The Mechanical Shim (MSHIM) core control strategy makes use of two independently controlled rod groups to provide fine control of both core reactivity and axial power distribution. This paper presents a reactor core fast simulation program (RCFSP) for AP1000 using MATLAB/SIMULINK. A nodal core model including xenon iodine dynamics is used to describe the core thermal power transient with the two group neutron diffusion equation for neutron kinetics modeling and an integral method for thermal-hydraulic calculation. Two closed loop rod controllers with implementation of the MSHIM core control strategy are developed to modulate the insertion of control rods. Based on the developed RCFSP, the MSHIM load follow operations with the original and revised MSHIM control strategies and two typical MSHIM load regulation operations with ten-percent step load change and five-percent per minute ramp load change are simulated. Results of these MSHIM operations demonstrate that the core reactivity and axial power distribution can be well-controlled via automatic rod control only. It has also been demonstrated that the MSHIM capabilities provided by the original MSHIM strategy are not diminished by the revised one. Moreover, the M-bank insertion for the original strategy is much deeper than that for the revised one. Thus, the power distribution perturbations associate with the M-bank movement for the revised strategy are not as pronounced as those for the original one during load change transients, which helps to alleviated peaking factor concerns associated with the control rod insertion. In view of its accuracy, simplicity and fast computation speed, the developed RCFSP can be used for dynamic simulations and control studies of the AP1000 reactor with application of MSHIM control strategy. With the adoption of modular programming techniques, the RCFSP code can be easily modified and applied to other pressurized water nuclear reactors that employs separate, independent control rod banks for respectively controlling core reactivity and axial offset within corresponding deadbands.

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