Actually youtube should be able to save the day.

http://www.youtube.com/watch?v=SQ4dJVHYWDU about 2 minutes in.

Also using two halves in two different bombs wouldn’t work. You see that gun-type design up there? The one with the hollow cylinder being fired at the smaller solid one? The total mass of both bits together is just over two critical masses. The hollow cylinder is therefore larger than one critical mass. It doesn’t go critical, though, because it’s a hollow cylinder rather than a sphere and it’s got lots of surface area to lose neutrons. The same applies for chopping a uranium sphere in half; doesn’t matter if the original sphere was two critical masses, changing the shape of the thing means that the criticality threshold for any non-sphere shape increases. So unless they specifically remould the half-sphere into a smaller sphere and then use it in an implosion design (and if they could do that then they wouldn’t need to purchase illicit uranium supplies anyway, because they’d either be dead or working for a governmental agency) it’s just not going to happen.

So I guess that’s not right then, huh?

I was going to ask why in movies when they get their uranium out of a padded suitcase it’s always shaped in two halves of a sphere but you’ve only gone and answered that bit. Is this a rare case of films getting things right?

*makes notes*