E9 Delayed Neutrons

Topic

Not all the neutrons released by the fission event described in the first model of this catalog appear the instant the nucleus splits. The vast majority—prompt neutrons—are emitted almost immediately, within an extraordinarily short time after fission occurs. In contrast, a tiny fraction of the total originates from the subsequent radioactive decay of specific fission products—a process that can take anywhere from a fraction of a second to tens of seconds to unfold; these are known as delayed neutrons.

Although they represent only a minute fraction, delayed neutrons are disproportionately important for making a reactor operable in the first place. If a reactor relied solely on prompt neutrons to sustain the criticality established by the chain-reaction model, the chain reaction would evolve on a timescale far too short for any mechanical control-rod system to respond in time to a deviation. Because a small portion of the neutrons needed to maintain precise criticality comes instead from this delayed process, the chain reaction's response to a change in rod position is extended to a timescale of seconds—perfectly compatible with mechanical control systems and the reaction times of human operators.

Thus, this module—which begins with a tiny fraction of late-arriving neutrons—concludes with the most important takeaway of this entire fundamentals section: this very delay, more than any engineering feature, is the fundamental reason why a fission reactor is a controllable machine in practice, and it serves as the basis for the reactivity control described in the next part of this catalog.

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