- S = Submarine platform
- 3 = Third generation core designed by the contractor
- W = Westinghouse was the contracted designer
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 USS Nautilus (SSN-571). 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.
The USS Halibut (SSGN-587) and the four Skate class boats were built with S3W reactors: USS Skate (SSN-578) , USS Swordfish (SSN-579) , USS Sargo (SSN-583) , and USS Seadragon (SSN-584) . The USS Swordfish (SSN-579)  and USS Seadragon (SSN-584)  had the S3W reactor installed in the S4W reactor plant, which was an alternate arrangement of the same components.