Abstract: The present invention is a system that updates a pressurized water reactor core model, or data file describing the core, periodically using measured values determinable from core instrumentation. The system starts with a calibrated model calibrated from a flux map and a boron concentration measurement both determined monthly. Reactor system measurements are performed and the model is updated from the measurements using a standard depletion calculation. Some of the measured values are also used to determine measured actual values of axial offset and pinch. Standard neutronics equations are used to calculate analytical values of axial offset and pinch from the model, and the measured and calculated values are compared. If a deviation beyond a certain magnitude is detected, buckling coefficients in the model are adjusted to compensate for model drift. The updated model is stored and a time-out period is entered until it is time for another drift compensation cycle to start.