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Nuclear marine propulsion is propulsion of a ship or submarine with heat provided by a nuclear reactor. The power plant heats water to produce steam for a turbine used to turn the ship's propeller through a gearbox or through an electric generator and motor. Nuclear propulsion is used primarily within naval warships such as nuclear submarines and supercarriers. A small number of experimental civil nuclear ships have been built. Compared to oil- or coal-fuelled ships, nuclear propulsion offers the advantage of very long intervals of operation before refueling. All the fuel is contained within the nuclear reactor, so no cargo or supplies space is taken up by fuel, nor is space taken up by exhaust stacks or combustion air intakes. The low fuel cost is offset by high operating costs and investment in infrastructure, however, so nearly all nuclear-powered vessels are military.