The Tube Alloys Directorate checked whether Britain could produce a bomb without American assistance. A gas diffusion facility to produce 1 kg of armed uranium per day cost up to $3 million in research and development and up to $50 million to build during the British war. In Canada, a nuclear reactor would have to be built that produced 1 kg of plutonium per day. Construction and the cost of $5 million would take up to five years. The project would also require heavy water generation facilities for the reactor, which costs between $5 million and $10 million, and for the production of uranium metal, which would cost an additional $1.5 million. The project would need an overwhelming priority, as it is estimated that 20,000 workers are needed, many of whom are highly skilled, 500,000 tons of steel and 500,000 kW of electricity. An interruption of other war projects would be inevitable and it was unlikely that it would be ready in time to influence the outcome of the war in Europe. The unanimous response was that before these measures were launched, an additional effort should be made to achieve American cooperation.  The British bomb was based on the design of the American plutonium implosion bomb, brought by Penney and Fuchs. Fuchs, a Soviet spy, passed on information similar to the Soviet project. The plutonium was produced primarily from Hinton reactors, although some were imported from the Chalk River site in Canada.
The first British plutonium plant was built at Sellafield, then renamed Windscale, Cumberland. Its first production reactor was criticized in 1950 and produced usable plutonium in 1952. During World War II, Britain and America worked closely together on the development of the atomic bomb, and many British scientists participated in the Manhattan Project. It is important that, as part of the War-in-War Quebec Agreement, Britain has relinquished the veto over the use of nuclear weapons by the United States. The agreements of the Quebec Conference in 1943, in Hyde Park in September 1944 and 1945 in Washington promised the continuation of nuclear cooperation after the end of the war. On August 19, Roosevelt and Churchill signed the Quebec Agreement, which was carved out of four pages of the Citadel note document and officially titled “Article of the Cooperation Agreement between the United States authorities and the United Kingdom on the issue of pipeline alloys.”  The United Kingdom and the United States agreed that it was essential for our common security in the current war that the Tube Alloys project be successful as soon as possible and that the best way to achieve this was to pool their resources.  The Quebec Agreement provided that it was essential for our common security in the current war that the “pipeline alloys” project be implemented in the early stages; and on April 13, 1944, the Combined Policy Committee decided to continue planning and construction of a heavy water nuclear reactor in Canada. It was agreed that a full exchange of information useful for the design of the reactor and the extraction of the plutonium it will produce will take place. However, General Leslie Groves insisted that Halban and the other scientists, who were not British subjects, leave the project and that the English physicist John Cockcroft be the director of the laboratory. The only part of the Quebec agreement that put Stimson in trouble was the requirement of mutual consent before nuclear bombs were used.  If Congress had known, they would never have supported it. The AMERICAN veto on British commercial and industrial uses after the war showed that Britain was the junior partner of the Great Alliance.
Churchill considered the Quebec agreement, among other things, to be the best deal he could have reached in the present circumstances, and the restrictions were the price to pay for obtaining the technical information necessary for a successful post-war nuclear project.  Margaret Gowing noted that “the idea of independent deterrence is already well entrenched.”  A heavy burden for the agreement was laid in 1944, when the United States learned that the Roya