Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Devin Purvis on 02/06/2011 06:01:03
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Devin Purvis asked the Naked Scientists:
Hi everyone,
My name is Devin, I live in Winnipeg, Manitoba, Canada. I never miss an episode (http://www.thenakedscientists.com/HTML/podcasts/). Not only are the topics intriguing, but the hosts deliver with a sexy yet calm enthusiasm. I love it!
While watching a documentary about the Hiroshima bombing I began to ponder... How small can you make an atomic blast? Could they have atomic hand grenades?
Thank you.
Sincerely,
Devin
What do you think?
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http://en.wikipedia.org/wiki/Californium
"Californium-251 has a very small critical mass (about 5 kg),[54] high lethality, and a relatively short period of toxic environmental irradiation. The low critical mass of californium led to some exaggerated claims about possible uses for the element. In an article entitled "Facts and Fallacies of World War III" in the July, 1961 edition of Popular Science magazine, the claim was made that "A californium atomic bomb need be no bigger than a pistol bullet. You could build a hand-held six-shooter to fire bullets that would explode on contact with the force of 10 tons of TNT."
Californium is vastly expensive to produce $10,000,000 Per gram so no one could afford these mini bombs even if they worked.
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Devin, I suggest that you have a careful read of the wikipedia article http://en.wikipedia.org/wiki/Nuclear_chain_reaction (http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
The basic fission reaction that occurs in a nuclear explosion involves
(Neutron) + (Fissionable Isotope of large atom) → (Isotope of medium sized atom) + (Isotope of medium sized atom) + 3 or 4 (Neutron)
For a nuclear explosion to occur, the average number of the product neutrons that trigger another reaction needs to be greater than 1.
There are several things that can happen to neutrons that are produced as a product of a fission reaction
(1) They can move out of the region where the fissionable material is
(2) They can be absorbed by atoms other than the fissionable ones
(3) They are usually very fast moving -- they can bounce around off other atoms a bit and gradually get slowed down, OR
(4) If conditions are just right, they can collide with another fissionable atom, and produce another fission reaction.
If each reaction produces, say, 3.5 neutrons out for each neutron in, then there will only be a nuclear explosion if about 30% or more of those neutrons trigger another fission.
With a small lump of fissionable material, you will not get a nuclear hand grenade, Nearly all of the neutrons will travel straight out of the lump, maybe having a few collisions and warming things up, but more than 90% will take route (1) above.
For a medium size lump, routes (2) and (3) become more important. The lump will probably melt and make a bit of a mess, but not an explosion.
Quite a sizable lump of material is required for there to be any chance of an explosion. The actual size needed depends on shape and purity, but for a 100% pure sphere of fissionable material there is a "critical mass" (you may be able to find how much on the web -- I think it is somewhere in the region of 2-10 kg) required.
Of course it is not possible to put together that critical mass in any straightforward way. The technical secrets in using fissionable material to make a nuclear explosion are to do with how to get that critical mass of fissionable material together and hold it together for a few milliseconds.
But I think that the minimum possible size of a nuclear explosion is roughly half the size of the Hiroshima bomb, Others may well be better informed than I am about this.
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Hello
The smallest size nuclear warhead I know of is that fitted to the 'Davey Crockett' (M28/M29) weapon system in the 1950s. About 40cm across with a yield of about 10-20 tonnes of TNT. There may be smaller ones today I don't know about. To produce explosive fission you either have to bring together enough mass of, or create a high enough density of, a fissile material such that a self sustaining 'explosive' reaction occurs. In principal you could make a nuclear explosive device very small indeed - say the size of a matchbox - but the engineering problems associated with it are enormous. As 'Damocles' says above, there are problems with generating sufficient additional neutrons fast enough from each single neutron 'impact' to give you a rapid fission if the mass of material is very low and/or the space within which its contained is very small - the material gets hot and melts before a fission chain reaction occurs - the sort of thing that happens with a reactor meltdown. In a nuclear weapon it’s rather evocatively called a 'fizzle.'
However, these are engineering problems - not limits set by the laws of physics. So you could, in theory, make a nuclear hand grenade. Of course what you'd really want is a nuclear warhead for, say, a grenade launcher so that the thing goes off a long way away.
You may also like to note that nuclear warheads were produced for air to air missiles and man-pack devices, about the size of a 'Jerrycan' - these latter are called Atomic Demolition Munitions.
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Mmmhh.
One could shoot a mini bullet of antimatter and when it hit the target...
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Nice idea but in fact 'anti-mater' doesn't behave like it does in the movies so nothing would happen.
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If you look at Project Orion.
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
They apparently designed 0.15 kt of TNT (600 MJ) Nuclear Bombs for the rocket propulsion. The Hiroshima bomb was about 15 kt. I presume the design work for the fission bombs was quite advanced.
At the end of the Wikipedia article, they talk about a theoretical micro-fusion explosive that presumably hasn't been built yet (or is still classified).
the Project Daedalus fusion microexplosion rocket. Daedalus class systems use pellets of one gram or less ignited by particle or laser beams to produce very small fusion explosions with a maximum explosive yield of only 10–20 tons of TNT equivalent.
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Mmmhh.
One could shoot a mini bullet of antimatter and when it hit the target...
Nice idea but in fact 'anti-mater' doesn't behave like it does in the movies so nothing would happen.
I don't know.
Antimatter (antihydrogen & antihelium and various subatomic particles) is real. But, unfortunately, now only produced in very small quantities. Everything I've read indicates that matter and antimatter do annihilate when they come into contact, releasing significant amounts of energy (but, at this moment only that of a few atoms).
Unfortunately, synthesizing, capturing, and containing, 1 gm or even a few mg of frozen antihydrogen or antihelium would be a pretty tall order. Even if you could get it frozen into a solid block, the containment vessel would not be trivial.
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Nice idea but in fact 'anti-mater' doesn't behave like it does in the movies so nothing would happen.
I don't know how it behaves "in the movies" but I know that antiprotons and antielectrons don't survive for a long time around normal matter...
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Nice idea but in fact 'anti-mater' doesn't behave like it does in the movies so nothing would happen.
I don't know how it behaves "in the movies" but I know that antiprotons and antielectrons don't survive for a long time around normal matter...
Too right Lightarrow. A toroidal electromagnetic containment with a gramme of antiprotons (cliff - much easier to use charged ions rather than neutral atoms would make) would make one hell of a splash.
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(cliff - much easier to use charged ions rather than neutral atoms)
Good point, the ions are more magnetically active, and thus easier to contain (although more difficult to concentrate).
With neutral hydrogen/helium, even if you managed to make a frozen solid, the vapor pressure would be way too high.
Perhaps one could make a salt... For example, one could drop the vapor pressure down significantly on anti-Lithium Hydride. Or, for that matter, even anti-Lithium wold be much easier to contain than either Anti-Hydrogen or Anti-Helium. Lithium metal is considered paramagnetic, which would help with containment.
But, all of this is a bit theoretical as we don't have a good method for synthesizing large quantities of antimatter, nor have we found a good place to mine it. And, so far haven't been able to produce anything larger than anti-hydrogen or anti-helium in very small quantities, lasting for a very brief period of time.
The "smallest atomic blast" might be the annihilation of a single electron/positron, or proton/anti-proton. But, at this point we have no method of concentrating it to make the equivalent of 1 pound TNT yield, or directing it for use at an arbitrary location.
Thus we get back to the scale used/planned for Project Orion, with fission explosions on the order of 0.15 kt TNT, and smaller fusion explosions planned, but not actually built.
Do you count small nuclear reactors such as those in many Universities. Capable of sustained fission, but not designed to "make a blast"?
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"The "smallest atomic blast" might be the annihilation of a single electron/positron,"
There are no nuclei involved in that so how "nuclear" is it?
Would the decay of a tritium nucleus (with the production of something like 20KeV) count as a nuclear blast?