Light Fluoride Thorium Reactor), would need fast neutrons too. But apparently, it uses thermal neutrons (for example, you can see this page, which looks serious though it is not written by a specialist: http://www.2112design.com/blog/lftr/) It seems to me there will never be enough neutrons. Do you have any solution to that?