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There are two main types of nuclear reactions: fusion and fission. In fusion reactions, two light nuclei are combined to form a heavier, more stable nucleus. In fission reactions, a heavy nucleus is split into two nuclei with smaller mass numbers. Both processes involve the exchange of huge amounts of energy: about a million times more energy than that associated with ordinary chemical reactions. In either case, if the new particles contain more stable nuclei, vast quantities of energy are released.

Nuclear power plants rely on fission to create vast quantities of energy. For example, U-235 nuclides can be bombarded with neutrons, and the result is lots of energy, three neutrons, and two stable nuclei (Kr-92 and Ba-141). The three neutrons formed can collide with other U-235 atoms, setting off a chain reaction and releasing tons of energy.