What is neutron leakage and its effect on reactor criticality?

• A. Neutrons escaping the core without causing fission; leakage lowers k_eff and can drive the system subcritical.
• B. Neutrons produced by fission that escape the core cause supercriticality.
• C. Neutrons are absorbed only in the fuel, not leakage.
• D. Leakage increases the resonance absorption causing higher reactivity.

Answer: (A)

Neutron leakage is the escape of neutrons from the reactor core without causing fission. This creates a loss in the neutron population, which directly lowers the effective multiplication factor, k_eff. Since k_eff is the average number of neutrons in one generation that cause fission in the next, any neutrons that leak out are not available to continue the chain reaction. When leakage is significant, fewer neutrons induce fission in the next generation, so the reactor tends toward subcritical (k_eff below 1).

In practice, designers work to minimize leakage by choosing core size, geometry, materials, and using reflectors to bounce neutrons back into the core, improving the neutron economy. The other statements don’t fit because escaping neutrons do not cause supercriticality; leakage is a loss mechanism, not a source of increased reactivity. Neutrons are not only absorbed in the fuel—absorption happens in many materials, and leakage represents neutrons that simply leave the system. Finally, leakage does not increase resonance absorption to raise reactivity; it reduces the number of neutrons available for all interactions, including fission, thereby lowering reactivity.