Design of Decoupled Mechanical Shim Control System for a Generation III+ Pressurized Water Reactor Based on Feedforward Compensation and Multimodel Approach

The advanced Mechanical Shim (MSHIM) core control strategy employs two separate and independent control rod banks, namely the MSHIM control banks (M-banks) and axial offset (AO) control bank (AO-bank), for automatic reactivity/temperature and axial power distribution control respectively. The M-banks and AO-bank are independently controlled by two closed-loop controllers called the coolant average temperature (T_avg) controller and AO controller. Since the movement of M-banks and AO-bank can both affect the T_avg and AO, the T_avg controller is coupled with the AO controller. In order to avoid the interference between the two controllers, the MSHIM control system adopts an interlock design between them to avoid the simultaneous movement of the M-banks and AO-bank and make sure the priority of the M-bank movement. This design can enhance the stability of the MSHIM control system. However, the control performance is degraded at the same time. In the present study, the feedforward compensation decoupling method and multimodel approach are used to eliminate the coupling effect between the two controllers in the MSHIM control system during a wide range of power maneuvers. A multiple feedforward compensation system is designed with integration of feedforward compensators for the T_avg and AO controllers at five power levels using the multimodel approach. By implementing it in the MSHIM control system, the interlock between the M-banks and AO-bank can be released to realize the independent and decoupled control between T_avg and AO. The effectiveness of the decoupled MSHIM control system is verified by comparing its control performance with that of the original MSHIM control system during typical load change transients of the AP1000 reactor. The obtained results show that superior and decoupled control of T_avg and AO can be achieved with the proposed decoupled MSHIM control system.