Frisch – Peierls memorandum The Frisch – Peierls memorandum was the first technical exposition of a practical atomic weapon . Written by Otto Frisch and Rudolf Peierls while they were both working at the University of Birmingham in England , the memorandum contained new calculations about the size of the critical mass needed for an atomic bomb , and helped accelerate British and U . S . efforts towards bomb development during World War II . Background and influence . Given to Marcus Oliphant , Oliphant passed the document on to Henry Tizard , chairman of the Committee on the Scientific Survey of Air Defence who consequently requested the setting - up of what was to become the secret MAUD Committee . The memorandum ( a copy of which is held in the Public Record Office at Kew ) is dated March 1940 . The two men were the first to calculate that an atomic bomb would require about of the isotope uranium - 235 . ( The estimate of 1 kg turned out to be too low ; see Critical mass . ) Before it had been assumed that the bomb itself would require many tons of uranium , implying that it was theoretically possible , but not a practical military device . An earlier letter to President Franklin D . Roosevelt , signed by Albert Einstein ( but written by Leó Szilárd ) , had suggested it may need to be delivered by ship but " might very well " not be small enough to drop from the air . The memorandum helped galvanize both Britain and America down a path which led to a report by the British MAUD Committee , the Tube Alloys project , the Manhattan Project , and ultimately the atomic bombings of Hiroshima and Nagasaki . Contents . The memo was written in two parts . The second was an explanation of the science supporting their conclusions . The first was an elegant and comprehensive outline of the implications of their calculations . It included a proposal that the best defence against such a weapon would be to develop one before Germany did so . In a few short pages these two scientists had anticipated the policies of deterrence which would later shape Cold War geopolitics . The memorandum opens with : " Strictly Confidential " Memorandum on the properties of a radioactive “ super - bomb ” " The attached detailed report concerns the possibility of constructing a “ super - bomb ” which utilizes the energy stored in atomic nuclei as a source of energy . The energy liberated in the explosion of such a super - bomb is about the same as that produced by the explosion of 1000 tons of dynamite . This energy is liberated in a small volume , in which it will , for an instant , produce a temperature comparable to that in the interior of the sun . The blast from such an explosion would destroy life in a wide area . The size of this area is difficult to estimate , but it will probably cover the centre of a big city . " " In addition , some part of the energy set free by the bomb goes to produce radioactive substances , and these will emit very powerful and dangerous radiations . The effect of these radiations is greatest immediately after the explosion , but it decays only gradually and even for days after the explosion any person entering the affected area will be killed . " " Some of this radioactivity will be carried along with the wind and will spread the contamination ; several miles downwind this may kill people . "