A dual temperature hydrogen isotopic exchange reaction system between water and hydrogen gas is numerically analyzed. The system has two features; high efficiency of isotope exchange reaction and operation under atmospheric pressure. To achieve them, the low temperature section of the system is composed of wate,r mist and hydrogen gas cocurrent reactor units. For the high temperature section, a multistage-type reactor, in which a bubble plate, superheater and catalyst bed are alternatively arranged, is applied.From a material balance between these reactors, enrichment and decontamination factors for the system are expressed as functions of seven parameters: unit number of the low temperature co-current reactor ( X ) ; stage number of the high temperature section ( y ) ; flow ratio of tritium enriched water to decontaminated water ( P / w ) , flow ratio of feed water to hydrogen gas (F/G) ; reaction temperatures of the low and high temperature sections (Tc, Th) ; and bubble plate temperature (To). Numerical calculations show that enrichment factor depends remarkably on FIG and Tb as well as X and Y.In order to understand the separation characteristics visually, the McCabe-Thiele diagrams for the present system are drawn and compared with the results calculated.