|MadSci Network: Chemistry|
Many elements occur naturally as a mixture of isotopes. There are three natural isotopes of oxygen, for instance, because while most atoms of oxygen have eight neutrons in the nucleus, a few (0.04%) have nine neutrons and a few (0.2%) have ten neutrons in the nucleus. Oxygen with eight neutrons and eight protons in the nucleus is called "oxygen 16," because 16 is the total number of nucleons and thus weighs 16 atomic mass units. Uranium is a metal that occurs naturally as a component of most rocks and soils (usually, it is a very small component). Uranium, like oxygen, occurs naturally as a mixture of three isotopes. Most natural uranium (97.27%) is uranium 238, which has 146 neutrons in the nucleus. Uranium 235 (143 neutrons) is only about 0.72% of natural uranium, and uranium 234 (142 neutrons) makes up the rest, less than 0.001%. Uranium 235 is a more useful material than uranium 238, because it's a better fuel for some reactors, such as reactors that provide electrical power or medically useful isotopes. Therefore, the percentage of U-235 in the uranium is "enriched" for reactor fuel by selectively removing U-235 atoms from the mix of natural uranium isotopes (a difficult and expensive process). Common enrichments for power reactors is 3-to-5%, meaning that the uranium fuel is 3% to 5% U-235 by weight. The material FROM which the U-235 was removed, the "waste" of the enrichment process, is called depleted uranium, because it has less U-235 than natural uranium. Ref: "Basic Radiation Protection Technology, 3d ed.," Daniel Gollnick, Pacific Radiation Corporation 1994. Pg. 204.
Try the links in the MadSci Library for more information on Chemistry.