United States naval reactors are nuclear reactors used by the United States Navy aboard certain ships to generate the steam used to produce power for propulsion, electric power, catapulting airplanes in aircraft carriers, and a few more minor uses. Such naval nuclear reactors have a complete power plant associated with them. All U.S. Navy submarines and supercarriers built since 1975 are nuclear-powered by such reactors. There are no commissioned conventional (non-nuclear) submarines or aircraft carriers left in the U.S. Navy, since the last conventional carrier, , was decommissioned in May 2009. The U.S. Navy had nine nuclear-powered cruisers with such reactors also, but they have since been decommissioned. Reactors are designed by a variety of contractors, then developed and tested at one of several government (Department of Energy) owned and prime contractor-operated facilities: Bettis Atomic Power Laboratory in West Mifflin, Pennsylvania and its associated Naval Reactors Facility in Idaho, and Knolls Atomic Power Laboratory in Niskayuna, New York and its associated Kesselring site in West Milton, New York, all under the management of the office of Naval Reactors. Sometimes there were full-scale nuclear-powered prototype plants built at the Naval Reactors Facility, Kesselring, and Windsor (in Connecticut) to test the nuclear plants, which were operated for years to train nuclear-qualified sailors. Reactor designations Each reactor design is given a three-character designation consisting of: A letter for the type of ship the reactor is intended for ("A" for aircraft carrier, "C" for cruiser, "D" for destroyer, and "S" for submarine) A consecutive generation number A letter for the reactor's designer ("W" for Westinghouse, "G" for General Electric, "C" for Combustion Engineering, and "B" for Bechtel) For example, a S9G reactor represents a submarine (S), ninth-generation (9), General Electric designed reactor (G). History Conceptual analysis of nuclear marine propulsion started in the 1940s. Research on developing nuclear reactors for the Navy was done at Bettis Atomic Power Laboratory in West Mifflin, Pennsylvania starting in 1948. Under the long-term leadership of Admiral Hyman G. Rickover, the first test reactor plant, a prototype referred to as S1W, started up in U.S. in 1953 at the Naval Reactors Facility in Idaho. Bettis Laboratory and Naval Reactors Facility were operated initially and for many decades afterwards by Westinghouse. The first nuclear-powered vessel, the submarine , put to sea in 1955. USS Nautilus marked the beginning of the transition of submarines from relatively slow and short-ranged conventional submarines