The S2W reactor is a naval reactor used by the United States Navy to provide electricity generation and propulsion on warships. The S2W designation stands for: * S = Submarine platform * 2 = Second generation core designed by the contractor * W = Westinghouse was the contracted designer This nuclear reactor is the shipboard equivalent of the prototype S1W reactor, with minor design changes, that was installed on the USS Nautilus (SSN-571). As installed in Nautilus it generated . It was originally designated STR.