The pressurizer is the fundamental equipment in nuclear power plant, maintaining the pressure in the primary side. A U-shaped tube filled with water, as the water seal structure, was installed in front of the safety valve with the purpose of reducing the non-condensable gas leaked through the safety valve. When the safety valve opens, water slug in the U-shaped tube moves through the safety valve, causing a large number of thermal-hydraulic loads on the safety valve and downstream line. In order to reduce the thermal-hydraulic loads on the safety valve system, the U-shaped tube water seal structure was placed inside the pressurizer dome and replaced by a scoop-shaped structure. Thermal-hydraulics characteristics of the water seal structure are simulated based on the geometric model with a scale ratio of 1:1 to investigate the condensation and formation of water seal. The key parameters in the water seal formation process are investigated under different pressure and heat dissipating capacity. The species transport model is utilized to describe the impact of non-condensable gas under various mass fractions. Three-dimensional distributions of pressure and temperature are obtained from the calculation by using the CFD code ANSYS FLUENT. The water seal formation time is calculated by using the condensation rate and the geometric model. The result reveals that water seal formation can be completed within the required time, even under a high mass fraction of non-condensable gas. Water seal formation time reduces when the system pressure increases. The temperature difference across the water seal is lower than 20K.

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