Dounreay prototype fast reactor, 1982.
© Science Museum / Science & Society Picture Library
Model (scale 1:50). Fast Reactors, or Fast Breeder Reactors, can be used to produce more fuel by breeding plutonium 239 from the non-fisile uranium 238. They get their name from the fact that the chain reaction in the core is carried on by 'fast' neutrons. Usually these reactors are cooled by molten sodium and have no moderator. When a uranium 238 nucleus captures a neutron, it eventually changes by radioactive decay into plutonium 239. The prototype fast reactor at Dounreay in Scotland operated from 1974 to 1994.