An experimental study was carried out to delineate the flow characteristics in a closed countescurrent two-phase thermo syphon with concentric tubes. This is to be installed in the HANARO research reactor as a part of a Cold Neutron Source(CNS). In the present investigation, experiments ata room temperature with Freon-II3 as a moderator were performed. Results show that, based on the magnitude of pressure fluctuation, the flow regimes could be divided into 4 distinct ones in the () plane, where represents the volume of the charged liquid and the heat load: a stable flow regime, an oscillatory flow regime, a restablized flow regime and a dryout flow regime. For >2.5l, the flow is stable at low . However, as increases, the flow becomes oscillatory and finally restablizes As increases, the oscillation amplitude decreases, reaching to the restablized flow region at low , and the liquid level in the moderator cell remains high. In the oscillatory flow regimes, for a fixed VI; the oscillating period of time varies with , having a minimum value at a certain value of . The heat load, where the oscillating period of time is minimum, decreases as increases.