A2W reactor 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 , 2A - 2B , 3A - 3B , and 4A - 4B ( numbered according to shaft they power ) . 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 ( with a possible maximum speed up to approximately ) . 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 . Design and operation . The reactors are pressurized water reactors 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 neutrons , 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 . In the steam generators , the water from the feed system is converted to steam at and a pressure of about . 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 .