Typically, how many delayed-neutron groups are used in reactor kinetics, and what is the typical total delayed-neutron fraction (β_eff) for thermal reactors?

• A. Six or eight groups; about 0.6% to 0.7%
• B. Four groups; about 1.5% to 2.0%
• C. Ten groups; about 0.3% to 0.4%
• D. Two groups; about 3% to 4%

Answer: (A)

Delayed neutrons come from fission product decays, so reactor kinetics use several precursor groups, each with its own decay rate, to capture how the neutron population rises after a change. Grouping these delays lets the model reproduce the actual time response without tracking every individual precursor. Six groups is the standard balance between accuracy and simplicity; some more detailed treatments use eight groups to better reproduce the very slow decays.

The overall measure, β_eff, is the fraction of neutrons produced that are delayed rather than prompt. In thermal reactors, this value is about 0.0065 to 0.007, i.e., roughly 0.65% to 0.7%. That combination—six (or occasionally eight) delayed-neutron groups with a β_eff around 0.6–0.7%—is what is typically used.

The other options either propose too high or too low a β_eff or suggest an impractical number of groups for standard kinetics modeling, which is why they don’t align with common practice.