Depressurization Behavior and Strategy of Automatic Depressurization System for AP1000

Abstract This article through simulate small-break loss-of-coolant accidents (SBLOCA) specifically in the AP1000 and thoroughly analyze the behavior and strategy of the ADS in such accident scenarios. A comprehensive AP1000 model is constructed using the System Analysis Code, incorporating various components including the reactor coolant system (RCS), secondary system, passive safety injection system (PSIS), and automatic depresurization system (ADS). Five different types of conditions are simulated and evaluated to assess their thermal-hydraulic impact on the RCS, passive core cooling system, core degradation, and the ADS itself. The analysis results show that the design of two sets of independent ADS depressurization pipelines aims to achieve a redundancy level of 2 × 100%. Gradually opening the ADS pipelines increases the inlet pressure of the sprayer and widens its pressure curve, allowing for a sustained stable critical jet state over an extended duration. In the initial stage of the accident, opening the ADS stage with a large area stage can effectively improve the depressurization and cooling rate of the RCS and maintain the critical discharge state of ADS. ADS-4 can reduce the RCS pressure to the start-up pressure of In-containment refueling water storage tank (IRWST) to ensure long-term core cooling. These results hold significant meaning in comprehending the behavior and strategy of the ADS in typical SBLOCA scenarios, thereby contributing to the establishment of an effective theoretical foundation for the design of ADS pipelines and sprayers.