|Fig. 1: USS Nautilus (SSN 571). (Source: Wikimedia Commons)|
Ever since sailors aboard USS Nautilus relayed the words "underway on nuclear power" in 1955, the U.S. Navy has incorporated nuclear power plants onboard its submarines and aircraft carriers. Naval nuclear power plants differ from traditional propulsion systems in that nuclear fuel lasts decades without having to resupply and does not require oxygen to operate. This enables submarines and aircraft carriers to operate at sea for much longer durations, limited only to food, munitions, and the sanity of the crew. The Naval Nuclear Propulsion Program provides developmental and operational support to ensure the safe and reliable operation of nuclear power plants within the U.S. Navy. Over the past several decades, manufacturers and laboratories have investigated and improved reactor design concepts that have contributed to the success of nuclear power in both naval and civilian applications. 
Naval nuclear reactors are designated by a letter representing its powered vessel, a number referring to the generation of the core design, and another letter distinguishing its manufacturer. For example, the designator for the S9G reactor found on the Navy's current Virginia-class fast attack submarines contains an S for the submarine platform, a 9 representing the ninth generation core design, and a G for General Electric who manufactures the reactor.
Following WWII, the U.S. Navy contracted the Westinghouse Electric Corporation to create a prototype for a pressurized water reactor (PWR) submarine plant. In 1955, this reactor, named S1W, sustained a 66-day continuous full power simulation of sailing around the world. S1W's success led to its role as the Navy's first naval reactor prototype onboard USS Nautilus (SSN 571), shown in Fig. 1.  Admiral Hyman Rickover was instrumental in standing up the U.S. Navy's nuclear propulsion program, and is credited as the "father of the nuclear Navy." His efforts streamlined nuclear power into naval operations, and delivered dozens of nuclear powered ships to the Fleet.
Submarines rely on stealth to execute their missions, and the submarine force has long been nicknamed "the silent service" for its efforts to remain as quiet as possible while out to sea and reclusion from the national spotlight. As a prototype reactor for USS Narwhal (SSN 671), the S5G was capable of operating in forced or natural circulation flow modes.  Natural circulation enabled water to flow through the reactor plant without the use of pumps, thus reducing noise levels substantially. While successful, the S5G never became the mainstream reactor used in fast attack submarines other than USS Narwhal. However, General Electric continued the natural circulation concept with their S8G reactor, which was designed for the Ohio-class ballistic missile submarine. The Ohio-class submarine currently makes up the Navy's SSBN fleet, and serves as the moat survivable component of the nuclear triad.
|Fig. 2: USS Hawaii (SSN 776), a Virginia-class submarine. (Source: Wikimedia Commons)|
Manufactured by General Electric, the S9G reactor was created for the U.S. Navy's advanced Virginia-class submarine, shown in Fig. 2. The S9G contains new plant components, increased energy density, improved corrosion resistance, and reduced life cycle costs over its predecessors.  The reactor was designed to operate for 33 years without refueling, outliving the 30 year life expectancy of the submarine itself, and produces 40,000 shaft horsepower. 
In 1956, the A1W prototype reactor was created for U.S. Navy surface ships. The plant was installed on the aircraft carrier USS Enterprise (CVN 65), which served as the first nuclear-powered aircraft carrier.  Fast forwarding to today, the A4W reactor is used on the Nimitz-class aircraft carrier, carrying two reactors that each provide 140,000 shaft horsepower.  The Navy's next generation aircraft carrier, the Gerald R. Ford-class, will also have a two reactor design with the A1B. 
Nuclear power continues to play an integral part of operations within the U.S. Navy. As the U.S. Department of Defense continues to purchase fast attack submarines, contract the construction of aircraft carriers, and invest in the Ohio Replacement Program, nuclear power's role within the Navy will continue to grow.
© Christopher DiOrio. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. Ragheb, "Nuclear Naval Propulsion," in Nuclear Power: Deployment, Operation and Sustainability, ed. by P. Tsvetkov (InTech, 2011), p. 3.