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United States Naval reactor

United States Naval reactors are given three-character designations consisting of a letter representing the ship type the reactor is designed for, a consecutive generation number, and a letter indicating the reactor's designer. The ship types are "A" for aircraft carrier, "C" for cruiser, "D" for destroyer, or "S" for submarine. The designers are "W" for Westinghouse, "G" for General Electric, and "C" for Combustion Engineering[?].
    


History

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, 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 single reactors, 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, Enterprise remains in service.

By 1962 the US Navy had 26 nuclear submarines operational and 30 under construction. Nuclear power had revolutionised the Navy.

The technology was shared with Great Britain, while French, the Soviet Union, and Chinese developments proceeded separately.

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

The largest submarines are the 26,500 tonne Russian Typhoon-class[?].

Nuclear Naval Fleets

At the end of the Cold War, in 1989, there were over 400 nuclear-powered submarines operational or being built. Some 250 of these submarines have now been scrapped and some on order cancelled, due to weapons reduction programs. Russia and the United States had over one hundred each, with the United Kingdom and France less than twenty each and China six. The total today is about 160.

The United States is the main navy with nuclear-powered aircraft carriers (10), while both it and Russia have had nuclear-powered cruisers. Russia has eight nuclear icebreakers[?] in service or building. The US Navy has accumulated over 5400 reactor years of accident-free experience, and operates more than 80 nuclear-powered ships.

Civil Vessels

Development of nuclear merchant ships began in the 1950s but has not been commercially successful. The US-built NS Savannah, was commissioned in 1962 and decommissioned eight years later. It was a technical success, but not economically viable. The German-built Otto Hahn[?] cargo ship and research facility sailed some 650,000 nautical miles on 126 voyages in 10 years without any technical problems. However, it proved too expensive to operate and was converted to diesel.

The Japanese Mutsu[?] was the third civil vessel. It was dogged by technical and political problems and was an embarrassing failure. These three vessels used reactors with low-enriched uranium fuel.

In contrast, nuclear propulsion has proven both technically and economically feasible in the Soviet Arctic. The power levels and energy required for icebreaking, coupled with refueling difficulties for other types of vessels, are significant factors. The icebreaker Lenin[?] was the world's first nuclear-powered surface vessel and remained in service for 30 years, though new reactors were fitted in 1970. It led to a series of larger icebreakers, the 23,500 dwt Arktika-class[?], launched from 1975. These vessels have two reactors and are used in deep Arctic waters. 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[?] with one reactor are being 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.

Power plants

Naval reactors are pressurised water types, which differ from commercial reactors producing electricity in that:

  • they have a high power density in a small volume and therefore run on highly-enriched uranium (>20% U-235, originally c93% but apparently now c20-25% in western vessels, twice this in Russian ones),
  • the fuel is not UO2 but a metal-zirconium alloy (c15%U with 93% enrichment, or more U with lower enrichment),
  • they have long core lives, so that refueling 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,
  • the design enables a compact pressure vessel while maintaining safety.
The long core life is enabled by the relatively high enrichment of the uranium and by incorporating a "burnable poison" 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.

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 PWR designs where embrittlement occurs due to neutron bombardment of a very narrow pressure vessel.)

Reactor sizes range up to 190 MW (thermal) in the larger submarines and surface ships. The French Rubis-class submarines[?] have a 48 MW reactor which needs no refueling for 30 years.

The Russian, US and British navies rely on steam turbine propulsion, the French and Chinese use the turbine to generate electricity for propulsion. Most Russian submarines as well as all surface ships since Enterprise are powered by two reactors. US, British, French and Chinese submarines are powered by one.

Decommissioning nuclear-powered submarines has become a major task for US and Russian navies. After defuelling, US 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 the whole vessels, or the sealed reactor sections, remain stored afloat indefinitely.

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 is well advanced with plans to build a floating power plant for their far eastern territories. The design has two 35 MWe units based on the the KLT-40 reactor used in icebreakers (with refueling every 4 years).

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