US scientists investigate uranium cubes from Nazi Germany’s nuclear programme

26 August 2021

Robertson with PNNL’s cube, which is in a protective case. Credit: Andrea Starr/PNNL Scientists at the US Pacific Northwest National Laboratory (PNNL) have obtained some initial results from their investigations into cubes of uranium which were part of Nazi Germany’s nuclear weapons programme, the American Chemical Society (ACS) reported on 24 August. Brittany Robertson, a doctoral student who works at PNNL will present the work at ACS Fall 2021 meeting. 

During World War II, Nazi Germany was racing to develop nuclear technology but before it could succeed, Allied forces disrupted the programme and confiscated some of the cubes of uranium which were part of it. The ultimate fate of most of that uranium is unknown, said ACS, but a few cubes thought to be associated with the programme are in the US and Europe. PNNL scientists now have some initial results from new methods being developed to confirm their provenance. These techniques might also help with investigations into illicit trafficking of nuclear material.

One cube is at PNNL, but no one is sure how it got there, says Dr Jon Schwantes, the project's principal investigator. Through collaborators, including Dr Timothy Koeth at the University of Maryland, the lab also has access to a few other cubes. "We don't know for a fact that the cubes are from the German programme, so first we want to establish that," Schwantes says. "Then we want to compare the different cubes to see if we can classify them according to the particular research group that created them."

In the early 1940s, several German scientists were competing to exploit nuclear fission to produce plutonium from uranium for the war. The teams included Werner Heisenberg's group in Berlin (later moved to Haigerloch to try to avoid Allied troops) and Kurt Diebner's team at Gottow. Uranium cubes were produced to fuel nuclear reactors at these sites. Measuring about 2 inches on each side, hundreds of the cubes were hung on cables submerged in heavy water, in which deuterium replaces lighter hydrogen. The scientists hoped radioactive decay of the uranium in the assemblies would unleash a self-sustaining nuclear chain reaction—but the design failed.

US and British forces seized some of the Heisenberg uranium cubes at Haigerloch in 1945, and more than 600 of these cubes were shipped to the USA. Some may have been used in the US nuclear weapons effort—which was launched in part due to fears that Germany was developing nuclear weapons—and a few belong to collectors and sites including PNNL. The whereabouts of the others, including hundreds of Diebner cubes, are unknown.

PNNL uses its sample to help train international border guards and nuclear forensics researchers to detect nuclear material. Although it is labelled as a Heisenberg cube, support for that assertion is anecdotal, says Robertson. "We didn't have any actual measurements to back up that claim." To prove the cube's origins, she began modifying some analytical techniques to combine with Schwantes's established forensic methods. Robertson used radiochronometry, the nuclear field's version of a technique that geologists use to determine the age of samples based on radioactive isotope content.

When the cubes were first cast, they contained fairly pure uranium metal. As time passed, radioactive decay transformed some of the uranium into thorium and protactinium. Robertson is adapting a radiochronometry procedure to better separate and quantify these elements in PNNL's cube. Their relative concentrations will show how long ago the cube was made. She is also refining this method to analyse rare-earth element impurities in the object. They could reveal where the original uranium was mined, which might indicate whether it was produced for the Heisenberg or Diebner group.

Meanwhile, Robertson and Schwantes are collaborating with PNNL's Dr Carlos Fraga to test the coatings, which the Germans applied to the cubes to limit oxidation. The PNNL team recently discovered that the cube at the University of Maryland is coated in styrene—an unexpected finding since Heisenberg's group used a cyanide-based coating. However, the team has now learned that some of the cubes from Diebner's group, which used a styrene-based coating, were sent to Heisenberg, who was trying to amass more fuel for his reactor. "We're curious if this particular cube was one of the ones associated with both research programmes," Schwantes says. "Also, this is an opportunity for us to test our science before we apply it in an actual nuclear forensic investigation."

Photo: Robertson with PNNL’s cube, which is in a protective case. Credit: Andrea Starr/PNNL

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