Magnox is a now obsolete type of nuclear power reactor which was designed and used in Britain, and exported to other countries, both as a power plant, and, when operated correctly, as a producer of plutonium for nuclear weapons. The name magnox comes from the material used to clad the fuel rods inside the reactor. Magnox reactors are pressurised carbon dioxide cooled, graphite moderated reactors using natural uranium (i.e. unenriched) as fuel and magnox alloy as fuel cladding. The design was continuously refined, and very few units are identical. On-load refuelling was an economically essential part of the design, to maximise power station availability by eliminating refuelling downtime. This was particularly important for Magnox as the unenriched fuel had a low burn-up, requiring more frequent changes of fuel than most enriched uranium reactors.