I've been having a discussion with a friend about nuclear materials and whether bringing two bits of subcritical fissile material together at a decent enough speed would cause a spontaneous explosion.

I'm aware that there have been several nuclear accidents involving fissile material over the years, but none of these have ever resulted in an "explosion" in the traditional sense - just the release of heat/light and radiation enough to kill (you could argue that this is a very small explosion)

Imagine I have two half-spheres of enriched uranium, how quickly would I need to bring these together to observe a "grenade sized" bang?

I'm aware a lot of this is speculative since there are a lot of variables - let's say weapons grade uranium in standard atmospheric conditions.

  • $\begingroup$ -1. Unclear. What effect do you think the speed at which the halves are brought together will have? Were the nuclear accidents you refer to caused by bringing the halves together fast? $\endgroup$ – sammy gerbil Jun 15 '17 at 12:24
  • $\begingroup$ I think the speed will effect how much time the reaction has to revert to a sub-critical state again based on expansion of the material by heat etc. The design of most nuclear bombs requires that either material be compressed into a smaller area or for two pieces of subcritical material to be brought together quickly. If speed was not a factor, why would these bombs take this approach? $\endgroup$ – Charleh Jun 15 '17 at 12:44
  • $\begingroup$ e.g. To produce detonation, the pieces of uranium are brought together rapidly. In Little Boy, this was achieved by firing a piece of uranium (a 'doughnut') down a gun barrel onto another piece (a 'spike'). This design is referred to as a gun-type fission weapon. $\endgroup$ – Charleh Jun 15 '17 at 12:45
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    $\begingroup$ I think that this is hard to answer. The assembly times for nuclear weapons are critical (not meant as a pun) and are something like a microsecond or less (I suspect the times are not well known). The assembly of an implosion weapon also needs to be highly symmetrical. Clearly you can't do anything like that manually. On the other hand, when you bring the halves together it's going to disassemble itself somehow, obviously, and that's going to involve a flash and a bang. I just have no idea how big that flash and bang will be and I suspect it is not widely known. $\endgroup$ – tfb Jun 15 '17 at 13:18
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    $\begingroup$ @tfb Actually a scientifically very good description of the sequence of the process can the found in the chapter "Three Shakes" of the book "The sum of all Fears" by Tom Clancy. $\endgroup$ – ZeroTheHero Jun 19 '17 at 3:21

You've hit upon a fundamental engineering problem in designing nuclear weapons: you get more energy by putting more fissile material into a smaller volume, but the energy of the reaction tends to decrease the density of the fissile material. (That is, it's hard to keep things close together while they are exploding.) Figuring out this tradeoff is hard. Let the pieces approach each other too slowly and you get a criticality incident without explosive power; too fast and you'll fission the entire critical mass, which is what the high-yield bombs do.

One way to think of it: we usually describe nuclear weapon yields in terms of kilotons or megatons of TNT. (Even North Korea's fizzle in 2006 was kind of a kiloton.) A grenade is actually made of TNT (more or less) and has a mass of a milli-ton. So what you're asking is for a tweak to a weapon design that reduces its yield by a factor somewhere between a million and a billion, without reducing it to zero. I think you can appreciate that's a hard problem.

If you wanted to explore this, a good place to start is Serber's Los Alamos Primer.

  • $\begingroup$ The question isn't really "could you make a grenade sized bang" more so, could you cause an explosion of any magnitude without using other explosives to create a state which would result in said explosion. $\endgroup$ – Charleh Jun 19 '17 at 2:10
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    $\begingroup$ Well, one could duplicate the "gun-type" design where the moving component is dropped in an evacuated tunnel rather than launched with chemical propellant. That'd give you approach velocities from a few meters per second to a few kilometers per second, depending on the drop altitude. Predicting what happens would be an interesting project but for me it's beyond the scope of a Physics.SE answer. $\endgroup$ – rob Jun 19 '17 at 16:23

I am on the other side of this and i believe that if you had 2 half spheres whos total amount could potentially go critical and you were to force them together at even the best forces humans could hope to obtain for a instant such as throwing together or smashing together without mechanical assistance the instant their was impact the heat and force would repel eachother in essense acting as a regulator, and with the absense of neutron reflectors or casings much of the energy would simply be lost to the air relatively slow and so while you might get a heat and pressure wave hoping for even a grenade like effect is optimistic.

  • $\begingroup$ Do you have any evidence for this, or is this just a wild guess? $\endgroup$ – Peter Shor Jun 15 '17 at 12:51
  • $\begingroup$ id call it a educated guess, because in this instance we are talking about a brief moment of impact, not a force clamping them together, while that is long enough for quite a large number of neutrons to pass between the 2 halves you would think the more of a reaction you generate the less time it would take for them to separate or expand and become non critical again, and it doesn't much force to separate 2 objects whos only force keeping them together is momentum, little of that in this case. $\endgroup$ – thomas roberts Jun 15 '17 at 13:10
  • $\begingroup$ upload.wikimedia.org/wikipedia/commons/2/2a/… this is the closest case i could find and in this instance even with 2 halves of the sphere slightly contained however it does not mention how fast the pieces came together, but considering they were forced apart before any significant damage occurred even without being free to move about or expand fully i thing its a good case for heat/pressure overcoming proximity before notable damage is done in a totally open enviroment. $\endgroup$ – thomas roberts Jun 15 '17 at 13:36

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