The reaction that occurs in a nuclear reactor core is called nuclear fission.

Here's a breakdown:

* Nuclear Fission: A process where the nucleus of an atom is split into two or more smaller nuclei, releasing a tremendous amount of energy.

* How it works in reactors: Nuclear reactors use a fissionable material like uranium-235. Neutrons bombard the uranium atoms, causing them to split and release more neutrons. These neutrons then strike other uranium atoms, creating a chain reaction that sustains the reactor's energy production.

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