The S5G was a pressurized water reactor plant with two coolant loops and two steam generators. It had to be designed with the reactor vessel situated low in the boat and the steam generators high in order for natural circulation of the primary coolant to be developed and maintained. 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 space required by propulsion equipment. The S5G prototype was permanently shut down in May 1995.