[Aktungbby]

Originally Posted by Jimbuna

1954 US explodes Castle Bravo, a 15 megaton hydrogen bomb at Bikini Atoll, which accidentally became the most powerful nuclear device ever detonated by the US.

But even worse, it was a surface burst in a coral reef, which is just a really, really bad idea. Detonating nuclear weapons on a desert floor, like in Nevada, still presents significant fallout issues. But a coral reef is really an awful place to set them off, and not just because coral reefs are awesome and shouldn’t be blown up. They are an ideal medium for creating and spreading contamination: they break apart with no resistance, but do so in big enough chunks that they rapidly fall back to Earth. Particle size is a big deal when it comes to fallout; small particles go up with the fireball and stay aloft long enough to lose most of their radioactive energy and diffuse into the atmosphere, while heavy particles fall right back down again pretty quickly, en masse. So blowing up and irradiating something like coral is just the worst possible thing. And, of course, it became famous for raining nuclear fallout down on inhabited islands over a hundred miles downwind, and exposing a crew of Japanese fishermen to fatal levels of radiation.
It was this latter event that made BRAVO famous — so famous that the United States had to admit publicly it had a hydrogen bomb. And accidentally exposing the Japanese fishing supply to radiation, less than a decade after Hiroshima and Nagasaki, has a way of making the Japanese people understandably upset. So the shot led to some almost frank discussion about what fallout meant — that being out of the direct line of fire wasn’t actually good enough.

A BAD BUSINESS ALL ROUND AS THE FALLOUT EXCEEDED EXPECTATION BY 250%:
https://en.wikipedia.org/wiki/Castle_Bravo

Castle Bravo's yield was 15 megatons of TNT, 2.5 times the predicted 6.0 megatons, due to unforeseen additional reactions involving 7Li, 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 ("Lucky Dragon No. 5") were also contaminated by the heavy fallout, experiencing acute radiation syndrome. The blast incited international reaction over atmospheric thermonuclear testing

IN A NUTSHELL:

 

the yield of 15 megatons was 3 times 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 be inert..It was 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. Lithium-6 indeed reacted in this manner.
It was assumed that the lithium-7 would absorb one neutron, producing lithium-8, which decays (through beryllium-8) to a pair of alpha particles on a timescale of seconds, vastly longer than the timescale of nuclear detonation. When lithium-7 is bombarded with energetic neutrons, rather than simply absorbing a neutron, it captures the neutron and decays almost instantly 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....

BUT THEY SURE LOOK PRETTY
< AN ABJECT LESSON HOW TO CRAP IN OUR OWN (CORAL) SANDBOX 101...LITERALLY

Before testing began, the 167 Marshallese people who lived on Bikini were relocated to other islands. The U.S. military used the Marshall Islands to test nuclear weapons from 1946 to 1958.

Today, a handful of caretakers live on Bikini. Most do field work for the U.S. government, including sampling the soil, plants, water, and marine life around the island. Many people from the Bikini Atoll are still hoping to return to their homeland, although tests indicate that the amount of radiation present in the island's food web could be harmful to them. The type of radiation detected at Bikini (cesium-137) has been linked to an increased risk for several types of cancer.