Robustness of the MSHIM Operation and Control Strategy in the AP1000 Design
Authors: Drudy Keith J. , Morita Toshio , Connelley Barbara T. 
Published: February 2010

The Westinghouse AP1000™ nuclear power plant design uses several evolutionary and advanced components, systems and analysis methods in order to maximize plant operational safety and efficiency. One advanced component of the AP1000 is the integration of the MSHIM operation and control strategy within the plant design. As a summary, the MSHIM operation and control strategy is an operational philosophy that has evolved from the Constant Axial Offset Control (CAOC) strategy, which was originally developed for the current generation of Westinghouse PWRs. The term MSHIM is derived from the fact that reactivity control uses low worth (or gray) control rod banks as a “mechanical shim.” This is opposed to today’s PWRs, which depend largely on changes in the chemical shim (soluble boron) concentration in the reactor coolant in order to provide fine reactivity control. The MSHIM strategy is unique in that it makes use of two independently controlled rod groups to provide fine control of both the core reactivity and axial power distribution during a wide range of operational scenarios. In the AP1000, the MSHIM operation and control strategy has been fully integrated into the digital rod control system. Specifically, automatic control of core reactivity (or RCS temperature) is provided using four (4) banks of “gray” control rods and two (2) banks of traditional control rods, all moving in a defined overlap. Furthermore, automatic axial power distribution (i.e., ex-core ΔI) control is provided using a single, heavy bank of traditional control rods which move independently of the reactivity control banks. It has been demonstrated that fine control of core reactivity and axial power distribution control can be achieved during a wide range of operational scenarios while relying only upon the automatic rod control functionality. Changes in the concentration of the chemical shim within the reactor coolant can thus be limited to only that required to directly compensate fuel and/or burnable absorber depletion during a given fuel cycle. In the AP1000, the MSHIM operation and control strategy is complemented by the BEACON™ on-line core monitoring system. BEACON provides direct monitoring of power distribution related parameters such as departure from nucleate boiling ratio (DNBR), peak linear heat rate (PLHR), and shutdown margin (SDM). The integration of the BEACON monitoring system into the AP1000 plant increases operational flexibility by relaxing certain operational limits during BEACON operation. The net effect of BEACON integration, thus, is to maximize the ability of rod control under the MSHIM strategy to maintain reactor operation within the associated technical specification limits, even under adverse operational transients. This paper explores the robustness of the MSHIM operation and control strategy under a range of anticipated plant operational scenarios. Furthermore, this paper demonstrates the operational simplification associated with this strategy as it has been integrated into the AP1000 plant control systems.