Control rods are made of boron carbide. When inserted into the reactor core, they absorb neutrons and stop or slow down the fission reaction. By controlling the depth of insertion of the rods into the core, the fission rate and thus the reactor power can be controlled. The main control rods are inserted from top to bottom and allow fully automatic, manual or emergency control of the reactor. Automatic control is based on data from neutron flux detectors located in the core. To balance the power throughout the core, RBMK reactors use additional shorter control rods inserted into the core from the bottom of the reactor.
