NUCLEAR REACTOR

An assembly in which a nuclear fission chain reaction is maintained and controlled for the production of nuclear energy, radioactive isotopes, or artificial elements. The nuclear fuel used in a reactor consists of fissile material (e.g. uranium-235 which undergoes fission as a consequence of which two nuclides of approximately equal mass are produced together with between two or three neutrons and a considerable quantity of energy. These neutrons cause further fissions so that a chain reaction develops. In order that the reaction should not get out of control, its progress is regulated by neutron absorbers in control rods, only sufficient free neutrons being allowed to exits in the reactor to maintain the reaction at a constant level. The fissile material is usually mixed with a moderator which slows down, or thermalizes, the fast neutrons emitted during fission, so that they are more likely to cause further fissions of the fissile material than they are to be captured by the uranium-238 isotope.

In a heterogeneous reactor the fuel and the moderator are separated in a geometric pattern called a lattice. In a homogeneous reactor the fuel and the moderator are mixed so that they present a uniform medium to the neutrons (e.g., the fuel, in the form of a uranium salt, may be dissolved in the moderator).

Besides this classification, reactors may be described in a number of ways. They may be described in terms of neutron energy (see fast reactor and thermal reactor) or in terms of function, e.g., a power reactor for generating useful electric power, a production reactor for manufacturing fissile material (see also breeder reactor and converter reactor) and a propulsion reactor for supplying motive power to ships, submarines, or spacecraft. Reactors are also described in terms of their fuel (e.g., plutonium reactor), their moderator (e.g. graphite-moderated reactor), or their coolant (e.g., boiling-water reactor).