Nuclear Reactors

Early History

The first nuclear reactor was constructed by Enrico Fermi's team at the University of Chicago in 1942. Chicago Pile-1, as it was called, achieved a sustained nuclear chain reaction—a point called “criticality” or “critical mass,” referring to the smallest amount of fissile material required for such a reaction—on December 2, 1942. Reactors built for the Manhattan Project followed in order to produce plutonium for use in nuclear weapons. In the years after the war, nuclear reactor technology was shared by the military with the scientific community. U.S. President Dwight D. Eisenhower's 1953 speech “Atoms for Peace” emphasized the benefits of using nuclear technology for peaceful ends—as a power source—rather than focusing scientific endeavor purely on developing nuclear weaponry.

Nuclear Fission

Nuclear reactors depend on fssile materials as fuel. Fissile materials are those that can sustain a chain reaction of nuclear fission; fissionable materials are those that can undergo nuclear fission. All that is fissile is fissionable, but much of what is fissionable is not fissile. Fissile materials include uranium-233, uranium-235, plutonium-239, and plutonium-241. When used in documents dealing primarily with nuclear arms, such as treaty proposals, “fissile” confusingly means something slightly different: materials that can be used in a nuclear weapon to sustain a fast-fission chain reaction, which does not include all fissile materials used as fuel. The atomic nucleus of such a fissile material splits into two or [Page 616]more nuclei when it absorbs a neutron, producing fission products consisting of gamma radiation, kinetic energy, and free neutrons. Some of the neutrons released are then absorbed by other fissile atoms, which split and release more fission products, including free neutrons: a nuclear chain reaction. This chain reaction is controlled through the use of neutron moderators (usually water, but sometimes graphite), which reduce the velocity of fast neutrons and make them more likely to be absorbed, and by control rods composed of neutron poisons (also called “neutron absorbers”), which absorb the excess neutrons left over.

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