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Fission and fusion

Nuclear energy can be released in two ways: by splitting heavy atomic nuclei, a process called nuclear fission, or by fusing very light nuclei, which is called nuclear fusion.
While controlled fission has long been used for the production of electricity, science has yet to master nuclear fusion, which is difficult to accomplish because it involves the joining of two nuclei that have a natural tendency to repel each other.
In nuclear fission, the nucleus of a heavy atom (a nucleus containing many protons and neutrons, which is the case with uranium and plutonium) splits into two lighter fragments following absorption of a primary neutron. This fragmentation reaction results in the emission of two or three secondary neutrons and in a massive energy release.
Spontaneous fission is an extremely rare phenomenon. For example, for a nucleus like uranium-238, it only occurs once every 2 million decays. The only fissile nucleus found in nature is a uranium isotope (U-235) that is present in very small proportions (0.7%) in uranium ores. There are other fissile nuclei, but they must be produced in reactors. The most commonly used man-made isotopes are plutonium-239 (generated from uranium-238) and uranium-233 (generated from thorium-232).
Fission is most often triggered by the capture of a neutron into a very heavy fissile nucleus made unstable by an excessive number of protons and neutrons. This “overloaded” nucleus then splits into more stable nuclei, releasing energy.
This phenomenon, discovered by Otto Hahn and Lise Meitner in 1938, would have remained relatively insignificant had it not been for the possibility of multiplying it through a chain reaction: the fission releases neutrons that in turn trigger other fission reactions. Under these conditions, nuclear energy is released by considerable quantities of matter rather than just a few isolated atoms.
This chain reaction, which turns into an explosion in an atomic bomb, is controlled in nuclear reactors. The phenomenon of induced nuclear fission was first described by Otto Hahn and Fritz Strassmann on December 17, 1938.
In 1939, the chain reaction triggered by neutron emission during fission was discovered by Hans von Halban, Frédéric Joliot and Lew Kowarski in France, and Enrico Fermi in the United States . Nuclear fusion is a reaction in which two light atomic nuclei join to form a heavier nucleus, for example when a deuterium nucleus and a tritium nucleus fuse to form a helium nucleus and one neutron.
This is the reaction that produces the energy radiated by the Sun, which can reach temperatures of several million degrees.
As of today, there is no man-made device capable of producing energy by controlling nuclear fusion reactions. Researchers are investigating ways to make the energy produced by fusion greater than the energy needed to induce it by heating the atomic particles. An international project called ITER seeks to develop the civil use of nuclear fusion energy to generate electricity.