Nuclear-powered ships

This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).


Work on nuclear marine propulsion started in the 1940s, and the first test reactor started up in USA in 1953. The first nuclear-powered submarine, USS Nautilus, was put to sea in 1955.

This marked the transition of submarines from slow underwater vessels to warships capable of sustaining 20-25 knots submerged for weeks on end—the submarine had come into its own.

Nautilus led to the parallel development of further (Skate-class) submarines, powered by a single pressurized water reactor (PWR, and an aircraft carrier, USS Enterprise, powered by eight reactor units in 1960. A cruiser, USS Long Beach, followed in 1961 and was powered by two of these early units. Remarkably, the Enterprise remains in service today.

By 1962, the US Navy had 26 nuclear submarines operational and 30 under construction; nuclear power had revolutionized the Navy. The technology was shared with Britain, while French, Russian and Chinese developments proceeded separately.

After the Skate-class vessels, reactor development proceeded, and in the US, a single series of standardized designs was built by both Westinghouse and GE, one reactor powering each vessel. Rolls Royce built similar units for Royal Navy submarines and then developed the design further to the PWR-2.

Russia developed both PWR and lead-bismuth cooled reactor designs, the latter not persisting. Eventually, four generations of submarine PWRs were utilized, the last entering service in 1995 in the Severodvinsk class.

The largest submarines are the 26,500 tonne Russian Typhoon-class, powered by twin 190 MWt PWR reactors, though these were superseded by the 24,000 t Oscar-II class (e.g., Kursk) with the same power plant.

Compared with the excellent safety record of the US nuclear navy, early Soviet endeavors resulted in a number of serious accidents—five where the reactor was irreparably damaged—and more resulting in radiation leaks. However, by development of Russia's third generation of marine PWRs in the late 1970s, safety had become a paramount concern.

Nuclear Naval Fleets

Russia built 248 nuclear submarines and five naval surface vessels powered by 468 nuclear reactors between 1950 and 2003, and was then operating about 60.

Towards the end of the Cold War, in 1989, there were over 400 nuclear-powered submarines operational or being built. At least 300 of these submarines have now been scrapped, and some on order cancelled, due to weapons reduction programs. Russia and the US had over one hundred each in service, while the UK and France had less than twenty each, and China only six. The total number in service today is understood to be about 120, including new ones commissioned.

The US Navy has the most nuclear-powered aircraft carriers (11), and both the US and Russia have had nuclear-powered cruisers (USA: 9, Russia: 4). Russia also has seven nuclear icebreakers and a nuclear freighter in service.

The US Navy has accumulated about 6000 reactor years of accident-free experience, and operates more than 80 nuclear-powered ships (with 103 reactors as of early 2005). Russia has logged 6000 nautical reactor years.

Civil Vessels

Nuclear propulsion has proven technically and economically essential in the Russian Arctic where operating conditions are beyond the capability of conventional icebreakers. The power levels required for breaking ice up to 3 meters thick, coupled with refuelling difficulties for other types of vessels, are significant factors. The nuclear fleet has increased Arctic navigation from 2 to 10 months per year, and in the Western Arctic, to year-round.

The icebreaker Lenin was the world's first nuclear-powered surface vessel (20,000 deadweight tonnes (dwt)) and remained in service for 30 years, though new reactors were fitted in 1970. It led to a series of larger icebreakers, the six 23,500 dwt Arktika-class, launched from 1975. These powerful vessels have two 171 MW OK-900 reactors delivering 54 MW at the propellers and are used in deep Arctic waters. In 1977, the Arktika was the first surface vessel to reach the North Pole.  Rossija, Sovetskiy Soyuz and Yamal were in service towards the end of 2008, with Sibir decommissioned and Arktika retired in October 2008.

The seventh and largest Arktika class icebreaker - 50 Years of Victory (50 Let Pobedy) – was built by the Baltic shipyard at St Petersburg and after delays during construction it entered service in 2007 (twelve years later than the 50-year anniversary of 1945 it was to commemorate).  It is 25,800 dwt, 160 m long and 20m wide, and is designed to break through ice up to 2.8 metres thick.  Its performance in service has been impressive.

For use in shallow waters such as estuaries and rivers, two shallow-draft Taymyr-class icebreakers of 18,260 dwt with one reactor delivering 38 MW were built in Finland and then fitted with their nuclear steam supply system in Russia. They are built to conform with international safety standards for nuclear vessels and have been launched since 1989.

Development of nuclear merchant ships began in the 1950s, but on the whole has not been commercially successful. The 22,000 tonne US-built NS Savannah was commissioned in 1962 and decommissioned eight years later. It was a technical success, but not economically viable. It had a 74 MWt reactor delivering 16.4 MW to the propeller. The German-built 15,000 tonne Otto Hahn cargo ship and research facility sailed some 650,000 nautical miles on 126 voyages in 10 years without any technical problems. It had a 36 MWt reactor delivering 8 MW to the propeller. However, it proved too expensive to operate and in 1982 it was converted to diesel.

The 8000 tonne Japanese Mutsu was the third civil vessel, put into service in 1970. It had a 36 MWt reactor delivering 8 MW to the propeller. It was dogged by technical and political problems and was an embarrassing failure. These three vessels used reactors with low-enriched uranium fuel (3.7 - 4.4% uranium-235).

In 1988, the NS Sevmorput was commissioned in Russia, mainly to serve northern Siberian ports. It is a 61,900 tonne lash-carrier (taking lighters to ports with shallow water) and container ship with ice-breaking bow. It is powered by the same KLT-40 reactor used in larger icebreakers, delivering 32.5 propeller MW from the 135 MWt reactor and it needed refuelling only once to 2003.

Russian experience with nuclear-powered Arctic ships totalled 250 reactor-years in 2003. A more powerful icebreaker of 110 MW net and 55,600 dwt is planned, with further dual-draught ones of 32,400 dwt and 60 MW power at propellers. In 2008 the Arctic fleet was transferred from the Murmansk Shipping Company under the Ministry of Transport to Atomflot, under Rosatom.

Power plants

Naval reactors, with one exception, have been pressurized water types, which differ from commercial reactors producing electricity in that:

  • they deliver a lot of power from a very small volume and therefore run on highly-enriched uranium (U) (>20% uranium-235 (235U), originally about 97% but apparently now 93% in latest US submarines, about 20-25% in some western vessels, and up to 45% in later Russian ones);
  • the fuel is not uranium dioxide (UO2) but a uranium-zirconium or uranium-aluminium (U-Al) alloy (c15% U with 93% enrichment, or more U with less—e.g., 20%—uranium-235) or a metal-ceramic (Kursk: U-Al zoned 20-45% enriched, clad in zircaloy, with about 200kg 235U in each 200 MW core);
  • they have long core lives, so that refuelling is needed only after 10 or more years, and new cores are designed to last 50 years in carriers and 30-40 years in submarines (US Virginia class: lifetime);
  • the design enables a compact pressure vessel while maintaining safety. The Sevmorput pressure vessel for a relatively large marine reactor is 4.6 m high and 1.8 m diameter, enclosing a core 1 m high and 1.2 m diameter; and
  • thermal efficiency is less than in civil nuclear power plants due to the need for flexible power output, and space constraints for the steam system.

The long core life is enabled by the relatively high enrichment of the uranium and by incorporating a "burnable poison" such as gadolinium in the cores which is progressively depleted as fission products and actinides accumulate, leading to reduced fuel efficiency. The two effects cancel one another out.

However, it was reported in 2006 that France has dropped the enrichment level for its naval fuel to 6-7% U-235.

Long-term integrity of the compact reactor pressure vessel is maintained by providing an internal neutron shield. This is in contrast to early Soviet civil pressurized water reactor (PWR) designs where embrittlement occurs due to neutron bombardment of a very narrow pressure vessel.)

The Russian Alfa-class submarines had a single liquid metal cooled reactor (LMR) of 155 MWt and using very highly enriched uranium. These were very fast, but had operational problems in ensuring that the lead-bismuth coolant did not freeze when the nuclear reactor was shut down. The design was unsuccessful and used in only eight trouble-plagued vessels.

Reactor power ranges from 10 MWt (in a prototype) up to 200 MW (thermal) in the larger submarines and 300 MWt in surface ships such as the Kirov-class battle cruisers. The French Rubis-class submarines have a 48 MW reactor which needs no refuelling for 30 years. British Vanguard class submarines of 15,400 t have a single PWR2 reactor with two turbines driving a single pump jet of 20.5 MW.  New versions of this with "Core H" will require no refuelling over the life of the vessel. Russia's Oscar-II class has two 190 MWt reactors.

The Russian, US and British navies rely on steam turbine propulsion, and the French and Chinese use the turbine to generate electricity for propulsion. Russian ballistic missile submarines, as well as all surface ships since the Enterprise, are powered by two reactors. Other submarines (except some Russian attack subs) are powered by only one reactor. A new Russian test-bed submarine is diesel-powered but has a very small nuclear reactor for auxiliary power.

The French aircraft carrier Charles de Gaulle, commissioned in 2000, has two PWR units driving 61 MW Alstom turbines and the system can provide 5 years running at 25 knots before refuelling.  Areva TA (formerly Technicatome) will provide six naval reactors developed from these for France's Barracuda submarines, the first to be commissioned in 2014.  The last of its Le Triomphant class of nuclear submarines (14,000 DWT) was launched in 2008 with an Areva TA 150 MW PWR designated K15.  These units will also power the Barracuda class.

The larger Russian Arktika class icebreakers use two OK-900A (essentially KLT-40) nuclear reactors of 171 MW each with 241 or 274 fuel assemblies of 45-75% enriched fuel and 3-4 year refuelling interval.  They drive steam turbines and each produce up to 33 MW (44,000 hp) at the propellers, though overall power is 54 MW.  The two Tamyr class icebreakers have a single 171 MW KLT-40 reactor giving 35 MW propulsive power.  Sevmorput uses one 135 MW KLT-40 unit producing 32.5 MW propulsive, and all those use 90% enriched fuel.  (The now-retired Lenin's first OK-150 reactors used 5% enriched fuel but were replaced by OK-900 units with 45-75% enriched fuel.)  Most of the Arktika-class vessels have had operating life extensions based on engineering knowledge built up from experience with Arktika itself.  It was originally designed for 100,000 hours of reactor life, but this was extended first to 150,000 hours, then to 175,000 hours.  In practice this equated to a lifespan of eight extra years of operation on top of the design period of 25.  In that time, Arkitka covered more than 1 million nautical miles.

caption UK nuclear submarine layout

For the next generation of Russian icebreakers, integrated light water reactor (LWR) designs are being investigated possibly to replace the conventional PWR. Russia is developing a new icebreaker reactor – RITM-200 – to replace the current KLT reactors.  This is an integral 210 MWt, 55 MWe PWR with inherent safety features.  For floating nuclear power plants (see below) a single RITM-200 would replace twin KLT-40S (but yield less power).

Dismantling decommissioned nuclear-powered submarines has become a major task for US and Russian navies.  After defuelling, normal practice is to cut the reactor section from the vessel for disposal in shallow land burial as low-level waste.  In Russia the whole vessels, or the sealed reactor sections, sometimes remain stored afloat indefinitely, though western-funded programs are addressing this and all decommissioned subs are due to be dismantled by 2012.

Marine reactors used for power supply

A marine reactor was used to supply power (1.5 MWe) to a US Antarctic base for ten years to 1972, testing the feasibility of such air-portable units for remote locations.

Russia has under construction at Severodvinsk the first of a series of floating power plants for their northern and far eastern territories. Two OKBM KLT-40S reactors derived from those in icebreakers, but with low-enriched fuel (less than 20% U-235), will be mounted on a 21,500 tonne, 144 m long barge.  Refuelling interval is 3-4 years on site, and at the end of a 12-year operating cycle the whole plant is returned to a shipyard for a 2-year overhaul and storage of used fuel, before being returned to service. 

Future prospects

With increasing attention being given to greenhouse gas emissions arising from the combustion of fossil fuels for international air and marine transport, and the excellent safety record of nuclear-powered ships, it is quite conceivable that renewed attention will be given to marine nuclear propulsion.

Further Reading

  • WNA paper on Nuclear-powered ships
  • Jane's Fighting Ships, 1999-2000 edition;
  • J Simpson 1995, Nuclear Power from Underseas to Outer Space, American Nuclear Society
  • The Safety of Nuclear Powered Ships, 1992 Report of NZ Special Committee on Nuclear Propulsion
  • Bellona 1996, The Russian Northern Fleet and Civil Nuclear Powered Vessels (on web)
  • M B Maerli, in Bull. Atomic Scientists Sep-Oct 2001.
  • Rawool-Sullivan et al 2002, Technical and proliferation-related aspects of the dismantlement of Russian Alfa-class submarines, Nonproliferation Review, Spring 2002.
  • Thompson, C 2003, Recovering the Kursk, Nuclear Engineering Int'l, Dec 2003.
  • Mitenkov F.M. et al 2003, Prospects for using nuclear power systems in commercial ships in Northern Russia, Atomic Energy 94, 4.


Hore-Lacy, I., & Association, W. (2010). Nuclear-powered ships. Retrieved from


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