Power output of nuclear reactors is controlled by control rods that sit in between the fuel rods:
These are pulled out to increase fission rates - and lowered to decrease them.
The control rods are made from neutron absorbing materials - such as cadmium.
When lowered completely, so many neutrons are absorbed, that no chain reaction is maintained. Pulling them out increases the neutrons that are available for fission.
The lower tips of the fuel rods will almost always be exposed to neutrons, so these should burn down much faster (have their uranium fissioned) than the upper parts that rarely have the control rods removed from them. Is there some engineering going on to have the fuel rods burn down at the same rate along their whole length?