Nuclear fission is a process where the nucleus of an atom breaks apart into two or more smaller nuclei. This process often creates gamma photons and releases a large amount of energy, even compared to other radioactive processes.
Nuclear fission was discovered by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Hahn and Strassmann found evidence of fission on December 19, 1938, while Meitner and Frisch explained it in January 1939. Frisch named the process "fission" after a similar process in living cells. In February 1939, Hahn and Strassmann predicted that extra neutrons are released during fission, which made it possible for a chain reaction to occur.
For heavy atoms, fission is an exothermic reaction that releases energy as electromagnetic radiation and as the movement of the broken pieces, which heats the material around them. Like nuclear fusion, fission produces energy only if the total energy that holds the resulting atoms together is greater than the energy that held the original atom together. A barrier must also be overcome for fission to happen. Fissionable atoms usually split when hit by fast neutrons, while fissile atoms split more easily when hit by slow, or thermal, neutrons.
Fission is a type of nuclear transmutation because the pieces created (called daughter atoms) are different elements from the original atom. Most fission events produce two pieces of similar size, with a mass ratio of about 3 to 2. Occasionally, three pieces are produced in a process called ternary fission. The smallest of these pieces can range in size from a proton to an argon nucleus.
Fission can occur naturally in very heavy atoms without needing an external neutron, as these atoms already have too many neutrons. This natural fission was discovered in 1940 by scientists in Moscow. Unlike nuclear fusion, which helps form stars, fission has little role in the universe's development. However, natural nuclear reactors can form under rare conditions. Elements important for forming solar systems, planets, and life are not fission products but results of fusion processes.
The unpredictable types of fission products make this process different from other processes like alpha decay, which always produce the same results. Nuclear fission provides energy for power plants and causes nuclear explosions. This happens because certain materials called nuclear fuels split when hit by neutrons and then release more neutrons, allowing a chain reaction. This reaction can be controlled in a reactor or uncontrolled in a weapon.
The energy released from fissioning uranium is about a million times greater than burning methane or using hydrogen fuel cells.
However, the materials left after fission are more radioactive than the original fuel and remain radioactive for a long time, creating a waste problem. Only a small part of the waste consists of long-lived fission products. Neutrons that do not cause fission can create plutonium and other radioactive materials with higher risks than fission products. Concerns about waste and the power of nuclear weapons balance the benefits of using fission for energy. The thorium fuel cycle creates little plutonium and fewer radioactive materials but produces gamma rays from uranium decay. All actinides can be used in reactors, and reprocessing spent fuel helps reuse materials and reduce waste. A process that fissions nearly all actinides is called a "closed fuel cycle."
Physical overview
Younes and Loveland describe fission as the movement of protons and neutrons within the nucleus, which is different from other processes that break the nucleus apart. Nuclear fission is a type of large-scale movement that splits a nucleus into two or more smaller nuclei. This can happen naturally or be caused by a particle hitting the nucleus. Most of the energy from fission comes from the motion of the split pieces, about 85 percent. About 6 percent comes from neutrons and gamma rays, and about 3 percent comes from neutrinos produced during decay.
Nuclear fission can occur without needing neutrons, a process called spontaneous fission. This was first observed in 1940. When fission is caused by a particle, a compound system forms after the particle joins the nucleus. If the energy is high enough, the nucleus may split, releasing neutrons or gamma rays. Fission that splits into two pieces is called binary fission and is the most common type. Fission that splits into three pieces, called ternary fission, is rare. The third piece is often an alpha particle. In fission, more neutrons are released than absorbed, which allows chain reactions to happen.
Binary fission can create pieces with masses around 95 or 135 daltons. For example, when uranium-235 absorbs a neutron, it splits into strontium-95, xenon-139, and two neutrons, releasing 180 MeV of energy. Binary fission is the most likely outcome, but ternary fission can happen in about 2 to 4 cases out of 1,000 in a reactor. In these cases, three positively charged fragments are produced, with the smallest ranging from a proton to an argon nucleus. The most common small fragments are helium-4 nuclei with more energy than typical alpha particles. These fragments can build up as helium-4 and tritium gas in reactor fuel rods.
Bohr and Wheeler used the liquid drop model and other scientific ideas to explain how fission works. They compared the nucleus to a liquid drop, where forces like surface tension and the Coulomb force compete. They found that about 6 MeV of energy is needed to overcome the energy barrier that stops fission. John Lilley explained that this energy is called the activation energy or fission barrier. For nuclei with a mass number of about 240, this barrier is around 6 MeV. As the nucleus becomes larger, the activation energy decreases until fission happens easily.
Maria Goeppert Mayer later proposed the nuclear shell model, which helped explain how nuclei are structured. Fuels used in nuclear reactors, like uranium-235 and plutonium-239, are chosen because they can support chain reactions. These fuels split into two groups of elements with masses near 95 and 135 daltons. Most fuels undergo spontaneous fission very slowly, instead decaying through alpha and beta decay over long periods. In reactors or weapons, most fission events are caused by neutrons from previous fissions.
Fissionable isotopes like uranium-238 need fast neutrons, such as those from nuclear fusion, to split. While some neutrons from uranium-235 fission are fast enough to split uranium-238, most are not. This means uranium-238 cannot reach criticality. Thermal neutrons can rarely cause fission in uranium-238, but neutron absorption is much more likely.
Fission cross sections measure how likely fission is in a reaction. These cross sections depend on the energy of incoming neutrons. For uranium-235 and plutonium-239, cross sections are much higher than for uranium-238 at low energy levels. Absorbing a neutron gives the nucleus about 5.3 MeV of energy. Uranium-235 needs an extra 1 MeV to split, which it gets when it changes from an odd to an even mass. Younes and Lovelace explained that when a neutron is absorbed by uranium-238, the resulting nucleus has enough energy to split, but this does not happen for uranium-235.
About 6 MeV of energy comes from binding an extra neutron to the nucleus. However, this is not enough for fission in some isotopes. Uranium-238 has almost no chance of splitting with low-energy neutrons. If no extra energy is provided, the nucleus will absorb the neutron instead of splitting. Additional energy can come from faster neutrons or changes in the nucleus’s structure. In isotopes with an odd number of neutrons, like uranium-238, an extra neutron adds 1 to 2 MeV of energy due to neutron pairing effects. This helps some isotopes split more easily.
History
In 1938, scientists discovered nuclear fission in the buildings of the Kaiser Wilhelm Society for Chemistry, which is now part of the Free University of Berlin. This discovery followed many years of research on radioactivity and nuclear physics. In 1911, Ernest Rutherford proposed a model of the atom where a small, dense, positively charged nucleus made of protons was surrounded by orbiting, negatively charged electrons. Niels Bohr improved this model in 1913 by explaining how electrons behave according to quantum rules. In 1928, George Gamow introduced the liquid-drop model, which helped scientists understand how atoms split during fission.
In 1896, Henri Becquerel discovered radioactivity, and Marie Curie later named it. In 1900, Rutherford and Frederick Soddy studied radioactive gas from thorium and concluded that thorium was slowly turning into argon gas.
In 1919, Rutherford tested an idea from 1915 by using alpha particles to split nitrogen atoms into oxygen and protons. This was the first artificial nuclear reaction, where particles from one atom changed another atom’s nucleus. Rutherford and James Chadwick later used alpha particles to split other elements, but their experiments were limited by the energy of their sources. In 1932, Ernest Walton and John Cockcroft split lithium-7 into two alpha particles using protons. This experiment, called "splitting the atom," earned them the 1951 Nobel Prize in Physics.
In 1932, James Chadwick discovered the neutron. He used an ionization chamber to observe protons knocked out of elements by beryllium radiation. Rutherford had first suggested the neutron’s existence in 1920, and Chadwick explained how neutrons could help build large, positively charged nuclei.
Richard Rhodes and Philip Morrison described how neutrons could easily cause nuclear reactions in materials compared to protons. Enrico Fermi and his team studied how neutrons interacted with uranium and other elements. They found that hydrogen could slow down neutrons, which helped them study nuclear reactions.
In 1934, Fermi and his colleagues in Rome bombarded uranium with neutrons and thought they had created new elements with 93 and 94 protons, which they named ausenium and hesperium. However, Ida Noddack suggested that the uranium nucleus might split into smaller pieces instead of forming new elements.
After Fermi’s findings, Otto Hahn, Lise Meitner, and Fritz Strassmann repeated similar experiments in Berlin. Meitner, an Austrian Jew, fled Germany in 1938 and worked with Hahn by mail. In December 1938, Hahn shared results showing that uranium bombardment produced barium, which had a much smaller mass than uranium. Meitner and her nephew Otto Frisch interpreted this as proof that uranium nuclei had split. Frisch named the process "nuclear fission," comparing it to how cells divide in biology.
News of the discovery spread quickly. Niels Bohr shared the findings with scientists in the United States, where Columbia University researchers confirmed that uranium-235 was fissioning. On January 25, 1939, a Columbia team conducted the first U.S. nuclear fission experiment in Pupin Hall’s basement, proving fission released energy. The next day, the fifth Washington Conference on Theoretical Physics began, where scientists discussed the implications of nuclear fission.