Study Shows Nazi Germany's Final Reactor Could Not Achieve Criticality
Analysis of uranium cubes and records reveals the B8 reactor, built in 1945, lacked sufficient uranium and heavy water to sustain a nuclear chain reaction, disproving claims it was near success.
A new study of surviving uranium cubes and historical documents has definitively shown that Nazi Germany's final experimental nuclear reactor could not have achieved a self-sustaining chain reaction. The B8 reactor, constructed in 1945 under physicist Werner Heisenberg, would have needed approximately twice as much uranium and more than twice as much heavy water as Germany possessed to reach criticality.
The B8 reactor was a pile that used cubes of natural uranium suspended in heavy water. In a nuclear reactor, heavy water acts as a moderator, slowing neutrons released during fission. These slower neutrons are far more likely to trigger subsequent fission events in uranium atoms, increasing the chance of sustaining a chain reaction.
Germany's inability to secure necessary resources
The required quantities of uranium and heavy water for a working reactor vastly exceeded what Germany had available during the war. The only European facility capable of supplying heavy water to Germany was the Norsk Hydro plant in Vemork, Norway. Germany gained control of Vemork after occupying Norway in April 1940, but the heavy water needed for a functioning reactor was far beyond what the plant could produce for them.
Allied disruption of heavy water supply
Allied forces targeted the Norsk Hydro plant through sabotage and air raids to cripple this supply. A major American bombing raid struck Vemork on November 16, 1943. This bombing ultimately crippled heavy water production, severing Germany's primary source of the key moderator.
Why alternatives like graphite failed
Facing a shortage of heavy water, German researchers examined whether graphite could substitute for it. Graphite can also slow neutrons and was used successfully in the first sustained chain reaction achieved by American scientists in 1942. However, graphite available in German-controlled regions contained impurities, including boron. Boron absorbs neutrons, preventing a sustained chain reaction. This made the impure graphite entirely unsuitable as an alternative neutron moderator.
Final assessment of the reactor's capability
When American forces discovered and dismantled the B8 reactor in 1945, it had never achieved criticality-the point where a fission reaction becomes self-sustaining. Heisenberg later claimed the B8 experiment had come close to criticality. He argued that the device needed only a relatively modest increase in its uranium supply to become operational.
A recent study led by researcher Timothy Koeth challenges that assessment. By examining two surviving uranium cubes and studying historical records, technical reports, and archival documents, the Heisenberg significantly underestimated the additional material needed. The German scientists faced shortages they could not overcome with available resources. Based on World War II designs and materials, Nazi Germany lacked the resources to make either a heavy water or graphite reactor operational. The research explains why Germany's nuclear program failed to transition from experimental design to a working reactor.
