A reactor including a bundle of catalyst tubes through which a reaction mixture is passed, and wherein a heat exchange medium is passed through a space surrounding the catalyst tubes and including ring lines at both reactor ends with jacket orifices for feeding in and removing the heat exchange medium by at least one pump. The heat exchange medium is fed to the upper ring line and is sucked in via the lower ring line by the at least one pump having at least one vertical pump shaft mounted and operated at its upper end. The at least one pump includes a diagonal rotor and a restrictor gap in the longitudinal direction of the at least one pump shaft, within the heat exchange medium, on the pressure side of the at least one pump for sealing and mounting the at least one pump shaft and reducing axial shear of the diagonal rotor.