You Might Want to Know: How Big Can They Get?
You Might Want to Know: How Big Can Nuclear Weapons Get?
The single nuclear bombs that the United States dropped on Hiroshima and Nagasaki in August 1945 were more powerful than any bombs the world had ever seen, by many orders of magnitude. Each bomb destroyed and burned up the city it was dropped on. Together the two bombs killed some one-hundred forty thousand people right away. More died later from the fires and radiation.
The “yield” of nuclear bombs is usually given in terms how many tons of TNT—a conventional chemical explosive--would be needed to produce an equivalent blast. The Hiroshima bomb yielded the equivalent of about fifteen thousand tons of TNT. The Nagasaki bomb yielded the equivalent of twenty thousand tons, one-third more than the Hiroshima bomb had. The largest strategic bomb we’d had before this was a ten-ton bomb. That was its weight, not its yield. Its yield was less than that.
Some readers will remember the terrible damage that was caused to the multi-story federal building in Oklahoma City when a Ryder truck loaded with conventional chemical explosives was exploded in front of the building. One kiloton of TNT would require four hundred such Ryder trucks. Fifteen kilotons, one Hiroshima, would require six thousand Ryder trucks.
In 1946, in our first atomic bomb tests after the war, the two nuclear bombs detonated in the Pacific were Nagasaki-type bombs, and their yields were about the same, something over twenty kilotons.
Our next tests in the Pacific took place in Spring 1948. The three bombs detonated used new design ideas. Two of the bombs yielded much more than the Nagasaki bomb had. The largest yielded forty-nine kilotons, more than three times Hiroshima, nineteen-thousand six-hundred Ryder trucks, if you like.
In 1951, in another test series in the Pacific, one of the bombs yielded two-hundred twenty-five kilotons, fifteen times Hiroshima. It had used the design idea called “boosting” in which fusion fuel is shot into the cavity at the center of a fission bomb just before the fissile fuel implodes. This “boosted” the yield of the device by more than four-and-a-half times.
These were all fission bombs, however, and there is a limit to how big a fission bomb can get. That’s because when fissile fuel is compressed to a supercritical state, the condition required for a nuclear explosion, the “fishing” atoms release energy that pushes apart the atoms of the fissile fuel. In less than a microsecond, this expansion, of only a couple of centimeters will stop the chain reaction. No more energy will be released.
Starting in 1948, a physicist at the Los Alamos National Laboratory named Ted Taylor, who became one of our most important weapons designers, started to help us make our fission bombs smaller in size while also increasing their yield. He wanted to see how big a yield could be gotten from a fission bomb. In the Ivy King test in the Pacific in November 1952, his Super Oralloy Bomb, the SOB, yielded five hundred kilotons, thirty-three and a third Hiroshimas, two-hundred thousand Ryder trucks. The largest yield yet by far for a fission bomb .
Ted Taylor thought it might be possible to design a fission bomb that would yield one thousand kilotons, one megaton, but that would be it for fission bombs. Because of the way fission bombs worked, it wouldn’t be possible to make one that would yield more than that.
Not to worry. Two weeks before the test of the SOB, in the Ivy Mike test, a device named Sausage had yielded twenty times more than the SOB, 10.4 megatons, 693+ Hiroshimas, four-million one-hundred-sixty thousand Ryder trucks. Sausage was our first thermonuclear, or hydrogen, device. It was a “device,” not a “bomb,” because it was a big thing the size of a small factory, not anything you could drop on someone. SOB had at least been a bomb.
So how big could the new thermonuclear devices get? In our next testing operation in the Pacific, Operation Castle in 1954, in a test called Bravo, we surprised ourselves with a hydrogen device—still a device, not a deliverable bomb--that yielded fifteen megatons, half again what Ivy Mike had yielded and six billion Ryder trucks. Was that the limit?
Our physicists at Los Alamos and at a second weapons laboratory that had been started recently at Livermore in northern California didn’t think this was the limit. Hydrogen bombs are “staged.” They are triggered by a “primary,” a fission-bomb that heats and compresses the hydrogen fusion fuel in the “secondary” terrifically enough to produce fusion in that fuel. Would it be possible then to add a third stage that would be triggered by the secondary? And a fourth? Might there be no limit to how big these bombs could get?
The Soviet Union had exploded its first true thermonuclear bomb three years after our Ivy Mike test. Theirs had yielded only 1.6 megatons. It was a bomb though, not a device. Again, not to worry. We’d designed an “emergency capability” bomb on the cryogenic principles used in the Ivy Mike test. It weighed twenty tons but we had modified a B-36 bomber to carry it. We thought it would yield six- to eight megatons. We planned to test it in 1954 for the first time in the Castle Yankee test but canceled the test after the wonderful success of Castle Bravo.
But just one minute. On October 30, 1961, ten months after John Kennedy was inaugurated president, the Soviet Union, over their test site at Novaya Zemlya, north of the Russian mainland, detonated a three-stage bomb that yielded fifty megatons, fifty-seven, by some estimates. The Soviets had planned to have it yield a hundred megatons but had scaled it back because of concerns about the radioactive fission products that would be generated. Because of the prevailing winds, they would likely fall on the Scandinavian countries and elsewhere in Europe but a lot might also blow back into Russia. Even so, at fifty megatons, the Tsar Bomba was the biggest nuclear bomb ever detonated. It still is. One bomb. The equivalent of twenty billion Ryder trucks.
The United States wasn’t sitting on its hands. By 1956, at our Pacific Proving Ground, we had successfully tested three-stage devices in Operation Redwing. In 1961, the year the Tsar Bomba was dropped, we’d already deployed a three-stage bomb, the MK-41, that we hadn’t tested but that the designers thought would yield twenty-five megatons.
Why stop there? We knew now we didn’t have to. Why not more? Why not just keep going?
Not because of any technical limit. It now seemed that there was no technical limit to how much a staged hydrogen bomb could be made to yield.
The limit is, you could say, a practical one. We’d realized that the damage caused by ten well-placed five-megaton bombs, or even better, fifty one-megaton bombs, or better yet a hundred five-hundred kiloton bombs, or two-thousand five-hundred twenty-kiloton Nagasaki-size bombs, would be much greater than would the damage of any single bomb like the Tsar Bomba .
In fact, though we hadn’t realized this in the 60’s, and didn’t until the 80’s, a mere one-hundred fifty-kiloton bombs dropped on cities, any cities anywhere, might well generate a nuclear winter that would starve us all.
I can’t see any reason to go beyond that, can you?
Next: How were our nuclear weapons “miniaturized”?