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Britain contributed to the Manhattan Project by helping initiate the effort to build the first atomic bombs in the United States during World War II, and helped carry it through to completion in August 1945 by supplying crucial expertise. Following the discovery of nuclear fission in uranium, scientists Rudolf Peierls and Otto Frisch at the University of Birmingham calculated, in March 1940, that the critical mass of a metallic sphere of pure uranium-235 was as little as 1 to 10 kilograms (2.2 to 22.0 lb), and would explode with the power of thousands of tons of dynamite. The Frisch–Peierls memorandum prompted Britain to create an atomic bomb project, known as Tube Alloys. Mark Oliphant, an Australian physicist working in Britain, was instrumental in making the results of the British MAUD Report known in the United States in 1941 by a visit in person. Initially the British project was larger and more advanced, but after the United States entered the war, the American project soon outstripped and dwarfed its British counterpart. The British government then decided to shelve its own nuclear ambitions, and participate in the American project.

In August 1943, the Prime Minister of the United Kingdom, Winston Churchill, and the President of the United States, Franklin D. Roosevelt, signed the Quebec Agreement, which provided for cooperation between the two countries. The Quebec Agreement established the Combined Policy Committee and the Combined Development Trust to coordinate the efforts of the United States, the United Kingdom and Canada. The subsequent Hyde Park Agreement in September 1944 extended this cooperation to the postwar period. A British Mission led by Wallace Akers assisted in the development of gaseous diffusion technology in New York. Britain also produced the powdered nickel required by the gaseous diffusion process. Another mission, led by Oliphant who acted as deputy director at the Berkeley Radiation Laboratory, assisted with the electromagnetic separation process. As head of the British Mission to the Los Alamos Laboratory, James Chadwick led a multinational team of distinguished scientists that included Sir Geoffrey Taylor, James Tuck, Niels Bohr, Peierls, Frisch, and Klaus Fuchs, who was later revealed to be a Soviet atomic spy. Four members of the British Mission became group leaders at Los Alamos. William Penney observed the bombing of Nagasaki and participated in the Operation Crossroads nuclear tests in 1946.

Cooperation ended with the Atomic Energy Act of 1946, known as the McMahon Act, and Ernest Titterton, the last British government employee, left Los Alamos on 12 April 1947. Britain then proceeded with High Explosive Research, its own nuclear weapons programme, and became the third country to test an independently developed nuclear weapon in October 1952. (Full article...)