Background The German government began development of an atomic weapons programme in April 1939 when two scientists, Irène and Frédéric Joliot-Curie, declared in an issue of the scientific journal Nature that atomic fission was possible and could produce immense amounts of power. The Joliot-Curies argued that not only did it have possibilities as a power source, it might also be used to create an extremely powerful weapon. By September 1941, the German scientists assigned to the programme had determined how a nuclear reactor could be developed. This required significant quantities of heavy water to act as a neutron moderator to encase the uranium which would be used to provide the material for a nuclear weapon. Heavy water was extremely scarce and difficult to produce, and Germany only possessed a small amount which had been produced in laboratories. Norway possessed a large stockpile which was produced by the Vemork Norsk Hydro chemical plant near the village of Rjukan, but the Norwegian government would not sell more than three gallons of heavy water a month, becoming suspicious of the sudden increase in demand for the water by the German government. When Norway was invaded and occupied by Germany in April 1940 this obstruction ended; the Vemork plant was captured and began producing heavy water for the German atomic weapons programme. Production of heavy water was slowed initially due to the effects of the fighting in Norway and a drought in the area, which led to a lack of water to provide hydroelectric power for the plant. Once the weather improved and snow began to melt, providing enough water to create sufficient hydroelectric power, production continued. By mid-1942 it had come to the attention of the British government that the plant had begun to produce large quantities of heavy water and was stockpiling it for later use. The decision was therefore taken that the plant and the stockpiles of heavy water would have to be destroyed in order to impede the German programme. Several methods for destroying the plant were considered. The first was a mass raid by Royal Air Force (RAF) bombers, but this was rejected due to the difficulty in locating the plant during night bombing, the principal bombing tactic used by the RAF at the time, and the heavy casualties that would be inflicted on the local Norwegian population during a night raid. An attack by Norwegian saboteurs was also