[BBKing]

That was about it, EoB, uranium is *very* dense, so even a four-foot ball would be pretty heavy. The figure I have for Heisenberg's original calculation is a ball 160cm in diameter weighing 40 tons. Apparently the German nuclear physicists captured and interned in the UK after the war were amazed that the US managed to find enough uranium for a bomb.

The mistake was saying that the rate of escape has to be small in comparison to the rate of production - it just has to be a wee bit less, so the first fission releases slightly more neutrons internally than escape, resulting in more fissions resulting in BOOM. The difference doesn't have to be that large, and neither does the bomb (the mass comes down to kilograms of U235).