NuScale is an advanced light water reactor with 50MWe. In its design, there is no coolant pump and the reactor is passively cooled. Integral design is adopted. In the pressure vessel, there are reactor core, steam generators and pressurizer. The coolant inventory is large and it takes more time to respond to disturbances. Once-through steam generator is adopted and superheated steam can be generated and sent to turbine. The water volume in the steam generator is small and it is very sensitive to disturbances. The different designs in the NuScale make its dynamic characteristics unique. Therefore, it is necessary to develop a dynamic model to study its dynamic behaviours and the control system can be further designed.
A dynamic model of NuScale is developed using Matlab/Simulink. The plant model includes reactor core model, helical coil steam generator model, pressurizer model and turbine model. A steady-state simulation is carried out and the results are compared with its design parameters. The largest relative error is 0.91%, which proves that the dynamic plant model developed is correct. Control systems are considered: reactor power, coolant pressure and pressurizer pressure control systems. The control rods are applied to regulate the reactor power and average coolant temperature. The spray and heater in the pressurizer is used to adjust the coolant pressure. The feedwater control is the most complex. Feedwater flow rate is adjusted to control the steam pressure and meanwhile the feedwater flow should be in accordance with the average coolant temperature. Therefore, average coolant temperature is considered in the feedwater control. PI controllers are selected and the gains are tuned to match the control requirements. Different load patterns are introduced to test the control system performance.