The Uranium Cubes From a Nazi Nuclear Reactor

In the summer of 2013, Physicist Timothy Koeth of University of Maryland received an unexpected gift from one of his friends. It was a small metallic cube, 2 inches per side, wrapped in a piece of paper. Koeth recognized the cube immediately from old grainy photos he saw in books on nuclear history, and if there was any doubt, the accompanying note settled it. It read: “Taken from Germany, from the nuclear reactor Hitler tried to build. Gift of Ninninger.”

This cube was one of hundreds that German nuclear scientists built for an experimental reactor, before the cubes were confiscated by Allied forces and shipped to the United States. Aside from a handful of cubes, like the one sitting on Timothy Koeth’s desk today, the whereabouts of the rest are unknown.

nazi uranium cube reactor

Photo: John T. Consoli, University of Maryland

Germany had a two-year head start on nuclear technology over the Americans. Their physicists were toying with atoms even before the war started. On December 1938, German chemist Otto Hahn and his assistant Fritz Strassman discovered nuclear fission for the first time. Within four months, German scientists were discussing nuclear reactors. They called them Uranmaschine, or “uranium machine”, and themselves the Uranverein or “Uranium Club.”

Work began shortly at the Georg-August University of Göttingen. But then, Hitler invaded Poland, and many physicist were called to military training, and the Uranverein was disbanded—but not for long. The German Army had already heard about nuclear energy and its potential application. Two weeks after Word War 2 began, a meeting was called in Berlin and the country’s top physicists were invited, including Walther Bothe, Siegfried Flügge, Hans Geiger, Otto Hahn, and Werner Heisenberg.

In one of these meetings, Heisenberg famously said:

…in principle atomic bombs could be made.... it would take years.... not before five.

I didn't report it to the Fuehrer until two weeks later and very casually because I did not want the Fuehrer to get so interested that he would order great efforts immediately to make the atomic bomb. Speer felt it was better that the whole thing should be dropped and the Fuehrer also reacted that way.

Nevertheless, it was Heisenberg’s work that laid the theoretical foundations for all subsequent research on that topic. Heisenberg correctly foresaw that a nuclear reactor could be built in two different ways, each posing its own difficulties. One, by using enriched uranium and an easily obtained moderator such as ordinary water, and two, by using natural uranium and heavy water or extremely pure graphite as a moderator. For some reason, they chose the later.

nazi uranium cube reactor

Reconstructed research reactor in the “Atomkeller-Museum”, Haigerloch. Photo: LepoRello/Wikimedia Commons

To find out how this could work, German scientists devised an experimental reactor where they made 664 cubes of natural uranium and strung them together with aircraft cable. Then, they submerged the cubes in a tank of heavy water shielded by graphite to prevent radiation exposure. This was known as the B-VIII nuclear reactor, and it was built in a secret cave in Haigerloch, in southern Germany, between February and April 1945, just before the end of the war in Europe.

With the Allies closing in on Germany, Heisenberg’s scientists quickly disassembled B-VIII and buried the uranium cubes in a nearby field. The heavy water was hidden in barrels, and some of the more significant documentation was hidden in a latrine. Heisenberg himself escaped by bicycle, carrying a few cubes in a backpack. The Allies recovered the buried cubes and shipped them to America, but what happened after that is not entirely clear.

nazi uranium cube reactor

Members of the US Alsos mission to Germany digging up the uranium cubes from a field. Photo: Samuel Goudsmit

Recent analysis suggest that the reactor the Germans built could not have achieved criticality, because they did not have enough uranium. In Heisenberg’s own words, “The apparatus was still a little too small to sustain a fission reaction independently, but a slight increase in its size would have been sufficient to start off the process of energy production.”

As a matter of a fact, the Germans did have an additional supply of 400 cubes of the exact size and shape, that were being utilized on another reactor experiment. Had Germany pooled their resources together instead of dividing them, their combined inventory would have been more than enough to have achieved criticality in the B-VIII reactor, and Germany would have been significantly closer to their goal of building a working nuclear reactor, or maybe even a weapon.

After the war, the additional cubes the Allies failed to seize created a black market in uranium throughout Eastern Europe. For years, US officials would receive ransom notes asking hundreds of thousands of dollars, threatening to sell the uranium cubes to US’s enemies. The profit seekers were not aware that these were raw uranium, and not enriched, and not that valuable or radioactive. The global market price for the metal at that time was only about 6 dollars per pound. The majority of the cubes eventually ended up in the Soviet Union.

nazi uranium cube reactor

The entrance to the laboratory of the B-VIII reactor experiment, underneath a castle in Haigerloch, Germany. The site is now home to the Atomkeller Museum. Photo: AIP Emilio Segrè Visual Archives, Goudsmit Collection

What became of the cubes that were shipped to the United States is a mystery. It was possible that some of them may have ended up at Oak Ridge, where they were melted, processed and manufactured into American atomic weapons. Since the discovery of the cube by Timothy Koeth, ten others were tracked down to private and public collections. The Smithsonian Institution has one, and so does Harvard University.

Koeth says the “cubes represent a bygone era in science when researchers were just beginning to discover the subatomic world.”

“We hope that by finding the cubes and piecing together what happened to them we will return a small amount of context to forgotten objects that have played a monumental role in human history. The cubes and the science they represent still shape modern life decades later,” he added.

There is a museum in Haigerloch today, housed inside the same rock cellar where German scientists built their failed nuclear reactor. The Atomkeller Museum was opened in 1980 and shows the history of German atomic research from Otto Hahn to the present day. There is also a replica of the research reactor with the hanging cubes, as well as two of the original uranium cubes.

nazi uranium cube reactor