Estimation of pressure-equalizing time for a hybrid safety injection tank used in a passive safety injection system

In this paper, a method for estimating the time required to reach the equalizing pressure between two tanks for a hybrid safety injection tank (SIT) system is proposed. The hybrid SIT allows a coolant injection over a wide range of operating pressures without using active systems such as a high-pressure pump. The pressure of the SIT can be equated to that of the reactor vessel through a pipe connected between the SIT and the pressurizer (PZR). Thus, the coolant is injected into the reactor vessel via the gravitational head of water. In this study, a zero-dimensional calculation with a simple approach was employed to estimate the pressure-equalizing time of the hybrid SIT. The real gas equation of state was used to estimate the time with mixture properties. The mass of the injected steam was calculated using the relationship between the pressure difference and mass. The results showed that the difference in the pressure-equalizing times is less than 5% compared to the test results. Accordingly, a guideline is proposed for the design of hybrid SITs for new nuclear power plants. Appropriate pipe loss coefficients for a pressure balancing line (PBL) were suggested for an integral effect test facility.