A nuclear reactor consists of a containment vessel which surrounds the reactor vessel, a number of Uranium fuel assemblies inside the reactor vessel, a loop of pipe that carries water from the reactor to a steam generator and back to the reactor by means of a pump, another loop of pipe to take steam from the steam generator to the turbine generator and then take water back to the steam generator to be made into steam again by means of a pump. The key to the process is the heat generated in the reactor by the fissioning of Uranium²³⁵. The reactor is started by slowly withdrawing the control rods from the core to get the nuclear chain reaction started. The fuel begins to fission, each atom of U²³⁵ that is struck by a free neutron in turn produces free neutrons, which strike an atom of U²³⁵ to continue the chain reaction. The water in the reactor acts as a moderator to slow the neutrons and make it more likely that they will cause fissioning. The control rods can be moved in or out of the reactor to slow down or speed up the fission reaction. The control rods contain material that absorbs neutrons, such as cadmium or boron. When enough neutrons are absorbed, the reaction stops. In addition to moderation the reaction the water acts as coolant to control the temperature of the core and prevent the fuel from melting. The system operates under pressure, something like a kitchen pressure cooker. This allows the water to reach much higher temperatures, nearly 300°F, than it otherwise could without boiling. When this superheated water reaches the steam generator the cool water in the secondary loop is immediately brought to a boil and converted into steam to turn the blades of the turbine and generate electricity. This is the same principle that is used in plants that burn oil, coal or gas, the heat is used to boil water and turn a turbine, the only difference is the source of the heat. There are a number of different reactor designs in use, but most of the reactors in the United States are either Boiling Water (BWR) or Pressurized Water (PWR) reactors. Worldwide there are several other types including Gas Cooled, Pressurized Heavy Water, Graphite and Water, Fast Breeders, Light Water Breeders, Magnox Gas Cooled. Boiling Heavy Water, Light Water Cooled Heavy Water, and others. The various configurations and moderator/coolant combinations all operate on the basic principal of heat produced by a nuclear chain reaction being used to turn a turbine and generate power. There have been individual examples of several of these other types of reactors built in the US. Some of these have had operating problems or accidents and most are now closed.

The only Liquid Metal Fast Breeder Reactor in commercial operation in the U.S., Fermi Unit One in Monroe County Michigan was permanently shut down in September of 1972 due to denial of an extension of its operating license by the Atomic Energy Commission. Fermi I was a 200 megawatt experimental fast breeder power plant operated by Detroit Edison in Lagoona Beach Michigan on Lake Erie. The plant was ordered in 1956 by a consortium known as the Atomic Power Development Association. Liquid Metal Fast Breeder reactors are designed to produce fissionable Plutonium²³⁹ from the fertile isotope Uranium²³⁸ which represents over 99% of all natural Uranium deposits, and in so doing, theoretically, produce more fuel than they consume. They are cooled by liquid Sodium metal and no moderator is used, hence the term ‘fast’ since they use fast neutrons. The fuel is of two types, in the center of the reactor is the fissionable fuel made of Uranium²³⁵ and/or Plutonium²³⁹, around that is a layer of Uranium²³⁸, natural Uranium oxide, depleted Uranium and Thorium²³² in some combination. As the reactor operates, the outer blanket of fuel is converted to Plutonium²³⁹ and Uranium233 through neutron bombardment. In this way nonfissionable but fertile isotopes are converted to fissionable isotopes which can be reprocessed into additional fuel for the reactor. The liquid Sodium used for the coolant requires certain precautions due to Sodium’s violently reactive nature. It must be prevented from coming into contact with Oxygen or water, either of which would produce and explosive reaction and fire. The reactor vessel must be evacuated and refilled with Argon, an inert gas, and all the Sodium lines must be purged. The steam generator must be designed very carefully in order to prevent any water leaks that could result in an explosion. There is a dual cycle or two loop system of coolant. The primary loop extends from the reactor vessel to a heat exchanger and back to the reactor. The Sodium in this loop becomes radioactive from flowing through the core. The secondary loop goes from the heat exchanger to the steam generator and back again. In the steam generator the heat from the Sodium is transferred to water under 2400psi of pressure at 900°F Sodium has "excellent heat transfer characteristics….[although it] presents problems due to is induced radioactivity, its flammability, and its reactivity with water. Operation of component such as pumps, which are completely immersed in molten Sodium, is a further problem". Unlike Light Water Reactors, Fast Breeder Reactors are capable of reaching the type of critical mass required to produce a low order nuclear explosion as a result of a power excursion that results in a meltdown of the fuel.

Boiling Water Reactors are the second most common type in the United States, there are 35 operational BWRs at this time. The design of the BWR has two variants, Dual Cycle and Single Cycle. In a dual cycle BWR there are two loops for the coolant, in a single cycle there is only one. The vast majority of operating reactors are single cycle. The primary difference between the two is that with a single cycle, radiation precautions must be taken around the turbines since the water used to generate the steam used to turn the turbines has passed through the core of the reactor. In a single cycle BWR steam is generated directly in the core of the reactor at 1000 psia. A steam separator at the top of the reactor vessel directs the steam to the turbines and the water is recirculated through the core by recirculation pumps. "Orifices at the base of each fuel assembly control the coolant flow to the individual fuel assemblies". The water serves as coolant and moderator. The dual cycle system also produces steam in the core, but rather than being channeled to the turbine it goes to a steam generator where its heat is used to produce steam in the secondary loop which is then used to turn the turbines. All BWRs in the US have been designed and manufactured by General Electric. The predominant single cycle type is favored because it "allows a lower capital investment for piping, heat exchangers, pumps etc. It also has a somewhat better thermal efficiency because of the direct generation of steam at the maximum cycle temperature". The BWR also has comparatively low operating costs. Two problems have been identified with the BWR design used in the US, the General Electric designed Mark 1 containment was deemed faulty and a modification was added to prevent breech of containment in the event of a buildup of pressure within it. A manually operated vent was added with a 300 foot stack which can be activated by operators to release pressure from the containment directly to the environment. This prevents the complete loss of containment by intentionally exposing the public to potentially harmful radiation. The other difficulty is with cracking of the welds in the core shroud which surrounds the fuel core in the reactor. This is made of circumferentially welded stainless steel plates. It has been determined that the rate of failure in these welds has been abnormally high. When they fail the fuel bundles are allowed to shift which can affect the ability of the control rods to operate normally.