Nuclear Naval Propulsion (2011)

The largest experience in operating nuclear power plants has been in nuclear naval propulsion, particularly aircraft carriers and submarines.This accumulated experience may become the basis of a proposed new generation of compact-sized nuclear power plants designs.The mission for nuclear powered submarines is being redefined in terms of signal intelligence gathering and special operations.The nuclear powered vessels comprise about 40 percent of the USA Navy's combatant fleet, including the entire sea based strategic nuclear deterrent.All the USA Navy's operational submarines and over half of its aircraft carriers are nuclear-powered.The main considerations here are that nuclear powered submarines do not consume oxygen like conventional power plants, and that they have large endurance or mission times before fuel resupply; limited only by the available food and air purification supplies on board.Another unique consideration is the use of High Enriched Uranium (HEU) to provide a compact reactor system with enough built-in reactivity to overcome the xenon reactor dead time for quick restarts and long fuel burnup periods between refuelings.During World War II, submarines used diesel engines that could be run on the water surface, charging a large bank of electrical batteries.These could later be used while the submarine is submerged, until discharged.At this point the submarine had to resurface to recharge its batteries and become vulnerable to detection by aircraft and surface vessels.Even though special snorkel devices were used to suck and exhaust air to the submarine shallowly submerged below the water's surface, a nuclear reactor provides it with a theoretically infinite submersion time.In addition, the high specific energy, or energy per unit weight of nuclear fuel, eliminates the need for constant refueling by fleets of vulnerable tankers following a fleet of surface or subsurface naval vessels.On the other hand, a single refueling of a nuclear reactor is sufficient for long intervals of time.With a high enrichment level of 93 percent, capable of reaching 97.3 percent in U 235 , modern naval reactors, are designed for a refueling after 10 or more years over their 20-30 years lifetime, whereas land based reactors use fuel low-enriched to 3-5 percent in U 235 , and need to be refueled every 1-1 1/2 years period.New cores are designed to last 50 years in carriers and 30-40 years in submarines, which is the design goal of the Virginia class of submarines.Burnable poisons such as gadolinium or boron are incorporated in the cores.These allow a high initial reactivity that compensates for the build-up of fission products poisons over the www.intechopen.comNuclear Power -Deployment, Operation and Sustainability 4 core lifetime, as well as the need to overcome the reactor dead time caused by the xenon poison changes as a result of operation at different power levels.Naval reactors use high burn up fuels such as uranium-zirconium, uranium-aluminum, and metal ceramic fuels, in contrast to land-based reactors which use uranium dioxide, UO 2 .These factors provide the naval vessels theoretical infinite range and mission time.For all these considerations, it is recognized that a nuclear reactor is the ideal engine for naval propulsion.A compact pressure vessel with an internal neutron and gamma ray shield is required by the design while maintaining safety of operation.Their thermal efficiency is lower than the thermal efficiency of land based reactors because of the emphasis on flexible power operation rather than steady state operation, and of space constraints.Reactor powers range from 10 MWth in prototypes to 200 MWth in large subsurface vessels, and 300 MWth in surface ships.Newer designs use jet pump propulsion instead of propellers, and aim at an all electrical system design, including the weapons systems such as electromagnetic guns. Historical evolutionIn the USA, initially the General Electric (GE) Company developed a liquid metal reactor concept; and the Westinghouse Company, a pressurized water reactor concept.Each company built an Atomic Energy Commission (AEC) owned and financed development laboratory.Westinghouse used the site of the Allegheny County Airport in a suburb of Pittsburgh, Pennsylvania for what became known as the Bettis Atomic Power Laboratory.GE built the Knolls Atomic Power Laboratory in the state of New York.The Westinghouse program used pressurized water as the coolant.It revealed how corrosive hot water could be on the metal cladding surrounding the fuel.It realized that the use of zirconium resisted such corrosion.The pure metal was initially used as the cladding for the fuel elements, to be later replaced by a zirconium alloy, Zircaloy that improved its performance.Zirconium has a low neutron absorption cross section and, like stainless steel, forms a protective, invisible oxide film on its surface upon exposure to air.This oxide film is composed of zirconia or ZrO 2 and is on the order of only 50 to 100 angstroms in thickness.This ultra thin oxide prevents the reaction of the underlying zirconium metal with virtually any chemical reagent under ambient conditions.The only reagent that will attack zirconium metal at room temperature is hydrofluoric acid, HF, which will dissolve the thin oxide layer off of the surface of the metal and thus allow HF to dissolve the metal itself, with the concurrent evolution of hydrogen gas.Jules Verne, the French author in his 1870 book: "20,000 Leagues Under the Sea," related the story of an electric submarine.The submarine was called the "Nautilus," under its captain Nemo.Science fiction became reality when the first nuclear submarine built by the USA Navy was given the same name.Construction of the Nautilus (SSN-571) started on June 14, 1952, its first operation was on December 30, 1954 and it reached full power operation on January 13, 1955.It was commissioned in 1954, with its first sea trials in 1955.It set speed, distance and submergence records for submarine operation that were not possible with conventional submarines.It was the first ship to reach the North Pole.It was