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A2W reactor
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A2W reactor

The A2W reactor designation for the nuclear reactors on the USS Enterprise (CVN-65) stands for:

 *  A = Aircraft Carrier
 *  2 = The second in the series (A1W was the prototype at the 
        Reactor Testing Station in Idaho)
 *  W = Westinghouse was the prime contractor that designed the reactor.

The four propulsion plants on Enterprise each contain two reactors, numbered 1A-1B, 2A - 2B, 3A - 3B, and 4A - 4B. 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 (61 km/h) - rumours of speeds of 60 knots (111 km/h) are just that - rumours). Both reactors would be on-line to simultaneously provide maximum ship speed and plane launching capability.

The reactors are pressurized water reactors fueled by enriched U-235. Light water is used as both neutron-moderator and reactor coolant. Control rods are used to control the operation of the reactor. Extracting the rods allows the reactor to reach "criticality" - the point at which the nuclear fission reactions reach a self-sustaining level. Thereafter, the rods are "shimmed" in or out to provide small power level adjustments.

Much of the control during steady state operation comes as a result of the negative temperature response of the water. As the water heats up, it expands and provides fewer molecules per volume to moderate the neutrons, hence fewer neutrons are slowed to the required thermal energies to sustain fission. This has the effect of reducing reactor power slightly, keeping the power at the level set by the Reactor Operator. 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.

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 and 285 C). In the steam generators, the water from the feed system is converted to steam at 535 F (279 C) and a pressure of about 600 lb/in² (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 lb/in² (4 MPa) 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.