Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Depleted uranium
About 95% of the depleted uranium produced until now is stored as uranium hexafluoride, (D)UF, in steel cylinders in open air yards close to enrichment plants. Each cylinder contains up to 12.7 tonnes (or 14 US tons) of UF. In the U.S. alone, 560,000 tonnes of depleted UF had accumulated by 1993. In 2005, 686,500 tonnes in 57,122 storage cylinders were located near Portsmouth, Ohio, Oak Ridge, Tennessee, and Paducah, Kentucky.
The long-term storage of DUF presents environmental, health, and safety risks because of its chemical instability. When UF is exposed to moist air, it reacts with the water in the air and produces UOF (uranyl fluoride) and HF (hydrogen fluoride), both of which are highly soluble and toxic. Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades.