page_title: Castle Bravo
section_title: The primary system
hierarchical_section_title: Castle Bravo / Bomb design / The primary system
caption_attribution_description: English: Operation Castle, Bravo event. The parabolic projection of the Shrimp device.
context_page_description: Castle Bravo was the first in a series of high-yield thermonuclear weapon design tests conducted by the United States at Bikini Atoll, Marshall Islands, as part of Operation Castle. Detonated on March 1, 1954, the device was the most powerful nuclear device detonated by the United States and its first lithium deuteride fueled thermonuclear weapon. Castle Bravo's yield was 15 megatons of TNT, 2.5 times the predicted 6.0 megatons, due to unforeseen additional reactions involving lithium-7, which led to the unexpected radioactive contamination of areas to the east of Bikini Atoll. At the time, it was the most powerful artificial explosion in history.
  Fallout, the most heavy of which in the form of pulverized surface coral from the detonation, fell on residents of Rongelap and Utirik atolls, while the more particulate and gaseous fallout spread around the world. The inhabitants of the islands were not evacuated until three days later and suffered radiation sickness. Twenty-three crew members of the Japanese fishing vessel Daigo Fukuryū Maru were also contaminated by the heavy fallout, experiencing acute radiation syndrome.
context_section_description: The Castle Bravo device was housed in a cylinder that weighed 23,500 pounds (10.7 t) and measured 179.5 inches (456 cm) in length and 53.9 inches (137 cm) in diameter.
  The primary device was a COBRA deuterium-tritium gas-boosted atomic bomb made by Los Alamos Scientific Laboratory, a very compact MK 7 device. This boosted fission device was tested in the Upshot Knothole Climax event and yielded 61 kilotonnes of TNT (260 TJ) (out of 50–70 kt expected yield range). It was considered successful enough that the planned operation series Domino, designed to explore the same question about a suitable primary for thermonuclear bombs, could be cancelled. The implosion system was quite lightweight at 410 kg (900 lb), because it eliminated the aluminium pusher shell around the tamper and used the more compact ring lenses, a design feature shared with the Mark 5, 12, 13 and 18 designs. The explosive material of the inner charges in the MK 7 was changed to the more powerful Cyclotol 75/25, instead of the Composition B used in most stockpiled bombs at that time, as Cyclotol 75/25 was denser than Composition B and thus could generate the same amount of explosive force in a smaller volume (it provided 13 percent more compressive energy than Comp B). The composite uranium-plutonium COBRA core was levitated in a type-D pit. COBRA was Los Alamos' most recent product of design work on the "new principles" of the hollow core. A copper pit liner encased within the weapon-grade plutonium inner capsule prevented DT gas diffusion in plutonium, a technique first tested in Greenhouse Item. The assembled module weighed 830 kg (1,840 lb), measuring 770 mm (30.5 in) across. It was located at the end of the device, which, as seen in the declassified film, shows a small cone projecting from the ballistic case. This cone is the part of the paraboloid that was used to focus the radiation emanating from the primary to the secondary.
caption: SHRIMP’s parabolic projection
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