Depleted uranium (DU; also referred to in the past as Q-metal, depletalloy or D-38) is uranium with a lower content of the fissile isotope U-235 than natural uranium. (Natural uranium is about 0.72% U-235—the fissile isotope, and the DU used by the U.S. Department of Defense contain less than 0.3% U-235). Uses of DU take advantage of its very high density of 19.1 g/cm3 (68.4% denser than lead). Civilian uses include counterweights in aircraft, radiation shielding in medical radiation therapy and industrial radiography equipment and containers used to transport radioactive materials. Military uses include defensive armor plating and armor-piercing projectiles.
Most depleted uranium arises as a byproduct of the production of enriched uranium for use in nuclear reactors and in the manufacture of nuclear weapons. Enrichment processes generate uranium with a higher-than-natural concentration of lower-mass uranium isotopes (in particular U-235, which is the uranium isotope supporting the fission chain reaction) with the bulk of the feed ending up as depleted uranium, in some cases with mass fractions of U-235 and U-234 less than a third of those in natural uranium. U-238 has a much longer halflife than the lighter isotopes, and DU therefore emits less alpha radiation than the same mass of natural uranium: the US Defense Department states DU used in US munitions has 60% of the radioactivity of natural uranium.
Since the U-235 content of nuclear reactor fuel is reduced by fission, uranium recovered by nuclear reprocessing from spent nuclear reactor fuel made from natural uranium will have a lower-than-natural U-235 concentration. Such 'reactor-depleted' material will have different isotopic ratios from enrichment byproduct DU, and can be distinguished from it by the presence of U-236. Trace transuranics (another indicator of the use of reprocessed material) have been reported to be present in some US tank armor.
The use of DU in munitions is controversial because of questions about potential long-term health effects.Normal functioning of the kidney, brain, liver, heart, and numerous other systems can be affected by uranium exposure, because uranium is a toxic metal. It is only weakly radioactive because of its long radioactive half-life (4.468 billion years for uranium-238, 700 million years for uranium-235). The biological half-life (the average time it takes for the human body to eliminate half the amount in the body) for uranium is about 15 days. The aerosol or spallation frangible powder produced during impact and combustion of depleted uranium munitions can potentially contaminate wide areas around the impact sites, leading to possible inhalation by human beings.