Could you cause a small "nuclear" explosion manually 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.
 A: 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.
A: 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.
A: 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.
