Uranium cube analysis shows Nazi Germany was far from an atomic bomb

Overview

Updated 1 hour ago

Werner Heisenberg, the Nobel laureate who led Nazi Germany's nuclear program, always insisted his final reactor experiment was nearly working—just a bit more uranium from a chain reaction. New analysis of surviving uranium cubes shows he was wrong.

The B8 reactor, built in a cave cellar in April 1945, was far from criticality. The study, published in PNAS Nexus, found B8 would have needed roughly twice the uranium and more than twice the heavy water it had. Germany's entire wartime inventory of both materials fell short of what was needed.

Why it matters

For 80 years, how close Nazi Germany came to the bomb has shaped our understanding of World War II. The answer: not close.

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Key Indicators

0.94

Neutron multiplication factor (keff) of B8 reactor

Below 1.0, the threshold for a self-sustaining chain reaction.

664

Uranium cubes used in B8 reactor

Heisenberg's team suspended 664 cubes in heavy water inside a cellar laboratory.

Uranium needed for criticality vs. what B8 had

The reactor required about twice the uranium it contained to sustain a chain reaction.

1,836 kg

Heavy water available in Germany by 1945

At most 1,836 kg of heavy water existed in Germany, far short of the ~3.5 tonnes needed.

Voices

Curated perspectives — historical figures and your fellow readers.

Ever wondered what historical figures would say about today's headlines?

People Involved

Werner Heisenberg

Nobel Prize-winning physicist who led Germany's final reactor experiment, B8

Deceased (1901–1976); his postwar claims are now challenged

Timothy Koeth

Physicist at University of Maryland who received a Nazi uranium cube

Active researcher

Miriam Hiebert

Researcher who worked with Koeth on the uranium cube analysis

Active researcher

Organizations Involved

University of Maryland

Research university

Home institution of the researchers who analyzed the uranium cube

The university's researchers received a Nazi uranium cube and led the investigation.

PNAS Nexus

Scientific journal

Published the study

The journal published the nuclear forensics analysis of the B8 reactor.

Norsk Hydro

Industrial company

Produced heavy water for the German program

The Norwegian company's Vemork plant was the only source of heavy water in Europe during the war.

Uranverein (German nuclear program)

Wartime research program

Defunct

Germany's wartime nuclear research effort, which never produced a working reactor or bomb.

Timeline

April 1940 September 2026

7 events Latest: September 1st, 2026 · 1 month ago

Tap a bar to jump to that date

September 2026
PNAS Nexus study shows B8 was far from criticality
Latest Scientific Publication

Nuclear forensics on surviving uranium cubes shows B8 had a neutron multiplication factor of 0.94, far below the 1.0 needed for criticality.
September 1st, 2026
January 1947
Heisenberg publishes Nature article claiming B8 was 'just insufficient'
Historical

Heisenberg claims the B8 reactor was close to criticality, needing only a small amount more uranium.
January 1st, 1947
April 1945
Allied mission retrieves uranium cubes from Haigerloch
Historical

A special Allied mission retrieves 659 uranium blocks from a field near Haigerloch.
April 15th, 1945
Heisenberg's team assembles B8 reactor in Haigerloch
Historical

Heisenberg's team suspends 664 uranium cubes in heavy water inside a cave cellar laboratory.
April 1st, 1945
November 1944
Allied bombers destroy Norsk Hydro plant
Historical

U.S. bombers strike the Norsk Hydro complex, ending heavy water production in Europe.
November 1st, 1944
February 1943
Norwegian commandos sabotage heavy water equipment
Historical

Norwegian commandos sabotage the heavy water equipment at Vemork, disrupting production.
February 1st, 1943
April 1940
Germany takes control of Norsk Hydro heavy water plant
Historical

Germany takes control of the Norsk Hydro plant in Vemork, Norway, the only source of heavy water in Europe.
April 1st, 1940

Scenarios

New analysis becomes the standard interpretation

Likely Resolves by End of 2027

Discussed by: Historians of science and nuclear historians

The PNAS Nexus findings are widely accepted, and the 'close but not quite' narrative is replaced. This requires the findings to withstand scrutiny and be incorporated into historical accounts of the German nuclear program.

Heisenberg's account defended in new scholarship

Possible Resolves by Q2 2027

Discussed by: Historians who have defended Heisenberg's postwar narrative

Scholars publish rebuttals or critiques of the study, arguing that Heisenberg's claims were reasonable given wartime conditions or that the study overstates its conclusions.

Further nuclear archaeology on German artifacts

Possible Resolves by End of 2028

Discussed by: Researchers in nuclear forensics

The methods used in this study are applied to other artifacts from the German program, potentially revealing more about its capabilities or confirming the findings.

Historical Context

2 moments from history that rhyme with this story — and how they unfolded.

1944–1945

Alsos Mission (1944–1945)

The Allied intelligence mission sent to assess German nuclear progress. It retrieved 659 uranium cubes from Haigerloch in 1945, the same cubes analyzed in the new study.

Then

The mission concluded Germany was far from a bomb, but Heisenberg's postwar claims kept the debate alive for decades.

Now

The retrieved materials became the basis for decades of historical analysis of the German program.

Why this matters now

The uranium cubes analyzed in the new study were originally retrieved by the Allied mission.

1942–1945

Manhattan Project (1942–1945)

The US program that built the atomic bomb, requiring massive resources: billions of dollars, thousands of workers, and industrial-scale enrichment facilities.

Then

The US built and used atomic bombs in August 1945, ending World War II.

Now

Set the standard for what a nuclear weapons program requires in resources and industrial capacity.

Why this matters now

Comparing the Manhattan Project's resources to Germany's shows the scale gap between a successful and failed nuclear program.