In the equation keff = kinf × Lf × Lth, what does Lf denote?

In the equation keff = kinf × Lf × Lth, what does Lf denote?

• A. The fraction of neutrons which do not leak out while slowing down
• B. The fraction of neutrons that leak out during slowing down
• C. The fraction of neutrons absorbed in fuel
• D. The fraction of fast neutrons produced in fission

Answer: (A)

In this product of factors, the fast-leakage factor represents how many fast neutrons stay in the system during the slowing-down process. Specifically, Lf is the fraction of fast neutrons that do not leak out while they slow down from the fission-energy range to thermal energies. It encapsulates losses due to leakage during slowing down, so it must be a number less than or equal to 1; if there were no leakage, it would be 1. The smaller Lf is, the more neutrons are lost to leakage before they can contribute to further fission, and keff decreases accordingly. The complementary idea—neutrons that leak out during slowing down—would be 1 minus Lf, not Lf itself.

In this product of factors, the fast-leakage factor represents how many fast neutrons stay in the system during the slowing-down process. Specifically, Lf is the fraction of fast neutrons that do not leak out while they slow down from the fission-energy range to thermal energies. It encapsulates losses due to leakage during slowing down, so it must be a number less than or equal to 1; if there were no leakage, it would be 1. The smaller Lf is, the more neutrons are lost to leakage before they can contribute to further fission, and keff decreases accordingly. The complementary idea—neutrons that leak out during slowing down—would be 1 minus Lf, not Lf itself.