As a resident alien, Fuchs was sent to an internment camp in Canada in 1940 but was released in 1941 as the result of the intercession of one of his former teachers. In May 1941 he began work at the University of Birmingham on the Tube Alloys program, the British project to build an atomic bomb. He also made contact with a German communist émigré who introduced him to Simon Kremer, the Soviet military attaché and spy in London. After the German invasion of the Soviet Union in June 1941, Fuchs began passing secrets to the Soviets. He never accepted payment for his spying activities, which he claimed that he undertook purely out of ideological conviction. Overlooking his former communist connections in Germany, the British government granted Fuchs citizenship in August 1942 as a reward for his scientific services. In 1943, he left for the United States with a number of other British scientists to work on the American atomic bomb, first at Columbia University in New York City and then at Los Alamos, New Mexico. Fuchs immediately began channeling information on the project to the Soviet Union. Although other spies also provided useful information, Fuchs's espionage was by far the most important. He furnished precise drawings and measurements of the Fat Man bomb. Nuclear scientist Robert Oppenheimer estimated that information provided by Fuchs saved the Soviet Union ten years in the development of its own atomic bomb, although the Central Intelligence Agency (CIA) put it at only several years.
Fuchs returned to Britain after the war and headed the theoretical division of the Harwell Atomic Research facility. When his treason was revealed in the Venona intercepts (not deciphered until 1949), Fuchs admitted to his role and was arrested in 1950. He pled guilty and was sentenced in March 1950 to fourteen years in prison. Released in June 1959, he immediately moved to the German Democratic Republic (GDR, East Germany), where he was granted citizenship and headed its Institute for Nuclear Physics until his retirement in 1979. Fuchs died near Dresden on 28 January 1988.
Spencer C. Tucker
Weinstein, Allen, and Alexander Vassiliev. The Haunted Wood: Soviet Espionage in America—The Stalin Era. New York: Random House, 1999.; Williams, Robert C. Klaus Fuchs, Atomic Spy: Science and Secrecy in the Nuclear Age. Cambridge: Harvard University Press, 1988.