Nuclear Reactor Core

the space in which the controlled chain reaction of nuclear fission of heavy elements (uranium, plutonium) takes place. The chain reaction is accompanied by the release of the kinetic energy of the fission fragments, as well as the energy of neutron radiation, γ-radiation, and β-decay. The core contains the fissionable material, most often fabricated in the form of slugs or rods; a moderator, if the reaction is basically a slow-neutron reaction (there is no moderator in fast-neutron reactors); a coolant for removing the heat produced during the reaction; and components, instruments, and devices of the systems for controlling, monitoring, and shielding the reactor. The fissionable material may be located either apart from the rest of the reactor core’s components (heterogeneous reactor) or in a mixture with those components (homogeneous reactor). Commonly used moderators are water, heavy water, graphite, beryllium, and organic fluids. Reliable heat transfer is necessary to prevent nuclear accidents caused by overheating resulting from the heat produced in the reactor and to improve reactor efficiency. Water, steam, heavy water, organic fluids, helium, and carbon dioxide gas serve as coolants in thermal neutron reactors. Liquid metal (predominantly sodium) is used as a coolant in fast-neutron reactors. The reactor core is usually surrounded on all sides by a neutron reflector to minimize neutron leakage; reflector materials are usually the same as the moderator materials.

From a physical standpoint, the best reactor core shape is a sphere, but design considerations often dictate a cylindrical shape.