Hans Bethe. Felix Bloch. Niels Bohr. Albert Einstein. Enrico Fermi. James Franck. Otto Frisch. Samuel Goudsmit. Lilli Hornig. Rolf Landshoff. Maria Goeppert Mayer. Rudolf Peierls. George Placzek. Eugene Rabinowitch. Joseph Rotblat. Leo Szilard. Edward Teller. Stanislaw Ulam. John von Neumann. Victor Weisskopf. Eugene Wigner. These are just some of the scientists who had emigrated from Europe, many of whom later made critical contributions to the Manhattan Project — the American effort to build the world’s first nuclear weapon.
Many of these scientists had seen firsthand the horrors Nazi Germany was perpetrating in Europe; many were Jews who were escaping persecution themselves. They showed their contempt for Adolf Hitler and his armies by bringing a sense of urgency to their research on the bomb, and were often motivated by what they believed the stake was: the fate of the world itself.
The Manhattan Project tested its first atom bomb on July 16, 1945, and delivered two to the U.S. military in July-August. Part of their sense of urgency was rooted in doubts about whether Nazi Germany was also building a bomb, including some groups led by the physicist and Nobel laureate Werner Heisenberg. The anxieties were not unfounded: in April 1945, only weeks before Germany surrendered to Allied forces, Heisenberg’s team had assembled a nuclear reactor in a cellar beneath a church in Haigerloch, in the country’s south.
The setup, called the B8 pile, had 664 cubes of uranium suspended on chains in around 1,400 litres of heavy water. A 40-cm-thick shell of graphite surrounded the core, and was itself surrounded by a layer of aluminium and a 60-cm-thick layer of (ordinary) water. The team had taken six years to build B8 but had little to show for it. When a few neutrons were shot into the reactor, the uranium multiplied them into more — but no chain reaction followed.
Far from critical
After the war ended, Heisenberg said the team had come very close; he wrote in a 1947 article in Nature that Haigerloch didn’t have enough resources and that just a little more quantity of uranium would have done the trick.
According to a new study in PNAS Nexus, by researchers from the U.S., Heisenberg & co. were actually way off the mark.
Only around 14 of the 1,100 or so uranium cubes made for Heisenberg’s efforts still survive. Using a pycnometer, the team found the density of one cube to be 18.535 g/cm3, lower than the 19.01 g/cm3 of solid uranium, indicating the cube was porous. Another cube contained 0.724% of uranium-235 (by mass), meaning it was natural rather than enriched uranium.
In March 1944, a German physicist named Karl Wirtz had reported that Nazi Germany’s stocks of heavy water had accidentally been diluted to 98.85% purity. A post-war sample from Haigerloch in 1947 found only 96.8% purity. The team also started from the records of aluminium giant Norsk Hydro to estimate that Nazi Germany had a maximum supply of 1,836 kg of pure heavy water during the war.
When the team worked upwards from here, including the design of the B8 pile, they concluded it had a neutron multiplication factor of 0.942. A reactor needs to have a factor of 1 to be critical. A difference of 0.058 is actually large. The team found that the uranium cubes in B8 were arranged too close together, preventing the heavy water from efficiently slowing the neutrons. The researchers estimated that increasing the heavy water’s purity to 100% would improve the factor to only 0.95.
They also simulated over 1,200 combinations of uranium and heavy water and found that the B8 pile would have required at least 2,994 kg of uranium and 3,411 litres of heavy water to become critical. B8 however, contained 1,538 kg of uranium and 1,549 kg of heavy water when it was dismantled. More importantly, the team also estimated all German facilities at the time combined possessed only 2,572 kg of uranium and 1,890 kg of 96.8%-pure heavy water — a significant shortfall and which could support a factor of 0.959 at best.
Industrial capacity
The first U.S. reactor, called Chicago Pile-1, first became critical on December 2, 1942, using natural uranium as fuel and graphite, rather than heavy water, as the moderator. So some nuclear historians have opined that Heisenberg et al. had simply picked the wrong moderator.
A 2025 study in The European Physical Journal H demurred, however: while the U.S. was able to source petroleum coke extremely low in boron and turn it into reactor-grade graphite, the graphite made from metallurgical coke in Germany was high in boron, which absorbs neutrons. The B8 pile used the graphite from the predecessor B7 pile, and the new study also found that it contained 3.4 weight-parts per million (wppm) of equivalent boron contamination. The Siemens graphite from earlier contained around 6.6 wppm. On the other hand, the Manhattan Project used graphite with around 1.4 wppm of boron. In simulations, it didn’t matter how far apart the uranium cubes in the B7 pile or in any previous piles could have been: the contamination made criticality impossible.
The significant difference between the Nazi German and the Manhattan Project programmes, then, seems to have been industrial capacity. The Americans developed and built multiple technologies to enrich uranium, set up reactors and chemical separation units, and a weapons laboratory, which together created 1.3 lakh jobs and invested $2.2 billion at the peak. Nazi Germany had nothing of similar scale.
Understandable concerns
According to a 2015 study in Angewandte Chemie, some uranium from the Nazi nuclear programme had not experienced a significant neutron flux and had probably come from Joachimsthal in today’s Czechia, indicating (in a limited sense) the absence of an industrial uranium enrichment programme.
Many questions about the Nazi nuclear programme still remain open, often rooted in different interpretations of common data. Some important ones are centered on whether German scientists deliberately delayed the bomb project. Some technical questions are open, too. For example, the purity of the heavy water at Haigerloch itself is unknown and the graphite’s boron content is based on archival rather than direct, firsthand information.
The overall picture today is neither that German physicists were incompetent nor that they were tantalisingly close to building a bomb. Their base of materials fell short of what a nuclear programme required and their supply chains were not good enough. Ultimately, given the information available at the time, and more so given Germany was also home to many of the world’s best nuclear physicists, the Manhattan Project scientists’ concerns were understandable.
mukunth.v@thehindu.co.in
Published – October 06, 2026 08:15 am IST
