What does the effective delayed neutron fraction β_eff represent in reactor kinetics?

• A. The effective delayed neutron fraction
• B. The prompt neutron fraction
• C. The absorbed fraction in fuel
• D. The fast neutron production

Answer: (A)

In reactor kinetics, there are prompt neutrons and delayed neutrons. The effective delayed neutron fraction is the portion of neutrons that actually cause fission in the reactor that come from delayed-neutron precursors, after accounting for losses such as absorption and leakage. It is defined as the ratio of delayed-neutron–induced fission events to the total fission events occurring in the system. This “effective” value reflects the real neutron economy in the reactor—geometry, materials, and spectra—so it is typically smaller than the raw delayed-neutron fraction emitted by fuel. This quantity is crucial in the point-kinetics equations because it weightings the delayed-neutron source term and sets the timescale of the reactor’s response, underpinning controllability. So, it is not about the prompt-neutron fraction alone, nor about the fraction absorbed in fuel or the production of fast neutrons; it specifically tells how much of the fission neutrons that drive the chain reaction are delayed, when losses are included.