Nuclear marine propulsion ispropulsion of a ship or submarine with heat provided by anuclear reactor. The power plant heats water to produce steam for a turbine used to turn the ship'spropeller through agearbox or through an electric generator and motor. Nuclear propulsion is used primarily within naval warships such asnuclear submarines andsupercarriers. A small number of experimental civil nuclear ships have been built.^[1]

Compared to oil- or coal-fuelled ships, nuclear propulsion offers the advantage of very long intervals of operation before refueling. All the fuel is contained within the nuclear reactor, so no cargo or supplies space is taken up by fuel, nor is space taken up by exhaust stacks or combustion air intakes.^[2] The low fuel cost is offset by high operating costs and investment in infrastructure, however, so nearly all nuclear-powered vessels are military.^[2]

Most naval nuclear reactors are of thepressurized water type, with the exception of a few^[quantify] attempts^{[by whom?]} at using liquid sodium-cooled reactors.^[2] A primary water circuit transfers heat generated fromnuclear fission in the fuel to asteam generator; this water is kept under pressure so it does not boil. This circuit operates at a temperature of around 250 to 300 °C (482 to 572 °F). Anyradioactive contamination in the primary water is confined. Water is circulated by pumps; at lower power levels, reactors designed for submarines may rely on natural circulation of the water to reduce noise generated by the pumps.^{[citation needed]}

The hot water from the reactor heats a separate water circuit in the steam generator. That water is converted to steam and passes throughsteam driers on its way to thesteam turbine. Spent steam at low pressure runs through acondenser cooled by seawater and returns to liquid form. The water is pumped back to the steam generator and continues the cycle. Any water lost in the process can be made up bydesalinated sea water added to the steam generator feed water.^[3]

In the turbine, the steam expands and reduces its pressure as it imparts energy to the rotating blades of the turbine. There may be many stages of rotating blades and fixed guide vanes. The output shaft of the turbine may be connected to a gearbox to reduce rotation speed, then a shaft connects to the vessel's propellers. In another form of drive system, the turbine turns an electrical generator, and the electric power produced is fed to one or more drive motors for the vessel's propellers. TheRussian,United States, andBritish navies rely on direct steam turbine propulsion, while French and Chinese ships use the turbine to generate electricity for propulsion (turbo-electric transmission).^{[citation needed]}

Some nuclear submarines have a single reactor, but Russian submarines have two, and so hadUSS Triton. Most American aircraft carriers are powered by two reactors, butUSS Enterprise had eight. The majority of marinereactors are of thepressurized water type, although the U.S. and Soviet navies have designed warships powered withliquid metal cooled reactors.^{[citation needed]}

Marine-type reactors differ from land-based commercial electric power reactors in several respects.^{[citation needed]}

While land-based reactors in nuclear power plants produce up to around 1600 megawatts of net electrical power (thenameplate capacity of theEPR), a typical marine propulsion reactor produces no more than a few hundred megawatts. Somesmall modular reactors (SMR) are similar to marine propulsion reactors in capacity and some design considerations and thus nuclear marine propulsion (whether civilian or military) is sometimes proposed as an additional market niche for SMRs. Unlike for land-based applications where hundreds of hectares can be occupied by installations likeBruce Nuclear Generating Station, at sea tight space limits dictate that a marine reactor must be physically small, so it must generate higher power per unit of space. This means its components are subject to greater stresses than those of a land-based reactor. Its mechanical systems must operate flawlessly under the adverse conditions encountered at sea, including vibration and the pitching and rolling of a ship operating in rough seas. Reactor shutdown mechanisms cannot rely on gravity to drop control rods into place as in a land-based reactor that always remains upright. Salt water corrosion is an additional problem that complicates maintenance.^{[citation needed]}

As the core of a seagoing reactor is much smaller than a power reactor, the probability of a neutron intersecting with a fissionable nucleus before it escapes into the shielding is much lower. As such, the fuel is typically more highly enriched (i.e., contains a higher concentration of²³⁵U vs.²³⁸U) than that used in a land-based nuclear power plant, which increases the probability of fission to the level where a sustained reaction can occur. Some marine reactors run on relatively low-enricheduranium, which requires more frequent refueling. Others run onhighly enriched uranium, varying from 20%²³⁵U, to the over 96%²³⁵U found in U.S.submarines,^[4] in which the resulting smaller core is quieter in operation (a big advantage to a submarine).^[5] Using more-highly enriched fuel also increases the reactor's power density and extends the usable life of the nuclear fuel load, but is more expensive and a greater risk tonuclear proliferation than less-highly enriched fuel.^[6]

A marine nuclear propulsion plant must be designed to be highly reliable and self-sufficient, requiring minimal maintenance and repairs, which might have to be undertaken many thousands of miles from its home port. One of the technical difficulties in designing fuel elements for a seagoing nuclear reactor is the creation of fuel elements that will withstand a large amount of radiation damage. Fuel elements may crack over time and gas bubbles may form. The fuel used in marine reactors is a metal-zirconium alloy rather than the ceramic UO₂ (uranium dioxide) often used in land-based reactors. Marine reactors are designed for long core life, enabled by the relatively high enrichment of the uranium and by incorporating a "burnable poison" in the fuel elements, which is slowly depleted as the fuel elements age and become less reactive. The gradual dissipation of the "nuclear poison" increases the reactivity of the core to compensate for the lessening reactivity of the aging fuel elements, thereby extending the usable life of the fuel. The compactreactor pressure vessel is provided with an internalneutron shield, which reduces the damage to the steel from constant neutron bombardment.^{[citation needed]}

Decommissioning nuclear-powered submarines has become a major task for U.S. and Russian navies.^[7] After defuelling, U.S. practice is to cut the reactor section from the vessel for disposal in shallow land burial as low-level waste (see theship-submarine recycling program).^[8] In Russia, whole vessels, or sealed reactor sections, typically remain stored afloat, although a new facility nearSayda Bay is to provide storage in a concrete-floored facility on land for some submarines in the far north.^{[citation needed]}

Russia built afloating nuclear power plant for its far eastern territories. The design has two 35 MWe units based on theKLT-40 reactor used inicebreakers (with refueling every four years). Some Russian naval vessels have been used to supply electricity for domestic and industrial use in remote far eastern and Siberian towns.^{[citation needed]}

In 2010,Lloyd's Register was investigating the possibility of civilian nuclear marine propulsion and rewriting draft rules (see text underMerchant Ships).^[9][10][11]

Insurance of nuclear vessels is not like the insurance of conventional ships. The consequences of an accident could span national boundaries, and the magnitude of possible damage is beyond the capacity of private insurers.^[12] A special international agreement, theBrussels Convention on the Liability of Operators of Nuclear Ships, developed in 1962, would have made signatory national governments liable for accidents caused by nuclear vessels under their flag^[13] but was never ratified owing to disagreement on the inclusion of warships under the convention.^[14] Nuclear reactors under United States jurisdiction are insured by the provisions of thePrice–Anderson Act.^{[citation needed]}

By 1990, there were more nuclear reactors powering ships (mostly military) than there were generating electric power in commercial power plants worldwide.^[15]

Under the direction ofU.S. Navy Captain (later Admiral)Hyman G. Rickover,^[16] the design, development and production of nuclear marine propulsion plants started in theUnited States in the 1940s. The first prototype naval reactor was constructed and tested at theNaval Reactor Facility at the National Reactor Testing Station in Idaho (now called theIdaho National Laboratory) in 1953.

The firstnuclear submarine,USS Nautilus (SSN-571), put to sea in 1955 (SS was a traditionalhull classification symbol for U.S. submarines, while SSN denoted the first "nuclear" submarine).^[17]

The Soviet Union also developed nuclear submarines. The first types developed were the Project 627, NATO-designatedNovember class with two water-cooled reactors, the first of which, K-3Leninsky Komsomol, was underway under nuclear power in 1958.^[18]

Nuclear power revolutionized the submarine, finally making it a true "underwater" vessel, rather than a "submersible" craft, which could only stay underwater for limited periods. It gave the submarine the ability to operate submerged at high speeds, comparable to those of surface vessels, for unlimited periods, dependent only on the endurance of its crew. To demonstrate thisUSS Triton was the first vessel to execute a submergedcircumnavigation of the Earth (Operation Sandblast), doing so in 1960.^[19]

Nautilus, with apressurized water reactor (PWR), led to the parallel development of other submarines like a unique liquid metal cooled (sodium) reactor inUSS Seawolf, or two reactors inTriton, and then theSkate-class submarines, powered by single reactors, and a cruiser,USS Long Beach, in 1961, powered by two reactors.^{[citation needed]}

By 1962, theUnited States Navy had 26 operational nuclear submarines and another 30 under construction. Nuclear power had revolutionized the Navy. The United States shared its technology with theUnited Kingdom, whileFrench,Soviet,Indian andChinese development proceeded separately.^{[citation needed]}

After theSkate-class vessels, U.S. submarines were powered by a series of standardized, single-reactor designs built byWestinghouse andGeneral Electric.Rolls-Royce plc built similar units forRoyal Navy submarines, eventually developing a modified version of their own, thePWR2.^{[citation needed]}

The largest nuclear submarines ever built are the 26,500 tonne RussianTyphoon class. The smallest nuclear warships to date are the 2,700 tonne FrenchRubis-class attack submarines. The U.S. Navy operated an unarmed nuclear submarine, theNR-1 Deep Submergence Craft, between 1969 and 2008, which was not a combat vessel but was the smallest nuclear-powered submarine at 400 tons.^{[citation needed]}

The United States and France have built nuclearaircraft carriers.

The sole French nuclearaircraft carrier example isCharles de Gaulle, commissioned in 2001 (a successor is planned).^[20]

The French carrier is equipped with catapults and arresters. TheCharles de Gaulle has 42,000 tonnes, is theflagship of the French Navy (Marine Nationale). The ship carries a complement ofDassault Rafale M andE‑2C Hawkeye aircraft,EC725 Caracal andAS532 Cougar helicopters forcombat search and rescue, as well as modern electronics andAster missiles.^[21]

The United States Navy operates 11 carriers, all nuclear-powered:^[22]

• USS Enterprise: in service 1962–2012, powered by eight reactor units, is still the only aircraft carrier to house more than two nuclear reactors, with eachA2W reactor taking the place of one of the conventional boilers in earlier constructions.^[23]
• Nimitz class: ten 101,000-ton, 1,092 ft long fleet carriers, the first of which was commissioned in 1975. ANimitz-class carrier is powered by twonuclear reactors providing steam to foursteam turbines.
• Gerald R. Ford class, one 110,000-ton, 1,106 ft long fleet carrier. The lead of the classGerald R. Ford, came into service in 2017, with another nine planned.

Construction of theSoviet aircraft carrier Ulyanovsk began in 1988, but abandoned in 1991 after the dissolution of the Soviet Union.

TheKirov class, Soviet designation 'Project 1144 Orlan' (sea eagle), is a class ofnuclear-poweredguided-missile cruisers of theSoviet Navy andRussian Navy, the largest and heaviestsurface combatant warships (i.e. not anaircraft carrier,amphibious assault ship, orsubmarine) in operation in the world. Among modern warships, they are second in size only to largeaircraft carriers, and of similar size toWorld War II erabattleships. The Soviet classification of the ship-type is "heavy nuclear-powered guided missile cruiser" (Russian:тяжёлый атомный ракетный крейсер). The ships are often referred to asbattlecruisers by Western defence commentators due to their size and general appearance.^[24]

TheUnited States Navy at one time hadnuclear-poweredcruisers as part of its fleet. The first such ship wasUSSLong Beach (CGN-9). Commissioned in 1961, she was the world's first nuclear-poweredsurface combatant.^[25] She was followed a year later byUSSBainbridge (DLGN-25). WhileLong Beach was designed and built as a cruiser,^[26]Bainbridge began life as afrigate, though at that time the Navy was using thehull code "DLGN" for "destroyer leader,guided missile,nuclear".^[27]

The last nuclear-powered cruisers the Americans would produce would be the four-shipVirginia class.USS Virginia (CGN-38) was commissioned in 1976, followed byUSS Texas (CGN-39) in 1977,USS Mississippi (CGN-40) in 1978 and finallyUSS Arkansas (CGN-41) in 1980. Ultimately, all these ships proved to be too costly to maintain^[28] and they were all retired between 1993 and 1999.^{[citation needed]}

SSV-33 Ural (ССВ-33 Урал;NATO reporting name:Kapusta [Russian for "cabbage"]) was acommand and controlnaval ship operated by theSoviet Navy.SSV-33's hull was derived from that of thenuclear-poweredKirov-class battlecruisers with nuclear marine propulsion.^[29]SSV-33 served inelectronic intelligence, missile tracking, space tracking, and communications relay roles. Due to high operating costs,SSV-33 was laid up.^[29]

SSV-33 carried only light defensive weapons. These were two AK-176 76 mm guns, four AK-630 30 mm guns, and four quadruple Igla missile mounts.^{[citation needed]}

ThePoseidon (Russian:Посейдон, "Poseidon",NATO reporting nameKanyon), previously known by Russian codenameStatus-6 (Russian:Статус-6), is a nuclear-powered and nuclear-armedunmanned underwater vehicle under development byRubin Design Bureau, capable of delivering both conventional and nuclearpayloads. According to Russian state TV, it is able to deliver athermonuclearcobalt bomb of up to 200megatonnes (four times as powerful as the most powerful device ever detonated, theTsar Bomba, and twice its maximum theoretical yield) against an enemy's naval ports and coastal cities.^[30]

The following areships that are or were in commercial or civilian use and have nuclear marine propulsion.

Nuclear-powered civil merchant ships have not developed beyond a few experimental ships. The U.S.-builtNS Savannah, completed in 1962, was primarily a demonstration of civil nuclear power and was too small and expensive to operate economically as a merchant ship. The design was too much of a compromise, being neither an efficient freighter nor a viable passenger liner. The German-builtOtto Hahn, completed in 1968, a cargo ship and research facility, sailed some 650,000 nautical miles (1,200,000 km) on 126 voyages over 10 years without any technical problems.^{[citation needed]} It proved too expensive to operate and was converted to diesel. The JapaneseMutsu, completed in 1972, was dogged by technical and political problems. Its reactor had significant radiation leakage and fishermen protested against the vessel's operation. All of these three ships used low-enriched uranium.Sevmorput, a Soviet and later RussianLASH carrier with icebreaking capability, has operated successfully on theNorthern Sea Route since it was commissioned in 1988. As of 2021^[update], it is the only nuclear-powered merchant ship in service.^{[citation needed]}

Civilian nuclear ships suffer from the costs of specialized infrastructure. TheSavannah was expensive to operate since it was the only vessel using its specialized nuclear shore staff and servicing facility. A larger fleet could share fixed costs among more operating vessels, reducing operating costs.

Despite this, there is still interest in nuclear propulsion. In November 2010 British Maritime Technology and Lloyd's Register embarked upon a two-year study with U.S.-based Hyperion Power Generation (nowGen4 Energy), and the Greek ship operator Enterprises Shipping and Trading SA to investigate the practical maritime applications for small modular reactors. The research intended to produce a concept tanker-ship design, based on a 70 MWt reactor such as Hyperion's. In response to its members' interest in nuclear propulsion, Lloyd's Register has also re-written its 'rules' for nuclear ships, which concern the integration of a reactor certified by a land-based regulator with the rest of the ship. The overall rationale of the rule-making process assumes that in contrast to the current marine industry practice where the designer/builder typically demonstrates compliance with regulatory requirements, in the future the nuclear regulators will wish to ensure that it is the operator of the nuclear plant that demonstrates safety in operation, in addition to the safety through design and construction. Nuclear ships are currently the responsibility of their own countries, but none are involved in international trade. As a result of this work in 2014 two papers on commercial nuclear marine propulsion were published by Lloyd's Register and the other members of this consortium.^[10][11] These publications review past and recent work in the area of marine nuclear propulsion and describe a preliminary concept design study for a 155,000 DWT Suezmax tanker that is based on a conventional hull form with alternative arrangements for accommodating a 70 MWt nuclear propulsion plant delivering up to 23.5 MW shaft power at maximum continuous rating (average: 9.75 MW). The Gen4Energy power module is considered. This is a small fast-neutron reactor using lead–bismuth eutectic cooling and able to operate for ten full-power years before refueling, and in service last for a 25-year operational life of the vessel. They conclude that the concept is feasible, but further maturity of nuclear technology and the development and harmonisation of the regulatory framework would be necessary before the concept would be viable.^{[citation needed]}

Nuclear propulsion has been proposed again on the wave ofdecarbonization of marine shipping, which accounts for 3–4% of global greenhouse gas emissions.^[31]

• USNSAmerican Explorer; United States tanker, converted to conventional power while under construction
• Mutsu, Japan (1970–1992); never carried commercial cargo, rebuilt as diesel engine poweredRV Mirai in 1996
• Otto Hahn, Germany (1968–1979); re-powered with diesel engine in 1979
• NS Savannah, United States (1962–1972)
• Sevmorput, Russia (1988–present), ice-strengthened nuclear-poweredlighter aboard ship (LASH) carrier

In December 2023, theJiangnan Shipyard under theChina State Shipbuilding Corporation officially released a design of a 24,000 TEU-classcontainer ship — known as the KUN-24AP — at Marintec China 2023, a premier maritime industry exhibition held inShanghai. The container ship is reported to be powered by athorium-basedmolten salt reactor, making it a first thorium-powered container ship and, if completed, the largest nuclear-powered container ship in the world.^[32] For comparison, as of 2025 thelargest container ships can carry 24,346 TEUs

Nuclear propulsion has proven both technically and economically feasible fornuclear-powered icebreakers in theSoviet, and laterRussian,Arctic. Nuclear-fuelled ships operate for years without refueling, and the vessels have powerful engines, well-suited to the task of icebreaking.^{[citation needed]}

TheSoviet icebreakerLenin was the world's first nuclear-powered surface vessel in 1959 and remained in service for 30 years (new reactors were fitted in 1970). It led to a series of larger icebreakers, the 23,500tonArktika class of six vessels, launched beginning in 1975. These vessels have two reactors and are used in deep Arctic waters.NSArktika was the first surface vessel to reach theNorth Pole.^{[citation needed]}

For use in shallow waters such as estuaries and rivers, shallow-draft,Taymyr-class icebreakers were built inFinland and then fitted with their single-reactor nuclear propulsion system inRussia. They were built to conform to international safety standards for nuclear vessels.^[33]

All nuclear-powered icebreakers have been commissioned by the Soviet Union or Russia.^{[citation needed]}

• Lenin (1959–1989; museum ship)
• Arktika (1975–2008; decommissioned)
• Sibir (1977–1992; scrapped)
• Rossiya (1985–2013; decommissioned)
• Taymyr (1989–present)
• Sovetskiy Soyuz (1989–2014; decommissioned)
• Vaygach (1990–present)
• Yamal (1992–present)
• 50 Let Pobedy (2007–present)
• Arktika (2020–present)^[34]
• Sibir (2021–present)
• Ural (2022–present)^[35]
• Yakutiya (2024–present)

• Air-independent propulsion
• Aircraft Nuclear Propulsion
• Knolls Atomic Power Laboratory
• List of United States Naval reactors
• Naval Reactors
• Nuclear navy
• Nuclear-powered aircraft
• Nuclear Power School
• Soviet naval reactors
• United States naval reactors
• United States Navy Nuclear Propulsion

• The World Nuclear Association
• Naval Nuclear Power Training Command

• Marine propulsion
• Nuclear-powered ships
• Nuclear-powered ships of the United States Navy
• Nuclear technology-related lists

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