Neutron and gamma-ray attenuation have been studied in a cylindrical duct, length-to-diameter ratio = 7, partly (36 vol%) filled by a helical (screw-like) steel plug with an adjustable number of turns. The total neutron leakage through the duct decreased by a factor of 6 when the number of turns increased from 0 to 0.5; and by an additional factor of 3 in going from 0.5 to 2 turns (saturation value). The leakage with less than 0.5 turns is governed by the fast flux and above 0.5 turns by the epithermal flux., It is shown that the neutron attenuation (in the saturated case) can be satisfactorily predicted by homogenizing the plug and by combining the attenuation calculated in an infinite layer thus obtained with that predicted in a circular duct. The prerequisite is that the attenuation curves obtained are displaced in the streaming direction by the diameter of the duct., The attenuation of gamma rays in the saturation case (≈3 turns) equals that predicted in a homogenized infinite material without the duct attenuation.