Fluoride salt cooled High temperature Reactor (FHR) is a kind of Gen-IV reactor which possesses many attractive features, such as high temperature, low pressure etc. Thermal-hydraulic features of molten salt are different from coolants of traditional reactors, which dominate operation transient behavior of FHR. However, as a new type reactor with sphere fuel element and fluoride salt coolant, FHR has inadequate operating experience and data used for reactor control and the design of power regulating system. Therefore, research of power regulation strategy is very important for FHR in automatic control operation and commercial application.
A code programmed in Fortran platform is used for investigating the system transient behavior, control logic and strategy. Based on the transient analysis code OCFHR for FHR, power control logic strategy is studied on a model of 10 MW Fluoride salt cooled High Temperature Experiment Reactor. OCFHR is a specialized code in FHR transient analysis, which contains point reactor model, simplified core thermal-hydraulic model, molten salt-salt exchanger and molten salt-air exchanger with a tube-shell type, control rod system and power regulation and control model. The control module of OCFHR uses the incremental PID controller to regulate control parameters and adopts the compound mode of control rod adjustment, load adjustment and molten salt flow adjustment, so that it can adjust the control rod position, primary and secondary molten salt flow rate and air flow rate of load at different operating power levels.
Two kinds of steady operation strategies are studied in this paper, which are a) constant outlet coolant temperature and b) constant average coolant temperature. The power level is regulated by control rod while the working temperatures are adjusted by shifting the load with weight coefficients of power and temperature deviations.
The results show that the incremental PID controller with optimized parameters can achieve the control requirement. Both of temperature control strategies gain great performances under 10%FPand 50%FP power regulation. The target power is reached quickly and accurately by using the incremental PID controller while the temperature control is very time-consuming. Compared with b), strategy a) has less temperature overshoot but larger power overshoot and longer adjusting time. The step wise power regulation for FHER is doable when a wide power adjustment range is needed and the simulation 10%FP treated as a step works well. Besides, the preliminary study of varying secondary coolant flow rate also indicates that the secondary loop plays an important part in restraining the deviation of secondary coolant temperatures during the process of balancing the power and load, so it is better to adjust the secondary coolant flow in terms of the power regulation range.