In the four-factor formula, which statement correctly describes the fast fission factor e?

In the four-factor formula, which statement correctly describes the fast fission factor e?

• A. The ratio of fast neutrons produced by fission to those absorbed
• B. The ratio of total neutrons produced by fission to neutrons produced by thermal fission
• C. The fraction of neutrons that leak out before fission
• D. The ratio of neutrons absorbed in fuel to neutrons produced by fission

Answer: (B)

The fast fission factor measures how many neutrons come from fission when fast neutrons are involved, compared with the number produced by fission when fission is caused by thermal neutrons. In other words, it’s the ratio of total neutrons produced by fission (including fast-neutron–induced fission) to the neutrons produced by fission from thermal neutrons. This factor is typically greater than one because fast neutrons can cause additional fission events, adding more neutrons to the chain reaction and boosting the multiplication. That’s why the statement describing this ratio—the total neutrons produced by fission divided by the neutrons produced by fission from thermal fission—is the correct description. The other options don’t capture this specific comparison: one describes absorption rather than production, another describes leakage, and another describes absorption-to-production in a way that isn’t the fast fission factor.

The fast fission factor measures how many neutrons come from fission when fast neutrons are involved, compared with the number produced by fission when fission is caused by thermal neutrons. In other words, it’s the ratio of total neutrons produced by fission (including fast-neutron–induced fission) to the neutrons produced by fission from thermal neutrons. This factor is typically greater than one because fast neutrons can cause additional fission events, adding more neutrons to the chain reaction and boosting the multiplication.

That’s why the statement describing this ratio—the total neutrons produced by fission divided by the neutrons produced by fission from thermal fission—is the correct description. The other options don’t capture this specific comparison: one describes absorption rather than production, another describes leakage, and another describes absorption-to-production in a way that isn’t the fast fission factor.