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How close were the Nazis to developing an atomic bomb?
The truth is that National Socialist Germany could not possibly have built a weapon like the atomic bombs dropped on Hiroshima or Nagasaki. This was not because the country lacked the scientists, resources, or will, but rather because its leaders did not really try.
They were certainly trying to win the war. And they were willing to devote huge amounts of resources to building rockets, jet planes, and other forms of deadly and sometimes exotic forms of military technology. So why not the atomic bomb? Nazi Germany, it turns out, made other choices and simply ran out of time.
A nuclear program is born
In January of 1939, the German chemists Otto Hahn and Fritz Strassmann published the results of an historic experiment: after bombarding uranium with neutrons—neutrally charged particles—they found barium, an element roughly half the size of uranium. Their former colleague Lise Meitner, who a few months before had been forced to flee Germany and seek refuge in Sweden, and her nephew Otto Frisch realized that the uranium nucleus had split in two. These revelations touched off a frenzy of scientific work on fission around the world.
The German "uranium project" began in earnest shortly after Germany's invasion of Poland in September 1939, when German Army Ordnance established a research program led by the Army physicist Kurt Diebner to investigate the military applications of fission. By the end of the year the physicist Werner Heisenberg had calculated that nuclear fission chain reactions might be possible. When slowed down and controlled in a "uranium machine" (nuclear reactor), these chain reactions could generate energy; when uncontrolled, they would be a "nuclear explosive" many times more powerful than conventional explosives.
Whereas scientists could only use natural uranium in a uranium machine, Heisenberg noted that they could use pure uranium 235, a rare isotope, as an explosive. In the summer of 1940, Carl Friedrich von Weizsäcker, a younger colleague and friend of Heisenberg's, drew upon publications by scholars working in Britain, Denmark, France, and the United States to conclude that if a uranium machine could sustain a chain reaction, then some of the more common uranium 238 would be transmuted into "element 94," now called plutonium. Like uranium 235, element 94 would be an incredibly powerful explosive. In 1941, von Weizsäcker went so far as to submit a patent application for using a uranium machine to manufacture this new radioactive element.
Researchers knew that they could manufacture significant amounts of uranium 235 only by means of isotope separation. At first German scientists led by the physical chemist Paul Harteck tried thermal diffusion in a separation column. In this process, a liquid compound rises as it heats, falls as it cools, and tends to separate into its lighter and heavier components as it cycles around the column. But by 1941 they gave up on this method and started building centrifuges. These devices use centripetal force to accumulate the heavier isotopes on the outside of the tube, where they can be separated out. Although the war hampered their work, by the fall of the Third Reich in 1945 they had achieved a significant enrichment in small samples of uranium. Not enough for an atomic bomb, but uranium 235 enrichment nonetheless.
Nearing a Nazi bomb
Uranium machines needed a moderator, a substance that would slow down the neutrons liberated by chain reactions. In the end, the project decided to use heavy water—oxygen combined with the rare heavy isotope of hydrogen—instead of water or graphite. This was not (as one of the many myths associated with the German nuclear weapons effort had it) because of a mistake the physicist Walther Bothe made when he measured the neutron absorption of graphite. Rather, it appeared that the Norsk Hydro plant in occupied Norway could provide the amounts of heavy water they needed in the first stage of development at a relatively low cost.
The Norwegian resistance and Allied bombers eventually put a stop to Norwegian production of heavy water (see Norwegian Resistance Coup and See the Spy Messages. But by that time it was not possible to begin the production of either pure graphite or pure heavy water in Germany. In the end, the German scientists had only enough heavy water to conduct one or two large-scale nuclear reactor experiments at a time.
By the very end of the war, the Germans had progressed from horizontal and spherical layer designs to three-dimensional lattices of uranium cubes immersed in heavy water. They had also developed a nuclear reactor design that almost, but not quite, achieved a controlled and sustained nuclear fission chain reaction. During the last months of the war, a small group of scientists working in secret under Diebner and with the strong support of the physicist Walther Gerlach, who was by that time head of the uranium project, built and tested a nuclear device.
At best this would have been far less destructive than the atomic bombs dropped on Japan. Rather it is an example of scientists trying to make any sort of weapon they could in order to help stave off defeat. No one knows the exact form of the device tested. But apparently the German scientists had designed it to use chemical high explosives configured in a hollow shell in order to provoke both nuclear fission and nuclear fusion reactions. It is not clear whether this test generated nuclear reactions, but it does appear as if this is what the scientists had intended to occur.
Time runs out
All of this begs the question, why did they not get further? Why did they not beat the Americans in the race for atomic bombs? The short answer is that whereas the Americans tried to create atomic bombs, and succeeded, the Germans did not succeed, but also did not really try.
This can best be explained by focusing on the winter of 1941-1942. From the start of the war until the late fall of 1941, the German "lightning war" had marched from one victory to another, subjugating most of Europe. During this period, the Germans needed no wonder weapons. After the Soviet counterattack, Pearl Harbor, and the German declaration of war against the United States, the war had become one of attrition. For the first time, German Army Ordnance asked its scientists when it could expect nuclear weapons. The German scientists were cautious: while it was clear that they could build atomic bombs in principle, they would require a great deal of resources to do so and could not realize such weapons any time soon.
Army Ordnance came to the reasonable conclusion that the uranium work was important enough to continue at the laboratory scale, but that a massive shift to the industrial scale, something required in any serious attempt to build an atomic bomb, would not be done. This contrasts with the commitment the German leadership made throughout the war to the effort to build a rocket. They sunk enormous resources into this project, indeed, on the scale of what the Americans invested in the Manhattan Project.
Thus Heisenberg and his colleagues did not slow down or divert their research; they did not resist Hitler by denying him nuclear weapons. With the exception of the scientists working on Diebner's nuclear device, however, they also clearly did not push as hard as they could have to make atomic bombs. They were neither heroes nor villains, just scientists working on weapons of mass destruction for Hitler's Germany.
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There were heated arguments within the German scientific community over the direction of nuclear research. Heisenberg's group preferred a reactor using uranium and heavy water as moderator. Its research, however, had been going on at a snail's pace. Heisenberg just seemed unable to grasp some fundamental principles of making an atomic bomb. This group seemed to believe that a whole reactor would have to be dropped as a nuclear bomb. Even the scientists involved admitted that no atomic bomb could be built before the end of the war.
Another group, led by Paul Harteck and backed by Dr. Wilhelm Ohnesorge, head of the Reich Post Office, opted for the low-temperature (-80C) reactor. A low-temperature reactor would produce neither heat nor power, but would leave radioactive material behind in the forms of spent fuel, radioactive isotopes and plutonium. These by-products, except plutonium, of course, did not amount to an atomic bomb, but there was another possibility. Fine sand and dust could be mixed with the radioactive material to make themselves radioactive (such a device is now known as "dirty bomb"). Packed around the high explosive warheads of the V-1 and V-2, the radioactive dust could spread far and wide, and knock out large cities like London. Harteck, however, met oppositions from Heisenberg, who disagreed with Harteck and withheld crucial materials. As a result, Harteck and others' work did not amount to much.
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Building Hitler's Bomb
German effort to build nuclear weapons in
World War II
Declassified files reopen "Nazi bomb" debate
Did leading German physicists choose not to "know" how to build an A-bomb?
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German nuclear energy project
The German nuclear energy project was an endeavor by scientists during World War II in Nazi Germany to develop nuclear energy and an atomic bomb for practical use. Unlike the competing Allied effort to develop a nuclear weapon the German effort resulted in two rival teams, one working for the military, the second, a civilian effort co-ordinated by the German Post Office.
Overview
The nuclear research effort most widely discussed was that of the Kaiser Wilhelm Institute team led by the physicist Werner Heisenberg. The second was a military team under the scientific leadership of Prof. Kurt Diebner. This military team was also associated with Dr. Paul Harteck who helped to develop the gaseous uranium centrifuge invented by Dr. Erich Bagge in 1942. Their team was part of the German Army (Heereswaffenamt Forschungsstelle E), the Kriegsmarine (navy) had a subsidiary team looking at nuclear propulsion for U-boats under Dr. Otto Haxel. Konteradmiral Karl Witzell and Konteradmiral Wilhelm Rein were military leaders of the naval nuclear project.
The intentions of Heisenberg's team are a matter of historical controversy, centering on whether or not the scientists involved were genuinely attempting to build an atomic bomb for Nazi dictator Adolf Hitler. The project was not a military success by any measure.
Effectiveness and implications
It is generally accepted that the Nuclear Age began with the 1938 publication by Otto Hahn and Fritz Strassman of results that proved Enrico Fermi had observed the bursting of a uranium nucleus, in other words: nuclear fission. Immediately afterwards, Lise Meitner and Otto Robert Frisch described the theoretical mechanisms of fission and revealed that large amounts of binding energy was released in the process. Thus by the beginning of World War II the scientific community was well aware of the early German lead in this area of theoretical physics.
The threat of a Nazi atomic bomb was one of the primary driving forces behind the creation of the British TUBE ALLOYS project which would eventually lead to the Allied nuclear weapons effort under Robert Oppenheimer: the Manhattan Project. (Several Germans eventually would make significant contributions to the Allied nuclear effort.) The German government never did finance a full crash program to develop weapons, as they estimated it could not be completed in time for use in the war, thus the German program was much more limited in capacity and ability when compared to the eventual size and priority of the Manhattan Project. In 1945, a U.S. investigation called Project ALSOS determined that German scientists had only almost reached the point that Allied scientists had reached in 1942, the creation of a sustained nuclear chain reaction, a crucial step for creating a nuclear reactor (which in turn could be used for either peaceful purposes, or for creating plutonium, needed for nuclear weapons). There has been a historical debate, however, as to whether the German scientists purposefully sabotaged the project by under-representing their chances at success, or whether their estimates were based in either error or inadequacy.
Post war
After the war, a number of German scientists including Heisenberg, Otto Hahn (who had co-discovered nuclear fission), and Max von Laue (an ardent anti-Nazi), were taken captive by Allied forces and put under secret watch at Farm Hall, England, as part of Operation Epsilon. Their conversations were recorded as Allied analysts attempted to discover the extent of German knowledge about nuclear weapons. The results were inconclusive, but they allowed them to hear the results of the atomic bombing of Hiroshima, Japan, which sent Hahn into a near-suicidal despair. By the next morning, Heisenberg claimed to have worked out exactly how the American atomic bomb must have worked, judging from reports of the damage and explosive size, and gave a lecture to the rest of the captive scientists on the effort.
While it is clear that Heisenberg had a firm understanding of the principles involved, he most likely greatly overestimated the amount of fissionable material required by several orders of magnitude.
Heisenberg's 1941 meeting with Bohr
In 1941, Werner Heisenberg met with his former mentor Niels Bohr in occupied Denmark and had a conversation outside of any other witnesses. The exact content of their conversation has, since the 1950s, been a matter of some controversy. The meeting and its controversy was the subject of a Tony Award-winning play from 1998 by Michael Frayn, Copenhagen.
There is considerable speculation on what occurred at the real-life meeting, and the actual accounts of it from the parties involved differ. The pro-Bohr version of the story asserts that Heisenberg was seeking to recruit Bohr to the Nazi nuclear effort, and offering him academic advancement in return. The pro-Heisenberg version asserts that Heisenberg was attempting to give Bohr information about the state of the German atomic programme, in the hope that he might pass it to the Allies through clandestine contacts. At that point the German atomic programme was not progressing well (the Nazi government had decided not to undertake the investment required to develop a weapon during the war); Heisenberg may have suspected that the Allies had a viable atomic program, and hoped that by disabusing them of the idea that the German program was also successful he could dissuade the Allies from using an atom bomb on Germany.
Much of the initial "controversy" resulted from a 1956 letter Heisenberg sent to the journalist Robert Jungk after reading the German edition of Jungk's book Brighter than a Thousand Suns (1956). In the letter, Heisenberg described his role in the German bomb project. Jungk published an extract from the letter in the Danish edition of the book in 1956 which, out of context, made it look as if Heisenberg was claiming to have purposely derailed the German bomb project on moral grounds. (The letter's whole text shows Heisenberg was careful not to claim this.) Bohr was outraged after reading this extract in his copy of the book, feeling that this was false and that the 1941 meeting had proven to him that Heisenberg was quite happy with producing nuclear weapons for Germany.
After the play inspired numerous scholarly and media debates over the 1941 meeting, the Niels Bohr Archive in Copenhagen released to the public all heretofore sealed documents related to the meeting, a move intended mostly to settle historical arguments over what they contained. Among the documents were the original drafts of letters Bohr wrote to Heisenberg in 1957 about Jungk's book and other topics. The documents added little to the historical record but were interpreted by the media as supporting the "Bohr" version of the events. According to the archivists, the letters were released "to avoid undue speculation about the contents of the draft letter", which had been known about but not been open to historians previously.
Analysis and legacy
There have been numerous other cited factors for the failure of the German program. One is that the repressive policies under Hitler encouraged many top scientists to flee Europe, including many who worked on the Allied project (Heisenberg himself was a target of party propaganda for some time during the Deutsche Physik movement). Another, put forth by ALSOS scientific head Samuel Goudsmit, was that the stifling, utilitarian political atmosphere adversely affected the quality of the science done. Another is that the German homeland was nowhere as secure from air attack as was the USA. Had the many massive centralized factories and production facilities constructed for the US bomb project been built in Germany, they would have been prime targets for Allied bombing raids.
In 2005, Berlin historian Rainer Karlsch published a book, Hitlers Bombe (in German), which was reported in the press as claiming to provide evidence that Nazi Germany had tested crude nuclear weapons on Rügen island and near Ohrdruf, Thuringia, killing many war prisoners under the supervision of the SS. Some press reports, however, have reported the book as only having claimed to provide evidence that the Nazis have been successful with a radiological weapon (a "dirty bomb"), not a "true" nuclear weapon powered by nuclear fission. Karlsch's primary evidence, according to his publisher's reports, are "vouchers" for the "tests" and a patent for a plutonium weapon from 1941. Karlsch cites a witness to the Ohrdruf blast and another to the scorched bodies of victims afterwards. He also claims to have radioactive samples of soil from the sites.
At the Nuremburg trials in 1946 Nazi munitions minister Albert Speer was questioned by prosecutors about the Ohrdruf blast, in an attempt to hold Speer accountable for its victims.
Mainstream American historians have expressed skepticism towards any claims that Nazi Germany was in any way close to success at producing a true nuclear weapon, citing the copious amounts of evidence which seem to indicate the contrary. Others counter that Prof. Kurt Diebner had a project which was far more advanced than that of Dr. Werner Heisenberg. A recent article in Physics Today by the respected American historian Mark Walker has presented some of Karlsch's less controversial claims — that the Germans had done research on fusion, that they were aware that a bomb could potentially be made with plutonium, that they had engaged in some sort of test of some sort of device, that a patent on a plutonium device (of unspecified detail) had been filed and found — as substantiated.
The Germans’ only source of heavy water, a necessary component of some of their bomb research, was Norsk Hydros plant in Vemork, Norway. In February 1943, a Norwegian commando unit sabotaged the plant. Whether this affected the German program is not clear.
It is noteworthy, though, that Germany had already had a significant amount of heavy water and could have built a small reactor with it. The problem of the supply of uranium was solved in 1940 when over 1,000 tons of mixed uranium products were captured at Oolen in Belgium. Germany had everything ready, but just seemed unable to do anything with it.
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This is pretty amazing. It’s a Scientific American article from October 1939, describing the splitting of the atom. It was written just after Einstein had written his famous letter to F.D.R and before the initiation of the Manhattan Project, yet it is obvious that scientists were well aware of the potential uses of atomic fission:
It may or may not be significant that, since early spring, no accounts of research on nuclear fission have been heard from Germany — not even from discoverer Hahn. It is not unlikely that the German government, spotting a potentially powerful weapon of war, has imposed military secrecy on all recent German investigations. A large concentration of isotope 235, subjected to neutron bombardment, might conceivably blow up all London or Paris.
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New light on Hitler's bomb
Controversial new historical evidence suggests that German physicists built and tested a nuclear bomb during the Second World War. Rainer Karlsch and Mark Walker outline the findings and present a previously unpublished diagram of a German nuclear weapon
This year marks the 60th anniversary of the American nuclear attack on the Japanese cities of Hiroshima and Nagasaki. The atomic bombs that were dropped on Japan in August 1945 were the fruit of a herculean wartime effort by the American, British and émigré scientists involved in the Manhattan Project. They had to overcome great obstacles and were only able to test their first atomic bomb after Germany surrendered in May of that year. The main motivation for these scientists when the project began in 1941 was the possibility that they were engaged in a race with their German counterparts to harness nuclear fission for war.
Even Albert Einstein had been involved, signing a letter to President Roosevelt in 1939 urging that the US take nuclear weapons seriously. And in December 1943 the Danish physicist Niels Bohr visited Los Alamos - the home of the Manhattan Project - to offer both scientific and moral support. But when the war was over, it was clear that the Germans did not have atomic bombs like those used against Japan.
The German "uranium project" - which had been set up in 1939 to investigate nuclear reactors, isotope separation and nuclear explosives - amounted to no more than a few dozen scientists scattered across the country. Many of them did not even devote all of their time to nuclear-weapons research. The Manhattan Project, in contrast, employed thousands of scientists, engineers and technicians, and cost several billion dollars.
Not surprisingly, historians have concluded that Germany was not even close to building a working nuclear device. However, newly discovered historical material makes this story more complicated - and much more interesting.
Germany and the bomb: a turbulent tale
Our understanding of the German nuclear-weapons project during the Second World War has changed over time because important new sources of information keep turning up. For example, in 1992 the British government released transcripts of secretly recorded conversations between 10 German scientists who had been interned at Farm Hall near Cambridge in 1945. With the exception of Max van Laue, all the scientists - Erich Bagge, Kurt Diebner, Walther Gerlach, Otto Hahn, Paul Harteck, Werner Heisenberg, Horst Korsching, Carl Friedrich von Weizsäcker and Karl Wirtz - had been involved in the uranium project. What was most interesting was the surprise with which the scientists greeted the news that Hiroshima had been bombed. Ironically, at the end of the war German scientists had been convinced that they were ahead of the Allies in the race for nuclear energy and nuclear weapons.
Further intriguing material appeared in 2002 when the Niels Bohr Archives in Copenhagen released drafts of letters that had been written by Bohr in the late 1950s about a visit to occupied Denmark by Heisenberg and von Weizsäcker in September 1941. After the war, the two German physicists claimed that they had merely gone to Copenhagen to assist Bohr and enlist his help in their efforts to forestall all nuclear weapons. But in the letters, Bohr denied that their actions or motivations had been so noble. The intrigue surrounding the visit has been well dramatized in Michael Frayn's play Copenhagen.
We now have an extra twist to the tale with new documents that were recently discovered in Russian archives, including papers from the Kaiser Wilhelm Institute of Physics in Berlin. There are four particularly notable items among this material: an official report written by von Weizsäcker after a visit to Copenhagen in March 1941; a draft patent application written by von Weizsäcker sometime in 1941; a revised patent application in November of that year; and the text of a popular lecture given by Heisenberg in June 1942.
One of us (RK) has used these documents - as well as many other sources - as the basis of a new book Hitlers Bombe. The book, which was published in March, prompted a heated debate about how close Germany was to acquiring nuclear weapons and how significant these weapons were (see Physics World April 2005 p7). Working with the journalist Heiko Petermann, RK discovered that a group of German scientists had carried out a hitherto-unknown nuclear-reactor experiment and tested some sort of a nuclear device in Thüringia, eastern Germany, in March 1945. According to eyewitness accounts given at the end of that month and two decades later, the test killed several hundred prisoners of war and concentration-camp inmates. Although it is not clear if the device worked as intended, it was designed to use nuclear fission and fusion reactions. It was, therefore, a nuclear weapon.
Following the publication of Hitlers Bombe, another document has turned up from a private archive. Written immediately after the end of the war in Europe, the undated document contains the only known German drawing of a nuclear weapon.
What did German scientists know?
Over the years, several authors have concluded that Heisenberg and his colleagues did not understand how an atomic bomb would work. These authors include the physicist Samuel Goudsmit, who in 1947 published the results of a US Army investigation - entitled Alsos - into Germany's bomb effort. The historian Paul Lawrence Rose came to the same conclusion in his 1998 book Heisenberg and the Nazi Atomic Bomb Project 1939-1945. These critics argue that the German scientists did not understand the physics of a nuclear-fission chain reaction, in which fast neutrons emitted by a uranium-235 or plutonium nucleus trigger further fission reactions. Both Goudsmit and Rose also say the Germans failed to realize that plutonium can be a nuclear explosive.
These criticisms of the Germans' scientific incompetence are apparently reinforced by the Farm Hall conversations, which reveal that Heisenberg initially responded to the news of Hiroshima with a flawed calculation of critical mass, although within a few days he had improved it and provided a very good estimate. However, there was other evidence that, no matter how Heisenberg responded at Farm Hall, he and his colleagues understood that atomic bombs would use fast-neutron chain reactions and that both plutonium and uranium-235 were fissionable materials.
For example, in February 1942 the German army officials who were responsible for weapons development described the progress of the uranium project in a report entitled "Energy production from uranium". This overview, which was discovered in the 1980s, drew upon all classified material from Hahn, Harteck, Heisenberg and the other scientists working on the project. The report concluded that pure uranium-235 - which forms just 0.7% of natural uranium, the rest being non-fissionable uranium-238 - would be a nuclear explosive a million times more powerful than conventional explosives. It also argued that a nuclear reactor, once operating, could be used to make plutonium, which would be an explosive of comparable force. The critical mass of such a weapon would be "around 10-100 kg", which was comparable to the Allies' estimate from 6 November 1941 of 2-100 kg that is recorded in the official history of the Manhattan Project - the so-called Smyth report.
Von Weizsäcker's draft patent application of 1941, which is perhaps the most surprising find from the new Russian documents, makes it crystal clear that he did indeed understand both the properties and the military applications of plutonium. "The production of element 94 [i.e. plutonium] in practically useful amounts is best done with the 'uranium machine' [nuclear reactor]," the application states. "It is especially advantageous - and this is the main benefit of the invention - that the element 94 thereby produced can easily be separated from uranium chemically."
Von Weizsäcker also makes it clear that plutonium could be used in a powerful bomb. "With regard to energy per unit weight this explosive would be around ten million times greater than any other [existing explosive] and comparable only to pure uranium 235," he writes. Later in the patent application, he describes a "process for the explosive production of energy from the fission of element 94, whereby element 94...is brought together in such amounts in one place, for example a bomb, so that the overwhelming majority of neutrons produced by fission excite new fissions and do not leave the substance".
This is nothing less than a patent claim on a plutonium bomb.
On 3 November 1941 the patent application was resubmitted with the same title: "Technical extraction of energy, production of neutrons, and manufacture of new elements by the fission of uranium or related heavier elements". This submission differed in two significant ways. First, the patent was now filed on behalf of the entire Kaiser Wilhelm Institute, instead of just von Weizsäcker. Second, every mention of nuclear explosive or bomb had been removed.
The removal of any reference to weapons could reflect the change of fortunes in the Second World War: in November 1941 a quick German victory no longer appeared as certain as it had done earlier in the year. Another possible explanation is that von Weizsäcker and his colleagues had a change of heart - perhaps their initial enthusiasm for the military applications of nuclear fission had cooled. This would support Heisenberg's and von Weizsäcker's post-war claims that they had visited Bohr in September 1941 because they were ambivalent about working on nuclear weapons. Perhaps the most forceful exponent of this thesis is Thomas Powers in his 1993 book Heisenberg's War.
But another of the new Russian documents - von Weizsäcker's report on his visit to Copenhagen in spring 1941 - suggests that, at least at that time, he was enthusiastic about the uranium work. Indeed, we know that, after the war, scientists from Bohr's institute accused Heisenberg and von Weizsäcker of acting as German spies when they came to Copenhagen. There may at least be some truth to this because in March 1941, when Germany had not yet invaded the Soviet Union and victory appeared likely, von Weizsäcker reported the following to the Army.
"The technical extraction of energy from uranium fission is not being worked on in Copenhagen. They know that in America Fermi has started research into these questions in particular; however, no more news has arrived since the beginning of the war. Obviously Professor Bohr does not know that we are working on these questions; of course, I encouraged him in this belief...The American journal Physical Review was complete in Copenhagen up to the January 15, 1941 issue. I have brought back photocopies of the most important papers. We arranged that the German Embassy will regularly photocopy [make photographs of] the issues for us."
The spotlight turns to Diebner
RK's book Hitlers Bombe draws upon what was already known about the German wartime work on nuclear reactors and isotope separation, and uses documents from Russian archives, oral history and industrial archaeology to open up a new chapter in the history of German nuclear weapons. For most of the war, there were two competing groups working on nuclear reactors: a team under the Army physicist Kurt Diebner in Gottow near Berlin; and scientists directed by Werner Heisenberg in Leipzig and Berlin.
Whereas the experiments under Heisenberg used alternating layers of uranium and moderator, Diebner's team developed a superior 3D lattice of uranium cubes embedded in moderator. Heisenberg never gave Diebner and the scientists working under him the credit they were due, but the Nobel laureate did take up Diebner's design for the last experiment carried out in Haigerloch in south-west Germany. RK now reveals that Diebner managed to carry out one last experiment in the last months of the war. The exact details of the experiment are unclear. After a series of measurements had been taken, Diebner wrote a short letter to Heisenberg on 10 November 1944 that informed him of the experiment and hinted that there had been problems with the reactor. Unfortunately, no more written sources have been found relating to this final reactor experiment in Gottow. Industrial archaeology done at the site during 2002 and 2003 suggests that this reactor sustained a chain reaction - if only for a short period of time - and may have ended in an accident.
In 1955 Diebner submitted a patent application for a new type of "two-stage" reactor that could breed plutonium. An internal section would use enriched uranium to achieve a self-sustaining chain reaction, while a much larger external section would surround the internal reactor and run at a subcritical level. Plutonium could then be removed from internal section. It appears likely that Diebner's 1955 patent application drew upon his last wartime experiment.
More surprising, if not shocking, is another revelation in RK's book: a group of scientists under Diebner built and tested a nuclear weapon with the strong support of both Walther Gerlach - an experimental nuclear physicist who by 1944 was in charge of the uranium project for the Reich Research Council. (Hahn, Heisenberg, von Weizsäcker and most of the better-known scientists in the uranium project apparently were not informed about this weapon.) This device was designed to use fission reactions, but it was not an "atomic" bomb like the weapons used against Nagasaki and Hiroshima (figures 1a and b). And although it was also designed to exploit fusion reactions, it was nothing like the "hydrogen" bombs tested by the US and the Soviet Union in the 1950s.
Instead, conventional high explosives were formed into a hollow shape, rather than a solid mass, to focus the energy and heat from the explosion to one point inside the shell (figure 1c). Small amounts of enriched uranium, as well as a source of neutrons, were combined with a deuterium-lithium mixture inside the shell. This weapon would have been more of a tactical than a strategic weapon, and could not have won the war for Hitler in any case. It is not clear how successful this design was and whether fission and fusion reactions were provoked. But what is important is the revelation that a small group of scientists working in the last desperate months of the war were trying to do this.
Blueprint for a bomb
Shortly after the end of the war in Europe, an unknown German or Austrian scientist wrote a report that describes work on nuclear weapons during the war. This report, which RK discovered after Hitlers Bombe was published, contains both accurate information and less accurate speculation about nuclear weapons, and may well include some information from the Manhattan Project - the word "plutonium" is used, for example. Unfortunately, the title page is not included and there is no other evidence of who composed it. However, this individual does not appear to have been a member of either the mainstream German uranium project or the group working under Diebner.
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The only known German diagram of a nuclear weapon
The diagram is schematic and is far removed from a practical blueprint for an atomic bomb
Although the weapon is shown to be a fission device based on plutonium, the report also reveals that German scientists had worked intensively on the theory of a hydrogen bomb
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What the report does demonstrate is that the knowledge that uranium could be used to make powerful new weapons was fairly widespread in the German technical community during the war, and it contains the only known German diagram of a nuclear weapon. This diagram is schematic and is far removed from a practical blueprint for an "atomic bomb". The unknown author also mentions a critical mass of slightly more than 5 kg for a plutonium bomb. This estimate is fairly accurate, because the use of a tamper to reflect neutrons back into the plutonium would cut the critical mass by a factor of two. Moreover, this estimate is particularly significant because such detailed information was not included in the Smyth report.
The new report is also interesting because it makes clear that German scientists had worked intensively on theoretical questions concerned with the construction of a hydrogen bomb. Two additional sources confirm this. The papers of Erich Schumann, director of the Army's weapons-research department, include many documents and theoretical calculations of nuclear fusion. The Viennese physicist Hans Thirring also discussed this topic in his book The History of the Atomic Bomb, which was published in the summer of 1946.
Not the last word
Historians, scientists and others have debated for decades whether Heisenberg and von Weizsäcker wanted to build atomic bombs.Taken together, the new revelations change our picture of German nuclear weapons. None of this new information supports in any way either the interpretation of Heisenberg and his colleagues as resistance fighters (Powers) or as incompetents with Nazi sympathies (Rose).
However, these new documents and RK's revelations do place Heisenberg and von Weizsäcker in a different context by making their ambivalence about nuclear weapons much clearer. Although they continued to work on nuclear reactors and isotope separation, and dangled the prospect of nuclear weapons in front of powerful men in the Nazi state, they did not try as hard as they could to create nuclear weapons for Hitler's regime. Other scientists were doing that, notably Walther Gerlach,Kurt Diebner and the researchers working under him.
It would be rash indeed to believe that this is the last word on the matter. The German atomic bomb is like a zombie: just when we think we know what happened, how and why, it rises again from the dead.
Heisenberg's role
During the Second World War, Werner Heisenberg was one of the most influential scientists in Germany and its leading theoretical physicist. He had won a Nobel prize for his work on quantum mechanics and the uncertainty principle, had become one of the youngest full professors in Germany when he began teaching at the University of Leipzig, and in 1942 at the age of 40 was appointed director of the prestigious Kaiser Wilhelm Institute for Physics as well as professor at the University of Berlin.
However, in the early years of the Third Reich, Heisenberg had been attacked by his fellow Nobel laureate Johannes Stark in an SS publication for being a "white Jew" and "Jewish in spirit". A subsequent investigation by the SS ended in 1939 with his public and political rehabilitation. The result was that, by 1942, Heisenberg enjoyed the support of influential figures in the Nazi regime, including the armaments minister Albert Speer, as well as the industrialist Albert Vögler, who was president of the Kaiser Wilhelm Society.
Pulled both ways
In February 1942 Heisenberg gave a popular lecture to an influential audience of politicians, bureaucrats, military officers and industrialists. At the time, the future of Germany's uranium project was in doubt because the Army was only interested in weapons that could be delivered in time to influence the outcome of the war. As we know from a transcript of the talk, which was discovered by the historian David Irving in the 1960s, Heisenberg emphasized both the potential of nuclear weapons and how difficult it would be to make them. His conclusion was clear.
"1) Energy generation from uranium fission is undoubtedly possible, provided the enrichment of isotope uranium-235 is successful. Isolating uranium-235 would lead to an explosive of unimaginable potency. 2) Common uranium can also be exploited to generate energy when layered with heavy water. In a layered arrangement these materials can transfer their great energy reserves over a period of time to a heat-engine. It thus provides a means of storing very large amounts of energy that are technically measurable in relatively small quantities of substances. Once in operation, the machine can also lead to the production of an incredibly powerful explosive."
However, by the summer of 1942, the uranium project had been transferred from the German Army to the civilian Reich Research Council and the German uranium-project scientists once again enjoyed secure institutional support. In June of that year Heisenberg gave a lecture at the Kaiser Wilhelm Society in Berlin before Speer and other military and industrial leaders of the Nazi state. The lecture has become famous because of the story that Heisenberg responded to a question about the size of an atomic bomb by saying that it would be about as big as a pineapple.
This anecdote was first reported in Irving's 1968 book The Virus House, but a transcript of the talk had never been found. However, it has now been discovered in the new Russian documents. The text of the June lecture - entitled "The work on uranium problems" - differs significantly from the February talk. Heisenberg begins by mentioning the discovery of nuclear fission in 1939, noting that interest in this new development had been "exceptionally great", especially in the US. "A few days after the discovery," he notes, "American radio provided extensive reports and half a year later a large number of scientific papers had appeared on this subject."
Heisenberg continues by describing Germany's work on isotope separation and nuclear reactors since the start of the war, cautioning that "naturally a series of scientific and practical problems will have to be cleared up before the technical goals can be realized". Mid-way through the talk, Heisenberg makes his only mention of nuclear weapons in a rather understated way. "Given the positive results achieved up until now," he says, "it does not appear impossible that, once an uranium burner has been constructed, we will one day be able to follow the path revealed by von Weizsäcker to explosives that are more than a million times more effective that those currently available."
But even if that did not happen, the nuclear reactor would have an "almost unlimited field of technical applications". These include boats and even planes that could travel long distances on small amounts of fuel, as well as new radioactive substances that could be useful for many scientific and technical problems. Heisenberg concludes by saying that new discoveries of "the greatest significance for technology" will be made "in the next few years".
Since the Germans knew that "many of the best laboratories" in America were working on this problem, they could hardly afford "not to follow these questions", Heisenberg points out. Even if "most such developments take a long time", they had to reckon with the possibility that - if the "war with America lasted for several years" - the "technical realization of atomic nuclear energies" might "play a decisive role in the war".
Heisenberg was right about that, of course. But fortunately for him and his countrymen, the first atomic bombs fell on Hiroshima and Nagasaki instead of Frankfurt and Berlin.
A timeline to the bomb
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January 1933 |
Nazis come to power in Germany |
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December 1938 |
Otto Hahn, Lise Meitner and Fritz Strassmann discover nuclear fission in uranium |
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2 August 1939 |
Einstein warns President Roosevelt of dangers of an atomic bomb |
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1 September 1939 |
Germany invades Poland and launches "uranium project" |
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3 September 1939 |
Britain and France declare war on Germany |
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1941 |
Von Weizsäcker files a draft patent application that refers to a plutonium bomb |
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March 1941 |
Von Weizsäcker visits Bohr in Copenhagen |
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June 1941 |
Germany invades Soviet Union |
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September 1941 |
Von Weizsäcker visits Bohr again, this time with Heisenberg |
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6 December 1941 |
Manhattan Project begins in Los Alamos |
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7 December 1941 |
Japan attacks Pearl Harbour |
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8 December 1941 |
US enters Second World War |
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February/June 1942 |
Heisenberg gives popular lectures on nuclear weapons |
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December 1943 |
Bohr visits Los Alamos |
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March 1945 |
Germany tests a nuclear device in Thüringia, eastern Germany |
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7 May 1945 |
Germany surrenders |
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16 July 1945 |
Trinity test - world's first atomic blast |
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6 August 1945 |
US bombs Hiroshima |
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9 August 1945 |
US bombs Nagasaki |
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14 August 1945 |
Japan surrenders |
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