Fuel assemblies are loaded into a nuclear reactor where nuclear fission produces energy. The energy released is used to create steam which drives turbines and, connected to generators, produce electricity. After several years' operation, the fuel assemblies become less efficient and have to be replaced. The assemblies are now classified as "spent fuel" and after unload from the reactor, they are placed into ponds until they are ready to be transported for storage or reprocessing. Transported by road, rail and sea to the reprocessing sites, mainly based in France and the United Kingdom, they are shipped in robust containers complying with the strict international regulations.