Background With the global energy structure continuing to shift toward low-carbon development, small modular reactors have attracted increasing attention as an important direction for advanced nuclear energy systems due to their modular construction, enhanced inherent safety and flexible load regulation capability. Among different small modular reactor concepts, integral small modular pressurized water reactors are considered promising for early deployment because of their mature technical foundation. However, load-following operation of such reactors involves strong coupling among reactor kinetics, primary-loop thermal-hydraulic behavior, steam generator heat transfer and turbine power demand. Therefore, detailed dynamic modeling and transient analysis are required to evaluate their operational characteristics under variable load conditions.

Purpose This study aims to develop a nonlinear dynamic model of an integral small modular pressurized water reactor and investigate its transient response characteristics under representative load-following conditions.

Methods A nonlinear dynamic model of an integral small modular pressurized water reactor was developed on the Simulink/Simscape platform. The model consists of key components, including the reactor core, steam generator, turbine throttle valve, turbine and condenser, and describes the dynamic coupling among the reactor and secondary-loop system. Based on the lumped-parameter modeling framework, six-group delayed neutron kinetics and axial core nodalization were introduced to describe reactor transient behavior and axial thermal-hydraulic distributions more accurately. A coordinated control strategy was further designed for wide-range load variation conditions. Step-load and ramp-load simulations were then carried out to analyze the multi-loop regulation mechanism and the dynamic responses of key system parameters.

Results The steady-state results agreed well with the design values, indicating good model accuracy. Under step-load disturbances, turbine valve action caused rapid variations in steam flow rate and steam pressure, while feedwater regulation changed the secondary-side heat transfer and subsequently affected the primary-side average coolant temperature. With the coordinated action of the control loops, the system recovered to stable operation within several hundred seconds. Compared with step-load disturbances, ramp-load variations produced smoother transient responses and better synchronization between reactor power and turbine power. Under low-load conditions, a mismatch between the secondary-side flow rate and the corresponding power level was observed, leading to a reduction in overall thermal efficiency.

Conclusions The proposed model can effectively describe the coupled dynamic behavior of an integral small modular pressurized water reactor during load-following operation and provide a useful tool for transient analysis and control strategy optimization.