A2W reactor
Encyclopedia
The A2W reactor is a naval reactor
used by the United States Navy
to provide electricity generation
and propulsion
on warship
s. The A2W designation stands for:
This nuclear reactor
was used in the world's first nuclear-powered aircraft carrier, the USS Enterprise (CVN-65)
. The four propulsion plants on Enterprise each contain two reactors, numbered 1A-1B, 4A-4B, 2A-2B, and 3A-3B (numbered as they are located from fore to aft). Each propulsion plant is capable of operating on one reactor plant through most of the power range required to propel the ship at speeds in excess of 33 knots (64.7 km/h) (with a possible maximum speed up to approximately 35 knots (68.6 km/h). Both reactors would be on-line to simultaneously provide maximum ship speed and plane launching capability. The prodigious steam available from eight reactors led to many urban legends crediting Enterprise (and the later Nimitz-class
carriers) with maximum speeds substantially higher than this; however, since the turbines used on Enterprise are identical to those on previous oil-fired carriers, the maximum burst speed cannot be substantially higher.
s fueled by highly-enriched (upwards of 93%) uranium-235
. Light water is used as both neutron-moderator and reactor coolant. Hafnium
Control rod
s are used to control the operation of the reactor. Extracting the rods to a calculated height allows the reactor to reach criticality
— the point at which the nuclear fission
reactions reach a self-sustaining level. Thereafter, steam flow (from the steam generators) regulates reactor power as explained below. The control rods are "shimmed" in or out to regulate average coolant temperature or lowered to the bottom of the reactor vessel to shut the reactor down (either done in a slow controlled manner or dropped rapidly during what is referred to as a SCRAM
to immediately shut the reactor down.
Much of the reactor power control during steady state operation comes as a result of the coolant water's negative temperature coefficient. The power of the reactor is determined by the number of fission events that takes place in the fuel at any given moment. As the water heats up, it expands and becomes less dense which provides fewer molecules per volume to moderate the neutron
s, hence fewer neutrons are slowed to the required thermal energies to sustain thermal fission. Conversely, when the coolant water temperature decreases, its density increases and a greater number of neutrons reach the required thermal energy, increasing the number of fissions per unit of time, creating more heat. This has the effect of allowing "steam demand" to control reactor power, requiring little intervention by the Reactor Operator for changes in the power demanded by the ship's operations.
The hot water from the reactors is sent, via large pipes, into heat exchangers called steam generators. There the heat from the reactor coolant water is transferred, through tube walls, to water being fed into the steam generators from a separate feed system. In the A1W
and A2W systems, the pressurized water reactor coolant is kept between 525 and 545 °F (274–285 °C). In the steam generators, the water from the feed system is converted to steam at 535 °F (279.4 °C) and a pressure of about 600 psi (4 MPa). Once the reactor coolant water has given off its heat in the steam generators, it is returned, via large electric pumps (four per reactor), to the reactors to repeat the cycle.
Saturated steam at 600 psi is channeled from each steam generator to a common header, where the steam is then sent to the main engine, electrical generators, aircraft catapult system, and various auxiliaries. The main propulsion turbines are double-ended, in which the steam enters at the center and divides into two streams as it enters the actual turbine wheels, expanding and giving up its energy as it does so, causing the turbine to spin at high speed. The main shaft enters a reduction gear in which the high rotational velocity of the turbine shaft is stepped down to a usable turn rate for propelling the ship. The expended steam from the main engine and other auxiliaries enters condensers to be cooled into water and recycled to the feed system.
United States Naval reactor
United States Naval reactor refers to nuclear reactors used by the United States Navy aboard certain ships to produce power for propulsion, electric power, catapulting airplanes in aircraft carriers, and a few more minor uses. Such Naval nuclear reactors have a complete power plant associated with...
used by the United States Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
to provide electricity generation
Electricity generation
Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
and propulsion
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...
on warship
Warship
A warship is a ship that is built and primarily intended for combat. Warships are usually built in a completely different way from merchant ships. As well as being armed, warships are designed to withstand damage and are usually faster and more maneuvrable than merchant ships...
s. The A2W designation stands for:
- A = Aircraft carrierAircraft carrierAn aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...
platform - 2 = Second generation coreNuclear reactor coreA nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...
designed by the contractor - W = WestinghouseWestinghouse Electric (1886)Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
was the contracted designer
This nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
was used in the world's first nuclear-powered aircraft carrier, the USS Enterprise (CVN-65)
USS Enterprise (CVN-65)
USS Enterprise , formerly CVA-65, is the world's first nuclear-powered aircraft carrier and the eighth US naval vessel to bear the name. Like her predecessor of World War II fame, she is nicknamed the "Big E". At , she is the longest naval vessel in the world...
. The four propulsion plants on Enterprise each contain two reactors, numbered 1A-1B, 4A-4B, 2A-2B, and 3A-3B (numbered as they are located from fore to aft). Each propulsion plant is capable of operating on one reactor plant through most of the power range required to propel the ship at speeds in excess of 33 knots (64.7 km/h) (with a possible maximum speed up to approximately 35 knots (68.6 km/h). Both reactors would be on-line to simultaneously provide maximum ship speed and plane launching capability. The prodigious steam available from eight reactors led to many urban legends crediting Enterprise (and the later Nimitz-class
Nimitz class aircraft carrier
The Nimitz-class supercarriers are a class of ten nuclear-powered aircraft carriers in service with the United States Navy. With an overall length of and full-load displacements of over 100,000 long tons, they are the largest capital ships in the world...
carriers) with maximum speeds substantially higher than this; however, since the turbines used on Enterprise are identical to those on previous oil-fired carriers, the maximum burst speed cannot be substantially higher.
Design and operation
The reactors are pressurized water reactorPressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...
s fueled by highly-enriched (upwards of 93%) uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
. Light water is used as both neutron-moderator and reactor coolant. Hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
Control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s are used to control the operation of the reactor. Extracting the rods to a calculated height allows the reactor to reach criticality
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
— the point at which the nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
reactions reach a self-sustaining level. Thereafter, steam flow (from the steam generators) regulates reactor power as explained below. The control rods are "shimmed" in or out to regulate average coolant temperature or lowered to the bottom of the reactor vessel to shut the reactor down (either done in a slow controlled manner or dropped rapidly during what is referred to as a SCRAM
Scram
A scram or SCRAM is an emergency shutdown of a nuclear reactor – though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads...
to immediately shut the reactor down.
Much of the reactor power control during steady state operation comes as a result of the coolant water's negative temperature coefficient. The power of the reactor is determined by the number of fission events that takes place in the fuel at any given moment. As the water heats up, it expands and becomes less dense which provides fewer molecules per volume to moderate the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s, hence fewer neutrons are slowed to the required thermal energies to sustain thermal fission. Conversely, when the coolant water temperature decreases, its density increases and a greater number of neutrons reach the required thermal energy, increasing the number of fissions per unit of time, creating more heat. This has the effect of allowing "steam demand" to control reactor power, requiring little intervention by the Reactor Operator for changes in the power demanded by the ship's operations.
The hot water from the reactors is sent, via large pipes, into heat exchangers called steam generators. There the heat from the reactor coolant water is transferred, through tube walls, to water being fed into the steam generators from a separate feed system. In the A1W
A1W reactor
The A1W reactor is a prototype nuclear reactor used by the United States Navy to provide electricity generation and propulsion on warships. The A1W designation stands for:* A = Aircraft carrier platform* 1 = First generation core designed by the contractor...
and A2W systems, the pressurized water reactor coolant is kept between 525 and 545 °F (274–285 °C). In the steam generators, the water from the feed system is converted to steam at 535 °F (279.4 °C) and a pressure of about 600 psi (4 MPa). Once the reactor coolant water has given off its heat in the steam generators, it is returned, via large electric pumps (four per reactor), to the reactors to repeat the cycle.
Saturated steam at 600 psi is channeled from each steam generator to a common header, where the steam is then sent to the main engine, electrical generators, aircraft catapult system, and various auxiliaries. The main propulsion turbines are double-ended, in which the steam enters at the center and divides into two streams as it enters the actual turbine wheels, expanding and giving up its energy as it does so, causing the turbine to spin at high speed. The main shaft enters a reduction gear in which the high rotational velocity of the turbine shaft is stepped down to a usable turn rate for propelling the ship. The expended steam from the main engine and other auxiliaries enters condensers to be cooled into water and recycled to the feed system.