Uranium is a naturally occurring, very hard, heavy, silvery, radioactive metallic element (atomic number 92).  It was discovered in 1789 by Martin Klaproth, a German chemist, who isolated an oxide of uranium while analyzing pitchblende samples from the Joachimsal silver mines in the former Kingdom of Bohemia, located in the present day Czech Republic.  He named his discovery “uran” after the planet Uranus. Uranium is referred to as a “special radioactive element” because it is capable of undergoing fission.

For many years, uranium was used primarily as a colorant for ceramic glazes and for tinting in early photography. Its radioactive properties were not recognized until 1866, and its potential for use as an energy source was not manifested until the mid-20th century. Now it is used to power nuclear reactors that produce electricity and for isotopes used for medical, industrial, and defense purposes around the world.

Uranium ore can be mined from open pits or underground excavations.   The ore can be crushed and treated at a mill to separate the valuable uranium from the ore, or uranium may be dissolved directly from the ore deposits in the ground (in-situ leaching) and pumped to the surface. Uranium mined from the earth is stored, handled, and sold as uranium oxide concentrate (U3O8).

Physical Properties of Uranium

• Concentration - Uranium ranks 48th among the most abundant elements found in natural crustal rock.
• Density - Uranium is very dense. At about 19 grams per cubic centimeter, it is 1.6 times more dense than lead. Density increases weight. For example, while a gallon of milk weighs about 8 pounds, a gallon container of uranium would weigh about 150 pounds.
• Melting Point - Uranium boils at about 3,818 degrees Celsius (about 6,904 degrees Fahrenheit).

Types of Uranium

• Natural Uranium - contains a 238U concentration of 99.3 percent and 235U concentration of 0.7 percent.
• Low Enriched Uranium – contains a 235U concentration between 0.7 percent and 20 percent. Most commercial reactor fuel uses low enriched uranium (LEU) enriched to between 3 and 5 percent 235U.  Uranium between 3 and 5 percent 235U is sometimes referred to as “reactor-grade uranium.”
• Highly Enriched Uranium – contains a 235U concentration greater than 20 percent. Highly enriched uranium (HEU) is used in research reactors, naval propulsion reactors, and nuclear weapons.
• Depleted Uranium – contains a 235U concentration of 0.71 percent or less.

What is Uranium Enrichment?

Uranium enrichment is a critical step in transforming natural uranium into nuclear fuel to produce energy. Uranium is a naturally occurring element containing 235U and 238U isotopes. Only the 235U isotope is fissionable. Enrichment is the process of increasing the concentration of 235U while decreasing the concentration of 238U. The stream with the greater 235U concentration is referred to as enriched uranium, while the stream that is reduced in its concentration of 235U is referred to as depleted uranium.

Currently, uranium is enriched commercially by using either (1) gaseous diffusion technology or (2) gas centrifuge technology. In both processes, the compound uranium hexafluoride (UF6) is heated and converted from a solid to a gas. At a gaseous diffusion plant, the UF6 gas is forced through a series of compressors and converters that contain porous barriers. Because 235U has a slightly lighter isotopic mass than 238U, it filters through the barriers at a slightly higher rate than the 238U. The operation of this equipment consumes a significant amount of electricity.

The gaseous diffusion process for enriching uranium was first developed on a large scale at the U.S. Department of Energy (DOE) plant in Oak Ridge, Tennessee. Two additional uranium enrichment plants were subsequently constructed in Paducah, Kentucky, and Portsmouth, Ohio. As of early 2009, USEC Inc. leases the only operating gaseous diffusion plant in Paducah, Kentucky, and is the only domestic U.S. supplier of uranium enrichment services.

In the gas centrifuge process, the UF6 gas is rapidly spun in cylinders. The difference in isotopic masses allows centrifugal forces to separate the 235U and 238U isotopes, concentrating the slightly heavier 238U closer to the cylinder wall. The gas centrifuge technology consumes only about five percent as much electricity as the gaseous diffusion technology. Two companies, Louisiana Enrichment Services and USEC Inc., have received licenses from the U.S. Nuclear Regulatory Commission to build and operate uranium enrichment facilities in the United States using centrifuge technology.