Radioactive Fallout

Radioactive fallout from nuclear weapons began in 1945 when the United States tested the first atomic bomb in New Mexico. Atomic bombs create devastating explosions by "splitting the atom," a process more properly referred to as nuclear fission. The powerful blast of an atomic bomb is the result of energy released when the nuclei of unstable heavy elements are split, such as uranium-235 and plutonium-239. Nuclear fission also generates unstable atoms that release subatomic particles and electromagnetic radiation, known as radioactivity. In some cases, neutrons released during fission can interact with nearby materials to create new radioactive elements.

Also in 1945, the United States exploded atomic bombs in Hiroshima and Nagasaki, Japan. There are the only nuclear weapons to have ever been used as an act of war. Since the end of World War II, the United States, the former Soviet Union, the United Kingdom, France, and China have test-exploded nuclear weapons above ground, and thereby contributed to worldwide fallout. Nuclear weapons testing was most intense between 1954 and 1961. (All of these countries have also undertaken numerous below-ground tests of nuclear weapons, as have India, Pakistan, and probably some other countries. However, below-ground testing carries little risk of causing atmospheric radioactive fallout.)

Another source of radioactive fallout is nuclear reactors. Like an atomic bomb, a nuclear reactor generates nuclear energy by splitting atoms. However, instead of releasing all of the energy in an instant, a reactor releases it slowly, in a controlled fashion. The heat generated by the carefully controlled nuclear reactions is used to make steam, which drives a generator that produces electricity.

After a cooling system failed at the Three Mile Island Nuclear power plant in Pennsylvania in 1979, a small amount of radioactive material was released into the atmosphere. Enormously larger amounts of dangerous radioactive materials were released in 1986, following a catastrophic accident at a poorly designed nuclear plant at Chernobyl in the Ukraine. After that catastrophe, significant amounts of fallout deposited over 52,000 square miles (135,000 sq km) in Belarus, Scandinavia, and elsewhere in Europe.