|Fig. 1: The Shrimp device in its shot cab.  (Source: Wikimedia Commons)|
On March 1, 1954, the United States detonated its first dry fuel hydrogen bomb on an artificial island built on a reef off Namu Island in the Bikini Atoll.  Castle Bravo was the code name given to this operation, and with a yield of 15 Mt, it turned out to be the most powerful nuclear device ever detonated by the US.  This high yield, far exceeding the predicted yield of 6 Mt, along with a powerful eastward wind current, led to the most significant accidental radioactive contamination ever caused by the US.  Fallout from the explosion fell on residents of Rongelap and Utirik atolls some 75 miles away, and quickly spread around the world. The islanders were not evacuated until two days later and suffered from serious radiation sickness. 
The device detonated for the Castle Bravo test was named "Shrimp" (Fig. 1), and had the same basic configuration as the Ivy Mike "Sausage" device but with a different type of fusion fuel. Whereas the Sausage used cryogenic liquid deuterium (which required elaborate cooling equipment), the Shrimp used lithium deuteride, a fuel that is is solid at room temperature.  This made Castle Bravo the first US test of a practical deliverable hydrogen bomb. The success of this detonation rendered the cryogenic design used by Ivy Mike and its derivate nuclear devices obsolete.
The Shrimp was a very large cylinder weighing about 23,500 pounds and measuring 4.56 meters in length and 1.37 meters in diameter.  Inside the cylindrical case was a smaller cylinder of lithium deuteride fusion fuel (secondary stage) with a fission atomic bomb (primary stage) at one end, with the latter employed to create the conditions needed to start the fusion reaction according to Teller-Ulam principles of staged thermonuclear explosion.  Running down the center of the secondary stage was a cylindrical rod of plutonium (the sparkplug), which fissioned with compression and neutrons from the primary and compressed the fusion material around it from the inside. Surrounding this assembly was a uranium tamper. The space between the tamper and the case formed a radiation channel to conduct X-rays from the primary stage to the secondary. This space was filled with plastic which turned to plasma from the X-rays, thus compressing the secondary stage externally, increasing the density and temperature of the fusion fuel to the level needed to sustain a thermonuclear reaction. 
|Fig. 2: The Castle Bravo Mushroom Cloud.  (Source: Wikimedia Commons)|
In terms of TNT tonnage equivalence, Castle Bravo was about 1,000 times more powerful than the atomic bomb dropped on Hiroshima in 1945.  Upon detonation, Bravo formed a fireball almost 4.5 miles across within a second, which was visible from Kwajalein atoll over 250 miles away.  The explosion left a crater 6,500 feet in diameter and 250 feet deep.  The mushroom cloud reached a height of 130,000 feet and a diameter of 70 miles in 10 minutes after hour zero (Fig. 2). The cloud contaminated more than 7,000 square miles of the surrounding Pacific Ocean, including the surrounding atolls of Rongerik, Rongelap and Utirik. 
The actual yield of 15 Mt was about 2.5 times more than the expected yield.  The cause of the higher yield was a theoretical error made by designers of the device at Los Alamos National Laboratory. They assumed only the Li-6 isotope in the lithium deuteride secondary stage to be reactive; the Li-7 isotope, accounting for 60% of the lithium content, was considered inert. 
It was predicted that the Li-7 would absorb one neutron, producing Li-8 which decays to a pair of alpha particles.  However, when the Li-7 was bombarded with energetic neutrons, rather than simply absorbing a neutron, it captured the neutron and instantly decayed into an alpha particle, a tritium nucleus, and another neutron.  As a result, much more tritium was produced than expected, with the extra tritium fusing with deuterium to produce an extra neutron. The extra neutrons directly released by the decay of Li-7 and those produced by fusion resulted in a much larger neutron flux.  The result was greatly increased fissioning of the uranium tamper and thus, a much higher- than-expected yield from the detonation.
© Aditya Singh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 C. Hansen, US Nuclear Weapons: The Secret History (Crown Publishers, 1988).
 R. Rhodes, Dark Sun: The Making of the Hydrogen Bomb (Simon and Schuster, 1995).
 G. Wendt, Atomic Energy and The Hydrogen Bomb, (Medill McBride Company, 1950).