The material used for construction of control rods in a nuclear reactor is

Control rods in a nuclear reactor are used to control the rate of fission of uranium or plutonium. They are made of chemical elements capable of absorbing many neutrons without themselves fissioning, such as Cadmium ($\text{Cd}$), Boron ($\text{B}$), or Hafnium ($\text{Hf}$). Graphite and Beryllium are typically used as moderators to slow down neutrons.