The S6G reactor used on the Los Angeles class attack submarines is a modified version of the D2G reactor plant first used on the CGN-25 Bainbridge class of guided missile cruisers. Starting with SSN 719 and beyond the last 31 hulls of the class had 12 vertical launch tubes for the Tomahawk cruise missile, along with an upgraded reactor core which did not require refueling.
The S6G submarine reactor plant originally was designed to use the 150 MWt D1G-2 core, similar to the 148 MWt D2G reactor used on guided missile cruisers (CGNs). There was no separate S6G prototype. The D1G-2 core had been tested previously in the D1G prototype at the Kesselring Site in West Milton, NY. The 1 x S6G reactor plant with a D1G-2 reactor core rated @ 150 MWt; 2 x main steam turbines delivering a combined 30,000 shp (est) to a single propeller.
Starting in the mid-1970s, the S6G with D1G-2 core was installed on all Los Angeles-class Flight I subs. The D1G-2 cores in the 17 688 Flight I boats were replaced with D2W cores during their mid-life refueling. The mid-life refuelings for the remaining 14 Flight I boats were cancelled and the boats were decommissioned early.
Starting in the mid-1980s, all Los Angeles-class Flight II and Improved (688i) submarines, starting with USS Providence (SSN-719), were built with a newer D2W core rated at 165 MW. The 1 x S6G reactor plant with a D2W reactor core rated @ 165 MWt; 2 x main steam turbines delivering a combined 33,500 shp (est) to a single propeller. All Flight II and 688i boats require a mid-life refueling. The reactor compartment measured 33 ft in diameter, 42 ft long, and weighted 1,680 tons.
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