Radioactive fallout is radioactive material produced by a nuclear explosion or a nuclear reactor accident that enters the atmosphere and eventually falls to Earth. This fallout consists of minute, radioactive particles of dust, soil, and other debris. While some fallout results from natural sources, the term is usually used in reference to radioactive particles that were released into the atmosphere by a nuclear explosion or reactor accident. Fallout refers to material that has fallen to Earth, and also to material that is still suspended in the atmosphere.
Radioactive fallout from nuclear weapons first appeared in 1945, when the United States tested the world’s first atomic bomb in New Mexico. Atomic bombs create devastating explosions through 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. Another class of weapons, fusion weapons or hydrogen bombs, uses a fission weapon as a trigger for a fusion process, in which atoms are forced to merge rather than being split apart. Fusion weapons are more powerful than fission weapons and also generate fallout.
Also in 1945, the United States exploded atomic bombs over the cities of Hiroshima and Nagasaki in Japan. There are the only nuclear weapons to have ever been used as an act of war. Since the end of World War II (1939–1945), 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 thousands of below-ground tests of nuclear weapons, as have India, Pakistan, North Korea, and Pakistan. Below-ground testing carries less 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 was deposited over 52,000 square miles (135,000 sq km) in Belarus, Scandinavia, and elsewhere in Europe.
Particles that make up radioactive fallout can be as small as the invisible droplets produced by an aerosol spray can, or as large as ash that falls close to a wood fire. The type of radioactivity in fallout depends on the nature of the nuclear reaction that emitted the particles into the atmosphere. More than 60 different types of radioactive substances may initially be present in fallout. Some of these decay into non-radioactive products in seconds, while others take centuries or longer to become non-radioactive. It takes 28 years, for example, for a sample of strontium-90 to lose one-half of its initial radioactivity. Strontium-90 is one of the most dangerous elements in fallout because it is treated by the metabolism of humans in the same manner as calcium, an important component of bone. If animals or humans eat food contaminated with strontium-90, it will accumulate in their bodies. Other particularly harmful products in fallout include cesium-134, cesium-137, and iodine-131.
Radiation damages and kills cells in the body. Large doses of radiation can result in burns, vomiting, and damage to the nervous system, digestive system, and bone marrow. Smaller doses can cause genetic mutations and cancer years after exposure.
Fallout from a nuclear explosion can be local, tropospheric, or stratospheric. Heavy objects caught in the wind fall to Earth before lighter objects. Under the same wind conditions, a large cinder will travel less distance than a small one. The same principle applies to fallout particles.
When a nuclear weapon explodes on or near the surface of Earth, huge quantities of soil, rock, water, and other materials are injected into the atmosphere, creating the familiar shape of the “mushroom cloud.” Depending on their size, particles in this cloud will fall to Earth relatively soon, or they may drift in the atmosphere for a long time. An underground nuclear explosion that does not break through the surface does not produce any fallout, and the radioactivity remains trapped below ground.
Local fallout deposits within about 10 mi (16 km) of a typical above-ground explosion. This material resembles ash or cinders that rise through a chimney and deposit nearby. Emitted particles greater than about 20 micrometers in diameter usually become local fallout. This fallout can be extremely radioactive, but only for a short time, after which its radioactivity is much less, though not zero.
Particles smaller than local fallout, as much as 200 times smaller, remain suspended in the lower atmosphere, or troposphere. Depending on the weather, these particles travel much farther than local fallout before being deposited to the surface, mostly within about one month.
Some fallout may reach the stratosphere, the high-altitude layer of atmosphere above the troposphere. To reach the stratosphere, fallout needs the force of the most powerful atomic explosions, caused by a hydrogen or thermonuclear bombs, to inject it that high. Stratospheric fallout can drift for years, and when it finally mixes with the troposphere and is deposited to the surface, it can fall-out anywhere in the world.
Isotopes— Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.
Nuclear fission— A nuclear reaction in which an atomic nucleus splits into fragments, with the release of energy, including radioactivity. Also popularly known as “splitting the atom.”
Nuclear reactor— A device which generates energy by controlling the rate of nuclear fission. The energy produced is used to heat water, which drives an electrical generator. By-products of the fission process may be used for medical, scientific, or military purposes, but most remain as radioactive waste materials.
Nuclear weapon— A bomb that derives its explosive force from the release of nuclear energy.
Radioactivity— Spontaneous release of subatomic particles or gamma rays by unstable atoms as their nuclei decay.
Radioisotope— A type of atom or isotope, such as strontium-90, that exhibits radioactivity.
and in outer space. France and China, however, have continued such tests. The United States and Russia further agreed in 1993 to eliminate two-thirds of their nuclear warheads by 2003. This agreement, made possible by the ending of the Cold War, greatly decreased the chances of nuclear warfare and the generation of enormous quantities of fallout.
Disastrous nuclear accidents, such as those at Three Mile Island and Chernobyl, have made nuclear reactors much less popular. No nuclear reactors ordered after 1973 have been completed in the United States, although several are under construction in Japan, Thailand, Turkey, and elsewhere. Some experts believe that new nuclear power plants may again be ordered in the United States in 2007 or 2008 because of generous government subsidies mandated by the energy bill of 2005.
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Dean Allen Haycock