How does the use of burnable absorbers reduce the need for control rods?

Burnable absorbers reduce the need for control rods by managing the excess reactivity present at the beginning of a fuel cycle. Fresh fuel loaded into a reactor core contains a higher concentration of fissile material (like Uranium-235), making the core more reactive than it needs to be for sustained operation. Control rods are used to absorb neutrons and reduce reactivity, but relying solely on control rods to manage this initial excess reactivity has drawbacks. Inserting control rods deeply into the core to compensate for the excess reactivity can distort the neutron flux distribution, leading to uneven power production and reduced fuel efficiency. Burnable absorbers are materials that also absorb neutrons, but they are designed to gradually deplete or "burn up" as the reactor operates. By incorporating burnable absorbers into the fuel assemblies, the initial excess reactivity is reduced without significantly distorting the neutron flux distribution. As the fuel burns up and its reactivity decreases, the burnable absorbers also deplete, compensating for the decrease in fuel reactivity. This allows the reactor to operate with a more uniform power distribution throughout the fuel cycle, maximizing fuel utilization. Because the burnable absorbers manage a significant portion of the initial excess reactivity, the control rods are needed to control smaller variations in reactivity and to provide shutdown capability. This reduces the amount of control rod insertion required, resulting in a flatter power distribution, increased fuel efficiency, and improved safety margins.