[en] Hydrogen permeation by heat-exchanging components is a problem of PNP plants (prototype plants for nuclear process heat) for which solutions are still being sought. In this study, the efficiency of an intermediate cycle for the reduction of permeation streams is examined on the basis of adjustable parameters such as speed, dampness of sweep gas, efficiency of an ancillary gas purification installation. The velocity constants determining the permeation were quantified for the assessment of the partial tritium pressures which developed in the presence of protium, and the influence of the build-up of counter-pressures was examined. A theoretical approach is confirmed by means of measurements. In this respect, surface effects play a great role for tritium. In experiments for embedding tritium in water vapour, the HTO/HT ratio almost becomes exactly equal to the H₂O/H₂ ratio. (RB)