What is the effect of neutron leakage on the effective multiplication factor k_eff?

• A. It increases k_eff.
• B. It has no effect on k_eff.
• C. It lowers k_eff, driving the system toward subcritical.
• D. It only affects reactor coolant temperature, not k_eff.

Answer: (C)

Neutron leakage acts as an extra loss term in the neutron balance: neutrons that escape the system don’t cause further fission. Since k_eff is the average number of neutrons in generation N+1 per neutron in generation N, any additional loss reduces the fraction of neutrons that can sustain the chain reaction. Therefore, leakage lowers k_eff, moving the system toward subcritical behavior. In other words, more leakage means fewer neutrons available for production in the next generation, so the reactor tends away from critical and toward subcritical.