Here's What You Need To Remember: Really huge nuclear weapons fell out of favor in America as missile accuracy increased. More accurate delivery platforms mean less need for big explosions to kill a target (and smaller delivery vehicles—a big warhead needs a big missile or plane or sub).

(This is a combination of two previous articles posted last year. They are reposted here for your reading pleasure.)

Russia: 

The Bomb represents the ultimate expression of might, so the biggest bombs express that power to its extremes. The United States and the Soviet Union built bigger and bigger bombs to deter each other. Eventually, Washington stopped testing such large weapons due to improved accuracy of their delivery methods. Only 3 percent of the superpowers' stockpiles consisted of nuclear weapons with yields greater than four megatons, but those weapons more than any others symbolize the terror of nuclear war.

The Soviet Union's swift strides towards nuclear arms alarmed American officials in the 1950’s. "First Lightning" (called "Joe-1" in the West)—the first Soviet nuclear test—announced the Moscow's new military power less than three months after lifting the Berlin Blockade in May 1949. The twenty-two-kiloton shot copied the U.S. Trinity test as closely as possible to achieve early success; the rushed development actually stalled the Soviet program for over a year, with the second test occurring in September 1951.

This phase of the Cold War went hot on the Korean Peninsula and President Truman gave the go-ahead for the "Super," as it was called. Edward Teller—a brilliant, vain and pugnacious physicist who fled Hungary for the United States to join the Manhattan Project—was smitten by the idea of thermonuclear fusion and argued forcefully for U.S. research into bigger bombs. Pursuit of the "Super" alarmed Soviet leaders and scientists. As they explored the possibility, however, the Soviets had an advantage over the Americans: lithium.

The first-ever hydrogen bomb blast, Ivy Mike, required American weaponeers to create an vast new industrial capacity for manufacturing liquid hydrogen in its "heavy" form of liquid deuterium. The Ivy Mike device was liquid-fueled and a handful of emergency-capability (EC) liquid-fuel bombs were later built; several B-36 bombers were modified to top off the liquid hydrogen in flight en route to their targets.

But the Soviets dispensed with liquid fuel for dry powdered lithium deuteride, a chemical compound of lithium metal and hydrogen gas. Lithium comes in two "flavors" or isotopes: Lithium-6 and Lithium-7. Lithium-7 was thought by both sides to be inert and unsuitable as bomb fuel. The Soviet Union had plentiful sources of Lithium-7, but the United States did not. As a result, Soviet weaponeers worked on dry-fuel bombs from the start. The fourth Soviet nuclear test in 1953 registered an impressive 400 kilotons from the "Sloika" design.

America’s disastrous Castle Bravo H-bomb test in 1954 revealed Lithium-6's fusion potential and provided the Soviet Union with needed information; a mere eighteen months elapsed between the Castle Bravo test and the Soviet test of November 1955: an air drop of a fully weaponized hydrogen bomb. At 1.6 megatons, the yield of RDS-37 was impressive—but it would be dwarfed by the monsters to come.

In 1958 the Soviet Union matched the America’s voluntary moratorium on nuclear testing; in September 1961, after raising the Berlin Wall, Nikita Khrushchev ordered testing resumed. On October 23, a 12.5-megaton airdrop pounded the high Arctic island of Novaya Zemlya. The resulting blast was nearly as large as Castle Yankee—America's second biggest test—but only the Soviet Union’s fifth-biggest.

The biggest bomb ever built or tested—the RDS-220 ("Big Ivan" or "Tsar Bomba")—rattled the planet one week later on October 30, 1961. The result of a crash program directed by Andrei Sahkarov, the Tsar Bomba was a conservative design accomplished with astonishing speed: in only four months. The 100-megaton (possibly 150-megaton) design was an impractical weapon—only a single modified Bear bomber could carry it, slowly—but a billy-hell of a propaganda show.

Even though Sakharov had the third-stage fission tampers replaced with inert lead due to concerns about fallout, the Tsar Bomba still yielded fifty-six megatons, enough to blow a hole in the atmosphere and cause damage hundreds of miles away. Had the third stage tampers been uranium this one bomb would have raised global fallout levels by 25 percent. Every person born before October 1961 has bits of the Tsar Bomba (and other bombs) in their bodies to this day.

If the Tsar Bomba explosion was a stunt, the other biggest Soviet tests weren't. 1962 was the last year of atmospheric nuclear tests for the Soviet Union and the United States, and both sides rushed to prove weapon designs. The fourth, third and second largest Soviet nuclear tests all seem related to ICBM-warhead development: Test 147 on August 5, 1962 yielded over twenty-one megatons; Test 173 nineteen megatons; and Test 219 a whopping 24.2 megatons—nearly half the Tsar Bomba's yield.

This city-busting warhead wound up on R-36 ICBMs (NATO designation SS-18 "Satan"), huge missiles capable of obliterating deeply buried targets or entire metro areas. Although photos and data from Test 219 are not publicly available, Alex Wellerstein's NUKEMAP drives home the power of such a bomb. Dropped on downtown Los Angeles, the air blast would level the Coliseum (four miles away) and burn down all of Beverly Hills (nine miles away).

America: 

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.