They show that he understood the difference between a reactor and a bomb and that he did realize that fast neutrons are essential for an explosion. But they also reveal some serious misconceptions. Heisenberg apparently believed that several tons of pure 235 U are needed to make a bomb. He made the astonishing disclosure that he had never calculated the critical mass. Bernstein gives a lucid analysis of the entire episode.
Confusion persists over Heisenberg's ideas concerning the critical mass.
My best guess is that, although Heisenberg never made a detailed calculation of the critical mass, he believed all along that it is very large. His Farm Hall calculation sounds like a reenactment of one previously performed, not like something concocted on the spur of the moment. If he had on some earlier occasion calculated a much smaller critical mass, he would have remembered it. The smaller numbers cited might refer to the amount of uranium that must fission in order to produce an appreciable weapon yield, a quantity quite different from the critical mass. Another possibility is that the smaller figures refer to plutonium rather than uranium.