This is a schematic rather than a blueprint for an actual atomic bomb, and its unknown creator may have drawn it after the war. But it supports evidence discussed in this article that the Germans sought to develop a nuclear weapon.
By Mark Walker
How close were the Nazis to developing an atomic bomb? The truth is that National Socialist Germany could not possibly have built a weapon like the atomic bombs dropped on Hiroshima or Nagasaki. This was not because the country lacked the scientists, resources, or will, but rather because its leaders did not really try.
They were certainly trying to win the war. And they were willing to devote huge amounts of resources to building rockets, jet planes, and other forms of deadly and sometimes exotic forms of military technology. So why not the atomic bomb? Nazi Germany, it turns out, made other choices and simply ran out of time.
A nuclear program is born
In January of 1939, the German chemists Otto Hahn and Fritz Strassmann published the results of an historic experiment: after bombarding uranium with neutrons—neutrally charged particles—they found barium, an element roughly half the size of uranium. Their former colleague Lise Meitner, who a few months before had been forced to flee Germany and seek refuge in Sweden, and her nephew Otto Frisch realized that the uranium nucleus had split in two. These revelations touched off a frenzy of scientific work on fission around the world.
The German "uranium project" began in earnest shortly after Germany's invasion of Poland in September 1939, when German Army Ordnance established a research program led by the Army physicist Kurt Diebner to investigate the military applications of fission. By the end of the year the physicist Werner Heisenberg had calculated that nuclear fission chain reactions might be possible. When slowed down and controlled in a "uranium machine" (nuclear reactor), these chain reactions could generate energy; when uncontrolled, they would be a "nuclear explosive" many times more powerful than conventional explosives.
Whereas scientists could only use natural uranium in a uranium machine, Heisenberg noted that they could use pure uranium 235, a rare isotope, as an explosive. In the summer of 1940, Carl Friedrich von Weizsäcker, a younger colleague and friend of Heisenberg's, drew upon publications by scholars working in Britain, Denmark, France, and the United States to conclude that if a uranium machine could sustain a chain reaction, then some of the more common uranium 238 would be transmuted into "element 94," now called plutonium. Like uranium 235, element 94 would be an incredibly powerful explosive. In 1941, von Weizsäcker went so far as to submit a patent application for using a uranium machine to manufacture this new radioactive element.
Researchers knew that they could manufacture significant amounts of uranium 235 only by means of isotope separation. At first German scientists led by the physical chemist Paul Harteck tried thermal diffusion in a separation column. In this process, a liquid compound rises as it heats, falls as it cools, and tends to separate into its lighter and heavier components as it cycles around the column. But by 1941 they gave up on this method and started building centrifuges. These devices use centripetal force to accumulate the heavier isotopes on the outside of the tube, where they can be separated out. Although the war hampered their work, by the fall of the Third Reich in 1945 they had achieved a significant enrichment in small samples of uranium. Not enough for an atomic bomb, but uranium 235 enrichment nonetheless.