Epithermal absorption by control rods in nuclear reactors is taken into account by replacing the black-grey-transparent rod by a black-transparent model. The lethargy boundary, in the model, is determined for 1/νabsorbers by the prescription that, at the boundary, half the incident partial current is absorbed by the rod. Application of the model to space-independent spectrum calculations improves the calculation to the extent that, provided the black-transparent boundary is the boundary between the groups, two-group methods can be used for control rods in bare reactors. For control rods in reflected reactors, three groups are indicated. Calculations are presented to demonstrate the accuracies of the black-transparent model and the small number of neutron energy groups.