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 device was called SHRIMP and had the same basic configuration (radiation implosion) as the "Ivy Mike" wet device, except with a different type of fusion fuel. "SHRIMP" used lithium deuteride (LiD), which is solid at room temperature; "Ivy Mike" used cryogenic liquid deuterium (D), which required elaborate cooling equipment. "Castle Bravo" was the first test by the United States of a practical deliverable fusion bomb, even though the TX-21 as proof-tested in the Bravo event was not weaponized. The successful test rendered obsolete the cryogenic design used by "Ivy Mike" and its weaponized derivative, the "JUGHEAD", which was slated to be tested as the initial "Castle Yankee". It also used a 7075 aluminum 9.5 cm thick ballistic case. Aluminum was used to drastically reduce bomb's weight and simultaneously provided sufficient radiation confinement time to raise yield, a departure from the heavy stainless steel casing (304L or MIM 316L) employed by contemporary weapon-projects.