You Might Want to Know: What’s the difference between a “clean” nuclear bomb and a “dirty” one?
You Might Want to Know: What’s the difference between a “clean” nuclear bomb and a “dirty” one?
Nuclear bombs have a “yield,” an explosive force usually given in tons of TNT equivalent. The Hiroshima bomb, for openers, yielded the equivalent of fifteen thousand tons of TNT. The largest device the U.S. has ever detonated--the Shrimp device in the Castle Bravo test in 1954 at our Pacific Proving Ground--yielded the equivalent of fifteen million tons of TNT. The largest bomb ever exploded was detonated by the Soviets in 1961, at the beginning of President Kennedy’s administration and not long before the Cuban Missile Crisis. It yielded fifty million tons, some say fifty-seven million. The Soviets had designed it to yield one hundred million tons, but had scaled it back. That bomb, which we call the Tsar Bomba, is still the largest nuclear bomb ever exploded.
But these numbers refer only to the “explosive yield” of the nuclear bomb. Unlike TNT, nuclear bombs also yield ionizing radiation. Some of it is “prompt radiation,” which is the single blast of radiation produced when the device explodes. Some of the radiation arrives later with the nuclear fission products, usually called “fallout.” These fission products are what make a nuclear bomb not just deadly but “dirty.”
A nuclear detonation produces hundreds of different fission products. The radioactivity in some of them stops being dangerous in only a few seconds. For others, it takes a few minutes, or few hours. Some fission products can be dangerous for months or years. For generations.
If the fireball of the nuclear explosion touches the ground or water, the amount of fallout will be greatly increased. That means you can make a nuclear bomb “dirtier” by detonating it where its fireball will hit the surface of the earth.
Thermonuclear bombs can be designed to be especially “dirty” no matter where they are exploded. This is done by placing inside the bomb, between the imploding conventional explosive and the nuclear material in the core, a shell of uranium. If the shell (the “tamper-pusher”) is uranium instead of something inert like lead, the neutrons produced by the fission and fusion reactions will generate more fission in the shell. More fission means more explosive yield and more radioactive fission products. Bigger and dirtier.
According to Chuck Hansen, the “clean” version of the big MK-36 bomb, which the United States had in service from the late 50’s to the early 60’s, would have yielded the explosive equivalent of maybe 6,000,000 tons of TNT. The dirty version would have yielded the equivalent of 19,000,000 tons, and, of course, much more fallout.1 The dirty version was the one favored by our Strategic Air Command.
The bigger one-hundred million ton version of the Tsar Bomba was probably a dirty version. The Soviets might have decided to scale it back from a hundred megatons because they didn’t want to have all those fission products from the bomb being blown around who knows where up there, Scandinavia and Europe probably but maybe even south into Russia.
A “clean” version any bomb is just clean-er. In spite of what the physicist Edward Teller wanted us and our leaders to believe in the 50’s, there’s no such thing as a “clean” nuclear bomb. All nuclear bombs, like all nuclear power plants, employ fission and therefore produce fission products that release radioactivity. The only questions are what kind, how much, and for how long the fission products will be radioactive. And whether they can be contained. And if not, where they will land.
There’s a kind of “dirty bomb” that is something completely different. This kind of “dirty bomb” would use a conventional chemical explosive—ten or a hundred pounds of TNT, say—to scatter radioactive fission products that had been packed in with the conventional explosive. The explosive yield of such a bomb would be nothing like that of a nuclear bomb. It would do its damage not by heat and blast but by poisoning a place with dangerous radioactivity—the D. C. Mall, say, or lower Manhattan, or the Long Beach docks--for a good while. Months, maybe, or years.
Dirty bombs of this kind are easier to make than the kind that maximizes fission. Not exactly easy to make. You still have to figure out how to handle safely the highly radioactive fission products you want to pack with the conventional explosives and how to disperse them effectively where you want them to be dispersed.
You’d have to get hold of the highly radioactive material first, but there’s a lot of it around now. Every operating nuclear power plant produces it. Last time I checked, our nuclear plants had produced about one-hundred thousand tons of high-level nuclear waste. At this writing, most of it is being stored in pools or in large “dry casks” around the power plants that produced it. We’ve been trying for a while now to think of a better way to store it. We haven’t been able to but we are still trying.
Forty four countries now have power reactors. Each of them is also producing this waste. Storing it safely and securely, we hope.
The works of Chuck Hansen (1947-2003), produced by him from unclassified sources, as are the columns of You Might Want to Know, are one of the most reliable sources available to the public of specific technical information about U.S. nuclear weapons during the Cold War. Hansen, Chuck (2007) [1995]. Swords of Armageddon: U.S. Nuclear Weapons Development since 1945 (PDF: CD-ROM & download available) (2nd ed.). Sunnyvale, CA: Chukelea Publications. ISBN 978-0-9791915-0-3