In the DEMO reactor highly efficient separation systems are required for the recovery of tritiated species to ensure a continuous re-fuelling of the plasma. For some of these systems, porous inorganic membranes have been proposed due to their promising modularity and cost-effectiveness for gas separation. For instance, zeolite membranes have been considered for the pre-concentration of Q2 in He (Q = H, D, T) in the European helium-cooled pebble bed (HCPB) to decrease the dimensions and costs of Pd/Ag membranes implemented downstream. Due to the similar sizes and masses of Q2 and He, the separation is limited with one membrane, hence a membrane cascade is required to achieve the desired performance requirements. In this paper, a numerical model is presented, which was developed to calculate the number of stages, surface area and power consumption of a membrane cascade using as input parameters the selectivity of the membrane α, permeances Π, pressures-ratio γ and the target separation requirements, the enrichment factor E and recovery fraction R. A sensitivity study was conducted to evaluate the impact of these parameters on the dimensioning of the membrane cascade.The pre-concentration of H2 in He using the zeolite membrane cascade with αH2/He=2 was first used as reference scenario for an inlet flow of Ffinj=8.06×103m3h−1 with E≥20 and R≥90%. The resulting membrane cascade consists of 16 stages, with 2563 m2 and 33.8 MW. For an improved selectivity of α = 10, the membrane cascade would have only 6 stages, with 260 m2 and 6 MW. The sensitivity analysis revealed that the cost-effectiveness and technological relevance of a membrane cascade are highly dependent on α, E and R.