Definition

Uranium is a natural and commonly occurring radioactive element.

Where Found

Uranium is found in very small amounts in nature in the form of minerals, but it may be processed into a silver-colored metal. Rocks, soil, surface and underground water, air, and plants and animals all contain varying amounts of uranium. Typical concentrations in most materials are a few parts per million.

Milling and Radioactive Wastes

If the amount and concentration of uranium is great enough, it may be commercially mined. After the uranium-bearing ore is mined, it is converted into uranium dioxide or other chemical forms by a series of chemical processes known as milling. The residue remaining after the uranium has been extracted is called mill tailings. Mill tailings contain smaller concentrations of uranium, other naturally radioactive waste products such as radium and thorium, and process chemicals. The milled uranium is then processed to further purify it and prepare a portion for manufacture into nuclear reactor fuel or weapons material. The residual is used in industry and military applications. Specific information regarding locations and status of uranium mill sites and tailings can be found at the U.S. Nuclear Regulator Commission Fact Sheet on Uranium Mill Tailings [NRC 2006].

Three Types of Natural Uranium

Natural uranium is a mixture of three types (or isotopes) of uranium, written as ²³⁴U, ²³⁵U, and ²³⁸U, and read as uranium two thirty-four, etc. All three isotopes behave the same chemically, so any combination of the three would have the same chemical effect on a person's health.

But they are different radioactive materials with different radioactive properties. That is why we must look at the actual percentages of the three isotopes in a sample of uranium to determine how radioactive it is.

Although natural and depleted uranium (discussed next) are primarily chemical hazards, the next few paragraphs describe the radiological nature of the toxicologically important uranium isotopes, because isotopes are addressed in some of the health effects studies.

Radioactive elements are those that undergo spontaneous transformation (decay), in which energy is released (emitted) either in the form of particles, such as alpha or beta particles, or as electromagnetic radiation with energies sufficient to cause ionization, such as gamma or X-rays. This transformation or decay results in the formation of different elements, some of which may themselves be radioactive, in which case they will also decay. The process continues until a stable (nonradioactive) state is reached.

When an atom of any of these uranium isotopes decays, it emits an alpha particle (the nucleus of a helium atom) and transforms into a radioactive isotope of another element. The process continues through a series of radionuclides until reaching a stable, non-radioactive isotope of lead. The radionuclides in these transformation series (such as radium and radon) emit alpha, beta, and gamma radiations with energies and intensities that are unique to the individual radionuclide [ATSDR 2008b].

In addition, each isotope has a different radiological half-life or the amount of time it takes for one-half of the atoms of the radionuclide to transform. ²³⁴U has the shortest half-life and is, therefore, the most radioactive, followed by ²³⁵U and ²³⁸U.

Processing

The processing of uranium for industrial and governmental use changes the ratios of the different isotopes. If the fraction of ²³⁵U is increased, it is called enriched uranium. However, if the portion of ²³⁵U is decreased, it is called depleted uranium. Depleted uranium is less radioactive than natural uranium, and enriched uranium is more radioactive than natural uranium. When enriched uranium is 97.5% pure ²³⁵U, the same weight of enriched uranium is about 75 times more radioactive than natural uranium. This is because enrichment also increases the concentration of ²³⁴U, which is the most radioactive of the three uranium isotopes. Natural uranium is typically about two times more radioactive than depleted uranium. Industrial processes produce other isotopes of uranium called ²³²U, ²³³U and ²³⁶U. These are also much more radioactive than natural uranium.

Natural Uranium

Natural uranium is radioactive but poses more of a chemical hazard because its half-life is very long. When uranium gives off radiation, it transforms into another substance (such as radon). Unlike other kinds of radiation, the alpha radiation ordinarily given off by uranium cannot pass through solid objects, such as paper or human skin. Very small amounts of natural uranium are used to make some ceramic ornament glazes, light bulbs, photographic chemicals, and household products. Some fertilizers contain slightly higher amounts of natural uranium.

Enriched Uranium

Enriched uranium contains more of the fissionable ²³⁵U that is the energy source for reactors and weapons. Low-enriched uranium is used in civilian reactors. High-enriched uranium is used in Naval reactors and for weapons production.

Depleted Uranium

Depleted uranium (DU) is created as a byproduct of the uranium enrichment process and possesses about 60% of the radioactivity of natural uranium. Due to its high density, it can be used as radiation shielding material, as penetrators in cannon rounds, and as counterweights for aircraft control surfaces.

Therefore, DU is used on helicopters and airplanes. It is also used by the armed forces as shielding to protect Army tanks and as parts of bullets and missiles to help them go through enemy armored vehicles. DU can contaminate wounds and become imbedded in soft tissue where direct contact with its high-energy alpha particles can be a source of systemic exposure over time.

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