For a variety of reasons, the reactor core can become unstable to power oscillations, e. g., initiated by instability in the reactivity control systems or turbines. In addition, spatial effects caused by the fission product xenon can also be a trigger across, as well as up and down the core. Instability is particularly an issue for large boiling water reactors (BWRs) due to the large core size, although it can occur in smaller cores, but will be least significant in those cases.

Stability in the control systems for the reactor hardware means that total core power oscillations are not usually possible, and nuclear design of the core and control rods can ensure that the xenon effects are self-limiting.