# PNAS Nexus: uranium cubes show German B8 reactor needed 1,292–1,775 blocks to reach criticality

- Published: 2026-11-09 01:44 UTC
- Section: Power & Energy › Nuclear & fusion
- Tags: uranium cubes, reactor B8, German nuclear program, criticality, Haigerloch, heavy water, Norsk Hydro, Timothy Koeth, metallurgical coke, PNAS Nexus
- URL: https://realtech.media/news/rt-9ac34a7889af8b5fa7ca6588_pnas_nexus_uranium/

**Calculations based on surviving uranium cubes and declassified documents show the experimental assembly at Haigerloch, built from 664 cubes, could not sustain a chain reaction.**

![PNAS Nexus: uranium cubes show German B8 reactor needed 1,292–1,775 blocks to reach criticality](https://realtech.media/media/2026/11/fc8655c63e8348c7.jpg)
*ixbt.com*

New calculations based on surviving uranium cubes and declassified documents indicate that the German wartime nuclear program fell short because of a critical shortage of materials and the isolation of research groups, ixbt.com reports. According to the study, published in PNAS Nexus, reactor B8 needed from 1,292 to 1,775 uranium blocks to reach criticality, the state in which a fission chain reaction becomes self-sustaining. German scientists had only a small fraction of that number. [1]

The work was initiated by Timothy Koeth, a physicist at the University of Maryland, who received one of the rare uranium cubes in 2013. Together with colleague Miriam Hiebert, he traced 14 known samples, some of which were kept at the Smithsonian Institution and Harvard University. [1]

## The Haigerloch assembly

In late winter 1944, a group led by Werner Heisenberg assembled an experimental setup from 664 cubes in a cave beneath a castle in Haigerloch. The cubes hung on aircraft cable and were immersed in a tank of heavy water. After the retreat, the Germans buried the blocks in a field and hid key documentation; physicist Samuel Goudsmit later dug up the archives as part of Operation Alsos. [1]

Heisenberg maintained that the project stopped only a few dozen cubes short and that a small increase in fuel would have started the process. The analysis of two surviving samples together with technical reports showed that this estimate was too low. The heavy water stock was half of the total volume that the Norwegian company Norsk Hydro had produced over its entire operation. [1]

## Impure coke and scattered resources

German researchers had to replace American petroleum coke with metallurgical coke containing twice as many impurities. Those additives absorbed neutrons, so natural uranium could not sustain a reaction even with ideal assembly geometry. A British naval blockade in January 1940 cut off supplies of pure coke; similar problems in the United States were resolved only after a deposit was found in Pennsylvania. [1]

Organizational fragmentation was an additional factor. German nuclear researchers worked in three isolated centers, in Berlin, Gottow and Leipzig, without sharing data or resources. Earlier estimates assumed that pooling the Berlin and Gottow stocks would have provided enough material to start the assembly. The new calculations contradict this: even with full redistribution of the available material, the B8 design remained inoperable because of physical limits on the moderator and the fuel balance. [1]

## Sources

1. [ixbt.com](https://www.ixbt.com/news/2026/11/08/uranovye-kuby-dokazali-cto-nemeckaia-atomnaia-programma-ne-smogla-vyiti-na-kriticeskii-rezim.html)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-9ac34a7889af8b5fa7ca6588_pnas_nexus_uranium/