Which specific group of neutrons, which are born a short time after a fission event rather than immediately, allow a reactor's power to be safely controlled by humans?

The specific group of neutrons that allow a reactor's power to be safely controlled by humans are called delayed neutrons. While the vast majority of neutrons, known as prompt neutrons, are emitted immediately, or instantaneously, during a nuclear fission event, a small but vital fraction are released after a measurable delay. These delayed neutrons do not come directly from the fission process itself but are instead produced by the radioactive decay of certain unstable atomic nuclei, called fission products, which are created when a heavy nucleus like uranium-235 splits. These particular fission products are known as delayed neutron precursors. After a fission event, these precursors undergo a process called beta decay, and in some cases, the resulting nucleus is left in an excited state and immediately emits a neutron. The time taken for this entire sequence, from the formation of the precursor to the emission of the neutron, ranges from milliseconds to tens of seconds, depending on the specific precursor's radioactive half-life. This time delay is absolutely critical for the safe and stable operation of a nuclear reactor. If all neutrons were prompt, any slight increase in reactivity – which is a measure of the balance between neutron production and neutron absorption in the reactor core – would cause the reactor power to increase exponentially and almost instantaneously. Such a rapid power surge would leave no time for mechanical control rods, which absorb neutrons to regulate the fission rate, or human operators to react and prevent an uncontrolled rise in power. The presence of delayed neutrons, even though they constitute a very small percentage of the total neutrons produced (typically less than one percent), effectively introduces a time constant into the reactor's behavior. This means that changes in reactor power, in response to changes in reactivity, occur much more slowly, over a period of seconds. This crucial time window allows mechanical control systems and human operators to safely adjust the control rods to absorb more or fewer neutrons, thereby precisely regulating the fission rate and maintaining the reactor's power at a desired, stable level.