E2 Chain Reaction

Topic

Of the various free neutrons released by the fission event described by the previous model, some may be absorbed by other nearby fissile nuclei, triggering further fission events that, in turn, release more free neutrons; this self-sustaining process is the chain reaction, driven not by a single neutron but by the entire population of neutrons passed from one generation of fissions to the next.

Whether this chain grows, is sustained, or dies out depends entirely on the average number of neutrons produced by each fission event that actually go on to trigger another fission: if that average number exceeds one, the fission rate increases over time; if it is exactly one, the rate remains constant—a condition known as criticality, which is precisely the normal, stable operating state of a power reactor; and if it is less than one, the reaction gradually dies out.

Not all released neutrons are available to sustain this chain: some escape the fuel volume without encountering another fissile nucleus, while others are absorbed by non-fissile materials within the reactor—such as the structure, coolant, or accumulated fission products—without triggering further fission. The greater the losses via these two pathways, the more neutrons must be produced to maintain the same level of criticality; consequently, the entire reactor design—from core geometry to material selection—is focused on carefully managing this balance.

Thus, this module—which begins with neutrons passing from one fission generation to the next—concludes by showing that maintaining criticality is not a state that occurs spontaneously, but rather an active balance between neutrons produced, neutrons lost, and neutrons that actually continue to trigger fission—a balance that the moderation and control models in this catalog explore next.

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