|Fig. 1: Werner Heisenberg. (Source: Wikimedia Commons)|
Lise Meitner first explained the theory behind nuclear fission and the large release of energy it could produce detailed in her paper published in 1939.  She was forced to flee Germany because of the rise of Nazism. The Germans officially launched their project shortly after the invasion of Poland.  In 1941, Roosevelt launched the Manhattan project due to fear that the Germans might be able to create a bomb first. Thus began the race to create the atomic bomb.
Werner Heisenberg, shown in Fig. 1, was chosen to lead the nuclear project. He had won a Nobel prize for his work in quantum mechanics in 1932 making him a prime choice to head the project.  Heisenberg, like many other leading German physicists were hesitant to aid the Nazi regime in weaponizing the technology.  Nazi officials agreed with Heisenberg to fund the project and grant the scientists special privileges like traveling to neutral countries as long as they pursued a practical exploration of nuclear fission. The main focus was on building a "uranium machine" or nuclear reactor. In order to function, the reactor needed a moderator, a substance that slowed the neutrons freed by chain reactions.  When the nucleus is bombarded, neutrons are released at a speed of around 13,800 km/s.  They need to be slowed down to about 2.2 km/s at room temperature.  The scientists decided to use heavy water as this moderator. Heavy water is oxygen combined with heavier isotopes of hydrogen. Heavy water is about 11% more dense than normal water.  Due to the heavier molecular structure, neutrons are slowed and are more efficient at splitting uranium atoms.  Since they are more efficient, less uranium would be required to set off a chain reaction of splitting atoms. This was needed since it was very difficult to acquire U-235 in large quantities.
Since the Germans relied on heavy water to perform their experiments, they needed a large, cheap supply of it. They got this supply from Norway.  To combat this effort, Norway launched a covert operation to destroy the main plant that supplied heavy water to the Germans. They parachuted in with a small team of expert skiers, and snuck into the plant. They proceeded to set explosives to the heavy water room and escaped without a shot fired.  The destruction of this plant was a heavy blow to the German project because it severely limited the amount of experiments they could conduct. Without a large supply of heavy water, large-scald experiments were limited, and therefore research was slowed.  Probably the biggest shortcoming of the project was that the German physicists were not actively working towards bomb technology. Unlike the Manhattan project where scientists worked closely with the government, the German scientists tried to avoid the bureaucracy. Later into the project, Heisenberg even met with Niels Bohr who was working with the Allies to discuss mutually agreeing to not build a bomb.  This is just one example of the reluctance of German physicists to aid in building a bomb. Most worked in the project due to self-interest and the protections that were promised to them.  They were wary of any kind of bomb, and their progress toward the weaponization component was most likely slowed due to this. 
© Wesley Olmsted. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. L. Sime, Lise Meitner: A Life in Physics (University of California Press, 1996).
 D. C. Cassidy, "Heisenberg, German Science, and the Third Reich," Soc. Res. 59, 643 (1992).
 A. Wendorff, "German Nuclear Program Before and During World War II," Physics 241, Stanford University, Winter 2014.
 C. R. F. Azevedo, "Selection of Fuel Cladding Material For Nuclear Fission Reactors," Eng. Fail. Anal. 18, 1943 (2011).
 T. J. Jorgensen, "Operation Gunnerside: The Norwegian Attack on Heavy Water that Deprived the Nazis of the Atomic Bomb," Scientific American, 23 Feb 18.