The government decided on 27 July 1954 to begin development of a thermonuclear bomb, and announced its plans in February 1955. British knowledge of thermonuclear weapons was based on the work done at the Los Alamos Laboratory during the war. Two British scientists, Egon Bretscher and Klaus Fuchs, had attended the conference there on the Super (as it was then called) in April 1946, and Chadwick had written a secret report on it in May 1946, but the design was found to be unworkable. Some intelligence about Joe 4 was derived from its debris, which was provided to Britain under the 1948 "Modus Vivendi". Penney established three megaton bomb projects at Aldermaston: Orange Herald, a large boosted fission weapon; Green Bamboo, an interim thermonuclear design similar to the Soviet Layer Cake used in Joe 4 and the American Alarm Clock; and Green Granite, a true thermonuclear design.