Control rod absorber configuration

A control rod construction especially for a boiling water nuclear reactor is disclosed in which at least some individual control rod elements include neutron absorbing poison, typically boron carbide (30), with hafnium cladding (32). In prior art construction, the entirety of the control elements have claddings of stainless steel; in the construction disclosed herein hafnium-clad, boron-carbide-containing control rods are used, preferably selectively interleaved with conventional stainless steel clad rod elements (33). The interleaving of the hafnium clad boron carbide with the stainless steel clad boron carbide maintains an operational balance between the desired worth of the control rod on one hand and the total weight of the control rod unit on the other hand. The combination of the hafnium and boron carbide, both of which constitute neutron absorbers, is complementary. The boron carbide and hafnium each have neutron absorbing capabilities independent of one another. The result is that the neutron absorption of the hafnium clad boron carbide exceeds the neutron absorption of either the boron carbide alone or the hafnium alone.