What is the primary purpose of burnable absorbers in a reactor core?

The primary purpose of burnable absorbers in a reactor core is to control the excess reactivity present at the beginning of a fuel cycle. Reactivity refers to the measure of how much the nuclear chain reaction in a reactor core can sustain itself. At the start of a fuel cycle, newly loaded fuel contains a larger amount of fissile material, like Uranium-235, making the core more reactive. This excess reactivity, if uncontrolled, can lead to large power increases and potentially damage the reactor. Burnable absorbers are materials ....