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
Irradiated debris fell up to deep over the island. A United States military medical team visited the island with geiger counters the day after the fallout, but left without telling the islanders of the danger they had been exposed to. Virtually all the inhabitants experienced severe radiation sickness, including itchiness, sore skin, vomiting, diarrhea, and fatigue. Their symptoms also included burning eyes and swelling of the neck, arms, and legs. The inhabitants were forced to abandon the islands, leaving all their belongings, three days after the test. They were relocated to Kwajalein for medical treatment. Six days after the Castle Bravo test, the U.S government set up a secret project to study the medical effects of the weapon on the residents of the Marshall Islands.