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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.

Francis Perrin of the Paris Group 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, and then had it transferred to England by Charles "Jack" Howard, Earl of Suffolk, on the steamer SS Broompark, just before Germany invaded Norway in April 1940. The heavy water was subsequently secretly stored in the Tower of London in the same place as the Crown Jewels were kept. 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.