The first method is to enrich natural uranium by separating U-235 from U-238. The U-235 isotope is fissile, but makes up only 0.72% of natural uranium by mass. If uranium is to be used in a bomb, its U-235 concentration must be raised to 90%. The second method is to bombard natural uranium with neutrons and transmute it into plutonium. The U-238 isotope is fertile, and if it captures a neutron, it will turn into U-239, which then decays into Pu-239. If plutonium is to be used in a bomb, its Pu-239 concentration must be raised to 93%. Uranium can be enriched to weapons grade by a variety of techniques, but uranium can only be transmuted into plutonium by a reactor. In hindsight, the german nuclear program made significant steps towards uranium enrichment, but were lagging in their efforts to make a reactor. The details of this subject are complicated and sometimes convoluted, since many historians have offered many appraisals that are mutually exclusive. Authors like Paul Lawrence Rose, for instance, have such a prejudice against the nazis that it interferes with their ability to even tell a coherent narrative. Other writers go in completely the opposite direction. As always, though, only some of these appraisals can be corroborated. This article will focus on a number of myths about the german nuclear program and how it measured up to the manhattan project.
A german uranium pile of 1945
The germans never measured the fission cross section of uranium-235: Hence, they were never able to properly estimate the critical mass for an atomic bomb.
This claim is not only illogical, but also contradicted by certain facts. After the conquest of denmark and france in mid 1940 (among other unfortunate victims of the blitzkrieg), germany had access to machines called cyclotrons at vienna, copenhagen, and paris. Cyclotrons are a type of particle accelerator that can be used to separate U-235 from U-238, thus enabling scientists to perform experiments on the isolated sample: In this case, measuring the fission cross section. Moreover, it is known that each of these laboratorys had been visited by german researchers at several points in the war. Apparently, we are expected to believe that the nazis never took advantage of the very machines that would have enabled them to determine the critical mass of U-235, a key parameter for which much of their subsequent work would hinge on! But of course, such a claim is unsupported by the evidence. In August of 1941, an individual named Fritz Houtermanns (who was employed in the laboratory of Manfred von Ardenne) wrote a paper which discussed runaway chain reactions and the possibility of transmuting uranium into plutonium. This paper was circulated among members of the uranverein, eliciting a flurry of discussion. By February 1942, the HWA team run by Kurt Diebner had published a document outlining the critical mass for a U-235 bomb: The estimate was 10 to 100 kilograms, comparable to the american estimate of 2 to 100 kilograms! However, the HWA report also stated that the difficultys of separating U-235 from U-238 were such that a crash program could not be recommended, because there was no guarantee that such investments would yield a bomb before the wars end. This pessimistic interpretation was reinforced by a meeting that Werner Heisenberg () had with Albert Speer in June of 1942, when he flat out stated that a bomb could not be delivered in a reasonable time, and that the nuclear program should only receive modest funding.
The uranverein never developed an effective means of enriching uranium to weapons grade.
This is simply not true. Early in 1943, the research team under Paul Harteck had created a double centrifuge which was being used to separate isotopes of xenon gas, and then to separate uranium hexafluoride. This machine was able to enrich several grams of uranium to 7%, good enough to warrant funding from the reich research council (RRC). More centrifuges were built, and the design was constantly tinkered with. By May of 1944, a company in freiburg had built and successfully tested the MK III centrifuge, which persuaded Harteck to move his laboratory there. The team set up a facility in the nearby town of kandern, where a few centrifuges were assembled into a cascade that could enrich several kilograms of uranium to 0.9% each day. After a few months, however, allied bombings forced them to stop work and relocate to a town called celle. Early in 1945, the facility only had 30 or 40 of these machines, but was still enriching 50 grams of uranium to 15% each day! This is a remarkable performance, well above any feat achieved by the allied centrifuges. While there were plans to put the MK III centrifuge into mass production again, the war ended before this could take place. Other research teams in germany had experienced similar ups and downs. By June of 1943, Erich Bagge had created an 'isotope-sluice' machine that ran uranium hexafluoride through two shutters revolving at high speed, allowing the lighter U-235 to be separated. This was a totally novel approach which never occurred to the americans, using a combination of electromagnetism, centrifugal force, and thermal diffusion.
While his first two prototypes were destroyed by air raids, Bagge was able to relocate to butzbach and set up another machine. By July of 1944, the 'isotope-sluice' had undergone an endurance test lasting 120 hours, yielding several grams of much enriched uranium. The models indicated its efficiency could be greatly increased. At around this same time, Manfred von Ardenne was testing a magnetic isotope separator, not unlike the calutrons used at the Y-12 plant at oak ridge. Both machines used magnetic fields to deflect charged particles and separate them based on differences in mass, but the germans used an ion source to sublimate the uranium, a fact which made it superior to the calutrons. Ardennes laboratory was located underground in his manor, which protected it from air raids. And since he was financially supported by the post office, work on it was able to continue unimpeded. Fortunately for the allies, however, only one of these small machines were made during the war. Putting the technical details aside for now, it should be clear that the uranverein had made major strides in their knowledge and ability to produce U-235. The problem was that these efforts were all confined to laboratorys, and were never expanded to the industrial scale that was needed for an atom bomb. There were not enough scientists and engineers working on uranium enrichment, and there was not enough funding from the RRC to produce these machines in anywhere near the numbers required.
The germans never generated a self-sustaining nuclear chain reaction, much less a working reactor.
This point requires some background. One of the things needed for a nuclear reactor is a substance which can act as a neutron moderator, and allow a chain reaction to continue unabated. During WW2 there were only two known substances that could fulfill this role: Graphite and heavy water. Allied and axis scientists investigated each of them. In January 1941, Walther Bothe had performed experiments on the purest graphite available, to see whether it could slow down the neutrons without absorbing them. Eventually, he determined that the capture cross-section of graphite was too large to make it an effective moderator. The americans actually came to the same conclusion as him, but would quickly learn that this was due to trace amounts of boron, which could be removed by making the graphite out of petroleum instead of coke. The germans never did this extra step, and were now totally dependent on a supply of heavy water, which was synthesized at only one location in all of europe: The norsk hydro plant. By May of 1942, enough heavy water had been assembled to make a uranium pile at leipzig. Heisenbergs teams detected a neutron increase of 13 percent, meaning that the pile emitted more neutrons than what had been injected into it. This was a step in the right direction.
Unfortunately, the containment vessel exploded soon after the experiment, leaving the scientists with no heavy water left. While the basic research problems had been overcome, no new reactors could be built without an adequate supply of heavy water. Progress on this area stalled as a result, and results came at an agonisingly slow pace. Conditions were only worsened when the allys conducted raids against the norsk hydro plant, interrupting the supply of heavy water. The months and years dragged on, and optimism soon gave way to pessimism. By early 1945, the germans had two crude 'reactors' that were on the verge of sustaining a chain reaction. One of these uranium piles was at stadtilm under Kurt Diebner, while the other uranium pile was at haigerloch under Heisenberg. Both of these reactors were assembled under very difficult circumstances while the scientists were on the run from allied armys, and contained only what materials could be loaded onto truck. The subteranean cavitys they were lowered into had been pre-constructed, but everything else was basically assembled on the fly. Given these constraints, the results they achieved were impressive. Heisenbergs reactor could have went critical if he had been given 50% more uranium and heavy water, which was locked away in a warehouse for safety. And Diebners reactor may have briefly went critical, before being forced to shut down in an emergency.