Uranium, a metal, is a natural element, silvery white in colour, shiny, dense and weakly radioactive. It is omnipresent in nature and is found in variable but small quantities in rocks, the ground, air, plants, animals and human beings.
Natural uranium is composed of three radioactive isotopes with the following atomic masses: 238U (99.27 % of the total mass), 235U (0.72 %) and 234U (0.0054 %).
In the human organism there are on average approximately 90 μg (micrograms) of uranium from absorbed air, water and food. Around 66% is in the skeleton, 16% in the liver, 8% in the kidneys and 10% in the other tissues.
Uranium is mainly used in nuclear power stations. Most reactors run on enriched uranium having 1.5 to 3% of U235 instead of the normal 0.72%.