A rod-shaped component containing a material that is a strong neutron absorber. The chain reaction is controlled by inserting the control rod deeper into the reactor core or withdrawing it.
Different reactors have different types of control rod.
Boiling-water reactor: Fig. Control rod 1 shows the design of control rods for boiling-water reactors. They are cruciform and made of stainless steel with pellets of boron carbide inlaid. B10 is a strong neutron absorber and hence is the active material in this construction. The control rod is inserted through the bottom of the reactor tank and glides up and down in the water gap between four adjacent fuel elements. The slow movement of the control rod regulating the output is implemented by means of electromechanical pinions. For rapid insertion of the rods in an emergency, use is made of a pneumatic-hydraulic system (fig. Control rod 2).
Pressurized-water reactor: Fig. Control rod 3, shows a corresponding control rod for a pressurized-water reactor. In this case the control rods are designed as a number of thin absorbing \"fingers\", from which is derived the name \"finger control rod\". The absorber is frequently an alloy of silver-indium-cadmium. Cadmium in particular is an efficient neutron absorber. The control rods are inserted from above and the \"fingers\" move in empty fuel rod positions. Here too electro-mechanical pinions are used for the slow control rod movement. In an emergency situation the electromagnetic coupling in which the rod hangs is released, and the rod falls by gravity into the reactor core. (Fig. Control rod 4).
In Russian reactors (both RBMK and VVER) the effect of the control rods is increased by combining an absorber rod with a fuel element, which means that insertion/withdrawal implies simultaneous insertion/withdrawal of a fuel element.
See BWR description