The U-235-enriched uranium is transformed into uranium oxide, a black powder that is compressed and sintered (baked) to form ceramic pellets (see illustration). The pellets are stacked in tubes about 4 metres long, known as 'pencils'.
These tubes are made of zircaloy, an alloy which allows the nuclear reaction, but which is resistant to heat and corrosion. The pencils are then assembled into a fuel assembly. The fuel assemblies are placed in the reactor. The assemblies are replaced every four years or so.
The stages involved in preparing natural uranium for use in a nuclear reactor are referred to as the 'front end of the nuclear fuel cycle'.
