Mark 21 nuclear bomb
The Mark 21 nuclear bomb was a United States nuclear gravity bomb first produced in 1955. It was based on the TX-21 "Shrimp" prototype that had been detonated during the Castle Bravo test in March 1954. While most of the Operation Castle tests were intended to evaluate weapons intended for immediate stockpile, or which were already available for use as part of the Emergency Capability Program, Castle Bravo was intended to test a design which would drastically reduce the size and costs of the first generation of air-droppable atomic weapons (the Mk 14, Mk 17 & Mk 24). At long, in diameter, and weighing , the Mk-21 was half the length and one-third the weight of the Mk-17/24 weapons it replaced. Its minimum yield was specified at four megatons.
Joseph Rotblat
At St Bartholomew's, Rotblat worked on the effects of radiation on living organisms, especially on ageing and fertility. This led him to an interest in nuclear fallout, especially strontium-90 and the safe limits of ionising radiation. In 1955, he demonstrated that the contamination caused by the fallout after the Castle Bravo test at Bikini Atoll nuclear test by the United States had been far greater than that stated officially. Until then the official line had been that the growth in the strength of atomic bombs was not accompanied by an equivalent growth in radioactivity released. Japanese scientists who had collected data from a fishing vessel, the "Lucky Dragon", which had inadvertently been exposed to fallout, disagreed with this. Rotblat was able to deduce that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity by forty times. His paper was taken up by the media and contributed to the public debate that resulted in the ending of atmospheric tests by the Partial Nuclear Test Ban Treaty.