One common misconception, and one that seems to have plagued even such esteemed scientists as
Werner Heisenberg before the
war, is that all you have to do is get a bunch on
radioactive stuff in one place and
run away real fast. This would indeed produce an
explosion, but it would be a
conventional explosion caused by the heat of the
super-critical mass, rather than the release of
heaping buttloads of
neutrons. To get a real
nuclear explosion, you have to bring the
plutonium or
enriched uranium together fast enough for the
chain reaction to go through several generations before the heat blows it apart like an
egg in the microwave.
As mentioned in previous writeups, there are two basic types of nuclear bombs:
The Little Boy design- Uranium 235 (U238 won't do) is formed into two parts that are brought together. This can be done as either a wedge into a sphere or a plug into a cylinder. By far the simplest design (though with a lower yield), this kind wasn't even tested before being dropped on Nagasaki, so confident were the designers of it's design. As mentioned above, however, the trick is to bring the two pieces fully together before the simple mechanical heat caused by the beginnings of a chain reaction blows the whole thing apart in a minor nuclear fizzle rather than a genuine nuclear explosion.
the Fat Man design- Plutonium formed into a spherical shell is imploded. The trick is that explosives aren't very good for forming things into dainty shapes. Generally something inside a spherical configuration of high explosives does indeed go inward, but not very precisely- bits of it tend to be ejected out, some areas are more compressed than others, all sorts of problems. The solution to this is to use different kinds of explosives in the outer shell. Like all materials, different explosives transmit force with different speeds. So high density explosives are packed in strategic locations throughout the shell that otherwise consists of lower density explosives. These high density nodes act as lense to focus the force on the center, so that instead of having these roughly interacting waves of force coming from every which way, the plutonium is instead hit by neatly focused force that pushes everything in to the center.
In both configurations, a neutron source is placed in the center of the configuration to ensure that the chain reaction actually starts. Chances are it would anyway, but why take the chance?