A simple I-125 generating device at the research reactor, HANARO, is developed. Xenon-124 gas circulates continuously in the generator producing I-125 by neutron capture and the I-125 is adsorbed at the upper part of the generator, which is to be chemically recovered in a hot-cell. The continuous xenon circulation technology is devised by the design of the generator structure with the consideration of thermal hydraulics and neutron physics involved. A computational fluid dynamics technique is used for the conceptual design of the generator.