Thermal-neutron spectrum effects of plane boron-steel absorbers, covering a wide range of optical thicknesses, have been studied in the UO2/H2O zero power assembly NORA. An integral spectrum property has been determined in terms of a spectrum index defined by the activation ratio of the isotopes 176Lu and 164Dy. The experimental results demonstrated that the thermal spectrum is considerably hardened near the various absorbers. The hardening effects were found to extend 1 to 2 lattice pitches away from the absorbers. The experimental results have been interpreted by the Monte Carlo code MONTROSA which makes use of the Brown-St. John energy transfer model. When the computed values were adjusted to correspond with the Koppel-Young scattering model, and the experimental values were corrected for the influence of foil perturbations in moderator and epithermal activations, satisfactory agreement between computed and experimental spectral indexes was obtained in nearly all cases.