The S5G was a pressurized water reactor plant with two coolant loops and two steam generators. The design featured a reactor vessel situated low in the boat and the steam generators located high in the boat to support natural circulation of the primary coolant.
This nuclear reactor was installed both as a land-based prototype at the Nuclear Power Training Unit, Idaho National Laboratory near Arco, Idaho, and on board the USS Narwhal (SSN-671); both have been decommissioned. It was intended to test the potential contribution of natural circulation technology to submarine quieting. Reactor primary coolant pumps are a main source of noise from submarines. The elimination of coolant pumps and associated equipment would also reduce mechanical complexity and the space required by propulsion equipment. Its design was the direct ancestor of the S8G reactor used on the Ohio class ballistic missile submarines, another very quiet submarine.
The S5G had primary coolant pumps, but they were only needed for very high speeds. And since the reactor core was designed with very smooth paths for the coolant, the coolant pumps were smaller and quieter than the pumps used by the competing S5W core. In most cases, the submarine could be operated without using coolant pumps at all. The reduction in electrical requirements enabled this design to use two lower-capacity, but quieter electrical turbine generators. The quiet design resulted in a larger hull diameter and a larger primary coolant intake pipe than the competing S5W reactor.  Due to the larger size, the S5G was not used in subsequent attack submarines, but was a precursor to the S8G reactor used in the larger Ohio class submarines.
To further reduce engine plant noise, the typical propulsion setup of two steam turbines driving the screw through a reduction gear unit was changed instead to one large propulsion turbine with no reduction gears. This eliminated the noise from the main reduction gears, but at the cost of a huge main propulsion turbine. The turbine was cylindrical, about 12 feet in diameter, and about 30 feet long. The massive size was a result of the slow rotations needed to directly drive the screw. The same propulsion setup was used for both the USS Narwhal and the land-based prototype.
The concept of a natural circulation plant was relatively new when the Navy requested this design. The prototype plant in Idaho was therefore given quite a rigorous performance shakedown to determine if such a design would work for the U.S. Navy. It was largely a success, although the design never became the basis for any more fast-attack submarines besides the Narwhal. The prototype testing included the simulation of the entire engine room of a submarine. Floating the plant in a large pool of water allowed the prototype to be rotated on its long axis to simulate a hard turn. The effects on natural circulation were evaluated at various angles and during simulated hard maneuvers.
The S5G prototype was permanently shut down in May 1995.