A nuclear fuel used in a nuclear reactor that has been compressed into pellets and sealed into metal tubes. There are various designs of rod depending on the type of nuclear reactor. The pellet is formed from an enriched form of the easily fissionable material such as the isotope of uranium-235, which has been converted to uranium dioxide powder. The powder is calcined, compressed into pellets, and loaded into the tubes. The rods used in nuclear reactors are made into an assembly called a fuel element. At the end of the life of the fuel rod in the nuclear reactor, once fissile impurities have built up, the rod is removed, and the unused fuel is recovered and reprocessed for reuse. This involves cutting the rod and recovering the fissile material inside. See nuclear reprocessing.