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 than natural uranium. Natural uranium contains about , while the DU used by the U.S. Department of Defense contains or less. The less radioactive and non-fissile constitutes the main component of depleted uranium. Uses of DU take advantage of its very high density of ( denser than lead). Civilian uses include counterweights in aircraft, radiation shielding in medical radiation therapy and industrial radiography equipment, and containers for transporting radioactive materials. Military uses include armor plating and armor-piercing projectiles. Most depleted uranium arises as a by-product of the production of enriched uranium for use as fuel in nuclear reactors and in the manufacture of nuclear weapons. Enrichment processes generate uranium with a higher-than-natural concentration of lower-mass-number uranium isotopes (in particular , 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 and less than a third of those in natural uranium. Since has a much longer half-life than the lighter isotopes, DU emits less alpha radiation than natural uranium. DU from nuclear reprocessing has different isotopic ratios from enrichment–by-product DU, from which it can be distinguished by the presence of . The only known natural source of uranium with a content significantly different from 0.72% is found in the natural nuclear fission reactor at Oklo, Gabon. It can be "fingerprinted" as different in origin from manmade depleted uranium by the content, which is 55 ppm in uranium from the Oklo Mine as well as all other natural sources, but will be lower in depleted uranium in accordance with the degree of depletion. DU is about 60% as radioactive as natural uranium. Most of the alpha radiation comes from and whereas beta radiation comes from and that are formed within a few weeks. The use of DU in munitions is controversial because of concerns about potential long-term health effects. Normal functioning of the kidney, brain, liver, heart, and numerous other systems can be affected by exposure to uranium, a toxic metal. It is only weakly radioactive because of the long radioactive half-life of (4.468 × 109 or 4,468,000,000 years) and the low amounts of (half-life about 246,000 years) and (half-life 700 million years). The biological half-life (the average time it