Water-moderated Reactors
Inside the core of a typical pressurized water reactor or boiling water reactor are nuclear fuel rods equivalent to the diameter of a large gel type ink-pen, each about 12 feet (3.7 m) 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 boron or 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. Conversely, when the control rods are lifted out of the way, more neutrons strike the fissile uranium-235 (U-235) or plutonium-239 (Pu-239) nuclei in nearby fuel rods, and the chain reaction intensifies.
The heat of the fission reaction is removed by the water, which also acts to moderate the neutron reactions.
An alternative form of nuclear fuel would be fissile uranium-233 (U-233) made by the neutron-bombardment of the common thorium-232.
Read more about this topic: Nuclear Reactor Core