Plutonium is not found in nature, but it is produced in reactors by bombarding urani-um-238 with neutrons. Plutonium-239 (²³⁹Pu) can be fissioned by slow neutrons, and so could be used in weapons. In December 1942, Hanford, Oregon, was chosen as a site for reactors that would produce plutonium. Hanford was selected because it was isolated but also on the Columbia River, which afforded a source of cooling water.