Experimental Study and Numerical Simulation Analysis on Thermal Response Characteristics under High Pressure of Nitrogen Pressurizer

As an important component of nuclear reactors, the pressurizer plays a crucial role in controlling the pressure of the reactor coolant system during normal operation, transient conditions, and accident scenarios. Currently, steam-type pressurizers are commonly used in pressurized water reactors (PWRs) both domestically and internationally. Its working principle relies on phase equilibrium and the compressibility of saturated steam for pressure control. Compared to steam-type pressurizers, nitrogen pressurizers, which utilize a mixture of nitrogen and steam or a high volume fraction of nitrogen for pressure stabilization, offer advantages such as compact size, flexible arrangement, and rapid response. Therefore, some compact small PWRs adopt nitrogen pressurizers for pressure control of the coolant system. Nitrogen, as a non-condensable gas, exhibits significant differences in physical properties compared to steam. It is necessary to conduct experimental research on the thermal response characteristics of nitrogen pressurization systems under high pressure. This paper focuses on the experimental investigation of the thermal response characteristics of a pure nitrogen-water nitrogen pressurization system under high pressure. The experimental setup includes a water tank, a surge line, and a nitrogen pressurizer. During the experiment, the water tank was heated or cooled to simulate the temperature rise and fall processes of the reactor. The numerical simulation analysis utilizes the LOCUST 1.0.2 thermal-hydraulic system analysis program, referencing the experimental circuit diagram to establish a one-dimensional nitrogen pressurization system calculation model. The modeled components include the surge line and the pressurizer, both of which are represented as tubular components. The inlet of the surge line is connected to a time-dependent control volume, with the input boundary conditions being the temperature of the water tank and the flow rate at the inlet of the surge line. The results indicate that, compared with the heating rates of +20, +40 and +60 ℃/h, the higher the heating rate, the faster the response of parameters such as pressure in the nitrogen pressurizer, and the smaller the difference in the response rate of the pressurizer under different heating rates. For the heating condition, when the temperature measurement point of the pressurizer is submerged in water, a sudden temperature rise occurs, with the maximum temperature increase being 9%. For the cooling condition, when the temperature measurement point of the pressurizer is exposed to the nitrogen space, a sudden temperature drop occurs, with the maximum temperature decrease being 3%. Compared with the calculated values of LOCUST 1.0.2 and the experimental values, the relative deviation of the pressure value of the pressurizer is 2.8%, which is within the acceptable range.