I push two half spheres of enriched uranium together after which the resultant sphere has a mass above the critical value. Will it explode? Suppose I hold in both my hands a piece of enriched uranium, in the form of a half-sphere and both having equal mass as well as radius. The mass of one piece lies below the critical value. Now I push the smooth surfaces of both half spheres with all the force I possess against each other, resulting in one spherical piece (which isn't true, obviously, but I don't know how to put it in words otherwise) of enriched uranium with a mass above the critical value. Will the last thing I see be a killing flash? By which I mean an atomic bomb explosion. Of course, I'll try not doing this at home! ;)
 A: It won't explode in the sense of 'be a nuclear bomb': what you are describing is reasonably close to one of the accidents that befell the 'demon core', and in particular the Slotin criticality incident (it's not actually the same: the Slotin incident was due to a reflector being allowed to fall over the core, but it's close).
What you would get would be prompt criticality followed, I assume, by either you dropping the thing or it explosively disassembling itself.  In either case you would get a really large burst of neutrons from the thing, which would kill you.  If it explosively disassembled itself then you'd presumably die in the explosion, which would be the better (quicker) outcome for you.
What it won't do is to be a nuclear weapon: in order to construct such a thing you need to assemble the thing really quickly and keep it assembled for as long as you can.  This generally involves some kind of explosive to assemble the thing and various other tricks.  The worst case is something like a 'fizzle', which would be a fairly small explosion.  Such a thing would still not be good since it would scatter bits of the device everywhere, requiring a lot of cleanup.  As far as I know the Slotin incident didn't result in any kind of explosion at all, although I think they had the presence of mind to disassemble the thing rather quickly.
As far as I am aware criticality incidents exactly as you describe haven't happened (list on Wikipedia), although almost exactly what you are describing has been shown in the 1980s BBC serial Edge of Darkness, which I think was relatively well-researched.
In any case: don't do this, obviously.
