Heisenberg Said Nazi Germany Nearly Built A Working Nuclear Reactor: The Math Says Otherwise

Nazi Germany Could Not Have Built a Nuclear Reactor With Its Wartime Materials, Study Finds

After World War II, physicist Werner Heisenberg told the world that Nazi Germany’s final nuclear reactor had barely missed working. A new analysis in PNAS Nexus says the gap was far wider. Getting the machine to run on its own would have taken about twice the uranium and more than twice the heavy water it held.

Heisenberg designed that reactor, the B8 pile, and his team assembled it in April 1945 in the village of Haigerloch, just before Germany surrendered. The design was a tank of heavy water with hundreds of uranium cubes hanging inside on chains. It never produced a self-sustaining chain reaction, the cycle of atom splitting that keeps a reactor going on its own. Writing in the journal Nature in 1947, Heisenberg said the material on hand was “just insufficient,” and that adding only “a relatively small amount of uranium” would probably have done the job.

Those claims shaped decades of debate. Some historians took Heisenberg at his word. Others, including author Richard Rhodes, argued Germany’s real blunder was choosing heavy water, a rare form of water that slows the particles driving a chain reaction, over graphite, the carbon material American scientists used to build the world’s first working reactor in 1942. From 1939 to 1942, Americans framed their own program as an effort to catch up with Germany. Yet few technical evaluations of what Germany actually built exist.

Only 14 of Roughly 1,100 Uranium Cubes Are Known to Survive

Checking Heisenberg was difficult because the physical evidence scattered. Uranium and equipment from Germany’s program landed in American and Soviet intelligence collections or on the black market, and much of the original data stayed classified into the 1970s. Of roughly 1,100 uranium cubes Germany’s program possessed, only 14 are known to exist today.

A team of six researchers, including physicist Patrick Park of Columbia University, examined two of those cubes. Weighing and measuring one showed it was slightly less dense than pure uranium because of tiny air pockets inside the metal. A radiation scan of another confirmed it was natural uranium, meaning it had not been processed to make it more reactive.

Next came the paper trail. Wartime German documents, Norwegian heavy water production records, and postwar Allied surveys of German industry filled in details the original files never recorded. Heavy water purity came from a 1947 American measurement of samples recovered from the B8, since no wartime record appears to survive. Researchers fed everything into a computer simulation of the reactor and checked it against readings Heisenberg’s own team took in 1945. The two matched closely.

Nazi Germany’s Nuclear Reactor Needed Double the Uranium and 2.2 Times the Heavy Water

As built, the B8 scored 0.94 on the scale physicists use, where 1.0 marks a reactor that keeps itself going. A gap of about 6% sounds easy to close. The study’s model says otherwise. A working version would have needed roughly 6,900 pounds of uranium and 820 gallons of heavy water, far more than the 3,400 pounds and 370 gallons the B8 held. Heisenberg’s own 1948 estimate, growing the core “by not quite half,” amounted to about 50% more material, by the authors’ reading.

Pooling every bit of uranium and heavy water that Germany’s reactor teams are estimated to have had still left the B8 short of a chain reaction. Heavy water was the tighter bottleneck: a working reactor needed about 1.8 times what Germany had available, compared with 1.2 times for uranium. Its only European source, a plant in Vemork, Norway, made about 3.1 tons during the war. The French took some in 1940, a 1942 experiment exploded and destroyed more, and Norwegian commandos sank another batch in 1944, leaving at most about 2 tons for Germany by 1945.

Boron-Contaminated Graphite Ruled Out a German Nuclear Reactor Too

Graphite offered no escape route. The study modeled reactors built with the graphite Germany actually had, reused from an earlier experiment called B7, and paired with German uranium. No such reactor could reach a chain reaction at any realistic spacing between cubes. The culprit was boron, a natural impurity that soaks up the particles a chain reaction depends on. The graphite carried impurities estimated at the equivalent of 3.4 parts per million of boron, a level nearly identical to that of a commercial graphite that physicists Enrico Fermi and Leo Szilard tested in 1940 and rejected.

American graphite came out cleaner because it started with a rare low-boron petroleum coke, a refining byproduct, found in Pennsylvania in 1941, and then got careful purification. After a British naval blockade began in January 1940, German makers could no longer import American petroleum coke and turned to coal-based coke from the Ruhr region, which carried up to roughly twice the impurities of petroleum coke. Only purer uranium or access to American-style coke could have rescued a German graphite design, the authors found.

Heisenberg’s near-miss story does not hold up against the calculations. With heavy water or graphite, the materials Germany had in 1945 could not have produced a working reactor.

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