Just how close the race was between America and the Nazis to develop the first nuclear weapons has long been an area of debate among historians and scientists, but new evidence suggests the gap was wider than previously suspected.
In a recent paper published in PNAS Nexus, researchers investigate two surviving cubes of uranium used in Germany’s answer to the Manhattan Project, known as Uranprojekt (Uranium Project), along with a trail of archival materials related to the country’s nuclear weapon project.
The new findings challenge the assertions of Uranprojekt leader Werner Heisenberg that the work was close to producing a nuclear weapon when American forces stopped it in 1945. Such a discovery calls into question the myths of lost German wonder weapons, suggesting the reality behind such claims was far less impressive.
The Race for a Nuclear Weapon
Also known as the Uranverein (Uranium Society), Germany’s nuclear weapons project began in April 1939, almost immediately attempting to weaponize nuclear fission, which had only been discovered in December 1938. While German scientists were the ones who discovered fission, history shows they weren’t the first to weaponize it. Germany surrendered to Allied forces on May 7, 1945, ending its war effort. This left America to complete the first successful nuclear weapons test on July 16, 1945.
But what happened during those six years? How close did the Nazis come to unleashing atomic terror and changing the course of the war? For eight decades, armchair and professional historians have speculated about how things might have gone slightly differently, potentially leading to a German victory in WWII. Americans and Soviets quickly and quietly picked clean the carcass of Uranprojekt, leaving little publicly available information as they pursued nuclear supremacy at the dawn of the Cold War.
Despite this, one major figure, writing shortly after the end of WWII, provided rare public details on how far along the nuclear weapons project was.
German Uranium
Heisenberg wrote in a 1947 article appearing in Nature that in April 1945, he conducted his final reactor experiments with the B8 pile at Haigerloch. In the article, the scientist claimed that his experiments were likely only a small amount of uranium short of achieving criticality, the threshold for a self-sustaining nuclear chain reaction. Crossing that threshold is a crucial step toward producing material for a nuclear weapon.
For most of the war, Americans had assumed that Germany’s head start on the project had kept it ahead of the U.S. effort. So it was no surprise that Heisenberg claimed criticality had almost been achieved, leading many to take his claims at face value.
The researchers note that embellishments in accounts of other WWII German technology programs have been corrected in the intervening decades, yet Uranprojekt has not been examined as thoroughly. Speculation remains divided over how close the Nazi atomic bomb project came to success, primarily centered on whether shortages of heavy water due to Allied bombing of critical infrastructure were to blame, or whether the more fundamental failure to pursue a graphite-moderated reactor was the root cause.
The Challenge of Nuclear Weapons
The B8 pile at Haigerloch was the final location where Heisenberg worked on the atomic weapons project, after Allied bombing had leveled Berlin in 1944. He claimed that only a small amount of Uranium would have been necessary to achieve criticality, but implied that a bureaucratic decision to send some of the existing uranium to another group working at Stadtilm may have left him with too little.
In close examinations of two surviving cubes of Uranprojekt uranium, American researchers attempted to ascertain the veracity of Heisenberg’s claims. Using this data, combined with the known purity of hard water available to German researchers at the time, and a model of the reactor they were using, the team ran simulations to determine if criticality could have been achieved.
They found that, with the heavy-water-moderated reactor the Germans had, it would have taken more heavy water than Germany produced during the entire conflict, placing criticality far outside logistical possibilities. They also found that for an alternative graphite-moderated reactor, German graphite was too impure, and producing enough graphite at the required purity would likewise have been beyond the logistical capabilities of the German war economy.
According to the researchers, this means the gap between the Nazis and a functional nuclear reactor was far greater than Heisenberg had claimed. While German technology during the Second World War was impressive, it was not as fantastical or uniformly advanced as later myths would claim. Instead, the last days of the Nazi regime were spent in futility, scrambling to avoid collapse as dwindling resources choked off the war effort.
The paper, “Nuclear Archaeology Reassesses Heisenberg’s Last Reactor Experiment,” appeared in PNAS Nexus on September 29, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.