What is a delayed neutron and why are they important for reactor control?

• A. Neutrons emitted instantly during fission.
• B. Neutrons emitted by fission fragments after a delay (precursors) with mean lifetimes on the order of seconds; they provide a time scale for reactor control.
• C. Neutrons emitted by moderator decay.
• D. Delayed neutrons are emitted by fuel decay.

Answer: (B)

Delayed neutrons are neutrons released by the decay of certain fission fragments (precursors) some time after a fission event. These precursors are created in fission and do not emit neutrons immediately; as they beta-decay, they occasionally emit a neutron. The resulting neutrons appear with lifetimes on the order of seconds (distributed over several groups), and they constitute only a small fraction of all neutrons—yet they are essential. That small, time-delayed component slows the overall neutron population growth and defines a usable time scale for reactor control. Because the reactor responds on seconds-to-minutes scales due to delayed neutrons, control rods and other reactivity mechanisms can adjust the reactor safely and predictably. Without delayed neutrons, the reactor would react essentially with prompt neutrons alone, making the dynamics too fast to manage and making stable control extremely difficult.