Nuclear Marine Propulsion

Nuclear marine propulsion is propulsion of a ship or submarine with heat provided by a nuclear reactor. The power plant heats water to produce steam for a turbine used to turn the ship's propeller through a gearbox or through an electric generator and motor. Nuclear propulsion is used primarily within naval warships such as nuclear submarines and supercarriers. A small number of experimental civil nuclear ships have been built.

Compared to oil- or coal-fuelled ships, nuclear propulsion offers the advantages 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. However, the low fuel cost is offset by high operating costs and investment in infrastructure, so nearly all nuclear-powered vessels are military.

Power plants

Basic operation of naval ship or submarine

Most naval nuclear reactors are of the pressurized water type, with the exception of a few attempts at using liquid sodium-cooled reactors. A primary water circuit transfers heat generated from nuclear fission in the fuel to a steam generator; this water is kept under pressure so it does not boil. This circuit operates at a temperature of around . Any radioactive 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.

The hot water from the reactor heats a separate water circuit in the steam generator. That water is converted to steam and passes through steam driers on its way to the steam turbine. Spent steam at low pressure runs through a condenser 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 by desalinated sea water added to the steam generator feed water.

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. The Russian, U.S. and British navies rely on direct steam turbine propulsion, while French and Chinese ships use the turbine to generate electricity for propulsion (turbo-electric transmission).

Some nuclear submarines have a single reactor, but Russian submarines have two, and so had . Most American aircraft carriers are powered by two reactors, but had eight. The majority of marine reactors are of the pressurized water type, although the U.S. and Soviet navies have designed warships powered with liquid metal cooled reactors.

Differences from land power plants

Marine-type reactors differ from land-based commercial electric power reactors in several respects.

While land-based reactors in nuclear power plants produce up to around 1600 megawatts of net electrical power (the nameplate capacity of the EPR), a typical marine propulsion reactor produces no more than a few hundred megawatts. Some small 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 acres can be occupied by installations like Bruce 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.

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 of ²³⁵U 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-enriched uranium which requires more frequent refueling. Others run on highly enriched uranium, varying from 20% ²³⁵U, to the over 96% ²³⁵U found in U.S. submarines, in which the resulting smaller core is quieter in operation (a big advantage to a submarine). 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 to nuclear proliferation than less-highly enriched fuel.

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 compact reactor pressure vessel is provided with an internal neutron shield, which reduces the damage to the steel from constant neutron bombardment.

Decommissioning

Decommissioning nuclear-powered submarines has become a major task for U.S. and Russian navies. 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 the ''ship-submarine recycling program''). In Russia, whole vessels, or sealed reactor sections, typically remain stored afloat, although a new facility near Sayda Bay is to provide storage in a concrete-floored facility on land for some submarines in the far north.

Future designs

Russia built a floating nuclear power plant for its far eastern territories. The design has two 35 MWe units based on the KLT-40 reactor used in icebreakers (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.

In 2010, Lloyd's Register was investigating the possibility of civilian nuclear marine propulsion and rewriting draft rules (see text under ''Merchant Ships'')..

Civil liability

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. A special international agreement, the ''Brussels 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 but was never ratified owing to disagreement on the inclusion of warships under the convention. Nuclear reactors under United States jurisdiction are insured by the provisions of the Price–Anderson Act.

Military nuclear ships

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

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

Submarines

The first nuclear submarine, , put to sea in 1955 (SS was a traditional designation for U.S. submarines, while SSN denoted the first "nuclear" submarine).

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

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 this was the first vessel to execute a submerged circumnavigation of the Earth (Operation Sandblast), doing so in 1960.

''Nautilus'', with a pressurized water reactor (PWR), led to the parallel development of other submarines like a unique liquid metal cooled (sodium) reactor in , or two reactors in ''Triton'', and then the s, powered by single reactors, and a cruiser, , in 1961, powered by two reactors.

By 1962, the United 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 the United Kingdom, while French, Soviet, Indian and Chinese development proceeded separately.

After the ''Skate''-class vessels, U.S. submarines were powered by a series of standardized, single-reactor designs built by Westinghouse and General Electric. Rolls-Royce plc built similar units for Royal Navy submarines, eventually developing a modified version of their own, the PWR-2 (pressurized water reactor).

The largest nuclear submarines ever built are the 26,500 tonne Russian . The smallest nuclear warships to date are the 2,700 tonne French attack submarines. The U.S. Navy operated an unarmed nuclear submarine, the NR-1 Deep Submergence Craft, between 1969 and 2008, which was not a combat vessel but was the smallest nuclear-powered submarine at 400 tons.

Aircraft carriers

The United States and France have built nuclear aircraft carriers.

* The sole French nuclear aircraft carrier example is , commissioned in 2001 (a second one is planned).
* The United States Navy have a much broader experience. , in service 1962–2012, powered by eight reactor units, is still the only aircraft carrier to house more than two nuclear reactors, with each A2W reactor taking the place of one of the conventional boilers in earlier constructions. Recent U.S. vessels include the and successor es.

French Navy

The French Navy operates one carrier equipped with catapults and arresters. The is a 42,000 tonne nuclear-powered aircraft carrier, commissioned in 2001 and is the flagship of the French Navy (Marine Nationale). The ship carries a complement of Dassault Rafale M and E‑2C Hawkeye aircraft, EC725 Caracal and AS532 Cougar helicopters for combat search and rescue, as well as modern electronics and Aster missiles.

United States Navy

The United States Navy operates 11 carriers, all nuclear-powered:
*: ten 101,000-ton, 1,092 ft long fleet carriers, the first of which was commissioned in 1975. A ''Nimitz''-class carrier is powered by two nuclear reactors providing steam to four steam turbines and is long,
*, one 110,000-ton, 1,106 ft long fleet carrier. The lead of the class , came into service in 2017, with another nine planned.

Destroyers and cruisers

Russian Navy

The ''Kirov'' class, Soviet designation 'Project 1144 Orlan' (sea eagle), is a class of nuclear-powered guided-missile cruisers of the Soviet Navy and Russian Navy, the largest and heaviest surface combatant warships (i.e. not an aircraft carrier or amphibious assault ship) in operation in the world. Among modern warships, they are second in size only to large aircraft carriers, and of similar size to a World War II era battleships. 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 as battlecruisers by Western defence commentators due to their size and general appearance.

United States Navy

The United States Navy at one time had nuclear-powered cruisers as part of its fleet. The first such ship was USS ''Long Beach'' (CGN-9). Commissioned in 1961, she was the world's first nuclear-powered surface combatant. She was followed a year later by USS ''Bainbridge'' (DLGN-25). While ''Long Beach'' was designed and built as a cruiser, ''Bainbridge'' began life as a frigate, though at that time the Navy was using the hull code "DLGN" for "destroyer leader, guided missile, nuclear".

The last nuclear-powered cruisers the Americans would produce would be the four-ship . was commissioned in 1976, followed by in 1977, in 1978 and finally in 1980. Ultimately, all these ships proved to be too costly to maintain and they were all retired between 1993 and 1999.

Other military ships

Communication and command ships

''SSV-33 Ural'' (''ССВ-33 Урал''; NATO reporting name: ''Kapusta'' Russian for "cabbage"]) was a command and control naval ship operated by the Soviet Navy. ''SSV-33''s hull was derived from that of the nuclear-powered s with nuclear marine propulsion. ''SSV-33'' served in electronic intelligence, missile tracking, space tracking, and communications relay roles. Due to high operating costs, ''SSV-33'' was laid up.

''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.

Nuclear-powered UUV

The Poseidon (russian: Посейдон, "Poseidon", NATO reporting name Kanyon), previously known by Russian codename Status-6 (russian: Статус-6), is a nuclear-powered and nuclear-armed unmanned underwater vehicle under development by Rubin Design Bureau, capable of delivering both conventional and nuclear payloads. According to Russian state TV, it is able to deliver a thermonuclear cobalt bomb of up to 200 megatonnes (four times as powerful as the most powerful device ever detonated, the Tsar Bomba, and twice its maximum theoretical yield) against an enemy's naval ports and coastal cities.

Civilian nuclear ships

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

Merchant ships

Nuclear-powered civil merchant ships have not developed beyond a few experimental ships. The U.S.-built , 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-built , completed in 1968, a cargo ship and research facility, sailed some on 126 voyages over 10 years without any technical problems. However, it proved too expensive to operate and was converted to diesel. The Japanese , 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 Russian LASH carrier with icebreaking capability, has operated successfully on the Northern Sea Route since it was commissioned in 1988. , it is the only nuclear-powered merchant ship in service.

Civilian nuclear ships suffer from the costs of specialized infrastructure. The ''Savannah'' 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 (now Gen4 Energy

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