Under the Mutual Defence Agreement 5.4 tonnes of UK produced plutonium was sent to the US in return for 6 kilograms of tritium and 7.5 tonnes of highly enriched uranium between 1960 and 1979, replacing production of the British uranium enrichment facility at Capenhurst in Cheshire, although much of the highly enriched uranium was used not for weapons, but as fuel for the growing UK fleet of nuclear submarines.