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Cryogenic adsorption is a promising technology for removing trace levels of hydrogen isotopologues (H 2 , HT, D 2 and T 2) from helium purge gas in the breeder zone of a fusion reactor. While Zeolites 4A and 5A are commonly used in packed bed cryogenic adsorption systems, MS 13X zeolite, despite its larger surface area and pore volume, has been less explored for adsorbing hydrogen isotopes. This study discusses the development of the lab scale cryogenic molecular sieve bed (CMSB) system and experimentally investigates the adsorption of hydrogen (H 2) - deuterium (D 2) on MS 13X zeolite at 77.4 K. From the breakthrough experiments, the adsorption capacity at bed saturation for the single-component system containing 1000 ppm of H 2 is 0.101 mol/kg, while that for 1000 ppm D 2 is 0.243 mol/kg. For a binary mixture with 1000 ppm each of H 2 and D 2 in helium gas, the observed saturation capacities are 0.1 mol/kg and 0.237 mol/kg for H 2 and D 2 , respectively with an isotopic selectivity of 2.1 for D 2 over H 2. Considering the challenging as well as hazardous nature of experiments with radioactive isotopes, a numerical model employing the extended Langmuir dual-site and linear driving force model is implemented to simulate the binary and ternary breakthrough curves for various hydrogen isotopologues mixtures in He gas that are relevant to fusion fuel cycle system. Analysis reveals the reduced separability of the H 2 -HT mixture when compared to H 2 -D 2 , and this may be linked to the difference in the respective binary pair's zero-point energies. At 77.4 K, a maximum selectivity of 4 is observed through simulations for T 2 over H 2 at equimolar partial pressures of 10 Pa in helium gas on MS 13X zeolites. A comparative evaluation of commercial LTA and MS 13X zeolite beads reveals that MS 13X exhibits performance that is intermediate to LTA 4A and LTA 5A under identical experimental conditions. This study demonstrates that MS 13X zeolite may be gainfully deployed for the cryosorption of low concentrations of hydrogen isotopes in a continuously flowing He gas. [Display omitted] • Lab-scale system developed for H 2 -D 2 cryosorption on MS 13X zeolite beads. • Radial distribution function plots confirmed dual sites with unequal affinities. • D 2 eluting slower than H 2 at 77.4 K is favorably adsorbed due to quantum effect. • Breakthrough curves for radioactive H 2 isotopologues obtained from simulations. • Macropore diffusion is dominant mass transfer mechanism in MS 13X beads. [ABSTRACT FROM AUTHOR]