Analysis of steam generator blowdown system flash tank liquid level instability and study on blowdown system modification

•Transient simulations of the steam generator blowdown system were conducted by RELAP5.•Causes and impact factors of the blowdown system flash tank liquid level instability were investigated.•Parallel flash tank arrangement and individual tank level signal control lead to unstable flash tank levels.•A modification plan to combine two blowdown flash tanks into one was proposed to address the issue of unstable liquid level. The steam generator blowdown system is one of the critical auxiliary systems in Pressurized Water Reactor (PWR) nuclear power plants. However, due to the complexity of its control system, the system often experiences issues with unstable operation of control parameters. The blowdown system of a steam generator is modeled by RELAP5 in this study. The present study investigates the impact patterns of parameters (such as blowdown flow rate, blowdown temperature, and key component resistance characteristics) on the liquid level of the flash tank. When the A/B two blowdown flash tanks are operated in parallel, the level of only one flash tank is used as the control signal for the blowdown pump. That is, the level of flash tank A controls the discharge pumps on both A/B branches. When the temperature drops, the flow rate increases, and the liquid phase resistance increases, it causes an imbalance in the pressure distribution in the A/B branch, which leads to an uncontrolled level in the flash tank and prevents normal discharge. Based on actual system operational data and computational results, this article proposes a modification plan to combine two blowdown flash tanks into one and the liquid level of the combined flash tank provides the control signal for the liquid level control system. In conclusion, this article proposes a theoretical solution to address the issue of unstable liquid level in the steam generator blowdown system’s flash tanks and provides support for the subsequent implementation of power plant modifications.