A nuclear reactor, formerly known as an atomic pile, is a device used tae initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear pouer stations for electricity generation and in propulsion o ships. Heat from nuclear fission is passed to a working fluid (watter or gas), which in turn runs through steam turbines. These either drive a ship's propellers or turn Electric generator shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical an industrial use, or for production o weapons-grade plutonium. Some are run only for research. As of April 2014, the IAEA reports there are 435 nuclear power reactors in operation, in 31 countries around the world. By 2017, this increased to 447 operable reactors according tae the World Nuclear Association.