4.9 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Castle Bravo | 5/9 | https://en.wikipedia.org/wiki/Castle_Bravo | reference | science, encyclopedia | 2026-05-05T13:10:17.178423+00:00 | kb-cron |
=== Use of boron === Boron was used at many locations in this dry system; it has a high cross-section for the absorption of slow neutrons, which fission 235U and 239Pu, but a low cross-section for the absorption of fast neutrons, which fission 238U. Because of this characteristic, 10B deposited onto the surface of the secondary stage would prevent pre-detonation of the spark plug by stray neutrons from the primary without interfering with the subsequent fissioning of the 238U of the fusion tamper wrapping the secondary. Boron also played a role in increasing the compressive plasma pressure around the secondary by blocking the sputtering effect, leading to higher thermonuclear efficiency. Because the structural foam holding the secondary in place within the casing was doped with 10B, the secondary was compressed more highly, at a cost of some radiated neutrons. By contrast, the Castle Koon MORGENSTERN device did not use 10B in its design; as a result, the intense neutron flux from its RACER IV primary predetonated the spherical fission spark plug, which in turn "cooked" the fusion fuel, leading to an overall poor compression. The plastic's low molecular weight is unable to implode the secondary's mass. Its plasma-pressure is confined in the boiled-off sections of the tamper and the radiation case so that material from neither of these two walls can enter the radiation channel that has to be open for the radiation transit.
== Detonation ==
The device was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in Bikini Atoll. A sizable array of diagnostic instruments were trained on it, including high-speed cameras trained through an arc of mirror towers around the shot cab. The detonation took place at 06:45 on 1 March 1954, local time (18:45 on 28 February GMT). When Bravo was detonated, within one second it formed a fireball almost 4.5 miles (7.2 km) across. This fireball was visible on Kwajalein Atoll over 250 miles (400 km) away. The explosion left a crater 6,500 feet (2,000 m) in diameter and 250 feet (76 m) in depth. The mushroom cloud reached a height of 47,000 feet (14,000 m) and a diameter of 7 miles (11 km) in about a minute, a height of 130,000 feet (40 km) and 62 mi (100 km) in diameter in less than 10 minutes and was expanding at more than 160 meters per second (580 km/h; 360 mph). As a result of the blast, the cloud contaminated more than 7,000 square miles (18,000 km2) of the surrounding Pacific Ocean, including some of the surrounding small islands like Rongerik, Rongelap, and Utirik.
In terms of energy released (usually measured in TNT equivalence), Castle Bravo was about 1,000 times more powerful than the atomic bomb that was dropped on Hiroshima during World War II. Castle Bravo is the sixth largest nuclear explosion in history, exceeded by the Soviet tests of Tsar Bomba at approximately 50 Mt, Test 219 at 24.2 Mt, and three other (Test 147, Test 173 and Test 174) ≈20 Mt Soviet tests in 1962 at Novaya Zemlya.
=== High yield ===
The yield of 15 (± 5) Mt was triple that of the 5 Mt predicted by its designers. The cause of the higher yield was an error made by designers of the device at Los Alamos National Laboratory. They considered only the lithium-6 isotope in the lithium deuteride secondary to be reactive; the lithium-7 isotope, accounting for 60% of the lithium content, was assumed to participate in reactions that were too slow to contribute to the yield. It was correctly expected that the lithium-6 isotope would absorb a neutron from the fissioning plutonium and emit an alpha particle and tritium in the process, of which the latter would then fuse with the deuterium and increase the yield in a predicted manner. It was assumed that the lithium-7 would absorb one neutron, producing lithium-8, which decays (through beta decay into beryllium-8) to a pair of alpha particles on a timescale of nearly a second, vastly longer than the timescale of nuclear detonation. However, when lithium-7 is bombarded with energetic neutrons with an energy greater than 2.47 MeV, rather than simply absorbing a neutron, it undergoes nuclear fission into an alpha particle, a tritium nucleus, and another neutron. As a result, much more tritium was produced than expected, the extra tritium fusing with deuterium and producing an extra neutron. The extra neutron produced by fusion and the extra neutron released directly by lithium-7 decay produced a much larger neutron flux. The result was greatly increased fissioning of the uranium tamper and increased yield. Summarizing, the reactions involving lithium-6 result in some combination of the two following net reactions:
1n + 6Li → 3H + 4He + 4.783 MeV 6Li + 2H → 2 4He + 22.373 MeV But when lithium-7 is present, one also has some amounts of the following two net reactions: