A reactor with keff less than 1 is subcritical; a reactor with keff greater than 1 is ______.

A reactor with keff less than 1 is subcritical; a reactor with keff greater than 1 is ______.

• A. Supercritical
• B. Subcritical
• C. Critical
• D. Lethal

Answer: (A)

The key idea is how keff describes the growth of the neutron population from one generation to the next. When keff is greater than 1, each generation of neutrons causes more than one generation to appear, so the neutron population grows and the reactor is in a supercritical state. Critical means keff equals 1, giving a steady neutron population, while subcritical means keff is less than 1 and the reaction dies away. Lethal isn’t a standard technical term for this state. So the correct description for keff > 1 is supercritical.

The key idea is how keff describes the growth of the neutron population from one generation to the next. When keff is greater than 1, each generation of neutrons causes more than one generation to appear, so the neutron population grows and the reactor is in a supercritical state. Critical means keff equals 1, giving a steady neutron population, while subcritical means keff is less than 1 and the reaction dies away. Lethal isn’t a standard technical term for this state. So the correct description for keff > 1 is supercritical.