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.
Radiophobia
March 1, 1954, the operation Castle Bravo testing of a then, first of its kind, experimental thermonuclear "Shrimp" device, overshot its predicted yield of 4–6 megatons and instead produced 15 megatons; this resulted in an unanticipated amount of "Bikini snow" or visible particles of nuclear fallout being produced, fallout which caught the Japanese fishing boat the Daigo Fukuryū Maru or "Lucky Dragon" in its plume, even though it was fishing outside the initially predicted ~5 megaton fallout area which had been cordoned off for the Castle Bravo test. Approximately 2 weeks after the test and fallout exposure, the 23-member fishing crew began to fall ill, with acute radiation sickness, largely brought on by beta burns that were caused by direct contact between the "Bikini snow" fallout and their skin, through their practice of scooping the "Bikini snow" into bags with their bare hands. One member of the crew, Kuboyama Aikichi the boat's chief radioman, died 7 months later, on September 23, 1954. It was later estimated that about a hundred fishing boats were contaminated to some degree by fallout from the test. Inhabitants of the Marshall Islands were also exposed to fallout, and a number of islands had to be evacuated.