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Substances such as ordinary water (light water), deuterium oxide (heavy water), and graphite have been found to be effective in slowing down neutrons during the fission process without reducing their number by absorption. The Uranium 235 rods are kept in rods in order to be able to insert them into the reactor where they are turned into energy. If the fissions begin to proceed at too great a rate, the result would be the release of an excessive amount of thermal energy and radiation and possibly cause meltdown of the core. Hence the term Nuclear Meltdown.

As a chain reaction continues, fission products accumulate in the reactor core. Most of these fragments are extremely radioactive and emit harmful gamma rays and neutrons. Consequently, thick, heavy concrete shielding to protect operators and other people in the vicinity against radiation must surround the reactor.

The disposal of radioactive fission products and spent-fuel assemblies poses a more difficult problem than does the containment of radiation in the reactor core. Some of these nuclear wastes remain dangerously radioactive for thousands of years and thus must be eliminated or stored permanently. We currently have no practical method of permanent disposal of nuclear waste.

The world’s first nuclear reactor was built at the University of Chicago under the direction of the Italian-born physicist Enrico Fermi. The reactor produced a chain reaction on Dec. 2, 1942. Following World War II, scientists and engineers in various other countries undertook efforts to develop reactors for large-scale power production. In 1956 the first full-scale commercial nuclear power plant was opened in Calder Hall, England. The following year the first American power station went online in 1957. Since then there were a lot of new power plants built until the late 1970s when there was a significant slowdown in the construction of new plants. The slow down can be attributed to the not in my back yard attitude, the rising cost of nuclear power plant construction and a drop in the projected rate of increase in electric-power demand. Major accidents at the Three Mile Island power station, near Harrisburg, Pa., in the United States, and the Chernobyl installation in the Soviet Union have left many afraid of nuclear power. France, Japan, South Korea, and Taiwan, which have few alternative-energy resources, however, have continued to increase their use of nuclear power.

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