Nuclear fuel assembly. Close-up of a partially assembled fuel pin bundle at a nuclear fuel assembly factory. Each nuclear fuel pin is a hollow metal tube, up to 3 metres long, which contains small pellets of nuclear fuel. In this case the fuel is mixed oxide (MOX) fuel, which is a mixture of plutonium and uranium. Hundreds of these fuel pins will be bundled together to form a unit containing hundreds of thousands of pellets, which is then loaded into a nuclear reactor to initiate a controlled nuclear fission reaction. This is the Melox factory in Marcoule, France. Photographed in 2004.