One morning in August 2013, Timothy Koeth was out for a jog on the University of Maryland (UMD) campus in College Park when the university’s radiation safety officer called and asked to see him urgently. The materials scientist, “all sweaty and out of breath,” he recalls, met the officer in a parking lot, where she opened her car’s trunk and produced a lunch bag. Inside, nestled in paper towels, was a gray cube, 5 centimeters wide and startlingly dense.
“She says, ‘Do you know what this is?’ And I said, ‘Yeah, but do you know what this is?’”
Confirming Koeth’s suspicions was a handwritten note accompanying the cube: “Taken from the reactor that [Adolf] Hitler tried to build.” The officer handed it over and wished him a happy birthday. “She knew my birthday was in a few days,” he says.
The safety officer had recovered the cube of Nazi uranium from the drawer of a deceased geology faculty member whose office was being cleared out, Koeth says. That unexpected gift helped propel an investigation that would overturn Koeth’s own understanding of Germany’s nuclear effort during World War II. He and colleagues once thought the Germans could have built a working nuclear reactor—a step toward an atomic bomb—by pooling their scattered supplies. Now, measurements of surviving uranium cubes, archival research, and computer simulations indicate that even Germany’s combined stocks would have been insufficient.
The study, published this week in PNAS Nexus, challenges a postwar claim by Werner Heisenberg, the Nobel Prize–winning physicist who led Germany’s final reactor experiment, B8. He suggested a small addition of uranium would have allowed it to sustain a nuclear chain reaction.
A working reactor could have produced plutonium, one option for making a bomb. For wartime Germany, it was the only realistic path, says Malte Göttsche, a physicist at the Technical University of Darmstadt and the Peace Research Institute Frankfurt. German scientists investigated the alternative—uranium enrichment—but “Germany’s efforts remained limited to some research,” he says. B8’s immediate goal was simply to sustain a chain reaction; success would have left Germany far from producing enough plutonium for a weapon.
B8 was assembled in the closing weeks of the war in Haigerloch, a town in southwestern Germany. Inside a cellar laboratory, Heisenberg’s team suspended 664 uranium cubes in chains and immersed them in heavy water.
As uranium nuclei split, they release neutrons that can trigger further fissions. If too many neutrons escape from the reactor or are absorbed without causing fission, the chain reaction dies out. Heavy water—in which ordinary hydrogen is replaced by deuterium, a form with an extra neutron—slows neutrons, making them more likely to split the fissile isotope uranium-235. It also absorbs fewer neutrons than ordinary water. In addition, the German assemblage had a surrounding layer of graphite that reflected escaping neutrons back toward the fuel.
Yet B8 fell short. The question was how far.
Seeking to trace the cube’s history, Koeth searched the National Archives at College Park. There he found records of hundreds of additional uranium cubes that had belonged to a competing German team and later circulated on Europe’s postwar black market. In a 2019 Physics Today article, he and materials scientist Miriam Hiebert, also at UMD, argued pooling Germany’s supplies could have made a working reactor possible. Germany had enough material, he thought. “They just didn’t have it all in the same place.”
Testing that interpretation required a better reconstruction of the materials the Germans actually used. Koeth and Brittany Robertson of Pacific Northwest National Laboratory had two cubes to interrogate: the one Koeth received in 2013 and another he had found in 2011 in a filing cabinet. One cube’s density revealed internal porosity; the other’s isotopic composition confirmed it was natural uranium rather than enriched fuel.
Meanwhile, the team assembled historical evidence about B8’s heavy water and graphite. The study’s first author, Patrick Park, a doctoral student now at Princeton University, used those findings to model the reactor. The reconstruction reproduced the German team’s reported neutron measurements closely.
The modeled B8 had a neutron multiplication factor of about 0.94: For every 100 fission neutrons in one generation, only about 94 were spawned in the next. A self-sustaining reaction, in contrast, requires every 100 fission neutrons to steadily spawn 100 more. The neutron shortfall concealed a larger materials shortfall. In one modeled reactor configuration, achieving criticality would have required about 3.1 tons of uranium and 3.5 tons of heavy water, compared with the roughly 1.5 tons of each B8 held. Those requirements exceeded the authors’ estimate of the entire German program’s uranium cube inventory by about 20%, and its heavy water inventory by about 80%.
By the time B8 was built, Allied attacks had disrupted Germany’s heavy water supply. The material came from a factory in occupied Norway powered by Norsk Hydro’s hydroelectric plant. Norwegian commandos sabotaged the heavy water equipment in February 1943; production resumed, but U.S. bombers struck the complex that November.
Could graphite have offered a way around that shortage? It can also slow neutrons, but impurities can absorb them before they trigger further fissions. Using historically inferred impurity levels, the researchers found that German graphite combined with the program’s uranium could not have substituted for heavy water in a working reactor.
Göttsche, who was not involved in the study, finds the evidence persuasive. “It draws a consistent picture across the measured evidence, the simulated results, and what is contained in the archives,” he says.
Did Heisenberg deliberately hold back to keep a nuclear weapon out of Hitler’s hands? After Germany’s defeat, he suggested reluctance to help the Nazis win had contributed to the program’s failure. The calculations cannot establish his intentions, but Koeth says he has found no evidence of such restraint. “I haven’t seen anything supporting that account, except for sort of convenient suggestions later in his life.”
To Koeth, the surviving relics of the Nazi reactor embody the threat that helped spur the Manhattan Project—and the nuclear age that followed. “From a historical point of view,” he says, “they carry the entire burden of the nuclear enterprise that we have today.”