Past exam of the MPhil in nuclear energy course of the University of Cambridge / 2019 / Module NE1 / 4 / a / Solution

Xenon poisoning is neutron removal by xenon-135, which reduces reactor reactivity. Its important precursor is iodine-135. At a long-established constant neutron flux, production through precursor decay is balanced by xenon radioactive decay and neutron capture. A steady model must account for that additional absorber in its neutron balance.

After a rapid removal of the imposed neutron flux, capture removal becomes small immediately, while the existing iodine population continues supplying xenon. The xenon population can therefore rise before eventually falling as the precursor reservoir is exhausted. The resulting extra negative reactor reactivity is a delayed effect, rather than an instantaneous disappearance of the absorber when nuclear fission stops.

An intermediate post-shutdown maximum is possible. This explains why the reactivity associated with a previously operating system can differ substantially from its long-time shutdown value. The population model below quantifies that mechanism; a population maximum is not by itself an operating or restart prescription.