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.
asserted
many → emigrate → weapon
Many of these scientists had seen firsthand the horrors Nazi Germany was perpetrating in Europe; many were Jews who were escaping persecution themselves.
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who → see → persecution
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.
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stake → show → world
The Manhattan Project tested its first atom bomb on July 16, 1945, and delivered two to the U.S. military in July-August.
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Project → test → August
Part of their sense of urgency was rooted in doubts about whether Nazi Germany was also building a bomb, including an effort led by physicist and Nobel laureate Werner Heisenberg.
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Germany → root → Heisenberg
Their 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.
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team → surrender → south
The setup, called the B8 pile, had 664 cubes of uranium suspended on chains in around 1,400 litres of heavy water.
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setup → call → 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.
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itself → surround → water
Heisenberg’s team had taken six years to build B8 but had little to show for it.
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team → take → it
When a few neutrons were shot into the reactor, the uranium multiplied them into more — but no chain reaction followed.
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reaction → shoot → more
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.
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quantity → end → trick
According to a new study in PNAS Nexus, by researchers from the U.S., Heisenberg & co. were actually way off the mark.
uncertain
Heisenberg → accord → mark
Only around 14 of the 1,100 or so uranium cubes made for Heisenberg’s efforts still survive.
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14 → make → efforts
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.
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cube → use → uranium
Another cube contained 0.724% of uranium-235 (by mass), meaning it was natural rather than enriched uranium.
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it → contain → 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.
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stocks → name → purity
A post-war sample from Haigerloch in 1947 found only 96.8% purity.
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sample → find → 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.
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Germany → start → 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.
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it → work → 0.942
A reactor needs to have a factor of 1 to be critical.
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reactor → need → 1
The team found that the uranium cubes in B8 were arranged too close together, preventing the heavy water from efficiently slowing the neutrons.
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cubes → find → neutrons
The researchers estimated that increasing the heavy water’s purity to 100% would improve the factor to only 0.95.
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increasing → estimate → 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.
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pile → simulate → water
B8 however, contained 1,538 kg of uranium and 1,549 kg of heavy water when it was dismantled.
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it → contain → water
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.
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which → estimate → 0.959
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.
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reactor → call → moderator
So some nuclear historians have opined that Heisenberg et al. had simply picked the wrong moderator.
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. → opine → 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.
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which → demur → 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.
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it → use → contamination
The Siemens graphite from earlier contained around 6.6 wppm.
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graphite → contain → wppm
On the other hand, the Manhattan Project used graphite with around 1.4 wppm of boron.
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Project → use → 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.
uncertain
criticality → matter → piles
The significant difference between the Nazi German and the Manhattan Project programmes, then, seems to have been industrial capacity.
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difference → seem → German
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.
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which → develop → peak
Nazi Germany had nothing of similar scale.
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Germany → have → scale
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.
uncertain
uranium → accord → programme
Many questions about the Nazi nuclear programme still remain open, often rooted in different interpretations of common data.
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questions → remain → data
Some important ones are centered on whether German scientists deliberately delayed the bomb project.
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scientists → center → project
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.
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content → base → information
The overall picture today is neither that German physicists were incompetent nor that they were tantalisingly close to building a bomb.
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they → build → bomb
…and 2 more, not listed.
The Aporia
