Radiation is energy in the form of photons or high-speed atomic particles (ionizing) or electromagnetic waves (nonionizing). Atoms release radiation as they change from unstable, energized forms to more stable forms. All matter is composed of elements, and each element can take many different forms (called isotopes). Some of these isotopes are unstable and emit radiation; these unstable isotopes are known as radioisotopes or radionuclides. Stable isotopes do not undergo radioactive decay and therefore do not emit radiation.

Types of Radiation

Figure 3. Three Types of Radiation
and How They Are Contained

Source: U.S. Department of Energy, Carlsbad Area Office.

Radiation is either ionizing or nonionizing. Ionizing radiation has three main forms:

Alpha particles can travel only a few inches in the air and lose their energy almost as soon as they collide with anything. They are easily shielded by a sheet of paper or the outer layer of a person’s skin.

Beta particles, which are identical to electrons, are more energetic than alpha particles. They can travel in the air for a distance of a few feet. Beta particles can pass through a sheet of paper but can be stopped by a sheet of aluminum foil or glass.

Gamma rays are waves of pure energy and are similar to x-rays. They travel at the speed of light through air or open spaces. Concrete, lead, or steel is necessary to block gamma rays.

Measurement of Radiation

Radiation is measured in different ways. Measurements used in the United States include the following:

Roentgen is a measure of exposure; it describes the amount of radiation energy, in the form of gamma or x-rays, in the air.

Rad (radiation absorbed dose) measures the amount of energy actually absorbed by a material, such as human tissue.

Rem (roentgen equivalent man) measures the biological damage of radiation. It takes into account both the amount, or dose, of radiation and the biological effect of the type of radiation in question. A millirem is one one-thousandth of a rem.

Curie is a unit of radioactivity. One curie refers to the amount of any radionuclide that undergoes 37 billion atomic transformations a second. A nanocurie is one one-billionth of a curie.

Everyday Exposure to Radiation

Individual exposures vary, but humans are exposed routinely to radiation from both natural sources, such as cosmic rays from the sun and indoor radon, and from manufactured sources, such as televisions and medical x-rays. Even the human body contains natural radioactive elements.

Because individual human exposures to radiation are usually small, the millirem (one one-thousandth of a rem) is generally used to express the doses humans receive. The following table shows average radiation doses from several common sources of human exposure.

Effects on Humans

Ionizing radiation is powerful enough to alter cellular chemicals and disrupt normal cell functioning. All three types of ionizing radiation are potentially harmful to humans. Alpha and beta particles can cause damage to tissue primarily through inhalation or ingestion. Inhaling or ingesting particles that emit gamma rays is also potentially harmful; in addition, gamma rays from outside sources can penetrate and cause damage throughout the human body.

Two types of cellular damage can result from exposure to ionizing radiation:

Genetic damage, which alters–or mutates–reproductive cells, resulting in damage to future generations.

Somatic damage, which alters ordinary, nonreproductive cells, harms the exposed individual during his or her lifetime, but is not passed on to offspring. Cancer, including some leukemias and bone, thyroid, breast, skin, and lung cancer, is the dominant type of somatic damage resulting from exposure to ionizing radiation. Other types of somatic damage include burns and cataracts.

The nature and extent of damage caused by ionizing radiation depend on a number of factors, including the amount of exposure, the frequency of exposure, and the penetrating power of the radiation to which an individual is exposed. Rapid exposure to very large doses of ionizing radiation is rare but can cause death within a few days or months. The sensitivity of the exposed cells also influences the extent of damage. For example, rapidly growing tissues, such as developing embryos, are particularly vulnerable to harm from ionizing radiation.

Duration of Radioactivity of Waste at the WIPP

A half-life measures the amount of time it takes for half the radioactive atoms in a radioisotope to decay to a more stable form. After one half-life, for example, half the radioactive atoms in a sample remain radioactive; after two half-lives, one-quarter remain radioactive; after three half lives, one-eighth remain radioactive; and so on. Each element has a unique half-life. Half-lives range from a fraction of a second to billions of years.

The half-lives of the radioisotopes in transuranic wastes vary, but some transuranic elements have very long half-lives. For example, the half-life of plutonium-239 (a predominant isotope in transuranic waste) is approximately 24,000 years, and it takes plutonium-239 nearly 240,000 years to decay by 99.9 percent. Some small amount of radioactivity will remain in the waste indefinitely, but the amount of radiation will continually decrease.

Because of the long half-lives of some transuranic radioisotopes, transuranic wastes must be isolated and controlled for many, many years. Before the WIPP may be opened to accept waste, DOE must obtain certification from EPA that the waste can be isolated from the human environment for at least 10,000 years.