A nuclear power reactor controls the fission chain reaction by moderating the neutrons and with the use of control rods which may be inserted in the reactor core to absorb neutrons and slow down the reaction. A fast reactor uses no moderator, but relies on fission produced by un-moderated fast neutrons to sustain the chain reaction. In some fast reactor designs, up to 20% of fissions can come from direct fast neutron fission of uranium-238, an isotope which is not fissile at all with thermal neutrons.
Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are made of chemical elements, such as boron, silver, indium and cadmium, that are capable of absorbing many neutrons without fissioning themselves.