## 3 Control System Design of the MSR Plant  
### Operation and Control Scheme  
Before designing the control system, it is necessary to determine the temperature and mass flow rate curves for each subsystem of the MSR plant in steady-state operation. In this work, the system operation parameters were determined by performing numerical simulations under steady-state operating conditions at partial loads. The major system parameters of this plant are listed in Table 1. The set value of the outlet temperature of the super-heated steam is constant at 810.9 K, and the inlet temperature of the feedwater is also set to be constant at 644.3 K so that the corrosion of turbine blades by droplets from steam is minimized. The outlet temperature of the primary salt in the reactor core varies linearly with power, while the inlet temperature is unchanged until the power is increased to 50% full power (FP), and then the temperature is increased from 810.9 K to 838.7 K for a further increase in power (from 50% FP to 100% FP). The inlet and outlet temperatures of the secondary salt in IHX are derived by applying the energy conservation law at steady state. To minimize the neutronic fluctuation resulting from the drifting effect of the delayed neutron precursors, the transit times of fuel salt inside and outside the core, \(\tau_{c}\) and \(\tau_{L}\), are set to be constant under transients. To achieve this, the primary salt flow rate is set to be constant at its nominal value. The relation between the secondary salt flow rate and the load is described by a fitting function, which was determined by performing numerical simulations under steady-state operating conditions with various loads. An innovative control system consisting of two load-tracking control modules--the thermal power controller and the steam temperature controller--was developed in this work for the realization of the desired operation scheme. The thermal power controller was optimized for a fast load transient response performance, which means that the overall thermal power should follow the load variation with as low a latency as possible so that the fission energy generated in the reactor core is maximized and the system stability is ensured. The objective of the steam temperature controller is to maintain a constant outlet temperature (810.9 K) of the steam for the steady-state operation and as low as possible fluctuation under transients so that the steam provided to the turbine has a stable quality and less damage to the blades of the turbine is expected. The schematic of the entire control system is illustrated in Figure 3. The measurements of the neutron flux (as an indicator of the power) and the outlet temperature of the primary salt are compared with the set values corresponding to the real-time load demand (desired values). Keeping the mass flow rate of the primary salt constant, the set values of the reactor power are a function of the outlet temperature during the transient process. The deviation signal of the neutron density (\(\Delta N_{r}\)) and the outlet temperature (\(\Delta T_{f0}\)) are processed and then passed to the control rod servo to introduce the desired reactivity. Meanwhile, a feedforward control scheme of the secondary pump is applied so that the temperature of the secondary salt is adjusted through the regulation of its mass flow rate corresponding to the desired power. Steam temperature control is realized by regulating the mass flow rate of the feedwater to ensure that the steam temperature is constant.