The First War of Physics (6 page)

In his own mind Heisenberg had already dismissed the prospect of an atomic super-weapon as a remote one, but the German military authorities were nevertheless willing to engage the services of nuclear physicists and provide research funds and facilities to explore the possibilities. Here was an opportunity to contribute to the war effort and at the same time carry out fundamental research. ‘We must make use of physics for warfare’, was the official slogan of the Nazi government. Heisenberg thought to turn this on its head: ‘We must make use of warfare for physics’, he wrote years later of his reaction to the news.

Many scientists down the centuries have fallen prey to such impeccable, but arrogant, logic. When the ends are deemed to be improbably achievable or irrelevant, the means become the most important consideration. But these same scientists have tended to fail spectacularly to see all possible ends. So, Werner Heisenberg, Nobel laureate, discoverer of quantum mechanics and the uncertainty principle, and one of the most talented theoretical physicists of his time, accepted the challenge to work on atomic weapons for Hitler’s Nazi Germany. He accepted eagerly and without hesitation. A darker and potentially much more dangerous Faustian bargain had now been struck.

The Uranverein was to meet again in Berlin the very next day, 26 September. Heisenberg journeyed to Berlin that night.

Reactors and bombs

The second meeting of the Uranverein, whose very existence was now classified as a military secret, was held in the research offices of German Army Ordnance in Berlin. The research branch of Army Ordnance was headed
by Erich Schumann,
¹
who had wrested control of the Uranverein from Esau at the Reich Research Council, part of the Ministry of Education. Schumann appointed Diebner to direct the project, supported by Bagge. Diebner had studied physics in Innsbruck and in Halle before joining the German Bureau of Standards and the Army Weapons Bureau in 1934. Bagge had studied in Munich and Berlin and gained his doctorate under Heisenberg at Leipzig in 1938. Both were loyal Nazis.

Heisenberg now joined Diebner, Bagge, Harteck, Hahn and other Uranverein physicists, including Carl Friedrich von Weizsäcker, one of Heisenberg’s former students and a close friend. Weizsäcker had studied in Berlin and Copenhagen before gaining his doctorate under Heisenberg in Leipzig in 1933. He was a talented young theoretical physicist and philosopher, the son of Ernst von Weizsäcker, Secretary of State under Foreign Minister Joachim von Ribbentrop. Just 24 days prior to the Uranverein meeting, on the second day of the war, his younger brother Heinrich had been killed fighting with the Ninth Infantry Regiment near Danzig.

Diebner and Bagge had drafted an outline of the research programme a few days prior to the meeting, and had allocated tasks to each of the scientists involved. There was still considerable uncertainty regarding the physical principles of a fission chain reaction in uranium and there were few hard measurements available, but there was enough understanding to make a start.

Bohr and Wheeler had argued that U-235 is responsible for fission in uranium, and that fission can be triggered by bombardment of U-235 with slow neutrons. Fission in the much more abundant isotope U-238 requires much faster, higher-energy neutrons. However, there are certain characteristic neutron energies, called ‘resonant’ energies, at which a U-238 nucleus will capture a neutron to form the unstable isotope U-239 without undergoing fission. At these relatively high energies, neutrons would therefore be removed from any chain reaction, the U-238 nuclei acting as a ‘sink’, preventing the neutrons from going on to fission more U-235 nuclei.

Obtaining a self-sustaining chain reaction in a nuclear reactor based on naturally-occurring uranium was therefore a simple matter of population statistics. Secondary neutrons produced by fission of U-235 would be formed with a range of energies, or speeds. If, on average, one or more neutrons survived long enough to encounter other U-235 nuclei, then there was a chance that these would cause fission, sustaining the chain reaction. If, on the other hand, the secondary neutrons were captured by the more abundant U-238, leaving, on average, less than one neutron to find a U-235 nucleus, then the chain reaction would be unsustainable and would quickly fizzle out.

The solution was obvious. To give the secondary neutrons as much chance as possible of finding and fissioning more U-235 nuclei, it would be necessary to incorporate a
moderator
in the reactor design. This would be a material containing light atoms capable of slowing the neutrons down without absorbing them. By slowing the neutrons down to an energy below the threshold of the U-238 resonance, they would be prevented from being absorbed in their turn by the U-238 nuclei. Suitable candidates for a moderator included so-called ‘heavy’ water, in which the hydrogen atoms of ordinary water are replaced by heavier deuterium isotopes,
²
or pure carbon in some readily available form, such as graphite. Harteck had already done some preliminary work on a reactor design consisting of alternating layers of uranium and heavy water.

It was also fairly clear at this early stage that a compact reactor or a bomb could not be built without separating U-235 from U-238 or, at the very least, greatly enriching the proportion of U-235 present in a mixture. There were few options available and, as Bohr had observed months earlier to his colleagues in Princeton, the prospects for large-scale separation of U-235 were dim. A thermal diffusion method, based on a process devised in 1938 by German chemists Klaus Clusius and Gerhard Dickel, appeared to be the best bet. This process relies on the tiny differences in the diffusion properties of gaseous forms of the isotopes when exposed to a temperature differential. To get uranium into a gaseous form would require working with uranium hexafluoride, a highly unpleasant substance that corrodes just about anything it comes into contact with.

The Uranverein physicists were faced with two hurdles. They needed to make some basic measurements to assess the suitability of various materials for use as a moderator and so figure out the optimal configuration for a nuclear reactor. They also needed to work out how to separate U-235 on a large scale.

Bagge was assigned the task of determining the suitability of heavy water as a moderator. Harteck was asked to continue with some preliminary work on isotope separation using thermal diffusion methods and to examine the effect of different reactor configurations on the production of secondary neutrons. Heisenberg was asked to assess the feasibility of achieving a self-sustaining chain reaction in uranium based on the known physical properties of the materials likely to be required.

Schumann announced that the War Office had requisitioned the Kaiser Wilhelm Institute for Physics in Berlin to house the uranium project and those Uranverein physicists based in other cities were now asked to relocate. Almost all of them refused, preferring instead to remain where they were and if necessary travel to Berlin once or twice a week. Although they were all keen to make their contribution to the effort, from their perspective this was simply another research project to be added to their existing projects and teaching commitments. There was as yet no sense of the kind of urgency that would warrant a major disruption to their academic schedules.

Heavy water

Heisenberg immersed himself in the scientific literature and in December 1939 produced the first part of a detailed report to the German War Office entitled
The Possibility of Technical Energy Production from Uranium Fission.
This paper was to chart the future course of the German nuclear programme.

From the outset Heisenberg focused his attentions on the physics of a nuclear reactor, or ‘uranium burner’. He saw no need to differentiate between this physics and the physics of a uranium bomb, perceiving them as extreme ends of a continuous spectrum. At one end of this spectrum would be a reactor formed from naturally-occurring uranium and a suitable moderator. At the other end would be an explosive formed from uranium greatly enriched in U-235, to the point of being ‘almost pure’.

Heisenberg estimated that a reactor capable of achieving a self-sustaining chain reaction would require over a ton of uranium and around a ton of heavy water combined in a spherical configuration. Such a reactor should settle down to stable operation at a temperature of around 800° Celsius. Adopting the layer configuration advocated by Harteck could be expected to reduce the size of the reactor somewhat. Heisenberg concluded his report with the observation that enriching the proportion of U-235 would help to reduce the size of the reactor further, and that enrichment was ‘the only method of producing explosives several orders of magnitude more powerful than the strongest explosives yet known’. At this stage Heisenberg expressed no preference for either heavy water or graphite as a moderator.

The War Office issued a contract for the production and delivery of quantities of refined uranium oxide to the Berlin-based Auer company, which had access to uranium from the Joachimsthal mines in Czechoslovakia. The director of Auer’s Radiological Laboratory was Russian chemist Nikolaus Riehl, who had studied nuclear chemistry and physics under Hahn and Meitner. Riehl immediately established production facilities at Oranienburg, about twenty miles north of Berlin, and the first ton of uranium oxide was delivered in early 1940.

Sourcing suitable quantities of heavy water was more problematic. The only facility producing heavy water in commercial quantities was a fertiliser plant owned by the Norwegian company Norsk Hydro, which produced it as a by-product. The plant, which had begun production in 1934, was perched high in the fjords at Vernork near the town of Rjukan in the remote Telemark region of Norway, about 150 miles west of Oslo.

Graphite was very much the preferred candidate for the choice of moderator because of its ready availability in large quantities, in pure form. But initial data from German chemist Walther Bothe’s team in Heidelberg – supported by theoretical predictions from Weizsäcker and his group in Berlin – suggested that graphite might absorb neutrons too readily and so prove to be unsuitable.

In Heisenberg’s second report to the War Office, delivered in February 1940, he was already leaning towards use of heavy water as a moderator. This was a much less attractive option because of the problems of isolating enough of this substance to meet the needs of the research project. Diebner asked if it was necessary for Germany to construct its own production facility. Heisenberg suggested that they first acquire a few litres of heavy water with which to check its suitability, and Diebner promised to procure ten litres from the Norsk Hydro plant.

However, the Norwegians were not very co-operative. Norsk Hydro was approached by a representative of the German chemicals giant I.G. Farben, which owned stock in the Norwegian company, with an offer to buy up all the available heavy water. At this stage the Vemork plant was producing about ten litres a year, more than enough to meet the esoteric needs of the research laboratories which were its principal customers. When asked why so much heavy water was needed, the I.G. Farben representative would not say. The Norwegians gave their regrets: they would not comply with the German request.

When Norsk Hydro was subsequently approached by Jacques Allier the response was very different. Allier was a representative of the Banque de Paris et des Pays Bas, which had a controlling interest in the company. He was also a lieutenant in the Deuxième Bureau, the French military intelligence agency. Joliot-Curie in Paris had also identified heavy water as a potential moderator and had advised the French Ministry of Armaments of its importance in nuclear research.

Arriving in Oslo under an assumed name and carrying a credit note for FF36 million, Allier had intended to negotiate the purchase of all the available heavy water. But when it became clear what purpose the heavy water served, the Norsk Hydro managing director Axel Aubert pledged the entire
stock to the French government at no cost: ‘Say that our company will accept not one centime for the product you are taking, if it will aid France’s victory.’ The heavy water was removed from Vemork and smuggled first by air to Edinburgh, then by rail and ferry to Paris.

The fall of France

The situation changed dramatically on 9 April 1940, when German forces invaded Denmark and Norway in Operation Weserübung. The Danish government quickly capitulated under threat from the Luftwaffe, and signed a non-aggression pact to secure a measure of political independence. Niels Bohr, long aware of the impending catastrophe, had now become trapped in Copenhagen.

The German forces met more resistance in Norway. King Haakon VII, together with other members of the Norwegian royal family and key government ministers, were eventually able to escape to Britain with the nation’s gold reserves. They formed a government in exile, leaving the Nazi sympathiser Vidkun Quisling to declare himself premier in a coup d’etat, broadcast on radio.

Fighting had been fierce around Rjukan, and this was the last town to yield in southern Norway. German troops entered the town on 3 May. This time there would be no negotiations. The Germans now learned that all existing stocks of heavy water had been smuggled to France, but there appeared little to prevent production from being accelerated to meet the needs of the German nuclear project. An increase in output to 1.5 tons a year was promised.

On 10 May German forces invaded France and the Low Countries. German armoured divisions scythed through the Ardennes forest, cutting off Allied units that had taken up positions in Belgium, including the British Expeditionary Force, consisting of ten infantry divisions despatched to the Franco-Belgian border following the German invasion of Poland. The Luftwaffe quickly gained superiority in the air over Belgium and Holland. Following the carpet bombing of Rotterdam, the Dutch army surrendered on 14 May. The encircled British Expeditionary Force,
and many French soldiers, were evacuated from Dunkirk on 26 May, in a rout that was heralded as little short of a miracle. Belgium capitulated on 28 May.