Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. The uranium nucleus binds between 141 and 146 neutrons, establishing six isotopes (233U through 238U), the most common of which are uranium-238 (146 neutrons) and uranium-235 (143 neutrons). All isotopes are unstable and uranium is weakly radioactive. Uranium has the second highest atomic weight of the naturally occurring elements, lighter only than plutonium. Its density is about 70% higher than that of lead, but not as dense as gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth. The main use of uranium in the civilian sector is to fuel nuclear power plants. One kilogram of uranium-235 can theoretically produce about 80 terajoules of energy (8×1013 joules), assuming complete fission; as much energy as 3000 tonnes of coal.Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235. The CANDU and Magnox designs are the only commercial reactors capable of using unenriched uranium fuel.
The uranium market, like all commodity markets, has a history of volatility, moving not only with the standard forces of supply and demand, but also to whims of geopolitics. It has also evolved particularities of its own in response to the unique nature and use of this material.The only significant commercial use for uranium is to fuel nuclear reactors for the generation of electricity. There are 440 reactors operating worldwide, and a total of 60 new reactors that are under construction, with over 150 power reactors (with a total net capacity of some 172,000 MWe) planned and over 340 more proposed. The world's present measured resources of uranium, economically recoverable at a price of $130/kg, are enough to last for some 80 years at current consumption.