Nuclear-powered submarines can remain submerged for long periods because of self–contained nuclear propulsion systems. Most, including Royal Navy submarines, incorporate a pressurised water reactor (PWR) to raise steam to drive turbines for propulsion and for on-board electricity production. For Britain's latest nuclear-powered submarines, the Royal Navy is now using the second generation of a larger Rolls Royce PWR2 core that lasts four times longer than predecessors, eliminating the need for removal from service for mid-life refuelling.
Elsewhere in the world, nuclear power is also used for surface ship propulsion, for example US aircraft carriers and Russian ice breakers. Nuclear power was used for some civilian cargo ships in the 1960's, but was not found to be economic and only a few vessels were made. However, present economic realities of much higher fuel prices and global concerns over CO2 emissions have brought about renewed interest in nuclear power for some civilian applications.