[calenel]

"It's not trivial. Nuclear weapon design, in public, has been deliberately sanitized and *over* simplified. But even what is in the Public domain is complex. Here's Fermi's reactor patent for getting a mere chain reaction (note: a chain reaction is insufficient for an atomic bomb):"

What ever you say, I guess, but a chain reaction - a stabile one - is much more difficult than a bang.

[Southack]

"a chain reaction - a stabile one - is much more difficult than a bang." - calenel

That's incorrect.

Chain reactions have occured naturally on Earth, something that one can't say for atomic explosions. See this link: http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml

Fission, after all, is a natural process. Atoms in many elements emit neutrons on their own.

There are chain reactions,
There are self-sustaining chain reactions,
and there are supercritical reactions (i.e. bombs). Mere "chain reactions" are simple enough to have occured naturally. Check out the link above. It's light reading.