How do boron and cadmium control a nuclear chain reaction?

In a nuclear reactor, boron and cadmium are used to control the nuclear chain reaction by regulating the number of neutrons available to induce further fission.1

These materials are highly effective because they possess a large neutron-capture cross section, meaning they readily absorb neutrons. By positioning movable control rods made of these substances into the reactor core, operators can absorb excess neutrons, thereby lowering the multiplication factor (k) to maintain a steady-state reaction or to shut the reactor down entirely.123

Specifically, boron-10 is particularly notable for its ability to capture thermal neutrons. In addition to their use in control rods, boron and other neutron-absorbing materials like gadolinium can be used as "burnable poisons" to regulate excess reactivity over the lifetime of the fuel. By mechanically adjusting the depth of these rods within the core, operators can precisely manage the reactor's power level or initiate an emergency shutdown, known as a "scram," by fully inserting the rods to render the assembly subcritical.431