This document was released to the general public by the United States War Department on August 12th, 1945, just days after nuclear weapons had been dropped on Japan (Hiroshima on August 6th and Nagasaki on August 9th). The author, Prof. Henry D. Smyth of Princeton University, had worked on the Manhattan Project since early 1941, was involved in a variety of theoretical and practical aspects of the effort, and possessed security clearances which gave him access to all of the laboratories and production facilities involved in the project. In May, 1944, Smyth, who had suggested such a publication, was given the go ahead by the Manhattan Project’s Military Policy Committee to prepare an unclassified summary of the bomb project. This would have a dual purpose: to disclose to citizens and taxpayers what had been done on their behalf, and to provide scientists and engineers involved in the project a guide to what they could discuss openly in the postwar period: if it was in the “Smyth Report” (as it came to be called), it was public information, otherwise mum’s the word.
The report is a both an introduction to the physics underlying nuclear fission and its use in both steady-state reactors and explosives, production of fissile material (both separation of reactive Uranium-235 from the much more abundant Uranium-238 and production of Plutonium-239 in nuclear reactors), and the administrative history and structure of the project. Viewed as a historical document, the report is as interesting in what it left out as what was disclosed. Essentially none of the key details discovered and developed by the Manhattan Project which might be of use to aspiring bomb makers appear here. The key pieces of information which were not known to interested physicists in 1940 before the curtain of secrecy descended upon anything related to nuclear fission were inherently disclosed by the very fact that a fission bomb had been built, detonated, and produced a very large explosive yield.
- It was possible to achieve a fast fission reaction with substantial explosive yield.
- It was possible to prepare a sufficient quantity of fissile material (uranium or plutonium) to build a bomb.
- The critical mass required by a bomb was within the range which could be produced by a country with the industrial resources of the United States and small enough that it could be delivered by an aircraft.
None of these were known at the outset of the Manhattan Project (which is why it was such a gamble to undertake it), but after the first bombs were used, they were apparent to anybody who was interested, most definitely including the Soviet Union (who, unbeknownst to Smyth and the political and military leaders of the Manhattan Project, already had the blueprints for the Trinity bomb and extensive information on all aspects of the project from their spies.)
Things never disclosed in the Smyth Report include the critical masses of uranium and plutonium, the problem of contamination of reactor-produced plutonium with the Plutonium-240 isotope and the consequent impossibility of using a gun-type design with plutonium, the technique of implosion and the technologies required to achieve it such as explosive lenses and pulsed power detonators (indeed, the word “implosion” appears nowhere in the document), and the chemical processes used to separate plutonium from uranium and fission products irradiated in a production reactor. In many places, it is explicitly said that military security prevents discussion of aspects of the project, but in others nasty surprises which tremendously complicated the effort are simply not mentioned—left for others wishing to follow in its path to discover for themselves.
Reading the first part of the report, you get the sense that it had not yet been decided whether to disclose the existence or scale of the Los Alamos operation. Only toward the end of the work is Los Alamos named and the facilities and tasks undertaken there described. The bulk of the report was clearly written before the Trinity test of the plutonium bomb on July 16, 1945. It is described in an appendix which reproduces verbatim the War Department press release describing the test, which was only issued after the bombs were used on Japan.
This document is of historical interest only. If you’re interested in the history of the Manhattan Project and the design of the first fission bombs, more recent works such as Richard Rhodes’ The Making of the Atomic Bomb are much better sources. For those aware of the scope and details of the wartime bomb project, the Smyth report is an interesting look at what those responsible for it felt comfortable disclosing and what they wished to continue to keep secret. The forward by General Leslie R. Groves reminds readers that “Persons disclosing or securing additional information by any means whatsoever without authorization are subject to severe penalties under the Espionage Act.”
I read a Kindle edition from another publisher which is much less expensive than the Stanford paperback but contains a substantial number of typographical errors probably introduced by scanning a paper source document with inadequate subsequent copy editing.
Smyth, Henry D. Atomic Energy for Military Purposes. Stanford, CA, Stanford University Press,  1990. ISBN 978-0-8047-1722-9.