The Pacific Northwest National Laboratory (PNNL), has housed a unique artifact from World War II for decades: a cube of solid uranium. It measures approximately two inches in each direction and weighs just under 2.5 kgs. Although it is believed that the cube was taken from Nazi Germany's nuclear reactor experiments that failed in the 1940s, this has never been tested.
Scientists at PNNL are working on new nuclear forensic methods that will allow them to confirm the pedigrees of this cube and others like it once and for all. These methods could be used in the future to trace illicit nuclear material trafficking. Jon Schwantes from PNNL and Brittany Robertson, a graduate student, presented their initial findings at the American Chemical Society's fall meeting (a hybrid virtual/in person event).
Timothy Koeth, University of Maryland physicist, is one of the outsider collaborators in this ongoing study. After receiving one as a gift, he spent seven years searching for these rare artifacts from Nazi Germany's nuclear program. Miriam Herbert, a UMD colleague, and he had already found 10 cubes in the US, one at Harvard University, one at Smithsonian, and one at Harvard University. There were also a few cubes in private collections, as well as the PNNL cube.
These cubes are unique because of their historical significance. We have already reported:
The Manhattan Project was based on the fear that Nazi-era German scientists would defeat the Allies to a nuke bomb. Although the Germans had a two year lead, Koeth says that "fierce competition over finite resource, bitter interpersonal rivalries and ineffectual scientific administration" caused significant delays in progress towards a sustained nuclear reaction. German nuclear scientists were divided into three separate groups, with headquarters in Berlin (B), Gottow(G), and Leipzig. The Berlin group was headed by Werner Heisenberg, a well-known physicist. As the Allies advanced in 1944, Heisenberg moved the team to a cave below a castle in Haigerlochnow home of the Atomkeller museum. The group then built the B-VIII nuclear reactor. The 664 uranium cubes were strung together using aircraft cable. They were then placed in a tank filled with heavy water and shielded with graphite to protect them from radiation exposure. The German scientists were racing against the clock, so Lieutenant General Leslie Groves, Manhattan Project leader, launched a covert mission called "Alsos" with the express purpose to gather information and materials about Germany's scientific research. Heisenberg dismantled the B-VIII experiment, buried the uranium pieces in a field and retrieved key documents in a latrine. Samuel Goudsmit was the poor physicist that had to excavate those cubes. Heisenberg escaped on a bicycle with a few cubes and a backpack.
Heisenberg acknowledged that the German scientists failed to succeed in their final experiment because the amount of uranium contained in the cubes was too low to cause a sustained nuclear reaction. Heisenberg believed that a slight increase in the cube's size would have been enough to initiate the process of energy production. The model, which was described in a 2009 paper, shows that the group would have only needed 50% more uranium cubes for the design to work. It might have made a huge difference in our world today.
According to reports, the Alsos team brought the Berlin-seized cubes to the United States to be used in Oak Ridge's uranium processing plant. Koeth discovered that the US did not require additional feedstock material by April 1945. There is no record of cubes entering the United States, so many of them are not known to have been found. The same goes for the approximately 400 uranium cubes used by the Gottow group, headed by Kurt Diebner.
According to PNNL legend, the cube was kept at DOE headquarters up until 1989. It was then brought to the laboratory for radiation training, RadCAD. RadCAD is a series of courses that teaches how to detect and intercept illicit radioactive material trafficking.
Like its brethren the PNNL cube is made from solid natural uranium. They are not radioactive and pose no health risk. Because uranium has such a dense structure, it acts as a shield. Measured radiation is emitted from the surface. Robertson states that the PNNL cube should be kept in a double-plexiglass box to protect it from radiation exposure during handling or contamination by oxidization.
Scientists at PNNL were confident that they had a Heisenberg cube. The cube was notched to allow it to be attached to the cables used in German reactor attempts. Robertson and Schwantes say that this evidence is largely anecdotal. However, high-resolution Gamma spectroscopy was used to analyze the cube in 2002 in order to estimate its age. The results were not conclusive. Schwantes stated that this method is not sensitive enough to give an accurate age for the cube.
Schwantes and a colleague took small pieces of the metal off the PNNL cube to analyze it. This was many years ago. They wanted to be certain that the cube was one of the Heisenberg cubes or a "Diebnercube". Robertson's doctoral thesis research focuses on studying those samples using modified analytic techniques in combination with PNNL's nuclear forensic methods.
Radiochronometry, for example, is a very popular method among geologists. This method is used to determine the age and radioactivity of uranium-rich materials by measuring the radioactive byproducts of their decay, such as thorium-230 or protactinium. Robertson's modified method involves simultaneously separating the protactinium and thorium in the hope that their relative concentrations will provide some indication as to when the cube was created. PNNL scientists could also benefit from analyzing rare-earth element impurities to determine the origin of uranium.
It's a long shot.
Initial findings at PNNL have shown that natural uranium is at least one of the cubes under test. Robertson also has preliminary results of his analysis of the coatings that the Germans used to prevent oxidation. The Berlin group used cyanide-based coatings, while the Gottow group used Styrene-based coats. It would be possible for the team to determine if a cube is from the Gottow or Berlin group if one could measure the signatures.
Robertson stated, "As far we know, nobody else has done this measurement." It was a very long shot, and I must be honest. It was impossible for me to believe that an organic could be detected after so many decades of sitting alongside uranium metal.
It was worth the effort. Koeth's cube was one of the tested. It revealed a styrene coat, which is somewhat surprising considering that Koeth had done historical sleuthing to track the cube back to the Berlin group. It turns out that Diebner actually sent some cubes from his group to Heisenberg in Berlin, when he needed more fuel for his reactor. Both groups may have used Koeth's cube.