Which statement best defines keff?

Which statement best defines keff?

• A. The ratio of neutrons produced at the end of generation to the neutrons present at the start of generation.
• B. The ratio of the number of neutrons resulting from fission in the current generation to the total number lost by absorption and leakage in the preceding generation.
• C. The ratio of fission events to absorption events in the current generation.
• D. The ratio of leakage neutrons to produced neutrons in the same generation.

Answer: (B)

Keff measures how the neutron population changes from one generation to the next by comparing production to losses. It is defined as the ratio of neutrons produced by fission in the current generation to the neutrons lost by absorption and leakage in the preceding generation. This captures the balance that determines whether the chain reaction will sustain itself, grow, or die out. When keff is about 1, the system is critical; above 1 it’s supercritical; below 1 it’s subcritical. The statement matches this production-to-loss balance across generations, which is why it’s the best definition. The other options either mix generations incorrectly or only compare one type of interaction (fission vs absorption) or leakage vs production, without reflecting the full generation-to-generation relationship keff describes.

Keff measures how the neutron population changes from one generation to the next by comparing production to losses. It is defined as the ratio of neutrons produced by fission in the current generation to the neutrons lost by absorption and leakage in the preceding generation. This captures the balance that determines whether the chain reaction will sustain itself, grow, or die out. When keff is about 1, the system is critical; above 1 it’s supercritical; below 1 it’s subcritical.

The statement matches this production-to-loss balance across generations, which is why it’s the best definition. The other options either mix generations incorrectly or only compare one type of interaction (fission vs absorption) or leakage vs production, without reflecting the full generation-to-generation relationship keff describes.