The present invention relates to a process for the continuous preparation of a polyolefin in a reactor from one or more α-olefin monomers of which at least one is ethylene or propylene, wherein the reactor comprises a fluidized bed, an expanded section located at or near the top of the reactor, a distribution plate located at the lower part of the reactor and an inlet for a recycle stream located under the distribution plate, wherein the process comprises—feeding a polymerization catalyst to the fluidized bed in the area above the distribution plate—feeding the one or more α-olefin monomers to the reactor—withdrawing the polyolefin from the reactor—circulating fluids from the top of the reactor to the bottom of the reactor, wherein the circulating fluids are cooled using a heat exchanger, resulting in a cooled recycle stream comprising liquid, and wherein the cooled recycle stream is introduced into the reactor using the inlet for the recycle stream wherein a stream comprising a thermal run away reducing agent (TRRA-containing stream) is introduced into the expanded section during at least part of the polymerization process, wherein said TRRA-containing stream is brought into contact with at least part of the interior surface of the expanded section.