the former had a yield of and that of the Nagasaki bomb of about . He wanted to return to academic life, and he was offered a chair of mathematics at the University of Oxford. British Nuclear Weapons Programme At the end of the war the British government, now under the Labour Prime Minister, Clement Attlee, believed that America would share the technology that British leaders saw as a joint discovery under the terms of the 1943 Quebec Agreement and the 1944 Hyde Park Agreement. In December 1945, Attlee ordered the construction of an atomic pile to produce plutonium and requested a report to detail requirements for Britain's atomic bombs. Penney was approached by C. P. Snow and asked to take up post as Chief Superintendent Armament Research (CSAR, called "Caesar") at Fort Halstead in Kent, as he suspected Britain was going to build an atomic bomb of its own and he wanted Penney in this job. As CSAR he was responsible for all types of armaments research. In 1946, at the request of Groves and the United States Navy, Penney returned to the United States where he was put in charge of the blast effects studies for Operation Crossroads at Bikini Atoll in the Marshall Islands in July 1946. He wrote the after action reports on the effects of the two nuclear detonations. His reputation was further enhanced when, after the sophisticated test gauges failed in the Able test due to the bomb missing the intended target, he was able to determine the yield using observations from simple devices made from petrol tins. The second test, Baker, was an underwater test, and it impressed Penney greatly. He speculated on the possible effects if an atomic bomb was exploded near a port city in the UK. The passage of the McMahon Act (Atomic Energy Act) in August 1946 made it clear that Britain would no longer be allowed access to US atomic research. Penney left the United States and returned to the United Kingdom where he drew up plans for an atomic weapons section in his department, submitting them to the Controller of Production, Atomic Energy, Marshal of the Royal Air Force Lord Portal, in November 1946. In January 1947, Penney went to the United States to attend a symposium on the Crossroads tests. With almost all other aspects of atomic co-operation between the countries at an end, his personal