## 1 Introduction
Based on this fact, a real-time dynamic model with high accuracy has to be developed for the control system design and the performance assessment. In the present study, an investigation of control systems of MSR concepts was conducted based on a simplified dynamic model developed by several researchers. A lumped-parameter system model was developed by [PERSON] [23] to preliminarily investigate the transient behavior and control strategy of a single-fluid Molten-Salt Breder Reactor (MSBR). [PERSON] et al. [24] developed a point-reactor coupled with a heat transfer model of the MSBR core for a temperature control investigation. [PERSON] [25] applied a classic PID controller for the power control system based on a nonlinear dynamic model. Fuzzy-PID control with a linear MSBR core model was proposed by [PERSON] et al. [26], and the performance of the PID controller was improved by applying the Internal Model Control (IMC) method [27]. [PERSON] et al. [28] presented different control strategies for an MSFR power plant based on a linearized model. However, the transient behaviors represented by the space distribution of the system parameters cannot be captured by simplified lumped-parameter models, and more deviations are expected when the linearization procedure is conducted. Thus, the control performance and precision are limited. Meanwhile, these studies mainly focused on the control of the thermal power of this reactor, and the entire plant system was not modeled to take into account the coupling effect between different circuits, which has a large influence on the system behavior. In addition, the input saturation and the dead zone have to be considered during the design process of the controllers, as is needed for engineering realization. In this study, an MSR plant, consisting of a reactor with liquid fuel and moderated by graphite, a heat exchanger, and a steam generator, was selected as the system to be modeled. Based on the 2250 MW MSR concept presented by [PERSON] [29], a nonlinear distributed-parameter model was established. Taking into account its dynamic characteristics, a new control system was designed to achieve the load-following capability of the whole plant, which is the objective of this study. A real-time nonlinear model of the whole system was established, and the system operation scheme was proposed. A joint feedforwardand feedback scheme was applied as a novel control strategy. In addition, the amplitude-limiting block and dead zone block were integrated into the control system, consisting of the power controller and the steam temperature controller, to ensure stable control performance under various engineering circumstances. The developed nonlinear system model was verified by comparing its transient responses with those from the reference data [30]. A stability analysis of the power and steam temperature control systems was performed with the Nyquist criterion, and system simulations were carried out with four different perturbations at the nominal power level. In addition, two types of load changes were selected for the investigation of the load-following capability: a 10% load step reduction and a linear load change at a speed of 5% FP/min.