Typical fuel rods in a boiling water reactor.
A fuel rod is a long, slender tube in which nuclear fuel (fissionable material), such as uranium-235, surrounded by cladding material, such as zirconium, is inserted into a reactor. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.
The rods must be surrounded by coolant, otherwise temperatures can rise to levels hot enough to melt metallic components over a prolonged period. This opens the possibility of a serious meltdown, in which molten, highly radioactive material from the reactor core falls through the floor of the containment vessel and into the ground below.