Depleted uranium (DU) is uranium which has had most of the fissile isotope U-235 removed, and consists of mostly U-238. The U-235 is concentrated into enriched uranium through the process of isotope separation for use in nuclear reactors and nuclear weapons. The remaining, less radioactive (40% of natural uranium), U-238 is waste material from this enrichment process. During the Manhattan Project depleted uranium had the codename, tubealloy, and this term is still occasionally used. The mining of uranium is mainly for its U-235 content, so the excess U-238 can be obtained cheaply and is utilized for its extremely high density, only slightly less than that of tungsten.
A major use of DU is as the head of a kinetic projectile fired to penetrate armour, it is therefore used by tanks and other military platforms. Depleted uranium is very dense, at 19.05 g/cm³ it is 70% more dense than lead, allowing it to penetrate most conventional armor. A DU projectile will burn and melt as it penetrates steel, becoming 'sharper' rather than blunting. As the projectile passes through armor the heat build up causes it to catch fire and disintergrates into fine particles on re-encountering air.
The US military is a major user of DU projectiles. It uses the DU in an alloy with around 3.5% titanium. By the US Army it is used in 120 mm or 105 mm caliber by the M1 Abrams tanks and in 25 mm by the M919 mounted on the M2, M3 Bradley and the LAV-AT. The US Navy use it in their 25 mm CIWS and the Mk 38 machine gun. The airforce use it in 30 mm caliber on the A-10 Thunderbolt and in 25 mm on the Marine Corps Harrier.
Some modern tanks, including the Abrams, "fight fire with fire" by using depleted uranium as one of the layers of their complex, multi-layered armor systems.
Other uses of depleted uranium include as keels for sailboats, counterweights in oil drills, and in other places where there is a need to place a weight that occupies as little space as possible, such as aircraft ballast.
Health concerns
Environmental groups have raised concerns about the use of this material. Whilst its radioactivity per se is not of concern, uranium is also chemically toxic, roughly the same as lead. The chemical toxicity is of concern to the occupants of the vehicle hit and it may be a major problem for troops or civilians who come close later. Health effects from depleted uranium has been speculated to be one of the possible causes of Gulf war syndrome. This possibility has been widely denied by authorities and studies in countries that use DU weapons.