New irradiation facilities for the neutron transmutation doping of silicon crystals with a diameter of up to 110 mm have been installed in the Danish heavy-water reactor DR 3. Cross-sectional uniformity is easily achieved by rotation, but axial variation is affected by different reactor operating conditions. A computer program has been developed as an aid in designing a stainless steel absorber that limits the maximum deviation with a minimum reduction of the average neutron fluence. Comprehensive experimental verification has been carried out to ascertain the quality of the NTD silicon within close specifications.