Some of the fission products generated during the operation of a reactor have a large capture cross-section for neutrons (e.g. Xe-135). The control device must be adjusted to compensate for the reactivity equivalent of the reactor poisons to keep the reactor at its power level. reactor poisons (e.g. boron acid solution) are injected into water-moderated reactors for emergency shutdown. Boric acid solution is used to compensate for excess reactivity in pressurized water reactors.