About 20 Fast Neutron Reactors (FNR) have already been operating, some since the 1950s, and some supplying electricity commercially.
In contrast to most normal nuclear reactors, a fast reactor uses a coolant that is not an efficient moderator, such as liquid sodium, so its neutrons remain high-energy. Although these fast neutrons are not as good at causing fission, they are readily captured by an isotope of uranium (U238), which then becomes plutonium (Pu239). This plutonium isotope can be reprocessed and used as more reactor fuel or in the production of nuclear weapons.
Fast reactors deliberately use uranium-238 as well as the fissile U-235 isotope used in most reactors.
They can be designed* to produce more plutonium than they consume, called Fast Breeder Reactors (FBR)
There are major problems with breeder reactors mainly that Plutonium-239 is extremely toxic. If an individual inhales a small amount, he or she will contract lung cancer. Also, the half-life of the material is extremely long, about 24,000 years. This could create an almost impossible disposal problem with large amounts of this material being generated.
Main types of Fast Breeder:
liquid-metal, fast-breeder reactor (LMFBR)
Gas-Cooled Fast Reactor (GFR) cooled by helium
Sodium-Cooled Fast Reactor (SFR)
Lead-Cooled Fast Reactor (LFR)
* If the ratio of final to initial fissile content is less than 1 they are burners, consuming more fissile material (U-235, Pu and minor actinides) than they produce (fissile Pu), if more than 1 they are breeders. This is the burn ratio or breeding ratio.