The control rods in a nuclear reactor control the reaction by

Explanation

The moderator helps slow down the neutrons produced by fission to sustain the chain reaction. Control rods can then be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it. The heat created by fission turns the water into steam, which spins a turbine to produce carbon-free electricity. A rod, plate, or tube containing a material such as hafnium, boron, etc. is used to control the power of a nuclear reactor. By absorbing neutrons, a control rod prevents the neutrons from causing further fissions.