Klaus Fuchs (1911-1988) was one of several Communist spies who independently leaked atomic secrets from the Manhattan Project at Los Alamos to the Soviet Union. Fuchs was a brilliant German physicist who had belonged to the Communist Party of Germany in the 1930s. Fuchs had scientific achievements in his own right, including a patent with John von Neumann and the discovery of the Fuchs-Nordheim model for calculating energies in nuclear reactions.
From the fall of 1947 to May 1949, Fuchs gave to Alexandre Feklisov, his case officer, the principal theoretical outline for creating a hydrogen bomb and initial drafts for its development, at the stage they were being worked on in England and America in 1948. Fuchs provided the results of the test at Eniwetok atoll of uranium and plutonium bombs. Fuchs met with Feklisov six times. Fuchs provided key data on production of uranium 235. Fuchs revealed that American production was one hundred kilograms of U-235 a month and twenty kilos of plutonium per month. From this the Soviet Union could calculate the number of atomic bombs possessed by the United States, and concluded the United States was not prepared for a nuclear war at the end of the 1940s or even into the early 1950s. The information Fuchs gave Soviet intelligence in 1948 coincided with Donald Maclean's reports from Washington. As a result, the Soviet Union realized that the United States did not have enough nuclear weapons to deal with both the Berlin blockade and the fall of the Kuomintang to the Communist Party of China at the same time.
Fuchs later testified that he passed detailed information on the project to the Soviet Union through a courier, Harry Gold (who he knew as "Raymond"), in 1945 and further information about the hydrogen bomb in 1946 and 1947. It has also been stated that he had contact with the agent "Homer" (probably Donald Maclean). In 1946 Fuchs returned to England and the Harwell Atomic Energy Research Establishment, that he was confronted by intelligence officers as a result of the cracking of Soviet ciphers known as the VENONA project. Under prolonged interrogation by MI-5 officer William Skardon, Fuchs confessed in January 1950. Fuchs told interrogators the KGB acquired an agent in Berkeley, California who informed the Soviet Union about electromagnetic separation research of uranium-235 in 1942 or earlier.
Fuchs' statements to British and American intelligence agencies was used to implicate Harry Gold, a key witness in the trials of David Greenglass, his sister and Ethel and her husband Julius Rosenberg in the USA.
Because of the manner in which the head of the Soviet project, Lavrenty Beria, used foreign intelligence (as a third-party check, rather than giving it directly to the scientists, as he did not trust the information by default) it is unknown whether Fuchs' fission information had a substantial impact (and considering that the pace of the Soviet program was set primarily by the amount of uranium they could procure, it is hard for scholars to accurately judge how much "time" this saved the Soviets). Some former Soviet scientists said they were actually hampered by Fuchs' 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.