The 15-megaton thermonuclear blast, roughly 1,000 times the size of the Hiroshima bomb, was nearly triple the yield scientists expected, due to an unanticipated reaction in the lithium-7 component of the device. Fallout drifted over inhabited atolls and a Japanese fishing vessel, causing acute radiation sickness among Marshall Islanders and fishermen and triggering an international uproar over U.S. nuclear testing.
Castle Bravo was meant to test a deliverable hydrogen bomb design using solid lithium deuteride fuel rather than the cumbersome cryogenic liquid deuterium used in earlier tests. Physicists had calculated a yield around 4 to 6 megatons. They were catastrophically wrong: the lithium-7 isotope, assumed to be largely inert, fissioned far more readily than predicted, roughly tripling the explosion's power to about 15 megatons, the largest nuclear detonation the U.S. ever set off.
The fireball vaporized three islands outright and sent a plume of irradiated coral dust drifting eastward over Rongelap and Rongerik atolls, whose residents had not been evacuated because planners hadn't anticipated fallout reaching that far. Islanders reported an odd snow falling from the sky; children played in it. Many later suffered burns, radiation sickness, thyroid damage, and elevated cancer rates for decades.
A Japanese tuna trawler, the Daigo Fukuryū Maru, sailed through the fallout zone outside the supposed danger area; its crew fell ill and one crewman died months later, reigniting Japanese anti-nuclear sentiment less than a decade after Hiroshima and Nagasaki. The incident also inspired Godzilla, released later that year as a direct allegory for the bomb.