Nuclear-weapons designers require plutonium with a very high plutonium 239 isotopic content, whereas plutonium from commercial power plants usually contains substantial quantities of the other isotopes of plutonium, making it difficult to use in a bomb. Nevertheless, use of plutonium from spent fuel in weapons is not inconceivable. Hence, President Jimmy Carter banned civilian reprocessing of nuclear fuel in the U.S. in 1977. He reasoned that if plutonium were not recovered from spent fuel it could not be used to make bombs. Carter also wanted America to set an example for the rest of the world. France, Japan, Russia and the U.K. have not, however, followed suit, so plutonium reprocessing for use in power plants continues in a number of nations.
An Alternative Approach
When the ban was issued, “reprocessing” was synonymous with the PUREX (for plutonium uranium extraction) method, a technique developed to meet the need for chemically pure plutonium for atomic weapons. Advanced fast-neutron reactor technology, however, permits an alternative recycling strategy that does not involve pure plutonium at any stage. Fast reactors can thus minimize the risk that spent fuel from energy production would be used for weapons production, while providing a unique ability to squeeze the maximum energy out of nuclear fuel. Several such reactors have been built and used for power generation—in France, Japan, Russia, the U.K. and the U.S.—two of which are still operating [see “Next-Generation Nuclear Power,” by James A. Lake, Ralph G. Bennett and John F. Kotek; Scientific American, January 2002].
Fast reactors can extract more energy from nuclear fuel than thermal reactors do because their rapidly moving (higherenergy) neutrons cause atomic fi ssions more effi ciently than the slow thermal neutrons do. This effectiveness stems from two phenomena. At slower speeds, many more neutrons are absorbed in nonfi ssion reactions and are lost. Second, the higher energy of a fast neutron makes it much more likely that a fertile heavymetal atom like uranium 238 will fi ssion when struck. Because of this fact, not only are uranium 235 and plutonium 239 likely to fi ssion in a fast reactor, but an appreciable fraction of the heavier transuranic atoms will do so as well.
Water cannot be employed in a fast reactor to carry the heat from the core— it would slow the fast neutrons. Hence, engineers typically use a liquid metal such as sodium as a coolant and heat transporter. Liquid metal has one big advantage over water. Water-cooled systems run at very high pressure, so that a small leak can quickly develop into a large release of steam and perhaps a serious pipe break, with rapid loss of reactor coolant. Liquid-metal systems, however, operate at atmospheric pressure, so they present vastly less potential for a major release. Nevertheless, sodium catches fire if exposed to water, so it must be managed carefully. Considerable industrial experience with handling the substance has been amassed over the years, and management methods are well developed. But sodium fi res have occurred, and undoubtedly there will be more. One sodium fire began in 1995 at the Monju fast reactor in Japan. It made a mess in the reactor building but never posed a threat to the integrity of the reactor, and no one was injured or irradiated. Engineers do not consider sodium’s flammability to be a major problem.
Researchers at Argonne National Laboratory began developing fast-reactor technology in the 1950s. In the 1980s this research was directed toward a fast reactor (dubbed the advanced liquidmetal reactor, or ALMR), with metallic fuel cooled by a liquid metal, that was to be integrated with a high-temperature pyrometallurgical processing unit for recycling and replenishing the fuel. Nuclear engineers have also investigated several other fast-reactor concepts, some burning metallic uranium or plutonium fuels, others using oxide fuels. Coolants of liquid lead or a lead-bismuth solution have been used. Metallic fuel, as used in the ALMR, is preferable to oxide for several reasons: it has some safety advantages, it will permit faster breeding of new fuel, and it can more easily be paired with pyrometallurgical recycling.