These are nuclear reactors that produce more fuel than they consume. Traditional reactors use moderators such as water or gas to slow down the neutrons to optimise energies for causing fission Fast Breeder Reactors use a coolant that is not an efficient moderator, such as liquid sodium, so its neutrons remain high-energy. In the process, the neutrons are 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.