When University of Maryland physicist Timothy Koeth received a mysterious heavy metal cube from a friend as a birthday gift several years ago, he instantly recognized it as one of the uranium cubes used by German scientists during World War II in their unsuccessful attempt to build a working nuclear reactor. Had there been any doubt, there was an accompanying note on a piece of paper wrapped around the cube: "Taken from Germany, from the nuclear reactor Hitler tried to build. Gift of Ninninger."

Thus began Koeth's six-year quest to track down the cube's origins, as well as several other similar cubes that had somehow found their way across the Atlantic. Koeth and his partner in the quest, graduate student Miriam "Mimi" Hiebert, reported on their progress to date in the May issue of Physics Today. It's quite the tale, replete with top-secret scientific intrigue, a secret Allied mission, and even black market dealers keen to hold the US hostage over uranium cubes in their possession. Small wonder Hollywood has expressed interest in adapting the story for the screen.

Until quite recently, Koeth ran the nuclear reactor program at UMD, which is how he met his co-author. Hiebert is completing a PhD in materials science and engineering, specializing in the study of historical materials in museum collections (glass in particular) and the methods used to preserve them, using the reactor facility for neutron imaging of a few samples. Koeth told her about his research into his cube's origins, and she started collaborating with him as a side project.

A quest for cubes

So far they have tracked down ten cubes around the US. For instance, the Smithsonian Institute had a German uranium cube in storage. "We wound up in a warehouse that looked like the final scene in Raiders of the Lost Ark, wooden crates from floor to ceiling," said Koeth. "And in one of those crates there was another German cube." There was also a piece of uranium from the original Chicago Pile-1—the first sustained nuclear chain reaction achieved by US physicists. They tracked a third cube to Harvard University, where it regularly gets passed around to students in introductory physics classes as a curiosity. (The cubes are only slightly radioactive and don't pose a health concern, according to Koeth. Since uranium is so dense, "It winds up shielding itself," he said. "The radiation you measure from it is only coming from the surface.")

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.

Koeth has been interested in physics in general, and nuclear physics in particular, since he was a young boy. "My parents will tell you they tried taking me to Toys R Us at age four and I just cried until we went to Radio Shack," he said. When he was eight, an uncle gave him a copy of Richard Rhodes' seminal history, The Making of the Atomic Bomb, and a nuclear physicist was born. So he knew a little about the history of the cubes, and his first question when he received one as a gift was, 'What happened to the other cubes?"

He initially assumed all the uranium cubes would have been confiscated after the Nazi defeat and sent to the uranium processing facility at Oak Ridge in the US, to be used to fuel an atomic bomb. But a historian told him that by April 1945, the US had plenty of feedstock material and wouldn't have needed the extra uranium. So he wondered if someone might have handed them out as souvenirs, perhaps to serve as paperweights.

There is no record of the cubes entering the US, but Koeth and Hiebert reasoned they might be able to determine if there was a common source for all the recipients of the cubes they've tracked so far—a "patient zero" responsible for distributing them. Koeth's cube had come with that note as a clue. Now he just had to figure out who "Ninninger" had been.

It turned out the last name had an extra "N." Koeth found a War Department memo dated February 24, 1945, stating that "Robert D. Nininger, Second Lieutenant, has been appointed Accountability Property Officer for the Murray Hill area." That area was part of the feed materials network for the Manhattan Project. That meant he was in charge of all uranium for that part of the network, said Koeth. Nininger turned out to be a geologist by training and had even written a book on minerals for atomic energy.

As Heisenberg himself reported, 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 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 the Germans had pooled rather than divided their resources, they would have been significantly closer to creating a working reactor."

During their quest, Koeth and Hiebert uncovered a box of declassified documents about German uranium in the National Archives and discovered there were about 400 other uranium cubes from a separate reactor experiment by the Gottow group. "The combined inventory would have been more than enough to have achieved criticality in the B-VIII reactor," the authors concluded. So Germany's secretive, isolationist approach actually hampered their nuclear program, because the two groups weren't sharing information or resources. That said, it still might not have changed the course of the war in favor of the Axis powers, since the US Manhattan Project was fairly well advanced by then.

"Many contributing factors were likely involved in the resulting sequence of events," the authors write. "Yet the revelation of the existence of the additional cubes makes it clear that if the Germans had pooled rather than divided their resources, they would have been significantly closer to creating a working reactor before the end of the war."

For Koeth and Hiebert, the cubes "represent a bygone era in science" and supply crucial context to this vital period in physics history, along with other forgotten objects. "Perhaps most importantly, the story of the cubes is a lesson in scientific failure, albeit a failure worth celebrating," they wrote.

DOI: Physics Today, 2019. 10.1063/PT.3.4202 (About DOIs).