No. The B8 experimental assembly built at Haigerloch in March–April 1945 could not sustain its own chain reaction. A 2025 computer reconstruction estimates its neutron multiplication factor at 0.952, below the threshold of 1. The study’s redesigned configurations approach that threshold, but require far more heavy water than the German team is estimated to have had—and reaching criticality would not, on its own, have made the assembly a practical power reactor.
What “working” means for B8
B8 was an experimental reactor pile, not a power station or an atomic bomb. For this question, “work” can mean two different things: becoming critical, so that each fission generation produces enough neutrons to sustain the next, or operating as a useful reactor with systems to control the reaction, remove heat and protect people from radiation. The evidence supports a clear answer on the first: the historical B8 configuration was subcritical. The second would have required engineering that the wartime assembly did not have.
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The neutron multiplication factor, keff, expresses whether a chain reaction grows, holds steady or dies away. At 1, each generation of fissions produces, on average, enough neutrons to sustain the next; below 1, the reaction dwindles. The 2025 Serpent 2 simulation by Wiesenmüller, Huaccho Zavala, Buck and Thorwart estimates keff at 0.952 for its reconstruction of B8.
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The final B8 experiment was assembled in a former beer cellar at Haigerloch, in southwest Germany, during the closing weeks of the war. Accounts differ on the precise start: the municipal history places it in late March or early April 1945, while a 2023 historical analysis describes the final experiment as taking place in March. “March–April 1945” is the most careful concise date.
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The design brought together natural uranium metal, heavy water (deuterium oxide, D2O) and graphite. Uranium cubes were suspended on aluminum wires in a vessel of heavy water; graphite acted as a neutron reflector. In the 2025 study’s best reconstruction, the pile contained 664 uranium cubes, each 5 cm on an edge, arranged in 78 chains. Its reference model estimates 1,360 litres of heavy water. These are reconstructed values, not a complete inventory taken from an intact machine: the apparatus was destroyed, records are incomplete, and the original April 1945 report is unavailable.
The material gap: too little heavy water
The 2025 study identifies heavy water as the clearest quantified constraint. In the reconstructed historical arrangement, the modeled keff is 0.952. The authors also examined changes to the geometry and fuel arrangement. Their optimized scenarios use about 3,170 litres of heavy water to approach criticality—roughly 2.33 times the estimated 1,360 litres available for B8.
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| Configuration in the 2025 study | Estimated heavy water | Modeled neutron multiplication | What the figure establishes |
|---|---|---|---|
| Reconstructed historical reference | 1,360 litres | keff = 0.952 | The modeled B8 arrangement remains below criticality. |
| Optimized modeled scenarios | About 3,170 litres | Near criticality | Under the study’s assumptions, altered arrangements with substantially more moderator can approach the threshold. |
The authors’ conclusion is that, under their assumptions, the amount of heavy water—not the total amount of uranium—was the limiting material. This is a result from modern modeling, not proof that the German team could have obtained the extra moderator, built the modeled arrangement or completed it in time. The study does not establish that any such redesign would have been ready for wartime operation.
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Why the historical number differs from the modern estimate
Wiesenmüller and colleagues also report that a value of keff = 0.85 is attributed to Heisenberg and Wirtz’s 1947 report. They compare it with their own estimate of 0.952, but the original report is no longer available and they say the difference remains unexplained. Neither figure should be presented as an uncontested direct measurement of a surviving B8 machine: the 0.952 is a modern simulation estimate, while 0.85 is a value attributed to the missing historical report.
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One input examined in the reconstruction is moderator purity. A 96.8% D2O purity measurement was made in 1947 on B8 heavy-water samples held at the U.S. National Institute of Standards and Technology (NIST). The simulation considers how light-water and boron contamination could affect neutron balance. The purity result informs that analysis, but it does not resolve the discrepancy between the two multiplication figures.
Why criticality would not have meant a working power reactor
The 2023 historical analysis by Popp and de Klerk describes the German wartime assemblies as subcritical and says they did not attempt the stated goal of a self-sustaining chain reaction. It identifies further gaps between an experiment and an operable reactor: the assemblies had no control rods and no provisions for heat removal or radiation protection. A modeled route toward criticality therefore answers only a question about neutron balance, not whether the system could safely generate useful power.
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The same analysis counts 22 experiments from 1940 to 1945 in Berlin, Leipzig, Hamburg, Gottow and Haigerloch. That broader record helps put B8 in context as one stage of experimental work; it does not show that the program had already solved the reactor engineering problems required for operation.
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Was Germany close to an atomic bomb?
B8’s status as a subcritical experimental reactor does not support describing it as a near-miss bomb project. The figures discussed here concern whether an experimental uranium-and-heavy-water pile could sustain a chain reaction. They do not establish a bomb design, a completed weapon or an imminent ability to build one. Keep the reactor-criticality question separate from claims about weapons.
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Where to see the Haigerloch site
The City of Haigerloch documents the experiment and the local Atomkeller Museum, which presents the historical site and a reconstruction. It offers a place-based way to explore the story beyond the modeled neutron calculations.
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