In order to establish a proper control method of the DT fuel isotope balance in ITER, isotopic composition of hydrogen, which was released rapidly from the metal hydride bed by vacuum pump, was investigated using a ZrCo bed (1/10 scale of ITER fuel storage & delivery system) as functions of initially stored H/D ratio and temperature. The equilibrium pressure (P) of hydrogen - metal system has large isotope effect such as PH2 < PD2 < PT2 for ZrCo, however, the difference of H,D isotope fractions was within about 5%, during rapid delivery of about 90% hydrogen gases at 623 K and initial H:D of 1:1. In cases of initial H:D of 9:1 or 1:9, the differences of H,D isotope fractions were rather small of a few %. Even if the fluctuation of the isotope ratio is less than 5%, depending on the requirements from plasma physics experiments and fuel accountancy of tritium plant, batch fuel delivery from adequate gas tanks after isotope composition adjustment will be preferable to direct rapid delivery from storage bed.
