[en] In the tritium breeding system the addition of a large flow rate of hydrogen (H/sub 2/) or deuterium (D/sub 2/) to the helium purge gas is considered essential to avoid a large amount of tritium inventory. However, the tritium concentration in the hydrogen isotope mixture to be separated is reduced by two or three orders of magnitude by the addition. The effects of this drastic dilution on the isotope separation by cryogenic distillation are analyzed for the Japanese Fusion Engineering Reactor where the tritium production rate is 3 g/h. It is shown that the dilution requires a specific cascade in addition to the cascade in the mainstream fuel circulation system. H/sub 2/ addition is much more favorable than D/sub 2/ addition in terms of the cascade scale needed, tritium inventory within the cascade, and refrigeration capacity required. The dilution of tritium by H/sub 2/ by two orders of magnitude requires a two-column cascade, and the tritium inventory and refrigeration capacity required are approximately 8 g and 65 W, respectively. The dilution by three orders of magnitude requires a three-column cascade, and the values of the two parameters are approximately 12 g and 630 W, respectively