On October 3, 1952, at 9.59 in the morning local time, Britain detonated its first atomic bomb in the lagoon of the Monte Bello Islands off the Pilbara coast of Western Australia. The device, codenamed Hurricane, was a plutonium implosion weapon with a yield of about 25 kilotons. It was fired inside the hull of the frigate HMS Plym, partly to test the effect of a ship-borne explosion and partly because the British had no other way to measure what they had built.
The American connection had been cut in 1946. The McMahon Act, passed that year, ended the wartime cooperation under which British scientists had worked at Los Alamos and contributed to the Manhattan Project, and it forbade passing weapons information to anyone at all, ally included. Britain had started its own programme, High Explosive Research, in 1947 under William Penney, who had been at Los Alamos and had gone to Nagasaki after the bombing.
Aldermaston built it. It was flown out. The device was closely related to Fat Man, the Nagasaki weapon, because that was the design the British team knew, and it used a plutonium core with an external neutron initiator of the same kind. About 1,500 people took part in the operation, the Royal Navy supplied most of the ships, and the site was chosen because it was empty, in Australian territory, and a long way from anything.
Australia agreed. It did so more or less without argument. The Menzies government was told in May 1952 and consented inside a week, and the British were given the Monte Bello Islands for a year. Later tests were held at Emu Field in 1953 and at Maralinga from 1956, and the full extent of the contamination at Maralinga did not become public until the McClelland Royal Commission reported in 1985.
Britain went on to test at Christmas Island and in Nevada, and conducted 45 tests in all before signing the Partial Test Ban Treaty in 1963. The 1958 Mutual Defence Agreement restored the exchange of nuclear weapons data with the United States, and the British deterrent has rested on American Trident missiles since 1998.