[en] In the absorber rod the excess reactivity existing at the beginning of the core residence time is compensated by depletion during the core lifetime of a burnable neutron absorber, in this way stretching the residence time. For this purpose the absorber rod may be filled with a solution of boron carbide, the cladding may be provided with a coating of boron carbide, and the interior may be connected with the coolant corcuit. Or the cladding may be designed as a coaxial tube and contain annular pellets from neutron-absorbing material in the interspace. In this case the interior of the cladding is also in contact with the coolant through the end caps. As material for the annular pellets there may be used boron carbide/aluminium oxide, ziconium diboride or gadolinium oxide. (DG)