What is a fast reactor?

Nuclear reactors produce energy by a process called fission. Fission occurs when an atom of fissile material is struck by a neutron, becomes unstable and splits, producing fission fragments and high-energy neutrons. In an operating reactor, one of these neutrons will strike another fissile atom to maintain a steady chain reaction. Fission also releases heat, which is used to produce steam to spin an electrical generator.

Reactors can be classified according to the energy of the neutrons that cause fission. Present day commercial power reactors are called "thermal" reactors because the neutrons have been slowed to thermal energy using a "moderator"—usually water. By contrast, a "fast" reactor uses neutrons of much higher energy to cause fission. A fast reactor does not have a moderator.

The only fissile material found in nature is uranium-235, which makes up less than 1 percent of natural uranium. While some fissile plutonium is produced in a thermal reactor, it is not enough to replace the uranium-235 used. In a fast reactor, however, enough plutonium can be produced and fissioned to more than make up for the uranium-235 used. In addition, many of the long-lived actinides that cannot be fissioned in a thermal reactor can be burned in a fast reactor, so the fast reactor is capable of destroying the major source of long-lived radiotoxicity in spent fuel. Thus, the fast reactor can create new fuel and destroy long-lived nuclear waste and plutonium while it produces electricity.

For more information, please contact David Baurac.