Technical Topic Papers
Rocky Flats Public Exposure Studies

Plutonium

Rocky Flats workers machined or shaped "pits" or triggers for weapons from 1952 through 1989. The pits were then shipped to Texas to be assembled into nuclear weapons.

What is plutonium?

How is it used?

The radioactivity level of an isotope such as plutonium is determined by its half-life, which is the time it takes for half of an original quantity to decay. the half-life of plutonium-239 is about 24,000 years.

Plutonium metal is difficult to handle and store safely, because it is radioactive and "pyrophoric" meaning it oxidizes and can become very hot when exposed to air. It can ignite nearby flammable materials, causing fires that can result in plutonium exposure of workers and the public. In addition, workers must avoid storing more than a few pounds in close proximity to prevent runaway fission, an uncontrolled nuclear chain reaction. Such a reaction’s burst of energy, known as a "criticality event," would not become a nuclear explosion, but could release radiation very dangerous to nearby workers. Such an event can also result in uncontrolled releases of both plutonium and fission products to the environment.

Plutonium was created in nuclear reactors in Hanford, Washington, and the Savannah River Site, South Carolina. It is then shipped to the Rocky Flats Plant northwest of Denver, Colorado for use in the manufacture of nuclear weapons components. Rocky Flats workers machined or shaped "pits" or triggers for weapons from 1952 through 1989. The pits were then shipped to Texas to be assembled into nuclear weapons. After weapons production ceased, about 14.2 U.S. tons of plutonium were left at Rocky Flats, now known as the Rocky Flats Environmental Technology Site. The site is currently undergoing cleanup and closure.

Some plutonium particles were released into the environment through roof vents and stacks during routine nuclear weapons production activities. Plutonium was also released during industrial fires in 1957 and 1969, and some was released through wastewater discharges into holding ponds and creeks that flowed off the site. More than 5,000 barrels of contaminated waste oil and solvents stored outdoors at the Rocky Flats 903 Area leaked onto the soil beginning in the late 1950s. The barrels were removed in the late 1960s, exposing the soil to the elements. Windstorms blew the plutonium-contaminated soil particles into the air and off-site. Resuspension of these particles is ongoing. The contaminated 903 Area was covered with asphalt in 1969. Soil sampling around the Rocky Flats Plant has shown that the area east and southeast of the plant has the highest plutonium levels. This contamination is the result of the routine releases, the fires and the 903 Area spill.

Before 1945, plutonium was virtually nonexistent in the human environment. Then in the 1950s and 1960s, plutonium was released into the environment during atmospheric testing of nuclear weapons. Plutonium can now be found in very small amounts in the soil throughout the Northern Hemisphere because of fallout from the atmospheric testing. Plutonium has also been found in soil near nuclear weapons production plants such as Rocky Flats due to accidents and spills.

Plutonium remains radioactive in the environment for a very long time. Plutonium decays by releasing small amounts of energy through fast-moving alpha particles. The radioactivity level of an isotope such as plutonium is determined by its half-life, which is the time it takes for half of an original quantity to decay. The half-life of plutonium-239 is about 24,000 years.

Plutonium particles may be released to the air where people may inhale them, or the particles may deposit on soil, plants and water. The plutonium particles landing on soil either attach to clays or stay near the soil surface where winds can pick them up and redistribute them. Plutonium particles can deposit on plants, but are not readily absorbed by the roots into the plants. Plutonium is usually insoluble in water, so plutonium particles that land on lakes and streams usually settle to the bottom in the sediment.

Members of the public living or working near nuclear weapons production plants can be exposed when equipment fails, accidents happen or mistakes are made, causing releases that move off the plant site through the air or in water. Plutonium particles in the air can deposit on the soil, where adults or children may work or play, or on water, which may be a source for drinking, irrigation of crops or recreation. Particles can also deposit on vegetables or on grass eaten by cows that may later provide milk and meat for human consumption.

Plutonium emits alpha radiation and low-energy x-rays, which are easily absorbed by tissue. The alpha radiation travels only about a quarter of an inch in air and cannot penetrate the skin. Therefore, if plutonium remains outside the body, it is generally not harmful. Plutonium is very toxic if it enters into the body because the alpha radiation can damage living tissue. The larger the "dose" in the body, the greater the toxicity.

Human exposure occurs mainly by breathing contaminated air or ingesting contaminated food or drink. Breathing is generally the route of most concern. When plutonium particles are inhaled and lodge in lung tissue, they continue to give off radiation internally. They can remain in the lungs or enter the gastrointestinal tract and the bloodstream. About 80 percent of the plutonium that enters the bloodstream goes either to the liver, bone or bone marrow, where it is retained for years, damaging tissue nearby. That damage may later develop into cancer. Common forms of plutonium do not dissolve significantly in water or body fluids, so little ingested material is actually absorbed into the blood from the gastrointestinal tract.

How do scientists estimate human health risks from low-level exposure to plutonium?

1) Results of studies of Russian workers exposed to plutonium at the Mayak weapons plant, a production facility similar to Rocky Flats;

2) Data from populations exposed to other alpha-emitting radioactive materials such as radium, radon and thorium;

3) Data from Japanese World War II atomic bomb survivors exposed briefly to high levels of the gamma and neutron radiation from the atomic explosions; and

4) Results of controlled experiments on animals exposed to plutonium and other alpha-emitting materials.

Based on this information, the researchers developed revised estimates of the risks associated with plutonium exposure. (See the technical report Assessing Risks of Exposure to Plutonium prepared by Radiological Assessments Corporation for the Historical Public Exposures Studies on Rocky Flats in February 1999.) Because plutonium is retained in the lung, liver, bone and bone marrow, rather than in the reproductive organs, genetic risks are not the primary risks from plutonium.

Can an individual's exposure to plutonium be measured externally or internally?

Are there standards for worker and public exposure to plutonium?

Where can I get more information about cancer risks from Rocky Flats?