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 with a high neutron absorption cross-section, meaning they readily capture neutrons. By capturing neutrons, they reduce the number of neutrons available to sustain the fission chain reaction, thereby decreasing the overall reactivity of the core. As the fuel depletes during the fuel cycle, the burnable absorbers also gradually deplete or "burn up" through neutron absorption. This depletion of the absorber compensates for the decrease in fuel reactivity as the fissile material is consumed. Common burnable absorber materials include boron, gadolinium, and erbium. They are either mixed directly within the fuel pellets or incorporated into separate absorber rods inserted into the fuel assembly. The use of burnable absorbers helps to maintain a more uniform reactivity level throughout the fuel cycle, which leads to a flatter power distribution within the core, more efficient fuel utilization, and improved safety margins. Without burnable absorbers, more control rods would be needed to manage the excess initial reactivity, which can distort the power distribution and reduce the efficiency of the fuel cycle. Therefore, burnable absorbers are a crucial component in managing reactor core reactivity and ensuring safe and efficient operation.