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? $\endgroup$ Commented Jun 15, 2017 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
    Commented Jun 15, 2017 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
    Commented Jun 15, 2017 at 12:45
  • 3
    $\begingroup$ 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. $\endgroup$
    – user107153
    Commented Jun 15, 2017 at 13:18
  • 1
    $\begingroup$ @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. $\endgroup$ Commented Jun 19, 2017 at 3:21

3 Answers 3


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
    Commented Jun 19, 2017 at 2:10
  • 1
    $\begingroup$ 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
    Commented Jun 19, 2017 at 16:23

You have to really work hard to assemble the correct material to create a nuclear weapon; you need to create a system that is super prompt critical using fast neutrons and remains so sufficiently long for the chain reaction to produce enough energy before pressure causes dis-assembly into a non-critical configuration. By super prompt critical is meant super critical on the prompt neutrons alone without having to wait for the delayed neutrons to contribute. As mentioned in an earlier answer, see the Los Alamos Primer by Serber, available from Amazon: the early notes on the physics of a fission weapon from Los Alamos at the beginning of the Manhattan project.

Manual assembly cannot maintain a super prompt condition sufficiently long for any appreciable release of energy from fission.

Older uranium based nuclear weapons - like Fat Man used on Hiroshima- were gun assembled devices. Plutonium based nuclear weapons - like Little Boy used on Nagasaki- use high explosives to cause assembly more rapidly; this is because the Pu contains 238 Pu and 240 Pu in addition to 239 Pu and spontaneous of these isotopes would cause pre-ignition in a slower gun assembly design. The 238 Pu and 240 Pu are unavoidably produced along with the 239 Pu from irradiating 238 U in a nuclear reactor. The book Building the Bombs by Loeber provides a good entry-level discussion of the physics and engineering aspects of nuclear weapons.


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? $\endgroup$ Commented Jun 15, 2017 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. $\endgroup$ Commented Jun 15, 2017 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. $\endgroup$ Commented Jun 15, 2017 at 13:36
  • $\begingroup$ Thought experiment: a 500 tonne block with a hemisphere of weapons-grade material and half of an initiator embedded in the top. A second 500 tonne block, with a similar hemisphere embedded in the bottom. Somebody manually releases the winch holding them apart. $\endgroup$ Commented Sep 8, 2020 at 17:01
  • $\begingroup$ @MarkMorganLloyd I think that just blew my mind (pun intended) $\endgroup$
    – Michael
    Commented Sep 9, 2020 at 1:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.