In a nuclear reactor, what is the function of a fuel rod?

In a nuclear reactor, the primary function of a fuel rod is to undergo fission. Fuel rods contain fissile material, typically uranium-235 or plutonium-239, which can sustain a nuclear chain reaction. When the nuclei of these atoms absorb a neutron, they become unstable and split into smaller nuclei, releasing a significant amount of energy in the process along with additional neutrons. These newly released neutrons can then induce further fission in nearby fuel nuclei, perpetuating the chain reaction necessary for generating heat.

This heat is essential, as it is used to produce steam that drives turbines for electricity generation. The choice of this answer reflects an understanding of the basic principles of nuclear fission and the role of fuel in sustaining the reactions within a reactor core.