Radionuclides are radioactive elements. Radioactivity can be defined as the release of alpha and beta particles from atoms, and/or gamma rays that takes place when the nuclei of certain unstable substances spontaneously disintegrate. It is during this disintegration process that they emit radiation. The two main types of radiation released during such processes are termed ionizing , or of sufficient strength to forcibly eject electrons from their orbit around an atoms nucleus, producing ions, and non-ionizing ,of less strength and incapable of displacing electrons and forming ions. Ionizing radiation is highly significant because when it occurs within the atoms of molecules in living things, it is capable of causing biological damage such as the death of cells or the unnatural reproduction of cells. We term this unnatural reproduction of cells cancer .
Radionuclides occur in the environment both naturally and as a result of human industry. Some naturally-occurring radionuclides exist all over the earth and have been present since its formation 4.5 billion years ago. But most elements can be made artificially radioactive by bombarding them with high-energy particles such as neutrons. Each radionuclides forms and behaves in unique ways, with its own method and rate of decay. Such decay is measured by what is called half-life , or the time it takes for a group of atoms to decay to half of their original number.
One of the substances that occurs naturally in the earth's crust is uranium , a radionuclide used often as fuel to produce nuclear power . Uranium is the heaviest element found on earth with the exception of tiny amounts of an element called neptunium. When the German chemist Martin H. Klaproth discovered uranium in 1789, he named it in honor of the recently discovered planet Uranus. The scientific community showed little interest in uranium until 1896, when Henri Becquerel discovered radioactivity, and uranium was named as one of only two known radioactive elements of the time. In 1938, scientists discovered nuclear fission , a reaction that produces energy. In fact, uranium can produce energy at nearly three million times that of coal (1 lb of uranium can produce the same amount of energy produced by 3 million lb [1.4 million kg] of coal).
The most abundant form (isotope ) of uranium boasts a half-life nearly identical to that of the earth itself. This allows scientists to use the substance's disintegration to date other geological features of the earth by comparison. However, uranium use also has many drawbacks. Most notably, use of the substance produces nuclear waste that must be carefully transported and stored. In addition, easily obtainable supplies of uranium on Earth are limited, therefore the costs of locating and refining uranium can be extremely high.
Radium occurs as a result of uranium disintegration. Marie Curie, the first woman to win a Nobel Prize, and one of the few scientists to ever receive the Nobel prize twice, along with her husband Pierre, discovered the substance. She had followed up on Becquerel's observations on uranium's radioactive properties for her doctoral dissertation in the late 1890s. The Curies began refining pitchblende, a waste ore that was commonly found around uranium mines, known to emit radiation. The Curies first discovered the radioactive salt they named polonium, and then radium, found to be thousands of times more radioactive than any other substance discovered to date.
Radium is found in water, soil , plants, and food, but at low concentrations. Drinking water holds the highest potential for human exposure to radium. When humans are exposed to radium orally, they are at risk for lung, bone, brain and nasal passage tumors. Chronic exposure has led to acute leukemia and other complications. Radium has been classified by the Environmental Protection Agency (EPA) as a human carcinogen . Today, radium is still considered among the most radioactive metals on Earth, and requires careful attention and handling. Its use has been limited in many products, but the substance still helps treat cancers through radiation therapy and aids in some forms of research. As evidence of the validity of the EPA's finding, its discoverer, Marie Curie died at the age of 67 of leukemia, caused by her prolonged exposure to radiation.
Radon is a gas that can diffuse out of uranium (from radium) in the ground from uranium and thorium present in minerals, ores and rocks. A German physics professor named Friedrich Dorn discovered radon in 1900, after he followed the experiments of Marie Curie. He found that radium emitted a radioactive gas he termed radium emanation . Dorns discovery was important because it showed that an element could be transmuted from metal to gas as part of the radioactive decay process. The hazards of radon were not discovered until late in the twentieth century, and since that time, testing houses for the gas has become an important precaution. Radon has shown the capability of seeping into groundwater and contaminating public drinking supplies.
As Radon is colorless and odorless, it can prove particularly dangerous. Radon exposure occurs mostly through inhalation of the gas in indoor locations such as schools, homes, or office buildings. Chronic exposure produces serious respiratory effects, and smokers are at particular risk for lung cancer, estimated at 10–20 times the risk of nonsmokers. Although hazardous, radon has been useful in predicting earthquakes. In 1918, the discovery by a group of Chinese scientists that radon levels in groundwater rise just before earthquakes led to the prediction of several earthquakes by monitoring radon concentrations in well water. Radon has also proves useful in detecting leaks, inspecting metal welds, and measuring flow rates. However, radons high risk status as a carcinogen outweighs its beneficial uses.
Plutonium is an example of an artificial radioactive substance. It was discovered around the early 1940s during experiments on nuclear fission conducted by a number of scientists. Plutonium is used in breeder reactors when uranium undergoes nuclear fission to produce energy and occurs as a waste product of uranium. Like uranium, plutonium produces large amounts of energy and its properties were put to the test in the first nuclear weapons test in New Mexico in 1945.
Knowledge of the properties and risks of all radionuclides have made it necessary for people all over the world to become more aware of the inherent dangers they possess and the necessary preventative measures needed for protection. People that work in factories that process uranium or with phosphate fertilizers are at increased risk of exposure to radionuclides, as are those living in close proximity to uranium mines. Tests can measure radioactivity levels in the body and elimination of radium and radon in exhaled breath. Uranium, radon, and radium levels can be measured in the urine.
One of the greatest challenges facing scientists and policymakers throughout the world is the safe transport and disposal of the hazardous waste resulting from radionuclides. The subject has been steeped in controversy for many years, as no place seems safe enough to handle storage of a substance that lives for millions of years. However, scientists continue to develop new methods for storage of nuclear byproducts. In early 2001, a Los Alamos National Laboratory team announced that certain ceramic materials held up against radiation damage and could potentially offer new solutions to resist leaching and radiation for thousands of years.
A consortium of international agencies promotes international cooperation in managing nuclear waste, since nearly every developed country has some sort of nuclear power or weaponry project in place. Most of these countries have developed plans to safely dispose of nuclear waste early in the twenty-first century. The United States leads most countries in storage efforts, with the 1982 passage of the Nuclear Waste Policy Act. The act identified objectives for developing geologic repositories for high-level nuclear waste.
The federal government identified Yucca Mountain in Nevada for deep underground storage of wastes to replace storage that now lies at commercial nuclear power plants and research reactor sites in 43 states. The EPA continues to develop public health and environmental standards to set safe limits for the long-term storage of highly radioactive waste . However, a coalition of environmental groups also continues to watch and question the agency to ensure that the radiation standard remains protective enough. Like the Waste Isolation Pilot Plant (WIPP) storage facility in southern New Mexico, the Yucca Mountain site will no doubt remain controversial for some time.
The EPA provides printed fact sheets on radionuclides that warn of environmental and occupational exposure to the substances and acceptable levels of contact for humans. According to the agency, uranium is present in rocks and soil and throughout the environment, and although exposure can occur through air, higher levels generally occur in food or drinking water. Chronic long-term exposure to uranium and radon has been linked to both lung and kidney diseases. The EPA is currently in the process of promulgating new drinking water standards (the first update since 1976) regarding (non-Radon) radionuclides. These standards will become effective December 8, 2003, and will affect only Community Water Systems (CWSs), or those systems that serve more than 25 residents regularly all year.
[Joan M. Schonbeck ]
Alexander, D. E., and R. W. Fairbridge. Encyclopedia of Environmental Science. Dordrecht, The Netherlands: Kluwer Academic Publishers, 1999.
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Environmental Protection Agency Office of Air and Radiation, Ariel Rios Building, 1200 Pennsylvania Ave. NW, Washington, DC USA 20460 (202) 564-7400, <http://www.epa.gov/air/oarofcs.html>
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