Answer
Cadmium and boron rods
Explanation
In a nuclear power plant, nuclear fission is the main process that generates nuclear energy. The radioactive decay of the fission products and transuranic elements produces heat. The continued chain reaction of a nuclear fission reactor depends on at least one neutron from each fission that is absorbed by the other fissionable nucleus. The reaction is controlled by using control rods of materials that absorbs neutrons. As the reaction continues, the number of uranium 235 nuclei reduces and fission by-products absorb the neutrons and accumulate. To be able to maintain this chain reaction, the control rods are withdrawn further.
