Reduction of the reactivity of a reactor by very high neutron capture in the fission product Xe-135. The increase in Xe-135 concentration after the shutdown of a reactor – Xe-135 is generated by the decay of the precursor nuclide I-135 with a half-life of 6.6 hours – may have the effect that the reactor cannot be restarted until the decay of the Xe-135 concentration – the maximum of Xe-135 concentration is reached after about 12 hours.