Boron Carbide Control Rod Operation

The reactivity that a control rod can control is characterized by the reactivity value of the control rod. The ratio of the difference between the effective proliferation coefficient of a control rod before and after it is inserted into the reactor to the effective proliferation coefficient of the reactor before insertion is called the reactivity value of this control rod or this group of control rods. That is, the share of the effective proliferation of neutrons absorbed by the control rod in the total effective proliferation of the reactor. For pressurized water reactors, the total reactivity value of each control rod group is generally about 7% to 10%.
The deeper the control rod is inserted into the reactor, the more neutrons it absorbs, which slows down the reaction rate and even stops it; when the rod is pulled out of the reactor, the less neutrons are absorbed, which can start the reactor or speed up the reaction. This adjustment is generally carried out through automatic devices.