Free Hitler's Terror Weapons by Geoffrey Brooks

‘moderator’, which slowed down the neutrons in the reaction without absorbing them. The deceleration increased the chances of a neutron finding a U 235 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 (D 2 O, 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 U 238 isotopes. This was due to the nuclear Doppler Effect. The widening of these U 238 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 U 238 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 obvious 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