The First War of Physics (15 page)

After the war, Heisenberg claimed he was trying – through Bohr – to establish a commitment from nuclear scientists not to develop atomic weapons. Whether this was really his intention or not, Bohr interpreted his efforts as those of a brashly confident representative of an aggressive occupying power bent on delivering the ultimate weapon to his masters. In another letter to Heisenberg that was never sent, Bohr wrote:

It had to make a very strong impression on me that at the very outset you stated that you felt certain that the war, if it lasted sufficiently long, would be decided with atomic weapons … You added, when I perhaps looked doubtful, that I had to understand that in recent years you had occupied yourself almost exclusively with this question and did not doubt that it could be done.

Heisenberg later recalled Bohr’s observation that it would be hopeless to try to influence the activities of physicists now working in various countries, ‘and that it was, so to speak, the natural course in this world that the physicists were working in their countries on the production of weapons’.

Despite this exchange, they appeared to have parted amicably. Heisenberg met with Weizsäcker on the Langelinie, the picturesque walk
near Copenhagen harbour. Heisenberg admitted: ‘You know, I’m afraid it went badly wrong.’

Heisenberg and Weizsäcker participated in the conference on 19 September. It was attended by just five local astronomers from the observatory in Copenhagen. In his obligatory report on the visit, Heisenberg commented that ‘our relations with scientific circles in Scandinavia have become very difficult’.

After attending a lunchtime reception at the German embassy, Heisenberg joined Weizsäcker for a last visit to the Bohr household. This final visit was free of political or scientific discussions. Bohr read aloud and Heisenberg played a Mozart sonata.

Faustian bargain

If Heisenberg had really been trying to stop the development of an atomic super-weapon, he had failed. If he had been seeking absolution, he had failed. But the manner of his failure was to have profound implications. The wartime efforts of nuclear scientists in Britain and America were driven by a deep-rooted fear of what the Nazis might do with such a superweapon. Against this background of fear, the interpretation of the
intent
of the German physicists, especially of Heisenberg, and the determination of the Nazi military authorities provided a critical moral justification for the work that Allied physicists were now doing.

In the end, it did not matter much precisely what Heisenberg had really intended to say to Bohr. As a result of the meeting, one of the most respected and revered of nuclear physicists, a Danish half-Jew living under Nazi occupation, had been left with the firm impression that Heisenberg was working earnestly to deliver an atomic bomb to Hitler’s arsenal.

Heisenberg had been unable to see all possible ends. His dangerous Faustian bargain was beginning to have unexpected consequences.

¹
See Mark Walker,
German National Socialism and the Quest for Nuclear Power, 1939–1949
, p. 26.

²
Charlotte was no relation to Leni Riefenstahl, the German film-maker and propagandist.

³
Narodnyi Komissariat Vnutrennikh Del (NKVD), or the People’s Commissariat for Internal Affairs, was the main directorate for state security in the Soviet Union. In addition to managing all of the USSR’s prisons (including the Gulag – the network of forced labour camps), the NKVD was also responsible for administering the Soviet Union’s foreign intelligence service and covert operations overseas. It was renamed the Ministerstvo Vnutrennikh Del (MVD) in 1946 and, after the arrest and execution of Lavrenty Beria, became the Komitet Gosudarstvennoy Bezopasnosti (KGB) in 1954.

⁴
He was just in time. All Jews were banned from emigrating from Germany in May 1941.

⁵
Also known as Military Intelligence – Section 6, or MI6.

⁶
This should be read in its proper context. In drafting this, and subsequent letters and notes to Heisenberg, he was reacting – many years after the event – to Heisenberg’s version of events that had been incorporated in the 1957 Danish translation of Robert Jungk’s popular book,
Brighter Than a Thousand Suns.

Chapter 5

TUBE ALLOYS

March–December 1941

T
he work of the MAUD Committee had proceeded apace through the last few months of 1940. Much more was now understood about the critical mass of a U-235 bomb and the challenges of separating the less abundant isotope from natural uranium on an industrial scale. And yet, the entire project still rested on little more than an intelligent guess.

All the physicists’ calculations depended on an assumed rate of fission of U-235 nuclei by fast neutrons. It had not proved possible to separate even a small amount of U-235 on which to make some direct measurements. Chadwick’s team in Liverpool used the cyclotron to measure how the rate of fission of natural uranium varied with the energy – or speed – of the neutrons fired at it. These rates depend on the combined fission rates for both U-235 and U-238, and their variation with neutron energy gives clues to the underlying behaviour of the individual isotopes. The experimental results closely fit theoretical predictions. ‘The first test of theory had given a completely positive answer and there is no doubt that the whole scheme is feasible’, Peierls wrote in March 1941.

The results confirmed independent measurements available from physicists working at the Carnegie Institution in Washington. The possibility
of spontaneous fission in U-235, which Frisch had discovered nearly a year earlier, was potentially worrying as it was thought that it might lead to premature release of neutrons and hence premature detonation of any putative bomb. However, the rate of spontaneous fission was found to be insufficient to threaten the bomb’s practical feasibility.

By April 1941 Simon’s team in Oxford had experimented with a halfscale model of a single stage of the proposed gaseous diffusion plant, and a full-scale model was under construction. The results were sufficiently encouraging for Simon to propose building a twenty-stage pilot plant. Towards the end of May, Metropolitan-Vickers was awarded a contract for the design of this plant, to be constructed by the end of the year at the Valley Works site in Rhydymwyn, near Mold in North Wales.
¹
ICI was contracted to supply quantities of uranium hexafluoride and to provide chemical engineering support for the plant’s construction and operation.

It seemed that a method was now available for the large-scale separation of U-235 and it seemed certain that a bomb could be fashioned from a relatively small, super-critical mass of fissionable material. Attention turned to further questions about how best to bring such a mass together and what kind of explosion would result.

The most obvious way of creating an explosive super-critical mass of U-235 is to bring two sub-critical pieces together. This, it was reasoned, would need to be done very quickly. Assembled too slowly, the mass would release neutrons and detonate prematurely – it would simply blow itself apart – yielding much less than the explosive force potentially available. The proposed solution was to shoot a small sub-critical mass of the active material into another sub-critical mass, in a process that was later to become known as the ‘gun method’. Thomson was assured by British armaments experts that constructing such a gun would be entirely practicable.

What kind of damage would such a bomb do? The calculations began to crystallise. The physicists estimated that a U-235 bomb consisting of just 25 pounds of active material would explode with a force equivalent to 1,800 tons of TNT. There was just one precedent for such an explosion. During the First World War the French munitions ship SS
Mont Blanc
had entered the harbour of Halifax, Nova Scotia. On deck were about 2,300 tons of wet and dry picric acid, 200 tons of TNT, ten tons of gun cotton and many drums of high octane fuel. On 6 December 1917 the
Mont Blanc
collided with a Norwegian ship, the SS
IMO.
The fuel spilled on the deck of the
Mont Blanc
and was soon ignited.

The resulting blast completely destroyed the ship and its surroundings over an area of nearly a square mile. Structural damage extended out a further half a mile. A mushroom-shaped cloud rose several miles into the sky as debris was thrown a distance of up to four miles and windows were shattered up to ten miles from the blast. The ship’s gun landed over a mile away, near Alboro Lake. About 1,600 people were killed immediately, the death toll rising to 2,000 from secondary effects of the blast.

The physicists had little doubt that the effort required to build bombs capable of delivering such devastating effects could be justified. No nation would want to be caught without such a decisive weapon in its arsenal.

In Cambridge, Halban and Kowarski continued their research on a uranium–heavy water reactor. They confirmed that a nuclear reactor was no longer just a possibility, it was a near certainty. Although this was work not directly related to the design or production of a bomb, the MAUD physicists were by now aware of the potential for production of element 94. Perhaps surprisingly, they tended to play down its significance. Some argued that element 94 would be unsuitable for a bomb. Besides, it was clear that this material could be produced only in a working nuclear reactor, and a working nuclear reactor would need large quantities of heavy water (the physicists believed they would need several tons of both uranium oxide and heavy water to make a working reactor). There was no heavy water plant in Britain, and sourcing this substance in such quantities from the Vemork plant in occupied Norway was clearly out of the question. Separation of the small amount of U-235 needed for a uranium
bomb seemed a much more practical and immediate route to an atomic weapon.

Work on various reactor designs continued, but the MAUD Committee judged that this was work that promised peacetime, rather than wartime, dividends. Halban and Kowarski were studying different reactor configurations, possible cooling systems and control systems. This work was obviously too important for it to be shelved for the duration of the war. And yet, when set against the wartime imperatives that the MAUD Committee physicists now faced in Britain, it was equally clear that the materials and resources required to support it properly could not be afforded.

Discussions about the future of Halban and Kowarski’s work grew into a wider debate about what the MAUD Committee could hope to achieve in a country at war and under direct attack. It became clear that the next step for the committee physicists was to draft a compelling report and seek support for its conclusions from the British government.

Penny-in-the-slot

Peierls had continued with his MAUD Committee work through the winter of 1940–41, but he had greatly missed his associate Frisch. With pressing problems to address concerning the physics of gaseous diffusion, he decided that he needed a new assistant. He recruited a quiet, somewhat reserved and unassuming émigré who had been working at Edinburgh University. This was a physicist with some considerable skill in mathematics, just the kind of talented theorist that Peierls needed. He joined the group in Birmingham in May 1941 and took the room in the Peierls’ home in Edgbaston that Frisch had vacated nearly a year before.

His name was Klaus Fuchs.

Fuchs had arrived in Britain on 24 September 1933, part of the first wave of émigrés looking to escape Nazi Germany. However, in Fuchs’ case, this was not emigration forced by anti-Semitism and new Nazi regulations. Fuchs was a Roman Catholic. He was also a Communist. As a young boy he had adopted the socialist leanings of his father Emil, a clergyman in the Lutheran Church, but in 1932 had come to reject the Socialist Democratic
Party in favour of the harder line sponsored by the German Communist Party. He had sensed that the disciplines imposed by party membership and party activism were the only meaningful response to the Nazi threat.