Editing Klaus Fuchs (section)

==Post-war activities==
At the request of [[Norris Bradbury]], who had replaced [[Robert Oppenheimer]] as director of the Los Alamos Laboratory in October 1945, Fuchs stayed on at the laboratory into 1946 to help with preparations for the [[Operation Crossroads]] weapons tests. The US [[Atomic Energy Act of 1946]] (McMahon Act) prohibited the transfer of information on nuclear research to any foreign country, including Britain, without explicit official authority, and Fuchs supplied highly classified U.S. information to nuclear scientists in Britain and to his Soviet contacts.

{{As of|2014}}, British official files on Fuchs were still being withheld.<ref>{{cite news |url=https://www.theguardian.com/books/2014/jun/13/the-spy-who-changed-the-world-mike-rossiter-review |newspaper=[[The Guardian]] |location=London |title=The Spy Who Changed the World by Mike Rossiter – review |date=13 June 2014 |first=Richard |last=Norton-Taylor |access-date=5 October 2014}}</ref>{{sfn|Laucht|2012|pp=78, 101}} {{As of|2020}}, the National Archives listed one dossier on Fuchs, KV 2/1263, including the "Prosecution file. With summary of early interrogations ... and details of the scientifical/technical information passed to the Russians". The date of release of this material was not stated.<ref>{{cite web |url=https://discovery.nationalarchives.gov.uk/details/r/C11134994 |title=mil Julius Klaus FUCHS: German/British. The atom spy, Klaus FUCHS, was an anti-Nazi... |date= |work=New Statesman |access-date=2 January 2021 |quote=}}</ref> According to an October 2020 book review, author Nancy Thorndike Greenspan "appears to have had access to some of the Fuchs files that have been withheld at Kew, such as the AB/1 series, which has been closed for access for most human beings".<ref>{{cite web |url=http://www.coldspur.com/tag/fuchs/ |title=Five Books on Espionage & Intelligence|date=31 October 2020 |work=New Statesman |access-date=2 January 2021 |quote=}}</ref>

Fuchs was highly regarded as a scientist by the British, who wanted him to return to the United Kingdom to work on Britain's [[High Explosive Research|postwar nuclear weapons programme]].{{sfn|Laucht|2012|pp=75–76}} He returned in August 1946 and became the head of the Theoretical Physics Division at the [[Atomic Energy Research Establishment]] at [[Harwell, Oxfordshire|Harwell]].{{sfn|Moss|1987|pp=93–94}} From late 1947 to May 1949 he gave [[Alexander Feklisov]], his Soviet case officer, the principal theoretical outline for creating a hydrogen bomb and the initial drafts for its development as the work progressed in England and America. Meeting with Feklisov six times, he provided the results of the [[Operation Sandstone|test at Eniwetok Atoll]] of uranium and plutonium bombs and the key data on production of [[uranium-235]].{{sfn|Rhodes|1995|p=259}}

Also in 1947, Fuchs attended a conference of the [[Combined Policy Committee]] (CPC), which was created to facilitate exchange of atomic secrets at the highest levels of governments of the United States, United Kingdom and Canada. [[Donald Duart Maclean|Donald Maclean]], another Soviet spy, was also in attendance as British co-secretary of CPC.{{sfn|Rhodes|1995|pp=300–301}}