In a basic study on reducing a conventional atmospheric detritiation system (ADS) to a small one, performances of H2(+HT), H2O(+HTO), and N2 (or air) permeating through a gas separation membrane (GSM) module were numerically analyzed. The calculations were carried out in (3 × 2 + 1) patterns of cross, countercurrent, and cocurrent flows based on the differential and plate models and the complete mixing flow. Previous experimental results of the H2/H2O/N2(air) three-component system were found to be well correlated by the differential cross-flow model. Applications of the GSM to the ADS were investigated, and the effectiveness of using the GSM in place of the first detritiation system of the ADS was discussed.