The A4W reactor is a naval reactor used by the United States Navy to propel warships and generate onboard electricity.

The A4W designation stands for:

• A = Aircraft carrier platform

• 4 = Fourth-generation core designed by the contractor

• W = Westinghouse, the contracted designer

These nuclear fission pressurized water reactors (PWRs) were jointly designed by Bettis Atomic Power Laboratory and Knolls Atomic Power Laboratory and built by Westinghouse Electric Corporation. Their reactor cores are expected to operate for about 23 years. The only ships to use these nuclear reactors are the Nimitz class supercarriers, which have two apiece, each of which generates enough steam to produce 140,000 shaft horsepower (104 MWe).