I have mentioned breeder reactors several times in recent posts so I decided that I should go into detail about exactly what a breeder reactor is. Basically a breeder reactor generates more than enough neutrons to generate power. The extra neutrons create more fissile materials in the reactor. The net result is that breeder reactors actually generate more radioactive materials than they consume. When a conventional light water reactor burns uranium fuel, it only extracts about one percent of the energy in the uranium. A breeder reactor can theoretically extract almost one hundred percent of the energy in uranium.
Early in the Atomic Age, they were attractive because they generated fuel. After the 1960s, interest waned because more uranium deposits were found and new uranium enrichments methods were developed, both of which increase the supply of uranium fuel and reduced the costs. Recently there has been renewed interest in breeder reactors as a possible way of dealing with nuclear waste due to their ability to burn a wide variety of nuclear fuels. There is also a desire to close the fuel cycle by recycling waste into fuel and generating more fuel. The fact the world uranium production has reached a peak and will decline in the future raising nuclear fuel prices also makes breeders attractive.
When uranium fuel is burned in a reactor, there are two types of waste products generated. One type is called fission products which includes different atom that are fragments of heavier atoms that have undergone nuclear decay. There are dozens of elements and hundreds of isotopes in fission products, all of them lighter than the uranium atoms which have been broken up to form them. They cannot undergo fission and so cannot be used as fuel. Most of them have very short half-lives with only a few lasting one hundred years. The other type of waste consists of elements heavier than uranium referred to transuranics that are formed when atoms in the fuel absorb neutrons but do not fission. These transuranics can have half-lives in the thousands of years. So geological disposal of fission products is much less problematic than geological disposal of transuranics. If the transuranics are removed from spent nuclear fuel, most of the long-term radioactivity will be gone.
Light water reactors do generate fissile isotopes of plutonium as they burn nuclear fuel. The fission of the plutonium isotopes provides about one third of the energy generated by the reactor but the plutonium created does not replace the uranium-235 that has been consumed. Even so, transuranics are still left in the spent fuel. If this fuel is recycled once as what is called mixed oxide fuels, most of the transuranics still remain. The term "conversion ratio" is used to indicate the relationship of the number of fissile atoms created per fission event to the number of fissile atoms destroyed. The conversion ratio of a common light water reactor is about .6. The conversion ratio of a pressurized heavy water reactor is about .8. Breeder reactors have conversion ratios of that vary from 1.01 to 1.2. Theoretical models indicate that conversion ratios of as much as 1.8 are possible. Obviously, the idea of getting twice as much fuel out of a reactor as you put in will be increasingly attractive as the supply of mined uranium goes down and the price of uranium fuel increases.
