Stephanie South | An account of William Penney’s report “Plutonium Weapon – General Description”, July 1947

William Penney, a British physicist who had worked on the Manhattan Project, and who was referred to as a ‘key member’ of the Los Alamos staff by General L. Groves, was asked in May 1947 to lead Britain’s nuclear weapons programme as Chief Superintendent of Armament Research (CSAR). In July of that year he published a report which would form the basis of the British project. Penney’s report, Plutonium Weapon – General Report, describes and discusses the key features of the American plutonium bomb, Fat Man, which was dropped on Nagasaki on 9 August 1945. The report outlined how Britain could reproduce such a weapon and indicates where further research and guidance was required; including how each component could be manufactured and who should be tasked with the job, along with specific details of areas where accuracy in design and manufacture would be of particular importance. Penney states that “the description can only give an overall picture and so does not profess scientific or technical detail”. The original report, accompanied by diagrams of the proposed design, offered a similar level of information as that given to the British and Soviets by Klaus Fuchs. This account will discuss the importance of Penney’s report and its significance to our understanding of the Nuclear Age.

The report raised important issues regarding production and development of the weapon. Firstly, who should manufacture each component and how it should be done; whether Penney’s group (CSAR) at the Armament Research Department and other scientific groups would be able to undertake the research, design, experimentation and production, or whether some parts, particularly those involving metalwork, could be ‘put to the trade’. Penney also enquires about the anticipated number of components required in order to judge whether it would be cost effective to have pressings and moulds produced for mass production, or alternatively they should be handmade. Secondly, a method for processing plutonium had not yet been developed in Britain and so new plants were urgently required; hence there were unresolved issues regarding this and the radiochemistry other components. Importantly, British scientists had no real knowledge of plutonium production nor had they any access to American plants.

To put Penney’s report into context, it was written after the Atomic Energy Act of August 1946, the McMahon Act, had been passed in America; which prevented the sharing of sensitive atomic information and effectively severed ties between Britain and the United States regarding atomic weapons. Britain believed that nuclear weapons were necessary in order to protect national security and industrial development, and so the project was launched as a matter of urgency.

Britain had been the first country to investigate the possibility of nuclear weapons. Otto Frisch and Rudolf Peierls studied chain reactions of Uranium-235 and the potential application in ballistic weapons in 1940, before publishing a memorandum on the subject later that year. The MAUD Committee was set up in Britain and commissioned both theoretical and experimental research into the possibility of a bomb. Also in 1940, it had been agreed that co-operation between Britain and America on the atomic weapons project would be mutually advantageous; although during World War II Britain had limited resources due to economic strain and heavy bombing, and so had to rely on American to cover the massive expense incurred. Until the middle of 1942, atomic information was passed between American scientists and the MAUD Committee, although according to D. C. Fakley the British were ambivalent about complete integration of the bomb project and expressed reservations. By the end of the year, the American project had become the responsibility of the U.S. Army, headed by General L. Groves, with strict security restrictions preventing atomic information being shared freely between the two nations.

In August of 1943, the Quebec Agreement was signed, agreeing that the bomb project was again to be a wholly joint effort, with neither country using the weapon against the other nor disclosing any information to a third party without mutual consent. At this point, British scientists began arriving in America to work on the project and any notion of a wartime project running concurrently in Britain was abandoned. In August 1945, Prime Minister Attlee organised a secret cabinet committee to consider nuclear policy, initially labelled GEN.75 and later Atomic Bomb Committee. Later a subcommittee, GEN.163, began making key decisions regarding the atomic bomb programme. By January 1947, GEN.163 had decided to proceed with the acquisition of atomic weapons, although the remainder of Parliament was not notified until May 1948. Britain’s first nuclear reactor for the production of plutonium went critical on 3 July 1948, the first atomic device, Hurricane, detonated on 3 October 1952 and the first nuclear weapon, Blue Danube, delivered to the Royal Air Force in November 1953.

Penney’s report is significant to our understanding of the Nuclear Age as the document was a key part of the development of the British atomic weapons programme. When it was published, planning had already begun for a British programme but Penney’s report was the first to outline how this may actually be carried out. The document highlights many issues which had to be overcome before acquisition of fissile plutonium and manufacture of the weapon could begin; it shows us quite how far behind America we were at this point in time following the McMahon Act. The report is also important because of whom it was written by; Penney had a great knowledge of the subject and had been an important and credible member of the Manhattan Project. Therefore he was an appropriate candidate to head our own programme and advise our Government on how to join the Nuclear Age.

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