If the multiplication factor k is less than 1, what must be applied to bring the reactor to criticality?

• A. Positive reactivity
• B. Negative reactivity
• C. Increased absorbing material
• D. Decreased fuel

Answer: (A)

When the multiplication factor is less than one, the reactor is subcritical—the neutron population is shrinking each generation. To reach criticality (where the neutron population remains steady), you must insert positive reactivity, which moves the system toward k = 1. Positive reactivity increases neutron production relative to losses, bringing the reactor to the critical state. Increasing absorbing material or decreasing fuel would reduce reactivity further, so they wouldn’t help. In practice, positive reactivity is achieved by actions that reduce absorption or increase neutron production, such as withdrawing absorbing control material or adding fuel.