Little Boy—the very first nuclear weapon America dropped in war—killed eighty thousand people on Hiroshima. But the destruction could have been much worse—only 1.38 percent of its nuclear core fissioned. Three days later, when Americans dropped their second bomb—codename “Fat Man”—on Nagasaki, it accidentally detonated one mile off target, though it still managed to kill forty thousand. A 1954 test of a hydrogen bomb in the Bikini atoll produced a yield of fifteen megatons—triple the amount America’s nuclear scientists anticipated—blanketing hundreds of square miles in the Pacific—and, as a
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