Nuclear reactors for shipboard propulsion can be of any type used for the production of useful heat. Nuclear power is particularly suitable for vessels which need to be at sea for long periods without refueling or for powerful submarine propulsion. Only the pressurized water reactor and the liquid metal reactor have actually been applied to operating vessels. The pressurized water reactor has been most widely applied since it uses a readily available coolant and has a relatively simple cycle and control system and a large industrial and technical base. The supposed advantages of a liquid metal reactor (compactness, fast response, and higher propulsion plant efficiency) have not been proven in application, and liquid metal reactors are not now in marine service. See also Nuclear power; Nuclear reactor; Reactor physics.
In all the shipboard nuclear power plants that have been built, energy conversion is based on the steam turbine cycle, and that portion of the plant is more or less conventional. There are two types in use: a steam turbine geared to a fixed-pitch propeller (called a geared turbine), and a steam turbine generator whose output drives an electric motor connected to a propeller (called a turboelectric unit). Any energy conversion process that converts heat into mechanical energy could be used to propel a ship. For example, a closed-cycle helium gas turbine has been studied, but none has been built for ship propulsion. See also Marine engine; Marine machinery; Propeller (marine craft); Ship powering, maneuvering, and seakeeping.
