The production and removal of xenon can be characterized by the following differential equations:
Note the coupling between the two equations. Decay of iodine 135, λINI, in the equation for iodine, produces xenon in the equation for xenon. As can be seen, we have neglected the burnup of 135I since its cross-section is very small. The xenon burnup term above refers to neutron absorption (or neutron capture) by xenon 135 by the following reaction. Xenon 136 is not a significant neutron absorber. Therefore, the neutron absorption by xenon 135 constitutes the removal of poison from the reactor. The burnup rate of xenon 135 depends on the neutron flux and the xenon 135 concentration (i.e., the reaction rate).