Breeder reactors are a type of nuclear reactor which produce more fissionable materials than they consume. They are designed to extend the nuclear fuel supply for the generation of electricity, and have even been mistakenly called a potential renewable energy source. Breeder reactors certainly have the ability to make nuclear fuels quite sustainable though which was Dr. Cohen's main point, see renewable and sustainable energy for a more thorough explanation. Concerns about nuclear weapons proliferation have been one large impediment to creating commercial breeder reactors.
Unlike normal reactors which only use uranium-235 as their fuel, which is only available in scarce concentrations of around 0.7% of natural uranium without enrichment, breeder reactors also make use of natural uranium-238 which is much more common. They can use approximately 70% of the uranium-238 for production of power, whereas normal reactors can only use around 1% of it. They can also use thorium-232 to breed uranium-233, another fissionable product.
The first experimental breeder reactor (EBR-1) developed was in 1951 in Idaho, U.S.A. Subsequently Russia, Japan, Great Britain and France all developed experimental breeder reactors, however no nation has developed one suitable for high-capacity commercial use. So far, France has made the largest implementation of breeder reactors with their Super-Phenix fast breeder reactor.
There are two categories of breeder reactors, based on the speed of the neutrons. Fast breeder reactors which use uranium-238 as fuel and thermal breeder reactors which use thorium-232 as fuel. Fast breeders do not require moderation since the neutrons need to be moving fast, whereas thermal breeders make us of moderation to achieve slower-moving neutrons.
The most promising type of breeder reactor is the Liquid Metal Fast Breeder Reactor (LMFBR), which operates by using liquid sodium as its coolant, and breeds plutonium from uranium-238. It works by using highly enriched uranium, between 15-20% uranium-235 content, surrounded or "blanketed" by natural uranium-238 in the reactor core. No moderator is used to slow down the neutrons, because fast neutrons transmute uranium-238 much more efficiently than slow neutrons.
Using water as a coolant would reduce the neutron abundance, since neutrons are absorbed by water. Therefore liquid sodium is used instead. This immediately raised concerns of safety when initially thought of, since sodium is a highly reactive element. It is important to keep the liquid sodium from contact with air or oxygen to avoid explosions, however they aren't any more dangerous than pressurized water reactors. This is because the sodium doesn't need to be pressurized to remain in a liquid state like water does; its boiling point is 892oC. This makes the bursting of pipes far less likely than in other water-reactors. Liquid sodium is also a very good choice because of its heat transfer capabilities, due to its high specific heat capacity.
The possibility to breed fissile material in slow neutron reactors is unique to thorium, as uranium cannot use thermal neutrons to do so. These reactors would need a fissile material to start the breeding just like fast breeder reactors, however the neutrons produced from this fissile material would need to be slowed down by a moderator.
The technology is much simpler than that of the liquid metal fast breeder; light water is used as the coolant to remove the heat produced by the continuous series of fission reactions rather than a liquid metal system.
Thorium hasn't been used in large scale reactors, however some reactors have used it successfully in the past. A light water breeder reactor in Shippingport, Pa. USA operated for 5 years, and by the end of its operation it had 1.4% more fissile fuel than it began with.
An important concept for a breeder reactor is how much fissionable fuel is being produced compared to how much fuel is being used. This is known as the breeding ratio. For example for the breeding of plutonium, the ratio would be the amount of plutonium produced to the amount of uranium-235 used. In the liquid metal fast breeder reactor (LMFBR), the breeding ratio is 1.4, however the actual achieved ratio is around 1.2.
The number 1.4 is based off of the average number of neutrons given off by a fission reaction of uranium-235, which is 2.4. Only 1 neutron is needed for the fission chain reaction to be stable, so the remaining 1.4 neutrons (on average) could be used for the breeding of uranium-238.
The amount of time for a breeder to produce enough material to fuel a second reactor is called its doubling time. Present goals for a breeder reactor's doubling time is 10 years, which means it would operate for 10 years producing energy and breeding fissionable material, after which this material could be used to produce the same energy output for another 10 years.