A silvery-gray metallic chemical element with the highest atomic weight of the naturally occurring elements, it is approximately 70% denser than lead. Uranium is weakly radioactive and occurs naturally in low concentrations (a few parts per million) in soil, rock and water. It is commercially extracted from uranium-bearing minerals such as uraninite. When used in nuclear reactors, uranium is enriched which means that the content of the isotope U235 has been increased.