A condition in which a number of neutron-absorbing elements accumulate in the core of a nuclear reactor slowing down the fission reaction. These neutron-absorbing substances are also called reactor poisons or neutron poisons. They are produced either directly as fission products or by their radioactive decay. They are isotopes with short half-lives, or they change to non-neutron-absorbing substances after neutron absorption. Their concentration can even lead to the cessation of the fission reaction. A well-known example of reactor poisoning is xenon poisoning. Xenon, the strongest known neutron absorber, is produced at low reactor power and leads to reactor poisoning, but when power is increased, it is rapidly burned to a stable isotope that no longer absorbs neutrons.