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No. The B8 experimental reactor 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 critical threshold of 1. A redesigned version might have approached criticality, but the modeled design required far more heavy water than the German team had. And reaching criticality would not, by itself, have made the assembly a safe, useful power reactor.
What “working” means for the B8 reactor
For B8, “work” can mean two different things: whether the assembly could sustain a chain reaction, or whether it could operate as a practical power reactor. The answer to the first is no for the historical configuration. The second would have required engineering systems the experimental assembly did not have.
In a fission chain reaction, neutrons released by one generation of fissions cause enough fissions in the next generation to keep the process going. The effective neutron multiplication factor, keff, describes that balance: below 1, the chain reaction dies away; at 1, it is self-sustaining; above 1, it grows.
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B8 was assembled in a former beer cellar at Haigerloch in southwest Germany during the final weeks of the war. Accounts place its start in March–April 1945: the City of Haigerloch dates it to late March or early April, while a 2025 reconstruction describes the final experiment as occurring in March.
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The assembly used natural uranium metal cubes as fuel, heavy water (deuterium oxide, D2O) as a moderator to slow neutrons, and graphite as a neutron reflector. The cubes hung on aluminum wires inside a vessel of heavy water. The surviving apparatus was destroyed, and the records are incomplete, so the exact arrangement has to be reconstructed rather than inspected directly.
What the modern reconstruction models
A 2025 Serpent 2 study by Wiesenmüller, Huaccho Zavala, Buck, and Thorwart models 664 uranium cubes, each 5 cm on an edge, arranged in 78 chains. Its reference case assumes 1,360 litres of heavy water, an estimated amount for the historical assembly. These are reconstruction inputs, not a complete inventory confirmed by intact machine records.
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Why the historical B8 design fell short
For the reconstructed historical configuration, the 2025 simulation calculates keff = 0.952. That is close to 1 compared with a widely subcritical assembly, but it remains below the threshold: the model does not predict a self-sustaining chain reaction in B8 as built.
The study identifies heavy water as the clearest quantified material bottleneck under its assumptions. Its modeled arrangements that came close to criticality used about 3,170 litres—approximately 2.33 times the estimated 1,360 litres available. The authors conclude that moderator supply, rather than total uranium quantity, limited the modeled design.
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This is a result of a modern simulation, not a demonstration that the wartime team could have built a critical reactor if given that quantity. The redesign depends on selected geometry and material assumptions; the study does not establish that the additional heavy water could have been obtained in time or that a practical operating reactor could then have been engineered.
Why historical and modern neutron figures differ
The 2025 paper also reports a heavy-water purity measurement of 96.8%, measured in 1947 on B8 samples held at the U.S. National Institute of Standards and Technology. Because impurities can affect neutron balance, the authors examine contamination by light water and boron in their reconstruction.
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The paper attributes a value of keff = 0.85 to a 1947 report by Heisenberg and Wirtz. That is lower than the modern study’s calculated 0.952 for its historical reference case. The original April 1945 B8 report is unavailable, and the authors say they cannot explain the discrepancy. Neither figure should be presented as an uncontested direct measurement of an intact reactor: 0.952 is a modern simulation estimate, while 0.85 is a value attributed to the historical report by the 2025 authors.
Criticality would not have made B8 a power reactor
A reactor that reaches criticality sustains a chain reaction; that does not automatically mean it can produce useful power safely. The 2023 historical analysis by Popp and de Klerk describes the German wartime assemblies as subcritical and says they did not attempt to achieve a self-sustaining chain reaction. It also notes that the experimental assemblies lacked control rods and provisions for heat removal or radiation protection.
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Those omissions matter independently of the heavy-water shortfall. The evidence supports a conditional conclusion about a modeled reactor concept approaching criticality with more moderator. It does not show that the German program had an operational power-reactor design, much less a route from B8 to a nuclear weapon.
Where B8 fits in Germany’s wartime experiments
Popp and de Klerk count 22 German reactor experiments between 1940 and 1945, at Berlin, Leipzig, Hamburg, Gottow, and Haigerloch. Their analysis places B8 in a series of subcritical experiments rather than describing it as a reactor that nearly operated. The key distinction is between measuring neutron multiplication in an experimental assembly and actually achieving a self-sustaining chain reaction.
Readers interested in the site can visit the Atomkeller Museum, which presents the Haigerloch experiment and a reconstruction of the apparatus.
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