During the Second World War, the United States raced to build an atomic bomb, fearing that Nazi Germany would beat it to the prize. A new scientific study suggests that the German nuclear programme was far further from success than its leading physicist, Werner Heisenberg, claimed.
By examining surviving uranium cubes and archival documents, the scientists concluded that starting up Germany's last experimental reactor would have required far more uranium and heavy water than the country had at its disposal.
One route to an atomic bomb is to build a nuclear reactor capable of producing plutonium. Inside a reactor, uranium atoms split, releasing energy and additional particles called neutrons. Under certain conditions, these neutrons trigger new fission reactions, sustaining a chain reaction.
Germany's final attempt was the experimental reactor B-8, built in 1945. It used cubes of natural uranium immersed in heavy water, a substance that slows neutrons and helps sustain the chain reaction. Heavy water resembles ordinary water, but the hydrogen atoms in it contain an additional neutron.
Obtaining sufficient heavy water posed a serious problem for Germany. The only plant in Europe capable of supplying it in the quantities the German programme required was the Norsk Hydro facility at Rjukan, Norway. After Germany occupied Norway in April 1940, the plant came under German control. The Allies understood its significance and carried out sabotage and bombing raids. A major American air raid on 16 November 1943 helped halt heavy water production at the plant.
When American troops discovered and dismantled the B-8 reactor in 1945, it had never reached criticality, the state in which the chain reaction becomes self-sustaining. Heisenberg maintained that the reactor needed only a slight increase in uranium to start. However, a new study by a group of scientists led by Timothy Koeth, published in the journal PNAS Nexus, refutes this assessment.
According to the researchers' calculations, reaching a self-sustaining chain reaction in B-8 would have required roughly twice as much uranium and more than twice as much heavy water as the reactor contained. Germany had no such quantities of these materials during the war.
The scientists also tested whether German physicists could have used graphite instead of the scarce heavy water, as their American colleagues did when they launched the world's first reactor in 1942. It turned out that graphite available in territories under German control contained impurities, notably boron, which absorbs neutrons and disrupts the chain reaction. Thus, replacing heavy water with graphite was impossible.
The study shows that German scientists faced an insurmountable shortage of resources. More than 80 years after the war's end, the surviving uranium cubes have helped establish how far Nazi Germany was from building a working nuclear reactor.
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