Figure 1: Captain Hyman G. Rickover, the “Father of the Nuclear Submarine,” inspects the USS Nautilus in 1954.

Nuclear marine propulsion, like microwaves, was a brainchild of war. While research into atomic fission had started in the late 19th century, World War II provided a new sense of urgency for scientists. Nuclear technology made leaps in the six years leading to the conclusion of WWII, culminating in the  creation of the atomic bomb.

In the wake of WWII, the world entered its “atoms for peace” days. People envisioned revolutionizing the world with nuclear powered cars, fridges, and ships (1). By the end of the decade, The Cold War continued to push the progress of nuclear research, although by the end of the decade, even the sea had adopted nuclear energy.

On the seas, leagues away from a port, the scarcity of resources limited warship capabilities. The longer the time spent at sea, the more equipment and food was needed. However, these obstacles were small in relation to the thousands of pounds in bunker fuel that ships demanded.

Nuclear power was perfect for the largest of ships spending months away from home. While merchant and commercial ships proved too small for a nuclear reactor to power efficiently, hulking icebreakers and submarines were suitable for nuclear energy. Unlike conventional fuel sources, reactor cores do not require refueling for over 10 years (2). It took the world’s superpowers years and millions of dollars of research to arrive at these conclusions, but the introduction of nuclear reactors to ships began a new era of seafaring. The limits on how long vessels could stay away from port would soon shift to depend on their human occupants.

The History of Nuclear Marine Propulsion

First Steps

Rising tensions between the United States and the Soviet Union caused the United States to bulk up its military resources. The nation needed bigger and better planes and ships, and in the wake of the destruction caused by “Little Boy” and “Fat Man” over Hiroshima and Nagasaki, respectively, the focus was on the atom. The United States managed to show that nuclear fission could indeed power a jet engine, but the project was cancelled in 1955 and an operational aircraft was never developed (3). Research into nuclear fission for the seas was more fruitful, and models today continue to use a system used by many nuclear land reactors.

The United States Navy beat the Soviet Union to the nuclear powered ship. The development of a nuclear propulsion plant was authorized by Congress in July 1951.  Captain Hyman G. Rickover led the Naval Reactors Branch of the Atomic Energy Commission and would go on to be known as the father of the nuclear submarine (see Fig. 1).

By 1955, the navy had placed a nuclear propulsion reactor in the USS Nautilius. The first lady, Mamie Eisenhower, christened the ship with champagne. After the ship set sail, its operators signaled back “UNDERWAY ON NUCLEAR POWER” in Morse code. The next month, Nautilus departed for southern waters, travelling from New London to San Juan, Puerto Rico. It travelled 1381 miles, a distance ten times longer than any other previous submarine, in less than 90 hours, setting a record for the longest period of submergence for a submarine. And for the first time, the submarine maintained an average speed of 16 knots for more than an hour, two times faster than the speed of most submarines during WWII (4).


By 1962, the U.S. had 26 nuclear submarines with 30 more on the way. The U.S. shared its technology with the United Kingdom while Russian, French, and Chinese developments progressed independently. By this time, the nation was eager to see its work profit. The work to apply the new technology to commercial ships had begun. That same year, a retired naval captain aboard the NS Savannah (see Fig. 2), the world’s first nuclear merchant ship, observed the following:

“I stood in the Savannah’s swept-back superstructure as she moved out into the Atlantic. She slipped along at an effortless 17 knots with only 60 percent power. I heard no noise save the swish of water at her sides. She was clean as a sailing ship; the nuclear plant gave off no smut or smoke or exhaust” (5).

Germany’s Otto Hahn and Japan’s Mutsu soon followed, but these ships actually proved that using fossil fuels for commercial ships was more economically efficient (6). Issues of liability, which could span across numerous nations in the case of an accident, made commissioning nuclear ships tedious. Nuclear energy systems aboard ships would also need to be extremely small, thus the uranium fuel had to be highly enriched. In the wrong hands, the fuel could be used to power atomic bombs (1).

As a research team in the United Kingdom in the 1960s reported, processing nuclear waste was difficult, and storing it for long lengths aboard was impossible. The ships would need access to a port where the waste could be dropped off and handled. At the time, they concluded that nuclear reactors could only be used on large ships that were run intensively on a few long routes with established terminal facilities (7).

The Brussels Convention on the Liability of Operators of Nuclear Ships was a multinational agreement specific to nuclear ships and would have made operation more feasible for commercial ships. After its initial stages of development, disagreements regarding the operation of military vessels blocked the convention, and the agreement was never ratified (8). Nuclear merchant ships ended up as a luxury that could not be diplomatically afforded, and only one more was ever produced.

How Nuclear Propulsion Works

Adaptations for the Seas

Nuclear reactors aboard ships are not immune to the perils of sea. Although the ship can move with degrees of freedom that land reactors cannot endure, space and weight aboard is limited, and the ship is isolated at sea. The reactor must be able to withstand the rolling and pitching seas, be able to generate higher power per unit of space, and fulfill stability requirements. While land reactors produce thousands of megawatts of power, a marine reactor only produces a few hundred megawatts.

The reactor also needs to be extremely sturdy and reliable. However skilled the personnel, maintenance away from port is difficult, and a breakdown can leave a ship stranded at sea with its contaminants seeping across international waters.

The reactor is made to minimize risk. In U.S. Naval submarines, the reactor is typically housed in a cylindrical section in the belly of the submarine, sandwiched between shielded bulkheads. One hundred tons of lead shielding surrounds each reactor. Only the inside of the reactor, roughly the size of a garbage can, is inaccessible for inspection and replacement (1).  It relies on its long core life and the ship’s relatively low energy demand to continue operation. By contrast, a land reactor is inspected and refueled every eighteen months (10).

Pressurized Water Reactor

In a typical nuclear reactor, a pressurized water reactor generates steam that rushes through turbines to produce energy. Pellets of uranium oxide arranged in tubes form the reactor core, and fission in these rods release neutrons from the nucleus of uranium. This release generates energy, and a moderator, typically water, slows down these neurons in order to further fission in other atoms. Neutron-absorbing material such as cadmium and hafmium are inserted or withdrawn from the core to control the reaction rate (9).

The massive amounts of energy released by the core heat up the water in the primary coolant loop of the reactor. The loop holds the water at high pressures and prevents it from evaporating until the water reaches the steam generator. Once the water evaporates, the steam travels through the main turbine. The turbine’s spin generates electricity.

The steam occupies almost 2,000 times the volume that an equal amount of water would.  It moves to the condenser, where electricity-powered pumps circulate water through the cooling system. The water then cools the steam enough so that the steam condenses, later to be reheated and sent through the reactor again.

The primary danger of nuclear energy lies in this condensation process. In the case that the system is cut off from the outside power grid, as in the recent Fukushima explosion, heated steam does not return to water. The build-up of this high-volume steam ultimately leads to a pressure explosion of steam—not, contrary to a common misconception, of the core itself. Every nuclear energy system is equipped with backup upon backup in the case of failure, and the failure of all of these systems is extremely rare (Fig. 3).

Figure 2: NS (Nuclear Ship) Savannah, the first commercial nuclear power cargo vessel, enroute to the World’s Fair in Seattle.

Nuclear Marine Propulsion Today

At the end of the Cold War in 1989, 400 nuclear submarines were operational or under construction. Since then, at least 300 have been scrapped due to weapons reduction programs, and there are currently about 120 vessels in operation, including newly constructed ones.

Since the commercial nuclear ship industry closed in the 1970’s, lack of competition and incentive have driven naval suppliers to a lull in development. Unlike in markets of other military weapons systems, such as aerospace and electronics, prices have crept up. Producers of civilian reactors could not and did not have the financial incentive to match the complexity of maritime reactors (10).

However, recent changes have returned interest to this market. Shipping currently accounts for 5% of greenhouse gas emissions, and the world is focused on climate change. Gen4 Energy, an outgrowth of Los Alamos National Laboratory, has developed a small modular reactor that produces 25 MW using low enriched uranium, while a typical reactor generates 1500 MW (1). The power and fuel choice of the reactor makes what this company calls “nuclear batteries” a breakthrough for the nuclear world.

Nuclear power creates close to zero emissions and is cost-effective. While the per-unit cost of fuel is higher than that of traditional fuels such as coal or gas, the overall cost per unit of energy is much lower in nuclear energy. Regardless, concerns about safety continue to impede the development of nuclear power. Gen4’s nuclear battery is also under scrutiny because its small size makes it vulnerable to theft and abuse.

Nuclear technology has made massive strides since its conception in the early 20th century. Its applications have been limited due to safety concerns, but the new war against global warming is poised to drive its progress. In a span of a few years, safety risks may be minimized and nuclear power—aboard ships and even in cars—may be the norm. Perhaps a reprisal of the Atoms for Peace days will come at last.

Contact Shinri Kamei at



1. E. Harrell, Nuclear Cruise Ships Ahoy? Time (15 November 2010). Available at (05 January 2013).

2. A. R. Newhouse, Ship nuclear propulsion, AccessScience@McGraw-Hill (2012). Available at: (01 January 2013).

3. Ben Sandilands. Cold war photo: Why nuclear jetliners never took off. Crikey. 2012. Available at: (01 January 2013).

4. USS Nautilus. Available at: (01 January 2013).

5. R. Adams, Historical Repetition?: Will Nuclear Propulsion Follow Steam Propulsion?  (01 July 1995).Available at follow-steam-propulsion.html (01 January 2013).

6. Alan R. Newhouse, Ship nuclear propulsion, AccessScience@McGraw-Hill (2012). Available at: (01 January 2013).

7. A Policy for Nuclear-Powered Marine Propulsion, Nature 202, 1272 (1964).

8. Brussels Convention on the Liability of Operators of Nuclear Ships (2010). Available at (05 January 2013).

9. Nuclear Power reactors. World Nuclear Association. December 2012. Available at: (01 January 2013).

10. FAS Military Analysis Network. Available at: (03 January 2013).