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A Nuclear reactor core is that an portion of a nuclear reactor containing the fuel components where the nuclear reactions take place.

Inside the core of a typical 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 are stacked end to end. Also inside the core are control rods, filled with pellets of substances like hafnium or cadmium 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 control rods are lifted out of the way, more neutrons strike ²³⁵U nuclei in nearby fuel rods, and the level of reaction intensifies.

The core of the molten salt reactor is a block of graphite through which holes are bored in which the 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 are water in which souble nuclear salts (usually uranyl sulfate or uranyl nitrate) have been dissolved. The fuel is also the coolant and the moderator, thus the name "Homogenous" ('mixed together') 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.

Nuclear Reactor Analysis, John Wiley & Sons Canada, Ltd.