Fast Breeders
Certain types of reactors are called “fast breeders”. These use plutonium 239 (239Pu) as a primary fuel. They then add 238U rods into the core, or surround it with a “blanket” of 238U. (The plutonium is extracted from used reactor fuel bundles.) Plutonium fission produces heat, plus a surplus of neutrons. These convert 238U into new 239Pu, “breeding” fresh fuel. Fast breeders make more plutonium than they consume – a supply for other reactors or for military (and terrorist?) purposes!
Fast breeders sound like a good idea. However, left in the reactor, 239Pu continues to absorb neutrons becoming 240Pu & 241Pu, which are not useful fuels. Because of this, a fast breeder must be refueled frequently. These fuel rods are processed to remove 239Pu. This is a poor prescription for cheap energy, but a good way to accumulate 239Pu for weapons! No fast breeders are being built now. Today, the world has a large surplus of plutonium from the nuclear weapons the former Soviet Union and the U.S.A. have been dismantling.
CANDU reactors also produce 239Pu – about half as much as the 235U they consume. They could be called “slow breeders”. CANDUs are also capable of using uranium fuel enriched with plutonium. Not long ago it was proposed that some of the world’s surplus (from dismantled H-Bombs) plutonium might be combined with natural uranium to fuel CANDU reactors. A public outcry against this idea killed it.
Although uranium and other fissionable materials are quite rare in the earth’s crust, their potential energy content is enormous. This may even exceed the energy available from known supplies of fossil fuels.