 A succession of prime ministers since the Second World War had been determined to persuade the United States to revive wartime co-operation in the area of nuclear weapons research.Macmillan believed that one way to encourage such co-operation would be for the United Kingdom to speed up the development of its own hydrogen bomb, which was successfully tested on 8 November 1957.Macmillan's decision led to increased demands on the Windscale and (subsequently) Calder Hall nuclear plants to produce plutonium for military purposes.