The S6G reactor plant consists of the reactor coolant, steam generating and other support systems that supplies steam to the engine room of United States Navy's Los Angeles class submarines. The 688 class engine room powers two steam turbines that propel a single-shaft rated at 30,000 horsepower (22 MW) plus the electrical generation system. The S6G reactor plant was originally designed to use the D1G core 2 reactor first used on the CGN-25 Bainbridge class of guided missile cruisers rated at 148 MW. All submarines following the SSN-719 were built with a D2W reactor rated at 165 MW. The D1G-2 reactors are being replaced with D2W reactors when the ships are refueled.

U.S. Naval reactors are assigned three-character designations consisting of a letter representing the ship type the reactor is designed for, a consecutive generation number, and a letter indicating the reactor's designer.

The ship types are: "A" for aircraft carrier, "C" for cruiser, "D" for destroyer, or "S" for submarine.

The designers are: "W" for Westinghouse, "G" for General Electric, and "C" for Combustion Engineering.

Specifications are classified, but the S6G can propel a Los Angeles Class submarine at 25+ knots in open water and 30+ knots while submerged.