text: In reactor fuel, the fission product xenon tends to migrate to form bubbles in the fuel. As caesium 133, 135, and 137 are formed by the beta particle decay of the corresponding xenon isotopes, this causes the caesium to become physically separated from the bulk of the uranium oxide fuel.
  Because Xe is a potent nuclear poison with the largest cross section for thermal neutron absorption, the buildup of Xe in the fuel inside a power reactor can lower the reactivity greatly. If a power reactor is shut down or left running at a low power level, then large amounts of Xe can build up through decay of I. When the reactor is restarted or the low power level is increased significantly, Xe will be quickly consumed through neutron capture reactions and the reactivity of the core will increase. Under some circumstances, control systems may not be able to respond quickly enough to manage an abrupt reactivity increase as the built-up Xe burns off. It is thought that xenon poisoning was one of the factors which led to the power surge which damaged the Chernobyl reactor core.