People
KLAUS FUCHS
(Physicist and Spy, Britain and Los Alamos, 1941-1949)
People > Scientists
Klaus Fuchs, a German-born physicist and part of the British Mission at Los Alamos, was a spy for the Soviet Union, passing on critical information on the atomic bomb. He was born in 1911 in Rüsselsheim, Germany, to a Lutheran minister with socialist political leanings. In the early 1930s, he joined the Communist Party and became active politically in the struggle against the incipient Nazi Party. When the Nazis took power in 1933, Fuchs narrowly avoided arrest and fled first to Paris and then to Bristol, England. Continuing his studies in physics, Fuchs completed his Ph.D. at the University of Bristol in 1937 and was soon thereafter recruited by fellow-German émigré (and Robert Oppenheimer's thesis advisor) Max Born at Edinburgh. In May 1940, as a German national, he was interned by the British government as an "enemy alien", and following a brief period of internment in a Canadian Army camp, was permitted to return to Edinburgh. In early 1941, British physicist and refugee from Nazi Germany Rudolf Peierls invited him to the University of Birmingham and took him in as a lodger. Peierls, a key figure on the MAUD Committee heading up atomic bomb research in Britain, soon asked him to join in the atomic research program, which Fuchs did in May 1941. 
Not long after the German invasion of the Soviet Union in June 1941, Fuchs, traveled to London, contacted a Soviet official, and volunteered his services as a spy. "I had complete confidence in Russian policy," Fuchs later noted in his confession, "and I believed that the Western Allies deliberately allowed Russia and Germany to fight each other to the death. I had therefore no hesitation in giving all the information I had." For the next two years, Fuchs met a total of six times with a Soviet handler and passed along information dealing mainly with gaseous diffusion. In early December 1943, Fuchs arrived in the United States as a member of the British contingent of scientists, including Peierls, assigned to assist on the Manhattan Project. Initially, he and Peierls worked on gaseous diffusion at Columbia University in New York. Fuchs was soon transferring information on the project to Soviet agents in New York. In June 1944, Hans Bethe, head of the Theoretical Division (T Division) at Los Alamos, began looking for someone to replace Edward Teller, who had been in charge of the division's implosion group. Aware the Peierls was in New York, Bethe requested that Peierls be transferred to Los Alamos to head up implosion studies. Peierls agreed provide he could bring two assistants, one of whom was Fuchs. Arriving at Los Alamos August 14, 1944, Fuchs worked on problems relating to design of the implosion bomb. Bethe would later describe Fuchs as "one of the most valuable men in my division." In February 1945, Fuchs, according to his confession, began to pass along central information regarding the need for an implosion design for plutonium weapons, the critical mass for such an assembly, and design concepts. In June 1945, just before Trinity, he passed along a sketch of the bomb, providing its components and dimensions, and described the design in great detail. "When combined with information from other sources," the British Secret Service, MI5, notes, "this helped the Russians to make rapid progress in developing what was effectively a copy of the American atomic bomb design." 
After the war, Fuchs stayed on at Los Alamos. In April 1946, he participated in a three-day secret conference on the hydrogen bomb (the Super). The conference reviewed progress made during the war, explored the path for future development, and concluded that it was likely that the Super could be constructed and that it would work. Information on the Super passed on by Fuchs helped spur the Soviet's thermonuclear program, but there is some debate as to the usefulness of the information itself. Bethe in May 1952 prepared a brief classified history of the U.S. thermonuclear program that considered if "the Russians may have been able to arrive at a usable thermonuclear weapon by straightforward development from the information they received from Fuchs in 1946." He concluded that subsequent developments had shown that "every important point of the 1946 thermonuclear program had been wrong. If the Russians started a thermonuclear program on the basis of the information received from Fuchs, it must have led to the same failure." Ironically, revelations of Fuchs's betrayal of American nuclear secrets helped accelerate and expand the U.S. thermonuclear program. Four days after Fuchs confessed to treason, President Harry Truman directed the Atomic Energy Commission to proceed with the development of the hydrogen bomb. In the summer of 1946, Fuchs left Los Alamos and returned to Britain where he was offered a post working on the development of nuclear energy at the Atomic Energy Research Establishment at Harwell. He continued to pass secret information to the Soviet Union intermittently until he was finally caught, largely due to VENONA. On December 21, 1949, a British intelligence officer informed the physicist that they suspected him of passing classified information to the Soviet Union. Fuchs was arrested and, in January 1950, confessed. He was convicted of espionage and given a sentence of 14 years. He was released in 1959, after having served only nine years, and shortly thereafter moved to East Germany to resume his physics career. Fuchs remained an ardent communist and received numerous honors from his new home, including the Order of Karl Marx in 1979. Fuchs died on January 28, 1988.
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources (OHHR). Major sources consulted include the following. Lillian Hoddeson, et. al. Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945 (Cambridge: Cambridge University Press, 1993), 53, 98, 279, 317, 331; Richard G. Hewlett and Francis Duncan, Atomic Shield, 1947-1952: Volume II, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972), 312-14, 412, 415, 523; Hans Bethe, "Memorandum on the History of the Thermonuclear Program," May 28, 1952, declassified with deletions copy, OHHR. The British Secret Service has a webpage on Fuchs at https://www.mi5.gov.uk/klaus-fuchs, documents on the case, including Fuchs's confession are at https://www.nationalarchives.gov.uk/education/resources/cold-war-on-file/klaus-fuchs-confesses/. Federal Bureau of Investigation documents on Fuchs, put together as part of the Rosenberg case, are at http://vault.fbi.gov/rosenberg-case/klaus-fuchs. Robert C. Williams, Klaus Fuchs: Atomic Spy (Cambridge, MA: Harvard University Press, 1987), Bethe quote on Fuchs on p. 76. On the value of Fuchs's information on the Super, see Daniel Hirsch and William T. Mathews, "The H-Bomb: Who Really Gave Away the Secret?" Bulletin of the Atomic Scientists 46 (January/February 1990): 22-29; see also comments in "What Fuchs didn't know, and when he didn't know it," Bulletin of the Atomic Scientists 46 (May 1990): 51-52. A brief biography is at http://www.pbs.org/wgbh/amex/bomb/peopleevents/pandeAMEX54.html. Fuchs's work as a Los Alamos spy is described in Gregg Herken's, Brotherhood of the Bomb: The Tangled Lives and Loyalties of Robert Oppenheimer, Ernest Lawrence, and Edward Teller (New York: Henry Holt, 2002). The role intelligence provided by Fuchs played in the Soviet bomb project is explored in David Holloway, Stalin and the Bomb: The Soviet Union and Atomic Energy, 1939-1956 (New Haven: Yale University Press, 1994). See also Christopher Andrew and Vasili Mitrokhin, The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB (New York: Basic Books, 1999); John Earl Haynes and Harvey Klehr, Venona: Decoding Soviet Espionage in America (New Haven and London: Yale University Press, 1999); Jeffrey T. Richelson, A Century of Spies: Intelligence in the Twentieth Century (New York: Oxford University Press, 1995); and Allen Weinstein and Alexander Vassiliev, The Haunted Wood: Soviet Espionage in America - the Stalin Era (New York: Random House, 1999). The Los Alamos ID badge photograph of Fuchs was taken in 1944; it is courtesy the Los Alamos National Laboratory (LANL). The photograph of Fuchs in East Germany in 1960 is courtesy the Federation of American Scientists. The image of the VENONA intercept is courtesy the National Security Agency.