I saw the movie Copenhagen. It was interesting in its own right, but
it turns out it was pretty much a fantasy based loosely on
Heisenberg's postwar statements. The actual history is more
accurately captured by the Farm Hall recordings and by Bohr in a
series of letters he wrote to Heisenberg but never sent. See:
http://www.nba.nbi.dk/papers/introduction.htm
http://www.nba.nbi.dk/papers/docs/cover.html
This one in particular seems authoritative:
http://www.nba.nbi.dk/papers/docs/d11ctra.htm
You cannot blame the playwright for the inaccuracies because the Bohr
family only decided to release the letters after the play became
popular. Actually, the story would not be compelling without the
ambiguity and lack of firm historic knowledge. A reenactment of what
actually happened would be a dull footnote to history.
I think the movie got some details wrong. There was confusion about
fast and slow neutrons. Also, as I understand it, Heisenberg did not
forget to compute the mean free path for U-235. He failed to realize
that neutrons lost from the mass would not prevent the reaction, they
would only increase the number of generations. That's how he came up
with ~1000 kg critical mass instead of the actual number, ~20 kg.
Anyway, it is clear from the Farm Hall recordings that he did not
think about the technical issues in depth, so I conclude they did not
seriously intend to make a bomb.
- Jed
