What does keff = 1 signify in reactor physics?

What does keff = 1 signify in reactor physics?

• A. Exactly critical
• B. Subcritical
• C. Supercritical
• D. Delayed critical

Answer: (A)

Keff tells you how the neutron population changes from one generation to the next. When keff is 1, the neutron population stays the same generation to generation, so the chain reaction is self-sustaining but not growing or shrinking. That's what "exactly critical" means in practice: a steady-state condition where power remains constant (ignoring small feedbacks). If keff were greater than 1, the population would increase each generation (supercritical), and if it were less than 1, the population would dwindle (subcritical). The term delayed critical is used to describe a transient case where criticality depends on delayed neutrons; at keff = 1 you’re at a steady state with all neutrons considered, so it’s simply critical.

Keff tells you how the neutron population changes from one generation to the next. When keff is 1, the neutron population stays the same generation to generation, so the chain reaction is self-sustaining but not growing or shrinking. That's what "exactly critical" means in practice: a steady-state condition where power remains constant (ignoring small feedbacks). If keff were greater than 1, the population would increase each generation (supercritical), and if it were less than 1, the population would dwindle (subcritical). The term delayed critical is used to describe a transient case where criticality depends on delayed neutrons; at keff = 1 you’re at a steady state with all neutrons considered, so it’s simply critical.