In August 1939, Fuchs applied to become a
British subject, but his application had not been processed before the
Second World War broke out in Europe in September 1939. There was a classification system for
enemy aliens, but Born provided Fuchs with a reference that said that he had been a member of the SPD from 1930 to 1932, and an anti-Nazi. There matters stood until June 1940, when the police arrived and took Fuchs into custody. He was first interned on the
Isle of Man. In July, he was sent to internment camps in Canada, first on the
Plains of Abraham in
Quebec City and later at a site near
Sherbrooke, Quebec. During his internment in 1940, he continued to work and published four more papers with Born:
The Mass Centre in Relativity,
Reciprocity, Part II: Scalar Wave Functions,
Reciprocity, Part III: Reciprocal Wave Functions and
Reciprocity, Part IV: Spinor Wave Functions, and one by himself,
On the Statistical Method in Nuclear Theory. While interned in Quebec, he joined a communist discussion group led by
Hans Kahle. Kahle was a KPD member who had fought in the
Spanish Civil War. After fleeing to Britain with his family, Kahle had helped
Jürgen Kuczynski organise the KPD in Britain. Kristel arranged for mathematics professor
Israel Halperin, the brother-in-law of a friend of hers,
Wendell H. Furry, to send Fuchs some magazines, likely scientific journals. Max Born lobbied for his release. On Christmas Day 1940, Fuchs and Kahle were among the first group of internees to board a ship to return to Britain. Fuchs returned to Edinburgh in January, and resumed working for Born. In May 1941, he was approached by
Rudolf Peierls of the
University of Birmingham to work on the "
Tube Alloys" programme – the British atomic bomb research project. Despite wartime restrictions, he became a British subject on 31 July 1942 and signed an
Official Secrets Act declaration form. As accommodation was scarce in wartime Birmingham, he stayed with Rudolf and Genia Peierls. Fuchs and Peierls did some important work together, which included a fundamental paper about isotope separation. Soon after, Fuchs contacted Jürgen Kuczynski, who was now teaching at the
London School of Economics. Kuczynski put him in contact with Simon Davidovitch Kremer (codename: "Alexander"; 1900-1991), the secretary to the military attaché at the
Soviet Union's embassy, who worked for the
GRU (Russian:
Главное Разведывательное Управление), the
Red Army's foreign military intelligence directorate. After three meetings, Fuchs was teamed up with a courier so he would not have to find excuses to travel to London. She was
Ursula Kuczynski (codename: "Sonya"), the sister of Jürgen Kuczynski. She was a German communist, a major in Soviet Military Intelligence and an experienced agent who had worked with
Richard Sorge's spy ring in the Far East. In late 1943, Fuchs (codename: "Rest"; he became "Charles" in May 1944), transferred along with Peierls to
Columbia University, in New York City, to work on
gaseous diffusion as a means of
uranium enrichment for the
Manhattan Project. Although Fuchs was "an asset" of GRU in Britain, his "control" was transferred to the
NKGB (Russian:
Народный Kомиссариат Государственной Безопасности), the Soviet Union's civilian intelligence organisation, when he moved to New York. He spent Christmas 1943 with Kristel and her family in Cambridge. He was contacted by
Harry Gold (codename: "Raymond"), an NKGB agent in early 1944. From August 1944, Fuchs worked in the Theoretical Physics Division at the
Los Alamos Laboratory, under
Hans Bethe. His chief area of expertise was the problem of imploding the fissionable core of the
plutonium bomb. At one point, Fuchs did calculation work that
Edward Teller had refused to do because of lack of interest. He was the author of techniques (such as the still-used Fuchs-
Nordheim method) for calculating the energy of a fissile assembly that goes highly
prompt critical, and his report on blast waves is still considered a classic. Fuchs was one of the many Los Alamos scientists present at the
Trinity test in July 1945. Socially, Fuchs was later judged as someone who kept to himself, and never talked about politics. He was fairly well-liked. He dated grade school teachers Evelyn Kline and Jean Parker, and occasionally served as a
babysitter for other scientists. He befriended
Richard Feynman. Fuchs and Peierls were the only members of the British Mission to Los Alamos who owned cars, and Fuchs lent his
Buick to Feynman so Feynman could visit his dying wife in a hospital in
Albuquerque. When Fuchs was discovered to be a spy, his former colleagues were shocked. The postwar director of Los Alamos,
Norris Bradbury, later said that: :Fuchs was a strange man. I knew him, though not well. A very popular, very reticent bachelor, who was welcome at parties because of his nice manners. He worked very hard; worked very hard for us, for this country. His trouble was that he worked very hard for Russia, too. Basically, he hated the Germans bitterly. He had an undying hatred and he simply thought this country was not working hard enough to assist the Russians to defeat the Germans. Well, he was in his own odd way loyal to the United States. He suffered from a double loyalty. Fuchs's main courier in the United States was
Harry Gold, a chemist who lived in
Philadelphia, but was willing to travel to wherever Fuchs was.
Allen Weinstein, the author of
The Haunted Wood: Soviet Espionage in America (1999), has pointed out: "The NKVD had chosen Gold, an experienced group handler, as Fuchs's contact on the grounds that it was safer than having him meet directly with a Russian operative, but
Semyon Semyonov was ultimately responsible for the Fuchs relationship." Gold reported after his first meeting with Klaus Fuchs: After the end of the war, in April 1946, he attended a conference at Los Alamos that discussed the possibility of a
thermonuclear weapon; one month later, he filed a patent with
John von Neumann, describing a thermonuclear weapon design the two had collaborated on. Though it was not a viable design, it was the first instance of the idea of
radiation implosion being part of a weapon design. Radiation implosion would later become a core part of the successful
Teller–Ulam design for thermonuclear weapons, but its importance was not appreciated at the time. Bethe considered Fuchs "one of the most valuable men in my division" and "one of the best theoretical physicists we had." ==Post-war activities==