Nuclear Fission

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When a neutron strikes the nucleus of certain isotopes, the nucleus breaks apart into two roughly equal parts in a process known as nuclear fission. The two parts into which the nucleus splits are called fission products. In addition to fission products, one or more neutrons is also produced. The fission process also results in the release of large amounts of energy.

The release of neutrons during fission makes possible a chain reaction. That is, the particle needed to initiate a fission reaction—the neutron—is also produced as a result of the reaction. Each neutron produced in a fission reaction has the potential for initiating one other fission reaction. Since the average number of neutrons released in any one fission reaction is about 2.3, the rate of fission in a block of material increases rapidly.

A chain reaction will occur in a block of fissionable material as long as neutrons (1) do not escape from the block and (2) are not captured by nonfissionable materials in the block. Two steps in making fission commercially possible, then, are (1) obtaining a block of fissionable material large enough to sustain a chain reaction—the critical size—and (2) increasing the ratio of fissionable to nonfissionable material in the block—enriching the material.

Atomic bombs, developed in the 1940s, obtain all of their energy from fission reactions while hydrogen bombs use fission reactions to trigger nuclear fusion. A long-term environmental problem accompanying the use of these weapons is their release of radioactive fission products during detonation.

The energy available from fission reactions is far greater, pound for pound, than can be obtained from the combustion of fossil fuels. This fact has made fission reactions highly desirable as a source of energy in weapons and in power production.

Many experts in the post-World War II years argued for a massive investment in nuclear power plants. Such plants were touted as safe, reliable, nonpolluting sources of energy. When operating properly, they release none of the pollutants that accompany power generation in fossil fuel plants. By the 1970s, more than a hundred nuclear power plants were in operation in the United States.

Then, questions began to arise about the safety of nuclear power plants. These concerns reached a peak whenthe cooling water system failed at the Three Mile Island Nuclear Reactor at Harrisburg, Pennsylvania, in March 1979. That accident resulted in at least a temporary halt in nuclear power plant construction in the United States. No new plants have been authorized since that time. A much more serious accident occurred at Chernobyl, Ukraine, in 1986 when one of four reactors on the site exploded, spreading a cloud of radioactive material over parts of the USSR, Poland, and northern Europe.

Perhaps the most serious environmental concern about fission reactions relates to fission products. The longer a fission reaction continues, the more fission products accumulate. These fission products are all radioactive, some with short half lives, other with longer half lives. The former can be stored in isolation for a few years until their radioactivity has reduced to a safe level. The latter, however, may remain hazardous for hundreds or thousands of years. As of the early 1990s, no completely satisfactory method for storing these nuclear wastes had been developed.

Nuclear Fusion; Nuclear Weapons; Radiation Exposure; Radioactive Pollution; Radioactive Waste; Radioactive Waste Management; Radioactivity

Fowler, J. M. Energy-Environment Source Book. Washington, DC: National Science Teachers Association, 1975.

Inglis, D. R. Nuclear Energy: Its Physics and Social Challenge. Reading, MA; Addison-Wesley, 1973.

Joesten, M. D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

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