(EN) THE INVENTION RELATES TO A REACTOR FOR CARRYING OUT AN EXOTHERMIC REACTION IN THE GAS PHASE, WHICH COMPRISES A VESSEL HAVING AN OUTER WALL (13) COMPOSED OF A METALLIC MATERIAL, WHERE AN INNER SHELL (17) IS ACCOMMODATED IN THE INTERIOR OF THE REACTOR (1) AND THE INNER SHELL (17) IS AT A DISTANCE OF AT LEAST 50 MM FROM THE INSIDE OF THE OUTER WALL (13) (FIG.1)