Recent reports of collapse at the North Korean nuclear test site under Mt. Mantap provide a sobering reminder of the sheer power of nuclear weapons. North Korea’s sixth (and final?) test had a likely yield well above one hundred kilotons, enough to corroborate the regime's claim of thermonuclear capability.
Really big nuclear explosions require the lightest elements rather than the heaviest. Fission explosions rely on the energy released by splitting heavy atoms of uranium or plutonium into lighter elements, and require lots of fissionable material. Fusion explosions release vastly more energy than fission blasts by fusing together lightweight hydrogen nuclei into slightly heavier helium. However, getting hydrogen to fuse requires immense temperatures and pressures, which only a fission explosion can readily generate.
Thus fusion (“H-bomb”) weapons rely on fission (“A-bomb”) triggers or primaries to heat and compress fusion fuel enough for thermonuclear fusion. In the fractions of a second between the primary's detonation and the bomb's explosion the primary's immense energy gets channeled into the fusion fuel. Hydrogen bombs are thus two-stage weapons: first a small fission explosion drives a much bigger fusion explosion.
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Both the United States and the Soviet Union saw fusion or thermonuclear weapons as the vital next step in trumping each other's nuclear arsenals. In pursuit of weaponizing the power of the sun both nations created the largest artificial explosions yet witnessed by humans. Edward Teller, who along with Stanislaw Ulam conceived the successful H-bomb concept, began thinking about “the Super,” as it became known, even as the Manhattan Project struggled to build a working fission bomb.
As the Korean War flared both the United States and USSR accelerated their nuclear programs. Despite serious reservation by some in the nuclear weapons community, President Truman launched the U.S. thermonuclear effort in 1950. Soviet scientists pursuing a different design had access to ideal fusion fuel—the chemical compound of the lightest metal, lithium, and a heavy form of the lightest gas, hydrogen. This powdery substance, lithium deuteride , itself came in two "flavors," one incorporating the Lithium-6 isotope and another incorporating Lithium-7. The Soviets swiftly built facilities for manufacturing Li-7 deuteride while the Americans, lacking sources of Li-7 focussed on liquid deuterium.
By late 1952 the Soviets had tested their first fusion-driven device with limited success. After an immense investment the Americans succeeded in producing abundant liquid heavy hydrogen (deuterium) and an eighty-ton science experiment to use it. Not in any sense a weapon, the Mike (for "megaton") shot of Operation Ivy yielded 10.8 megatons of explosive force when detonated November 1, 1952. Ivy Mike proved the Teller-Ulam design by leaving a mile-wide hole in Eniewetak Atoll.
And yet, Ivy Mike was only the fourth biggest U.S. nuclear test.
Operation Ivy’s other test puts Mike in perspective. Weapons designer Ted Taylor thought the H-bomb to be overkill and to prove his point designed the biggest pure-fission bomb ever tested. Ivy King (for “kiloton”) yielded five hundred kilotons at the cost of four critical masses of uranium alloy—so much fissile material that the weapon had to be transported with chains of neutron-absorbing boron and aluminum to prevent a premature chain-reaction. Ivy King worked, but wasn't the future.
By early 1954, hot on the heels of the Korean conflict, the United States had new H-bomb designs ready to test. The six-shot series of Operation Castle would prove these new designs at Bikini Atoll. As with Operation Ivy, an armada of ships and an army of people shipped out the Central Pacific to dredge islands, build test equipment and ready bunkers.
The bombs prepared included a weaponized version of the Ivy Mike device—the liquid-fueled EC-16 hydrogen bomb, along with others trying out the new American dry fuel made of mixed Li-6 and Li-7 hydride. Scientist thought Lithium-6 was far less reactive to neutrons thrown from fission primaries and thus expected yields of six to ten megatons.
Nature (only partly) surprised them on the first test. The Castle Bravo shot yielded fifteen megatons—the biggest American nuclear test ever—and the worst radiological disaster in American history. It was two hundred and fifty times bigger than Hiroshima, Castle Bravo vaporized an island, contaminated three atolls and hundreds of people, caused major diplomatic incidents and introduced the word “fallout” into public discourse.
The size of the explosion resulted from Li-6's previously-unknown affinity for neutrons; the supposedly regular-gas bomb fuel turned out to be premium hi-test. Nature wasn't involved in another factor, the wind: despite warnings from meteorologists Test Director Alvin Graves ordered the shot fired. Graves, who had survived near-lethal radiation exposure , had little patience with those urging caution.