Control strategy study for once-through direct cycle Canadian SCWRs

Abstract Canadian Supercritical Water-cooled Reactor (SCWR) is a pressure-tube type SCWR under development in Canada. Once-through direct cycle is adopted in the design. The reactor and the turbine are tightly coupled. A suitable strategy is necessary to deal with the relationship between the reactor and the turbine for safe and smooth plant operation. A turbine dynamic model is derived together with the reactor model to complete the nonlinear plant model. Three basic control strategies: reactor-leading, turbine-leading and feedwater-leading are applied to the Canadian SCWR. Input-output pairings of the control systems are determined based on the different strategies and Proportional and Integral (PI) controllers are tuned to meet the control requirements. A typical load pattern is introduced to test the control performance. Through simulation, it is found that the load following capacity with the reactor-leading strategy is low while the fluctuations in the thermal parameters are large with the turbine-leading and feedwater-leading strategies. Therefore, a new control strategy is needed to deal with these drawbacks. A coordinated control strategy is proposed to achieve high load following capacity with small variation in the thermal parameters. It is established based on the turbine-leading strategy by adding a feedforwad control system to balance the reactor and the turbine. In the feedforward control system, the input signal is the difference between the steam pressure and its setpoint, and the outputs are the control rod reactivity and the feedwater flow rate. In such a way, the power-to-flow ratio can be maintained in a correct proportion to reduce the fluctuations in the thermal parameters. Simulations are performed by introducing a typical load pattern. From the responses, it is found that the turbine power can closely follow the load demand and the variation in the steam temperature is efficiently suppressed compared to that with the turbine-leading strategy. It can be concluded that the proposed control strategy is applicable for the Canadian SCWR and smooth operation can be achieved.