from the Department of Scientific and Industrial Research to the Ministry of Supply effective 1 November 1945. To coordinate the atomic energy effort, the Gen 75 Committee decided to appoint a Controller of Production, Atomic Energy (CPAE). Wilmot suggested Marshal of the Royal Air Force Lord Portal, the wartime Chief of the Air Staff. Portal was reluctant to accept the post, as he felt that he lacked administrative experience outside the Royal Air Force, but eventually accepted it for a two-year term, commencing in March 1946. In this role he had direct access to the Prime Minister. Portal ran the project until 1951, when he was succeeded by Sir Frederick Morgan. It was hidden under the cover name High Explosive Research. Nuclear reactors An early debate among the scientists was whether the fissile material for an atomic bomb should be uranium-235 or plutonium. Tube Alloys had performed much of the pioneering research on gaseous diffusion for uranium enrichment, and Oliphant's team in Berkeley were well-acquainted with the electromagnetic process. The staff that had remained in Britain strongly favoured uranium-235; but the scientists that had worked in the United States argued for plutonium on the basis of its greater efficiency as an explosive, despite the fact that they had neither the expertise in the design of nuclear reactors to produce it, nor the requisite knowledge of plutonium chemistry or metallurgy to extract it. However, the Montreal Laboratory had designed and was building pilot reactors, and had carried out some work on separating plutonium from uranium. The Manhattan Project had pursued both avenues, and the scientists who had worked at Los Alamos were aware of work there with composite cores that used both; but there were concerns that Britain might not have the money, resources or skilled manpower for this. In the end, it came down to economics; a reactor could be built more cheaply than a separation plant that produced an equivalent quantity of enriched uranium, and made more efficient use of uranium fuel. A reactor and separation plant capable of producing enough plutonium for fifteen bombs per year was costed at around £20 million. It fell to the Gen 75 Committee to decide how many reactors should be built at its meeting on 18 December 1945. Considering the demands that reactors would make on scarce skilled labour and materials, the Gen 75 Committee decided to defer a decision on building