 Removing the most significant neutron poison xenon-135 made the reactor safer and easier to restart.In solid-fuel reactors, on restart the 135Xe in the fuel absorbs neutrons, followed by a sudden jump in reactivity as the 135Xe is burned out.Conventional reactors may have to wait hours until xenon-135 decays after shutting down and not immediately restarting (so-called "iodine pit").