Variable overlap control

In a nuclear reactor having at least two control rod groups residing in the core for controlling the power level generated by the core, wherein one group is at a higher elevation in the core than the other, an improved method of controlling the core axial power distribution. The method comprises the steps of monitoring the core axial power distribution to provide a measured index, comparing the measured index with an axial power distribution target index, determining whether the measured index exceeds the target index, determining which axial portion of the core is generating an excessive proportion of the core power output, moving each group farther apart by substantially the same distance if the lower portion of the core is generating excessive power, and moving each group closer together by substantially the same distance if the upper portion of the core is generating excessive power, whereby the axial imbalance in the power distribution is reduced and the axial power distribution is brought within the target index