|Fig. 1: USS Enterprise (CVAN-65) was the world's first nuclear aircraft carrier. Decommissioned in 2013, it served for 51 years, longer than any other aircraft carrier. (Courtesy of the U.S. Naval History and Heritage Command).|
A nuclear powered ship - be it a surface ship or a submarine - receives its propulsion energy from a nuclear power plant on board. These ships have been widely used in the military since the 1950s as submarines and aircraft carriers, but only a few experimental civil nuclear ships exist, such as the N.S. Savannah, which was built as a part of the U.S. Atoms for Peace program. 
Naval power plants are quite similar to land-based nuclear power reactors. They produce heat through a nuclear reaction that is used to boil water, which then turns a turbine. This spinning turbine either directly turns the propeller of the ship, or is connected to a turbo-electric drive system that produces electric power to turn the propeller.  Naval nuclear power reactors still have a few distinct properties from land-based ones. Due to space considerations, these reactors are small (a few hundred megawatts compared to a few gigawatts on land), and have a higher output power density. They generally use pressurized water, run on highly enriched uranium, and use uranium-zirconium or uranium-aluminum alloy, or a metal-ceramic.  They have long core lives, therefore need little refueling, and they contain a safe yet compact pressure vessel. Furthermore, due to the need of flexible power output and space constraints, the thermal efficiency of naval reactors are less than land-based nuclear power plants.
Naval nuclear reactors provide more miles per unit of raw fuel compared to combustion-driven power sources. Thus, nuclear ships are much faster, need to carry much less fuel, and do not need an oxygen source.  Furthermore, with rising oil prices, nuclear ships might be more economical than conventionally powered ships, since savings in fuel costs might offset the additional up-front costs of nuclear-powered ships. 
Additionally, nuclear ships are environmentally friendly: nuclear fission releases no greenhouse gasses, the confinement against radioactive leakage is excellent, and nuclear ships are even recyclable. For example, after the ship's life cycle ends, the U.S. Navy recycles the ship by defueling the reactor, inactivating the ship, removing the reactor compartment for land disposal, recycling the remainder of the vessel, and disposing of the non-recyclable materials. 
Through the usage of the newest technologies, good training programs, and high levels of standardizations nuclear powered propulsion is one of the safest means of transportation. The United States Navy has had an accident-free record up to this day, operating for 5500 reactor-years.  To further perfect this technology, there are many laboratories in the U.S. that specialize on naval nuclear propulsion work, such as Bettis and Knolls Atomic Power Laboratories. 
Yet, there are problems with this technology, with the leading one being the expense. Nuclear powered ships are much more expensive than those with conventional sources of power (petroleum based), and especially during peace times, it is not entirely clear which technology is cheaper in the long run.  Plus, there are additional problems with the stress on the mechanical system, salt-water corrosion, and operation under shaking and vibrations.
Research on the possibility of nuclear-powered ships started in the 1940s, and the first nuclear powered submarine, USS Nautilus, was completed in 1955. This marked a remarkable change in the capabilities of underwater vessels: nuclear powered submarines could sustain 20-25 knots for weeks without surfacing.  USS Enterprise, and aircraft carrier followed, and in less than a decade, the United States Navy had 26 functioning submarines, and 30 more under construction.
The United States and Russia became the first and leading countries to use this technology, peaking at the end of Cold War. By that time, there were around 400 nuclear powered submarines operational or under construction. Following weapons reduction programs, this number has decreased dramatically, but other countries, such as UK, France, China, and India have joined in. Today, the Navy's combat submarine and carrier forces are entirely nuclear-powered. 
As for civilian use, nuclear propulsion is essential in icebreaking in the Russian Arctic. Only nuclear power can provide the power levels for breaking thick ice with little fueling. Nuclear technology has also helped increase Arctic navigation from two to ten months per year, and year round in the Western Arctic.
Nuclear merchant ships, on the other hand, have not been successful. The main reason of failure is due to high cost.  Though, with decreasing availability of fossil fuels, nuclear technology might enter our daily lives, and might even open way for ships to compete with aircrafts in the market for international deliveries.
© Melis Tekant. 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.
 J. G. Wirt, "N. S. Savannah: A Federal Demonstration Project," in Case Studies in Maritime Innovation, U.S. National Research Council, 1978, pp.29-36.
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 "The Cost-Effectiveness of Nuclear Power for Navy Surface Ships," U. S. Congressional Budget Office, Pub. No. 4028, May 2011.
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 J. G. Wirt, "A Federal Demonstration Project: N.S. Savannah," in Case Studeies in Maritime Innovation (U.S. National Academy of Sciences, 1979), p 29.
 Ronald O'Rourke, "Navy Nuclear-Powered Surface Ships: Background, Issues, and Options for Congress," Congressional Research Service, RL33946, September 2010.