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 minor actinides accumulate, leading to reduced fuel efficiency. The two effects cancel each other out. One of the technical difficulties is the creation of a fuel which will tolerate the very large amount of radiation damage. It is known that during use the properties of nuclear fuel change; it is quite possible for fuel to crack and for fission gas bubbles to form.
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.)
The Russian, U.S. and British navies rely on steam turbine propulsion, while the French and Chinese use the turbine to generate electricity for propulsion (turbo-electric propulsion). Most Russian submarines as well as most American aircraft carriers since CVN-65 are powered by two reactors (although Enterprise has eight). U.S., 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, 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, the whole vessels, or the sealed reactor sections, typically remain stored afloat, although a new facility near Sayda Bay is beginning to provide storage in a concrete-floored facility on land for some submarines in the Far North.
Russia is well advanced with plans to build a floating nuclear power plant for their far eastern territories. The design has two 35 MW 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.
Under the direction of Admiral Hyman G. Rickover, the design, development and production of nuclear marine propulsion plants started in the USA in the 1940s, with the first test reactor being started up in 1953. The first nuclear-powered submarine, USS Nautilus (SSN-571), put to sea in 1955. Much of the early development work on naval reactors was done at the Naval Reactor Facility on the campus of the Idaho National Laboratory.
Nautilus led to the parallel development of further Skate-class submarines, powered by single reactors, and a cruiser, USS Long Beach (CGN-9), in 1961, powered by two reactors. The aircraft carrier USS Enterprise, commissioned in 1962, was powered by eight reactor units in 1960. Enterprise remains in service.
By 1962 the United States Navy had 26 nuclear submarines operational and 30 under construction. Nuclear power had revolutionized the Navy. The technology was shared with the United Kingdom, while French, Soviet, Indian and Chinese developments proceeded separately.
After the Skate-class vessels, reactor development proceeded and in the USA a single series of standardized 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 (pressurized water reactor).
The largest nuclear submarines ever built are the 26,500 tonne Russian Typhoon class.
Development of nuclear merchant ships began in the 1950s, but has not generally 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 (1,200,000 km) 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. All three vessels used reactors with low-enriched uranium fuel.
The fourth nuclear merchant ship, Sevmorput, operates successfully in the specialised environment of the Northern Sea Route. 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. Using the new micro nuclear reactors, other existing cargo ships could potentially be converted to nuclear propulsion as well.
Nuclear propulsion has proven both technically and economically feasible for nuclear powered icebreakers 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 Soviet icebreaker Lenin was the world's first nuclear-powered surface vessel 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, launched from 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 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.