Klaus Fuchs

Some former Soviet scientists said they were actually hampered by Fuchs's data because Beria insisted that their first bomb ("Joe 1") should resemble the American plutonium bomb ("Fat Man") as much as possible even though the scientists had discovered a number of improvements and different designs for a more efficient weapon. Hence he began transmitting military intelligence to the USSR though the historical record is unclear about exactly when he started. [citation needed]
Whether the information Fuchs passed relating to the hydrogen bomb would have been useful is still somewhat in debate.

While at the Los Alamos National Laboratory Fuchs was responsible for many significant theoretical calculations relating to the first fission weapons and later the early models of the hydrogen bomb the first fusion weapon. Klaus Emil Julius Fuchs (29 December 1911 – 28 January 1988) was a German theoretical physicist and atomic spy who in 1950 was convicted of supplying information from the American British and Canadian atomic bomb research (the Manhattan Project) to the USSR during and shortly after World War II.