№371 Letter from the USSR People’s Commissar for State Security to L P Beria on the preparation of atomic bomb test in the United States1 № 4305 m July 10 19452 Top Secret Urgent We have received information from several reliable USSR NKGB agent sources that in July of this year3 the U S has scheduled the first experimental explosion4 of the atomic bomb The explosion is expected to take place on July 10th We have the following information on this bomb The bomb is made with element 94 plutonium which is analogous to uranium-235 in its nuclear fission properties Element 94 is a product of nuclear fission of uranium-238 which takes place in atomic machines uranium boilers The plutonium is in the shape of a sphere weighing 5kg At the center of the sphere there is a socalled initiator a beryllium-polonium source of alpha-particles that activates the active material in the bomb at the necessary moment The plutonium mass is surrounded by an 11cm thick 1 The letter was prepared by E M Potapova there is a note from the typing office based on information received – see document № 367 Preparations for the Potsdam Conference were underway during this time for the three allied powers – the USSR the U S and Great Britain scheduled for July 17-August 2 1945 According to the FSB Central Archive of Russia Stalin left Moscow for Potsdam on July 16 1945 at 5 30p m It is quite possible that Beria gave him this information before his departure An indirect confirmation of this assumption is the fact that the original copy of the message about the testing of the atomic bomb was not found in Beria’s archives A search of Stalin’s archives for the original has not been conducted In Potsdam on July 16 Truman received a telephone message and on July 21 a full report on the successful test of the atomic bomb As is known during the conference Truman informed Stalin about the creation of the atomic bomb According to James Byrnes it happened as follows from the record of conversation between Byrnes and Herbert Feis 1958 “Byrnes said that he came to the conclusion that it would be catastrophic for the U S and China to draw the Soviet Union into the war in the Pacific This led him to think that it would be nice to put it mildly to not fully inform Stalin about the atomic bomb’s potential Otherwise Stalin could speed up the Soviet Union’s entry into the war This is why the decision was made to tell Stalin about the test results in passing at the end of one of the meetings of the heads of government After agreeing on what to say Truman and Bohlen who was present as an interpreter walked around the table and in the most casual manner Truman said to Stalin that he wanted to inform him that a new and powerful weapon was developed in the U S which the U S decided to use against Japan According to Byrnes the entire conversation between Truman and Stalin lasted no more than a minute ” Quoted from the book by V L Malkov “The Manhattan Project” Razvedka i diplomatiya – M Nauka 1995 pg 234 According to recollections there was no reaction from Stalin to this information from Truman 2 The typing office note shows this document was printed on July 6 1945 3 A reference to Soviet espionage directed at the Manhattan Project 4 Hereafter italics indicate parts of the text that were handwritten in empty spaces on the printed document aluminum casing The aluminum casing is in turn surrounded by a layer of the explosive material pentolite 46cm in thickness The total weight of the bomb including the weight of the pentolite the case etc is around 3 tons The estimated explosive power of the bomb is equivalent to 5 thousand tons of TNT 5 The same sources provide the following information on the reserves of active materials for the atomic bomb a Uranium-235 As of April of this year the U S had 25kg of uranium-235 The production is approximately 7 5kg per month b Element-94 There are 6 5kg of this substance in Camp-26 Its production in atomic machines is established and the production plan is being overfulfilled The USSR People’s Commissar for State Security Commissar of State Security First Rank V Merkulov7 5 Tests have shown that the explosive power is equivalent to 15-20 thousand tons of TNT A reference to Los Alamos 7 There is no signature 6