The enigma of the uranium cubes that the Nazis used to create their nuclear program

Este es uno de los 664 cubos de uranio del reactor nuclear que los alemanes intentaron construir durante la Segunda Guerra Mundial.

This is one of 664 nuclear reactor uranium cubes that the Germans attempted to build during WWII.

In World War II, Germany and the United States competed in a fierce battle to see who could develop a nuclear program first.

In the early 1940s, several teams of German scientists began producing thousands of uranium cubes which would be the core of the reactors that were being developed as part of the nascent Nazi nuclear program.

The Germans were far from achieving a atomic bombBut they hoped these experiments would give them an advantage over the United States.

In fact, the Nuclear fision It was discovered in 1938 in Berlin.

The Germans Otto Hahn and Fritz Strassmann were the first to know how it could be split an atom, and that doing so would release a large amount of energy.

Years later, however, the Manhattan Project and his atomic bomb showed that in reality the Americans were far ahead of the Germans in atomic technology.

The uranium cubes, however, hold clues about the secrecy and the distrust between the two countries over the nuclear race.

Device to achieve nuclear fission.

Nuclear fission was discovered in Germany in 1938.

Today the whereabouts of the vast majority of the thousands of cubes that were manufactured is a mystery.

“It is hard know what happened to these cubes ”, tells BBC Mundo Alex Wellerstein, a historian specializing in nuclear weapons at the Stevens Institute of Technology in the United States.

“The records that exist are not the best.”

In the United States, only a dozen of them have been identified, which makes them a Precious treasure for researchers trying to reconstruct the beginnings of the nuclear age.

Failed experiment

One of the teams experimenting with uranium cubes was led by physicist Werner Heisenberg, a pioneer of quantum mechanics and a 1932 Nobel Prize winner.

Werner Heisenberg

Werner Heisenberg led one of the laboratories where uranium cubes were experimented with.

The project by Heisenberg and his colleagues was to tie 664 of these 5 cm cubes to hanging cables and submerge them in heavy water.

Heavy water is made up of oxygen and deuterium, an isotope of hydrogen that has twice the mass of ordinary hydrogen.

The idea is that submerging the cubes will unleash a chain reaction, but the experiment did not work.

According to Timothy Koeth, a researcher at the University of Maryland who has tracked the cubes, Heisenberg would have needed 50% more uranium and more heavy water for the design to work.

“Despite being the birthplace of nuclear physics and having almost two years of advantage over the US, there was not a imminent threat of a nuclear Germany at the end of the war ”, says Koeth in an article of the American Institute of Physics.

Nuclear bomb

The development of the atomic bomb demonstrated that the United States had a much more advanced nuclear program than Germany.

Confiscated material

In 1945, as the Germans tried to refine their efforts, the United States and the Allies they won the war.

At that time, the United States formed a mission to collect information and confiscate material related to the progress of the Germans in nuclear matters.

This is how american troops they reached Heisenberg’s laboratory in the small town of Haigerloch.

More than 600 uranium cubes were seized and shipped to the United States, according to a report from the US Pacific Northwest National Laboratory (PNNL).

The idea was to know how advanced the Germans were in nuclear technology and also to prevent the cubes from falling into the hands of the soviets, as Wellerstein explains.

In the end, the discovery of the cubes served to the American scientists to realize that the Germans were laggards in nuclear matters.


Still today the whereabouts are unknown of the vast majority of cubes.

Several of them are believed to have been used in the nuclear weapons development from the United States.

According to Wellerstein, some people began giving away the cubes as souvenirs, other scientists used them as testing material, and others fell for it. black market.

Still others remain as collector’s items.

In 2019, the magazine Physics Today managed to trace the location of 7 cubes that according to those who have them belonged to the nuclear experiments of the Nazis.

Three of them are in Germany: one in the Atomkeller Museum in Haigerloch, where Heinsenberg’s laboratory used to be; another is in the Museum of Mineralogy at the University of Bonn; and the third at the Federal Office for Radiation Protection in Berlin.

Two others are in the National Museum of American History in Washington DC; and another at Harvard University.

The magazine indicates that apparently a sixth cube was Rochester Institute of Technology, but due to a change in the rules of storage of radioactive material, the bucket was discarded.

A seventh cube is in the hands of the PNNL, and although it is known as “Heisenberg’s cube”, researchers are not 100% sure where it came from.

Another of the cubes is owned by Koeth himself, who received it as a curious birthday present in 2013.

Brittany Robertson

Brittany Robertson works on the identification of the uranium cubes.

Koeth collaborates with the PNNL to find out the whereabouts of the hundreds or thousands of cubes that still remain lost and to learn more about how they got to the United States.

In search of the pedigree

Beyond its historical and symbolic value, “really the cubes are not very valuable, you can’t do anything with them“, dice Wellerstein.

They are not dangerous either, since they generate a worksaction very weak. After grabbing one of them, “just wash your hands,” says the expert.

In August 2021, Jon Schwantes and Brittany Robertson, PNNL researchers, presented a project in which they describe how they work to identify the “Pedigree” of several of the cubes that have been found.

As Schwantes explains, the idea is to compare different cubes and try classify them.


The United States developed its nuclear program in part out of fear of Nazi advances in this technology. (Photo of Hiroshima after the atomic bomb of 1945).

To do this, they combine forensic methods and radiochronometry, which is the nuclear version of the technique that geologists use to determine age of a sample based on the content of radioactive isotopes.


Experts agree that the United States rapidly developed its nuclear program largely because of afraid for the Germans to do it before them.

And while some see these cubes as a historical curiosity, others see it as the trigger for the dangerous was nuclear weapons in which the world is trapped today.

“Nuclear weapons, nuclear energy, the Cold War, the planet as a nuclear hostage, all this was motivated by the effort that was generated from these 600-odd cubes “says Koeth in an article on the NPR chain.

In any case, the two big questions about hundreds or thousands of these cubes remain unanswered: how many still exist and where are they.

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