Nuclear Reactor

When neutrons strike the nucleus of a large atom, they cause that nucleus to split apart into two roughly equal pieces known as fission products. In that process, additional neutrons and very large amounts of energy are also released. Only three isotopes are known to be fissionable, uranium-235, uranium-233, and plutonium-239. Of these, only the first, uranium-235, occurs naturally. Plutonium-239 is produced synthetically when nuclei of uranium-238 are struck by neutrons and transformed into plutonium. Since uranium-238 always occurs along with uranium-235 in a nuclear reactor, plutonium-239 is produced as a byproduct in all commercial reactors now in operation. As a result, it has become as important in the production of nuclear power as uranium-235. Uranium-233 can also be produced synthetically by the bombardment of thorium with neutrons. Thus far, however, this isotope has not been put to practical use in nuclear reactors.

Nuclear fission is a promising source of energy for two reasons. First, the amount of energy released during fission is very large compared to that obtained from conventional energy sources. For example, the fissioning of a single uranium-235 nucleus results in the release of about 200 million electron volts of energy. In comparison, the oxidation of a single carbon atom (as it occurs in the burning of coal or oil) releases about four electron volts of energy. When the different masses of carbon and uranium atoms are taken into consideration, the fission reaction still produces about 2.5 million times more energy than does the oxidation reaction.

Second, the release of neutrons during fission makes it possible for a rapid and continuous repetition of the reaction. Suppose that a single neutron strikes a one gram block of uranium-235. The fission of one uranium nucleus in that block releases, on an average, about two to three more neutrons. Each of those neutrons, then, is available for the fission of three more uranium nuclei. In the next stage, about nine neutrons (three from each of three fissioned uranium nuclei) are released. As long as more neutrons are being released, the fission of uranium nuclei can continue.

A reaction of this type that continues on its own once under way is known as a chain reaction. During a nuclear chain reaction, many billions of uranium nuclei Submerged in water, the fuel element is removed from the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory. Cerenkov radiation is causing the blue glow because light moves at a slower pace than energetically charged particles when in water. The HFIR is used exclusively to research manmade elements heavier than plutonium. U.S. Dept. of Eng., National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. may fission in less than a second. Enormous amounts of energy are released in a very short time, a fact that becomes visible with the explosion of a nuclear weapon.

Arranging for the uncontrolled, large-scale release of energy produced during nuclear fission is a relatively simple task. Fission (atomic) bombs are essentially devices in which a chain reaction is initiated and then allowed to continue on its own. The problems of designing a system by which fission energy is released at a constant and useable rate, however, are much more difficult.