The A2W reactor is a naval nuclear reactor used by the

United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

to provide

electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its s ...

and

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived f ...

warship A warship or combatant ship is a naval ship that is built and primarily intended for naval warfare. Usually they belong to the armed forces of a state. As well as being armed, warships are designed to withstand damage and are usually faster ...

s. The A2W designation stands for: * A = Aircraft carrier platform * 2 = Second generation

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...

History

This

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

was used in the world's first nuclear-powered aircraft carrier, the . The four propulsion plants on ''Enterprise'' each contained two reactors, numbered according to the shaft they powered, 1A-1B, 2A-2B, 3A-3B, and 4A-4B. Each propulsion plant was capable of operating on one reactor plant through most of the power range required to propel the ship at speeds in excess of . Both reactors would have been on-line to simultaneously provide maximum ship speed and plane launching capability.

Design and operation

The reactors are

pressurized water reactor A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) i ...

s fueled by highly enriched (upwards of 93%)

uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exi ...

. Light water is used as both neutron-moderator and reactor coolant.

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 many zirconium minerals. Its existence was predicted by Dmitri M ...

Control rods 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 called a

SCRAM A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor ...

to immediately shut the reactor down in an emergency. 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 instantaneous rate of fission events that take place in the fuel. As the water heats up, it expands and becomes less dense, which provides fewer molecules per volume to moderate the

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

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 pressurised, superheated 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 reactor coolant water 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. There are two main propulsion turbines, one high pressure turbine and one low pressure turbine, with a moisture separator in place between the two. The Low Pressure main propulsion turbine is double-ended, whereby 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 liquid water and recycled to the feed system.

References

{{Westinghouse United States naval reactors Pressurized water reactors