A significant number of current PWRs are experiencing axial offset anomaly (AOA), a condition that continues to elude prediction and thus creates an operational difficulty of being unable to accurately estimate safety margin. The cause of AOA is complex, involving both thermal-hydraulics and chemistry of core operation and design. This paper considers the structure of crud layers, which lead to AOA, and presents a thermal performance and neutron absorber concentration model for the crud layer that satisfactorily predicts the amount of boron accumulated to match that required to cause the observed power shifts.

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