A substance which has a large neutron absorption cross section is loaded into the reactor core or is directly mixed in fuel to suppress the large excess reactivity

at the initial burnup. This neutron absorber is converted into a nuclide with a low absorption cross section as the result of neutron absorption. Thus, the increase in reactivity accompanying the burnup of the poison compensates to some extent for the decrease in reactivity due to fuel burnup. This poison is called a burnable poison or a burnable neutron absorber.

Gadolinium (Gd) and boron (B) are representative burnable poisons. Boiling water reactors (BWRs) and pressurized water reactors (PWRs) use gadolinia (Gd2O3) mixed with the UO2 fuel. In past PWRs, borosilicate glass (B2O3-SiO2) was inserted into the control rod guide tubes which were not being used for control rods. Recently, investigation showed that erbium (Er), which has a lower neutron absorption cross section compared with Gd but can suppress the excess reactivity for a long time, can be mixed in the form of erbia (Er2O3) with the UO2 fuel as a burnable poison.

Burnable poisons are zoned both radially and axially for flattening neutron flux in the more advanced core designs.