The S5G nuclear reactor, the fifth submarine reactor designed by General Electric, was installed only on Narwhal (SSN-671). It was a natural-circulation powerplant intended to test the potential contribution of this technology to submarine quieting. Reactor primary coolant pumps are one of the primary sources of noise from submarines, and the elimination of coolant pumps and associated equipment would also reduce mechanical complexity and the volume required by propulsion equipment. Its design was the direct ancestor of the S8G reactor used in the Ohio-class submarines.