Nuclear Technology / Volume 187 / Number 3 / September 2014 / Pages 243-259
Technical Paper / Fission Reactors / dx.doi.org/10.13182/NT13-111
This paper presents the performance evaluation of an improved mechanical shim (MSHIM) control strategy that is implemented in the AP1000 reactor by a digital rod control system. The MSHIM control system automatically controls the core reactivity and axial power distribution using gray and black M control banks (M-banks) and an axial offset (AO) control bank (AO-bank). The M-banks and AO-bank are independently controlled by the power control subsystem and the AO control subsystem. In the original MSHIM strategy, the power control subsystem takes precedence, and the AO-bank is blocked from moving when a demand signal exists for the movement of the M-banks. This rod control logic can minimize the potential for interactions between the two rod control subsystems and guarantee the safety and stability of the MSHIM control system. However, the AO control capability is weakened at the same time. Thus, Westinghouse has improved this core control strategy, which gives preference to the AO-bank when both the AO-bank and the M-banks have a demand to move in the same direction. In this paper, first, the coupling characteristic of the MSHIM control strategy is analyzed to illustrate the coupling effect between the two rod control subsystems. Then, both the original and the improved MSHIM control strategies are applied to AP1000. It has been demonstrated by the MSHIM load-follow and load regulation simulation results that the improved strategy not only can provide much tighter AO control but also can reduce the total control rod movement without compromising the coolant average temperature control. Therefore, the improved MSHIM strategy can provide much better reactor control capabilities than the original strategy.