Mechanical Shim(MSHIM) mode is used in AP1000 nuclear power plant to control core reactivity and power distribution during load maneuver operations.In MSHIM mode,the motion of MSHIM control rods changes the neutron flux distribution in core,and consequently changes the core power and its distribution.The relationship between the core power and the ex-core Power Range(PR) detector Axial Flux Difference(AFD) is affected significantly,and the current methodology that uses the ex-core PR detector AFD as the input makes too many errors in reactor protection set-point calculation.In this article,the process of measuring axial flux difference by the PR detectors is introduced,and the analysis of effect on AFD in MSHIM mode is discussed.At last,the mathematical modeling of the core power difference and the ex-core PR detector AFD is given to find a method to reduce the error.%AP1000采用机械补偿模式在负荷跟踪过程中对堆芯反应性和功率分布进行控制，在机械补偿模式下，由于机械补偿控制棒的移动，堆芯中子注量率分布发生变化，从而改变了堆芯功率及其分布，作为反应堆保护重要参数的堆外核测测量结果无法满足停堆保护的要求。本文主要介绍了堆外核测堆芯轴向功率偏差的测量方法及其在确定反应堆超温和超功率停堆保护整定值中的作用，分析了采用堆外核测功率量程探测器测量轴向功率偏差在机械补偿模式下受到的影响，提出一种修正测量结果的方法。
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