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A nuclear reactor core is that portion of a nuclear reactor containing the fuel components where the nuclear reactions take place.
Description
Inside the core of a typical pressurized water reactor are pencil-thin nuclear fuel rods, each about 12 feet long, which are grouped by the hundreds in bundles called fuel assemblies. Inside each fuel rod, pellets of uranium or, more commonly, uranium oxide are stacked end to end. Also inside the core are control rods, filled with pellets of substances like hafnium or cadmium or cobalt that readily capture neutrons. When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chain reaction and start more fission. In reverse, when the control rods are lifted out of the way, more neutrons strike 235U nuclei in nearby fuel rods, and the level of reaction intensifies. The heat of the fission reaction is removed by the water which also acts to moderate the neutron energies.
In an advanced gas-cooled reactor the core is made of a graphite neutron moderator in which the fuel assemblies are located. Carbon dioxide gas acts as a coolant and circulates through the graphite removing the fission heat.
The core of the molten salt reactor is a block of graphite through which holes are bored in which molten salt circulates. The graphite serves as a neutron moderator, it is the solid structure of the reactor. The molten salt that circulates in the channels is both the fuel and the coolant, it contains the fissionable material needed to sustain the chain reaction.
Aqueous homogeneous reactors cores employ water in which soluble nuclear salts (usually uranyl sulfate or uranyl nitrate) have been dissolved. As the water serves as the solvent for the uranium salts, it serves as the fuel. As it is water, it serves to cool the reactor as well- hence the name 'homogeneous' (as coolant and fuel are one homogeneous substance). The water can be either heavy water or ordinary light water.
In a gaseous fission reactor the reaction takes place in a core which is bounded and created by magnetic field. The fuel is supplied and fission occurs in the gas phase.
Reference
Nuclear Reactor Analysis, John Wiley & Sons Canada, Ltd. Nuclear technology
"Nuclear reactor core"