Uranium is a metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has the highest atomic weight of the naturally occurring elements. It occurs in variable concentrations in a range of minerology and orebody types around the world. Uranium is commercially extracted from minerals such as uraninite (pitchblende), and its use is almost exclusively as nuclear fuel in commercial reactors, which are responsible for about 18% of world electricity production. In most reactors, the slightly enriched uranium fuel rods are bombarded with neutrons to produce a sustained chain reaction, in which the uranium atoms are split, releasing more neutrons and heat in a water-moderated pressure vessel. The heat is used to produce steam which powers turbine generators to produce electricity. The countries with the largest reserves and production of natural uranium include Australia, Canada, Kazakhstan, Namibia, Russia, Niger, Uzbekistan, and the United States. Uranium is principally mined via conventional methods (underground and open pit), as well as by in situ leaching, in which acid or alkaline fluid is injected into the orebody, and the uranium leaches into the fluid and is pumped to the surface and extracted. It is often found in conjunction with other minerals such as copper, gold, molybdenum, and vanadium. Natural uranium is mildly radioactive, emitting alpha particles, and has a very long half-life.