Diana Preston - Before the Fallout

In July 1940 Walther Bothe arrived at the College de France in occupied Paris, followed soon afterward by Kurt Diebner and Erich Schumann. The three men were keenly interested in the fate of the heavy water shipped out of Bordeaux on the Broompark a month previously by von Halban and Kowarski. A nervous Frederic Joliot-Curie, who had recently returned to Paris, leaving a frail Irene to continue her recuperation in the country, convinced them that the heavy water had been loaded onto another ship known to have been sunk by the Germans. He also persuaded them that a substantial quantity of uranium ore purchased by the French from the Belgians before the war had been taken south by the fleeing French government. He assured his visitors that its whereabouts were unknown, although, as he knew, it was, in fact, in Algeria, where it would remain throughout the war.

Bothe, Diebner, and Schumann also wanted to know about Joliot-Curie’s cyclotron. Though unfinished, it was one of only two in occupied Europe. The other was in Niels Bohr’s laboratory in Copenhagen. The visitors realized they could not reveal the military-related motives behind their interest in Joliot-Curie’s facilities, but they also knew they needed his cooperation. They therefore blandly proposed some joint nuclear studies and offered Joliot-Curie a compromise. They would leave him in virtual control of his laboratory and help him complete his cyclotron. In return, he had to agree to accept a German research team under the direction of Wolfgang Gentner.

Gentner had worked at Berkeley with Ernest Lawrence, and his motivation for returning to Germany had been, like that of Heisenberg, to protect German science rather than any enthusiasm for the regime. He had worked with Joliot-Curie in the mid-1930s and regarded him as a friend. At a private meeting he sought and received Joliot-Curie’s blessing to come to his laboratory. Despite their friendship, both their situations were fraught with ambiguity. Gentner might not always be able to protect Joliot-Curie, and the results of Joliot-Curie’s work would inevitably be known to the Germans.

Although in 1940 German scientists were short of heavy water, they had excellent sources of raw uranium. The Nazi occupation of the formerly Czechoslovak Sudetenland on the borders of Bohemia had delivered them the world’s richest uranium mines at Joachimsthal. From 1940 onward, slave workers mined the uranium ore for the Nazis. The Auer Company, which produced the radioactive toothpaste used by James Chadwick in his experiments during his internment in the First World War, organized the processing of the uranium into a usable form at its works at Oranienburg, near Berlin. Their laborers included two thousand female inmates from the Sachsenhausen concentration camp. In April 1940, when Heisenberg complained about the time it was taking to obtain processed uranium, Auer requisitioned more slave workers and stepped up production.

While awaiting sufficient quantities of suitable materials, German scientists addressed two main tasks: assessing techniques for separating U-235 from natural uranium and working out the optimum size and configuration for a reactor. Of nine research teams controlled by Kurt Diebner, two were detailed to work on reactor construction: the experimental physics section of Heisenberg’s Physics Institute at Leipzig University and the Kaiser Wilhelm Institute for Physics in Berlin. Heisenberg, who was also exploring the properties of heavy water, commuted between the two.

By the summer of 1940 a new laboratory for reactor experiments was under construction among a pleasant grove of cherry trees on the grounds of the Kaiser Wilhelm Institute for Biology and Virus Research in Berlin, located next to the Institute for Physics. To deter unwanted visitors, the wood-framed building was named the “Virus House.” Rumors spread that scientists there were conducting deadly experiments with bacteria. In fact, Heisenberg was directing some early reactor experiments using whatever was available—paraffin as a moderator and limited amounts of uranium. The results suggested that, with heavy water and enough uranium, a self-sustaining chain reaction might indeed be achievable.

German scientists were also exploring the potential applications of elements heavier than uranium—the “transuranics”—which had so fascinated and perplexed Lise Meitner, Otto Hahn, and others. Some had spotted the article by the Berkeley cyclotroneers Edwin McMillan and Philip Abelson published in the American journal the Physical Review in June 1940 reporting the discovery of the transuranic element 93—neptunium—created when U-238, the most common isotope in natural uranium, captures a neutron and transmutes into U-239, which, in turn decays into element 93. However, at the Kaiser Wil­helm Institute for Chemistry, others were already and independently on the trail. The young radiochemist Kurt Starke had stumbled on element 93, and Otto Hahn and Fritz Strassmann immediately began dissecting the new element’s chemical characteristics.

Strassmann had remained true to the principles that had first endeared him to Lise Meitner. He not only despised the Nazi regime but was prepared to risk his life and that of his wife, Maria, and baby son, Martin, to protect others. At the very time he was working on one of the most sensitive and secret projects of the German war effort, he was secretly sheltering the Jewish pianist Andrea Wolffenstein in his Berlin apartment. She later wrote that he and his wife helped her “in full knowledge of all the dangers” they were running, sharing their meager food with her and taking her to safety during air raids, and that Otto Hahn knew that the Strassmanns were hiding her. The risks to them all were heightened by the presence of a staunch and watchful Nazi living in the apartment directly beneath, so that when all the Strass­manns were known to be out, Andrea Wolffenstein had to be careful not to make a sound. After she managed to escape Berlin undetected, the Strass­manns also helped her sister Valerie. [25] Fritz Strassmann is commemorated at the Holocaust center in Jerusalem, Yad Vashem, as one of the “righteous gentiles” who came to the aid of persecuted Jews.

If Hahn’s and Strassmann’s preoccupation was, as Hahn claimed after the war, simply with the chemistry of element 93, von Weizsacker, at least, was working on a broader canvas, and he quickly grasped the element’s bomb-making potential. He deduced that it was highly fissionable and could be manufactured in a reactor and used to fuel an atom bomb. Because element 93 could be separated by conventional chemical rather than isotopic processes, it would be easily retrievable from other fission products. This would overcome the greatest technical obstacle in the path of a German atom bomb: developing isotopic separation techniques to squeeze enough of the rare isotope U-235 out of natural uranium to fuel an explosive device. On 17 July 1940 von Weizsacker wrote a five-page paper to the army authorities, which he also copied to Werner Heisenberg. In it, he suggested that element 93 could be as useful as U-235 in making Sprengstoff —“explosive.”

Like their counterparts in the United States and Britain, German scientists also began seeking element 93’s fissionable, longer-lived, more stable daughter, element 94—plutonium. In early 1941 the theoretical physicist Fritz Houtermans concluded that a reactor fueled by natural uranium could manufacture plutonium, which could be chemically extracted and used to make a bomb—a discovery that both excited and disturbed him.