small nuke-a-ler 'splosions

We hear of nuclear devices having “X times the amount of the Hiroshima bomb”. I’m wondering, how SMALL can you make a nuclear explosion? Is this something that only works on a big scale, like pyramid marketing? Can I build a device small enough to take out the neighbor kid’s boombox car without contaminating the whole neighborhood?

Jus’ curious, and no, I don’t currently have access to enriched plutonium or gas centrifuges.


The article above refers to 0.01 kT devices, but also mentions some 1994 law banning development of devices smaller than 5 kT

See also

which has photos and other info on a 0.01 kT device.

In theory, you can make a nuclear explosion as small as you like, right down to a firecracker equivalent! However, you can’t make a nuclear bomb as small as you like.

I’m sure you are familiar with the concept of critical mass. A critical mass of fissile material DOESN’T give you an explosion - it is merely an arrangement of fissile material where the quantity of neutrons generated by fission is exactly equal to the number of neutrons that escape.

To get a fission explosion, you need a supercritical mass, so that the number of neutrons goes up and up and up…The amount of energy you get out of it depends on how long you can maintain supercriticality before the thing blows itself to bits. To get a decent yield you need a large supercriticality, e.g. 2-3 critical masses. And you need to put it together fast, which is what that business with conventional explosives and converging shockwaves is all about.

To get a little fission explosion, you can take a nice Nagasaki-style spherical- implosion plutonium bomb and reduce the amount of explosive in the implosion assembly. When you light it off, instead of getting a 2-3 critical mass supercriticality, aim for 1.001 critical mass, and you will get a satisfying pathetic nuclear explosion.

The trouble with this approach is, it still costs you 1-15kg of plutonium (depending on your bomb design), most of which doesn’t fission at all. It’s very dirty, since all your non-fissioned plutonium is scattered over the neighbourhood. You still have a fairly clunky physical bomb, as opposed to being able to scale it down to grain-of-rice size. And for the size and weight of the thing, you can do better with conventional explosives for taking out the neighbour’s car.

As it goes, the Davy Crockett is pretty small - less that 12 inches diameter and about 60lbs, with a minimum design yield of 10 tonnes TNT equivalent. It’s not at the lower limit, though. Ted Taylor, who did much of the development work for small fission devices in Los Alamos, has been quoted as saying “…less that a kilogram. Quite bit less” when talking about minimum masses of plutonium that can be made to explode. When talking about the smallest bomb that had been produced, he said “It was a full implosion bomb that you could hold in one hand that was about six inches in diameter.” The yield of the thing wasn’t mentioned. (Both quotes from “Project Orion” by George Dyson.)

If you want to take out something a bit bigger than cars but smaller than city blocks (say, a large bridge) then a six-inch nuke with reduced power will have weight and size advantages over the equivalent backpack full of C4, but the cost

matt: Yes, your final paragraph says it. Why bother?

If your target is a car, you don’t use a nuke.

If you are a terrorist, you aren’t interested in smallest yield, you want biggest bang for the buck.

Does it make a little mushroom cloud?

Libertarian: You probably know the mechanics of mushroom cloud formation. Are you making a joke or asking a question?

Apparently just another stupid “hit and run enigmatic comment that is supposed to sound really zen” from Libertarian.

Why is asking about creating a little mushroom cloud a bad thing? I happen to NOT know the quantim mechanics or any other form of mechanics of nuclear explosion cloud formation and it may have been a joke or may not but still an interesting side questionand certianly not deserving of such strong beratement.

Desmo’s right about Lib.

It’d probably make a mushroom cloud… I’d think any hot, quick, explosive force in the right environment would. I’ve seen small “mushroom” type clouds from movie-set explosions on tv. But they’re nothing like the mammoth clouds you’d see for a large nuke, which, IIRC, can create gale-force winds as they suck up the surrounding air at ground level.

I have no opinion one way or the other on Lib and maybe your taking his actions based on past threads or something but i dont see it here. 1 hour and 20 min passed before being accused of hit and running <not at ALL a big deal IMHO> and his question wasnt even THAT far off base. Please continue your regularly scheduled thread.

If you wish to take exception to another member’s posting style, you will do so in the BBQ Pit or by private e-mail. Don’t do it in this forum again.

moderator GQ

wow vindication from a moderater< i feel all warm and fuzzy inside now> ok im stopping now…really

Somebody needs to point out that it isn’t the terrorists who build the nukes–the best they could hope to do is acquire one and apply it to a target different from the nuke’s original purpose.

For the states who develop the bombs, wouldn’t there be a broad range of uses for miniature nuclear devices, like busting caves and bunkers, destroying an enemy aircraft carrier at one blow, or taking out an incoming ballistic missile?

BTW, I also found an interesting link on this topic, this one from the Bulletin of the Atomic Scientists. It goes into “micro-nukes”, “mini-nukes”, and “tinynukes”.

I would assume that the question would have to be based on what mass constitutes a critical mass for every fissionable isotope. I believe that there is a 22-pound figure as a publicly releasted value for minimum critical mass for either U-235 or Pu-239 (presumably pure or nearly so). But if the critical mass for Cf-261 turned out to be 13 grams or such, and the stuff had a sufficient half-life to justify manufacturing and transporting it, an “atomic bullet” could be a distinct possibility.

A critical mass isn’t strictly a material constant. That’s why I referred to it as an “arrangement” earlier. Published values are generally for a spherical mass at room temperature. The same mass flattened out into a sheet will not be critical. The same spherical mass surrounded by a neutron reflector will be super-critical. The same spherical mass at 500 deg. C will have expanded to a lower density and not be critical.

In modern fission bombs, the quantity of fissile material is generally rather smaller than the published spherical critical mass. Nevertheless, this can be used to form a highly supercritical mass by elastically compressing it into a much smaller volume using convergent shockwaves. The plutonium atoms are temporarily closer together, changing the escape probabilities of the neutrons signifcantly and giving a supercriticality. I was surprised that less than 1kg of plutonium could be made to fission, but Ted Taylor is arguably the foremost authority on the subject so I accept it.

There are isotopes that are more fissile that plutonium 239, isotopes of californium and americanium in particular. If you apply shockwave implosion to these, you could theoretically get physically very small bombs, and maybe even small yields without having to cripple the efficiency. I still think you’ll cause a fair bit of addtional damage to the neighbourhood when you pop a 0.5" BMG “Atomic” into someone’s car.

Building an implosion arrangement into a bullet would be some feat, but if enough californium were available, someone would do it.

I don’t see why these “Davy Crockett” devices did not become more poplar. Immagine being able to have thousands of those buggers on a ship. Hundreds on a single bomber or dozens on a single jet fighter. You could destroy things with a razor accuracy using such a small package. Instead of a single 5000 pound missle, you could have 100 50 pound missles.

How is californium made? This idea of nuclear weapons fitting into small shells is interesting.

Hmm. A single M16 round that, just before it strikes, explodes with the force of one ton of dynamite.

"I don’t see why these “Davy Crockett” devices did not become more poplar"

My guess is straight economics. Remember, we’re talking about a fission bomb with an artificially low efficiency. You could get a bigger bang from the same amount of plutonium if you imploded it to the maximum density achievable. You get a bigger bang still if you boost the fission with tritium, and a monster bang if you use that boosted bomb as the trigger for an H-bomb.

Not just economics. The stigma associated with any type of nuclear weapons does not disappear with a reduction in yield. The political ramifications of using ANY nuclear weapon, no matter the yield, outway the tactical military benifits.
Any nation that used such a weapon would be uniformly condemned by the international community, likely facing sanctions of one kind or another.
Additionally, as previously mentioned, the radioactive fallout from a low yeald fission device would produce many civilian casualties and long term contamination of the local environment.