Sixty years (and three months-ish) ago today, the United States conducted a nuclear test called Castle Bravo. The Castle Bravo device It was the first practical, self-contained, potentially aircraft-deliverable fusion bomb, with a predicted explosive yield of somewhere around five megatons. The test location was Bikini Atoll in the Marshall Islands, which the U.S. had been nuking since 1946. and they thought they had things pretty well figured out. Someone pushed the button just before 7am on March 1, 1954, and then presumably said something along the lines of "oh ****" while trying to find a big lump of lead to hide behind, as Castle Bravo "went big."
Castle Bravo's expected yield of five megatons was left in the radioactive dust as the bomb instead delivered something like fifteen megatons. Within a second, the explosion resulted in a fireball four and a half miles wide, visible from 250 miles away. The mushroom cloud topped out at a staggering 62 miles wide, reaching 130,000 feet into the stratosphere. This was not the largest nuclear explosion that humans have set off (it's the fifth, with the biggest four courtesy of the USSR), but it was certainly the most unexpected.
The Castle Bravo device was powered (in part) by lithium deuteride fusion fuel. The bomb's designers calculated the potential yield based on one isotope of lithium: lithium-6. When the bomb was set off, the lithium-6 atoms would absorb neutrons from the plutonium fission igniter, emitting alpha particles and tritium. The tritium then fuses with deuterium (contained in the lithium deuteride), releasing energy. However, 60% of the lithium deuteride fuel was lithium-7 (since lithium-6 is expensive and hard to make), which was thought to be inert. As it turned out, the lithium-7 wasn't inert at all, and during the detonation, it started absorbing neutrons and decaying into tritium while releasing more neutrons. More tritium meant more fuel to fuse with the deuterium, and more neutrons ended up causing the uranium case of the bomb to undergo fission, adding even more energy to the mix.
The discovery that the cheaper, readily available lithium-7 had usability as fuel was a valuable outcome of the test, but nobody was expecting a 15 megaton nuclear explosion, especially not the inhabitants of nearby islands or one particularly unfortunate Japanese fishing boat. The Castle Bravo test was supposed to be a secret, but as fallout spread as far as the mainland United States and Europe, people started asking questions, and it was eventually determined that the secondary fission that occurred increased the radioactive fallout by a factor of about 1,000. Most of the worst of it was confined to the area immediately around the blast site, which was good for most of the rest of the world, but bad for the Marshall Island natives, who had to be evacuated.
You can still see the crater left by the Castle Bravo test on Google Earth; it's 6,500 feet wide and 250 feet deep.
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