Nuclear Naval Propulsion

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 U235, 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 U235, 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