Radioactive pollution and fallout

DEFINITIONS: Radioactive pollution is environmental contamination resulting from the release of radioactive materials; fallout is the radioactive particles that fall to Earth after detonation of nuclear weapons

The use of radioactive materials for civilian and military applications has helped to disseminate radioactivity throughout the environment. Because of the long half-lives of some radioactive pollutants, their detrimental effects on ecosystems and human health can persist for generations and pose major challenges for safe, long-term disposal efforts.

Many people are well aware of the use of radioactive elements in nuclear weapons and reactors. Likewise, the general public understands that nuclear war, nuclear weapons testing, and accidents such as the disastrous Chernobyl plant meltdown of 1986 can have profound, far-reaching, and long-lasting environmental impacts. It may be less well known, however, that radioactive elements are sometimes used for their physical or chemical properties rather than their radioactivity, and that these less familiar applications can cause small amounts of radioactive pollution. For example, uranium and thorium compounds have been used for centuries to give ceramic glazes brilliant orange and yellow hues. (Although pieces produced in the United States after about 1950 are generally safe to use, it is recommended that other ceramics with uranium or thorium glazes be left for occasional show unless they are tested and known to be safe for use.)

In addition, until the 1980s trace amounts of uranium were used to color porcelain teeth, and tiny amounts of uranium or thorium are present in some tinted contact lenses, eyeglass lenses, and other optical glass. Gas lantern mantles have long used thorium to produce a bright, white glow. Although the hazard to the public is believed to be negligible, nonthorium mantles are now available as an alternative. Tungsten electrodes for arc welding may contain 2 percent thorium for easier starting and greater weld stability. The resulting dose to both the welder and the public is very small. Ionization detectors use tiny amounts of radioactive americium to ionize air in a small chamber. This allows a current to flow. Smoke particles reduce this current and trigger the detector. Americium has a 458-year half-life, so it will remain radioactive long after the smoke detector’s service life is over. While a smoke detector poses little threat when installed and used properly, its radioactive content can present a health hazard once the smoke detector is discarded. In order to keep americium out of the stream, many US state and local governments conduct roundups of ionization smoke detectors or encourage consumers to return used smoke detectors to their manufacturers.

Industrial, Medical, and Scientific Uses

Because small amounts of are easily detected, both industries and the medical field use radioactive elements as tracers. For example, radioactive technetium may be injected into a patient’s vein so that its progress can be followed with radiation detectors. Imaging systems can then reveal constrictions in the patient’s heart or arteries. The technetium used has a six-hour half-life. The nondecayed portion of technetium is eventually eliminated from the body and passes into the system. No special precautions are taken because the radioactivity quickly decays and is greatly diluted with normal waste. Worldwide, more than 30 million diagnostic procedures using various radioactive elements are performed each year.

Radioisotope thermoelectric generators (RTGs) were developed to supplement the power from solar cells or replace them on space missions where sunlight is too weak. The Jupiter mission’s Galileo spacecraft carried two RTGs, while the Saturn mission’s Cassini spacecraft carried three. A standard RTG uses the heat from the radioactive decay of 11 kilograms (24 pounds) of plutonium dioxide to produce electricity. Because the process involves no moving mechanical parts, it is very reliable. RTGs are built to withstand the explosion of the spacecraft during launch as well as the heat of reentry.

Although the United States has used RTGs in many space missions, it has had only three accidents involving these power sources. In 1964 a navigational satellite failed to reach orbit and burned up over the Indian Ocean. Its RTG was of an earlier design that also burned up, as was then intended. The resulting plutonium oxide dust settled out of the over the next several years. Greatly diluted across the globe, it barely increased background radiation. In 1968 a spacecraft was destroyed after launch by the range safety officer, and its RTGs were recovered intact. In 1970 the lunar module of the damaged Apollo 13 spacecraft reentered the atmosphere over the South Pacific. Its RTG plunged into the Tonga Trench, which is 6 kilometers (3.7 miles) deep. Although it was never recovered, surveys have shown no of radioactivity.

The Soviet Union used RTGs not only in spacecraft but also as power sources for lighthouses and other navigational beacons in remote locations. Since the breakup of the Soviet Union in late 1991, many of these RTGs have been unattended and have fallen into disrepair. Looters hoping to strip metal parts and sell them to recyclers have stolen unsecured RTGs and received high doses of radiation. Stolen RTGs have been abandoned in forests, dumped in the sea, and in one case left at a bus stop. The United States has taken in interest in helping to decommission Russian RTGs, as they are a source of radioactive material that could be exploited by terrorists.

Depleted Uranium

Uranium is a widely distributed trace element. Its estimated in the earth’s crust is 2.7 parts per million. Pure uranium is a lustrous, silver-white metal, and although it is radioactive, the activity consists of alpha particles and low-energy gamma rays that can be readily shielded to safe levels. The ease with which uranium can be safely handled under favorable conditions has led to its increasing use. Many view this as dangerous.

Natural uranium consists of three different forms, or isotopes: about 0.7 percent is uranium 235, 99.3 percent is uranium 238, and only a trace amount is uranium 234. Weapons-grade uranium must be enriched to at least 90 percent uranium 235, while reactor fuel is generally enriched to 3 to 5 percent uranium 235. Extraction processes leave uranium that has only 0.2 to 0.3 percent uranium 235. This is called depleted uranium. Depleted uranium is only 60 percent as radioactive as natural uranium, but there is a lot of it. The United States entered the twenty-first century with an estimated 480,000 metric tons of depleted uranium; the total world inventory at the turn of the century was more than 1.1 million metric tons.

Rather than provide storage for it as low-level radioactive waste, the US Department of Energy actively seeks uses for depleted uranium. Almost twice as dense as lead, it makes a good radiation shield, and it is used to shield radioactive isotopes shipped to hospitals. Ducrete, a special concrete containing depleted uranium, is used as radiation shielding in shipping casks for spent reactor fuel.

Depleted uranium packs a great deal of weight into a small volume; therefore, it is used to make counterweights for commercial aircraft and the tips of Tomahawk cruise missiles. Its and hardness led to its use in the armor of the M1A1 Abrams tank and armor-piercing ammunition. Powdered uranium is pyrophoric (that is, it burns spontaneously in air). Some of the depleted uranium in an armor-piercing round burns upon impact with a hard surface. This makes the round more effective against tanks because it helps the round to penetrate armor and often ignites a secondary explosion. While most of the uranium is expected to remain within several meters of the target, a significant amount may not. Some uranium oxide particles formed from burning uranium are smaller than 5 microns and can be carried by the wind for long distances. Such small radioactive particles can also lodge in the lungs of organisms and are potentially hazardous.

Hundreds of metric tons of depleted uranium ammunition have been used in Middle Eastern conflicts such as the 1991 Persian Gulf War and the 2003 invasion and subsequent occupation of Iraq. Depleted uranium ammunition was also employed during the 1992-1995 Bosnia-Herzegovina conflict and the 1999 Balkan war. While depleted uranium rounds are highly effective, their potential impact on the and the health of soldiers and civilians makes them a controversial weapon. During the mid-1980’s, concerns about possible radiation of ship crews led the US Navy to switch from depleted uranium to tungsten rounds for the Phalanx guns used to defend military vessels against planes and missiles. The long-term health effects of depleted uranium use during wartime have yet to be established. The World Health Organization recommends that and cleanup operations be carried out in impact zones following military conflict if there is a reasonable possibility that depleted uranium could enter the or in sufficiently high quantities to pose a health and environmental hazard.

Fallout from Nuclear Weapons

Fallout is the name given to radioactive particles that rain down from the debris cloud of a nuclear explosion. Whether the is local or global depends chiefly on the yield of the weapon. Nuclear weapon yields are measured in terms of how many tons of the high-explosive trinitrotoluene (TNT) would be required to release the same energy. The bomb used in the American attack on the Japanese city of Hiroshima during World War II, for example, had a yield of approximately 13 kilotons. The largest nuclear weapons built by the United States had an estimated yield of 25 megatons (25,000 kilotons).

Yields of less than 100 kilotons produce local fallout, which consists of particles that fall to the ground within twenty-four hours of the explosion. Local fallout is generally most intense near ground zero; however, winds may carry local fallout hundreds of kilometers or more. Larger weapons loft debris higher into the air, and small particles may drift for several days before falling to the ground. Particles lifted into the stratosphere may remain there for months or longer and be carried around the world. The radioactivity of fallout decreases with time, so that the longer it remains aloft, the less dangerous it is. Therefore, local fallout is far more hazardous than global fallout.

When a nuclear weapon explodes, it instantly becomes an expanding fireball of radioactive vapor. The radioactivity chiefly comes from the debris of the nuclear of uranium. The explosion also produces a torrent of neutrons that can transform some normal elements into radioactive elements. Wherever the fireball touches the ground, dirt and debris are sucked into the air. As the fireball expands and cools, radioactive vapor condenses into radioactive particles and debris that are pulled into the fireball. Because hot air rises, the fireball rises and forms the hallmark mushroom-shaped cloud. Fallout begins within minutes as the heaviest radioactive pebbles rain down near the stem of the mushroom cloud. Fine particles are carried downwind from ground zero and continue to fall to the earth over the next several hours.

If the wind is steady, the radioactivity of the fallout accumulated on the ground could be described with a series of concentric, elongated ovals. The near ends of all of the ovals would touch at ground zero. The innermost oval would have the highest radioactivity. The next oval outward would mark lower activity, and its far end would extend farther from ground zero. For a 1-megaton bomb, the oval that contains a lethal dose of fallout during the first forty-eight hours would be 1,000 square kilometers (386 square miles) in area. However, fallout radioactivity decays relatively quickly, so that after one year the lethal oval would cover only 1 square kilometer (0.38 square mile). At first, 20,000 square kilometers (7,722 square miles) would be contaminated badly enough that an unprotected person might show signs of radiation sickness within two weeks. After one year, that area would decrease to about 20 square kilometers (7.72 square miles).

Depending on such factors as targeting strategies, timing, and weather, fallout from a large-scale nuclear attack could kill tens of millions of people and endanger hundreds of millions more. Because fallout is radioactive dust, it accumulates most readily on horizontal surfaces such as the ground and the roofs of buildings. If caught in a fallout zone, the best strategy would be to get as far away from the fallout as possible and place as much mass between the person and the fallout as possible. For example, the shelter of a simple basement can reduce the radiation dose to 5 or 10 percent of that of a person in the open.

Bibliography

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Leopold, Ellen. Under the Radar: Cancer and the Cold War. Piscataway, N.J.: Rutgers University Press, 2009.

Miller, Alexandra C., ed. Depleted Uranium: Properties, Uses, and Health Consequences. Boca Raton, Fla.: CRC Press, 2007.

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