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 (PWR) and the liquid metal reactor (LMR) 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. For basic information on fission reactors