For decades, the Pacific Northwest National Laboratory (PNNL) has been home to an unusual artifact from World War II: a small cube of solid uranium metal, measuring about two inches on each side and weighing just under 2.5 kilograms. Lab lore holds that the cube was confiscated from Nazi Germany's failed nuclear reactor experiments in the 1940s, but that has never been experimentally verified.

PNNL scientists are developing new nuclear forensic techniques that should help them confirm the the pedigree of this cube—and others like it—once and for all. Those methods could also eventually be used to track illicit trafficking of nuclear material. PNNL's Jon Schwantes and graduate student Brittany Robertson presented some of their initial findings this week at the fall meeting of the American Chemical Society (a hybrid virtual/in-person event).

University of Maryland physicist Timothy Koeth is among the outsider collaborators in this ongoing research. He has spent over seven years tracking down these rare artifacts of Nazi Germany's nuclear research program, after receiving one as a gift. As of 2019, he and a UMD colleague, Miriam Herbert, had tracked down 10 cubes in the US: one at the Smithsonian, another at Harvard University, a handful in private collections—and of course, the PNNL cube.

What makes these cubes so special is their historical significance. As we reported previously:
https://arstechnica.com/science/2019/06/physicists-hunt-uranium-cubes-to-shed-light-on-germanys-failed-nuclear-reactor/

Underpinning the Manhattan Project in the US was the fear that German scientists under Adolf Hitler's Nazi regime would beat the Allies to a nuclear bomb. The Germans had a two-year head-start, but according to Koeth, "fierce competition over finite resources, bitter interpersonal rivalries, and ineffectual scientific management" resulted in significant delays in their progress toward achieving a sustained nuclear reaction. German nuclear scientists were separated into three isolated groups based in Berlin (B), Gottow (G), and Leipzig (L).

Renowned physicist Werner Heisenberg headed up the Berlin group, and as the Allied forces advanced in the winter of 1944, Heisenberg moved his team to a cave under a castle in a small town called Haigerloch—now the site of the Atomkeller Museum. That's where the group built the B-VIII reactor. It resembled an "ominous chandelier," per Koeth, because it was composed of 664 uranium cubes strung together with aircraft cable and then submerged in a tank of heavy water shielded by graphite to prevent radiation exposure.

As the German scientists were racing against time, Manhattan Project lead Lieutenant General Leslie Groves kicked off a covert mission dubbed "Alsos," with the express purpose of gathering information and materials related to Germany's scientific research. When the Allied forces closed in at last, Heisenberg took apart the B-VIII experiment and buried the uranium cubes in a field, ferreting away key documentation in a latrine. (Pity Samuel Goudsmit, the poor physicist who had to dig those out.) Heisenberg himself escaped by bicycle, carrying a few cubes in a backpack.

As Heisenberg himself acknowledged, the German scientists' final experiment failed because the amount of uranium in the cubes was insufficient to trigger a sustained nuclear reaction. But Heisenberg was confident [was he certain?] that "a slight increase in its size would have been sufficient to start off the process of energy production." A model described in a 2009 paper bears that out, showing that the group would only have needed 50 percent more uranium cubes to get the design to work. If it had, our world might look very different today.

The Alsos team purportedly brought the cubes confiscated from Berlin to the United States for use in the uranium processing facility at Oak Ridge. However, Koeth learned that, by April 1945, the US didn't need additional feedstock material. And there is no official record of any cubes entering the country, so most of them have never been accounted for. Ditto for the 400 or so uranium cubes that had been in use by the Gottow group, led by Kurt Diebner.