Diana Preston - Before the Fallout

Heisenberg reported to the war office, where, as he later wrote, he was told that he had been conscripted into the new nuclear physics research group “to work on the technical exploitation of atomic energy.” The group became known, with surprising casualness about security, as the Uranverein (the Uranium Club). According to von Weizsacker, Heisenberg joined the club without hesitation in order to protect German science. His argument was “Well, we must do it. Hitler will lose this war. It is like the end game in chess, with one castle less than the others…. Consequently, much of Germany will be destroyed, or its value will have disappeared. The value of science will still be there and it is necessary that science should live through the war, and we must do something for that.”

Heisenberg’s own subsequent recollections described sessions of deep soul-searching with von Weizsacker, during which both men agreed that the prospect of successfully building an atomic bomb was very remote. The technical problems were formidable, probably insuperable, at least over the likely lifespan of the war. The greatest challenge was obtaining enough fissionable material to create an explosive device. Niels Bohr and John Wheeler had shown that it could not be done with natural uranium; only a sufficient quantity of the rare and highly fissionable isotope U-235. would do. But to separate this from U-238, the less-fissionable isotope of which natural uranium was chiefly composed, would, in Heisenberg’s words, require “a gigantic technical feat” that would take until “the distant future.” However, according to Heisenberg’s postwar account, the two men agreed that it might well be possible to use natural uranium to trigger a chain reaction capable of yielding controllable amounts of energy that could be used for “power stations, ships and the like.” They also agreed that when the war ended such technology would be important for the rebuilding of Germany. They could, they convinced themselves, work on it “with a clear conscience.”

The two broad thrusts of the Uranium Club’s research were how to separate enough U-235. and how to build a chain-reacting nuclear pile—a “reactor.” Meanwhile, Army Ordnance swiftly requisitioned the Kaiser Wilhelm Institute for Physics, which became the heart of the army project, and gave the institute’s Dutch director, Peter Debye, an ultimatum: renounce his Dutch nationality and take German citizenship or resign his directorship. Debye departed for the United States to teach at Cornell University.

Heisenberg’s role was to drive the theoretical side of the project. Still only thirty-seven years old and brimming with drive and energy, the man whom James Chadwick would identify later in the war as “the most dangerous possible German in the field because of his brain power” got quickly to work. His first priority was to develop a theoretical basis for a workable reactor. By December 1939, just weeks after his appointment, he submitted to the army a secret twenty-four-page report, which suggested that the production of power through nuclear fission in a reactor was technically possible using natural uranium. But “enriched” uranium, where the percentage of the rare isotope U-235. had been increased by means of isotopic separation, would be better. He offered an alluring scenario: Enriched uranium could be used to run a smaller reactor at a higher temperature than achievable with natural uranium and to generate enough power to drive German warships and submarines.

Heisenberg also suggested, in a statement in his report somewhat at odds with his later justification of his motives, that enriching natural uranium could create an explosive surpassing “the explosive power of the strongest existing explosive materials by several orders of magnitude.” Isotopic separation was, he said, the “surest method” for achieving a nuclear reactor but the “only method for producing explosives.”

Heisenberg’s report and a follow-up paper in February 1940 would provide the template for the Nazi fission research program until the end of the war. However, he made a critical misjudgment over the choice of a suitable material to use as a moderator to slow neutrons down and thus to enhance their chances of hitting their target uranium nuclei, causing fission and thereby triggering the release of more neutrons to sustain a chain reaction. Heisenberg had initially focused on two substances as a moderator: heavy water or carbon, which, as he later wrote, “I had suspected, for theoretical reasons… could be used as a moderator in place of heavy water.” However, in his second report to the army, he declared it doubtful whether the uranium machine (i.e., a reactor) could be built with carbon.

Heisenberg had been misled by imprecise data from experiments he had had conducted. The error was compounded by von Weizsacker, whose calculations in Berlin supported Heisenberg’s views. So did measurements made in early 1941 by Walther Bothe, by then Germany’s leading experimental physicist despite some difficult times. In 1933 he had been ejected from his professorship at Heidelberg University for failing to show due enthusiasm for the Nazi Party. However, he had managed to obtain a post at the Kaiser Wilhelm Institute for Medical Research in Heidelberg.

At first Bothe believed that carbon was a promising material for a moderator: It did not absorb neutrons and was freely available. However, just as von Weizsacker had done, Bothe chose, as his form of carbon, industrial graphite. Both men failed to realize that even the best industrial graphite contains too many impurities to function well as a moderator. In particular, it contains boron, which absorbs, or mops up, neutrons. Had they experimented with completely pure graphite, they would have discovered, as had Enrico Fermi in his experiments at Columbia University, that it was an excellent moderator. Thanks, however, to Leo Szilard’s persistence, Fermi’s results had not been published, so Bothe remained unaware of his mistake.

In the spring of 1940 at Hamburg University, Paul Harteck came close to devising a carbon-based moderator. He conceived the brilliant notion of using carbon dioxide and persuaded industrial giant IG Farben to loan him a chunk of frozen carbon dioxide—dry ice. However, the dry ice, whose excellent credentials as a moderator would have been revealed in experiments, arrived before Harteck could obtain sufficient uranium. Consequently, the limited tests he was able to perform were inconclusive. [24] Ironically, Heisenberg refused to lend him any of his own uranium stockpile.

The net result, as Heisenberg wrote, was that German scientists “abandoned the whole idea” of carbon “prematurely” and turned, instead, to heavy water. Had they pursued carbon, the first self-sustaining chain reaction using a carbon-based moderator might have been achieved not in the United States but in Nazi Germany.

The German scientists’ immediate problem in the early stages of the war was how to obtain sufficient stocks of heavy water. In April 1940, after invading Norway, the Germans had seized the Norsk-Hydro plant at Vemork, where they quickly increased production from 5.28 gallons a year to one ton. However, the amount the Germans estimated they needed for one reactor per year was closer to four or five tons. Paul Harteck designed a catalytic exchange process to increase the plant’s production to those levels, but it would still take time for significant quantities of heavy water to be produced and shipped.