In the case of loss of residual heat removal systems under mid-loop operation during shutdown of a PWR plant, reflux cooling by a steam generator (SG) is expected, and steam generated in the reactor core and water condensed in the SG form a countercurrent flow in a hot leg. The flow is highly complicated because the hot leg consists of a horizontal pipe, an elbow, and an inclined pipe. In this study, a scale model of the PWR hot leg (1/15th of the actual plant size) is used to investigate flow patterns and characteristics of countercurrent flow limitation (CCFL). The following conclusions are obtained.

Degradation of condensation HTC (Heat Transfer Coefficient) under an air presence in a vertical tube was explored both experimentally and analytically, with the aim of developing evaluation methods for the design of passive containment cooling systems in the next generation reactors. Measurements were done using a stainless steel tube of 49.5 mm I. D. and 2.0 m length. enclosed by a cooling jacket. Flow rates of steam. air and cooling water, and the system pressure were varied as the experimental parameters. First, condensation HTC was correlated to a function of mixture Reynolds number and air partial pressure ratio, in which thermal re&tance of the condensate film was excluded. Secondly, an analogy between heat and mass transfer was applied. The calculated values agreed well with the measured values of condensation HTCs in turbulent flow, while an obvious underestimation was observed for the flow in which mixture Reynolds number was lower than 2,300. Finally, ratios of calculated to experimental HTCS. which include thermal resistances of the condensate film, averaged 1.01 for turbulent steam flow.

When reflux cooling is executed in the case of loss of residual heat removal systems under mid-loop operation of PWR, steam generated in the reactor core and water condensed in the steam generator may form a complicated countercurrent flow in the hot leg. Numerical simulations of air-water countercurrent flows in a 1/15-scale model of the hot leg are carried out in this study to examine the capability of predicting two-phase flow patterns and CCFL characteristics in the hot leg. A three-dimensional two-fluid model implemented in FLUENT6.3.26 is used in the simulations. Good agreements between measured and predicted flow patterns and CCFL characteristics are obtained by using an appropriate set of correlations for the interfacial friction coefficient in the momentum equation of the two-fluid model.

Flow accelerated corrosion (FAC) rate downstream from an orifice was measured in a high-temperature water test loop to evaluate the effects of flow field on FAC. Orifice flow was also measured using laser Doppler velocimetry (LDV) and simulated by steady RANS simulation and large eddy simulation (LES). The LDV measurements indicated the flow structure did not depend on the flow velocity in the range of Re = 2.3×10 4 to 1.2×10 5 . Flow fields predicted by RANS and LES agreed well with LDV data. Measured FAC rate was higher downstream than upstream from the orifice and the maximum appeared at 2D (D: pipe diameter) downstream. The shape of the profile of the root mean square (RMS) wall shear stress predicted by LES had relatively good agreement with the shape of the profile of FAC rate. This result indicates that the effects of flow field on FAC can be evaluated using the calculated wall shear stress.

An experimental study on countercurrent flow limitation (CCFL) in vertical pipes is carried out. Effects of upper tank geometry and water levels in the upper and lower tanks on CCFL characteristics are investigated for air-water two-phase flows at room temperature and atmospheric pressure. The following conclusions are obtained: (1) CCFL characteristics for different pipe diameters are well correlated using the Kutateladze number if the tank geometry and the water levels are the same; (2) CCFL occurs at the junction between the pipe and the upper tank both for the rectangular and cylindrical tanks, and CCFL with the cylindrical tank occurs not only at the junction but also inside the pipe at high gas flow rates and small pipe diameters; (3) the flow rate of water entering into the vertical pipe at the junction to the rectangular upper tank is lower than that to the cylindrical tank because of the presence of low frequency first-mode sloshing in the rectangular tank; (4) increases in the water level in the upper tank and in the air volume in the lower tank increase water penetration into the pipe, and therefore, they mitigate the flow limitation.