Negative power coefficients keep the PWR cores stable against core power oscillation. The stability of PWR cores is focused on the core power spatial oscillation induced by xenon, which tends to become larger and gives a less convergence for a larger core and a flatter power distribution.

(1) Radial Power Distribution Oscillation

A radial power distribution oscillation may occur by abnormal operation of rod cluster control assemblies (for example, only one rod cluster control assembly works in an abnormality). Core equivalent diameter is smaller than one causing an oscillation divergence and the radial power distribution oscillation has an inherent convergence feature due to the feedback effect of moderator temperature coefficient. Therefore, it is not necessary to suppress the radial power distribution oscillation which can be continu­ously monitored and detected by quadrant core power tilt (a symmetry index of core four-quadrant powers) through the coolant outlet temperature distribution and ex-core neutron detectors.