The 135Xe buildup occurs when a reactor is brought to a low power level as well as at shutdown and then the 135Xe concentration begins to decrease soon, or vice versa, when the power level is increased. This is called the “Xe transient” which is characterized by times of the order of about 8 h [9] and therefore it should be controlled properly.

In a large power reactor, local changes in the power distribution by such factors as control rod movement can lead to spatial oscillations even though the reactor power is kept constant. In the region of increased power density, 135Xe burns out more rapidly and then its concentration decreases. This decrease leads to a higher reac­tivity in this region, which, in turn, leads to an increased neutron flux. This again leads to a more increased power density in this region. Meanwhile, in the region of decreased power density, the 135Xe concentration increases due to its reduced burnup. This increased concentration decreases the local reactivity, which reduces the neutron flux, in turn, decreasing the local power density again. In a short time, however, the decreased 135Xe concentration begins to increase and contrarily the increased concentration begins to decrease. In this way, the local power continues to oscillate between different regions. Calculations show that these oscillations have a period from about 15-30 h. Actually this type of oscillation is easily controlled in practical reactor operations by negative reactivity feedback (moderator void coeffi­cient and Doppler coefficient) and a procedure of control rod movement.