Many fuel pellets are stacked end-to-end to form a fuel rod. Each fuel rod is approximately 12 feet long and is encased in a metal tube called the fuel cladding.

The cladding prevents fission products produced in the fuel pellets from escaping into the reactor cooling water. Most radioactive fission products remain in the fuel, very close to where they are formed. However, certain fission products such as krypton and xenon gases or iodine atoms are mobile and may move out of the fuel and become trapped in the narrow gaps between the fuel pellets and the fuel cladding.