The US and NATO militaries used DU penetrator rounds in the 1991 Gulf War, the Bosnia war, bombing of Serbia, the 2003 invasion of Iraq, and 2015 airstrikes on ISIS in Syria. It is estimated that between 315 and 350 tons of DU were used in the 1991 Gulf War. Production and availability Natural uranium metal contains about , , and about . The production of enriched uranium using isotope separation creates depleted uranium containing only 0.2% to 0.4% . Because natural uranium begins with such a low percentage of , enrichment produces large quantities of depleted uranium. For example, producing of enriched uranium requires of natural uranium, and leaves about of depleted uranium having only . The Nuclear Regulatory Commission (NRC) defines depleted uranium as uranium with a percentage of the isotope that is less than by weight (see 10 CFR 40.4). The military specifications designate that the DU used by the U.S. Department of Defense (DoD) contain less than . In actuality, DoD uses only DU that contains approximately . Depleted uranium is further produced by recycling spent nuclear fuel, in which case it contains traces of neptunium and plutonium. Quantities are so small that they are considered to be not of serious radiological significance (even) by ECRR. Uranium hexafluoride Most depleted uranium is stored as uranium hexafluoride, a toxic crystalline solid, (D), in steel cylinders in open air storage yards close to enrichment plants. Each cylinder holds up to of . In the U.S. of depleted had accumulated by 1993. In 2008, in 57,122 storage cylinders were located near Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky. The storage of (D) presents environmental, health, and safety risks because of its chemical instability. When is exposed to water vapor in the air, it reacts with the moisture to produce (uranyl fluoride), a solid, and (hydrogen fluoride), a gas, both of which are highly soluble and toxic. The uranyl fluoride solid acts to plug the leak, limiting further escape of depleted . Release of the hydrogen fluoride gas to the atmosphere is also slowed by the plug formation. Like any other uranium compound, it is radioactive, and precautions should be taken. It is also highly toxic. Uranyl fluoride is corrosive and harmful upon inhalation, ingestion, or skin absorption. Ingestion or inhalation may be fatal. Effects of exposure may be delayed. There have been several accidents involving uranium hexafluoride in