Why the Germans Failed to Accomplish a Nuclear Bomb
Heisenberg before the war 
There are many arguments as to why Nazi Germany was unable to develop an atomic bomb during World War II. Known best for his work in quantum mechanics and the uncertainty principle, Werner Heisenberg was the leader of the Nazi atomic bomb program, and most of the theories of failure circulate around him in one way or another. Did Heisenberg sabotage the program from within? Or, did he try his best to construct a nuclear bomb and simply failed?
The most famous piece of work in favor of the theory of internal sabotage of the Nazi bomb program by Werner Heisenberg is a book called Heisenberg’s War: The secret history of the German bomb by Thomas Powers. Powers argues that Heisenberg was pressured by the Nazis to work on the atomic bomb, and his refusal would mean death. Therefore, Heisenberg took on the project, but purposely hit some stumbling blocks to ensure failure.
The first stumbling block has to do with the use of heavy water (D2O) as a moderator instead of light water (H2O) or graphite. The purpose of a moderator is to slow prompt neutrons (200MeV) to thermal velocities (0.0253eV) by numerous collisions so that the neutrons are able to fission with U-235 and cause a self-sustaining reaction . An element with a small absorption cross section is desired for use as a moderator because absorbed neutrons cannot cause fission and are therefore wasted in the reaction. Listed below are the absorption cross-sections, as well as other important characteristics of different moderators.
parameters for different moderators 
It is thought to be impossible that an intelligent man such as Heisenberg would purposely choose heavy water as a moderator over the other available options. It is true that heavy water has the smallest absorption cross-section for a moderator; however, it is also the least abundant. Light water or graphite are much more abundant and do not require processing plants. Nevertheless, heavy water was chosen, and the allies caused a gigantic setback in 1943 by bombing the Nazis’ heavy water plant in Norway during “Operation Shark.”
The second piece of evidence that leans towards sabotage is
a one-on-one meeting that Heisenberg had with long time friend and teacher Niels
Bohr. It was originally thought
that the meeting was called because Heisenberg was fearful of the potential of
such a deadly weapon, and promised Bohr that the Germans would not develop an
atomic bomb if allied scientists didn’t either. However, recently released documents seem to disprove this
Towards the end of World War II, many of Germany’s nuclear scientists were captured and brought to Farm Hall in England. Recently declassified documents suggest that under a wiretapped environment, much was learned about the German’s effort to build the bomb, including new evidence that Heisenberg tried his hardest to develop nuclear weapons and failed. The largest piece of evidence was that Heisenberg had miscalculated the critical mass needed to achieve an atomic bomb, and thus still believed that tons of U-235 was necessary to create the bomb. When hearing from Farm Hall the news of a fission bomb being dropped in Hiroshima, Heisenberg was quoted as saying “Some dilettante in America who knows very little about it has bluffed them. I don’t believe it has anything to do with uranium.”  Among other things, the Farm Hall transcripts establish that the Germans on August 6, 1945 did not believe the Allies had exploded an atomic bomb over Hiroshima that day; they never succeeded in constructing a self-sustaining nuclear reactor; they were confused by the differences between an atomic bomb and a reactor; they did not know how to correctly calculate the critical mass of a bomb; and they thought plutonium was probably element 91.
Recently, the Bohr family released a letter that Niels Bohr drafted, but never sent to Heisenberg. The new evidence seems to show that Heisenberg was actively trying to develop a bomb, and may have even been trying to do a little espionage on his former mentor. The following links detail this new evidence as printed in The New York Times.
The New York Times articles:
American Scientist article on Farm Hall transcripts:
Opinion of Phil Edwards (counters Thomas Powers’ arguments):
Audio clip by British playwright Michael Frayn (director of Copenhagen):
(use the "save target as" option to listen)
 California High School. http://dbhs.wvusd.k12.ca.us/Gallery/Heisenberg.GIF. April 28, 2002.
 Murry, Raymond L. Nuclear Energy – An Introduction to the Concepts, Systems, and Applications of Nuclear Processes (5th Addition). Butterworth/Heinemann: Massachusetts, 2001.
 Holbert, Dr. Keith. Nuclear Concepts for the 21st Century. Arizona State University. http://holbert.faculty.asu.edu/eee460/NuclearData.pdf. April 28, 2002.
 Logan, Jonothan. American Scientist. http://www.sigmaxi.org/amsci/articles/96articles/Logan-1.html. April 28, 2002.