A nuclear reactor exploits the innate instability of some atoms—in general, those that have a large atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium number or that contain an imbalance of protons and neutrons—which break apart (fission) at random times, releasing photons, neutrons, electrons, and alpha atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium. For some atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium (atomic species having a specific number of protons and neutrons in the nucleus), the average wait until a given atom spontaneously fissions is shorter. When enough atoms of such an unstable isotope are packed close together, the neutrons released by fissioning atoms are more likely to strike the nuclei of nearby unstable atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium. These may fission at once, releasing still more neutrons, which may trigger still other fission events, and so forth. This is the atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium on which nuclear reactors and fission-type nuclear bombs depend. In a reactor, however, the fission rate is approximately constant, whereas in a bomb it grows exponentially, consuming most of the fissionable atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium in a small fraction of a second.
To produce a sustained chain reaction rather than a nuclear explosion, a reactor must not pack its fissionable atoms too closely atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium. They are therefore mixed with less-fissionable atoms that do not sustain the chain reaction. For example, in a reactor utilizing 235U as its primary fuel, only 3 atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium of the fuel is actually 235U; the rest is mostly 238U, a much less fissionable isotope of atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium. The higher the ratio of active fuel atoms to inert atoms in a given fuel mix, the more "enriched" the fuel is said to be; commercial nuclear power plant fuel is atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium only 3 to 5 percent 235U, and so cannot explode. For a fission bomb, 90 percent atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium would be typical (although bombs could be made with less-enriched uranium). Naval nuclear reactors, discussed further below, have used fuels enriched to atomicatomsbetweenchain reactionenrichedenrichmentmaterialnuclidesparticlespercenttogetheruranium 20 and 93 percent.