What happens to the reactivity if the concentration of fission product X is high?

The correct answer indicates that the reactivity decreases when the concentration of fission product X is high. This decrease in reactivity can be attributed to the nature of fission products and their impact on neutron economy within a reactor.

Fission products are the fragments that result from the nuclear fission process, and many of them are neutron absorbers or have properties that affect the balance of neutrons in the reactor core. When the concentration of fission products increases, particularly those that capture neutrons effectively, they tend to absorb neutrons that would otherwise contribute to sustaining the chain reaction. This increased absorption reduces the overall number of neutrons available to continue the fission process, leading to a reduction in reactivity.

In a nuclear reactor, maintaining a critical state requires a delicate balance of neutron production and absorption. An elevated concentration of fission products disrupts this balance, causing the reactor to become less reactive, which is crucial for reactor control and safety management.

Understanding this relationship is essential for reactor operation, as it helps engineers predict how changes in fission product concentrations can affect reactivity and, by extension, the power output of the reactor.