Tube 8 in News

Tube Alloys was the code-name for the British nuclear weapon directorate during World War II, when the very possibility of nuclear weapons was kept at such a high level of secrecy that it had to be referred to by code even in the highest circles of government. Later in the war, tube alloy came to refer specifically to the synthetic 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 American Manhattan Project. However the programme had its origins in France and Germany.

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.

Francis Perrin of the Paris Group then 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 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 via England, just before Germany invaded Norway in April 1940. When Germany invaded France in May 1940, the Paris Group moved to Cambridge and brought the heavy water inventory of 188 litres. Joliot-Curie remained in France and became an active worker in the French Resistance movement.

At first British research correctly concluded that an atomic bomb using natural uranium is impossible with fast neutrons, because too many neutrons get lost or captured by the uranium-238 atoms. However, in February 1940, Otto Frisch and Rudolf Peierls, two exiled German scientists living in England realised that an atomic bomb could be built and detonated using only a few kilograms of uranium-235, the lighter rare isotope of uranium, using only fast neutrons. Frisch and Peierls reported in their famous Frisch-Peierls memorandum that if uranium-235 is completely separated from uranium-238, there is no need to slow the neutrons down, so no moderator was required.

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