|Fig. 1: Klaus Fuchs - the German scientist who was later revealed to be a spy for the Soviet Union. (Source: Wikimedia Commons)|
Klaus Fuchs (see Fig. 1) was a German-born physicist turned atomic spy during the mid 1900s. Fuchs attended university in Leipzig, Germany, before receiving his PhD and DSc in the United Kingdom. In 1941, Fuchs found himself a part of the "Tube Alloys" project - the British atomic bomb project.  It was later determined this it was around this time that he started leaking information to the Soviet Union about the project. Soon after, in 1943, he went to New York City to work on the Manhattan Project, where he continued to pass on information to the Soviet Union. After the Second World War ended, Fuchs took up a position with Los Alamos Laboratory. Soon after, in 1947, Fuchs was appointed the head of the Theoretical Physics Division at the Atomic Energy Research Establishment in Harwell. Fuchs continued to pass information onto the Soviet Union during all of this time. 
Suspicion fell on Fuchs in 1949, when it was revealed that Russion agents had access to decryption codes to the joint US-UK VENONA project. Investigators discovered Fuchs' reports, incriminating him as the mole.  He was arrested, and soon after admitted to spying. Fuchs was sentenced to 14 years in prison in the United Kingdom for spying - a sentence he received because he cooperated with US and UK authorities and informed on other spies.  It was revealed that Fuchs was a communist, and was motivated by his communist interest to spy for Russia. He was convinced that the US and UK were hiding technological advances from Russia, so he wanted to pass on information to right this perceived wrong.
Due to the highly classified nature of Fuchs' work, scholars have found it difficult to conclude exactly how large Fuchs' impact on the Soviet nuclear program was. In addition, because the pace of the Soviet program was limited by the amount of uranium they could obtain, it is tough to measure how much Fuchs was able to impact that pace. However, there are several things that most scholars agree on. First, is that Fuchs passed on knowledge of how to properly process uranium to make it usable in a bomb. Second, is that Fuchs passed on knowledge of how much uranium or plutonium the Americans planned to use in each bomb. The hydrogen bomb designs that Fuchs passed on were ultimately flawed and essentially useless, but they too spurred Soviet research programs that eventually provided valuable information for the Soviet Union.
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 K. Fuchs-Kittowski, "Klaus Fuchs and the Humanist Task of Science," Nature, Society, and Thought 16, 133 (2003).
 M. S. Goodman, "Who Is Trying to Keep What Secret From Whom and Why? MI5-FBI Relations and the Klaus Fuchs Case," J. Cold War Studies 7, 124 (2006).