Nuclear reactor physics
Modern deposits of uranium contain only up to ~0.7% U (and ~99.3% U), which is not enough to sustain a chain reaction moderated by ordinary water. But U has a much shorter half-life (700 million years) than U (4.5 billion years), so in the distant past the percentage of U was much higher. About two billion years ago, a water-saturated uranium deposit (in what is now the Oklo mine in Gabon, West Africa) underwent a naturally occurring chain reaction that was moderated by groundwater and, presumably, controlled by the negative void coefficient as the water boiled from the heat of the reaction. Uranium from the Oklo mine is about 50% depleted compared to other locations: it is only about 0.3% to 0.7% U; and the ore contains traces of stable daughters of long-decayed fission products.Fermi age theory
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.