A reactor (1) is proposed, comprising a bundle of catalyst tubes (2) through which a reaction mixture is passed and through whose space surrounding the catalyst tubes (2) a heat exchange medium (3) is passed, comprising ring lines (4, 5) at both reactor ends with jacket orifices (6, 7) for feeding in and removing the heat exchange medium (3) by means of one or more pumps (8), the heat exchange medium (3) being fed to the upper ring line (4) and being sucked in via the lower ring line (5) by the pump(s) (8) having a vertical pump shaft (9) or vertical pump shaft(s) (9) which is or are mounted and operated at their upper end, and the pump(s) (8) having a diagonal rotor (10) and a restrictor gap (11) in the longitudinal direction of the pump shaft(s) (9), within the heat exchange medium (3), on the pressure side of the pump(s) (8) for sealing and mounting the pump shaft(s) (9) and reducing the axial shear of the diagonal rotor (10).