Nuclear reactor control rod cluster for enthalpy rise compensation

This patent describes a control arrangement for use with a nuclear fuel assembly having guide thimbles to provide enthalpy rise compensation, comprising: (a) a spider assembly; and (b) control rodlets containing neutron absorber material, supported at their upper ends from the spider assembly in a pattern which matches that of the guide thimbles and extending generally parallel to one another downwardly for insertion into the guide thimbles of the fuel assembly; (c) the control rodlets arranged in a cluster having a higher axial neutron absorption capability at an upper portion than at a lower portion; each control rodlet varying in axial neutron absorption capability from every other rodlet