The UK produced HEU at its facility in Capenhurst, but production for military purposes ceased there in March 1963. Thereafter, uranium oxide was imported from Australia, Canada, Namibia, South Africa, the United States and Zaire and processed into uranium hexafluoride at Springfields. It was then shipped to the US, where it was enriched at the Portsmouth Gaseous Diffusion Plant near Piketon, Ohio. HEU was then flown back to the UK in RAF aircraft. In 1994, with the Portsmouth plant about to close, the treaty was amended with the US requirement to "provide" uranium enrichment services changed to one to "arrange" them. By March 2002, the UK had a stockpile of 21.86 tonnes of HEU, about 80 years' supply for the Royal Navy's nuclear-powered submarines.