| Mosaic | |
|---|---|
 Mosaic G2 test | |
| Information | |
| Country | United Kingdom |
| Test site | Montebello Islands, Western Australia |
| Period | May–June 1956 |
| Number of tests | 2 |
| Test type | tower |
| Max. yield | 60 kilotonnes of TNT (250 TJ) |
| Test series chronology | |
Operation Mosaic was a series of two British nuclear tests conducted in the Montebello Islands in Western Australia on 16 May and 19 June 1956. These tests followed the Operation Totem series and preceded the Operation Buffalo series. The second test in the series was the largest ever conducted in Australia.
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. Although a boosted fission weapon is not a hydrogen bomb, which the British Government had agreed would not be tested in Australia, the tests were connected with the British hydrogen bomb programme.
The Operation Totem tests of 1953 had been carried out at Emu Field in South Australia, but Emu Field was considered unsuitable for Operation Mosaic. A new, permanent test site was being prepared at Maralinga in South Australia, but would not be ready until September 1956. It was decided that the best option was to return to the Montebello Islands, where Operation Hurricane had been conducted in 1952. To allow the task force flagship, the tank landing ship HMS Narvik, to return to the UK and refit in time for Operation Grapple, the planned first test of a British hydrogen bomb, 15 July was set as the terminal date for Operation Mosaic. The British Government was anxious that Grapple should take place before a proposed moratorium on nuclear testing came into effect. The second test was therefore conducted under time pressure.
At the time of the Royal Commission into British nuclear tests in Australia it was claimed that the second test was of a significantly higher yield than suggested by the official figures: 98 kilotonnes of TNT (410 TJ) as compared to 60 kilotonnes of TNT (250 TJ), but this remains unsubstantiated.