The Discovery of Nuclear Fission

By Nekud October 17, 2026 23 min read

In the late afternoon of December 17, 1938, in a chemistry laboratory on the outskirts of Berlin, two chemists at the Kaiser Wilhelm Institute for Chemistry opened a sealed glass tube and discovered, to their considerable astonishment, that the radioactive substance they had been painstakingly trying to identify was not, after all, the heavy element they had expected but rather a much lighter element altogether. The substance behaved chemically like radium, an element only slightly lighter than uranium, and the chemists had been trying for weeks to separate it from a barium carrier used to precipitate it from solution. But the substance would not separate. It followed the barium through every fractional crystallisation, behaving in every respect like the lighter element rather than the heavier one. The substance was, in fact, barium. The implication of the discovery was immediate and shocking. The uranium nucleus that the chemists had been bombarding with neutrons had not, as everyone had assumed, been chipped away at the edges to produce a sequence of slightly lighter elements. It had, instead, split roughly in two. The energy that bound the uranium nucleus together had been liberated in a single act, and the chemists had produced two medium-weight nuclei, several free neutrons, and an enormous release of energy. The discovery of nuclear fission, as the splitting of the atom would soon be called, was one of the most consequential scientific findings of the twentieth century. It was the discovery that made the atomic bomb possible, that made nuclear power possible, and that opened a new chapter in the long history of human understanding of matter.

The Setting of the Kaiser Wilhelm Institute

The Kaiser Wilhelm Institute for Chemistry was one of the foremost centres of nuclear physics and radiochemistry in the world in the 1930s. The institute had been founded in 1912 as a private research institute funded by the Kaiser Wilhelm Society, and it was located in Berlin in a group of buildings that included laboratories, workshops, and the residences of some of the principal scientists. The institute had been a centre of radiochemistry from its earliest years, and the institute was particularly strong in the chemistry of the radioactive elements, the chemistry of the heavy elements, and the chemistry of nuclear reactions. The institute was also closely connected with the German university system, and the institute’s principal scientists held teaching positions at the University of Berlin.

The principal figures in the institute’s nuclear research were Otto Hahn, a chemist who had been trained at the University of Marburg and who had joined the institute in 1928 as the head of a small radiochemistry group, and Lise Meitner, a physicist who had been trained at the University of Vienna and who had been at the institute since 1907. Hahn and Meitner had worked together for more than thirty years, and their collaboration was one of the most productive in the history of nuclear science. Their collaboration had produced, among other things, the discovery of several new radioactive isotopes, the identification of the phenomenon of nuclear isomerism, and a series of careful studies of the radioactive decay chains of the heavy elements.

Hahn and Meitner were joined in the mid-1930s by Fritz Strassmann, a younger chemist who had been trained at the Technical University of Hannover and who had joined the institute in 1929. Strassmann was a skilled radiochemist who had developed a particular expertise in the chemistry of the alkaline earth elements, and his expertise in the chemistry of barium and radium would prove to be crucial in the events that led to the discovery of fission. The three scientists formed a tightly knit group that was unusual in combining a physicist of the first rank with two of the leading radiochemists of the younger generation. The collaboration would produce the discovery of fission within four years of Strassmann’s arrival at the group.

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The Transuranic Hypothesis and the Italian Experiments

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The discovery of nuclear fission grew out of an attempt to understand the products of the neutron irradiation of uranium, and the attempt itself grew out of a famous series of experiments that Enrico Fermi and his collaborators in Rome had performed in 1934. Fermi had begun bombarding uranium with neutrons in March 1934, and he had identified several new radioactive species that could not be explained by the known products of neutron capture by uranium. Fermi had proposed that the new species were transuranic elements, elements heavier than uranium, that had been produced by neutron capture followed by beta decay. The Fermi hypothesis was widely accepted in 1934, and the identification of the transuranic elements became one of the most active fields of nuclear research.

Hahn and Meitner had repeated the Fermi experiments in the spring of 1934, and they had confirmed the existence of the new radioactive species. By the end of 1934, Hahn and Meitner had begun a systematic study of the products of neutron irradiation of uranium, and they had published a series of papers on the chemistry of the presumed transuranic elements. The Berlin group was not the only group working on the problem. Irene Curie and Pavel Savitch in Paris, and a group at the Imperial University of Tokyo, were also working on the problem, and the groups exchanged samples and information in an informal international collaboration.

The transuranic hypothesis was based on the assumption that the uranium nucleus, when bombarded with neutrons, would behave in the same way as other heavy nuclei. Other heavy nuclei, when bombarded with neutrons, captured the neutrons and underwent beta decay to produce a nucleus one position higher in the periodic table. The assumption seemed reasonable, and the chemistry of the presumed transuranic elements was consistent with the assumption. The transuranic elements, if they existed, would have chemical properties similar to those of the elements just below them in the periodic table, and the chemistry of the new radioactive species was consistent with that prediction. The transuranic hypothesis was, in retrospect, an extraordinary example of a near-universal scientific misreading of evidence, and it would be overturned in less than five years by the discovery of fission.

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The Curie Anomaly and the Crisis of the Transuranic Hypothesis

The transuranic hypothesis began to come under pressure in 1937 and 1938, when Irene Curie and Pavel Savitch in Paris reported the discovery of a new radioactive species with a half-life of three and a half hours. The species did not behave like any of the known transuranic elements, and its chemistry was difficult to reconcile with the transuranic hypothesis. Curie and Savitch tentatively suggested that the species might be a transuranic element, but the suggestion was uncertain. The Curie anomaly was the first major crack in the transuranic hypothesis, and the anomaly attracted considerable attention from the Berlin group.

The Berlin group, including Hahn, Meitner, and Strassmann, undertook a careful re-examination of the chemistry of the products of neutron irradiation of uranium. The re-examination took several months, and the results were puzzling. The group identified several new radioactive species that did not fit the transuranic pattern, and the group began to consider alternative explanations. One possibility, raised by Hahn in a letter to Meitner, was that the radioactive species might be isotopes of radium, an element in the same chemical group as barium but slightly heavier than uranium. The hypothesis was testable, because radium has distinctive chemistry that can be distinguished from that of barium and from that of the transuranic elements.

Strassmann undertook a careful series of fractional crystallisations to separate the suspected radium from the barium. The technique of fractional crystallisation had been developed by Marie Curie in the early twentieth century, and it is a method of separating two similar elements by exploiting the small differences in the solubility of their salts. Strassmann’s fractional crystallisations were careful and painstaking, and they were carried out over several weeks. The results were unambiguous. The suspected radium did not separate from the barium. Whatever the species was, it was not radium. It was, in fact, barium itself. The result was published in January 1939, in a paper by Hahn and Strassmann that announced, with characteristic German understatement, that the presumed transuranic elements were not transuranic at all but were instead the much lighter elements of the middle of the periodic table.

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The Theoretical Interpretation by Meitner and Frisch

The chemical identification of barium among the products of neutron irradiation of uranium was, by itself, a striking finding. But the chemical finding raised an immediate theoretical question. If the barium had been produced by the splitting of the uranium nucleus, the uranium nucleus must have split in two, and the energy that bound the uranium nucleus together must have been liberated in a single act. The magnitude of the energy release was enormous, on the order of two hundred million electron volts per fission. The release was far larger than the energies typically involved in nuclear reactions, and the release implied that the uranium nucleus was an unusually energetic structure that could release large amounts of energy when disrupted.

Lise Meitner, who had been forced to flee Germany in July 1938 because of her Jewish ancestry, was now in Sweden, working at the Nobel Institute in Stockholm. Meitner had been in close contact with Hahn and Strassmann throughout the experiments, and she had been kept informed of the results by letter. When Hahn wrote to Meitner in December 1938 to report the barium finding, Meitner recognised immediately that the finding implied the splitting of the uranium nucleus. Meitner discussed the finding with her nephew, Otto Robert Frisch, a physicist who was then working in Copenhagen with Niels Bohr. Meitner and Frisch met in the Swedish countryside in late December 1938, and over the course of a long walk they worked out the theoretical interpretation of the Hahn-Strassmann finding.

The Meitner-Frisch interpretation was based on the liquid drop model of the nucleus, which had been developed by the Danish physicist Niels Bohr and the Soviet physicist Yakov Frenkel in the mid-1930s. The liquid drop model treated the atomic nucleus as a drop of incompressible nuclear fluid, and the model predicted that a heavy nucleus could split into two lighter nuclei under the right circumstances. Meitner and Frisch calculated that the energy released by the splitting of a uranium nucleus into two medium-weight nuclei would be on the order of two hundred million electron volts, which was consistent with the observed energies of the particles produced in the experiments. The Meitner-Frisch interpretation was published in February 1939 in the journal Nature, in a paper that coined the term nuclear fission.

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The Spread of the News and the Confirmation

The discovery of nuclear fission was communicated to the scientific community in two stages. The first stage was the publication of the Hahn-Strassmann paper in the German journal Naturwissenschaften on January 6, 1939. The paper reported the chemical identification of barium among the products of neutron irradiation of uranium, and the paper concluded, with characteristic German caution, that the findings could not be reconciled with the transuranic hypothesis. The paper did not explicitly state that the uranium nucleus had split, and the paper left the theoretical interpretation open. The paper was widely read by nuclear physicists in Europe and the United States.

The second stage was the publication of the Meitner-Frisch paper in Nature on February 11, 1939. The Meitner-Frisch paper proposed the theoretical interpretation of the Hahn-Strassmann finding, and the paper coined the term nuclear fission. The Meitner-Frisch paper was widely read and widely discussed, and the paper established the interpretation that the uranium nucleus had split into two lighter nuclei. The Meitner-Frisch paper also estimated the magnitude of the energy release, and the paper noted that the release of neutrons in the fission process could, in principle, lead to a chain reaction.

The news of the discovery spread rapidly through the international physics community. Niels Bohr learned of the discovery in January 1939, just before he was due to travel to the United States for a conference at George Washington University. Bohr communicated the news to his American colleagues at the conference, and the news was repeated to physicists at Columbia University, Princeton University, and other institutions. Within weeks, the discovery of nuclear fission had been confirmed in laboratories in the United States, in Europe, and in the Soviet Union. The confirmation was particularly important in the United States, where several groups had been working on the transuranic problem and where the resources for the experimental work were considerable.

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The Possibility of a Chain Reaction

One of the most important implications of the discovery of nuclear fission was the possibility of a nuclear chain reaction. A chain reaction is a process in which the products of one nuclear reaction trigger further nuclear reactions, and the process becomes self-sustaining. In the case of nuclear fission, the chain reaction would be triggered by the neutrons released in the fission of one uranium nucleus, which would then trigger the fission of further uranium nuclei, which would release further neutrons, and so on. The chain reaction could, in principle, release enormous amounts of energy, and the chain reaction could, in principle, be made to occur either in a controlled manner, in a nuclear reactor, or in an uncontrolled manner, in a nuclear weapon.

The possibility of a chain reaction was recognised by several physicists in the early months of 1939. Frederic Joliot-Curie in Paris, Leo Szilard and Enrico Fermi at Columbia University, and other physicists in the United States and Europe all recognised the possibility, and the recognition led to a series of experimental and theoretical studies of the chain reaction. The studies were carried out in secret in some cases, particularly in the United States, where the possibility of a nuclear weapon was recognised as having military implications. The studies were also carried out in secret in the Soviet Union, where the physicist Igor Kurchatov began a programme of fission research in 1939.

The first experimental demonstration of a controlled chain reaction was achieved by Enrico Fermi and his collaborators at the University of Chicago on December 2, 1942. The experiment, known as Chicago Pile-1, was carried out in a squash court under the stands of the Stagg Field stadium. The experiment used natural uranium as the fuel and graphite as the moderator, and the experiment achieved a self-sustaining chain reaction for a period of about twenty-eight minutes. The experiment was the first practical demonstration of the controlled release of nuclear energy, and it was the foundation of the subsequent development of nuclear power and nuclear weapons.

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The Implications for the Atomic Bomb

The military implications of nuclear fission were recognised almost immediately after the discovery was announced in 1939. Leo Szilard, who had been one of the first physicists to recognise the possibility of a chain reaction, drafted a letter to President Franklin Roosevelt in August 1939, warning the President of the possibility that Nazi Germany might develop an atomic bomb. The letter was signed by Albert Einstein and was delivered to Roosevelt in October 1939. The letter initiated the American programme to develop an atomic bomb, and the programme eventually became the Manhattan Project.

The Manhattan Project was one of the largest and most secret scientific and engineering projects in the history of the world. The project was carried out at a network of sites in the United States, including Los Alamos in New Mexico, Oak Ridge in Tennessee, and Hanford in Washington state. The project involved more than a hundred thousand people, and the project cost more than two billion dollars in the money of the time. The project was led by the physicist J. Robert Oppenheimer, and the project produced the first atomic bombs, which were detonated in the Trinity test in July 1945 and in the bombings of Hiroshima and Nagasaki in August 1945.

The atomic bombings of Hiroshima and Nagasaki killed more than a hundred thousand people immediately, and many more in the years that followed from the effects of radiation. The bombings brought the Second World War to an end, but the bombings also opened a new era in human history. The bombings demonstrated that nuclear weapons could be produced, and the bombings demonstrated that nuclear weapons could be used in warfare. The bombings also raised the possibility that nuclear weapons could be used again, and the possibility has shaped the international order ever since.

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The Recognition and the Legacy of the Discovery

The discovery of nuclear fission was recognised by the award of the 1944 Nobel Prize in Chemistry to Otto Hahn. The award was made in 1944, but the announcement was delayed until after the war, and Hahn received the prize in 1945. The award recognised Hahn’s role in the chemical identification of the products of neutron irradiation of uranium. The award did not recognise the contributions of Lise Meitner and Fritz Strassmann, and the omission has been a subject of historical controversy ever since. The controversy has been particularly intense with respect to Meitner, who had been forced to flee Germany and who had made the theoretical interpretation of the experimental finding, and who has been widely regarded by many historians and physicists as a co-discoverer of fission who was unjustly denied the Nobel Prize.

The legacy of the discovery of nuclear fission has been profound. The discovery led to the development of nuclear weapons, which have shaped the international order ever since. The discovery also led to the development of nuclear power, which provides a substantial fraction of the world’s electricity and which has been the subject of intense political debate. The discovery has also led to the development of nuclear medicine, which has revolutionised the diagnosis and treatment of many diseases. The discovery has also led to the development of new branches of physics and chemistry, and the discovery has shaped the way that scientists approach the study of matter.

The discovery of nuclear fission is, in retrospect, one of the most consequential scientific discoveries of the twentieth century. The discovery was made in difficult circumstances, in a Germany that was in the grip of Nazi antisemitism, by scientists who were being persecuted for their ethnicity and their beliefs. The discovery was the result of a four-year collaboration between two chemists and a physicist, and the discovery depended on the combination of careful experimental chemistry and theoretical physics that the collaboration made possible. The discovery is a reminder of the power of science to transform the world, and the discovery is also a reminder of the moral responsibilities that come with the power of science.

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The Background of Nuclear Physics in the 1930s

The discovery of nuclear fission grew out of a quarter-century of progress in nuclear physics that had begun with Ernest Rutherford’s discovery of the atomic nucleus in 1911. By the mid-1930s, the basic structure of the atom was well understood. The atom consisted of a small, dense nucleus, which contained most of the mass of the atom, surrounded by a much larger cloud of electrons, which occupied most of the volume of the atom. The nucleus itself consisted of two kinds of particles, the positively charged protons and the electrically neutral neutrons, and the number of protons in the nucleus determined the chemical element. The forces that bound the protons and neutrons together in the nucleus were not yet fully understood, but the basic features of nuclear structure were clear.

The 1930s saw a series of remarkable advances in nuclear physics. James Chadwick had discovered the neutron in 1932, and the discovery had given nuclear physicists a new tool for probing the structure of the nucleus. The neutron had no electric charge, and it could penetrate the electron cloud surrounding the nucleus without being repelled. The neutron could also be captured by the nucleus, and the capture of a neutron could trigger nuclear reactions that produced new radioactive isotopes. The capture of a neutron could also produce new elements, and the production of new elements by neutron capture was one of the most active fields of nuclear research in the early 1930s.

The discovery of artificial radioactivity by Frederic and Irene Joliot-Curie in 1934 was another important advance. The Joliot-Curies had discovered that the bombardment of certain light elements with alpha particles could produce radioactive isotopes of other elements, and the discovery had shown that radioactivity could be induced artificially. The discovery had been recognised by the award of the 1935 Nobel Prize in Chemistry to the Joliot-Curies. The discovery of artificial radioactivity had opened a new field of nuclear research, and the field had grown rapidly through the mid-1930s.

The discovery of the neutron and the discovery of artificial radioactivity had together created the conditions for the discovery of nuclear fission. The neutron was the projectile that could penetrate the nucleus most easily, and artificial radioactivity was the technique that allowed scientists to detect the products of nuclear reactions. Without the neutron, the experiments that led to the discovery of fission could not have been performed. Without artificial radioactivity, the products of those experiments could not have been detected. The discovery of fission was, in a real sense, the discovery that the new tools of nuclear physics had made possible.

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The Fermi Group in Rome and the Transuranic Hypothesis

The experiments that led to the discovery of nuclear fission began in Rome in 1934, when Enrico Fermi and his collaborators began a systematic study of the products of neutron irradiation of uranium. Fermi was one of the leading nuclear physicists of the time, and he had been awarded the 1938 Nobel Prize in Physics for his work on the artificial radioactivity produced by neutron bombardment. The Fermi group included several of the leading Italian physicists of the time, including Franco Rasetti, Emilio Segre, and Edoardo Amaldi, and the group had access to the resources of the physics institute of the University of Rome, including a source of neutrons produced by the bombardment of beryllium with alpha particles from a radon source.

The Fermi group reported in June 1934 that the neutron bombardment of uranium produced several new radioactive species, and the group proposed that the species were transuranic elements, elements heavier than uranium. The transuranic hypothesis was widely accepted, and the identification of the transuranic elements became one of the most active fields of nuclear research in the mid-1930s. The transuranic hypothesis was based on the assumption that the uranium nucleus, when bombarded with neutrons, would behave in the same way as other heavy nuclei. Other heavy nuclei, when bombarded with neutrons, captured the neutrons and underwent beta decay to produce a nucleus one position higher in the periodic table.

The transuranic hypothesis was, in retrospect, an extraordinary example of a near-universal scientific misreading of evidence. The hypothesis was based on the assumption that the uranium nucleus was stable against splitting, and the assumption seemed reasonable at the time. The forces that bind the nucleus together were not well understood, but the assumption of stability was implicit in the transuranic hypothesis. The assumption would be overturned in less than five years by the discovery of fission, and the overturning of the assumption would be one of the most dramatic reversals in the history of science.

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The Experimental Verification of Fission in Multiple Laboratories

The discovery of nuclear fission was confirmed in a series of experiments in laboratories around the world in the months following the publication of the Hahn-Strassmann paper and the Meitner-Frisch interpretation. The most important of the confirmations were carried out in the United States, where several groups had been working on the transuranic problem. The first American confirmation was carried out at Columbia University by a group led by John R. Dunning and using the cyclotron at the Pupin Physics Laboratory. The Columbia group confirmed the barium finding within days of hearing of the discovery, and the confirmation was reported to the American Physical Society meeting in late January 1939.

The American confirmations were followed by confirmations in other countries. In France, Frederic Joliot-Curie confirmed the fission finding in February 1939, and the confirmation was reported in the Comptes Rendus of the French Academy of Sciences. In the Soviet Union, the physicist Igor Kurchatov confirmed the fission finding in early 1939, and the confirmation was reported in the Soviet physics journals. In the United Kingdom, the physicists at the Cavendish Laboratory in Cambridge confirmed the fission finding in February 1939. The confirmations were reported in rapid succession, and the discovery of nuclear fission was established as a fact of nature within weeks of the original publication.

The rapid confirmation of the fission finding in multiple laboratories around the world was a testament to the power of the international physics community in the 1930s. The community was small, and the members of the community were in close contact with each other. The community was also characterised by a culture of openness and rapid communication, and the culture allowed the members of the community to share their findings quickly. The rapid confirmation of the fission finding was, in a real sense, the product of the international character of the physics community.

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The Development of Nuclear Reactors and the First Sustained Chain Reaction

The development of nuclear reactors was one of the most important practical consequences of the discovery of nuclear fission. A nuclear reactor is a device that sustains a controlled nuclear chain reaction, and the development of such a device was the goal of a substantial fraction of the nuclear physics community in the years immediately after the discovery of fission. The most successful of the early reactor development efforts was the work of Enrico Fermi and his collaborators at the University of Chicago, which culminated in the Chicago Pile-1 experiment of December 2, 1942.

The Chicago Pile-1 experiment was the first sustained controlled nuclear chain reaction. The experiment used natural uranium as the fuel, graphite as the moderator, and cadmium rods as the control elements. The experiment was carried out in a squash court under the stands of the Stagg Field stadium at the University of Chicago, and the experiment achieved a self-sustaining chain reaction for a period of about twenty-eight minutes. The experiment demonstrated that a controlled nuclear chain reaction was possible, and the experiment laid the foundation for the subsequent development of nuclear power and nuclear weapons.

The Chicago Pile-1 experiment was followed by the development of a series of more sophisticated reactors in the United States, in the Soviet Union, in the United Kingdom, and in other countries. The reactors were used to produce plutonium for nuclear weapons, to produce tritium for thermonuclear weapons, and to produce electricity. The development of nuclear reactors was one of the most important technological developments of the twentieth century, and the development has had profound implications for the international order, for the global energy supply, and for the global environment.

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The Public Announcement and the Reaction of the Press

The discovery of nuclear fission was announced to the public in a series of newspaper articles in the United States and Europe in the spring of 1939. The most prominent of the newspaper articles was a front-page article in the New York Times on January 5, 1939, which reported the Hahn-Strassmann finding under the headline, “German Physicians Report Atom Is Smashed.” The article was based on the Naturwissenschaften paper and on interviews with American physicists who had heard of the discovery from Bohr. The article described the discovery in somewhat dramatic terms, and the article emphasised the possibility of a chain reaction and the potential for the release of nuclear energy.

The public reaction to the discovery was mixed. Some newspaper accounts emphasised the potential for the production of nuclear energy, and some accounts emphasised the potential for the production of nuclear weapons. The popular imagination was captured by the possibility of a chain reaction, and the possibility of an atomic bomb was widely discussed in the popular press in the months following the discovery. The discussion of the possibility of an atomic bomb was particularly intense in the United States, where the popular press had a long tradition of reporting on scientific developments, and where the discussions of the scientific community were widely covered.

The popular discussion of the atomic bomb was, in retrospect, a remarkable example of the way that the public can anticipate the implications of scientific discoveries. The popular discussion of the atomic bomb in the spring of 1939 was based on a relatively small amount of information, and the discussion was based on speculation rather than on detailed technical knowledge. The speculation, however, was largely correct. The popular discussion anticipated the basic features of the atomic bomb, including the use of uranium as the fuel, the use of a moderator to slow the neutrons, and the use of a chain reaction to release the energy. The popular discussion was a striking example of the way that the public can engage with the implications of scientific discoveries, even when the public does not have detailed technical information.

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The discovery of nuclear fission also led to the founding of the discipline of nuclear engineering, a field that has grown enormously since the end of the Second World War. The first generation of nuclear engineers were trained in the Manhattan Project, and they went on to design and build the reactors that produced plutonium for the first atomic weapons and that produced the electricity that powers much of the developed world. The field of nuclear engineering has continued to evolve since the war, and the field now encompasses a wide range of activities, including the design of nuclear reactors, the management of nuclear waste, the development of nuclear medicine, and the design of nuclear propulsion systems for ships and submarines. The discipline of nuclear engineering is, in a real sense, the most direct practical legacy of the discovery of nuclear fission.