Reactivity is defined as the change in neutron multiplication in a reactor. What does a multiplication factor k = 1 indicate?

• A. Critical
• B. Subcritical
• C. Supercritical
• D. Steady-state

Answer: (A)

The main idea is the neutron multiplication factor, k, which tells how many neutrons in the next generation come from the current generation’s fissions. When k equals 1, every generation produces the same number of neutrons as the previous one, so the neutron population remains constant over time. That means the reactor is in a steady state with zero reactivity and constant power. If k were greater than 1, the neutron population would grow (supercritical) and power would rise; if k were less than 1, it would decay (subcritical) and power would fall. So k = 1 is best described as critical. Steady-state describes the same condition in terms of constant power, but the standard term used for k = 1 is critical.