A closed counter-current two-phase thermosyphon loop of a cold neutron source in HANARO research reactor

An experimental study was carried out to delineate the flow characteristics in a closed counter-current two-phase thermosyphon 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 at a room temperature with Freon-113 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 Vf, Qi) plane, where Vf represents the volume of the charged liquid and Qi the heat load: a stable flow regime, an oscillatory flow regime, a restablized flow regime and a dryout flow regime. For Vf > 2.5 l, the flow is stable at low Qi. However, as Qi increases, the flow becomes oscillatory and finally restablizes. As Vf increases, the oscillation amplitude decreases, reaching to the restablized flow region at low Qi, and the liquid level in the moderator cell remains high. In the oscillatory flow regimes, for a fixed Vf, the oscillating period of time varies with Qi, having a minimum value at a certain value of Vi. The heat load, where the oscillating period of time is minimum, decreases as Vf increases. (author)