What was the last high-yield thermonuclear weapon design test conducted by the US at Bikini Atoll?
Castle Bravo
Castle Bravo was the first in a series of high-yield thermonuclear weapon design tests conducted by the United States at Bikini Atoll, Marshall Islands, as part of Operation Castle. Detonated on March 1, 1954, the device was the most powerful nuclear device detonated by the United States and its first lithium deuteride fueled thermonuclear weapon.[1][2] Castle Bravo's yield was 15 megatons of TNT, 2.5 times the predicted 6.0 megatons, due to unforeseen additional reactions involving 7Li,[3] which led to the unexpected radioactive contamination of areas to the east of Bikini Atoll.
Nuclear weapons and the United Kingdom
The Green Granite prototype, known as Short Granite, was tested in the first of the Operation Grapple test series, Grapple 1. The bomb was dropped from a height of by a Vickers Valiant piloted by Wing Commander Kenneth Hubbard, off the shore of Malden Island in the Pacific on 15 May 1957. It was Britain's second airdrop of a nuclear bomb after the Operation Buffalo test at Maralinga on 11 October 1956, and the first of a thermonuclear weapon. The United States had not attempted an airdrop of a hydrogen bomb until the Operation Redwing Cherokee test on 21 May 1956. Short Granite's yield was estimated at , far below its designed capability. Despite its failure, the test was hailed as a successful thermonuclear explosion, and the government did not confirm or deny reports that the UK had become a third thermonuclear power. When documents on the series began to be declassified in the 1990s, they were denounced as a hoax. The reports were unlikely to have fooled the American observers.