Every time an atom of uranium is split inside an atomic power plant, it releases three neutrons that move out at the speed of light in search of other uranium atoms to continue the chain reaction.
So rapid is this process and so much heat does it generate that conventional nuclear plants are built in ways to slow down and contain as many of these released neutrons as possible. One way to do this is to cover the uranium fuel core with graphite or water to catch the escaped neutrons. Another way is to clad the uranium fuel elements with stainless steel, which works to slow down the escaping neutrons.
In a fast breeder like the proposed $2 billion Clinch River plant, the nuclear fuel "bundle" is built to let as many neutrons as possible escape at the fastest speeds possible. No slowdowns. No captures.
One result is that the number of split atoms doubles, which comes close to doubling the heat generated by the fuel. Twice as many uranium atoms split apart in the process, which means that twice as many gain atomic weights and change from uranium to plutonium, the next heaviest element known to science.
A 2,200-pound fuel bundle of uranium will produce about 3,000 pounds of plutonium in less than four years. That's enough to fuel an entire new atomic power plant, which itself will generate a heavier isotope of plutonium to fuel still a third plant. The trouble is, this is also enough plutonium for 100 atomic bombs, the reason the plutonium breeder is so controversial.