The only ships to use the A4W nuclear reactors are the Nimitz class supercarriers, which have two reactors . The A4W designation stands for: A = Aircraft carrier platform 4 = Contractor's fourth core design generation W = Westinghouse, the contracted designer. These nuclear fission pressurized water reactors were jointly designed by Bettis Atomic Power Laboratory and Knolls Atomic Power Laboratory and built by Westinghouse Electric Company.
A figure of 550 MWt each is quoted for two A4W units in the Nimitz-class carriers, and these supply 104 shaft MW each (USS Enterprise had eight A2W units of 26 shaft MW and was refuelled three times).
The steam from the A4W reactors is passed through four General Electric turbines, that power the four, 25-foot diameter bronze propellers. The power of the four propeller shafts produce a maximum power output of 280,000 horsepower and a top speed of over 36 knots (56 km/h).
Their reactor cores are expected to operate for about 20 years [some sources cite a life span of 23 years]. Fuel life spans depend on how much the ship is operated, and how accurately one can predict reactor performance. Current cores for the NIMITZ Class aircraft carrier, LOS ANGELES Class SSN, and OHIO Class SSBN last on average about 20 years. Efforts to extend the lives of operating reactor plants are resulting in longer ship lifetimes; NIMITZ-Class carriers are now expected to last 50 years, and OHIO-Class submarines 40 years, versus their original design lifetimes of 30 years.
CVN 78 comes with a brand new propulsion plant design. The A1B reactor plant and its associated propulsion plant will require 50% fewer people to operate and maintain than a Nimitz class. The new A1B reactors also have 25% more energy than legacy A4W reactor plants, allowing for more steaming days and increased operational availability for the Combatant Commander over a 50-year service life.
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