Delayed neutrons

Definition

Delayed neutrons are neutrons that are emitted by certain radioactive nuclei after a brief delay following the fission process. This delay occurs because some fission products are themselves radioactive and undergo beta decay before emitting neutrons. Understanding delayed neutrons is crucial for controlling chain reactions, particularly in nuclear reactors, as they provide a means to regulate the reactivity of the system.

5 Must Know Facts For Your Next Test

1. Delayed neutrons make up a small fraction of the total neutron population produced during fission, typically around 0.5% to 0.7%.
2. The presence of delayed neutrons significantly enhances the controllability of nuclear reactors by allowing operators to manage reactivity changes more effectively.
3. Different fission products contribute varying amounts of delayed neutrons, with certain isotopes, like Iodine-135 and Xenon-135, being particularly significant.
4. Delayed neutrons are crucial for the time it takes for a reactor to respond to changes in reactivity, providing safety margins during transient conditions.
5. The study of delayed neutrons is essential for reactor design and safety analysis, influencing critical parameters such as the reactor's prompt neutron lifetime and reactivity feedback mechanisms.

Review Questions

• How do delayed neutrons impact the control of nuclear chain reactions?
  • Delayed neutrons play a vital role in controlling nuclear chain reactions because they allow for more manageable changes in reactivity. Unlike prompt neutrons, which are released instantaneously during fission, delayed neutrons provide a time buffer that enables operators to respond to reactivity fluctuations more effectively. This delay helps prevent rapid increases in power levels, enhancing overall reactor safety and stability.
• Discuss the significance of specific fission products that contribute to the emission of delayed neutrons.
  • Certain fission products, such as Iodine-135 and Xenon-135, are significant contributors to the emission of delayed neutrons. Their beta decay processes lead to the release of neutrons after a short delay following fission events. The presence of these isotopes is critical for understanding the neutron economy in reactors and influences how operators manage reactivity. Their behavior impacts the reactor's overall stability and safety during operation.
• Evaluate how understanding delayed neutrons contributes to reactor design and safety protocols in nuclear engineering.
  • Understanding delayed neutrons is essential for effective reactor design and safety protocols as it allows engineers to predict and manage reactivity changes with greater accuracy. By incorporating knowledge of delayed neutron emissions into reactor models, designers can establish safety margins that account for transient conditions. This understanding also informs operational procedures that ensure safe handling of unexpected fluctuations in power output, thus reinforcing the reliability and safety of nuclear reactors.

Related terms

Fission:

The process by which a heavy nucleus splits into two or more lighter nuclei, along with the release of energy and neutrons.

Prompt neutrons: Neutrons that are released immediately during the fission process, crucial for initiating subsequent fission events.

Neutron economy: The balance and management of neutron production and absorption in a nuclear reactor to maintain a stable chain reaction.