Water formation test loop was constructed with a scale ratio of 1:1 to investigate the condensation and analogous U-shaped tube water seal in front of pressurizer safety valve. The water seal features were investigated under different pressure and various concentrations of non-condensable gases. The test data reveals that water seal can completely formed even in the presence of non-condensable gases in high content. Water formation time reduced when increased system pressure and refilling of the pipe is much more rapid than the initial one due to a lower content of non-condensable gases remaining in the steam phase. A simple criterion to determine the water seal starting point has been put forward in this paper. The strong natural convection in the water seal keeps the temperature difference lower than 20K.
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