Research on the Influence Mechanism and Transient Characteristics of the Replication Loop in the Steam Generator Water Level Control System of Nuclear Power Plants

In the steam generator water level control system, the replication loop is a key design feature enabling bumpless transfer of the flow control valve from manual to automatic mode. However, under certain operating conditions, this loop may trigger unintended control actions. To systematically identify and mitigate associated risks, this study investigates the impact of the replication loop under various operational scenarios through a combination of theoretical analysis and simulation. Analysis of the control logic reveals that when the secondary circuit load exceeds 20%, the main control valve is in manual mode, and the bypass control valve is in automatic mode, a positive disturbance in the steam-water mismatch signal will cause the replication loop to drive the bypass valve towards closure. This action exacerbates the declining trend of the steam generator water level and increases the risk of an unplanned reactor trip. Transient simulation results from a simulation platform further validate the dynamic response characteristics of key parameters such as flow rate, water level, and valve opening during this process. The study proposes two improvement measures: First, strictly avoid the operational configuration where the “main valve is in automatic while the bypass valve is in manual” during mode switching above 20% load. Second, introduce an integrated operation function into the control system by embedding optimized sequential control logic into the system process. These measures enhance operational standardization while effectively eliminating potential adverse effects caused by the replication loop, thereby providing an effective technical approach to improve the operational reliability of the water level control system.