In 1932, James Chadwick, working in Rutherford's Cavendish Laboratory at Cambridge University, discovered a previously unknown particle, the neutron. In 1934, Irne and Frdric Joliot-Curie discovered that by bombarding certain elements with alpha particles they were able to produce nuclei that decayed spontaneously, emitting nuclear radiation. Enrico Fermi, a young Italian physicist, read the Joliot-Curies' report on this work and decided to try similar experiments on uranium, using the neutron as a bombarding particle. An important result of Fermi's experiments was the discovery that slowing down neutrons makes them much more effective in causing uranium to decay spontaneously.

However, one result of the experiments was misinterpreted. After attempting unsuccessfully to determine the atomic numbers of the atoms formed by the neutron bombardment, Fermi concluded that he had created several transuranium elements (elements with atomic numbers larger than that of uranium).

In 1938, the German scientists Otto Hahn and Fritz Strassmann repeated Fermi's experiments. Among the products resulting from the bombardment of uranium nuclei with neutrons they identified barium, which has an atomic weight only slightly more than half that of uranium. Hahn notified Lise Meitner, a former associate, of the results of the experiments. She realized that Fermi had not created new elements in his 1934 experiments; instead, he had split the uranium nucleus into two smaller nuclei of approximately equal mass.

This news spread rapidly throughout the scientific world. Within a few months, Frdric Joliot-Curie with Hans von Halban and Lew Kowarski proved that, in the process of splitting, the uranium nucleus gives off free neutrons. This finding suggested that a self-sustaining fission reaction might be possible if the freed neutrons would, in turn, cause other uranium atoms to split. Albert Einstein in 1905 proved that matter was changed into energy during the reaction.

World War II began in 1939, and these advances in nuclear physics assumed military and political importance as it was realized that a fission reaction could be used for a devastating new weapon—an atomic bomb. Both the United States and Germany set to work in developing such a weapon, but only the United States succeeded. On December 2, 1942, the world's first nuclear reactor (built under Fermi's direction at the University of Chicago) produced a controlled chain reaction, marking the beginning of the nuclear age. On July 16, 1945, the United States detonated the first atomic bomb at a test site in the desert near Alamogordo, New Mexico.

Even before the end of World War II, scientists and political leaders in the United States began to plan the development of atomic energy for nonmilitary purposes. Control of atomic energy in the United States was passed from a military to a civilian agency by the Atomic Energy Act of 1946, which created the Atomic Energy Commission (AEC).

President Dwight D. Eisenhower proposed international sharing of the benefits of atomic energy with his Atoms-for-Peace program, presented to the United Nations in 1953. This program led to the formation, in 1957, of the International Atomic Energy Agency. In 1958 Belgium, France, West Germany, Italy, Luxembourg, and the Netherlands organized the European Atomic Community (EURATOM). Other international organizations to promote the peaceful uses of nuclear energy have since been formed.

In 1946, the United States government for the first time released for private use radioisotopes produced in government-owned nuclear reactors. In 1951 an experimental nuclear reactor at the AEC testing station at Idaho Falls, Idaho, produced the first useful amounts of electricity derived from nuclear energy.

In 1954, Congress passed legislation aimed at encouraging private industry to build nuclear reactors and to use reactor-produced electricity. An important military advance was also made in 1954 when the United States launched the world's first nuclear powered vehicle, the submarine USS Nautilus. The first nuclear-powered surface vessel, the Soviet icebreaker Lenin, was launched in 1957.

In the early 1960s, several demonstration nuclear power plants for the commercial production of electrical power went into operation in the United States. By the late 1960s, electric utilities, perceiving nuclear power as economically attractive, began to order large numbers of nuclear power plants.

In 1974, the Atomic Energy Commission was abolished and replaced by the Nuclear Regulatory Commission (NRC) and the energy Research and Development Administration (ERDA). The NRC assumed the regulatory functions of the AEC; the ERDA, its research and development functions. In 1977, the ERDA became part of the newly created Department of Energy.

The cost of power from nuclear plants increased rapidly through the early 1970s. Factors contributing to this increase included rising construction costs, high interest rates, and construction delays caused by licensing procedures and environmental lawsuits. By the late 1970s, few orders were being placed for nuclear power plants in the United States.

In May, 1979, a major accident occurred in a pressurized-water nuclear reactor at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania. There were no casualties, but residents in the area were exposed to radiation by a leakage of radioactive contaminants. The reactor was left crippled, and lengthy and expensive clean-up operations were required. The accident greatly raised public concern over the safety of nuclear reactors.

In 1986, a major accident at a nuclear power plant in Chernobyl, Ukraine, resulted in the release of a dangerous amount of radioactive material, causing a number of deaths and contaminating a large area. The reactor had no containment building, and the explosion tore through the roof of the reactor building. The accident again raised concerns over the safety of nuclear reactors.

By the mid-1990s, the building of new nuclear power plants in the United States and in many western European countries had largely ceased. Many older ones were being dismantled. There were several reasons for abandoning nuclear energy. In the wake of several nuclear accidents, such as the one at Chernobyl, many nuclear plants were investigated and found to be unsafe. Also, the cost of producing energy from nuclear reactors had risen higher than the cost of producing energy from fossil fuels. The disposal of nuclear waste materials also became a growing and increasingly costly problem.

In certain countries—particularly Japan, France, and those of eastern Europe—new nuclear energy plants continued to be built. Other nuclear capable countries include China, India, South Africa, United Kingdom, and Pakistan.

Nuclear energy today is used for powering ships and submarines, apart from producing electricity. Nuclear radiation is used in scientific studies and medicine. Around 29 countries use electricity generated through nuclear energy produced in 440 reactors.

During the 1920s and 1930s physicists, in experiments using high-energy particles, were able to fuse nuclei of hydrogen and other light atoms. In 1938, the German-American physicist Hans Bethe and the German physicist Carl von Weizscker, working independently, calculated the details of fusion processes that occur in stars.

The development of the atomic bomb led some scientists, including the United States physicist Edward Teller, to believe that a similar weapon could be made that would utilize the fusion process. Such a weapon—the hydrogen bomb—was developed by Teller and other U.S. scientists, and the first one was detonated in 1952. Since that time physicists have experimented with various techniques for making nuclear fusion a source of energy for commercial use, but the controlled and continuous fusion reactions required for such use have yet to be achieved.

Antoine Henri Becquerel, a French physicist, discovered the radioactivity of uranium in 1896. In 1898, two other French physicists, Marie and Pierre Curie, discovered two previously unknown radioactive elements, radium and polonium. Soon scientists all over the world were studying radioactivity. Among them was a British physicist, Ernest Rutherford. In the early twentieth century he helped establish that radioactive substances gave off three kinds of radiation—alpha particles, beta particles, and gamma rays. He showed that alpha particles were essentially atoms of helium without electrons. To account for the scattering behavior of alpha particles in passing through a metal foil, Rutherford theorized that an atom consists of a relatively heavy nucleus surrounded by tiny electrons. This theory was published in 1911. The studies of radioactivity made by Rutherford and other scientists showed that the nucleus is not indivisible, and that the nucleus of an atom of one element can be changed into that of another element.