Uranium (U) is a fairly common, weakly radioactive element with a
naturally low radiation level.
The Earth’s crust contains an average of four grams of uranium per ton, and seawater contains three milligrams per ton. The uranium found in nature is mainly U-238.
Nature’s own nuclear reactors have been active for periods of millions of years in particularly rich uranium deposits. The best known of these is the Oklo in Gabon, West Africa, where a natural nuclear fission reactor was active nearly two billion years ago. At the time, the fissile isotope uranium-235 constituted approximately three percent of the Earth’s crust, providing ample fuel for a natural light water reactor. The Oklo reactor is estimated to have been active for hundreds of thousands of years. The reactor produced the same radioactive fission products produced by today’s nuclear reactors. The nuclear waste from the natural reactor has mostly decayed into stable elements that remain at the location where they were created.