Nuclear marine propulsion

Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships (see Nuclear navy). Very few experimental civil nuclear ships have been built.



Power plants
Operation of a civil or naval ship power plant is similar to land-based nuclear power reactors. A sustained nuclear reaction in the reactor produces heat that is used to boil water. The resulting steam spins a turbine. The turbine shaft may be coupled through a gearbox speed reducer to the ship's propeller, or in a turbo-electric drive system may run a generator that supplies electric power to motors connected to the propellers.

The Russian, US and British navies rely on steam turbine propulsion, while the French and Chinese ships use the turbine to generate electricity for propulsion (turbo-electric transmission). Most nuclear submarines have a single reactor, but Russian submarines and the USS Triton (SSRN-586) had two. Most American aircraft carriers are powered by two reactors, but the USS Enterprise (CVN-65) had eight. The majority of marine reactors are of the pressurized water type, although the US and Soviet navies have designed warships powered with liquid metal cooled reactors.

Nuclear power for propulsion has several operating and logistic characteristics that appeal to the designers of ships for both civil and military purposes. A small amount of nuclear fuel can provide energy equivalent to millions of times its weight in coal or oil. It is quite practical to build a reactor which will operate a vessel for several years without refuelling. Although the cost of manufacturing nuclear fuel elements is high, the overall cost of fuel is much lower than that of the amount of fossil fuel required to generate the same amount of energy. Like sailing ships, nuclear vessels are independent of the vagaries of procurement of fuel at every port. The laborious and costly process of loading and burning fuel is largely eliminated for most of the vessel's operating life.

Because of its high power density and the elimination of the need for large fuel bunkers, a nuclear propulsion plant allows more space for paying cargo. It also allows a vessel to operate at higher speeds for years without refuelling. This improves the speed and efficiency of ocean-going commerce. Military vessels, such as submarines and aircraft carriers, can travel at high speeds over vast distances, limited only by the endurance of their crews. Arctic vessels can operate for months, independent of fuel supplies.

Nuclear reactors require no oxygen for combustion and emit no exhaust gas. This is a minor benefit for surface vessels, eliminating the ducts, exhaust stacks and machinery needed to support the burning of fossil fuels. For submersible vessels this is the most important advantage. With nuclear power, a submarine can be propelled at speeds comparable to those of surface ships for protracted periods, limited only by crew endurance instead of fuel supply or battery capacity. Although not a motivation for the original development of maritime nuclear power, environmental concerns have sparked increased interest on the part of some who are concerned about effects of CO2, SO2 and other air pollutants emitted by cargo ships. (For economic reasons, ships operate on the lowest-quality fuel their engines can tolerate, resulting in very high emissions of pollutants. (See Environmental impact of shipping.)

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 thousands of megawatts of power, a typical marine propulsion reactor produces no more than a few hundred megawatts. Space considerations 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.

The fuel in a seagoing reactor is typically more highly enriched (i.e., contains a higher concentration of U235 vs. U238) than that used in a land-based nuclear power plant. Some marine reactors run on relatively low-enriched uranium which requires more frequent refueling. Others run on highly enriched uranium, varying from 20% U235, to the over 96% U235 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 which 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 oxide) 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 lengthening the usable life of the fuel. The life of the compact reactor pressure vessel is extended by providing an internal neutron shield, which reduces the damage to the steel from constant bombardment by neutrons.

Decommissioning
Decommissioning nuclear-powered submarines has become a major task for US 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 is well advanced with plans to build a floating nuclear power plant for their 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.

Lloyd's Register is investigating the possibility of civilian nuclear marine propulsion and rewriting draft rules.

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
Under the direction of Admiral (then Captain) Hyman G. Rickover, the design, development and production of nuclear marine propulsion plants started in the USA 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. The first nuclear-powered submarine, USS Nautilus (SSN-571), put to sea in 1955.

The Soviet Union also developed nuclear submarines. The first types developed were the November class, the first of which, K-3 "Leninskiy Komsomol", was underway under nuclear power on July 4, 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.

Nautilus led to the parallel development of further Skate-class submarines, powered by single reactors, and a cruiser, USS Long Beach, in 1961, powered by two reactors. The aircraft carrier USS Enterprise, commissioned in 1961, is powered by eight reactor units.

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, US 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 Typhoon-class submarine. The smallest naval nuclear submarines to date are the 2,700 tonne French Rubis-class attack submarines. The US Navy operated a 400 ton, unarmed nuclear submarine, the NR-1 Deep Submergence Craft, between 1969 and 2008, which was not a combat vessel.

The United States and France have built nuclear aircraft carriers. By 1990 there were more nuclear reactors powering ships (mostly military) than there were generating electric power in commercial power plants worldwide. Many of these submarines and other vessels were decommissioned in the 1990s.

Merchant ships
Nuclear-powered, civil merchant ships have not developed beyond a few experimental ships. The US-built NS 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-built Otto Hahn, a cargo ship and research facility, sailed some 650000 nmi on 126 voyages over 10 years without any technical problems. However, it proved too expensive to operate and was converted to diesel. The Japanese Mutsu 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 specialized infrastructure. The Savannah was expensive since it required many initial costs for the first ship of its class and a nuclear civilian ship, as well as costs for a nuclear shore staff, and servicing facility. As there was only one ship, this was an expensive infrastructure for one. A larger nuclear fleet would be able to use the same infrastructure reducing successive incremental costs: each ship would be cheaper than the last.

Recently there has been renewed interest in nuclear propulsion, and some proposals have been drafted. For example, the cargo coaster is a new design for a nuclear cargo ship.

Icebreakers
Nuclear propulsion has proven both technically and economically feasible for nuclear-powered icebreakers in the Soviet Arctic. Nuclear-fuelled ships operate for years without refueling, and the vessels have powerful engines, well-suited to the task of icebreaking.

The Soviet icebreaker Lenin 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 23,500 ton Arktika-class icebreaker of six vessels, launched beginning in 1975. These vessels have two reactors and are used in deep Arctic waters. NS Arktika was the first surface vessel to reach the North Pole.

For use in shallow waters such as estuaries and rivers, shallow-draft, Taymyr class icebreakers are being built in Finland and then fitted with their single-reactor, nuclear propulsion system in Russia. They are built to conform to international safety standards for nuclear vessels.

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

Merchant cargo ships

 * Mutsu, Japan (1970–1992; never carried commercial cargo)
 * Otto Hahn, Germany (1968–1979; re-powered with diesel engine in 1979)
 * Savannah, United States (1962–1972)
 * Sevmorput, Russia (1988–2012)

Nuclear-powered icebreakers
All nuclear-powered icebreakers have been commissioned by the Soviet Union or Russia.


 * Lenin (1959-1989; museum ship)
 * Arktika (1975-2008; inactive, awaiting refit or scrapping)
 * Sibir (1977–1992; inactive, awaiting refit or scrapping)
 * Rossiya (1985–present)
 * Sovetskiy Soyuz (1990–present)
 * Yamal (1986-present)
 * 50 Let Pobedy, formerly the Ural (2007–present)
 * Taymyr (1989–present)
 * Vaygach (1990–present)