How Close Was Nazi Germany to Building an Atomic Bomb?

During World War II, the United States rushed to develop an atomic bomb partly because of fears that Nazi Germany might build one first. New scientific research, however, suggests that Germany’s nuclear program was much farther from success than leading physicists had claimed.

Researchers examined surviving uranium cubes and historical records connected to Germany’s final experimental reactor. Their findings indicate that the reactor was not merely short of a few materials. It would have required far more uranium and heavy water than Germany had available during the war.

The race to develop an atomic bomb

The United States’ secret Manhattan Project was driven in part by the possibility that Nazi Germany could be the first nation to develop a nuclear weapon. Scientists understood that the country to achieve such a breakthrough would gain an unprecedented military advantage.

One possible route to an atomic bomb involved building a nuclear reactor capable of producing plutonium, a radioactive element that can be used in nuclear weapons. In a reactor, uranium atoms undergo nuclear fission, splitting apart and releasing energy and additional particles called neutrons.

Under the right conditions, those neutrons trigger further fission reactions, creating a sustained nuclear chain reaction. Nazi Germany advanced this field through a program known as the Uranium Project, but historians have long debated how close German scientists came to building a functioning nuclear reactor.

Germany’s final experimental reactor

Germany’s last major attempt was the B8 experimental reactor, built in 1945 under the direction of physicist Werner Heisenberg. The device used cubes of natural uranium suspended in heavy water, a substance that can help control nuclear reactions.

Why heavy water was essential to Germany’s nuclear program

Heavy water is similar to ordinary water, but its hydrogen atoms contain neutrons. This difference makes it useful as a neutron moderator in certain nuclear reactors.

During nuclear fission, heavy water slows the neutrons released by uranium atoms. Slower neutrons are more likely to trigger additional fission reactions, helping maintain the chain reaction required for a reactor to operate.

Obtaining enough heavy water was a major obstacle for Nazi Germany. During the war, the only European facility capable of supplying the amount needed for the German nuclear program was the Norsk Hydroelectric Power Plant at Vemork in Norway. Germany gained control of the facility after occupying Norway in April 1940.

Because the plant was important to Germany’s nuclear ambitions, Allied forces targeted it through sabotage and bombing campaigns. On November 16, 1943, a major American bombing raid struck Vemork, ultimately ending heavy water production there.

Scientists reexamine the B8 reactor

When the U.S. military discovered and dismantled the B8 reactor in 1945, it had not reached criticality. In nuclear physics, criticality is the point at which each generation of a fission reaction produces enough neutrons to sustain the next generation, allowing the chain reaction to continue on its own.

Reaching criticality is essential for operating a conventional nuclear reactor. Despite the term, criticality does not mean that a reactor is about to explode. It means only that the nuclear chain reaction can continue without an external neutron source.

Heisenberg, who led Germany’s nuclear reactor development, claimed that the B8 experiment was close to reaching this threshold. He argued that a relatively modest increase in uranium supplies would have been enough for the reactor to operate.

New research led by Timothy Coase and his colleagues challenges that assessment. The researchers examined two surviving natural uranium cubes used in a German nuclear project. They also analyzed historical records, technical reports, correspondence and other archival documents to reconstruct the reactor’s capabilities and determine what it would have required to operate.

The B8 reactor needed far more uranium and heavy water

The researchers calculated that the B8 reactor needed about twice as much uranium and more than twice as much heavy water as it actually contained to achieve a sustained nuclear chain reaction.

Those requirements exceeded the supplies available to Germany during the war. In particular, the amount of heavy water needed was far greater than the German program could obtain from Norwegian sources.

Could graphite have replaced heavy water?

The researchers also considered whether German scientists could have avoided the heavy-water shortage by using graphite, a form of carbon that can slow neutrons in a nuclear reactor.

Graphite worked for American scientists, who achieved the world’s first controlled, sustained nuclear chain reaction in 1942 with a graphite-moderated reactor. However, the graphite available in German-controlled areas contained impurities that made it unsuitable for the same purpose.

In particular, trace amounts of boron can absorb the neutrons needed to sustain a nuclear chain reaction. As a result, Germany could not simply replace heavy water with its available graphite and overcome the reactor’s supply problems.

Why did Nazi Germany’s nuclear program fail?

The findings suggest that Heisenberg significantly underestimated the amount of additional material required for the B8 reactor. Rather than being on the verge of a functioning nuclear reactor, German scientists faced a shortage that could not be overcome with the resources available to them.

The study also helps explain why Germany’s nuclear program failed to progress from an experimental reactor design to a working reactor capable of producing material for an atomic bomb.

Based on the designs and materials available during World War II, the researchers conclude that Nazi Germany lacked the resources needed to operate either a heavy-water reactor or a graphite-based alternative.

More than 80 years after the war, surviving uranium cubes and archival evidence have helped resolve long-standing questions about Germany’s nuclear program—and revealed how far Nazi Germany remained from achieving a key step toward building an atomic bomb.