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.
Downwinders
One of these many tests, the March 1, 1954, explosion of Castle Bravo, a U.S. thermonuclear device, was responsible for most of the radiation the Marshall Islanders were exposed to. The fallout-related doses of this single test are believed to be the highest recorded in the history of worldwide nuclear testing. Many of the Marshall Islands which were part of the Pacific Proving Grounds remain contaminated by nuclear fallout, and many of those downwinders who were living on the islands at the time of testing have suffered from a highly increased incidence of several types of cancers and birth defects.