New evidence shows that this uranium cube is likely a relic of the Nazi A-bomb program

Enlarge / This is likely one of 664 uranium cubes from the failed nuclear reactor that German scientists tried to build in Haigerloch during World War II.

The Pacific Northwest National Laboratory (PNNL) has housed an unusual artifact from World War II for decades: a small cube made of solid uranium metal that measures about five centimeters on each side and weighs just under 2.5 kilograms. According to laboratory lore, the cube was confiscated from Nazi Germany’s failed nuclear reactor experiments in the 1940s, but this has never been experimentally confirmed.

PNNL scientists are developing new nuclear forensic techniques to help them confirm the pedigree of this cube – and others like it – once and for all. Eventually, these methods could also be used to track down the illegal trafficking of nuclear materials. PNNL’s Jon Schwantes and PhD student Brittany Robertson presented some of their first results this week at the American Chemical Society’s Fall Meeting (a hybrid virtual / in-person event).

University of Maryland physicist Timothy Koeth is an outside contributor to this ongoing research. He spent over seven years tracking down these rare artifacts from the Nazi Germany nuclear research program after receiving one as a gift. By 2019, he and a UMD colleague, Miriam Herbert, had tracked down 10 cubes in the United States: one at the Smithsonian, another at Harvard University, a handful in private collections – and of course the PNNL cube.

The special thing about these cubes is their historical significance. As we already reported:

The Manhattan Project in the USA was underpinned by the fear that German scientists under Adolf Hitler’s Nazi regime would strike the Allies with an atomic bomb. The Germans had a two-year lead, but according to Koeth, “fierce competition for finite resources, fierce interpersonal rivalries, and ineffective scientific management” caused significant delays in their progress towards a sustainable nuclear response. German nuclear scientists were divided into three isolated groups based in Berlin (B), Gottow (G) and Leipzig (L).

The renowned physicist Werner Heisenberg headed the Berlin group, and when the Allies advanced in the winter of 1944, Heisenberg moved his team to a cave under a castle in a small town called Haigerloch – today the location of the Atomkeller Museum. There the group built the B-VIII reactor. It resembled an “ominous chandelier,” Koeth said, because it consisted of 664 uranium cubes that were strung together with airplane cables and then immersed in a tank of heavy water that was shielded with graphite to prevent radiation exposure.

When the German scientists were fighting against time, the head of the Manhattan Project, Lieutenant General Leslie Groves, embarked on a covert mission called “Alsos” with the express aim of collecting information and materials on Germany’s scientific research. When the Allies finally approached, Heisenberg dismantled the B-VIII experiment and buried the uranium cubes in a field, tracking down important documents in a latrine. (Too bad Samuel Goudsmit, the poor physicist who had to dig them up.) Heisenberg himself escaped on his bike and carried a few cubes in a backpack.

As Heisenberg himself admitted, the last experiment by the German scientists failed because the amount of uranium in the cubes was insufficient to trigger a sustained nuclear reaction. But Heisenberg was confident that “a slight increase would have been enough to start generating energy”. A model described in a 2009 paper proves this, showing that the group would have needed only 50 percent more uranium cubes to get the design to work. If that were the case, our world could look very different today.

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The Alsos team is said to have brought the cubes seized from Berlin to the United States for use in the uranium processing plant in Oak Ridge. However, Koeth learned that the US did not need any additional raw material in April 1945. And there are no official records of the entry of dice into the country, so most of them were never recorded. The same applies to the approximately 400 uranium cubes that were used by the Gottow Group under the direction of Kurt Diebner.

According to PNNL records, your cube was stored in the DOE headquarters until 1989. At that point, it was brought into the lab as a radiation training tool for RadCAD, a series of hands-on courses in tracking down and intercepting illicit trafficking in radioactive materials.

Like its brothers, the PNNL cube is made of solid natural uranium metal. The cubes are only weakly radioactive and harmless to health. Because uranium is so dense, it essentially shields itself. All measured radiation comes from the surface. Nonetheless, the PNNL cube is kept in a double-plexiglass container to prevent exposure to radiation during handling and contamination of the cube through oxidation, according to Robertson.

Brittany Robertson with the PNNL cube in a protective case.Enlarge / Brittany Robertson with the PNNL cube in a protective case.

Andrea Starr / PNNL

The PNNL scientists were pretty sure they had a “Heisenberg Cube”; Among other things, the cube is notched to better hang on the cables used in the German reactor effort. But this evidence is largely anecdotal, according to Robertson and Schwantes. The cube was analyzed using high resolution gamma spectroscopy as early as 2002 to estimate its age, but these results were inconclusive. “That is usually not sensitive enough to provide an exact age for the cube,” said Schwantes.

A few years ago, when the PNNL cube was being repackaged, Schwantes and a colleague shaved a few small samples for analysis from the metal. They hoped to be able to confirm once and for all that it was one of the Heisenberg’s dice – or possibly a “thief’s dice”. Robertson’s work – part of her PhD – is to examine these samples using her own modified analytical techniques in conjunction with PNNL’s standard nuclear forensic methods.

For example, radiochronometry is a popular method among geologists. It is widely used to determine the age of a uranium-rich material by measuring the byproducts of uranium decay, namely the radioactive isotope thorium-230 and protactinium. Robertson’s modified approach involves the simultaneous separation of thorium and protactinium in the hope that the relative concentrations of the materials will provide an indication of when the cube was made. In addition, the analysis of the rare earth metal impurities could help PNNL scientists determine where the original uranium was mined.

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“Kind of a long shot”

So far, initial findings have confirmed that at least one of the three cubes tested at the PNNL is natural uranium. There are also preliminary results from Robertson’s analysis of the coatings the Germans applied to the cubes to keep oxidation in check. Cyanide-based coatings were used by the Berlin group, while the Diebner Gottow group used styrene-based coatings. If the relevant signatures could be measured precisely, the team could recognize whether a particular cube comes from the Berlin or Gottow group.

“As far as we know, no one else took this measurement,” said Robertson. “And I have to be honest, I thought it would be a long way. I never thought that an organic substance would sit next to uranium metal for so many decades and still be detectable.”

This long shot was worth it. Koeth’s cube was among those tested and showed a styrene coating – a bit of a surprise since Koeth’s historical detective work followed the cube to the Berlin group. It turns out, however, that Diebner sent some of his group’s cubes to Berlin when he was looking for more fuel for his reactor. So Koeth’s cube could possibly have been used by both groups.

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