to ones capable of sustaining submerged for weeks on end. Much of the early development work on naval reactors was done at the Naval Reactors Facility on the campus of the Idaho National Laboratory (INL, previously INEL). USS Nautilus was powered by the S2W reactor, and crew were trained on the land-based S1W reactor at INL. The second nuclear submarine was , which was initially powered by a sodium-cooled S2G reactor, and supported by the land-based S1G reactor at the Kesselring site under Knolls Atomic Power Laboratory operated by General Electric. A spare S2G was also built but never used. USS Seawolf was plagued by superheater problems, with the result that USS Nautilus delivered far superior performance. This and the risks posed by liquid sodium in the event of an accident at sea led Admiral Rickover to select the pressurized water reactor (PWR) as the standard U.S. naval reactor type. The S2G was removed from USS Seawolf and replaced by the S2Wa reactor, using components from the spare S2W that was part of the USS Nautilus program. All subsequent U.S. naval reactors have been PWRs, while the Soviet Navy used mainly PWRs, but also used lead-bismuth cooled liquid metal cooled reactors (LMFR) of three types in eight submarines: and the seven-member . Experience with USS Nautilus led to the parallel development of further () submarines, powered by single reactors, and an aircraft carrier, , powered by eight A2W reactor units in 1960. A cruiser, , followed in 1961 and was powered by two C1W reactor units. USS Enterprise remained in service for over 50 years, and was inactivated in 2012. Full-scale land-based prototype plants in Idaho, New York, and Connecticut preceded development of several types (generations) of U.S. naval nuclear reactors, although not all of them. After initial construction, some engineering testing was done and the prototypes were used to train nuclear-qualified sailors for many years afterwards. For example, the A1W prototype at Naval Reactors Facility led to development of A2W reactors used in USS Enterprise. By 1962, the US Navy had 26 nuclear submarines operational and 30 under construction. Nuclear power had revolutionized the U.S. Navy. The technology was shared with the United Kingdom, while technological development in France, China and the Soviet Union proceeded separately. After the Skate-class vessels, reactor development proceeded and in the U.S. a single series of standardized designs was built by both Westinghouse and