How hard is it to design The Bomb?

Recently the press has been all aflutter about how Pakistan was helping other nations get the Bomb, by providing knowledge and tools to enrich uranium.

Bad news. But then I read about how, horror of horrors, the Pakistanis had provided, gasp, an actual bomb design to some nations, including Libya and, possibly, Iran.

And now I’m confused.

Because growing up, I was under the impression that the only hard part about making a Hiroshima style atomic bomb was the enrichment of the uranium - that once you had the weapons grade uranium, making a straight A bomb was simply a matter of banging two pieces of it together very hard and very quickly.

Is this not true? Is there something about making an atomic bomb that requires more than a metal donut, a metal plug and some dynomite?

My understanding is that its not difficult to make a working bomb if you have the materials. But improving the yield of a device, or making a hydrogen bomb is a much more difficult task.

Like you say, enriching the uranium is the most difficult task.

FWIW, Tom Clancy postulates that motivated Arab terrorists could build a multi-stage nuke in a cave in five months if they already had most of the materials they needed. The novel “The Sum of All Fears” tells this exact story and also comments on how easy it would be to sneak such a device into the continental US. Fredrick Forsythe in “The Fist of God” postulates that it would take Iraq about 10 years of hard work and deception to build a nuke from scratch using their own methods of uranium enrichment. But both of these books are works of fiction so YMMV.

The geometry and timing of the HE charges is fairly critical too, especially for maximizing the yield of your bomb. For the typical A-bomb design, you want to have the conventional explosives compress the core uniformly and rapidly. But technically, it wouldn’t be terribly difficult. I seem to recall a story about a high-school student (in the 70’s? '80’s?) who designed an atomic bomb from publicly-available information. I believe it was determined that his design could very well have worked had it actually been constructed.

We can’t tell you. If we tell you, you’ll tell all your friends and soon everyone, even North Korea will have The Bomb. Oh, wait…

Q.E.D. – This may be what you’re referring to. Maybe not:

“The radioactive boy scout: when a teenager attempts to build a breeder reactor. (case of David Hahn who managed to secure materials and equipment from businesses and information from government officials to develop an atomic energy radiation project for his Boy Scout merit-badge)”

Nope, that’s not it. I’ve seen that story too. The one I’m referring to is about a kid who designed, on paper, an atomic bomb, that experts believe may have worked. This tale could very well be apocryphal, as I can’t find any references to it anywere on the internet yet.

This may be it, but I’m not sure as some of the details are quite different than I recall. It’s pretty close, though, and the time frame is about right (late '60’s).

You might be thinking of the guy who’s (true) story was told in the book Mushroom: The story of the Atomic Kid.

From what I remember, he was a college (not HS) student, in the mid-70s, when he designed an implosion-type A-Bomb as a “term paper.” (He passed.)

Yes, I think that might be it, Ranchoth. Thank you.

Yes & no. While you could more or less make a uranium bomb that way - with enough enriched uranium, you could even do away with the dynamite and use gravity(!) - it’s a wasteful design in that it takes a relatively large amount of fissile material. Weapons-grade materials are probably the hardest substances to come by bar absolutely none, and if the proper R&D lets you make 5 Bombs instead of 1 - or lets you make one with 1/5th the material - that’s a really good trade-off.

I remember the incident also. The link that Ranchoth provided didn’t have any details, but after a bit of googling I figured out that the student’s name was John Aristotle Phillips and he was a student at Princeton at the time. In a couple of pages of googling I didn’t find any details other than the bomb was about the size of a beach ball and was basically the same type of bomb as the one dropped on Nagasaki. From what I remember though, he built a complete bomb, with an empty space where the uranium was supposed to go. I recall quite clearly that experts who examined the bomb said that if uranium had been put into it, there was a very good chance it would have worked.

Thanks to everyone for the excellent answers - for the record I was thinking of a (possibly false) claim that scientists were so sure the Hiroshima bomb would work they didn’t even bother to test it.

Yeah, I guess the big question is what your goal for the bomb is. As you say, a country would probably want to get as many bombs (and as high a yield) as they could for the work, but a terrorist organization probably wouldn’t mind “only” getting a few hundred kilotons of yield out of their bomb… :eek:

“Analog,” the SF magazine, had a low-tech bomb design in the 1970s as a warning about the great ease of terrorist bomb-making. Fill an entire basement with concrete, with a several-inch diameter vertical iron pipe embedded in the center. Put half the bomb material at the bottom of the pipe (so it sits in the center of the concrete volume.) Drop the other half down the pipe with a huge concrete rod following behind. Mechanical confinement rather than explosive momentum confinement. The calculated yeild was impressive, but I don’t remember what it was.

When I was part-owner of a comic book store long ago, we were selling a-bomb plans …as a poster! As I recall, the only hard part of a design is to create a droppable bomb: a large yeild with a very small amount of explosive and of uranium/plutonium. If you can afford to pile on extra material (e.g. make one guaranteed good bomb instead of two or three maybe-failed bombs,) then it’s not difficult. The A-bomb plans used minimum material, but still had a couple hundred pounds of high explosive, a hollow Pu sphere the size of a bowling ball, and an external neutron emitter that “gooses” the reaction during maximum compression in order to knock it way upwards along the exponential growth curve.

The poster was from a grad-student’s research project that came years AFTER the famous one mentioned above. He was going for a lighter, more elegant version. Rather than an array of carefully-timed multiple detonators on the surface as with the Hiroshima bomb, he enclosed the spherical explosive in the center of a large plexiglas football, with a thin layer of high explosive on the surface of the football, and one detonator at each end. The blast wave of the exploding thin layer would create a perfectly spherical contracting wave that would ignite the entire surface of the HE sphere at once.

Found a link:

Elements of A-bomb design
http://www.stealthskater.com/Documents/Nuke_041.doc

Interesting, but I’d have to question if the compression would be fast enough to cause detonation. The setup you describe is similar to the original gun-type atomic bomb, in which two subritical masses are brought together by firing one into the other using high explosives. As I understand it, the two parts must be brought together very quickly to achieve detonation. Would merely dropping one part onto the other be sufficient?

They did indeed test the bomb back in WWII. One for testing, two for dropping.

Remember it is possible for fission to occur just by getting enough material together in the same place and has even occurred naturally (Oklo Fossil Reactor). Ok…so no big BOOM but it gives you the idea that the boom isn’t all that far off with a little extra work.

The first bomb was exploded at the trinity test site in New Mexico. Then they dropped the bombs on Heroshima and Nagasaki. The trinity bomb was similar to Fat Man, the bomb that was dropped on Nagasaki.

Google “Little Boy” “Fat Man” and “Trinity” for lots more details.

Based on my reading of Richard Rhodes excellent book “The Making of the Atomic Bomb”, I’d say that the most important part of making an atomic bomb is basically the engineering project required to enrich the uranium (as you said). Although I don’t recall any numbers that Rhodes may have mentioned, it appeared to me that roughly 99% (on that order) of the money/work that went towards the Manhattan Project went into the Uranium enrichment project.

But there have probably been new technologies over the past years since such huge projects today would easily be discovered.

Anyway, you are correct in that the media is giving us false information about atomic bomb technology passing hands. A a-bomb “design” is as trivial as this fork that’s on my desk and is of little importance in the grand scheme of things.

The way I understand it (going by an old “How it works” encyclopaedia dated 1977 :smiley: ) the key is to seperate the Uranium-235 from the U-238. U-235 can allow fission, 238 doesn’t. (Generally speaking- it’s an old book. '38 can, however, capture neutrons and convert to Plutonium.)

'35 is only 0.73% of naturally occurring Uranium (the rest being U-238) and most reactors need it to be 1.5 to 2.5% to be able to use it as a fuel- “weapons grade” is purer still, roughly 85 to 95%. The Hiroshima weapon used roughly 110 pounds of it, so you can see that making one bomb’s worth of Uranium takes a great deal of raw ore.

Now the problem is, the two isotopes are essentially identical, save for what, three electrons? Neutrons? (Someone will correct me, I’m sure.) So it’s not just finding a needle in a haystack, it’s more like finding a #2 needle in a stack of #3 needles. You can’t seperate them chemically, they have to be seperated mechanically.

This seperation process, to concentrate the '35, is complex and costly, involving substances like hydrogen fluoride, tributyl phosphate, nitric acid, zirconium, magnesium, and a great deal of complex machinery like gas centrifuges. Considering the size of the machinery, the chemicals used, the systems needed to store, transport and use the chemicals, and the support systems, it’s all a setup about the size of a small oil refinery.

It’s large enough that it’s very difficult to hide such a setup underground, but without on-site inspections, the setup looks the same from a satellite’s view whether the place is simply making fuel rods for a reactor or making enriched “weapons grade” material.

And once you have the fissile material, I understand that making a single-stage nuke is absurdly easy. The Hiroshima weapon was not a whole lot more than a stout pipe, two chunks of enriched Uranium, a handful of an explosive, and an altimeter-detonator.

The important point that Whack-a-Mole and engineer_comp_geek miss is that the Hiroshima bomb (Little Boy) had an entirely different design from the Nagasaki bomb (Fat Man).

Little Boy used a gun design, and it was never tested prior to its first use.