Nazi atomic bomb reactor was nowhere near ready, study finds

By LineZotpaper

Published21 October 2026

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A new study using historical documents and forensic analysis of uranium cubes has concluded that the German B8 nuclear reactor, the centrepiece of the Nazi atomic bomb program, was far from achieving a self-sustaining fission reaction, contradicting the postwar claims of physicist Werner Heisenberg that it was almost ready.

Heisenberg, who led the project, wrote in 1947 that the reactor lacked only a 'relatively small amount of uranium' to work. But research published in the journal PNAS Nexus argues he was off by a factor of about two.

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'My results argue that Heisenberg was off by roughly a factor of two,' said first author Patrick Park, a doctoral student at Princeton University who co-wrote the study while at Columbia University.

Park said Heisenberg's mistake was understandable: His calculations were only approximate, and the project was relocated to a rock cellar in the town of Haigerloch, Germany, after Berlin was bombed.

'People were dying or starving and so these reactor calculations were not his first priority,' while many details of neutron physics were unknown at that time, Park told Live Science in an email. 'Given these circumstances, I think he did the best he could.'

The B8 reactor consisted of five concentric circles of uranium cubes suspended on chains within a tank of heavy water. By combining archival records of the cubes, heavy water and graphite used by the German scientists with measurements of two surviving cubes, the researchers created a mathematical model of the reactor. It showed the reactor contained only about 370 gallons (1,400 litres) of heavy water, rather than more than 780 gallons (roughly 3,000 litres) needed, and about half the required uranium, 664 cubes totalling 3,390 pounds (1,540 kilograms), when at least 1,200 cubes would have been needed.

The study also examined whether the Germans could have built a graphite-moderated reactor instead, like the one developed in Chicago. The authors found that was impractical because German coal used to produce graphite contained high levels of neutron-absorbing boron.

'We conclude Germany could not have built heavy water nor graphite reactors during World War II,' the researchers wrote.

Mark Walker, a historian at Union College in New York and author of 'Hitler's Atomic Bomb,' who was not involved in the study, agreed that a graphite reactor was unfeasible but cautioned that the analysis is counterfactual. 'This paper argues that if the Germans had done something they did not do, then this would have failed,' he said. 'While this is counterfactual history, it is an interesting engineering analysis.'

Analysis

Why This Matters

Resolves a long-standing historical debate over how close Nazi Germany came to building an atomic bomb.
Reinforces that the Allies' atomic monopoly was not nearly challenged by German efforts, altering understanding of World War II's technological race.
Demonstrates how modern 'nuclear archaeology' methods can revisit historical claims with forensic evidence.

Background

The German nuclear program, known as the 'Uranverein' (Uranium Club), was led by Werner Heisenberg and other top physicists. During the war, they focused on building a nuclear reactor to achieve a self-sustaining chain reaction, a stepping stone to a bomb. The B8 reactor at Haigerloch was their most advanced attempt, using heavy water produced at a single Norwegian factory. After Germany's surrender, Heisenberg and other scientists were interned and later wrote accounts of their work, with Heisenberg claiming success was very close. The new study systematically tests that claim.

Key Perspectives

Werner Heisenberg (as recalled by 1947 writings): The reactor was nearly ready; only a small additional amount of uranium was needed to achieve a chain reaction. The researchers (Patrick Park and team): Quantitative analysis of surviving cubes and historical records shows the reactor had less than half the required heavy water and about half the uranium. Heisenberg's estimate was off by roughly a factor of two, though understandable given wartime conditions. Historian Mark Walker: The engineering analysis is interesting but the paper examines a counterfactual scenario (if Germany had pursued a graphite reactor they did not actually build). It does not settle all debates but agrees that a graphite reactor was impossible.

What to Watch

Whether further isotopic analysis of the remaining uranium cubes yields additional constraints on the reactor's potential performance.
Future historical studies may re-examine other German nuclear claims using similar forensic methods.
The study may prompt new assessments of how close other wartime nuclear programs (e.g., Japanese) came to success.

Sources

Uranium from World War II reveals how close the Nazis were to an atomic bomb · Latest from Live Science

Checked

9 claims in this story checked against 1 source: 8 supported.

Show each claim

SupportedThe B8 reactor was nowhere near ready, according to a study.Latest from Live Science: “Contrary to Heisenberg's claims, the heavy-water reactor was not nearly ready, and a working graphite reactor was impossible.”
Not in the sourcesThe B8 reactor was the centrepiece of the Nazi atomic bomb program.Latest from Live Science
SupportedThe study concluded the B8 reactor was far from achieving a self-sustaining fission reaction.Latest from Live Science: “The new research indicates that the German B8 reactor had less than half the amount of "heavy water" and about half the amount of uranium it would have needed for a continuous chain reaction.”
SupportedHeisenberg wrote in 1947 that the reactor needed only a relatively small amount of uranium to work.Latest from Live Science: “Heisenberg, who led the reactor's development, claimed the reactor was "just insufficient" to create a self-sustaining fission reaction and that it lacked only a "relatively small amount of uranium" to work.”
SupportedPark said Heisenberg was off by about a factor of two.Latest from Live Science: “"My results argue that Heisenberg was off by roughly a factor of two," said first author Patrick Park”
SupportedThe reactor contained about 370 gallons (1,400 litres) of heavy water, rather than the more than 780 gallons (roughly 3,000 litres) needed.Latest from Live Science: “the reactor contained only about 370 gallons (1,400 liters) of heavy water, instead of the more than 780 gallons (roughly 3,000 liters) it would have needed”
SupportedThe reactor contained 664 uranium cubes weighing 3,390 pounds (1,540 kilograms) in total, and would have needed at least 1,200 cubes.Latest from Live Science: “The reactor contained a total of 664 cubes, each weighing about 5.11 pounds (2.32 kilograms), for a total of 3,390 pounds (1,540 kilograms) of uranium. ... the B8 reactor would have needed at least 1,200 cubes to work.”
SupportedThe researchers concluded Germany could not have built heavy-water or graphite reactors during World War II.Latest from Live Science: “"We conclude Germany could not have built heavy water nor graphite reactors during World War II," the researchers wrote.”
SupportedWalker said the analysis was counterfactual history but an interesting engineering analysis.Latest from Live Science: “"While this is counterfactual history, it is an interesting engineering analysis."”