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Tube Alloys
Tube Alloys was a codename of the clandestine research and development programme, authorised by the Government of the United Kingdom with participation from Canada, aiming to develop nuclear weapons for the United Kingdom's nuclear weapons programme during the Second World War. Starting before the Manhattan Project (the Western Allies' nuclear bomb project) in 1942, the British efforts were kept highly classified such that they had to be referred to by code even within the highest circles of government. At the end of the Second World War, ''Tube Alloys'' came to refer specifically to the element plutonium, whose very existence was secret until its use in the atomic bombing of Nagasaki.
The Tube Alloys programme in Britain and Canada, effectively the first nuclear weapons project of its type, was eventually subsumed into the US-led Manhattan Project. Both programmes had some elements of earlier research accomplished in France and Germany. Despite reaching the understanding with the United States, the details of such efforts were not completely provided to the United Kingdom, although ironically the Soviet Union gained through espionage valuable details, scientific data and research documents critical for the development of nuclear weapons. This prompted the United Kingdom to (re)launch its own directorate to match the achievements of the Soviets and Americans. Production facilities were established under the auspices of the British Army and Royal Air Force and British scientists were re-called from the United States to continue their work under the auspices of an independent British programme. Finally in 1952, Britain performed a test under codename ''Hurricane'', a decade after conducting research on the field. In 1958, after British demonstration of a two-stage thermonuclear (fission-fusion) bomb, the United Kingdom and the United States signed a defence understanding agreement that resulted in the United Kingdom electing to resume its nuclear weapons co-operation with the United States.
==The Paris Group==
Otto Hahn in Germany and Lise Meitner, exiled in Sweden, reported nuclear fission in uranium in 1938. This was followed up by a group of scientists at the Collège de France in Paris: Frédéric Joliot-Curie, Hans von Halban, Lew Kowarski, and Francis Perrin. In February 1939, the Paris Group showed that when fission occurs in a uranium nucleus, two or three extra neutrons are also given off. This important observation suggested that a self-sustaining chain reaction might be possible. It was immediately apparent to many scientists that, in theory, an extremely powerful explosive could be created, an ''atomic bomb'', but many scientists thought a practical bomb was an impossibility.
Perrin defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction. However, it was found that natural uranium cannot sustain a chain reaction without a moderator to slow down the fast-moving neutrons given off by the fission. Early in 1940, the Paris Group decided on theoretical grounds that heavy water would be an ideal moderator. They asked the French Minister of Armaments to obtain as much heavy water as possible from the only source, the large Norsk Hydro hydroelectric station at Vemork in Norway. The French then discovered that Germany had already offered to purchase the entire stock of Norwegian heavy water, indicating that Germany might also be researching an atomic bomb. The French told the Norwegian government of the possible military significance of heavy water. Norway then gave the entire stock to a French Secret Service agent, who secretly brought it to France, just before Germany invaded Norway in April 1940. On 19 June 1940 it was shipped to England by Charles Howard and Major Ardale Golding, aboard the steamer . The heavy water was subsequently secretly stored in the library at Windsor Castle.〔Danger UXB p. 69〕 When Germany invaded France in May 1940, the Paris Group moved to Cambridge and brought the heavy water inventory of . Joliot-Curie remained in France and became an active in the Resistance.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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