A device in which a nuclear fission chain reaction is sustained and controlled in order to produce nuclear energy, radioisotopes, or new nuclides. The fuels available for use in a fission reactor are uranium–235, uranium–233, and plutonium–239; only the first occurs in nature (as 1 part in 140 of natural uranium); the others have to be produced artificially (see nuclear fuel). When a uranium–235 nucleus is made to undergo fission by the impact of a neutron it breaks into two roughly equal fragments, which release either two or three very high-energy neutrons. These fast neutrons need to be slowed down to increase the probability that they will cause further fissions of ²³⁵U nuclei and thus sustain the chain reaction. This slowing down process occurs naturally to a certain extent when the neutrons collide with other nuclei; unfortunately, however, the predominant uranium isotope, ²³⁸U, absorbs fast neutrons to such an extent that in natural uranium the fission reaction is not self-sustaining. In order to create a controlled self-sustaining chain reaction it is necessary either to slow down the neutrons (using a moderator in a thermal reactor) to greatly reduce the number absorbed by ²³⁸U, or to reduce the predominance of ²³⁸U in natural uranium by enriching it with more ²³⁵U than it normally contains. In a fast reactor the fuel used is enriched uranium and no moderator is employed.

In thermal reactors, neutrons are slowed down by collisions with light moderator atoms (such as graphite, deuterium, or beryllium); they are then in thermal equilibrium with the surrounding material and are known as thermal neutrons. In a heterogeneous thermal reactor the fuel and moderator are in separate solid and liquid phases (e.g. solid uranium fuel and a heavy water moderator). In the homogeneous thermal reactor the fuel and moderator are mixed together, for example in a solution, molten dispersion, slurry, or suspension.

Nuclear reactor. A schematic diagram of a gas-cooled reactor.

In the reactor core the fuel elements encase the fuel; in a heterogeneous reactor the fuel elements may fit into a lattice that also contains the moderator. The progress of the reaction is controlled by control rods, which when lowered into the core absorb neutrons and so slow down or stop the chain reaction. The heat produced by the nuclear reaction in the core is used to generate electricity by the same means as in a conventional power station, i.e. by raising steam to drive a steam turbine that turns a generator. The heat is transferred to the steam-raising boiler or heat-exchanger by the coolant. Water is frequently used as the coolant; in the case of the boiling-water reactor (BWR) and the pressurized-water reactor (PWR) water is both coolant and moderator. In the BWR the primary coolant drives the turbine; in the PWR the primary coolant raises steam in a secondary circuit for driving the turbine. In the gas-cooled reactor the coolant is a gas, usually carbon dioxide with an outlet temperature of about 350°C, or 600°C in the case of the advanced gas-cooled reactor (AGR).

In fast reactors, in which there is no moderator, the temperature is higher and a liquid-metal coolant is used, usually liquid sodium. Some fast reactors are used as converters or breeders. A converter reactor is one that converts fertile material (such as ²³⁸U) into fissile material (such as ²³⁹Pu). A breeder reactor produces the same fissile material as it uses. For example, a fast breeder reactor using uranium enriched with ²³⁹Pu as the fuel can produce more ²³⁹Pu than it uses by converting ²³⁸U to ²³⁹Pu. See also thermonuclear reactor.

http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/nuclear-power-reactors.aspx The website of the World Nuclear Association, an association of companies in the nuclear industry