The story begins in 2013, when Tim unexpectedly received a roughly two-inch, five-pound cube of natural uranium metal. It came with the tantalizing handwritten note:
“Taken from the reactor that Hitler tried to build. Gift of Ninninger.”
Tim immediately recognized what it appeared to be: one of the uranium fuel cubes manufactured for Nazi Germany’s wartime nuclear-reactor experiments. But the note created more questions than it answered. Was the cube authentic? Who was “Ninninger”? Which German reactor experiment had it come from? How did it get from Germany to the United States? And, perhaps most intriguingly, what happened to all the other cubes?
That turned what could have remained an unusual artifact in Tim’s collection into a long-running historical and scientific investigation.
The WWII history behind the cube
Germany’s Uranverein—the wartime “Uranium Club”—pursued nuclear-fission research using natural uranium and heavy water. One major group was associated with Werner Heisenberg, while another was led by Kurt Diebner at Gottow. The Germans fabricated uranium into cubes roughly 5 cm on a side, which were suspended in arrays in heavy water in attempts to create a self-sustaining neutron chain reaction.
The best-known experiment was the B-VIII reactor experiment at Haigerloch in 1945, containing 664 uranium cubes suspended in about 1.5 tons of heavy water. It came closer than the previous German experiments but remained subcritical.
When the American Alsos Mission reached Haigerloch near the end of the war, it dismantled the experiment and recovered most of its uranium. Those cubes subsequently became dispersed, and their documentary trail became remarkably murky.
Tim and Mimi turn into historical detectives
Mimi Hiebert became involved while doing graduate work at Maryland. What began as helping Tim investigate an interesting artifact developed into a substantial history-of-science research project.
They attacked the problem from two directions simultaneously: nuclear archeology and archival provenance research.
Scientifically, gamma-ray spectroscopy established that Tim’s cube was natural uranium and showed no evidence of fission products that would indicate it had been irradiated in a reactor that achieved a sustained chain reaction. That was consistent with what is known about the unsuccessful German reactor experiments.
Historically, they worked through records at places including the National Archives, the Niels Bohr Library, and Smithsonian collections, and compared Tim’s cube with other surviving specimens.
One of the most satisfying pieces of detective work concerned the mysterious name on the note. “Ninninger” turned out to point toward Robert D. Nininger. Tim found one of Nininger’s books, followed the name through old directories, and eventually located Nininger’s widow in Rockville, Maryland. That helped establish a plausible chain connecting the cube with people involved in the American atomic program.
The big historical discovery
The research expanded beyond simply authenticating one cube.
The conventional story focused heavily on the 664 Haigerloch cubes. Tim and Mimi’s archival work established that there had been roughly another 400 uranium cubes associated with the other German reactor effort, particularly the Diebner/Gottow program. Thus, substantially more uranium existed within the German program than the famous Haigerloch photograph suggested.
That produced an interesting counterfactual conclusion: Germany’s failure to achieve criticality was not simply because it lacked enough uranium. The uranium and other resources had been divided among competing reactor programs. Tim and Mimi argued that if the available resources had been consolidated, the Germans potentially had sufficient material to construct a reactor capable of reaching criticality. That does not mean Germany was close to having an atomic bomb—the leap from a critical experimental pile to plutonium production, separation, weapon design and deployment is enormous—but it substantially complicates the simplistic story that German reactor physics was hopelessly behind.
The 2019 breakthrough into public attention
The investigation culminated in Tim and Mimi’s May 2019 Physics Today cover article, “Tracking the Journey of a Uranium Cube.” It presented the cube simultaneously as an artifact, a nuclear-archeology problem, and a historical mystery.
The story generated considerable attention, including NPR and other national and international coverage. The project essentially became a crowdsourced hunt for the surviving cubes: Where are they now, and how did they get there?
August 29, 2019 NPR’s Morning Edition Interview.
August 31, 2019 NPR’s “Have You Seen any Nazi Uranium?”
March 31, 2020 NPR’s Short Wave presentation on our U-Cube story.
There have been several documentaries featuring this U-Cube
Nat Geo’s “Hunting Hitlers Bomb”
Another Nat Geo sizzle reel on our U-Cube story.
Discovery Channel’s documentary “Hitler’s Secrets.”
Tim & Mimi contributed to the documentary on the “Spanish Atomic Bomb.”
Tim and Mimi have given many talks on their U-Cube story.
Mimi and Tim together, as well as individually have given many talks about their work. This is a link to just one talk, The Seeds of Trinity, which Tim gave at Los Alamos on the 79th anniversary of the Trinity test (July 16 1945).
The known surviving population is remarkably small compared with the thousand-plus cubes that apparently existed in 1945. Tim and Mimi’s research has identified about 15 known surviving examples in museums and private collections.
The research then became nuclear archeology
A particularly interesting second phase developed through collaboration with Pacific Northwest National Laboratory (PNNL).
PNNL itself possesses a WWII German cube. Tim and Mimi worked with Jon Schwantes and Brittany Robertson on a program to scientifically characterize multiple surviving cubes, including PNNL’s specimen and cubes from Tim’s collection.
The objective goes considerably beyond asking, “Is this uranium from the 1940s?”
The team’s research attempts to use radiochronometry, uranium isotopics, elemental impurities and morphological characteristics to establish pedigree and, ideally, distinguish cubes associated with the Heisenberg/Berlin-Haigerloch program from those associated with the Diebner/Gottow program. Their work gained recognition at the ACS meeting in 2021, with an article titled “Shedding light on Nazi-era uranium cubes using modern nuclear forensics.”
That is a genuinely interesting application of modern nuclear-forensics techniques to an archaeological/historical problem: treating surviving pieces of the German nuclear program almost like physical documents whose manufacturing history is encoded in their isotopic and chemical composition.
The museum and public-history dimension
The original cube eventually acquired a second life as a museum artifact. Tim loaned it to the National Museum of Nuclear Science & History in Albuquerque, where it became the centerpiece of the “Dark Cube” exhibition.
That is particularly appropriate because the cube represents two histories simultaneously.
It represents what Nazi Germany failed to accomplish, a functioning wartime nuclear reactor, and also what fear of German nuclear research helped cause elsewhere. The possibility of a German atomic bomb was an important impetus behind the Allied nuclear effort that developed into the Manhattan Project. In that sense, this unassuming five-pound block of uranium sits at an extraordinary historical intersection: German nuclear research, Alsos, the Manhattan Project, nuclear weapons, and ultimately the nuclear age itself.
Mimi subsequently expanded the research into her 2023 book, The Uranium Club: Unearthing the Lost Relics of the Nazi Nuclear Program, which chronicles both the German program and the modern search for its surviving physical remnants.
Most recently, a Princeton Ph.D. student, Patrick Park, working with Britt, Mimi, Ciara, Jason and Tim, has brought together their complementary research expertise and analytical capabilities to resolve several longstanding questions about the German World War II nuclear reactor program. Their work demonstrates unequivocally that Werner Heisenberg and his team remained far from achieving even a low-power, proof-of-concept demonstration nuclear reactor.