Define keff qualitatively.

Define keff qualitatively.

• A. keff indicates the maximum reactor power of the system
• B. keff is a measure of thermal efficiency
• C. keff indicates whether the reactor is subcritical, critical, or supercritical
• D. keff equals the ratio of neutron production to losses per generation

Answer: (C)

The main idea here is that keff tells you where the reactor stands in terms of the neutron population from one generation to the next. Qualitatively, keff is the balance between neutrons produced by fission and neutrons lost to absorption and leakage. If keff is equal to one, the neutron population remains steady from generation to generation, so the reactor is critical. If keff is less than one, the population declines and the reaction dies away, which is subcritical. If keff is greater than one, the neutron population grows and the reaction becomes self-sustaining at a higher level, which is supercritical. This classification—subcritical, critical, or supercritical—is what the question is aiming to capture. The other options refer to power, thermal efficiency, or a specific production-to-loss ratio, which do not describe the qualitative state as directly as the criticality picture.

The main idea here is that keff tells you where the reactor stands in terms of the neutron population from one generation to the next. Qualitatively, keff is the balance between neutrons produced by fission and neutrons lost to absorption and leakage. If keff is equal to one, the neutron population remains steady from generation to generation, so the reactor is critical. If keff is less than one, the population declines and the reaction dies away, which is subcritical. If keff is greater than one, the neutron population grows and the reaction becomes self-sustaining at a higher level, which is supercritical. This classification—subcritical, critical, or supercritical—is what the question is aiming to capture. The other options refer to power, thermal efficiency, or a specific production-to-loss ratio, which do not describe the qualitative state as directly as the criticality picture.