Which statement best describes k_eff and its meaning for subcritical, critical, and supercritical conditions?

Which statement best describes k_eff and its meaning for subcritical, critical, and supercritical conditions?

• A. k_eff is the ratio of neutrons in one generation to the previous generation; subcritical k_eff<1 means the neutron population declines, critical k_eff=1 means a steady neutron population, and supercritical k_eff>1 means a rising neutron population.
• B. k_eff is the reactor's thermal efficiency.
• C. k_eff is the neutron flux at the core.
• D. k_eff is the energy release per fission.

Answer: (A)

k_eff tells you how the neutron population changes from one generation to the next. It is the ratio of neutrons produced in the next generation to those in the current generation, taking into account all births and losses (such as absorption and leakage). When k_eff is less than 1, the chain reaction fades away over generations because fewer neutrons cause fewer new fissions each time. When k_eff equals 1, the neutron population remains constant from generation to generation, giving a steady state. When k_eff is greater than 1, the neutron population grows with each generation, driving a rising reaction rate until something (like reactor control actions) brings it back toward balance. This concept is about the multiplication of neutrons, not about heat conversion, instantaneous neutron density, or energy released per fission.

k_eff tells you how the neutron population changes from one generation to the next. It is the ratio of neutrons produced in the next generation to those in the current generation, taking into account all births and losses (such as absorption and leakage). When k_eff is less than 1, the chain reaction fades away over generations because fewer neutrons cause fewer new fissions each time. When k_eff equals 1, the neutron population remains constant from generation to generation, giving a steady state. When k_eff is greater than 1, the neutron population grows with each generation, driving a rising reaction rate until something (like reactor control actions) brings it back toward balance. This concept is about the multiplication of neutrons, not about heat conversion, instantaneous neutron density, or energy released per fission.