Asside from the governments meddl’in ways, that is.
I have heard that there is a lower limit to the size of nuclear weapons, the suitcase nukes being the smallest possible. Why is this? It is easy to imagine the tactical usefulness of a bomb with the power of only a few tons of TNT (as opposed to several thousand) in a size you could carry in your pocket.
Why is physics denying me the atomic bullet?
Fairly briefly, if it’s too small the neutrons will get out too quickly.
As nearly as I understand it, that is. In any chunk of fissionable material, (uranium or plutonium,) say, there is an occasional stray incidence of nuclear fission going on every few seconds. That’s just one atom breaking apart, though, and doesn’t release much energy compared to ordinary radioactive decay.
To create a bomb, you need to squeeze your fissionable material into a very small space, so that the neutrons from any one fissioning atom will on average go and trigger one more nucleus to fission, creating an expanding chain reaction. ‘super-critical mass’ is the correct term I believe - the critical mass is a size and density of uranium so that each one reaction will, on average, trigger one more, and that’s what you want for a nicely ticking along nuclear power reactor. “Going supercritical” is when the number of nuclear reactions are increasing quickly. Bad news for a power plant. Exactly what you want for a bomb.
Generally, in an explosive device, you can create this pressure by setting off a bunch of conventional explosives in a sphere, in such a way that the pressure waves are all focused to converge on a fissionable core in a perfectly symmetrical way. You get a super-critical mass for a second or so, and then the heat and energy from the nuclear reactions make the core expand until you lose critical mass. By that time, though, you have enough energy and heat to blow up a bunchuv other stuff.
Creating a really tiny bomb runs into two big problems, though. One is organizing the spherical trigger with sufficient precision. The second is that a small enough lump of fissionable material just won’t go supercritical with the amount of pressure you can put on it, because neutrons that are released too close to the edge of the ‘lump’ are likely to escape rather than triggering other nuclear reactions.
Is all of this clear as mud now? 
So, I need to create an external source of neutrons that will bombard my fissionable material since that material will be unable of supplying the needed particles.
Is that at all possible of doing (forget about size for a moment) without being prohibitedly inefficient?
Your fission nuke starts off as a “supercritical” lump of fissile material, with neutrons zipping about in it. “Fissile” means that if its atoms are hit by neutrons, they’ll split and release more neutrons.
The nuke works by a chain reaction of atoms splitting after they’re hit by neutrons. E.g. one atoms splits, kicking out energy and a few neutrons. A couple of those neutrons split other atoms, which kick out energy and a few neutrons each, and they split other atoms, which split other atoms via neutrons…
Not all of the neutrons split atoms, though. Some of them hit atoms without splitting them, and some of them escape the fissionable material entirely.
If you have too small a chunk of fissionable material, more neutrons escape than are generated and the chain reaction peters out. That’s it, really.
“Supercritical” means that you have a big enough chunk that this doesn’t happen. You can make the chunk more supercritical by surrounding it with a “neutron reflector” that helps send some escaping neutrons back into the chunk, and you can also make it more supercritical by squashing it into a smaller volume so all its atoms are closer together. This is done via conventional explosives, which can really squash things hard, though not for very long. But even with a reflector and explosive compression, there’s a limit below which you can’t go.
Not really. You need nuclear reactions to generate a decent supply of neutrons.
I was going to ask if we could lower the size by reflecting the neutrons back in but this Wikipedia article talks about that and suggests you could.
The critical masses involved are pretty heavy, no idea how big a lump that translates to.
You’re problem with the golf ball nuke is that even if you could get the amount of material into the required size you’d have to stop it going critical too early, which is going to be difficult with so much stuff packed into such a small space. Plus you probably want some shielding as well to protect the users …
SD
Why?
Think in the most simple of situations, a single atom and a single neutron.
If I propel that neutron with sufficient energy to break apart the atom why would this not be fusion?
What if I did this for a zillion neutrons?
In my second post I am asking if it is possible to generate a bunch of neutrons of sufficient energy to break apart a large number of atoms without requiring an amount of energy near to what would be created by the splitting atoms.
Fission, but yes, it would.
That’d be great, but you can’t. Neutrons don’t come in paper packets, and being neutral, you couldn’t accelerate them even if they did. Whenever we need to generate neutrons, nuclear reactions are involved. The neutrons are bound up in atoms and you have to persuade them to spit them out.
For a big, consistent source of neutrons we use a whole fission reactor. Such reactors exist for scientific purposes such as neutron imaging, and buying time on one is expensive.
For smaller, short-lived sources of neutrons we can use a particle accelerator to bombard a suitable target, and the resulting nuclear reactions in the target generate neutrons. The neutron generators in current nukes work in this way, but they are there to make sure there’s a few neutrons in the supercriticality at the instant it’s compressed.
Maybe it’s possible, but with our current technology it’d be a neat trick!
A comparatively recent design of neutron generator uses small-scale fusion to generate neutrons. This is the cutting edge of neutron generation at the moment.
Yes, I have heard of it and it is what I was thinking of.
Because fusion means “sticking atoms together”. I’m guessing you really meant “fission”.
Your external, compact, neutron source is the problem. It’s not that it’s difficult to get atoms to break apart; a few atoms in a lump of uranium are doing it all the time. It’s getting the process to accelerate to a self-sustaining (even momentarily) runaway chain reaction that liberates loads of excess energy that is the problem. The simplest and most compact way to do this is to bang together a couple of chunks of U235 or Pu, which will bring their own random neutron emissions to the party and liberate increasing amounts of the same as the reaction gets under way.
Don’t forget that neutrons aren’t amenable to being accelerated the way that protons are. Not having an electrical charge, they don’t have anything to be conveniently grabbed hold of in order to energise them. You’re pretty much stuck with the energy they had when they were liberated from the nucleus, and the best way to get high-energy neutrons in large amounts is to start a runaway chain reaction.
Gah. A day late and a dollar short, again. Carry on.
Eep. Yes, I meant fission.
To create a bomb-type atomic weapon, you need a “fissionable substance” – and while everything but Hydrogen-1 is fissionable in the strict meaning of the term, in atomic physics “fissionable substance” is a term of art, meaning an isotope which will undergo a chain reaction to create self-sustaining fission. Only one isotope occurs naturally in more than trace quantities which qualifies as a “fissionable substance” – Uranium-235. Two other isotopes can be created in breeder reactors that are “fissionable” by this usage: Plutonium-239 and Uranium-233, which can be created from Uranium-238 and Thorium-232 (both relatively common nuclides) by slow-neutron bombardment.
All three “common” fissionable isotopes have supercritical masses high enough to call for delivery by bomb, missile, or other large-scale device.
The government has understandably not gone public with what else is available and its critical mass, but I recall discussions in the late 1950s or early 1960s of an “atomic shell” – an artillery-size device that could be fired from a field gun, which would produce a relatively small fission-type explosion on impact. My extremely hazy memory says that an Americium isotope was what was mentioned.
Producing something of bullet size that would reliably explode on impact, be stable enough to be stored until needed, and not explode before detonation by the proximity of the subcritical masses is a non-trivial problem. Presumably there are isotopes with a low enough supercritical mass – but they’re also likely to be subject to rapid breakdown, would be very difficult to store, and extremely difficult to manufacture and be safe for use.
Fusion, of course, complicates the matter even further. Hydrogen will fuse into helium spontaneously at the right temperature and pressure (with the heavier isotopes and helium-3 having lower temperature and pressure requirements than common light hydrogen-1), but the only ways of producing those stringent conditions are the core of a star and the center of a fission explosion, along with the experimental fusion reactors such as Tokamaks.
Well, if you do need a big explosion in a small package, you might want to look into antimatter. 1/2 gram antimatter (combined with normal matter) will give you a yeild of 22 kilotons TNT, if I did my math right. Of course, wikipedia gives the world production of antimatter at 1 to 10 nanograms a year, so you might have to wait awhile.
But of course antimatter is something less than trivial to store and transport. 
A simple cardboard box will do. Provided you have a protective forcefield.
The devil’s in the details, isn’t it? And of course, without the protective forcefield, even a General Products hull won’t do.
The easiest thing would be to (a) go to a pulsar, (b) scoop out a hunk of neutronium from the surface, and then © use explosives to propel a neutronium bullet at the uranium fuel.
That should get you more than enough neutrons to run the supercritical reaction.
Of course, if you can figure out how to pull off (a) and (b), you probably would laugh at any need for puny nuclear fission weapons.
I believe the idea in a fission reactor isn’t so much to create neutrons with a lot of energy but rather to provide a lot of neutrons. In a reactor the neutrons don’t “smash” the atoms. Instead the slow neutrons are captured by the atoms giving rise to an unstable nucleus which then splits releasing energy and more neutrons.
Fusion is something else entirely. The form of fusion most talked about is the combining of 4 hydrogen atoms into one helium atom under intense pressure and heat. The four hydrogens are slightly heavier than the one helium with the excess mass being released as energy.
Hijack.
I heard something about a ‘natural nuclear reactor’. In Africa? IIRC there was a large deposit of unranium that sometime in the past generated a nuclear reaction of some sort. Does anyone have any information on that?