Abstract: A reaction vessel for heating by helium under pressure comprising an upright container provided with an upper part and with a lower part constituting a heating space. In the lower container part are vertically spaced inlet and outlet openings for heated helium under pressure. The upper container part has spaced walls providing a space therebetween and two covers. The upper end of the lower container part is closed and catalyst-filled reaction tubes pass through such closure into the heating space. Inlet and outlet pipes for the reaction tubes extend in coaxial manner from the reaction tubes head through the inner wall of the upper reactor part below the cover of such inner wall, and thence through the space between the inner and outer walls and finally to the outside through the outer wall of the upper part below the cover for the outer wall. The length of the inlet and outlet pipes inside the space is more than the distance of the walls, and the coaxial pipes may be disposed in serpentine fashion.