One winter’s day in June 1956, Britain detonated the largest weapon ever exploded in Australia. The test, code-named Mosaic G2, was conducted at Western Australia’s remote Montebello Islands, site of the first British atomic test in 1952, over 100 kilometres off the Pilbara coast.

G2 was at least four times the size of the bomb that wiped out the Japanese city of Hiroshima in 1945 and perhaps six times as big. An earlier test held in May, Mosaic G1, was significantly less powerful.

The tests happened at a pivotal moment in the evolution of Britain’s weapons program, as it sought to match the growing nuclear arsenals of the United States (which had stopped sharing information about its nuclear technology with the UK after World War II) and the Soviet Union.

Britain was working towards developing a hugely powerful hydrogen bomb: the US had exploded its own in 1952. Operation Mosaic was made up of two tests, G1 and G2. Its devices were not hydrogen weapons, but “boosted fission” weapons, containing light elements that could potentially boost their explosive output. A component of G2, the “tamper” (a wrapping of dense material around the explosive core), was made of uranium, while G1’s was made of lead.

With Operation Mosaic, the British scientists wanted to test what effect, if any, could be achieved by including these elements, potentially enabling the creation of a high-yield weapon using predominantly fission technology. (It draws its power from splitting heavy atomic nuclei – in this case, plutonium – to release energy.)

‘What the bloody hell is going on?’

The Montebello tests sparked (brief) panic in Australia. A uranium prospector at Marble Bar in WA, 400 kilometres east of the test site, recorded significant radioactivity on his Geiger counters a few hours after the test. When he contacted the British authorities, word of his discovery leaked into the Australian media.

Some thought it was a nuclear accident. Acting prime minister, Arthur Fadden, reportedly sent a cable to British prime minister Anthony Eden bluntly demanding to know “what the bloody hell is going on, the cloud is drifting over the mainland”.

A furore erupted, calmed later by soothing........

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