Fukushima accidents emphasized the requirement of passive safety system for reactor. Especially, when accidents such as station black out (SBO) occur, the passive safety system which can ensure the safety of reactor is needed. A thermosyphon, wickless heat pipe, is a heat transfer device of high thermal conductance that functions passively on the principle of evaporation and condensation of a working fluid [1]. The heat-pipe concept was first proposed by Gaugler in 1942 [2]. After its independent invention by Grover [3] in the early 1960s, serious development progress was made, and the heat pipe concept was studied intensively for both space and terrestrial applications, because of its beneficial characteristics. Recently, as regards station black-out (SBO) scenarios due to beyond design-basis accidents (BDBA) in nuclear power plants, heat pipes have been recognized as alternative heat-removal devices for use in the passive safety system [4, 5]. As the requirement of applications to such systems with large scale increases, importance of exact simulation model of heat pipe also increases. However, only few studies for heat pipe and thermosyphon simulation have been made. Joudi and Al_Tabbakh [6] tried to analyze thermal characteristics of thermosyphon using mathematical modeling. Zhang et al.[7] studied two dimensional thermal and mass flux of disc type thermosyphon. Alizadhdakhel et al.[8] provided CFD modelling and experimental measurement of thermosyphon. Using VOF method, they investigated the effect of filling ratio to thermal performance of thermosyphon. Annamalai and Ramalingam [9] developed a CFD modeling for wick part of heat pipe using commercial code, ANSYS CFX. Khurram Kafeel [10] numerically studied thermal hydraulic characteristics of thermosyphon in both transient and steady state. Bandar Fadhl et al.[11] built a CFD modeling for boiling and condensing of thermosyphon using VOF method of ANSYS Fluent. They made user defined function (UDF) to define source term based on Lee model [12, 13].

In this study, CFD model of 1m-thermosyphon has been studied using VOF model. Unlike formal studies, vacuum pressure condition was applied because thermosyphon with vacuum inner pressure is much generally used. Furthermore, to check out hydraulic characteristics of the model, transparent thermosyphon experiment also has been conducted. Thermal