The A2W reactor designation for the nuclear reactors on Enterprise is a short-hand for
* A = Aircraft Carrier * 2 = The second in the series (A1W is the prototype at the Reactor Testing Station in Idaho) * W = Westinghouse is the prime contractor.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 - rumours of speeds of 60 knots are just that - rumours). Both reactors would be on-line to simultaneously provide maximum ship speed and plane launching capability.
The reactors are large water-cooled and moderated reactors fueled by enriched U-235. 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, 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 temperature decreases, the density of the medium 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 reactor is sent, via large pipes, into the heat exchanger, also called a steam generator. There the heat from the reactor coolant is transferred, through pipe walls, to water being fed into the S/G from the feed system. In the A1W and A2W systems, the pressurized water reactor coolant is kept in a band from about 525 degrees fahrenheit (cold side) to 545 degrees (hot side). As noted, the resulting steam (535 degrees F) has a pressure of about 600 pounds. Once the reactor water has given off its heat, it is returned, via large electric pumps (four per reactor), to the reactor to repeat the cycle.
Saturated steam (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.