During early April 1939, Dr. Paul Harteck, a German physical chemist at the University of Hamburg, wrote one of the most important letters of that year . Addressed to the Ministry of War, it was to result in a nuclear programme to develop the military applications of nuclear fission being formally established by the German army on 1 September, the day World War 2 began. This was a month after Einstein’s letter reached President Roosevelt, and almost a year before Frisch and Peierls realised that atom bombs could become practical weapons.
The first part of this article discusses the history of the discovery of nuclear fission and states that German engineers in charge of nuclear development prevented Nazi Germany from building an atom bomb because they believed that nuclear power should be used for the benefit of mankind.
“24 April, 1939. To the Reich Ministry of War. Berlin.
“We permit ourselves to direct your attention to the newest development in the field of nuclear physics, for in our estimation it holds perhaps a possibility for the creation of explosives whose effect would excel many times those presently in use” .
Opening sentence of the letter to the German Ministry of War by Dr. Paul Harteck.
The two men who signed the letter, Harteck and his assistant, Dr. Wilhelm Groth, sent it to General Erich Schumann. It alerted Schumann, head of the weapons research branch of German Army Ordnance (Heereswaffenamt) for whom Harteck worked as a consultant on chemical explosives, to the military potential of nuclear power [2,14]. In fact, the German establishment had been aware of the military applications of nuclear fission even earlier. A more informal Uranium Club had been formed on 29 April by the German Ministry of Education, but was discontinued in August 1939, after the Army Ordnance Office took over .
The Uranium club
“Please join the Uranverein [uranium club], not to help us, but to help yourself, because you will protect your institute by doing so. You will be doing something which is officially judged to be important for the war effort, and therefore your institute will continue” .
A key member of the German nuclear programme, Carl Friederich von Weizsäcker, explaining the importance of the uranium club to Otto Hahn.
Like most other nuclear programmes, the German one was based on fear. But this fear was a far more rational one. The German scientists and physicists working on the different aspects of uranium technology were not stupid, and saw they would be conscripted to fight in a looming war, a war many of them privately doubted that Germany would win. In fact, Werner Heisenberg, who later headed the nuclear programme, had already been called up (and later demobilised) in 1938 .
But, if the War Office could be convinced that nuclear fission had military potential, then these specialists would be classified as important to the war effort, and serve the state in the relative comfort of their university departments, while living at home. The War Office was also the only institution that would have had the money to provide funding for research .
Thus it was that Harteck, who had good relations with the German military and worked as a consultant on chemical explosives for German Army Ordnance, was asked to draw up the fateful letter of 24 April 1939. On 1 September, when many other German men had to report for military service, the key technical specialists were relieved to find they had been drafted to serve in what was called the “Uranium Club”: an organisation that covered the research on the different aspects of the nuclear programme.
There was no hesitation to join this programme and work in it, as the scientists concerned realised clearly how lucky they were.
What makes fascinating reading is how key members of the programme (Werner Heisenberg, Otto Hahn, Carl Friederich von Weizsäcker and Paul Harteck) quickly became involved in a delicate balancing act: selling the military relevance of the work, and thus the need for continued funding and support, while avoiding the massive programme they knew was the only way to achieve real results: and one that would have been regarded as treason.
Selling the uranium club’s work
“You see you had to be quite a politician with the [German military and leaders, and the] people. You couldn’t tell them they were losing the war or any such thing” (Harteck, being interviewed by Prof JJ Ermenc in New York in 1967) .
By December 1941, German forces had become bogged down in the war with Russia. Gone were the quick victories of “blitzkrieg”, which had enabled most of Europe to be conquered, and German authorities realised that they would now have to plan for a long war. All projects supported by the Army, including the nuclear programme, were now in danger of being shut down, unless they could prove their military relevance and attract support .
For this reason, General Schumann arranged for a high-level conference on the nuclear programme in February 1942. In parallel with this, a short, more popular conference was arranged on 26 February 1942.
This popular conference is particularly interesting because it showed that the Germans knew full well (in principle) that nuclear weapons could be made using uranium. Even a quick reading of the paper given by Werner Heisenberg (“The theoretical foundations for obtaining energy from fission of uranium”), who was basically coordinating the programme, shows that he and the others understood clearly that a runaway chain reaction could be used to produce a nuclear explosion, while a controlled chain reaction could be used to produce what he called a “uranium machine” – a nuclear reactor – that could generate power [1, 5].
Even then, in 1942, the Germans realised the potential of this “uranium machine” to power submarines. But, fortunately (as will see below) they did not realise the practical details of how to make a nuclear reactor, enrich uranium, make plutonium, and actually manufacture atom bombs.
Luckily for the scientists concerned, the popular lecture series was successful: the German Ministry of Education decided to take over the project, and provide financial support. The popular lecture series was almost too successful, as Hitler’s minister of propaganda, Joseph Goebbels, now thought that a nuclear weapon could be built quickly enough in Germany to win the war [1, 5].
To explain the actual facts, a smaller high-level conference was arranged some three months later on 4 June 1942. This time, Albert Speer (Hitler’s minister of Armaments and Munitions) and several top military people listened to the presentation given by Heisenberg and the other scientists . Heisenberg gave a presentation and explained that nuclear weapons would only be possible if maximum support were given, and would take several years, two years at the very earliest .
The end result was that, while Speer realised that a nuclear bomb was not a realistic option, he felt that nuclear energy (and the development of the “uranium machine”) was a technology that held great promise, and arranged for continued, if modest, support for the nuclear programme for the rest of the war . Speer later informed Adolf Hitler briefly about this meeting on 23 June 1942 during a routine meeting [2; 192]. Put another way, although the Germans never learned about the Manhattan Project, and thought they were far ahead of the Americans, Hitler did know about the military potential of nuclear fission.
The research stayed at a modest level at various universities, with “only about 70 scientists working on the project, with about 40 devoting more than half their time to nuclear fission research” [9, 13]. This was roughly comparable to the nuclear research done in Britain, before the United States took this over and applied massive resources and tens of thousands of people in the Manhattan Project.
It also explains why the German nuclear programme failed to make practical progress, as far bigger support was then given instead to two top-secret programmes: building V-1 flying bombs and V-2 missiles [2, 10], and making nerve gases and weapons to deliver them . As Harteck put it, after this conference, it would have been considered treason to motivate taking the large scale resources needed to make an atom bomb away from other projects with more direct military applications.
Problems of organisation
“On 7 April 1933 the German [Nazi] government announced the infamous … legal framework for the purge of the German government of all racial and political enemies or opponents of Hitler’s regime”.
One of the problems faced by the nuclear programme was its organisation. Because of Hitler’s anti-Semitic policies, all Jewish scientists that could do so, had left Germany. This meant that many of the best and brightest nuclear physicists were not available to the nuclear programme, because they had left the country.
And, because all civil servants who were not regarded as pure enough Aryans were forced out of the government (including university professors and lecturers of Jewish extraction) , even those who remained in Germany were not allowed to join the programme.
Harteck mentions the case of Gustav Hertz, who won the Nobel Prize for physics in 1925, and was seriously wounded while fighting for Germany in World War 1. But, because his paternal grandfather had been Jewish as a child (before the entire family converted to Lutheranism in 1834), Hertz was classified as “second degree part Jew” , and had to leave his academic position and work in research in industry, which he did at Siemens.
Because of this, despite Harteck’s request to include Hertz, he was not used in the nuclear programme. As Harteck put it, had Hertz been included in the nuclear programme, he might well have been able to develop a workable gaseous diffusion process: one of the most important ways of enriching uranium-235. In other words, leaving Gustav Hertz out probably cost the German programme any chance it might have had to devise a practical way to enrich uranium . As things turned out, Hertz’s expertise helped the Russians to produce enriched uranium after World War 2.
Another problem was that Werner Heisenberg, a brilliant theoretical physicist, with Carl Friederich von Weizsäcker (a former colleague) as deputy, were effectively placed in charge of the nuclear programme in June 1942 . Neither had run experiments before, and this resulted in practical problems when it came to developing the “uranium machines”.
The uranium machines
“Hence, concluded Bothe… pure graphite was out of the question as a moderator of neutrons in a chain-reacting pile”.
One of the key mistakes of the German nuclear programme was made in early 1941, when Professor Walter Bothe concluded that graphite could not be used to moderate (i.e. slow down) neutrons in a nuclear reactor. Bothe’s error was that he failed to realise that the graphite he was working with was not pure enough .
This result had huge implications. A year before the Allied programme decided to use graphite for its nuclear reactors, the German programme rejected graphite and decided to use heavy water as a moderator instead [1, 2]. This meant that Germany was dependent on the only practical source of heavy water then available: the plant at Vemork in Norway  (see Fig. 2).
The resulting campaign by the Allies to first sabotage and then bomb this plant is clearly set out in a very readable book “Heavy water and the wartime race for nuclear energy” , and will not be discussed here. The main point is that German attempts to build a working nuclear reactor were soon affected because not enough heavy water could be imported from Norway.
Another point was the reactor design. Because of his lack of experimental training, Heisenberg’s initial designs for the “uranium machine” used shells of uranium oxide (and later uranium metal), separated by layers of heavy water. This approach was used because it was easier for certain calculations to be done .
Only after about a year was a better design used, in which blocks of uranium were suspended in the heavy water. Had Heisenberg listened to Diebner and Harteck, and tried out their designs a year earlier, better progress might well have been achieved . As Harteck mentions, if he had access to more uranium and had more time, he might well have been able to produce a sustainable nuclear reactor before 1942 [2, 15].
Despite this, the Germans programme actually produced a nuclear reactor that produced more neutrons than were consumed about six months before Enrico Fermi’s CP-1 attained criticality on 2 December 1942. However, this triumph was lessened when the pile, L-IV, caught fire and exploded .
A “nuclear explosion” in 1942
“Upon removing one of the covers, there was a dull thud, and a jet of flame shot out of the machine” .
A few weeks after the special conference to Albert Speer and other decision makers had secured the survival of the nuclear programme, a major setback occurred. The pile L-IV, composed of about 750 kg of uranium metal powder, formed into two spherical shells, with 164 kg of heavy water in between. The whole contraption was covered by aluminium to form a sphere, and placed in a pit filled with ordinary water.
This meant that, for several weeks, the uranium powder had been exposed to water, albeit heavy water. Ordinary water also leaked through the aluminium covering into the outer sphere of uranium powder. As Heisenberg and Döpel, who had built the pile, soon discovered, uranium metal powder can react even with cold water, to form hydrogen gas and uranium oxide: and the reaction produces energy (in chemical terms, is highly exothermic).
U + 2 H2O → UO2 + 2 H2 + energy (1)
In other words, once the first hydrogen has been produced, the reaction can proceed at an ever quicker rate, and even run away: especially if the hydrogen starts to burn with the oxygen in the air. The uranium can then appear to “burn” in the water.
While uranium cast metal is less reactive, Heisenberg was a theoretical physicist, not a chemist, and did not realise that the finely divided uranium powder that was used would react more quickly with the water.
However, he and Döpel (also a physicist) soon learned the hard way. Döpel and his one technician saw bubbles emerging from the sphere, which was kept in a “pit” filled with ordinary water. After hoisting the sphere out of the pit, and then removing one of the aluminium covers, “there was a dull thud, and a jet of flame shot out of the machine” as the hydrogen caught fire .
Water (not the best choice) was then used to put out the fire, and the flames appeared to be dying. After pumping out the heavy water, the sphere was lowered back into the water in the pit (nobody had yet realised that water had also leaked into the sphere and reacted with the powdered uranium ).
Heisenberg looked in, on his way to give a lecture, and left. However, soon the reaction continued. At first bubbles (of hydrogen) again came from the sphere, and then steam. Heisenberg was called away from his lecture. As they watched inside the laboratory, the sphere began to bulge. Something was clearly very wrong.
Both men raced for the door, getting out the laboratory just in time to avoid the shower of burning uranium powder that sprayed the entire room as the pile burst. Although the fire brigade soon managed to put out the fire, the uranium powder inside kept burning for a day and a half, completely wrecking L-IV.
The German scientists did have a sense of humour. After the explosion, Heisenberg’s colleagues congratulated him on making a successful uranium bomb ! This accidental chemical explosion was closest the German scientists ever came to making an explosive device with uranium.
The British soon learned of this accident and some other details of the nuclear programme: through the courage of a spy who worked for them in Berlin throughout the war. The activities of this man, code named “Griffin”, now seem stranger than fiction.
The next, and final, part of this article will cover the work of a spy who leaked information of Germany nuclear programme to the allies and the developments that ultimately led up to the development of a nuclear-powered submarine.
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