E1 Neutron Induced Nuclear Fission

Topic

When a large, fissile atomic nucleus—such as uranium-235, uranium-233, or plutonium-239—absorbs a neutron, that additional neutron can destabilize it enough to split it into two or more lighter nuclei, known as fission products. This splitting simultaneously releases three distinct things: kinetic energy distributed among the fission products themselves, which are ejected at high speed; gamma radiation emitted at the very moment of the split; and several additional free neutrons, which no longer belong to any nucleus and become available for subsequent reactions.

Not all fissile nuclei behave the same way upon absorption: natural uranium consists primarily of uranium-238—which does not readily undergo fission when struck by low-energy neutrons—mixed with a small fraction of uranium-235, which does. This difference in behavior is why the fuel for most power reactors must undergo an enrichment process to increase the proportion of uranium-235 before it can sustain a reaction.

The fission products resulting from this split are not a fixed set of particles but a wide variety of lighter nuclei, most of which are themselves unstable; they undergo their own subsequent radioactive decay—a process that, in itself, explains the origin of the residual heat described later in this fundamentals section. Furthermore, the energy released by the fission of a single nucleus is extraordinarily large compared to any ordinary chemical reaction, precisely because it releases energy stored in the nuclear force holding the nucleus together—an interaction far more intense than the chemical bonds governing conventional combustion. That is why this module—which opens with a neutron destabilizing a nucleus—concludes by noting that this very splitting leaves behind two distinct legacies: free neutrons capable of repeating the process in another nucleus, and unstable fission products whose own decay will continue to release energy long after the initial splitting has occurred.

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