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.
Castle Bravo
The detonation took place at 06:45 on March 1, 1954, local time (18:45 on February 28 GMT).
When Bravo was detonated, within one second it formed a fireball almost across. This fireball was visible on Kwajalein atoll over away. The explosion left a crater in diameter and in depth. The mushroom cloud reached a height of and a diameter of in about a minute, a height of and in diameter in less than 10 minutes and was expanding at more than . As a result of the blast, the cloud contaminated more than of the surrounding Pacific Ocean, including some of the surrounding small islands like Rongerik, Rongelap, and Utirik.