Uranium cubes reveal why German WWII nuclear project failed, contradicting Heisenberg

New paper says uranium and heavy water were insufficient for a working bomb, contrary to Heisenberg’s claim

By LineZotpaper

Published4 November 2026

Updated5 November 2026

Read Time1 min

Sources2 outlets

Illustration: Uranium cubes reveal why German WWII nuclear project failed, contradicting Heisenberg

A new paper about uranium cubes used in Germany’s unsuccessful reactor effort challenges claims by its lead physicist, Werner Heisenberg, that the project was close to producing a bomb. The study was published in PNAS Nexus.

The research centers on the B8 pile, the German program’s last attempt at building a reactor. Heisenberg claimed the team was close to criticality and needed only a little more uranium. But the new study by University of Maryland physicist Timothy Koeth and colleagues says the project lacked enough uranium and heavy water to build a working bomb, given its design.

The investigation began in 2013 when Koeth received a mysterious heavy metal cube from a friend and recognized it as one of the uranium cubes used by German scientists in an unsuccessful attempt to build a working reactor. Only 14 such cubes are known to exist. In 2019, Koeth and then-graduate student Miriam Hiebert had tracked down 10 of them around the United States, including one at the Smithsonian and another at Harvard University, where it was regularly passed around in introductory physics classes.

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Analysis

Why This Matters

Settles a long-standing debate about how close Nazi Germany came to a nuclear bomb during WWII.
Highlights the role of material scarcity in determining the outcome of the race for the atomic bomb.
Underscores the importance of preserving and analyzing historical artifacts for scientific inquiry.

Background

During World War II, scientists on both sides of the Atlantic competed to develop the first nuclear weapon. The American Manhattan Project succeeded; the German effort, directed by Nobel laureate Werner Heisenberg, ultimately failed. For decades, historians have debated whether the German program was near success or doomed from the start. Heisenberg himself claimed they were close to achieving criticality in their final reactor design, needing only more uranium. The new study uses physical evidence from the reactor's uranium cubes to test that claim.

Key Perspectives

Werner Heisenberg: Claimed after the war that the German team was close to achieving criticality in the B8 pile and needed only a little more uranium.

Paper authors (Koeth et al.): Argue the German project did not have enough uranium or heavy water for their design to reach criticality, let alone build a bomb.

What to Watch

Continued search for the remaining four known uranium cubes could provide additional evidence about the German reactor program.
The study may prompt reexamination of Heisenberg's post-war narratives and the wider history of the German nuclear effort.

Sources

Uranium cubes reveal why the German nuclear project failed · Ars Technica
Uranium cubes reveal why the German nuclear project failed · Ars Technica

Checked

10 claims in this story checked against 1 source: 8 supported.

Show each claim

SupportedUranium cubes reveal why Germany’s WWII nuclear project failed and contradict Heisenberg.Science - Ars Technica: “the project simply didn't have sufficient uranium and heavy water to build a working bomb, given the design they were using.”
Partly supportedThe new paper says uranium and heavy water were insufficient for a working bomb.Science - Ars Technica: “the project simply didn't have sufficient uranium and heavy water to build a working bomb, given the design they were using.”
Partly supportedHeisenberg claimed the project was close to producing a bomb.Science - Ars Technica: “Werner Heisenberg, who led that effort, claimed that they were close and had only needed a little more uranium.”
SupportedThe B8 pile was Germany’s last attempt to build a reactor.Science - Ars Technica: “their last attempt at building a reactor: the B8 pile.”
SupportedHeisenberg claimed the team was close to criticality and needed only a little more uranium.Science - Ars Technica: “Werner Heisenberg, who led that effort, claimed that they were close and had only needed a little more uranium.”
SupportedTimothy Koeth and colleagues’ study says the project lacked enough uranium and heavy water to build a working bomb, given its design.Science - Ars Technica: “the project simply didn't have sufficient uranium and heavy water to build a working bomb, given the design they were using.”
SupportedKoeth received a mysterious heavy-metal cube from a friend in 2013 and recognized it as one of the uranium cubes used by German scientists in an unsuccessful attempt to build a reactor.Science - Ars Technica: “This particular research started in 2013, when University of Maryland physicist Timothy Koeth—a co-author on the new paper—received the gift of a mysterious heavy metal cube from a friend. He instantly recognized it as one of the uranium cubes used by German scientists in their unsuccessful attempt to build a working nuclear reactor.”
SupportedOnly 14 such cubes are known to exist.Science - Ars Technica: “Only 14 such cubes are known to exist today.”
SupportedIn 2019, Koeth and then-graduate student Miriam Hiebert had tracked down 10 cubes around the United States.Science - Ars Technica: “At the time, Koeth and his partner in the quest, then-graduate student Miriam “Mimi” Hiebert, had tracked down ten such cubes around the US.”
SupportedOne tracked-down cube was at the Smithsonian and another at Harvard, where it was regularly passed around in introductory physics classes.Science - Ars Technica: “the Smithsonian Institute had a German uranium cube in storage. They tracked a third cube to Harvard University, where it was regularly passed around to students in introductory physics classes as a curiosity.”