## 4 Results and Discussion
### Control Performance Analysis
#### 4.3.1 Step Load Change  
In this load variation transient, the electrical grid load demand was stepped down by 10% at t = 50 s after the steady-state operation with nominal power. The control system was actuated because of the mismatch between the actual power and the desired value. The transient behaviors of key system parameters and the control process are presented in Figure 13. The load signal, as the desired power, was transmitted to the control system to adjust the thermal power. The deviations of the measured values from the set values, \(\Delta N_{r}\) and \(\Delta T_{fo}\), were transmitted to the control system as error signals. Then, they were processed into a control signal and forwarded to the servo to drive the control rods. As shown in Figure 13a,c, the insertion of negative reactivity decreased the reactor power to around 87% FP in several seconds. As the fuel salt was less heated by the decreased power, an insertion of positive reactivity caused by the negative temperature feedback coefficient of the fuel salt took place, which increased the power up to around 90% FP. This coupling effect resulted in the fluctuation of the thermal power until it reached a new equilibrium point. Simultaneously, a feedforward scheme, which was achieved by adjusting the secondary salt flow rate, was integrated into the control system. The secondary salt flow rate was assumed to be determined by the load. The reactor outlet temperature was decreased due  
Figure 11: System responses to an increased fuel mass flow perturbation: 10%.  
to the reduced power, and the reactor inlet temperature was increased for the first stage because of the reduction in heat transferred to the secondary salt due to the decrease in its mass flow rate, and then it decreased as a consequence of the reduced power, as can be observed in Figure 13e,f. In addition, the load signal transmitted to the control system was also used to maintain a constant steam outlet temperature. To achieve this purpose, the mass flow rate of the feedwater was adjusted to limit the temperature variation, as shown in Figure 13d. In the first stage of the transient, the steam temperature decreased quickly due to the reduction in the heat absorbed by the feedwater in the steam generator. As a consequence, the deviation of the steam temperature was transmitted to the control system as an error signal, which was then processed and sent to the pump servo to change the feedwater flow rate. As Figure 13b shows, the steam outlet temperature dropped rapidly with the reduction in thermal power and then rose gradually since the feedwater flow rate was reduced until it reached its set value.  
As illustrated in Figure 13a, a high consistency between the actual value and the desired value (control target value) is observed, which means that, in this case, a suitable load-following capability was achieved. During the control process, the overshoot was lower than 2.6%, and a settling time of around 15 s was observed. The steam temperature was able to follow its set value within a difference of less than 4 K, as shown in Figure 13b, and this is acceptable from an engineering point of view. The reactivity inserted by the control rod was within a range of \(-20\) pcm at a speed bounded by \(\pm 10\) pcm/s, so it is achievable by the control rod servo. As the mass flow rate of the feedwater is approximately  
Figure 12: System responses to an decreased fuel mass flow perturbation: \(-10\%\).  
proportional to the transferred heat according to the energy balance, the final mass flow rate is around 90% of its nominal value. The inlet and outlet temperature changed within a range of 20 K as a consequence of the power change and heat balance. These results show that the control systems have an excellent performance for this step load reduction transient.