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Material corrosion in nuclear power plant (NPP) is not controlled only by amine injection but also by ion exchange (IX) which is the best option to remove trace Na+. This study was conducted to understand the Na+ leakage characteristics of IX beds packed with ethanolamine-form (ETAH-form) and hydrogen-form (H-form) resins in the simulated water-steam cycle in terms of intrinsic behaviors of four kinds of cation-exchange resins through ASTM test and Vanselow mass action modeling. Na+ was inappreciably escaped throughout the channel created in resin layer. Na+ leakage from IX bed was non-linearly raised because of its decreasing selectivity with increasing Na+ capture and with increasing the fraction of ETAH-form resin. Na+ did not reach the breakthrough earlier than ETAH+ and NH4+ due to the increased selectivity of Na + to the cation-exchange resin (H+ < ETAH+ < NH4+ ≪ Na+) at the feed composition. Na+ leakage from the resin bed filled with small particles was decreased due to the enhanced dynamic IX processes, regardless of its low selectivity. Thus, the particle size is a predominant factor among intrinsic properties of IX resin to reduce Na+ leakage from the condensate polishing plant (CPP) in NPPs.