Operation Mosaic was a series of two British nuclear tests, called G1 and G2, conducted in the Montebello Islands in Western Australia on 16 May and 19 June 1956.

The purpose of the tests was to explore increasing the yield of British nuclear weapons through boosting with lithium-6 and deuterium, and the use of a natural uranium tamper.

[2] Fearing a resurgence of United States isolationism, and of Britain losing its great power status, the British Government restarted its own development effort,[3] which was given the cover name "High Explosive Research".

[4] The first British atomic bomb was tested in Operation Hurricane at the Montebello Islands in Western Australia on 3 October 1952.

The technology mastered in Operation Hurricane was six years old and, with the hydrogen bomb in hand, the US Congress saw no benefit in renewing cooperation with the UK.

[7] Although Britain strove for independence, at the same time it sought interdependence, in the form of a renewal of the Special Relationship with the United States.

The scientists had also heard a rumour from American sources that the yield could be improved by up to 50 per cent through the use of a natural uranium tamper.

The Operation Totem tests of 1953 had been carried out at Emu Field in South Australia, but that location was considered unsuitable.

[11] It was too isolated, with the nearest road over 160 kilometres (100 mi) away, and only tracked vehicles, or those with special tyres, could traverse the intervening sand dunes.

[11] The Australian minister for supply, Howard Beale, responding to rumours reported in the newspapers,[15] asserted that "the Federal Government has no intention of allowing any hydrogen bomb tests to take place in Australia.

[21] Eden informed Menzies that the two shots would be from towers, which would produce a fifth of the fallout of that of Operation Hurricane, and there would be no danger to people or animals on the mainland.

[23] The Atomic Trials Executive in London, chaired by Lieutenant General Sir Frederick Morgan, had already begun planning Operation Buffalo.

[24] On 18 July 1955, a five-man mission, headed by Martell, that included Adams, Menaul and Lieutenant Commanders A. K. Dodds and R. R. Fotheringham, departed the UK for Australia.

[27] The Australian Government created a Montebello Working Party as a subcommittee of the Maralinga Committee, a counterpart to the British Mosex.

Mosex agreed that at least two members of the AWTSC would be present on board the Task Force 308 flagship, the Landing Ship, Tank, HMS Narvik, when the decision to fire was taken.

The frigate HMS Alert, normally the yacht of the Commander-in-Chief, Far East Fleet, was loaned to act as an accommodation ship for scientists and VIPs.

The sloop HMAS Warrego and boom defence vessel Karangi carried out a hydrographic survey of the Montebello Islands, laying marker buoys for moorings.

[35] Lieutenant-Colonel R. N. B. Holmes was in charge of the Royal Engineers, whose task including erecting the 91-metre (300 ft) aluminium towers for the shots.

[34] Five members of the AWTSC, Leslie H. Martin, Ernest Titterton, Cecil Eddy, Butement and Dwyer, arrived at Onslow and were flown to Narvik by helicopter on 14 May.

[39] Penney sent a message to Adams 10 May:[40] Strongly advise not showing Safety Committee any significant weapon details, but would not object to their seeing outside of cabled ball in centre section.

They could be told that fissile material is at centre of large ball of high explosive and that elaborate electronics necessary to get symmetrical squash.

A tent with a decontamination area was established ashore, and a water pump allowed the Radiological Group to wash themselves before they returned to Narvik.

The implosion system had performed flawlessly, but the boosting effect of the lithium deuteride had been negligible because the process had not been fully understood.

The fissile core of the device was delivered to Onslow by RAF Hastings on 6 June and, once again, was couriered to the Montebello Islands by HMS Alert.

To allow Narvik to return to the UK and refit for Operation Grapple, the first test of a British hydrogen bomb, 15 July was set as the terminal date for Mosaic.

As the deadline approached, William Cook, the scientist in charge of the hydrogen bomb project at Aldermaston, determined that, in view of the results of G1, G2 was now more important than ever.

[46] The weather improved on 8 June, and Martell ordered the countdown to begin the following day, but Beale objected to a test being carried out on a Sunday.

[56] Because fallout was detected over northern Australia by monitoring stations, in the light of Beale's announcement that G2 would be smaller than G1, an impression was generated that something had gone horribly wrong.

A physicist from the Commonwealth X-Ray and Radium Laboratory (CXRL), using a Geiger counter, found no evidence of radioactive contamination, and the deaths were determined to have resulted from red water disease caused by a malaria-like parasite.

[58] It was estimated that someone living in Port Hedland, where the contamination was highest, would have received a dose of 580 microsieverts (0.058 rem) over a period of 50 years, assuming that they wore no clothes.