The Frisch-Peierls memorandum: A seminal document of nuclear history

July 14, 2025, 12:00PMNuclear NewsCameron Reed
Plaque honoring Frisch and Peierls at the University of Birmingham in England. (Photo: Anthony Cox)

The Manhattan Project is usually considered to have been initiated with Albert Einstein’s letter to President Franklin Roosevelt in October 1939. However, a lesser-known document that was just as impactful on wartime nuclear history was the so-called Frisch-Peierls memorandum. Prepared by two refugee physicists at the University of Birmingham in Britain in early 1940, this manuscript was the first technical description of nuclear weapons and their military, strategic, and ethical implications to reach high-level government officials on either side of the Atlantic. The memorandum triggered the initiation of the British wartime nuclear program, which later merged with the Manhattan Engineer District.

Who were Frisch and Peierls, and how did their memorandum come to be?

Frisch’s Los Alamos ID photo. (Photo: LANL)

Otto Frisch completed his doctoral degree in physics and mathematics at the University of Vienna in the summer of 1926. His aunt was physicist Lise Meitner of fission discovery fame. He became an assistant to Otto Stern at the University of Hamburg and developed into a master instrument builder and experimentalist. Stern left for America in 1933 but secured a position for Frisch at Birkbeck College in London through an Academic Assistance Council grant. The grant was good for only one year, but the head of the physics department at Birkbeck, Patrick Blackett, persuaded Niels Bohr to offer Frisch a position in Copenhagen when it expired. The story of how Frisch and Meitner were the first to interpret the discovery of fission during their Christmas holidays in 1938 after Meitner had been informed of the experimental evidence by Otto Hahn is well-known; soon thereafter, Frisch set up the first experiment designed to deliberately demonstrate the newly discovered effect.

Rudolf Peierls in 1966. (Photo: GFHund)

In the summer of 1939, Frisch was offered a lectureship at the University of Birmingham by physics department chair Marcus Oliphant, an Australian native and student of Ernest Rutherford. When Frisch arrived at Birmingham, Rudolf Peierls was already there, having been recruited by Oliphant in 1937. Peierls, originally from Berlin, was an outstanding theoretical physicist who earned his doctorate under Werner Heisenberg in 1929. After serving as an assistant to Wolfgang Pauli for three years, he secured a Rockefeller Foundation fellowship and spent time with Enrico Fermi in Rome before moving to Cambridge University. After temporary postings at Cambridge and Manchester, he was offered a position by Oliphant. Prophetically, in the fall of 1939 Peierls published a paper on the theory of critical mass for a fast-neutron chain reaction of a pure fissile isotope. At the time, he did not substitute any numbers into his formulae, as cross sections and secondary neutron numbers were then only roughly known, but a few months later this work would play a significant role in the preparation of the memorandum.

Frisch knew that Bohr had predicted that uranium-235 was the fissile isotope of that element. Around February 1940, he approached Peierls to ask what would happen if one had a large sample of pure U-235. Making some rough estimates of relevant parameters, they used Peierls’s formula to estimate a critical mass on the order of a mere 600 grams. The modern value for this is about 46 kilograms; the underestimate was caused by an overestimate of the fission cross s­ection. Alarmed that German scientists might be thinking along the same lines, they felt it their duty to inform the British government of the possibility of nuclear weapons.

They prepared the memorandum, which they took to Oliphant, who forwarded it to Sir Henry Tizard, head of the British government’s Committee on the Scientific Survey of Air Warfare. Tizard asked George P. Thomson, a physicist at Imperial College London (and son of electron-discoverer J. J. Thomson), to convene a committee to investigate the matter. This group would include, among others, James Chadwick, the discoverer of the neutron. Ironically, as refugees, Frisch and Peierls were barred from serving on the committee, but Thomson arranged for them to serve as consultants.

The final report of the committee in the summer of 1941 would have significant influence on the American nuclear program.

Titled “Memorandum on the Properties of a Radioactive ‘Super-bomb,’” Frisch and Peierls’s document comprised two parts: a three-page qualitative summary (the memorandum proper) and a technical appendix. Both parts are reprinted in several sources, but these often contain various errors; the correct version appears in Peierls’s collected papers (see “Further reading” at the end of the article).

The memorandum opened with a striking but ultimately very conservative statement: That the likely power of a bomb based on “the energy stored in atomic nuclei” could be equivalent to the explosion of 1,000 tons of dynamite. In contrast, a typical World War II bomb weight was 500 pounds, including the casing and firing mechanism.

Frisch and Peierls further estimated that the resulting explosion would probably cover the center of a big city, which would prove to be the case at Hiroshima and Nagasaki. They also emphasized that a nuclear weapon would release immense amounts of radioactive fallout that would be spread by wind and rain. To trigger the weapon, they suggested the method that would be used in the Little Boy bomb at Hiroshima: Divide the fissile material into two subcritical pieces, and then bring them together when the bomb is detonated. They suggested relying on cosmic-ray neutrons to initiate the chain reaction; at Los Alamos, neutron-emitting “initiators” would be developed for a more controlled approach. They also touched on the issue that assembly of the two pieces must happen quickly to avoid premature detonation by spontaneous fissions or an alpha-decay-induced neutron emission.

Frisch and Peierls remarked that they did not feel competent to discuss the strategic value of such a bomb—but they did exactly that with jargon-free prose, describing the device as “practically irresistible.” Since radiation would likely kill large numbers of civilians, they remarked that “this may make it unsuitable as a weapon for use by this country.” The ethical dilemmas of nuclear warfare were apparent before a single gram of U-235 had been isolated.

Regarding what might be happening in Germany, Frisch and Peierls felt that since much research had already been openly published, it was entirely possible that such a weapon could be under development there. Conversely, if nobody in Germany had thought of the idea, it was vital to keep their memorandum secret lest any rumors leak out. In anticipation of the later Cold War, they remarked that if Germany was working on such a weapon, the only effective reply would be a counterthreat with a similar bomb: mutually assured destruction.

The last few paragraphs of the memorandum dealt with issues of radiological protection, suggesting that detection squads equipped with instruments and self-contained oxygen supplies would have to enter contaminated areas in order to determine when and for how long people could enter the affected area.

They closed with what must be one of the greatest understatements in history: Their conclusions were not based on direct experiments “since nobody has ever yet built a super bomb,” but they felt confident that the relevant physics was well-established.

The technical appendix elaborated on all these issues with quantitative detail; it can still serve as a primer for students interested in the physics of nuclear weapons. In seven pages, Frisch and Peierls discussed the differences between U-235 and U-238, the timescales of slow- and fast-neutron reactions, why U-235 had to be used for a bomb, the concept of critical mass, an estimate of the energy that would be liberated by a bomb of a given mass, how a critical mass could be assembled, how the technique of thermal diffusion could be used to enrich uranium, and the levels of radioactivity that would be created. They covered a remarkable amount of ground in a few pages.

In mid-1943, Franklin Roosevelt and Winston Churchill formalized atomic relations between the United States and Britain. As a result, a group of British scientists traveled to America to work at various Manhattan Project sites; Frisch and Peierls both transferred to Los Alamos. Frisch’s work revolved around instrumentation, but he was also involved in potentially dangerous critical assembly experiments. Peierls headed a theoretical group charged with carrying out calculations and simulations of implosion dynamics. (One of his colleagues was the spy Klaus Fuchs.) Both Frisch and Peierls witnessed the Trinity test on July 16, 1945, and were awed by the spectacle.

After the war, both men returned to Britain. Frisch spent two years at the new Atomic Energy Research Establishment, but in 1947 he was offered a position at Cambridge University. He remained at Cambridge for the rest of his career, passing away in September 1979. Peierls returned to Birmingham but in 1963 became the Wykeham Chair of Physics at Oxford; he passed away in September 1995, almost 16 years to the day after Frisch.

In their memorandum, Frisch and Peierls touched on every key aspect of the physics, strategy, and ethics of nuclear warfare; it covered much more than the Einstein letter. Even after 85 years, it still makes for fascinating and informative reading.

Further reading

  • R. H. Dalitz and R. Peierls, Selected Scientific Papers of Sir Rudolf Peierls with Commentary (World Scientific, 1997).
  • O. R. Frisch, What Little I Remember (Cambridge University Press, 1979).
  • R. Peierls, Bird of Passage: Recollections of a Physicist (Princeton University Press, 1985).
  • B. C. Reed, “Revisiting the Frisch-Peierls Memorandum,” European Physics Journal H 49, no. 6 (2024); doi.org/10.1140/epjh/s13129-024-00070-x.


Cameron Reed is an ANS member and the Charles A. Dana Professor of Physics Emeritus at Alma College in Michigan. He has published extensively on the Manhattan Project and is preparing a book on the Frisch-Peierls memorandum to be published with Springer in late 2025.


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