 Four universities provided the locations where the experiments were taking place. The laboratory at the University of Birmingham was responsible for all the theoretical work, such as what size of critical mass was needed for an explosion. It was run by Peierls, with the help of fellow German refugee scientist Klaus Fuchs. The laboratories at the University of Liverpool and the University of Oxford experimented with different types of isotope separation. Chadwick's group at Liverpool dealt with thermal diffusion, which worked based on the assumption that different isotopes of uranium are attracted to different temperatures. Franz Simon's group at Oxford investigated the gaseous diffusion of isotopes. This method works on the principle that at differing pressures uranium 235 would diffuse through a barrier faster than uranium 238. Eventually the most promising method of separation was gaseous diffusion. Egon Bretscher and Norman Feather's group at Cambridge investigated whether another element, now called plutonium, could be used as an explosive compound. Because of the French scientists, Oxford also obtained the world's only supply of heavy water, which helped them theorise how uranium could be used for power.
