Uranium is a common naturally occurring and radioactive substance. It is a normal part of rocks, soil, air, and water, and it occurs in nature in the form of minerals – but never as a metal. Uranium metal is silver-colored with a gray surface and is nearly as strong as steel. Natural uranium is a mixture of three types or isotopes called U-234/234U, U-235/235U and U-238/238U. All three are the same chemical, but they have different radioactive properties.
Typical concentrations in soil are a few parts per million ppm. Some rocks contain high enough mineral concentrations of uranium to be mined. The rocks are taken to a chemical plant where the uranium is taken out and made into uranium chemicals or metal. The remaining sand is called mill tailings. Tailings are rich in the chemicals and radioactive materials that were not removed, such as radium and thorium.
One of the radioactive properties of uranium is half-life, or the time it takes for half of the isotope to give off its radiation and change into another substance. The half-lives are very long around 200,000 years for 234U, 700 million years for 235U, and 5 billion years for 238U. This is why uranium still exists in nature and has not all decayed away.
The isotope 235U is useful as a fuel in powerplants and weapons. To make fuel, natural uranium is separated into two portions. The fuel portion has more 235U than normal and is called enriched uranium. The leftover portion with less 235U than normal is called depleted uranium, or DU. Natural, depleted, and enriched uranium are chemically identical. Du is the least radioactive and enriched uranium the most.