Hitler's Terror Weapons

by Brooks, Geoffrey

  In the preamble to the report, Heisenberg cited as his principal source of reactor theory an article published in the 9 June 1939 edition of the scientific periodical Die Naturwissenschaften 42 under the title Can Technical Use Be Made of the Energy Content of Nuclei? written by the physicist Dr Siegfried Flügge of the KWI for Physics at Berlin-Dahlem. Under a sub-title The Control of Chain Reactions Flügge had stated:

  “The decisive question for the technical application of the mechanism is manifestly this: is it possible to slow the chain cascade? Adler and Halban (Nature, Vol 143, 1939, p.739) have entered the debate and suggested the addition of cadmium salts to the mixture beforehand. In the absence of cadmium, the reaction would soar straightaway to a stationary temperature of 100,000 degrees C.”

  In the mentioned article Adler and Halban had warned:

  “The danger that a system containing uranium in high concentration might explode once the chain starts is considerable.”

  The idea of the instantaneous explosive chain reaction in a reactor is grounded in an error of theory caused by failing to take into the mathematical reckoning the small fraction of relatively long-lived neutrons which are emitted up to a minute after fission. What should have been done in the mathematical theory was to average the slowing down and diffusion time of the lifetime of the prompt neutrons liberated within a micro-second of fission added to the mean lifetime of the 0.75% of neutrons which emerge up to 80 seconds after fission occurs. That calculation would have shown that while neutron density increases exponentially with time, the stable Period of the Reactor is not “less than 1 second” but is about 54 seconds.

  The delayed neutrons play the decisive role in the safe control of modern atomic energy plants and without them nuclear power reactors would not be feasible. Heisenberg may have been genuinely under a misapprehension. On the other hand, he may have realized that this would be a useful error to have in hand if he wanted to obstruct the development of a nuclear reactor.

  In later reports he was never challenged when he relied on the argument as a reason for proceeding at slow ahead with the interminable low-level experiments which filled the next five years. Knowingly or not, Professor Heisenberg accepted that the Period of the Reactor was less than one second in length, after which the reactor blew up. After the war, in his reproduction report respecting the Haigerloch B8 experiment published in FIAT Review of German Science 1939–1945 Heisenberg acknowledged that his theory had been at fault, admitting:

  “American work shows that the Period of the Reactor is substantially extended by the delayed emergence of a number of those neutrons liberated during the fission process.”

  And in a report about the German project prepared by A. Weinberg and L. Nordheim for A. H. Compton on 8 November 1945 the authors were of the opinion that the importance of delayed neutrons for the stability of a nuclear reactor had probably not been considered. Even if Heisenberg knew all along, however, he could hardly say so in 1947. So, throughout the Second World War, Heisenberg believed, or let it be thought that he believed, that the Uranbrenner – the atomic pile for power – was impossible because the reactor would explode one sixth of a second after it went critical. He did not explain this fact in writing when setting down the theory originally, although one would think he must have informed his superiors at the Heereswaffenamt of his fears confidentially. To make some sense out of the fact that Heisenberg and the Uranium Project spent the war years performing interesting experiments of subreactor geometry, and obviously had no intention of actually bringing an experiment beyond the critical point since there was sufficient heavy water available in aggregate to moderate a working reactor by 1944 but no enthusiasm for doing so, Heisenberg must have convinced Hitler of the impossibility of building a working pile. Hitler did not want a nuclear reactor in any case because it was Jewish Physics. Probably he just waved a hand in dismissal, allowing Heisenberg and the reactor project to appear to be doing something useful to keep enemy Intelligence on the hop. That really is the only logical conclusion to be drawn from the manner in which the project was conducted.

  The Basis of Reactor Design

  The surest method of realizing energy production from the fissioning of uranium lay in enriching the U235 isotope, Heisenberg explained: the more the enrichment the smaller the reactor would be. If the proportion of theU235 isotope in the uranium material were to be enriched by 50%, from 0.7% to 1%, success was practically certain. However, such a proceeding was prohibitively expensive.

  Natural uranium could be used in the reactor vessel in conjunction with another substance, a ‘moderator’, which slowed down the neutrons in the reaction without absorbing them. The deceleration increased the chances of a neutron finding a U235 isotope to fission. Ordinary water and paraffin were not suitable as a moderator, since, being rich in hydrogen atoms, they absorbed neutrons. On the other hand heavy water and very pure carbon satisfied the requirements. Slight impurities in them could spoil the reaction, however.

  Heavy water (D2O, deuterium oxide) is four times more efficient at slowing neutrons than the purest graphite and thus a much smaller reactor is required. Surrounding the reactor vessel would be a ‘reflector’, a wall of material enclosing the core of a nuclear pile against which escaping neutrons are scattered back into the reaction. Heisenberg indicated that graphite blocks would be suitable for this.

  He then described a number of possible reactor arrangements. The most important was a configuration three cubic metres in size consisting of 30 tons of pure carbon in the form of graphite and 25 tons of uranium oxide which, according to his calculations, would reach the critical point and supply energy. In the supplementary paper to G-39 of 29 February 1940 Heisenberg confessed to some misgivings regarding his design for a graphite reactor and this may have been prompted by Professor Harteck’s interest in it.

  Professor Paul Harteck (1902–1985) had graduated in chemistry at the University of Vienna and at the age of 26 had obtained his PhD at the University of Berlin. He is credited with the discovery of parahydrogen.

  In 1933 he studied nuclear physics at the Cavendish Laboratory and during this period was set the task of producing a quantity of heavy water, which he achieved by spending several weeks passing an electric current through a small electrolytic cell. The amount was minute in comparison with all the gallons of water used in the process. Later he would have charge of Germany’s heavy water production process. Following his return to Germany in 1934, Professor Harteck was appointed Director of the Institute of Physical Chemistry at Hamburg. He was a Nazi Party member and his team of five co-workers were known as “the Hamburg Bomb Group”.

  Heisenberg had remarked that the uranium machine would shut down automatically at certain peaks of temperature, and then only resume when the temperature had fallen again. This would occur because of the expansion of metals on heating, resulting in a lowering of density and an alteration of the various cross-sections. This same increase in temperature would cause an increase in the width of the capture bands formed of U238 isotopes. This was due to the nuclear Doppler Effect. The widening of these U238 capture bands caused many more neutrons to be absorbed, resulting in a lessening of fissions until the chain reaction collapsed altogether.

  In earlier conversations with Heisenberg, Professor Harteck had suggested that uranium and moderator should be segregated into a heterogeneous design more favourable for the production of an efficient reactor. When he read the mention in Heisenberg’s two pioneering papers of the problems of heat, Harteck realized that there was a better way of building a nuclear reactor altogether. If a pure carbon moderator was used at extremely low temperatures, the nuclear Doppler Effect would ensure that the width of the U238 capture bands would shrink and the reactor would produce no heat. If it did heat up, the chain reaction would collapse. Thus all the troublesome engineering arrangements inherent in an energy-producing reactor, such as heat transfer, core and fuel cooling and temperature control, would be obviated. We may infer from his ob
vious disinterest that Professor Heisenberg was not honestly in favour of building a working nuclear reactor at all, for this simple experimental zero-energy design would have been a good way to look at the problem of reactor stability. But he knew the terrible danger it presented. In his initial report he had observed:

  “An extraordinarily intensive neutron and gamma radiation goes hand in hand with energy producion. Even in achieving only 10kW power, 1015 neutrons and gamma rays are created every second. The radiation is, therefore, 100,000 times greater than that produced in a large cyclotron. Even if a substantial amount of this radiation is absorbed in the core of the pile, nevertheless the working reactor would obviously require the provision of the most comprehensive biological shielding against radiation. This applies especially at the ‘switching-on’ of the machine, i.e. at criticality. At the moment when the temperature reaches the stationary value of 100°C, 108 calories are used to produce heat leaving an excess of 5 to 1019 neutrons and gamma rays liberated.”

  The very low temperature uranium pile would produce nothing but radioisotopes and the intensely radioactive decay products of nuclear fission. Radioisotopes do have modern applications in medicine, biochemistry, biology and industry, but Professor Harteck saw another use for them.

  After the war Harteck admitted43 that his idea in proposing to build a sub-zero uranium pile was to obtain nuclear waste for use against the populations of enemy cities. This seems to have been the first time that radiological material was being seriously suggested for military purposes. Such weapons are not outlawed by international treaties, since they are not classified as chemical. The evidence suggests that Hitler was prepared to entertain radiological warfare to stave off defeat but might not have resorted to it early on in the war unless he thought it would guarantee him victory.

  Professor Harteck set about building an experimental sub-critical pile immediately. His idea was simple. Dry ice sublimates slowly at a temperature of – 78°C and is as pure as one part in a million. Its oxygen atoms do not absorb neutrons in significant quantities at very low temperatures. Concluding that carbon dioxide ice was an ideal moderator for his proposed experiment, Harteck asked permission of the Heereswaffenamt to proceed and went ahead with it at once.

  He had useful contacts with the firm of I G Farben, and on 8 April 1940 he induced the firm’s research director, Dr Herold, to make him a gift of a 15-tonne block of dry ice to be delivered at the end of May. The War Office agreed to supply a railway wagon to expedite the consignment from Merseburg to Hamburg, and Harteck wrote to Diebner asking for 300 kilos of uranium. This figure was on the low side, but Harteck thought that it was all that was available.

  It must have been obvious that Harteck was expecting to perform his experiment within a week of receiving his 15-tonne block of dry ice. Diebner had only 150 kilos of uranium oxide at Berlin-Dahlem, but Heisenberg was waiting for a large delivery from the War Ministry and probably had a large hoard besides. Diebner promised Heisenberg that the large amount would arrive in June and requested him to settle privately with Harteck.

  Heisenberg suggested to Harteck in a letter that he was exaggerating the urgency of his experiment, since there were a number of preparations to be made first:

  “… of course if there is for any reason any urgency in your experiments, you can go first by all means. But I should like to suggest that for the time being you content yourself with just 100 kilograms.”

  Heisenberg concluded in a very reasonable vein that he was quite prepared to let Diebner make the final decision. Harteck replied by return, emphasizing the obvious urgency, and begged Heisenberg to loan him from 20 May, for three weeks at the most, as much of his Leipzig stock as he possibly could allow. In the expectation that Heisenberg would relent, Harteck asked Dr Herold to delay shipping the ice until the last possible moment and spoke to Diebner twice to emphasize his need for a minimum supply of 600 kilos of uranium oxide.

  At the end of May Diebner loaned him 50 kilos and Dr Riehl of the Auer Company brought him 135 kilos more. Heisenberg sent nothing. When the block of ice arrived at the beginning of June the experiment was doomed and the only useful information it yielded was criteria for the distribution of neutron density in certain arrangements of uranium oxide and dry ice44.

  Heisenberg’s non-cooperation prevented Harteck from obtaining a figure for neutron multiplication. This would have enabled Harteck to calculate the quantity of materials he needed for a working pile. Both Harteck44 and Wirtz45 made this point subsequently.

  A Windfall of Uranium Oxide: Harteck Tries Again

  There was no shortage of uranium oxide in German-occupied Europe. In May 1940 German forces arriving at Oolen in Belgium had discovered at the warehouses of the Union Minière Company over 1200 tons of uranium oxide and 1000 tons of other refined uranium metals. The British Government had known about this stock since early 1939 but had dropped a plan to purchase it outright and so remove it from proximity to Germany. The President of Union Minière, Edgar Sengier, appears to have made a purely business decision to leave the material for the Germans when they invaded so that his company would find favour with Hitler should he emerge victorious in the coming war in western Europe. Sengier then ordered the uranium mines at Katanga in the Belgian Congo flooded and had the mined ores shipped from Lobito to the United States. In October 1939 he transferred his offices to New York46.

  The Germans controlled the Joachimstal mines in Czechoslovakia and thus held virtually all the uranium in Europe. There was in fact so much uranium in their hands that Professor Harteck set about planning his ambitious second experiment, a heterogeneous design consisting of 20 tonnes of uranium oxide in a lattice of shafts embedded throughout a 30-tonne block of dry ice. As soon as he announced his intention, he ran up against the determined opposition of Heisenberg, who argued that the experiment was so big that all Harteck would learn from it was a great deal about 20 tonnes of dirty uranium oxide and 30 tonnes of dry ice. Why Harteck thought that was something worth knowing he could not imagine. He expected that it would not work, however, or at least not unless Harteck sent the uranium oxide to a factory for purification first.

  Harteck then came under growing pressure from other quarters, probably orchestrated by Heisenberg, and these argued that it was too extravagant for a first experiment to use 50 tonnes of materials to do the whole programme at once, while Heisenberg returned to the attack by remonstrating about the unprofessional approach to the experiment.

  Bitterly Harteck was forced to concede defeat, refusing to accept their opinion. He resented Heisenberg in particular, commenting that, to his knowledge, Heisenberg had never contributed a single basic idea leading to the solution of the problems of nuclear fission: he found it inexplicable that a theoretical physicist who had never been involved in a large experimental venture could be appointed as leader of a technological enterprise. It was worse than merely poor judgment. Harteck attributed Germany’s failure to produce a nuclear weapon to the antagonistic attitude existing between the theoretical physicists and the experimentalists: the former considered the latter as beneath them, “a few egoists pushed the others aside”.47

  What seems to have been Heisenberg’s real worry over Harteck’s proposed reactor was that since it operated at sub-zero temperatures it might be easier to stabilize it with control rods when it went critical: if this sort of primitive reactor worked, it would produce the nuclear waste which Harteck wanted to use in radiological weapons. Harteck felt sure that such a programme would have brought the war to a swift end in Hitler’s favour.

  Was Professor Harteck serious about radioactive bombing? One must profess astonishment and credit him for having come clean on the matter after the war. Few others were honest. Considering how the dry-ice low-temperature reactor would have placed Germany’s nuclear programme on an entirely different footing, he stated:

  “You must be thankful this didn’t occur. Not that an atomic bomb would have been made. But if you have a carbon dioxide reacto
r and you let it run for a certain time, the cubes or rods of uranium would have become highly radioactive. Much radioactive material could have been made which could have been thrown about. That would have been very bad47.”

  CHAPTER 4

  Plutonium, Paraffin and Moderators

  IN THE AMERICAN scientific periodical Physical Review of 1 September 1939 Niels Bohr of Copenhagen and John Wheeler of Princeton theorized that if during fission the U238 nucleus captured two successive neutrons then the new compound structure should be even more fissionable than U235, which of course suggested another kind of atom bomb.

  Professor von Weizsäcker obtained the June 1940 issue of the same journal, the last available internationally. It contained an article offering proof of the substance 239Np (neptunium) in research on the Berkeley cyclotron. This tended to validate the Bohr-Wheeler hypothesis. In his report to the Heereswaffenamt G59-Concerning the Possibility of Extracting Energy from U238 on 17 July 1940, Professor von Weizsäcker left open the question as to whether atomic decay proceeded beyond neptunium, but if it did it would probably be an explosive, he said. (In the event plutonium – element 94 – was confirmed as an explosive “with the same unimaginable effects as U235” and “much easier to obtain from uranium since it can be separated chemically” in Heisenberg’s paper Die theoretische Grundlagen für die Energiegewinnung aus der Uranspaltung presented to the Haus der Deutschen Forschung on 26 February 1942.) The Viennese experimenters Hernegger and Schintlmeister48 had reached virtually the same conclusion about the new transuranic substance at about the same time as von Weizsäcker, although they did not lodge their paper in Berlin until December 1940.