Fast breeder reactors are a type of reactor that can utilize natural Uranium-238 as fuel and do not moderate neutrons like other designs. High energy neutrons convert U-238 into fissile plutonium which generates heat. The main advantage of this system is that most of the products of nuclear fission can be bred into fissile material which can greatly reduce waste. Furthermore, the waste that is produced in general has a much shorter period of radioactivity due to the lack of transuranic elements.
Currently all fast breeder reactors use liquid sodium as a coolant. This is because water acts as a moderator for neutrons which slows the neutrons and prevents them from breeding more fuel.
Thermal Breeder Reactor
Thermal breeder reactors are similar too faster breeder reactors which breed fissile material from other non-fissile elements. In fast breeder reactors this is Uranim-238 while in thermal breeder reactors this is Thorium-232. These reactors also differ from fast breeder reactors in that neutrons must be moderated in order to breed fuel which allows the use of water as a coolant.