There are several clear stages in the life of nuclear fuel, known as the fuel cycle. Most nuclear fuel is made from enriched uranium (although UK Magnox stations use natural uranium). Uranium in the form of uranium ore concentrate is readily available on the world market, where commercial demand is principally determined by the requirements of nuclear electricity generation.
In 2008 the main five main producers were Canada (21%), Kazakhstan (20%), Australia (19%), Namibia (10%) and the Russian Federation (8%). World reactor-related uranium requirements by the year 2035 are projected to increase to between 87370 tU/year in the low case and 138165 tU/year in the high. This compares with 59065 tU global requirement in 2008.
The next step in producing fuel for advanced gas-cooled reactors (AGRs) and pressurised water reactors (PWRs) is to convert uranium into uranium hexafluoride. For AGR and PWR fuel (Magnox stations use natural, un-enriched uranium), this is then enriched to increase the proportion of the uranium 235 (U235) from approximately 0.7% to typically between 2.5 and 5.0%. The enriched material is then converted into either AGR or PWR ceramic fuel pellets, which are packed into stainless steel tubes to form fuel pins, zirconium alloy tubes or fuel rods.
After the AGR fuel elements are delivered to a power station, they are inspected and stored. Each station typically stores up to six months' supply of fuel. To refuel the reactor, several elements are stacked to form a fuel assembly, which is loaded into the reactor using a fuelling machine. AGRs typically contain 300 to 330 fuel assemblies per reactor.
A fuel assembly typically remains within an AGR for four to eight years. It is then removed from the reactor using the fuelling machine and stored for up to a month to allow some of its radioactivity to decay. It is then moved to an irradiated fuel cell for dismantling. The fuel elements are passed to water tanks (called cooling ponds), where they remain for three to six months before being loaded into specially designed flasks and transported off site for reprocessing or long-term storage.
In the UK, PWR fuel is delivered approximately six weeks before refuelling, inspection and storage. The PWR must be shut down for refuelling at intervals of 12 to 18 months, depending on the stations' output and fuel enrichment. PWR refuelling takes place with the primary circuit cooled and depressurised. The top of the pressure vessel is removed, the internal structures above the fuel are disconnected and the fuel elements are lifted from the pressure vessel and passed to the cooling ponds. Approximately two thirds of the discharged fuel and one third of new fuel is then returned to the reactor vessel and the structure is reassembled.
Spent PWR fuel is stored on site in cooling ponds pending transport off site or long-term storage.