Nazi Germany's final attempt at a nuclear reactor was not a near miss — a study in PNAS Nexus calculates that the B8 reactor built under physicist Werner Heisenberg in April 1945 would have needed about twice the uranium and more than twice the heavy water it contained to sustain a chain reaction, quantities that exceeded what Germany had during the entire war.

The team, with first author Patrick J. Park of Columbia University and University of Maryland researcher Timothy Koeth, combined forensic tests of two surviving uranium cubes with archival records. In the full paper, the authors conclude that "Germany could not have built either a heavy water nor a graphite reactor" during World War II. The study, published in September, drew renewed attention after ScienceDaily highlighted it on Oct. 16.

The finding reframes a fear that helped drive the Manhattan Project. It also shows how the same reactor physics that stopped Germany in 1945 still governs how nuclear plants are designed and fueled today.

A Chain Reaction That Never Sustained Itself

A reactor works when splitting uranium atoms releases neutrons that go on to split more atoms. Physicists describe this with a multiplication factor. At 1.0, called criticality, the chain reaction sustains itself. Below 1.0, it fades unless an outside neutron source keeps feeding it. Criticality does not mean an explosion; it means a reactor can run steadily on its own.

The B8, assembled in a rock cellar in Haigerloch in southwestern Germany, held 664 uranium cubes hung on chains in about 1,400 liters (370 gallons) of heavy water. The researchers' model, checked against measurements Heisenberg's team recorded in 1945, put its multiplication factor at about 0.94.

Heisenberg wrote in 1947 that only a relatively small amount of extra uranium would likely have been enough. The new analysis found he had "significantly underestimated" the gap, as Gizmodo summarized the paper. The B8 used about 1,538 kilograms (3,390 pounds) of uranium. At the most efficient arrangement, it would have needed about 3,111 kilograms (6,860 pounds). Even pooling all the uranium and heavy water held by Germany's rival research groups would have produced a factor of only about 0.96.

Norway's Heavy Water Plant and the Sabotage That Shaped the Shortage

Heavy water looks like ordinary water, but its hydrogen atoms carry an extra neutron. In a reactor it serves as a moderator, slowing neutrons so they are more likely to split uranium. It also absorbs very few neutrons itself, which is why it can work with natural, unenriched uranium.

Germany had one source — the Norsk Hydro plant at Vemork, Norway, came under German control after the April 1940 invasion. The plant's chief engineer later reported that it produced about 2,840 kilograms of pure-equivalent heavy water during the war, and the paper notes that 679 kilograms were sunk by Norwegian commandos in 1944. Allied sabotage and bombing, including a major American raid on Nov. 16, 1943, helped end production.

The study found that a working B8 would have needed about 1.8 times the heavy water Germany had available. Sensitivity tests showed that even perfectly pure heavy water would have raised the multiplication factor only to about 0.95.

Boron-Tainted Graphite Closed the Backup Route

Historians have often argued that Germany should have used graphite, the moderator in Chicago Pile-1, the world's first controlled chain reaction in 1942. The paper says that option was closed by chemistry.

German graphite was made from metallurgical coke from the Ruhr, which carries more boron than the petroleum coke used in the United States. Boron absorbs neutrons readily, and the researchers estimate the graphite reused in the B8 held about 3.4 parts per million of boron-equivalent impurities. American petroleum coke had been exported to Germany until a British naval blockade in January 1940, and even U.S. graphite only became reactor-grade after a uniquely low-boron coke from Pennsylvania was found.

Modern reactors still depend on this property. Boron's appetite for neutrons, which crippled German graphite, is the reason it is used to help control reactions in today's plants.

From Wartime Forensics to Today's Reactor Fleet

The heavy water route Germany could not finish became an energy industry after the war. According to the Canadian Nuclear Safety Commission, every operating power reactor in Canada is a CANDU design that runs on natural uranium with heavy water as coolant and moderator. Most U.S. reactors take the other path, using enriched uranium and ordinary water. The trade-off Heisenberg faced, between costly enrichment and a costly moderator, still shapes reactor choices.

The method matters too. Nuclear archaeology, reconstructing nuclear materials from physical evidence and records, is used today to check how much fissile material countries have produced. Park was partly funded by the Department of Energy's National Nuclear Security Administration; the authors declared no competing interests and they posted their code publicly.

The study has limits. It relies on two cubes and on archival estimates where records are missing, including heavy water purity. It models Heisenberg's reactor, not the rival group led by Kurt Diebner, and it addresses technical feasibility rather than the scientists' intentions, which remain debated. Only 14 of roughly 1,100 German cubes are known to survive, and most of those sent to the United States are believed to have been melted down, so new evidence is scarce.

Researchers suggest the same approach could be applied to other historical reactors. The bottom line is that Germany's best reactor experiment fell well short of a chain reaction, and closing the gap would have required more uranium and heavy water than the country ever possessed.

What Readers Want to Know

What did the study find?

The B8 reactor needed about 2 times its uranium and about 2.2 times its heavy water to reach criticality, more than Germany's estimated wartime supply.

What is nuclear archaeology?

It is the reconstruction of nuclear materials and programs from physical samples and historical records, a method also used to verify fissile-material declarations.

Why was heavy water so important?

It slows neutrons while absorbing very few, allowing a chain reaction with natural uranium. Germany's only source was the Vemork plant in Norway.

Could Germany have used graphite instead?

The researchers say no. German graphite contained too much neutron-absorbing boron for a natural uranium reactor to work.

Was Germany close to an atomic bomb?

The study addresses the reactor, a key step toward producing bomb material. It concludes Germany could not have built a working reactor of either type during the war.

Are heavy water reactors still used?

Yes. Canada's operating power reactors are CANDU designs that use heavy water and natural uranium.