The S3W reactor is a naval reactor used by the United States Navy to provide electricity generation and propulsion on warships. The S3W designation stands for:
This nuclear reactor was a variant of the Submarine Fleet Reactor (SFR), an advance on the pressurized water reactor (PWR) plant design employed on . The S3W plant utilized horizontal U-tube steam generators. This concept developed into vertical U-tube steam generators in the S5W reactor plant, and is in common use today on commercial PWR power plants.
and the four boats were built with S3W reactors: , , , and . USS Swordfish and USS Seadragon had the S3W reactor installed in the S4W reactor plant, which was an alternate arrangement of the same components.