What is the most important role of delayed neutrons in reactor operation?

The most important role of delayed neutrons in reactor operation is to make the reactor controllable and allow for a relatively slow response time to changes in reactivity. Neutrons produced directly from nuclear fission are called prompt neutrons and are released almost instantaneously. Delayed neutrons, on the other hand, are emitted from the decay of certain fission products, called delayed neutron precursors, and are released seconds or even minutes after the fission event. The fraction of delayed neutrons is small, typically less than 1% of the total neutron population, but their impact on reactor control is significant. Without delayed neutrons, any increase in reactivity would cause the reactor power to increase extremely rapidly, making it difficult or impossible for operators or automatic control systems to respond in time to prevent an overshoot or even a runaway reaction. Delayed neutrons effectively slow down the reactor's response to reactivity changes, providing operators and control systems with sufficient time to make adjustments and maintain stable operation. This slower response time is crucial for safe reactor startup, shutdown, and power maneuvering. The time constant associated with the delayed neutrons allows the reactor to be controlled using conventional mechanical control systems. In essence, delayed neutrons transform the reactor from a potentially unstable and uncontrollable system into a manageable and predictable one.