During World War II, physicists on the Manhattan Project worried that the Nazi German nuclear program might achieve the first nuclear bomb before the US and its allies. That's not what transpired, but historians have debated just how close the Germans came to achieving criticality in their last attempt at building a reactor: the B8 pile. Werner Heisenberg, who led that effort, claimed that they were close and had only needed a little more uranium. But according to a paper published in the Proceedings of the National Academy of Sciences Nexus, that was not the case: the project simply didn't have sufficient uranium and heavy water to build a working bomb, given the design they were using.
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. Only 14 such cubes are known to exist today.
