Palladium alloy permeators are foreseen for the retrieval of hydrogen in the fusion fuel cycle of the European DEMO power plant. Driven by a pressure gradient, unburned fuel permeates through a thin-walled metallic membrane within the permeator while other gases cannot pass this barrier. With a theoretically unlimited selectivity with regard to nonhydrogenic species, a very high proportion of unburned fuel can be recovered in a continuous process from the exhaust gas and reused after a very short time. A potential candidate for the design of such a permeator consists of a tube (l = 500 mm, d = 10 mm) with a 125-μm-thick, self-supporting membrane made of a palladium-silver alloy all combined in the shape of a so-called finger-type design. A two-stage process then connects several of these permeators in parallel and in series to match the required throughput of DEMO during plasma operation at a given degree of separation. As the first design point in the scope of the current preconceptual design phase, a model was developed using the commercial software ASPEN Custom Modeler to estimate important parameters such as the tritium inventory and the scale of the permeator unit. How the hydrogen pressure profile is calculated over the length of a permeator using the Sieverts’ Law and the Finite Volume Method is thoroughly described. As a result, the integral performance of the combined permeators is presented as well as all important boundary conditions and assumptions that led to it. For the current DEMO baseline scenario, the total number of permeators of the abovementioned shape is found to be about 50.