To the Editor:
In “Building Hitler’s Bomb” [May], Jeremy Bernstein has the physics of nuclear reactors wrong. A reactor operates with U-235, not U-238. The latter fissions not “when it is struck by slow neutrons” but when it is struck by fast neutrons. While it is true that U-235 and Pu-239 (the plutonium isotope used in the bomb) are fissionable by fast neutrons, they will also undergo fission by slow neutrons.
In addition, Mr. Bernstein states that Germany found heavy water hard to come by. In fact, however, Norway already had a heavy-water production plant in operation at the time of the German invasion and occupation in 1940. Whether its capacity would have been sufficient to support a German reactor-development program, I cannot say. But the Allies must have thought it was, since the plant was destroyed in 1943 by British commandos in concert with the Norwegian underground.
Western Springs, Illinois
To the Editor:
Jeremy Bernstein’s fine article on the role of Werner Heisenberg in Hitler’s atomic-bomb effort brings additional scholarship to bear on the myth that Heisenberg heroically thwarted Nazi attempts to develop the bomb. I only wish Mr. Bernstein had dealt more extensively with the secret recordings made by British intelligence of Heisenberg and his fellow German scientists when they were incarcerated at Farm Hall. These Farm Hall transcripts reveal Heisenberg’s spontaneous thoughts when he first learned of the bombing of Hiroshima from a news broadcast. According to Malcolm MacPherson, in his book Time Bomb (1986), this is what Heisenberg said after hearing the news:
More funds were first made available in Germany in the spring of 1942 . . . when we had absolutely certain proof that this business [the atomic bomb] was possible. . . . We did not muster the moral courage to recommend that the government should employ 120,000 people in spring 1942 to develop this business.
If “moral courage” is the correct translation in this context, the proposition that Heisenberg was trying to sabotage the data that would have made an atomic bomb feasible becomes untenable.
Additionally, in order to be approved by the Gestapo, Heisenberg’s meeting with Niels Bohr in Copenhagen in September 1941 must have served some Nazi purpose. It seems entirely possible that at that time, with the fall of Moscow a near certainly and with it, the resumption of the blitz of British cities, the German aim was to use Bohr as a conduit to warn of even more horrible devastation if the Allies refused to parley. Many in power in England had already quietly made known their favorable opinion of such discussions. It could hardly have been anticipated by the Germans that by the time Bohr’s fearful news reached the West, Russia would still be a mighty player, the U.S. would be in the war, and the response would be not capitulation but the Manhattan Project.
M. Donald Coleman
Mamaroneck, New York
Jeremy Bernstein writes:
Nuclear reactors operate with natural uranium, which is 0.7 percent U-235 and most of the rest U-238. It is, as Ira Charak says, the U-235 that is fissioned in a reactor. On the matter of heavy water: the Germans were committed to using heavy water, which was difficult to obtain, rather than the more readily available graphite, because of an error in an experiment conducted by the German scientist Walter Bothe. Bothe was trying to determine if graphite was a suitable moderator—a material that would slow down neutrons in a reactor—but his sample was contaminated with a tiny amount of boron, which absorbs neutrons. It was then concluded that graphite was not effective. Our program was spared such a mistake because Leo Szilard was a fanatic for obtaining purified graphite. And it worked! The real point here is that because the German program was not centralized, Bothe’s experiments were not widely known and so were never checked.
I might have noted that heavy water was not available in Germany but in Norway, an occupied and hostile country. As Mr. Charak points out, one of the great stories of the war is how the British and Norwegians managed to destroy the supply. But even if they had not, there might not have been enough heavy water for the job. Hans Bethe, who left Germany in 1935 and later headed the theoretical division at Los Alamos, told me that Heisenberg underestimated the amount that would have been needed in developing a nuclear reactor.
Here is what bothers me about M. Donald Coleman’s scenario. Let us assume that Heisenberg’s mission was to use Bohr as a conduit to intimidate the Allies. Why then did he not tell Bohr about the one thing that would really have intimidated them: the fact that the Germans knew about plutonium (as it came to be called), which at that time looked like the royal road to making a nuclear weapon? I do not know if it was the British who first told Bohr of the plutonium or whether he learned about it a little later at Los Alamos, but it is clear that he did not know of it before he left Denmark. Again, it was Hans Bethe who told me that when Bohr got to Los Alamos, he knew very little about the bomb. As a matter of fact, Robert Oppenheimer assigned the then very young Richard Feynman to bring Bohr up to speed on the project.
I have written an entire book on the Farm Hall transcripts, Hitler’s Uranium Club, in which, with the aid of David Cassidy, the whole history is revealed.