Castle Bravo
Castle Bravo was the first test of a Teller-Ulam thermonuclear dry fuel hydrogen bomb, detonated at the Bikini Atoll on Feburary 28, 1954. It was the most powerful nuclear device ever detonated by the United States, and the prototype of the first practically deliverable hydrogen bomb in the US arsenal. Castle Bravo had a yield of 15 megatons, far exceeding the 6 Mt that was expected. That, combined with other factors, resulted in the worst radiological disaster ever caused by the United States.
The device detonated for the test was called "Shrimp" and was practically identical to the "Runt" device later detonated on Castle Romeo, but using partially enriched lithium as fuel instead. The device was a very big cylinder weighting 23,500 pounds and measuring 179.5 inches in length and 53.9 inches in width. It used 40% enriched Lithium-6 and a natural uranium tamper. Of the total 15 megatons, 10 were from fast fission of the tamper. This device also implemented a light case design. The primary device–the fission bomb which would begin the fusion reaction of the fuel in the secondary device–was a standard RACER 4 fusion boosted bomb.
The factor that caused the extremely high and unexpected yield of the device was that most of the lithium used in it was Lithium-7, an isotope that is considered completely inert by itself but, when Lithium-7 is bombarded with high energy neutrons it splits into a tritium and a helium atom. This extra amount of tritium contributed greatly to the fusion reaction, and in this manner greatly increased the device's yield.
The explosion left a crater of 6,510 feet in diameter and 150 feet in depth. The mushroom cloud reached a height of 130,000 feet and 100 kilometers in diameter in less than 10 minutes. Radioactive fallout was spread eastward onto the inhabited Rongelap and Rongerik atolls, which were soon evacuated (many of the Marshall Islands natives have since suffered from birth defects and have received some compensation from the federal government). A Japanese fishing boat, the Lucky Dragon, also came into contact with the fallout, which caused many of them to grow ill and one of them to eventually die, which reignited Japanese concerns about radiation, especially in regards to the possibility of contaminated fish.
Shrimp later evolved into the Mk-21 bomb, of which 275 units were produced, weighting 15,000 pounds and measuring 12.5 feet long and 56 inches in diameter. This 4 megaton bomb was produced until July 1956. In 1957, it was converted into the Mk-36 and entered into production again.