Hitler's Terror Weapons (9 page)

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 reactor 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 bad
⁴⁷
.”

CHAPTER 4

Plutonium, Paraffin and Moderators

I
N 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 U
²³⁸
nucleus captured two successive neutrons then the new compound structure should be even more fissionable than U
²³⁵
, 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
²³⁹
Np (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 U
²³⁸
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 U
²³⁵
” 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 Schintlmeister
⁴⁸
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.

Many millions of words have been written by learned historians in an attempt to make some sense of the German uranium project during the Second World War. Thousands of documents have been inspected, but the story lacks coherency. The muddle over graphite is a good example. Many commentators have attempted to show that the German reactor project failed because of an error in materials measurement prejudicial to the use of graphite as a moderator. If the Germans had had graphite instead of heavy water, so their argument goes, then they would have had a critical reactor in 1941 and plutonium and a bomb, and so on. The evidence does not support this line of reasoning because in 1944 there was enough heavy water available for a working reactor but no attempt was made to reach the critical point. Moreover, to build and run such a reactor would have required a tremendous effort, and it would not have proceeded in Germany as rapidly as it did in the United States, where large manpower and enormous material resources free from constant aerial bombardment had still not produced a plutonium bomb which worked by the end of hostilities in Europe.

There is in any case rather more to the error in the graphite evaluation than meets the eye. Professor Walther Bothe (1891–1957) was an opponent of the Nazi regime and, after having been roughed up by the Nazis at Heidelberg in 1933, spent a long period in convalescence at a Badenweiler sanatorium from where he did not return to the University until 1937. Bothe had been frequently accused of scientific fraud by the Nazis before the war. There being no smoke without fire, he was probably quite prepared to repay them in his own currency if he got the chance. Professor Peter Jensen, his assistant, was a member of Heisenberg's intimate circle of three and was aware that Heisenberg wanted heavy water as the official moderator and not graphite.

In his first pioneering paper in December 1940 Heisenberg had said that graphite was suitable as a moderator and he was visualizing in his mind's eye a reactor of 25 tonnes of uranium oxide and 30 tonnes of graphite. He even had a
Most Favourable Design of Reactor
in which there would be layers of uranium oxide, heavy water and graphite. In his supplementary report of 29 February 1940 he had changed his mind and said that “the properties of graphite make it unsuitable as a neutron moderator”. Nobody else thought this at the time and he submitted nothing in writing to substantiate his statement about a matter which was obviously critical to the development of the reactor project.

How convenient it was then that when Bothe and Jensen reported their results on graphite in a bulletin under the title
G-71 The Absorption of Thermal Neutrons in Electro-Graphite
⁴⁹
dated 20 January 1941, their paper should state that experiments on the purest carbon available showed the rate of neutron capture to be so high that it was of no use as a moderator. Actually the measurements were incorrect. Professor Heisenberg explained in an interview postwar:

“Bothe's Heidelberg people got about a ton of graphite. An error slipped into his experiment. His values were too high but we assumed they were correct and so we did not think that graphite could be used. He had built a pile of graphite pieces but in between the graphite pieces there was always some air and the nitrogen of the air has high neutron absorption. Somehow he must have forgotten this. I don't know why but it is understandable.”
⁵⁰

So Professor Bothe, who had been involved in neutron research since it began in 1931, had forgotten that fresh air absorbs neutrons. Without Heisenberg's knowledge, Professors Joos and Hanle at the University of Göttingen submitted research papers to the
Heereswaffenamt
even though they were not affiliated to the official programme. Their treatises
G-46/G-85 Concerning the Existence of Boron and Cadmium in Graphite
⁵¹
dated 18 April 1941 contradicted Bothe's erroneous opinion. The
Heereswaffenamt
admitted the contradictory report and accepted that graphite was suitable as a moderator in papers G39(24) and G(40)(a) but decided against it on economic grounds.

Heavy Water

Neutrons are absorbed by the hydrogen atoms in ordinary water (H
²
O). Heavy water (D
²
O) is water with the hydrogen atoms removed so that the liquid which is left consists only of its deuterium atoms. Neutrons collide with deuterium molecules and lose much of their velocity but are not absorbed, which is why heavy water is an exceptionally efficient moderator when used with natural uranium in reactor processes. The production of D
²
O is extremely costly: at Vemork near Rjukan in Norway, 200 kilometres west of Oslo, in 1940 the world's only large production facility, 1000 KwH of electricity was required to turn out a single gramme of heavy water. Heavy water cannot be contaminated in the normal course of events and can be used indefinitely.

A Hypothetical Meeting

There is no record of Professor Heisenberg ever having met Hitler. Nevertheless, since the latter preferred to be addressed by people who knew exactly what they were talking about instead of intermediaries, it seems logical that Hitler would have asked him to call at some stage. In broad terms Professor Heisenberg would have explained why he believed a working reactor to be impossible. As the obvious corollary, a plutonium bomb would not be possible. A U
²³⁵
bomb would be prohibitively expensive. The Führer would then have asked, “Well what is possible?” to which Heisenberg could hardly have replied, “Nothing,” for such an answer would have jeopardized the continued existence of the Uranium Project. We simply have to assume that something was put on the table. Following the logical track proposed in this book, I suggest that Heisenberg would have informed him: “I could produce a very low-yield atom bomb built in the laboratory for a rocket hitting the ground at Mach 3.5.” Whereupon Hitler, rising and offering his hand in parting, would have replied, “Then we'll have to settle for that, won't we?” Actually the talk would probably have been far more circuitous than I have expressed it here, but certainly the gist must have been along those lines. Sometime in 1940 a schedule of experiments was drawn up by Heisenberg which were more useful for bomb-making than reactor-building.

Heisenberg's Mysterious Leipzig Experiments

In May 1940, at Leipzig, Professor Heisenberg took delivery of a tonne of uranium oxide sent by the
Heereswaffenamt
with which he was to start his reactor theory experiments. In
G23 Determination of the Diffusion Length of Thermal Neutrons in Heavy Water
⁵²
, dated 7 August 1940, Heisenberg described the results of examining how the velocity of neutrons emitted by a 480 mg radium-beryllium source was reduced during their passage through 9 litres of heavy water. In
G22 Determination of the Diffusion Length of Thermal Neutrons in Preparation 38
⁵³
, submitted in December 1940, he reported on work involving neutrons emitted into small samples of uranium oxide in a sphere of 12-cm radius. These experiments represented the preliminary work preparatory to designing a hypothetical uranium oxide reactor moderated by heavy water.

In March 1941 Heisenberg carried out experiment L-I at Leipzig University.
⁵⁴
He said he wanted to establish constants using uranium oxide with paraffin. The two materials were layered alternately in a cylindrical tank. Paraffin is so rich in hydrogen atoms that it is useless as a moderator. From the experiment there was, of course, no neutron multiplication to report. Paraffin is very useful as a reflector. Used to enclose the reactor core, it prevents neutrons escaping. If the uranium fuel is arranged in alternate layers with paraffin, there could never be a chain reaction, because neutrons would be confined and absorbed within their respective layers. If one had in aggregate a critical mass of uranium, as in a bomb for example, and wanted to keep the material in sub-critical quantities with no passage of neutrons between the layers, paraffin would be a good way to do it.

There was now a long wait of five months before sufficient heavy water would be available for experiment L-II at Leipzig. During this respite there suddenly burst on the scene a scientific paper which resurrected the spectre of Harteck's radioactive weapons and worse.

CHAPTER 5

The Open Road to the Atom Bomb

E
ARLY IN SEPTEMBER 1941 Professor Heisenberg received a copy of a scientific paper of 29 pages entitled
On the Question of Initiating Chain Reactions
⁵⁵
which explained how, from a practical standpoint, a chain reaction could be simply and effectively brought about by the use of methane as the moderator in a very low temperature reactor. The author was Professor Fritz Houtermans, a Jewish scientist who was perhaps the most brilliant physicist in the field of chain reaction theory in the Third Reich. He was employed at the Post Office Research Institute under Baron Manfred von Ardenne at the Lichterfelde-Ost laboratory in Berlin. The Post Office research was funded independently. The Postmaster-General, Dr Wilhelm Ohnesorge, was known as a pro-Bomb Cabinet Minister close to Hitler.

The primary purpose of the report was to show the quickest and most effective means of having a working nuclear pile. It was the obvious first stepping stone into the atomic age. Houtermans also explained how a new explosive U
²³⁹
(plutonium) could be bred in his reactor. He argued that if heat and energy were not required, then the use of heavy water or graphite as a moderator was not necessary. If the reactor was required only for the production of radioisotopes, then a carbon-based substance in a very low temperature was all that was needed.

What he had observed in the course of his experiments with moderators was that hydrogen molecules in carbon compounds absorbed far fewer neutrons in extreme sub-zero temperatures. It was almost certainly attributable to the nuclear Doppler Effect: he thought it probable that the Doppler Effect alone would enable a liquid carbon substance to be used as moderator. He considered the most favourable to be liquid methane, CH
⁴
, a colourless, odourless, flammable gas which is liquid in the temperature range – 164
^°
C to – 186
^°
C.

In common with all other Reich physicists, Houtermans was unaware of the stabilizing influence of the delayed neutrons of fission, but he had no concerns regarding the safety and stability of his design. Since the machine only operated at a very low temperature, the chain reaction would automatically collapse once the temperature began to rise substantially towards freezing point. While in theory he planned to use regulating rods to control neutron multiplication, in practice Houtermans would have found his reactor unexpectedly stable due to the unsuspected effect of the delayed neutrons.

Summarizing the paper in a section headed
The Significance of a Chain Reaction in a Low Temperature Environment as a Neutron Source and as an Apparatus for Transforming Isotopes
, Houtermans confirmed that because U
²³⁹
(i.e. Pu
²³⁹
plutonium) is a different element from uranium and, therefore, chemically distinct, concentrations of Pu
²³⁹
should be obtainable relatively easily by chemical separation from the spent reactor fuel, and this would be an explosive, since it was also fissionable.