Study on the water seal formation process in advanced PWR pressurizer using CFD method

Pressurizer is one of the most important equipment in pressurized water reactor (PWR) nuclear power plant. Its function is to maintain the pressure and water quantity of the primary circuit. In the early design, a U-tube was installed in front of the pressurizer safety valve and filled with water to reduce the leakage of non-condensable gas through the valve. This U-tube is called a water seal structure. However, when the safety valve is opened, the water in the U-shaped pipe passes through the safety valve, which generates a large number of thermal and hydraulic loads on the safety valve and downstream pipeline, which is not conducive to the safety of the system. In order to reduce the thermal and hydraulic loads in the safety valve, a new type of water seal structure was designed. The new water seal structure consists of a shovel-shaped structure placed in front of the safety valve pipeline in the dome of the pressurizer. In this paper, the thermo-hydraulic characteristics of the new type of water seal structure are simulated, and the process of steam condensation and water seal formation is studied. The key parameters of water seal formation under different pressure and heat dissipation conditions were studied. The species transport model is used to describe the effect of non-condensable gas on condensation process. The three-dimensional distributions of velocity, pressure and temperature are obtained. The time required for water seal formation was calculated by using condensation rate and water tank volume. The calculation results show that the water seal in the pressurizer can still be formed within the required time under the condition of the influence of non-condensable gas. The temperature difference between inside and outside of water seal is less than 20 K. The new water seal structure satisfies the requirement of the actual start-up process of the reactor.