What is the key parameter affecting the reactivity variation in a PWR?

In a Pressurized Water Reactor (PWR), the reactivity variation is significantly influenced by the time and fuel used in the reactor core. This encompasses a combination of factors that change the isotopic composition of the fuel as it undergoes fission reactions over time.

As the nuclear fuel is used, the concentration of fissile material (like U-235 or Pu-239) changes due to fission and neutron absorption processes. Over time, the buildup of fission products and the transmutation of the fuel affects the neutron economy of the reactor, influencing the overall reactivity. Specifically, some fission products are strong neutron absorbers, which can reduce reactivity, while others might contribute positively to the neutron population.

Moreover, the time factor relates to how long the fuel has been in the reactor, which impacts the balance of isotopes present, their capacity to engage in fission, and how quickly reactivity may change during operation and shutdown. The effective management of these changes is essential for safe and efficient reactor operation.

Understanding the dynamics of fuel cycling, its isotopic evolution, and the time-dependency of reactivity is crucial for reactor physics and operational safety, making time and fuel utilization a key parameter in reactivity control in PWR