Reactor channel characterization is commonly performed by irradiating bare and cadmium-covered fluence rate monitors, avoiding as much as possible the use of irradiation vials and spacers to position the monitors inside the channel. However, in routine k0-NAA often samples are packed in small polyethylene vials prior to irradiation. This work aims at studying the impact of typical polyethylene vials (1mm wall thickness) on the determination of the f and alpha channel specific parameters through the Bare, Cd-covered and Cd-ratio methods. The impact of these vials on each method was studied for 3 irradiation chanels of the BR1 reactor at SCK-CEN (Mol, Belgium) with low to high f and alpha values. The net impact was 1% on each parameter. Inconsistencies between the different methods were found when the impact of the polyethylene was neglected, implying that all determination methods must be pooled and thin monitors should be used for an accurate channel characterization.