Depleted Uranium (DU) is a by-product of the uranium enrichment process whereby the fissionable isotope ²³⁵U is extracted from natural uranium. After separation of ²³⁵U, the energy source material for reactors, the DU that remains is used in making military,  commercial and medical products.

Uranium occurs in nature as an oxide and is mined as U₃O₈, which contains about 0.7 wt% of the fissionable isotope ²³⁵U. The impure U₃O₈ is converted to UO₃. UO₃ is then hydrofluorinated to form UF₆, a gas, at slightly elevated temperature and reduced pressure. UF₆ is processed through a gaseous diffusion plant or gas centrifuge plant to separate the isotopes in the form of ²³⁵UF₆ and ²³⁸UF₆. The US Department of Energy enriches natural uranium from 0.7 wt% ²³⁵U to 3 -5 wt% ²³⁵U for commercial reactor fuel. For each kilogram of uranium that is enriched to 3 wt% ²³⁵U, five to six kilograms of DU containing about 0.2 wt% ²³⁵U are produced.

Historically, the enrichment process has produced more DU than is being used. Consequently, the Department of Energy has produced a large stockpile of material, estimated at 300,000 to 500,000 metric tons, in the form of uranium hexafluoride (UF₆). This waste by-product of the enrichment process becomes the basic raw material in the production of DU metal.

Uranium hexafluoride (UF₆) is chemically reduced with hydrogen to produce uranium tetrafluoride (UF₄), commonly referred to as "green salt". Green salt is reduced to metal by an exothermic reaction with magnesium. The product of the reaction is a high purity, uranium metal mass, referred to as a "derby".

Uranium products may be coated or clad to protect the surfaces and reduce exposure to personnel involved with subsequent handling in the field or to comply with regulatory requirements. Various coating materials such as acrylic paint, zinc, and nickel may be applied.

Depleted uranium is used in applications where its combination of high density, fabricability, relatively good mechanical properties and availability give it an advantage over other materials. There are several commercial and military non-nuclear uses of depleted uranium:

Calorimeters/Detectors: DU sheet is in wide-scale use as an absorber material in high-energy physics research at large accelerator laboratories. The high atomic number and density of DU presents a large number of atoms per unit volume to interact with the particles emerging from collisions in these detectors. Also the slight background radiation from DU enables insitu calibration of the electronic read out devices within such detectors, thereby improving the accuracy of measurement.

Radiation Shielding: Containers made of DU are used to transport highly radioactive, spent fuel elements and radioactive isotopes for medical and industrial applications. In addition, DU is used as shields in medical equipment for radiation therapy.

Counterweights: Counterweights made of DU are used in aerodynamic, control devices of airplanes, missiles and helicopters.

Miscellaneous: Flywheels have been made of DU for large, inertial, energy-storage devices and as sinker bars for oil well logging.

Kinetic Energy Penetrators: Kinetic energy penetrators are made of DU because of its high density, fabricability, pyrophoricity, availability and low cost compared to other heavy metals.

Shape Charge Liners and Explosively Formed Penetrators Lenses: Depleted uranium SCLs and EFP lenses are under investigation as a material for warhead applications in missiles, ammunition and submunitions.

Armor: The U.S. Army has revealed that depleted uranium is used as armor protection in the Abrams main battle tank.

Ownership, production and use of DU are subject to state and federal regulations. Title 10, Part 40, of the Code of Federal Regulations describes the requirements for obtaining a Radioactive Materials License.

In general, possession of more that 15 lbs. of uranium requires a license from the U.S. NRC or authorized Agreement State. However, users are exempt from this requirement for the following applications:

1. Uranium contained in counterweights installed in aircraft, rockets, projectiles or missiles, or stored or handled in connection with installation or removal of such counterweights when:
1. The counterweights are manufactured in accordance with the specifications contained in a specific license or equivalent licensing document issued by the NRC or Agreement State;
2. Each counterweight has been impressed with the following legend legible through any plating or other covering: "DEPLETED URANIUM";
3. Each counterweight is durably and legibly labeled or marked with the identification of manufacturer and the statement "UNAUTHORIZED ALTERATIONS PROHIBITED"; and
4. The exemption contained in this subparagraph shall not be deemed to authorize the chemical, physical, or metallurgical treatment or processing of any such counterweights other than repair or restoration of any plating or other covering.

 

1. Uranium used as shielding constituting part of any shipping container which is conspicuously and legibly impressed with the legend "CAUTION - RADIOACTIVE SHIELDING - URANIUM" and which is encased in mild steel or equally fire resistant metal of minimum wall thickness of 1/8 inch.
2. Uranium contained in detector heads for use in fire detection units, provided that each detector head contains no more than 0.005 microcurie of uranium.

    

In addition, other local, state, and federal regulations may apply and should be checked prior to possession or use of uranium.

There are three properties of the metal which require special precautions during fabrication and use:

• Radioactivity
• Toxicity
• Pyrophoricity

   

Radiation: Depleted uranium is a low specific activity (LSA) material. The radiation from DU is primarily non-penetrating; that is, it is very easy to shield so that it has little or no effect on people who handle it. In fact, DU is used as shielding for radioactive material.

Depleted uranium emits three types of radiation - alpha, beta, and gamma. While alpha radiation is insignificant as an external radiation hazard (skin stops alpha particles), it becomes a problem if inhaled or ingested. Once in the body, alpha rays, because of their short range and high ionization, are potentially more hazardous than beta or gamma radiation.

The permissible limits for radiation exposure are designated in Title 10, Part 20, Code of Federal Regulations. The limits are equivalent to an average of 100 mrem of exposure to whole body radiation per week and 625 mrem to the hands or feet. Operating experience using film badges and ring dosimeters indicate that persons working with DU receive very low levels of exposure to radiation compared to the permissible limits.

The greatest problem working with DU comes from finely divided airborne particles that can result from some manufacturing operations such as machining and grinding. It is essential to provide machine ventilation, area ventilation and special filtering equipment to protect workers from radioactive dust and particles that could be inhaled or ingested into the body where radiation may affect body organs. Machines are enclosed and ventilated, and air monitoring and frequent urinalysis are conducted to ensure controls are effective.

Toxicity: Depleted uranium, like lead, is a heavy metal poison that can be lethal if a sufficient amount of dust or fumes are ingested. The fact that DU is radioactive is helpful in this regard because it is much easier to detect its presence and protect against ingestion than it is for other non-radioactive, heavy metals like lead, tungsten or tantalum.

Pyrophoricity: A pyrophoric metal is one that oxidizes rapidly, that is, can "burn" in air. DU becomes pyrophoric only when finely divided. Because pyrophoric reactions take place at the surface of the metal, surface condition and the amount of exposed surface area are critical. Solid metal oxidizes slowly. A smoothly machined surface slowly turns to a tea color, and within a few days turns black.

Fires are extinguished by cooling the uranium and by restricting access of oxygen to the uranium by covering it with graphite powder or with a dry powdered chemical extinguisher. Water should never be used on uranium fires. Water reacts with the hot metal and generates hydrogen, which exacerbates combustion.