In reactor kinetics, which term describes a state where the chain reaction dies out because k<1?

In reactor kinetics, which term describes a state where the chain reaction dies out because k<1?

• A. Critical
• B. Subcritical
• C. Supercritical
• D. Prompt critical

Answer: (B)

Subcritical describes a state where the effective multiplication factor k is less than one, so each generation of neutrons produces fewer neutrons than the previous one. With N_next = k × N_now and k<1, the neutron population declines geometrically, causing the chain reaction to die out over time. When k equals one, the reaction is critical and self-sustaining; when k is greater than one, it is supercritical and grows. The term prompt critical refers to a rapid growth driven by prompt neutrons alone, which is a different situation than simply k<1. Even with delayed neutrons, a subcritical reactor still decays toward zero because the overall reproduction is insufficient to maintain the chain reaction.

Subcritical describes a state where the effective multiplication factor k is less than one, so each generation of neutrons produces fewer neutrons than the previous one. With N_next = k × N_now and k<1, the neutron population declines geometrically, causing the chain reaction to die out over time. When k equals one, the reaction is critical and self-sustaining; when k is greater than one, it is supercritical and grows. The term prompt critical refers to a rapid growth driven by prompt neutrons alone, which is a different situation than simply k<1. Even with delayed neutrons, a subcritical reactor still decays toward zero because the overall reproduction is insufficient to maintain the chain reaction.