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.
Operation Castle
Because "Castle Bravo" greatly exceeded its expected yield, JTF-7 was caught unprepared. Much of the permanent infrastructure on Bikini Atoll was heavily damaged. The intense thermal flash ignited a fire at a distance of on the island of Eneu (base island of Bikini Atoll). The ensuing fallout contaminated all of the atoll, so much so, that it could not be approached by JTF-7 for 24 hours after the test, and even then exposure times were limited. As the fallout spread downwind to the east, more atolls were contaminated by radioactive calcium ash from the incinerated underwater coral banks. Although the atolls were evacuated soon after the test, 239 Marshallese on the Utirik, Rongelap, and Ailinginae Atolls were subjected to significant levels of radiation. 28 Americans stationed on the Rongerik Atoll were also exposed. Follow-up studies of the contaminated individuals began soon after the blast as Project 4.1, and though the short-term effects of the radiation exposure for most of the Marshallese were mild and/or hard to correlate, the long-term effects were pronounced. Additionally, 23 Japanese fishermen aboard "Daigo Fukuryū Maru" were also exposed to high levels of radiation. They suffered symptoms of radiation poisoning, and one crew member died in September 1954.