In PWRs, the control rods for the 17 x 17 lattice fuel assembly are combined in clusters of 48 rods which are inserted from the top of the reactor vessel. Such a great number of rods with a relatively low reactivity worth keeps the neutron flux as
uniform as possible throughout the core. The control neutron absorber is an alloy of Ag-In-Cd, which features a somewhat weaker neutron absorption that produces less flux peaking and has a neutron absorption over a considerable range of neutron energies.

The control rods of BWRs consist of two crossed blades fitting into each corner between the fuel assemblies. The cruciform rods raise the surface-to-volume ratio of the control elements. Boron carbide (B4C) or hafnium (Hf) is employed as a control rod material. Hf loaded control rods absorb neutrons by resonance capture reactions of Hf isotopes. The control rods of BWRs are driven up from the bottom of the core. The bottom-entry arrangement makes the reactivity worth of control rods greater in the lower part of the core than in the upper part where the void ratio is high and the neutron spectrum is hard.

In fast reactors, the control assemblies composed of B4C rods, rather than the control rods inserted into fuel assemblies, replace some fuel assemblies in the core because the mean free path of neutrons is long.