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Three Mile Island
Nuclear Power Plant Accident

March 28, 1979

All nuclear explosions produce a giant fireball of intensely hot gases and dust. Everything inside the fireball or in contact with it is vaporized (turned into a gas). When an explosion occurs close to the earth's surface, the fireball vaporizes soil, vegetation, and buildings. The intensely hot gases of the fireball also draw in dirt, dust, and other small particles as the fireball rises into the atmosphere. The radioisotopes formed during fission then combine with the vaporized materials. As the vaporized materials rise and cool, some of them condense into solid particles ranging in size from fine invisible dust to ashes the size of snowflakes. These particles, to which radioisotopes have become attached, return to the earth as fallout.

From the mid-1940's to the early 1960's, the United States, the Soviet Union, and a few other nations exploded many experimental nuclear weapons. As a result, fallout increased to alarming levels. In 1963, more than 100 nations, including the United States and the Soviet Union, signed a treaty that banned the testing of nuclear weapons everywhere but underground. Fallout then decreased greatly. China and France did not sign the treaty. But they later stopped testing nuclear weapons aboveground.

Today, accidents at nuclear reactors pose the greatest risk of fallout. In 1986, an accident at the Chernobyl nuclear power plant in Ukraine, then part of the Soviet Union, created fallout consisting mainly of the radioisotopes iodine 131 and cesium 137. A power surge caused fuel in the nuclear reactor to overheat, resulting in a steam explosion and fire. The radioisotopes created by nuclear fission in the reactor escaped into the atmosphere through the smoke from the fire, which burned for 10 days.

The fallout from the Chernobyl accident was distributed by winds, but most of the debris remained in the troposphere. Areas that sustained the highest levels of radioactive deposits were in northwest Ukraine, southeast Belarus, and southwest Russia. In addition, precipitation created hot spots throughout the zone of fallout. Radioactive isotopes were carried into northern and central Europe. Traces of radioactivity were eventually measurable throughout the Northern Hemisphere.

The explosion at Chernobyl was the worst in history, but other serious accidents have also occurred. One of the worst happened at the Windscale plutonium production plant in northern England in 1957. Fallout from a fire in a reactor at the Windscale plant contaminated about 200 square miles of surrounding countryside. In the United States, an accident occurred at the Three Mile Island nuclear power plant near Harrisburg, Pa., in 1979. Overheating caused by an interruption in the reactor's cooling system resulted in severe damage to a reactor core. However, a protective building that contained the reactor largely prevented the radioactive debris from being released into the environment.

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