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.
Rongelap Atoll
From 1946 through 1958 the United States military conducted numerous atmospheric nuclear weapons tests, including hydrogen bomb tests, primarily at Bikini Atoll, about from Rongelap Atoll. On March 1, 1954, the testing of the Castle Bravo hydrogen device produced an explosion that was 2½ times more powerful than predicted, and produced unexpected amounts of fallout that resulted in widespread radioactive contamination. The blast cloud contaminated more than of the surrounding Pacific Ocean including some of the then inhabited surrounding islands including Rongerik Atoll, Rongelap Atoll ( away) and Utirik Atoll.