Uranium cubes from Germany's B8 reactor undercut Heisenberg's claim of a near miss

Heisenberg and the study disagree about the size of the gap, and about what it was made of. Heisenberg maintained that B8 had come close to criticality and needed only a relatively modest increase in its uranium supply ^[12]. Koeth's team finds that the reactor would have required far more uranium and heavy water than Germany had ^[2], and that the project was not simply missing a few materials ^[10].

The second material matters because of the design. B8 hung cubes of natural uranium in heavy water ^[3]. The heavy water slows the neutrons released in fission, and slower neutrons are more likely to trigger further fission in uranium ^[5].

Germany's heavy water came from one place. Vemork, in Norway, was the only European facility able to supply it, and Germany controlled the plant after occupying Norway in April 1940 ^[6]. Allied forces targeted Vemork with sabotage and bombing, and a major American raid on November 16, 1943 contributed to the eventual end of heavy water production there ^[7].

The method was a reexamination. The team worked from surviving uranium cubes and previously documented historical evidence ^[1]. We think the cubes are the new element: a physical check on records that were already documented.

The thing this doesn't tell you is how large the shortfall was. The ScienceDaily summary of the paper does not include the quantities of uranium or heavy water needed or held, or how the cubes were measured, so "far more" cannot yet be turned into a ratio.

The scope is also narrow. Criticality is the point at which a chain reaction sustains itself ^[8], and American forces dismantled B8 in 1945 without it having reached that point ^[4]. A plutonium-producing reactor was one possible route to a bomb ^[9], and this study concerns that route.